AP Biology Fall Review


Terms in this set (...)

Elements are substances that cannot be broken down into simpler substances by chemical means.
Oxygen is an element with symbol O and atomic number 8. It is a highly reactive nonmetal and oxidizing agent. In living organisms, oxygen is used in respiration and in a number of organic molecules.
Carbon is an non mental element with symbol O and atomic number 6. It is the second most abundant element in living organisms and is present in all organic compounds.
Hydrogen is an element with symbol H and atomic number 1. Hydrogen combines with non-metallic elements to form water and other organic compounds.
Nitrogen is an element with the symbol N and atomic number 7. In biology, nitrogen is important as it is found in a number of organic compounds and is used in fertilizers and antibiotics.
Trace elements
Trace elements are elements required by an organism in very small quantities. Trace elements include iron, iodine, and copper.
An atom is the smallest unit of an element that retains its characteristic properties; they are the building blocks of the physical world.
Protons are positively charged subatomic particles that are found in an atom's nucleus.
Neutrons are uncharged subatomic particles found in an atom's nucleus.
Electrons are negatively charged subatomic particles that spin around the nucleus.
The nucleus is the positively charged core in an atom made up of neutrons and protons.
Atoms that have the same number of protons but differ in the number of neutrons in the nucleus are called isotopes.
A chemical compound is formed when two or more different types of atoms are combined in a fixed ration.
Chemical reaction
A chemical reaction describes a chemical change in which reactants react to form products chemically different from the reactants.
Chemical bond
The atoms of a compound are held together by chemical bonds, which may be ionic bonds, covalent bonds, or hydrogen bonds.
Ionic bond
An ionic bond is formed between two atoms when one or more electrons are transferred from one atom to the other. In this case, one atom becomes negatively charged and one atom becomes positively charged.
An ion is a charged form of an atom.
Covalent bond
A covalent bond is formed when electrons are shared between atoms and can be polar or non-polar.
Non-polar covalent
A non-polar covalent bond is a covalent bond in which the electrons are shared equally between the atoms.
Polar covalent
A polar covalent bond is a covalent bond in which the electrons are shared unequally - one atom attracts electrons more than the other.
A molecule is polar if it has partially positive and partially negative charged ends, such as water.
Hydrogen bonds
Hydrogen bonds are intermolecular attractions that form when a hydrogen bond that is covalently bonded to one electronegative atom that it also attracted to another electronegative atom. Hydrogen bonds are individually weak but are strong when present in large numbers.
Cohesion refers to the tendency of water molecules to stick together and allows water to have a high surface tension.
Adhesion refers to the tendency of water molecules to stick to other substances, which accounts for the phenomenon of capillary action.
Capillary action
Capillary action is the ability of water, or other liquids, to travel against gravity in a thin vessel and accounts for the ability of water to rise up the roots, trunks, and branches of trees.
Heat capacity
Heat capacity refers to the ability of a substance to store heat and is the quantity of heat required to change the temperature of a substance by 1 degree. Water has a high heat capacity, allowing it to keep a fairly stable temperature in our bodies and in the environment.
Surface tension
Surface tension is a property of water, due to the cohesiveness of its molecules, that allows things (sometimes organisms) to float and stride on its surface without sinking.
A solution is acidic if it contains excess hydrogen ions. It will have a pH less than 7.
A solution is basic if it releases hydroxide ions when added to water. These solutions are said to be alkaline and are usually slippery.
A solution is neutral, neither basic nor acidic, if it has a pH of 7.
pH scale
The pH scale is a logarithmic scale to measure acidity, with 1 being most acidic, 14 being most basic, and 7 being neutral.
Organic compounds
Organic compounds are compounds that contain carbon and include carbohydrates, lipids, proteins, and nucleic acids.
Inorganic compounds
Inorganic compounds are compounds that do not contain carbon atoms (except for some simple carbon compounds such as carbon oxides)
Carbohydrates are organic compounds that contain carbon, hydrogen, and oxygen, usually in a ratio of 1:2:1.
Monosaccharides are the simplest sugars which serve as an energy source for cells. The two most common monosaccharides are glucose and fructose.
Disaccharides are formed by two sugar molecules combining together through dehydration synthesis. An example of a disaccharide is maltose, make by linking two glucose molecules.
Polysaccharides are made up of many repeated unites of monosaccharides, and is therefore a type of polymer. The most common polysaccharides are starch, cellulose, and glycogen. Polysaccharides are often storage forms of sugar.
