Living and nonliving things share common characteristics, such as being composed of atoms, the smallest units of natural substances.
- Living things, though, have many distinctive features:
- Living things take in and use energy and materials.
- Living things sense and respond to specific changes in their environment.
- Living things reproduce and grow.
- Living things consist of one or more cells.
- Living things maintain homeostasis (dynamic balance).
Our Place in the Natural World
- Humans have evolved over time.
- Human beings are a part of biological evolution—the change in organisms through the generations.
- Humans are mammals belonging to the animal kingdom, one of the four kingdoms of life in the domain Eukarya.
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Four kingdoms of life in the domain Eukarya. | Humans are related to all other organisms |
Click here for the Animation: Life's Diversity Please make sure that your sound is on and your volume is up.
- Humans are related to all other organisms—and humans also have some distinctive features.
- Humans share characteristics with our closest primate relatives.
- Humans also have distinctive features: increased dexterity, large brain, analytical skills, sophisticated communication, and culture.
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Life's Organization
Table 1.1 Summary of Life's Characteristics |
| 1. Living things take in and use energy and materials. |
| 2. Living things sense and respond to changes in their surroundings. |
| 3. Living things reproduces and grow based on information in DNA. |
| 4. Living things consist of one or more cells. |
| 5. Living things maintain the internal steady state called homeostasis. |
Life is organized on many levels. Atoms and molecules are nonliving materials from which all of nature is built. Cells are organized into increasingly complex levels: tissues >>> organs >>> organ systems >>> organisms. Organisms, in turn, form populations >>> communities >>> ecosystems >>> biosphere.
Organisms are connected through the flow of energy and cycling of materials. Energy flows from the sun. Plants (“producers”) trap this energy by photosynthesis. Animals (“consumers”) feed on the stored energy in plants, using cellular respiration. Bacteria and fungi (“decomposers”) break down the biological molecules of other organisms in order to recycle raw materials. All organisms are part of webs that depend on one another for energy and raw materials.
Science Is a Way of Learning about the Natural World
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Science is an approach to gathering knowledge. Biology, like all science, pursues a methodical search for information that reveals the secrets of the natural world. Explanations are sought using an approach known as the scientific method:
- Observe some aspect of the natural world and ask a question.
- Develop hypotheses (educated guesses) using all known information.
- Predict what the outcome would be if the hypothesis is valid.
- Test the hypothesis by experiments, models, and observations.
- Repeat the tests for consistency.
- Analyze and report objectively on the tests and conclusions.
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Experiments are major scientific tools. Experiments involve tests in which conditions are carefully controlled. Control groups are used to identify side effects during a test that involves an experimental group. The experimental group experiences all of the same conditions as the control except for the variable being studied. The sample size must be large enough to be representative of the whole.
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A scientific theory explains a large number of observations. A theory is a related set of hypotheses that form a broad-ranging explanation of many phenomena. Theories are accepted or rejected on the basis of tests and are subject to revision. Scientists must be content with relative certainty, which becomes stronger as more repetitions are made. Scientists must be prepared to change their minds in light of new evidence.
Science is limited to questions that can be tested; subjective questions do not readily lend themselves to scientific analysis or experiments. Science has the potential to be used for controversial endeavors, which means that all of society must commit to responsible use of scientific knowledge.
Critical Thinking
Critical thinking is an objective evaluation of information. Consider the source. Let credible scientific evidence, not opinions or hearsay, do the convincing. Question credentials and motives. Evaluate the content. Be able to distinguish between cause and correlation.Separate facts from opinions.
Critical Thinking Checklist Do's and Don'ts | |
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Do's | Don'ts |
| Do gather information or evidence. | Don't rely on heresay. |
| Do look for facts that can be checked independently and for signs of obvious bias (such as paid testimonials). | Don't confuse cause with correlation. |
| Do separate facts with opinions. | |
Herbal Supplements
Are herbal supplements safe? Controversy surrounds the use of herbal supplements. Some supplements have been linked to harm in humans. Other supplements have been shown to offer no biologically observed effect. Rigorous testing of supplements is currently being undertaken by the National Institute of Health and others.It's Elemental
Life depends on chemical reactions. An element is a fundamental form of matter that has mass and takes up space. Organisms consist mostly of carbon, oxygen, hydrogen, and nitrogen. Trace elements are needed only in small quantities.
Elements in the Human Body vs. the Earth's Crust | ||||||||||||
| Human Body | Oxygen | Carbon | Hydrogen | Nitrogen | Calcium | Magnesium | Potassium | Sodium | Sulfur | Chlorine | Phosphorus | Iron |
65% | 18 | 10 | 3 | 2 | 0.05 | 0.35 | 0.15 | 0.25 | 0.15 | 1.1 | 0.004 | |
| Earth's Crust | Oxygen | Silicon | Aluminum | Iron | Calcium | Magnesium | Potassium | Sodium | ||||
46.6% | 27.7 | 8.1 | 5.0 | 3.6 | 1.5 | 2.1 | 2.8 | |||||
Atoms, the Starting Point
Atoms are composed of smaller particles. An atom is the smallest unit of matter that is unique to a particular element. Atoms are composed of three particles: Protons (p+) are part of the atomic nucleus and have a positive charge. Their quantity is called the atomic number (unique for each element). Electrons (e-) have a negative charge. Their quantity is equal to that of the protons. They move around the nucleus. Neutrons are also a part of the nucleus; they are neutral. Protons plus neutrons = atomic mass number.Click here for the Animation: Atomic Particles. Please make sure that your sound is on and your volume is up.
Click here for the Animation: Animation: Atomic Number, Mass Number. Please make sure that your sound is on and your volume is up.
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Electron activity is the basis for organization of materials and the flow of energy in living things. Isotopes are varying forms of atoms. Atoms with the same number of protons (e.g., carbon has six) but a different number of neutrons (carbon can have six, seven, or eight) are called isotopes . Some radioactive isotopes are unstable and tend to decay into more stable atoms. They can be used to date rocks and fossils. Some can be used as tracers to follow the path of an atom in a series of reactions or to diagnose disease.
Medical Uses for Radioisotopes
Example of Radioactive Iodine |
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Click to enlarge |
Radioisotopes have many important uses in medicine. Tracers are substances containing radioisotopes that can be injected into patients to study tissues or tissue function. Radiation therapy uses the radiation from isotopes to destroy or impair the activity of cells that do not work properly, such as cancer cells. For safety, clinicians usually use isotopes with short half-lives (the time it takes the isotope to decay to a more stable isotope).
