Only $35.99/year

AP Biology Exam Review

Terms in this set (795)

a hydrogen bond forms when the slightly negatively charged oxygen of one water molecule is attracted to the slightly positively charged hydrogen of a nearby water molecule. hydrogen bonding between water molecules is the basis for water's properties

hydrogen bonding keeps water molecules close to each other, giving water cohesion. hydrogen bonding is also responsible for water's surface tension

water has high specific heat: heat is absorbed when hydrogen bonds break and is released when hydrogen bonds form. this helps keep temperatures relatively steady, within limits that permit life. evaporative cooling is based on water's high heat of vaporization. the evaporative loss of the most energetic water molecules cools a surface

ice floats because it is less dense than liquid water. this property allows life to exist under the frozen surfaces of lakes and seas

water is an unusually versatile solvent because its polar molecules are attracted to ions and polar substances that can form hydrogen bonds. hydrophilic substances have an affinity for water; hydrophobic substances do not. molarity, the number of moles of solute per liter of solution, is a certain number of molecules of a substance. the mass of a mole of a substance in grams is the same as the molecular mass in daltons

a water molecule can transfer an H+ to another water molecule to form H3O+ and OH-

the concentration of H+ is expressed as pH; pH = -log [H+]. a buffer consists of an acid-base pair that combines reversibly with hydrogen ions, allowing it to resist pH changes

the burning of fossil fuels increases the amount of CO2 in the atmosphere. some CO2 dissolves in the oceans, causing ocean acidification, which has potentially grave consequences for coral reefs
a. transport. a protein that spans the membrane may provide a hydrophilic channel across the membrane that is selective for a particular solute. other transport proteins shuttle a substance from one side to the other by changing shape. some of these proteins hydrolyze ATP as an energy source to actively pump substances across the membrane

b. enzymatic activity. a protein built into the membrane may be an enzyme with its active site exposed to substances in the adjacent solution. in some cases, several enzymes in a membrane are organized as a team that carries out sequential steps of a metabolic pathway

c. attachment to the cytoskeleton and extracellular matrix. microfilaments or other elements of the cytoskeleton may be noncovalently bound to membrane proteins, a function that helps maintain cell shape and stabilizes the location of certain membrane proteins. proteins that can bind to ECM molecules can coordinate extracellular and intracellular changes

d. cell-cell recognition. some glycoproteins serve as identification tags that are specifically recognized by membrane proteins of other cells. this type of cell-cell binding is usually short-lived

e. intercellular joining. membrane proteins of adjacent cells may hook together in various kinds of junctions, such as gap junctions or tight junctions. this type of binding is more long-lasting

f. signal transduction. a membrane protein (receptor) may have a binding site with a specific shape that fits the shape of a chemical messenger, such as a hormone. the external messenger (signaling molecule) may cause the protein to change shape, allowing it to relay the message to the inside of the cell, usually by binding to a cytoplasmic protein
1. a photon of light strikes a pigment molecule in a light-harvesting complex of PS II, boosting one of its electrons to a higher energy level. as this electron falls back to its ground state, an electron in a nearby pigment molecule is simultaneously raised to an excited state. the process continues, with the energy being relayed to other pigment molecules until it reaches the P680 pair of chlorophyll a molecules in the PS II reaction-center complex. it excites an electron in this pair of chlorophylls to a higher energy state

2. this electron is transferred from the excited P68- to the primary electron acceptor. P680 turns into P680+

3. an enzyme catalyzes the splitting of a water molecule into two electrons, two H+, and an oxygen atom. the electrons are supplied one by one to the P680+ pair, each electron replacing one transferred to the primary electron acceptor. the H+ are released into the thylakoid space. the oxygen atom immediately combines with an oxygen atom generated by the splitting of another water molecule, forming O2

4. each photoexcited electron passes from the primary electron acceptor of PS II to PS I via an electron transport chain, the components of which are similar to those of the electron transport chain that functions in cellular respiration. the electron transport chain between PS II and PS I is made up of the electron carrier Pq, a cytochrome complex, and a protein Pc

5. the exergonic fall of electrons to a lower energy level provides energy for the synthesis of ATP. as electrons pass through the cytochrome complex, H+ are pumped into the thylakoid space, contributing to the proton gradient that is subsequently used in chemiosmosis

6. meanwhile, light energy has been transferred via light-harvesting complex pigments to the PS I reaction-center complex, exciting an electron of the P700 pair of chlorophyll a molecules located there. the photoexcited electron is then transferred to PS I's primary electron acceptor, creating an electron "hole" in the P700 (P700+). in other words, P700+ can now act as an electron acceptor, accepting an electron that reaches the bottom of the electron transport chain from PS II

7. photoexcited electrons are passed in a series of redox reactions from the primary electron acceptor of PS I down a second electron transport chain through the protein ferredoxin

8. the enzyme NADP+ reductase catalyzes the transfer of electrons from Fd to NADP+. two electrons are required for its reduction to NADPH. this molecule is at a higher energy level than water, and its electrons are more readily available for the reactions of the Calvin cycle than were those of water. this process also removes an H+ from the stroma