Glucose is the most abundant monosaccharide and is a six-carbon sugar with the chemical formula C-6, H-12, O-6. Glucose comes in two forms: alpha glucose and beta glucose, which differ simply by a reversal of the H and OH of the first carbon.
Fructose is a monosaccharide that is a common sugar in fruits. It is a six-carbon sugar with the chemical formula C-6, H-12, O-6.
Glycosidic bond
A glycosidic bond is a covalent bond in which a carbohydrate binds to another group, which could also be a carbohydrate. A glycosidic bond is found between the two glucose molecules in maltose.
Dehydration synthesis
Dehydration synthesis, or condensation, is the process by which two molecules come together by the loss of a water molecule, such as the forming of maltose from two glucose molecules.
Hydrolysis is the opposite process of dehydration synthesis by which molecules are broken up by the addition of a water molecule, such as the formation of two glucose molecules from a maltose.
A polymer is a molecule with repeating subunits of the same general type, such as polysaccharides.
Starch is a polysaccharide of alpha glucose molecules bound together and is produced by most green plants as an energy store.
Cellulose is a polysaccharide of beta glucose molecules that is a major part of the cell wall in plants and is used to lend structural support.
Glycogen is a multi-branch polysaccharide of glucose that is the main storage of glucose in the body.
Plastids are double membrane bound organelles that temporarily store starch in plants. Plastids include chloroplasts, chromoplasts, and leucoplasts.
Amino acids
Amino acids are organic molecules that serve as the building blocks of proteins. They contain carbon, hydrogen, oxygen, and nitrogen atoms. Every amino acid has four parts: an amino group, a carboxyl group, a hydrogen, and an R group.
Amino group
An amino group is a functional group and is found in organic compounds known as amines.
Carboxyl group
Carboxyl groups are weak acids that are common in many organic molecules including amino acids and fatty acids.
R group
An R-group is any group in which the carbon or hydrogen is attached to the rest of the molecule.
Side chain
Side chain is another name for an R group, and is a group of atoms attached to the main part of a molecule and having a ring or chain structure.
Functional group
A functional group is a distinctive group of atoms that play a large role in determining the chemical behavior of the compound they are a part of. In amino acids, functional groups include the carboxyl group and the amino group.
When two amino acids join they form a dipeptide. In a dipeptide, the carboxyl group of one amino acid combines with the amino group of another amino acid.
Peptide bond
A peptide bond is the bond between two amino acids.
If a group of amino acids are joined together in a chain, the resulting organic compound is a polypeptide, which is the primary structure of a protein.
A protein is a polypeptide, a chain of amino acids, that twists and folds on itself.
A lipid is an organic molecule consisting of carbon, hydrogen, and oxygen atoms and includes fats, oils, phospholipids, and steroids. Lipids are important because they function as structural components of cell membranes, sources of insulation, and a means of energy storage.
A neutral fat is the simplest lipid and consists of three fatty acids and one molecule of glycerol, also known as a triglyceride.
Oils are a type of lipid and are triglycerides that are liquid.
Phospholipids are a class of lipids that contain two fatty acids tails and one negatively charged phosphate head. They are extremely important in their unique properties with regard to water.
Steroids are a class of lipids that have a basic structure of four linked carbon rings and include cholesterol, vitamin D, and a variety of hormones.
Neutral fats
Neutral fats are non polar, uncharged triglycerides that have no acidic or basic groups.
Glycerol is a simple polyol (sugar alcohol) compound that is found in fats.
Ester linkage
The linkage formed between the glycerol molecule and the fatty acids in a fat is the ester linkage. This bond is formed through dehydration synthesis.
If a fatty acid is saturated, it means it has a single covalent bond between each pair of carbon atoms.
If a fatty acid is unsaturated it means it has adjacent carbons that are joined by double bonds instead of single bonds.
A polyunsaturated fatty acid has many double bonds within the fatty acids.
A hydrophobic molecule is a molecule that does not mix with water because it is non polar, such as a fatty acids.
A hydrophilic molecule is a molecule that mixes with water because it is polar, such as a phosphate head in a lipid.
An amphipathic molecule has both a hydrophilic region and a hydrophobic region, such as a phospholipid.
Nucleic acids
Nucleic acids are organic compounds that contain carbon, hydrogen, oxygen, nitrogen, and phosphorus. They are made up of simple units called nucleotides and include deoxyribonucleic acid and ribonucleic acid.