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What is a Chemical Bond?
Interacting atoms: Electrons rule! In chemical reactions, an atom can share electrons with another atom, accept extra electrons, or donate electrons. Electrons are attracted to protons, but are repelled by other electrons. Orbitals can be thought of as occupying shells around the nucleus, representing different energy levels.
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| Electron Arrangements |  |
Chemical bonds join atoms. A chemical bond is a union between the electron structures of atoms. Having a filled outer shell is the most stable state for atoms. The shell closest to the nucleus has one orbital holding a maximum of two electrons. The next shell can have four orbitals with two electrons each for a total of eight electrons. Atoms with “unfilled” orbitals in their outermost shell tend to be reactive with other atoms - they want to "fill" their outer shell with the maximal eight electrons allowed.
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| Shell Model |
Click here for the Animation: The Shell Model of the Distribution of Electrons in Atoms. Please make sure that your sound is on and your volume is up.
Click here for the Animation: Shell Models of Common Elements. Please make sure that your sound is on and your volume is up.
Click here for the Animation: Electron Distribution. Please make sure that your sound is on and your volume is up.
Atoms can combine into molecules. Molecules may contain more than one atom of the same element; N2 for example. Compounds consist of two or more elements in strict proportions. A mixture is an intermingling of molecules in varying proportions.
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We use symbols for elements when writing formulas, which identify the composition of compunds. For example, water has the formula H2O. Symbols and formulas are used in chemical equations, which are representations of reactions among atoms and molecules.
In written chemical reactions, an arrow means "yields." Substances entering a reaction (reactants) are to the left of the arrow. Reaction products are to the right. For example the reaction between hydrogen and oxygen that yields water is summarized this way:
2H2 | + | O2 |  | 2H2O |
4 hydrogens | 2 oxygens | 4 hydrogens, 2 oxygens |
Note that there are as many atoms of each element to the right of the arrow as there are to the left. Although atoms are combined in different forms, none is consumed or destroyed in the process. The total mass of all products of any chemical reaction equals the total mass of all its reactants. All equations used to represent chemical reactions, including reactions in cells, must be balanced this way.
Important Bonds in Biological Molecules
An ionic bond joins atoms that have opposite charges. When an atom loses or gains one or more electrons, it becomes positively or negatively charged - an ion. In an ionic bond, (+) and (–) ions are linked by mutual attraction of opposite charges, for example, NaCl.
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Table Salt | Salt Crystals | Salt Molecule charges | Salt bonds |
A hydrogen bond is a weak bond between polar molecules. In a hydrogen bond, a slightly negative atom of a polar molecule interacts weakly with a hydrogen atom already taking part in a polar covalent bond. These bonds impart structure to liquid water and stabilize nucleic acids and other large molecules.
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Click here for the Animation: Bonds in Biological Molecules. Please make sure that your sound is on and your volume is up.
Major Chemical Bonds in Biological Molecules | |
| Bond | Characteristics |
| Ionic | Joined atoms have opposite charges |
| Covalent | Strong; joined atoms share electrons. In a polar covalent bond one end is positive, the other negative. |
| Hydrogen | Weak, joins a hydrogen (H+) atom in one polar molecule with an electronegative atom in another polar molecule. |
Antioxidants
Free radicals are formed by the process of oxidation. Oxidation is the process whereby an atom or molecule loses one or more electrons. Oxidation can produce free radicals that may “steal” electrons from other molecules. In large numbers, free radicals can damage other molecules in a cell, such as DNA. Antioxidants are chemicals that can give up an electron to a free radical before it does damage to a DNA molecule.
ydrogen bonding makes water liquid. Water is a polar molecule because of a slightly negative charge at the oxygen end and a slightly positive charge at the hydrogen end. Water molecules can form hydrogen bonds with each other.
Polar substances are hydrophilic (water loving); nonpolar ones are hydrophobic (water dreading) and are repelled by water (see image to the top left).
Water can absorb and hold heat. Water tends to stabilize temperature because it has a high heat capacity - the ability to absorb considerable heat before its temperature changes. This is an important property in evaporative and freezing processes.
Water is a biological solvent. The solvent properties of water are greatest with respect to polar molecules because “spheres of hydration” are formed around the solute (dissolved) molecules. For example, the Na+ of salt attracts the negative end of water molecules, while the Cl- attracts the positive end (See the image to the bottom left.)
Click here for the Animation: Structure of Water. Please make sure that your sound is on and your volume is up.
Acids, Bases, and Buffers: Body Fluids in Flux
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pH Scale Click to enlarge the image |
The pH scale indicates the concentration of hydrogen ions. pH is a measure of the H+ concentration in a solution; the greater the H+ the lower the value on the pH scale. The scale extends from 0 (acidic) to 7 (neutral) to 14 (basic).
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Acids give up H+ and bases accept H+. A substance that releases hydrogen ions (H+) in solution is an acid - for example, HCl. Substances that release ions such as (OH-) that can combine with hydrogen ions are called bases (example: baking soda). High concentrations of strong acids or bases can disrupt living systems both internal and external to the body.
Buffers protect against shifts in pH. Buffer molecules combine with, or release, H+ to prevent drastic changes in pH. Bicarbonate is one of the body’s major buffers.
Molecules of Life
Biological molecules contain carbon.Only living cells synthesize the molecules characteristic of life - carbohydrates, lipids, proteins, and nucleic acids. These molecules are organic compounds, meaning they consist of atoms of carbon and one or more other elements, held together by covalent bonds.
Carbon’s key feature: versatile bonding. Living organisms are mostly oxygen, hydrogen, and carbon. Much of the hydrogen and oxygen are linked as water. Carbon can form four covalent bonds with other atoms to form organic molecules of several configurations.
Functional groups affect the chemical behavior of organic compounds. By definition a hydrocarbon has only hydrogen atoms attached to a carbon backbone. Functional groups - atoms or groups of atoms covalently bonded to a carbon backbone - convey distinct properties, such as solubility, to the complete molecule.
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Click here for examples of Functional Groups
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Example of Condensation Reaction |
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Example of Enzyme Action |
Cells have chemical tools to assemble and break apart biological molecules. Enzymes speed up specific metabolic reactions. In condensation reactions, one molecule is stripped of its H+; another is stripped of its OH-. The two molecule fragments join to form a new compound; the H+ and OH- form water (dehydration synthesis). Cells use series of condensation reactions to build polymers out of smaller monomers.