Nucleotides are simple units that make up nucleic acids. A nucleotide consists of a nitrogen base, a phosphate group, and a five carbon sugar (ribose or deoxyribose).
Deoxyribonucleic acid (DNA)
DNA is a nucleic acid that has a deoxyribose sugar, a phosphate group, and a nitrogen base. DNA is important because it contains genes, and it is kept in the nucleus of cells.
Ribonucleic acid (RNA)
RNA is a nucleic acid that has a ribose sugar, a phosphate group, and a nitrogen base. RNA is important because it has an essential role in protein synthesis.
Miller and Urey
Miller and Urey were scientists who, in 1953, simulated the conditions of primitive Earth in a laboratory. They put the gases theorized to be abundant in the early atmosphere into a flask, struck them with electrical charges in order to mimic lighting, and organic compounds similar to amino acids appeared.
The cell is the basic unit of structure of function in life and is what all living things are composed of.
Eukaryotic cells
Eukaryotic cells contain a nucleus and cytoplasm filled with membrane bound organelles. Eukaryotic cells include fungi, protists, plant cells, and animal cells.
Prokaryotic cells
Prokaryotic cells are much smaller than eukaryotic cells and lack both a nucleus and membrane bound organelles. In a prokaryote, circular DNA lies free in the nucleoid . Most prokaryotes have a cell wall composed of peptidoglycan and may also have ribosomes or flagella.
Cytoplasm is a thick solution that fills each cell and contains the organelles. The cytoplasm is composed of water, salt, and proteins and helps contain the organelles and may have important enzymes to break down large molecules.
Organelles are small units suspended in the cytoplasm which carry out a specific function to help the cell.
The nucleoid is the area in a prokaryotic cell in which the circular DNA molecule lies free in the cell.
Flagella are long projections on a cell used for motility. They are often found on single cell organisms but are sometimes present in larger organisms, such as on sperm.
Plasma membrane
The plasma membrane is a double layered structure made up of phospholipids and proteins that serves as an outer envelope for cells. In the membrane, hydrophobic fatty acid tails face inwards and hydrophilic phosphate heads face outwards. The membrane is semi-permeable and regulated the movement of substances in and out of the cell.
Peripheral proteins
Peripheral proteins are proteins that are loosely associated with the lipid bilayer and only temporarily attach to the membrane. They are located on the inner or outer surface of the membrane.
Integral proteins
Integral proteins are firmly bound to the plasma membrane and are amphipathic.
Transmembrane proteins
Transmembrane proteins are integral proteins that do not extend all the way through the membrane.
Fluid-mosaic model
The fluid-mosaic model refers to the arrangement of phospholipids and proteins in the plasma membrane of cells.
Adhesion proteins
Adhesion proteins are membrane proteins that form junctions between adjacent cells.
Receptor proteins
Receptor proteins are membrane proteins that serve as docking sites for proteins of the extracellular matrix or hormones.
Transport proteins
Transport proteins are membrane proteins that form pumps that use ATP to actively transport solutes across the membrane.
Channel proteins
Channel proteins are membrane proteins that form channels that selectively allow the passage of certain ions or molecules.
Recognition and adhesion proteins
Recognition and adhesion proteins, such as glycoproteins, are exposed on the extracellular surface and play a role in cell recognition and adhesion
Carbohydrate side chains
A carbohydrate side chain is attached to the surface of some proteins found only on the outer surface of the plasma membrane.
Cholesterol is a steroid that is found in the phospholipid bilayer because it helps stabilize membrane fluidity in animal cells.
Phospholipid bilayer
The phospholipid bilayer is the double layered plasma membrane that surround cells.
DNA is organized into large structures called chromosomes in the nucleus.
The nucleolus is the most visible structure within a cell's nucleus and is where rRNA is made and ribosomes are assembled.
Ribosomes are organelles that are the sites of protein synthesis - they manufacture all the proteins required by the cell or secreted by the cell. Ribosomes are round structures composed of RNA and proteins and can be either free floating or attached to the endoplasmic reticulum.
Endoplasmic reticulum (ER)
The endoplasmic reticulum is a continuos channel that extends into many regions of the cytoplasm. The rough ER contains many ribosomes on its surface and generates proteins, which are then trafficked to or across the plasma membrane. The smooth ER lacks ribosomes and makes lipids, hormones, and steroids, and breaks down toxic chemicals.
Golgi bodies
Golgi bodies , which look like stacks of flatten sacs, are organelles that participate in the processing of proteins in the cell. After the rough ER completes the synthesis of proteins, the Golgi bodies modify, process, ad sort the products. They then package and distribute the proteins to be sent out of the cell, packaging the products into vesicles.