In hydrolysis reactions, the reverse happens: one molecule is split by the addition of H+ and OH- (from water) to yield the individual components.Click here for the Animation: Condensation and Hydrolysis. Please make sure that your sound is on and your volume is up.
Carbohydrates: Plentiful and Varied
A carbohydrate can be a simple sugar or a larger molecule composed of sugar units. Carbohydrates are the most abundant biological molecules. Carbohydrates serve as energy sources or have structural roles. Simple sugars - the simplest carbohydrates.
A monosaccharide - one sugar unit - is the simplest carbohydrate. Sugars are soluble in water and may be sweet-tasting. Ribose and deoxyribose (five-carbon backbones) are building blocks for nucleic acids. Glucose (six-carbon backbone) is a primary energy source and precursor of many organic molecules.
Oligosaccharides are short chains of sugar units. An oligosaccharide is a short chain resulting from the covalent bonding of two or three monosaccharides. Lactose (milk sugar) is glucose plus galactose; sucrose (table sugar) is glucose plus fructose. Oligosaccharides are used to modify protein structure and have a role in the body’s defense against disease.
Polysaccharides are sugar chains that store energy. A polysaccharide consists of many sugar units (same or different) covalently linked. Glycogen is a storage form of glucose found in animal tissues. Starch (energy storage in plants) and cellulose (structure of plant cell walls) are made of glucose units but in different bonding arrangements.
Lipids: Fats and Their Chemical Kin
Lipids are composed mostly of nonpolar hydrocarbon and are hydrophobic. Fats are energy-storing lipids. Fats are lipids that have one, two, or three fatty acids attached to glycerol. A fatty acid is a long, unbranched hydrocarbon with a carboxyl group (—COOH) at one end. Saturated fatty acids have only single C - C bonds in their tails, are solids at room temperature, and are derived from animal sources. Unsaturated fatty acids have one or more double bonds between the carbons that form “kinks” in the tails; they tend to come from plants and are liquid at room temperature.
Click here for the Animation: Fatty Acid Saturation. Please make sure that your sound is on and your volume is up.
Triglycerides have three fatty acids attached to one glycerol. They are the body’s most abundant lipids. On a per-weight basis, these molecules yield twice as much energy as carbohydrates. Trans fatty acids are partially saturated (hydrogenated) lipids implicated in some types of heart disease.
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Click here for the Animation: Triglyceride Formation. Please make sure that your sound is on and your volume is up.
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Phospholipid |
Phospholipids are key building blocks of cell membranes. A phospholipid has a glycerol backbone, two fatty acids, a phosphate group, and a small hydrophilic group. They are important components of cell membranes.
Click here for the Animation: Phospholidip Structure. Please make sure that your sound is on and your volume is up.
Sterols are building blocks of cholesterol and steroids. Steroids have a backbone of four carbon rings, but no fatty acids. Cholesterol is an essential component of cell membranes in animals and can be modified to form sex hormones.
Proteins: Biological Molecules with Many Roles
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Proteins |
Because they are the most diverse of the large biological molecules, proteins function as enzymes, in cell movements, as storage and transport agents, as hormones, as antidisease agents, and as structural material throughout the body.
Proteins are built from amino acids. Amino acids are small organic molecules with an amino group, an acid group, a hydrogen atom, and one of 20 varying “R” groups. They form large polymers called proteins.
Click here for the Animation: Structure of an Amino Acid. Please make sure that your sound is on and your volume is up.
The sequence of amino acids is a protein’s primary structure. Primary structure is defined as the chain (polypeptide) of amino acids. The amino acids are linked together in a definite sequence by peptide bonds between an amino group of one and an acid group of another. The final shape and function of any given protein is determined by its primary structure.
A Protein’s Function Depends on Its Shape
Primary structure determines the shape and function of proteins by positioning different amino acids so that hydrogen bonds can form between them and by putting R groups in positions that force them to interact.
Many proteins fold two or three times. Secondary structure is the helical coil or sheetlike array that will result from hydrogen bonding of side groups on the amino acid chains. Tertiary structure is caused by interactions among R groups, resulting in a complex three-dimensional shape.Proteins can have more than one polypeptide chain. Hemoglobin, the oxygen-carrying protein in the blood, is an example of a protein with quaternary structure - the complexing of two or more polypeptide chains to form globular or fibrous proteins. Hemoglobin has four polypeptide chains (globins), each coiled and folded with a heme group at the center.
Nucleotides and Nucleic Acids
Nucleotides: energy carriers and other roles. Each nucleotide has a five-carbon sugar (ribose or deoxyribose), a nitrogen-containing base, and a phosphate group. ATP molecules link cellular reactions that transfer energy. Other nucleotides include the coenzymes, which accept and transfer hydrogen atoms and electrons during cellular reactions, and chemical messengers. Click here for the Animation: Structure of ATP. Please make sure that your sound is on and your volume is up. Nucleic acids include DNA and RNA. In nucleic acids, nucleotides are bonded together to form large single- or double-stranded molecules. DNA (deoxyribonucleic acid) is double-stranded; genetic messages are encoded in its base sequences. RNA (ribonucleic acid) is single-stranded; it functions in the assembly of proteins.Click here for the Animation: Nucleotide Subunits of DNA. Please make sure that your sound is on and your volume is up. |  |
| Summary of the Main Carbon Compounds in the Human Body | |||
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Category | Main Subcategories | Examples | Functions |
| Carbohydrates contain an aldehyde or a ketone group and one or more hydroxyl groups. | Monosaccharides (simple sugars) | Glucose | Structural roles, energy source |
| Polysaccharides (complex carbohydrates) | Energy storage Structural roles | ||
| Lipids are largely hydrocarbon, generally do not dissolve in water but dissolve in nonpolar solvents | Lipids with fatty acids: | Fats (e.g., butter) | Energy storage |
| Key component of cell membranes | |||
| Lipids with no fatty acids: Sterols: four carbon rings; the number, position, and type of functional groups vary | Cholesterol | Component of animal cell membranes, can be rearranged into other steroids (e.g., vitamin D, sex hormones) | |
| Proteins are polypeptides (up to several thousand amino acids, covalently linked) | Fibrous proteins: Individual polypeptide chains, often linked into tough, water-insoluble molecules | Keratin Collagen | Structural element of hair, nails Structural element of bones and cartilage |
| Globular proteins: One or more polypeptide chains folded and linked into globular shapes; many roles in cell activities | Enzymes Hemoglobin Insulin Antibodies | Increase in rates of reactions | |
| Nucleic Acids (and Nucleotides) are chains of units (or individual units) that each consist of a five-carbon sugar, phosphate, and a nitrogen-containing base | Adenosine phonsphates Nucleotide coenzymes | ATP NAD+, NADP+ | Energy carrier Transport of protons (H+) and electrons from one reaction site to another |
Nucleic acids: | DNA, RNAs | Storage, transmission, translation of genetic information | |
What is a Cell?
The cell theory has three generalizations:
- All organisms are composed of one or more cells.
- The cell is the smallest unit having the properties of life.
- All cells come from pre-existing cells.
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- A plasma membrane separates each cell from the environment, but also allows the flow of molecules across the membrane.
- DNA carries the hereditary instructions.
- The cytoplasm containing a semifluid matrix (cytosol) and organelles is located between the plasma membrane and the region of DNA.
There are two basic kinds of cells. Prokaryotic cells (bacteria) do not have a separation of the DNA from the remainder of the cell parts. Eukaryotic cells have a definite nucleus and membrane-bound organelles.
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Click here for the Animation: Overview of Cells. Please make sure that your sound is on and your volume is up.
Most cells are so small they can only be seen by using light and electron microscopes. Cells are necessarily small so that the surface-to-volume ratio remains low; this means that the interior will not be so extensive that it cannot exchange materials efficiently through the plasma membrane.
Membranes enclose cells and organelles. A large portion of the cell membrane is composed of phospholipids, each of which possesses a hydrophilic head and two hydrophobic tails. If phospholipid molecules are surrounded by water, their hydrophobic fatty acid tails cluster and a lipid bilayer results; hydrophilic heads are at the outer faces of a two-layer sheet with the hydrophobic tails shielded inside.
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Surface to volume ratio | Lipid bilayer |
The Parts of an Eukaryotic Cell
All eukaryotic cells contain organelles. Organelles form compartmentalized portions of the cytoplasm.Organelles separate reactions with respect to time (allowing proper sequencing) and space (allowing incompatible reactions to occur in close proximity).
Common Features of Eukaryotic Cells | |
| Organelles and Their Main Functions | |
| Nucleus | Contains the cell's DNA |
| Endoplasmic reticulum (ER) | Routes and modifies newly formed polypeptide chains; also, where lipids are assembled |
| Golgi body | Modifies polypeptide chains into mature proteins; sorts and ships proteins and lipids for secretion or for use inside cell |
| Various vesicles | Transport or stor a variety of substances; break down substances and cell structures in the cell; other functions |
| Mitochondria | Produce ATP |
| Other Structures and Their Functions | |
| Ribosomes | Assemble polypeptide chains |
| Cytoskeleton | Gives overall shape and internal organization to cell; moves the cell and its internal parts |
The Plasma Membrane: A Double Layer of Lipids
The plasma membrane is a mix of lipids and proteins. Bilayers of phospholipids, interspersed with glycolipids and cholesterol, are the structural foundation of cell membranes. Within a bilayer, phospholipids show quite a bit of movement; they diffuse sideways, spin, and flex their tails to prevent close packing and promote fluidity, which also results from short-tailed lipids and unsaturated tails (kinks at double bonds).
Proteins perform most of the functions of cell membranes.
- The scattered islands of protein in the sea of lipids create a “mosaic” effect.
- Membrane proteins (most are glycoproteins) serve as enzymes, transport proteins, receptor proteins, and recognition proteins.
How Do We See Cells?
Microscopy allows us to see cells and their pieces. Many types of microscopes exist, which can produce many types of pictures (micrographs): Light microscopes use light to see samples; specimens usually must be thin and colored with dyes to be seen.
The Nucleus
The nucleus encloses DNA, the building code for cellular proteins. Its membrane isolates DNA from the sites (ribosomes in the cytoplasm) where proteins will be assembled. The nuclear membrane helps regulate the exchange of signals between the nucleus and the cytoplasm.
A nuclear envelope encloses the nucleus. The nuclear envelope consists of two lipid bilayers with pores. The envelope membranes are continuous with the endoplasmic reticulum (ER).
Click here for the Animation: Nuclear Envelope. Please make sure that your sound is on and your volume is up.
The nucleolus is where cells make the units of ribosomes. The nucleolus appears as a dense mass inside the nucleus. In this region, subunits of ribosomes are prefabricated before shipment out of the nucleus.
DNA is organized in chromosomes. Chromatin describes the cell’s collection of DNA plus the proteins associated with it. Each chromosome is one DNA molecule and its associated proteins. See image at the right.
Events that begin in the nucleus continue to unfold in the cell cytoplasm. Outside the nucleus, new polypeptide chains for proteins are assembled on ribosomes. Some proteins are stockpiled; others enter the endomembrane system.
Components of the Nucleus | |
| Nuclear envelope | Double membrane with many pores; it separates the interior of the nucleus from the cytoplasm |
| Nucleolus | Dense cluster of the RNA and proteins used to assemble ribosome subunits |
| Nucleoplasm | Fluid portion of the nucleus interior |
| Chromosomes | DNA molecules and proteins attached to them |
| Chromatin | All DNA molecules and their associated proteins in the nucleus |
The Endomembrane System
ER is a protein and lipid assembly line. The endoplasmic reticulum is a collection of interconnected tubes and flattened sacs, continuous with the nuclear membrane. Rough ER consists of stacked, flattened sacs with many ribosomes attached; oligosaccharide groups are attached to polypeptides as they pass through on their way to other organelles, membranes, or to be secreted from the cell. Smooth ER has no ribosomes; it is the area from which vesicles carrying proteins and lipids are budded; it also inactivates harmful chemicals and aids in muscle contraction. Golgi bodies “finish, pack, and ship.” In the Golgi body, proteins and lipids undergo final processing, sorting, and packaging. The Golgi bodies resemble stacks of flattened sacs whose edges break away as vesicles.
A variety of vesicles move substances into and through cells. Lysosomes are vesicles that bud from Golgi bodies; they carry powerful enzymes that can digest the contents of other vesicles, worn-out cell parts, or bacteria and foreign particles. Peroxisomes are membrane-bound sacs of enzymes that break down fatty acids and amino acids.
Mitochondria: The Cell’s Energy Factories
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Mitochondria |
Mitochondria make ATP. Mitochondria are the primary organelles for transferring the energy in carbohydrates to ATP; they are found only in eukaryotic cells. Oxygen is required for the release of this energy.
ATP forms in an inner compartment of the mitochondrion. Each mitochondrion has compartments formed by inner folded membranes (cristae) surrounded by a smooth outer membrane. Mitochondria have their own DNA and some ribosomes, which leads scientists to believe they may have evolved from ancient bacteria.