Vesicles are little membrane sacs which carry materials around and out of the cell.
The mitochondria is an organelle that converts the energy from organic molecules into useful energy for the cell through cellular respiration.
Adenosine triphosphate
Adenosine triphosphate, or ATP, is the energy molecule used by the cell, synthesized in the mitochondria.
Lysosomes are organelles that carry digestive enzymes, which they use to break down old, worn out organelles, debris, or large ingested particles. The lysosomes help keep the cytoplasm clear of unwanted materials. Lysosomes contain hydrolytic enzymes that function at acidic pH, which is enclosed inside the lumen of the lysosome.
Centrioles are small, paired cylindrical structures found within microtubule organizing centers. Centrioles are most active during cellular division, during which they produce microtubules, which pull apart the replicated chromosomes. Centrioles are not found in plant cells.
Microtubule Organizing Centers (MTOCs)
The MTOC is a structure found in eukaryotic cells from which microtubules emerge. MTOCs have two main functions: the organization of eukaryotic cilia and flagella, and the organization of the mitotic and meiotic spindle apparatus.
Vacuoles are fluid-filed sacs that store water, food, wastes, salts, or pigment. They are found in both animal and plant cells, but much larger in plant cells.
The cytoskeleton is a network of fibers that holds the cell together and enables it to keep its shape. These fibers include microtubules and microfilaments.
Microtubules, which are made up of the protein tubulin, participates in cellular division and movement. These small fibers are an integral part of centrioles, cilia, and flagella.
Microfilaments are thin, rodlike structures composed of the protein actin and are involved in cell mobility and muscle contraction.
Tubulin is the protein of which microtubules are made up of.
Cilia are threadlike structures made out of microtubules that provide locomotive properties in single-celled organisms.
Cell wall
A cell wall is a rigid layer just outside of the plasma membrane that provides support for the cell. It is found in plants, protists, bacteria, (made out of cellulose), and fungi (made out of chitin).
Chitin is a long polymer of a glucose derivative that is a principle component of the cell wall in fungi and of an arthropod's exoskeleton.
Chloroplasts are organelles involved in photosynthesis that are possessed by plants. Chloroplasts contain chlorophyll, a light capturing pigment that gives plants their green color.
Simple diffusion
Simple diffusion, or passive transport, refers to the movement of substance down the concentration gradient, which uses no energy.
Osmosis is the diffusion that involves the movement of water. When osmosis occurs through a lipid bilayer it moves through membrane proteins called aquaporins.
A solute is a substance that is dissolved in a solvent.
Facilitated transport
Facilitated transport is the movement of lipid insoluble substances across the plasma membrane through special channel proteins.
Active transport
Active transport is the movement of a substance against the concentration gradient, which required energy.
Sodium-potassium pump
The sodium-potassium pump is a channel protein that ushers out sodium ions and brings in potassium ions across the cell membrane, and depend on ATP to get ions across. This pump is found in neurons and skeletal muscle fibers.
Endocytosis is a means of a cell engulfing a substance too large to enter the cell. Endocytosis involves the cell membrane forming a pocket, pinching in, and forming either a vacuole or a vesicle.
Pinocytosis is endocytosis during which the cell ingests liquids.
Phagocytosis is endocytosis during which the cell takes in solids.
Receptor-mediated endocytosis
Receptor-mediated endocytosis involves cell surface receptors that are covered in clathrin-coated pits. When a particle bind to one of these receptors, the ligand is brought into the cell by the folding in of the cell membrane, forming a vesicle.
Bulk flow
Bulk flow is the one way movement of fluids brought about by pressure, such as the movement of blood through a blood vessel or fluids in xylem and phloem of plants.
Dialysis is the diffusion of solutes across a selectively permeable membrane.
Exocytosis is the transportation of large particles out of the cell, by the fusion of a vesicle with the plasma membrane.
Intercellular junctions
Intercellular junctions are strucutres between cells that allow neighboring cells to form strong connections with each other, prevent passage of materials, or establish rapid communication between adjacent cells. The three types of intercellular contact in animal cells are: desmosomes, gap junctions, and tight junctions.
Desmosomes hold adjacent animal cells tightly to each other and consist of a pair of discs associated with the plasma membrane of adjacent cells. Intermediate filaments within and outside the cells are attached to the discs.