The Cell’s Skeleton
The cytoskeleton is an interconnected system of bundled fibers, slender threads, and lattices extending from the nucleus to the plasma membrane in the cytosol. The main components are microtubules, microfilaments, and intermediate filaments - all assembled from protein subunits. The skeleton helps organize and reinforce the cell and serves in some cell functions.
Movement is one function of the cytoskeleton. Microtubular extensions of the plasma membrane display a 9 + 2 cross-sectional array and are useful in propulsion. Flagella are quite long, whiplike, and are found on animal sperm cells. Cilia are shorter, more numerous, and may function as “sweeps” to clear, as an example, the respiratory tract of dust or other materials. The microtubules of flagella and cilia arise from centrioles, which play a role in cell division.
How Diffusion and Osmosis Move Substances Across Membranes
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Selective Permeability | |
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Selectively permeable membrane between two compartments How a solute concentration gradient affects osmotic movement of water. |
The plasma membrane is “selective.” Lipid-soluble molecules and small, electrically neutral molecules (for example, oxygen, carbon dioxide, and ethanol) cross easily through the lipid bilayer. Larger molecules (such as glucose) and charged ions (such as Na+, Ca+, HCO3-) must be moved by membrane transport proteins. Because some molecules pass through on their own and others must be transported, the plasma membrane is said to have the property of selective permeability.
Click here for the Animation: Selective Permeability. Please make sure that your sound is on and your volume is up.
In diffusion, a solute moves down a concentration gradient. A concentration gradient is established when there is a difference in the number of molecules or ions of a given substance between two adjacent regions. Molecules constantly collide and tend to move from areas of high concentration to areas of low concentration. The net movement of like 
molecules down a concentration gradient (high to low) is called diffusion; when this occurs across a plasma membrane, it is called passive transport.
Molecules move faster when gradients are steep, and different solutes move independently according to their respective gradients. Electric gradients (gradients of electrical charge) are important to nerve function.
Click here for the Video: Diffusion. Please make sure that your sound is on and your volume is up.
Water crosses membranes by osmosis.Osmosis is the passive diffusion of water across a differentially permeable membrane in response to solute concentration gradients.
Click here for the Animation: Solute Concentration and Osmosis. Please make sure that your sound is on and your volume is up.
Osmotic movements are affected by the relative concentrations of solutes in the fluids inside and outside the cell (tonicity). An isotonic fluid has the same concentration of solutes as the fluid in the cell; immersion in it causes no net movement of water. A hypotonic fluid has a lower concentration of solutes than does the fluid in the cell; cells immersed in it may swell as water moves into the cell down its gradient. A hypertonic fluid has a greater concentration of solutes than does the fluid in the cell; cells in it may shrivel as water moves out of the cell, again down its gradient.
Click here for the Animation: Tonicity and Diffusion of Water. Please make sure that your sound is on and your volume is up.
Click here for the Video: Passive Transport Overview. Please make sure that your sound is on and your volume is up.
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Tonicity and the diffusion of water. In the sketches, membrane-like bags through which water but not sucrose can move are placed in hypotonic, hypertonic, and isotonic solutions. In each container, arrow width represents the relative amount of water movement. The sketches show what happens when red blood cells—which cannot actively take in or expel water—are placed in similar solutions. |
Other Ways Substances Cross Cell Membranes
Many solutes cross membranes through transport proteins. In facilitated diffusion, solutes pass through channel proteins in accordance with the concentration gradient; this process requires no input of energy. Channel proteins are open to both sides of the membrane and undergo changes in shape during the movement of solutes. The transport proteins are selective for what they allow through the membrane.
Click here for the Animation: Passive Transport. Please make sure that your sound is on and your volume is up.
In active transport, solutes move against their concentration gradients with the assistance of transport proteins that change their shape with the energy supplied by ATP.
Click here for the Animation: Active Transport. Please make sure that your sound is on and your volume is up.
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Membrane-crossing mechanisms. | |||||
Vesicles transport large solutes. Exocytosis moves substances from the cytoplasm to the plasma membrane during secretion, moving materials out of the cell. Endocytosis encloses particles in small portions of plasma membrane to form vesicles that then move into the cytoplasm; if this process brings organic material into the cell, it is called phagocytosis.
Metabolism: Doing Cellular Work
ATP is the cell’s energy currency. Metabolism refers to all of the chemical reactions that occur in cells; ATP links the whole of these reactions together. ATP is composed of adenine, ribose, and three phosphate groups. ATP transfers energy in many different chemical reactions; almost all metabolic pathways directly or indirectly run on energy supplied by ATP. ATP can donate a phosphate group (phosphorylation) to another molecule, which then becomes primed and energized for specific reactions.
The ATP/ADPcycle is a method for renewing the supply of ATP that is constantly being used up in the cell; it couples inorganic phosphate to ADP to form energized ATP.
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| (a) Structure of the energy carrier ATP. (b) ATP connects energy-releasing reactions with energy-requiring ones. In the ATP/ADP cycle, the transfer of a phosphate group turns ATP into ADP, then back again to ATP. |
Click here for the Animation: The ATP-ADP Cycle. Please make sure that your sound is on and your volume is up.
There are two main types of metabolic pathways. Metabolic pathways form series of interconnected reactions that regulate the concentration of substances within cells. In anabolism, small molecules are assembled into large molecules—for example, simple sugars are assembled into complex carbohydrates. In catabolism, large molecules such as carbohydrates, lipids, and proteins are broken down to form products of lower energy, releasing energy for cellular work.
Pathways exist as enzyme-mediated linear or circular sequences of reactions involving the following:
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| How enzymes and substrates fit together. When substrate molecules contact an enzyme’s active site, they bind to the site for a brief time and a product molecule forms. When the product molecule is released, the enzyme goes back to its previous shape. It is not changed by the reaction it catalyzed. |
Enzymes play a vital role in metabolism. Enzymes are proteins that serve as catalysts; they speed up reactions. Enzymes have several features in common:
- Enzymes do not make anything happen that could not happen on its own; they just make it happen faster.
- Enzymes can be reused.
- Enzymes act upon specific substrates, molecules which are recognized and bound at the enzyme’s active site.
Click here for the Animation: Induced Fit Model. Please make sure that your sound is on and your volume is up.
Because enzymes operate best within defined temperature ranges, high temperatures decrease reaction rate by disrupting the bonds that maintain three-dimensional shape (denaturation occurs). Most enzymes function best at a pH near 7; higher or lower values disrupt enzyme shape and halt function. Coenzymes are large organic molecules such as NAD+ and FAD (both derived from vitamins), which transfer protons and electrons from one substrate to another to assist with many chemical reactions.
How Cells Make ATP
Cellular respiration makes ATP. Electrons acquired by the breakdown of carbohydrates, lipids, and proteins are used to form ATP. Overall, the formation of ATP occurs by cellular respiration; in humans this is an aerobic process meaning it requires oxygen.
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Overview of glycolysis. |
- Step 1:
Glycolysis breaks glucose down to pyruvate.Glycolysis reactions occur in the cytoplasm and result in the breakdown of glucose to pyruvate, generating small amounts of ATP. Glucose is first phosphorylated in energy-requiring steps, then split to form two molecules of PGAL. Four ATP are produced by phosphorylation in subsequent reactions; but because two ATP were used previously, there is a net gain of only two ATP by the end of glycolysis. Glycolysis does not use oxygen.
Click here for the Animation: Overview of Glycolysis. Please make sure that your sound is on and your volume is up.
- Step 2:
The Krebs cycle produces energy-rich transport molecules. Pyruvate (produced in the cytoplasm) enters the mitochondria for the oxygen requiring steps of cellular respiration. The pyruvate is converted to acetyl-CoA, which enters the Krebs cycle to eventually be converted to CO2.
Reactions within the mitochondria and the Krebs cycle serve three important functions:
- Two molecules of ATP are produced by substrate-level phosphorylation.
- Intermediate compounds are regenerated to keep the Krebs cycle going.
- H+ and e- are transferred to NAD+ and FAD, generating NADH and FADH2.
Click here for the Video: Krebs Cycle. Please make sure that your sound is on and your volume is up.
- Step 3:
Electron transport produces many ATP molecules. The final stage of cellular respiration occurs in the electrontransport systems embedded in the inner membranes (cristae) of the mitochondrion.
NADH and FADH2 from previous reactions give up their electrons to transport (enzyme) systems embedded in the mitochondrial inner membrane. Electrons flow through the system eventually to oxygen, forming water; as they flow, H+ are pumped into the outer compartment of the mitochondrion to create a proton gradient. H+ ions move down their gradient, through a channel protein called ATP synthase, in the process driving the synthesis of ATP.
Click here for the Animation: Third Stage of Aerobic Respiration. Please make sure that your sound is on and your volume is up.
Alternative Energy Sources in the Body
How the body uses carbohydrates as fuel. Excess carbohydrate intake is stored as glycogen in liver and muscle for future use. Free glucose is used until it runs low; then glycogen reserves are tapped. Under some conditions a process called lactate fermentation can be used to produce ATP; here, pyruvate is converted directly to lactic acid with production of quick, but limited, energy.
Fats and proteins also provide energy. Lipids are used when carbohydrate supplies run low.Excess fats are stored away in cells of adipose tissue. Fats are digested into glycerol (which enters glycolysis) and fatty acids, which enter the Krebs cycle. Because fatty acids have many more carbon and hydrogen atoms, they are degraded more slowly and yield greater amounts of ATP.
Proteins are used as the last resort for supplying energy to the body. Amino acids are released by enzymatic digestion of proteins; protein is never stored by the body. After the amino group is removed, the amino acid remnant is fed into the Krebs cycle to produce energy (ATP), or is used to make fats and carbohydrates. Ammonia (from the amino group) is excreted as waste.
Summary of Energy Sources in the Human Body | ||
| Starting Molecule | Subunit | Entry Point into the Aerobic Pathway |
| Complex carbohydrate | Simple sugars (e.g., glucose | Glycolysis |
| Fat | Fatty Acids | Preparatory reactions for Krebs cycle |
| Glycerol | Raw material for key intermediate in glycolysis (PGAL) | |
| Protein | Amino acids | Carbon backbones enter Krebs cycle or preparatory reactions |
Cells combine to form four types of tissues:
- Muscular
- Connective
- Epithelial
- Nervous
Each these tissues combine to form organs and ultimately organ systems.
Impact/Issues: Stem Cells
Stem cells are the first to form when a fertilized egg starts dividing. Adults have stem cells in some tissues such as bone marrow and fat; these cells have shown some promise as therapy. Embryonic stem cells can be coaxed to differentiate into many different types of cells, which can replace damaged or worn out body cells perhaps to an extent greater than adult stem cells.
The human body is an orderly assembly of parts (anatomy). A tissue is an aggregation of cells and intracellular substances functioning for a specialized activity. Various types of tissues can combine to form organs, such as the heart. Organs may interact to form organ systems such as the digestive system. Homeostasis allows for the stable functioning (physiology) of all our combined parts.
Epithelium: The Body’s Covering and Linings
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Epithelial tissue covers the surface of the body and lines its cavities and tubes. One surface is free and faces either the environment or a body fluid; the other adheres to a basement membrane, a densely packed layer of proteins and polysaccharides. Cells are linked tightly together; there may be one or more layers.
Click here for the Animation: Structure of Epithelium. Please make sure that your sound is on and your volume is up.
There are two basic types of epithelia.
- Simple epithelium is a single layer of cells functioning as a lining for body cavities, ducts, and tubes. Simple epithelium functions in diffusion, secretion, absorption, or filtering of substances across the cell layer. Pseudostratified epithelium is a single layer of cells that looks like a double layer; most of the cells are ciliated; examples are found in the respiratory passages and reproductive tracts.
- Stratified epithelium has many layers—as in human skin.
Click here for the Animation: Types of Simple Epithelium. Please make sure that your sound is on and your volume is up.
Major Types of Epithelium | ||
Type | Shape | Typical Locations |
| Simple (one layer) | Squamous | Linings of blood vessels, lung alveoli (sites of gas exchange) |
| Cuboidal | Glands and their ducts, surface of ovaries, pigmented epithelium of eye | |
| Columnar | Stomach, intestines, uterus | |
Pseudostratified | Columnar | Throat, nasal passages, sinuses, trachea, male genital ducts |
| Stratified (two or more layers) | Squamous | Skin (keratinized), mouth, throat, esophagus, vagina (nonkeratinized) |
| Cuboidal | Ducts of sweat glands | |
| Columnar | Male urethra, ducts of salivary glands | |
Both simple and stratified epithelium can be subdivided into groups based on shape at the tissue surface:
- Squamous epithelium consists of flattened cells; examples are found in the lining of the blood vessels.