Gap juntions
Gap junctions are protein complexes that form channels in membranes and allow communication between the cytoplasm of adjacent animal cells or the transfer of small molecules and ions.
Tight junctions
Tight junctions are tight connections between the membrane of adjacent animal cells. They're so tight that there is no space between the cells, so they seal off body cavities and prevent leaks.
Enzymes are organic catalysts, molecules that speed up the rate of a reaction without altering the reaction itself.
Exergonic reactions
Exergonic reaction are those in which the products have less energy than the reactants, so energy is given off during the reaction.
Endergonic reactions
Endergonic reactions are those in which the products have more energy than the reactants, so energy is required as an input.
Activation energy
Activation energy is the energy needed to begin a chemical reaction.
Enzyme specificity
Enzyme specificity is the concept that each enzyme catalyzes only one kind of reaction.
Substrates are the molecules targeted by an enzyme in an enzymatic reaction
Active site
The active site is a region on the enzyme where the substrate binds.
Enzyme-substrate complex
An enzyme-substrate complex is the term for the enzyme and substrate(s) bound together.
Induced fit
Induced fit refers to when an enzyme has to change its shape slightly to accommodate the shape of the substrates.
Factors that assist enzymes in catalyzing a reaction are known as coenzymes. Vitamins are examples of organic coenzymes. Coenzymes accept electrons and pass them along to another substrate. NAD+ and NADP+ are two examples of such enzymes.
Cofactors are inorganic elements that help catalyze reactions and are usually metal ions.
Allosteric sites
An allosteric site is a region of the enzyme other than the active site to which a substance can bind, and can regulate enzymatic activity.
Allosteric regulators
Allosteric regulators are substances that can either inhibit or activate enzymes and that bind at the allosteric site.
Allosteric inhibitor
Allosteric inhibitors bind to an allosteric site and keep the enzyme in its inactive form.
Allosteric activator
Allosteric activators bind to an enzyme and induce its active form.
Feedback inhibition
Feedback inhibition is a system in which the formation of an end product inhibits an earlier reaction in the enzymatic sequence.
Competitive inhibition
Competitive inhibition is a process by which a chemical substance has a shape that fits the active site of an enzyme and competes with the substrate, effectively inhibiting the enzyme.
Noncompetitive inhibition
In noncompetitive inhibition, the inhibitor binds with the enzyme at a site other than the active site and inactivates the enzyme by altering its shape.
First law of thermodynamics
The first law of thermodynamics states that energy cannot be created or destroyed.
Second law of thermodynamics
The second law of thermodynamics states that energy transfer leads to less organization and the universe tends towards disorder (entropy).
Entropy is disorder, and is what the universe tends towards.
Cellular respiration
Cellular respiration is a process performed by all organisms that produced ATP through the breakdown of nutrients. In cellular respiration, a sugar is combined with oxygen and water to produce carbon dioxide, water, and energy in the form of ATP.
Aerobic respiration
When ATP is made in the presence of oxygen, the process is called aerobic respiration. Aerobic respiration consists of four stages: Glycolysis, formation of acetyl CoA, the Krebs cycle, and oxidative phosphorylation.
Anaerobic respiration
When ATP is made without oxygen, the process is called anaerobic respiration.
Glycolysis, the splitting of glucose, is the first step in aerobic respiration. During glycolysis glucose, a 6-carbon molecule, is broken into two three carbon molecules called pyruvic acid. This breakdown of glucose also results in the net production of two molecules of ATP.
Pyruvic acid
Pyruvic acid is a 3 carbon molecule produced by the splitting of a glucose in glycolysis. Pyruvate is important because it goes on to be broken down into acetyl CoA.
The matrix is the inner region of the mitochondria, inside the inner membrane. The citric acid cycle, or Krebs cycle, takes place inside the matrix of the mitochondria.
Inner mitochondrial membrane
The inner mitochondrial membrane is the innermost membrane of the mitochondria. Oxidative phosphorylation and chemiosmosis take place at the inner mitochondrial membrane, which produces ATP via the flow of protons across the membrane.
Inter-membrane space
The inter-membrane space is the space between the inner and outer mitochondrial membrane. In cellular respiration, hydrogen atoms are pumped across the inner mitochondrial membrane into the inter-membrane space, creating a proton gradient that is responsible for the production of ATP.
Outer membrane
The outer membrane is the outermost membrane in the mitochondria that protects and holds the form of the organelle.