- Cuboidal epithelium has cube-shaped cells; examples are found in glands.
- Columnar epithelium has elongated cells; examples are found in the intestine.
Glands develop from epithelium.Glands are secretory structures derived from epithelium that make and release specific substances, such as mucus. Glands are classified according to how their products reach the site where they are used. Exocrine glands often secrete through ducts to free surfaces; they secrete mucus, saliva, earwax, milk, oil, and digestive enzymes for example.Endocrine glands have no ducts but distribute their hormones via the blood.
Connective Tissue: Binding, Support, and Other Roles
Connectivetissue binds together, supports, and anchors body parts; it is the most abundant tissue in the body. Fibrous connective tissues and specialized connective tissues are both found in the body. Fiber-like structural proteins and polysaccharides secreted by the cells make up a matrix (ground substance) around the cells that can range from hard to liquid.
Fibrous connective tissues are strong and stretchy. Fibrous connective tissue takes different forms depending on cell type and the fibers/matrix produced.
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Characteristics of connective tissues. |
Types and examples of fibrous connective tissue:
- Loose connective tissue supports epithelia and organs, and surrounds blood vessels and nerves; it contains few cells and loosely arrayed thin fibers.
- Dense, irregular connective tissue has fewer cells and more fibers, which are thick; it forms protective capsules
around organs.
Blood - Dense, regular connective tissue has bundled collagen fibers lying in parallel; such arrangements are found in ligaments (binding bone to bone) and tendons (binding muscle to bone).
- Elastic connective tissue contains fibers of elastin; this tissue is found in organs that must stretch, like the lungs.
Click here for the Animation: Soft Connective Tissues. Please make sure that your sound is on and your volume is up.
Cartilage, bone, adipose tissue, and blood are specialized connective tissues. Cartilage contains a dense array of fibers in a rubbery ground substance; cartilage can withstand great stress but heals slowly when damaged. Hyalinecartilage has many small fibers; it is found at the ends of bones, in the nose, ribs, and windpipe. Elasticcartilage, because of its elastin component, is able to bend yet maintain its shape, such as in the external ear. Fibrocartilage is a sturdy and resilient form that can withstand tremendous pressure such as in the disks that separate the vertebrae.
Bone tissue is composed of collagen, ground substance, and calcium salts; minerals harden bone so it is capable of supporting and protecting body tissues and organs. Adipose tissue cells are specialized for the storage of fat; most adipose tissue lies just beneath the skin.Blood is a fluid connective tissue involved in transport; plasma forms the fluid “matrix” and blood proteins, blood cells, and platelets compose the “fiber” portion of the tissue.
Muscle Tissue: Movement
Muscle tissue contracts in response to stimulation, then passively lengthens; movement is a highly coordinated action. There are three types of muscle:
- Skeletal muscle tissue attaches to bones for voluntary movement; long muscle cells are bundled together in parallel arrays, which are enclosed in a sheath of dense connective tissue.
- Smooth muscle tissue contains tapered, bundled cells that function in involuntary movement; it lines the gut, blood vessels, and glands.
- Cardiac muscle is composed of short cells that can function in units due to the signals that pass through special junctions that fuse the cells together; cardiac muscle is only found in the wall of the heart.
Nervous Tissue: Communication
Nervous tissue consists mainly of cells, including neurons (nerve cells) and support cells; nervous tissue forms the body’s communication network. Neurons carry messages. Neurons have two types of cell processes (extensions): branched dendrites pick up chemical messages and pass them to an outgoing axon.
A cluster of processes from different neurons is called a nerve. Nerves move messages throughout the body. Neuroglia are support cells. Glial cells (neuroglia) make up 90 percent of the nervous system. Neuroglia provide physical support for neurons. Other glial cells provide nutrition (astrocytes), clean-up, and insulation services (Schwann cells).
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Nerve Tissue | Nerves |
Summary of Basic Tissue Types in the Human Body | |||
| Tissue | Function | Characteristics | |
| Epithelium | Covers body surface; lines internal cavities and tubes | One free surface; opposite surface rests on basement membrane supported by connective tissue | |
| Connective Tissue | Binds, supports adds strength; some provide protection or insulation | Cells surrounded by a matrix (ground substance) containing structural proteins except in blood | |
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| Loose | Elasticity, diffusion | Cells and fibers loosely arranged | |
| Dense | Support, elasticity | Several forms. One has collagen fibers in various orientations in the matrix; it occurs in skin and as capsules around some organs. Another form has collagen fibers in parallel bundles; it occurs in ligaments, tendons | |
| Elastic | Elasticity | Mainly elastin fibers; occurs in organs and that must stretch | |
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| Cartilage | Support, flexibility, low-friction surface | Matrix solid but pliable; no blood supply | |
| Bone | Support, protection, movement | Matrix hardened by minerals | |
| Adipose tissue | Insulation, padding, energy storage | Soft matrix around large, fat-filled cells | |
| Blood | Transport | Liquid matrix (plasma) containing blood cells, many other substances | |
| Muscle Tissue | Movement of the body and its parts | Made up of arrays of contractile cells | |
| Nervous Tissue | Communication between body parts; coordination, regulation of cell activity | Made up of neurons and support cells (neuroglia) | |
Cell Junctions: Holding Tissues Together
Epithelial cells tend to adhere to one another by means of specialized attachment sites. Tight junctions link cells of epithelial tissues to form seals that keep molecules from freely crossing the epithelium. Adhering junctions are like spot welds in tissues subject to stretching. Gap junctions link the cytoplasm of adjacent cells; they form communication channels. Sites of cell-to-cell contact are especially profuse when substances must not leak from one body compartment to another.
Tissue Membranes: Thin, Sheetlike Covers
Epithelium membranes pair with connective tissue. Mucous membranes line the tubes and cavities of the digestive, respiratory, and reproductive systems where embedded glands secrete mucus. Serous membranes such as those that line the thoracic cavity occur in paired sheets and do not contain glands. Cutaneous membranes are hardy and dry - and better known as skin.
Membranes in joints consist only of connective tissue. Synovial membranes line the sheaths of tendons and the capsule-like cavities around certain joints. Their cells secrete fluid that lubricates the ends of the moving bones.
Organs and Organ Systems
An organ is a composite of two or more tissue types that act together to perform one or more functions; two or more organs that work in concert form an organ system. The major cavities of the human body are: cranial, spinal, thoracic, abdominal, and pelvic.
Click here for the Animation: Major Body Cavities. Please make sure that your sound is on and your volume is up.
Click here for the Animation: Directional Terms and Planes of Symmetry. Please make sure that your sound is on and your volume is up.
Eleven organ systems (integumentary, nervous, muscular, skeletal, circulatory, endocrine, lymphatic, respiratory, digestive, urinary, and reproductive) contribute to the survival of the living cells of the body.
The Integument - Example of an Organ System
Humans have an outer covering called the integument, which includes the skin and the structures derived from epidermal cells including oil and sweat glands, hair, and nails. The skin performs several functions:
- The skin covers and protects the body from abrasion, bacterial attack, ultraviolet radiation, and dehydration.
- It helps control internal temperature.
- Its receptors are essential in detecting environmental stimuli.
- The skin produces vitamin D.
Epidermis and dermis - the two layers of skin.
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Click to Enlage | |
Epidermis refers to the thin, outermost layers of cells consisting of stratified, squamous epithelium. Keratinocytes produce keratin; when the cells are finally pushed to the skin surface, they have died, but the keratin fibers remain to make the outermost layer of skin (the stratum corneum) tough and waterproof. Deep in the epidermis are melanin-producing cells (melanocytes); melanin, along with carotene and hemoglobin, contribute to the natural coloration of skin. Langerhans cells and Granstein cells are two important cells in skin that contribute to immune function.
The dermis is the thicker portion of the skin that underlies the epidermis. The dermis is mostly dense connective tissue, consisting of elastin and collagen fibers. Blood vessels, hair follicles, nerve endings, and glands are located here. The hypodermis is a subcutaneous layer that anchors the skin; fat is also stored here.
Click here for the Animation: Structure of Human Skin. Please make sure that your sound is on and your volume is up.
Sweat glands and other structures are derived from epidermis. Sweat glands secrete a fluid (mostly water with a little dissolved salt) that is useful in regulating the temperature of the body. Oil (sebaceous) glands function to soften and lubricate the hair and skin; acne is a condition in which the ducts become infected by bacteria.
Hairs are flexible, keratinized structures rooted in the skin and projecting above the surface; growth is influenced by genes, nutrition, and hormones.
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Damaged Skin |
Sunlight permanently damages the skin. Ultraviolet (UV) radiation and the light from tanning beds stimulate melanin production in skin, resulting in a tan; too much UV exposure, however, can damage the skin. UV light can activate proto-oncogenes in skin cells, leading to cancer. Rates of skin cancer are on the rise due to continued destruction of the atmospheric ozone layer that normally protects the Earth from too much UV light.
Homeostasis: The Body in Balance
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Homeostasis |
The internal environment: A pool of extracellular fluid. The trillions of cells in our bodies are continuously bathed in an extracellular fluid that supplies nutrients and carries away metabolic wastes. The extracellular fluid consists of interstitial fluid (between the cells and tissues) and plasma (blood fluid).
The component parts of an animal work together to maintain the stable fluid environment (homeostasis) required for life. Homeostasis requires the interaction of sensors, integrators, and effectors. Homeostatic mechanisms operate to maintain chemical and physical environments within tolerable limits and to keep the body close to specific set points of function.
Homeostatic control mechanisms require three components:
- Sensory receptor cells detect specific changes (stimuli) in the environment.
- Integrators (brain and spinal cord) act to direct impulses to the place where a response can be made.
- Effectors (muscles and glands) perform the appropriate response.
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Feedback mechanisms are important homeostatic controls. A common homeostatic mechanism is negative feedback. It works by detecting a change in the internal environment that brings about a response that tends to return conditions to the original state. It is similar to the functioning of a thermostat in a heating/cooling system.Positive feedback mechanisms may intensify the original signal; childbirth is an example.
Click here for the Animation: Negative Feedback at the Organ Level. Please make sure that your sound is on and your volume is up.
Click here for the Animation: Homeostatic Control of Temperature. Please make sure that your sound is on and your volume is up.
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| Homeostatic controls over the internal temperature of the human body. The dashed line shows how the feedback loop is completed. The solid arrows indicate the main control pathways. | |
How Homeostatic Feedback Maintains the Body’s Core Temperature
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Homeostatic controls over internal body temperature. |
Humans are endotherms, heated from within by metabolic processes. Core temperature of the head and torso is roughly 37°C (98.6°F). Above this temperature (~41°C) proteins begin to denature; below this temperature (35°C and below) the body stops functioning.
Click here for the Animation: Human Thermoregulation. Please make sure that your sound is on and your volume is up.
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Click to Enlarge |
Click here for the Animation: Heat Denaturation of Enzymes . Please make sure that your sound is on and your volume is up.
Responses to cold stress. Cold responses are controlled by an area of the brain called the hypothalamus. Several things happen when the outdoor temperature drops:
- Peripheral vasoconstriction occurs when the hypothalamus commands the muscles around blood vessels to contract; this diverts blood flow away from the body surface.
- The pilomotor response causes your body hair to stand on end to trap air around the body to prevent heat loss.
- Skeletal muscle contractions cause you to shiver in an attempt to generate heat.
- In babies, who can’t shiver, hormones raise the rate of metabolism in a nonshivering heat production response; this response occurs in a special type of adipose tissue called brown fat.
If body temperature cannot be maintained, damage to the body occurs. Hypothermia is characterized by mental confusion, coma, and possibly death. Physical freezing can lead to frostbite and death of the affected tissues.
Responses to heat stress. Heat responses are also controlled by the hypothalamus. Peripheral vasodilation causes blood vessels to expand in the skin, allowing excess body heat to dissipate. Heat is also dissipated in sweat from sweat glands; water and salts both are lost to cool the body.
Various levels of heat stress (hyperthermia) can be experienced:
- Heat exhaustion occurs under mild heat stress; blood pressure drops as fluid is lost and the person can collapse.
- Heat stroke occurs when the body ceases to be able to control temperature; death is one possible outcome.
- A fever is a natural rise in core temperature used to fight off disease; severe fevers, however, should be controlled to avoid serious damage to the body.
Summary of Human Responses to Cold Stress and Heat Stress | ||
Environmental Stimulus | Main Responses | Outcome |
| Drop in Temperature | Vasoconstriction of blood vessels in skin; pilomotor response; behavior changes (e.g., putting on a sweater) | Heat is conserved |
| Increased muscle activity; shivering; nonshivering heat production | More heat is produced | |
| Rise in Temperature | Vasodilation of blood vessels in skin; sweating; changes in behavior; heavy breathing | Heat is dissipated from body |
| Reduced muscle activity | Less heat is produced | |
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