Acetyl coenzyme A
When oxygen is present, pyruvic acid is transported to the mitochondrion, where each pyruvic acid is converted into acetyl coenzyme A, a two carbon molecule. Two Acetyl CoA are produced from two pyruvic acid. Acetyl CoA then moves to the Krebs Cycle, where it combines with oxaloacetate to form citric acid.
Krebs cycle (citric acid cycle)
The Krebs Cycle, also known as the citric acid cycle, takes place in the matrix of the mitochondria. In the Krebs Cycle, each of the two acetyl coenzyme A molecules enter the cycle and combine with oxaloacetate to form citric acid, which then loses two carbons as carbon dioxide. The cycle is now ready to begin again with the second Acetyl CoA. For each Acetyl CoA, the Krebs Cycle produces 1 ATP, 3 NADH, and 1 FADH2.
Oxaloacetate is a four carbon molecule that combines with Acetyl CoA in the Krebs Cycle to form citric acid. This citric acid then loses two carbons as carbon dioxide, to form oxaloacetate again, so that the Krebs cycle can begin again.
Citric acid
Citric acid, or citrate, is a six carbon molecule formed in the Krebs cycle by the joining of an Acetyl CoA molecule with an oxaloacetate. Citric acid then loses two carbons, in the form of carbon dioxide, to become oxaloacetate again.
Cytochromes are iron-containing carriers that are carrier molecules in the electron transport chain. These carrier molecules hand down electrons to the end of the chain, where they are accepted by oxygen molecules. This system releases energy which is used to pump hydrogens across the inner membrane, setting up a proton gradient responsible for the production of ATP.
pH gradient (proton gradient)
The energy released from the electron transport chain is used to pump hydrogen ions across the inner mitochondrial membrane to the inter-membrane space. The pumping of hydrogen ions into the inter-membrane space creates a pH gradient, in which the inter-membrane space has a high hydrogen concentration and the matrix has a low hydrogen concentration. This gradient establishes the potential energy responsible for the production of ATP, as hydrogen molecules diffuse through the ATP synthase.
ATP synthase
The ATP synthase is a channel on the inner mitochondrial membrane. Protons diffuse through the ATP synthase channel, moving into the matrix of the mitochondria. The ATP synthase uses the energy from this diffusion to combine ADP and P on the matrix side of the channel, creating ATP, a process known as oxidative phosphorylation. The total number of ATP produced by oxidative phosphorylation is 32 ATP.
Oxidative phosphorylation
Oxidative phosphorylation refers to the process by which ATP synthase uses the flow of protons into the matrix to combine ADP and P, producing ATP. Oxidative phosphorylation produces 32 ATP.
Fermentation is the process of ATP production under anaerobic conditions. In this process, after glycolysis pyruvic acid is converted to either lactic acid or ethyl alcohol and carbon dioxide. This process also produced NAD+, which allows glycolysis to continue. This process is not very efficient, and only results in a gain of 2 ATP for each molecule of glucose broken down.
Lactic acid
Lactic acid is a by-product of fermentation. Two lactic acid molecules are produced for every one glucose that is broken down. Lactic acid is produced by some bacteria during anaerobic respiration and by muscles when they do not get enough oxygen.
Ethyl alcohol (ethanol)
Ethyl alcohol is a by-product of fermentation. Two ethyl alcohol molecules are produced for every one glucose that is broken down. Yeast cells and some bacteria make ethanol and carbon dioxide during anaerobic respiration.
Sodium-potassium pump
After a part of the neuron has been repolarized, the charges are in their original state, but the ions are on the wrong side of the axonal membrane, with potassium outside and sodium inside. The neuron reestablishes the order of the ions with the sodium-potassium pump, which pumps two potassium ions inside the axon for every three sodium ions it pumps out.
Homeostasis refers to the stable conditions of your body, including steady heart and respiratory rates. The parasympathetic system works to keep the body at homeostasis.
Differentiation is the process by which a less specialized cell becomes a more specialized cell type, through homeotic genes.
Hox genes
Hox genes are a subset of homeobox genes that specify the position of body parts in the developing embryo.
Apoptosis, or programmed cell death, plays a crucial role in normal differentiation and development. For example, in a human embryo, apoptosis allows for the removal of tissue between newly developing fingers and toes.
Genetic variability
The genetic differences in every individual is known as genetic variability. The survival of a species is dependent on genetic variation, since it allows a population to survive in a changing environment.

Flickr Creative Commons Images

Some images used in this set are licensed under the Creative Commons through Flickr.com.
Click to see the original works with their full license.