AP Biology
Membrane Structure and Transport of Molecules – Part 2
(Associated Learning Objectives: 2.6, 2.7, 2.8, 2.9, 2.10, 2.11, 2.12, 2.13, 2.15, 2.17, 4.4, 4.5, 4.6, 4.14, 4.16)
I.
Material Transport
A. CO2 and O2 (both gases) diffuse across the wet bi-layer because they are neutrally charged particles.
B. Ions and water move through the proteins because of particle charge. (Hence the name Transport proteins.)
a. Some proteins are Tunnels and some are Grabbers.
II. Passive Transport (NO E required for this process.)
A. Diffusion
1. This process operates upon an established concentration [ ] gradient.
2. Materials flow from high [ ] to low [ ] until equilibrium is achieved.
3. This is how the majority of materials are transported in cells. (Because it requires no E expenditure
by the cell…which saves E for maintaining homeostasis, repair, and reproduction.)
B. Osmosis (The diffusion of Water.)
1. Water ALWAYS flows from Hypotonic to Hypertonic until Isotonic.(Provided there is no
pressure being applied.)
a. Terms refer to the material dissolved in the water. NOT the water itself. (That is tonic.)
b. Water flows one way and the materials dissolved in the water flow the opposite direction.
c. Plants and Fungus have cell walls that may affect water movement, see below.
2. This process is crucial for all cells to control.
a. Osmoregulation (This term refers to water control)
b. Pure water vs. normal water. Pure water is ALWAYS the HYPO.
c. Turgid – This refers to a condition when there is plenty of water in the plant cell, so the
cells are rigid and the plant is stiff.
d. Flaccid – This refers to a condition when there is not enough water in the plant cell, so the
cells are limp and the plant is wilted.
e. Plasmolysis – This is when the cell membrane rips away from the cell wall killing the
plant cell. (“Plasmo” refers to the plasma membrane; “lysis” means “the process of
tearing”)
3. Water Potential (Represented by the Greek letter psi - Ψ) (After Poseidon’s Trident.)
a. It is basically water’s ability to perform work while passing through the cell membrane.
b. It flows from High Ψ to Low Ψ. (It can be affected by the pressure of a plant cell wall.)
c. Pressure Potential (Represented by ΨP)
i. Pushing is positive pressure being exerted on the cell. (+ΨP)
ii. Pulling away from is negative pressure (-ΨP) being exerted on a cell. (Important
when you consider a plant is having water pulled out of it by transpiration at
the stomata and pushed in to the xylem vascular cylinder in the root.
(Referred to as Root pressure.)
d. Solute Potential (Represented by the symbol ΨS)
i. ΨS = -iCRT
ii. i is the ionization constant for a molecule (Basically, the number of ion types
present. For example Na+ and Cl in water would be a constant of 2.
iii. C is the molar concentration
iv. R is the pressure constant (R= 0.0831literbars/mole-K)
v. T is the temperature in Kelvin (273+ ⁰C…Room temp is 298K)
e. Total Water Potential (Represented by ΨT)
i. ΨT= ΨS +ΨP
C. Facilitated Diffusion
1. This transport of molecules requires the help of a Transport Protein.
2. Aquaporins- These help move water (because it is a polar molecule) across a membrane.
3. Gated-ion channels are also examples.
III. Active Transport (This process REQUIRES ENERGY.)
A. This process is moving material against the [ ] gradient. (Like pushing a car up a hill…it will require energy.)
1. Na+/K+ Pump of the nervous system, is an example.
a. Energy from ATP by Phosphorylation (Attaching a phosphate ion to a structure to make it
work.) activates the protein to grab and move molecules.
IV. Membrane Potential
A. This is the ability for a cell membrane to do work involving transport of molecules.
B. Voltage Gradient (This gradient works on + and – Ions.)
1. The inside of a cell is negative because of anions in excess, mostly DNA; the outside of a cell is
positive because of cations in excess, mostly Na+. (A [ ] gradient or “hill” is created. So now we
have the “potential” to perform work moving molecules.)
2. A.K.A. Electochemical gradient (“electro” referring to charge; “chemical” referring to the ion)
C. Electrogenic Pump (A.K.A. Proton {H+} Pump)
1. This is the most important active transport protein for all life forms. They are important in processes
involved in the electron transport chain of photosynthesis and cellular respiration.
2. Hydrogen ions, H+, move out of the cell to create a gradient. (Outside is + and inside is -) Diffusion
can now occur based on charges into and out of cell.
3. Co-transport
a. Protons (H+) acts like party invitations. (Very little energy is required to send them out of
the cell. They then bring in INVITED guests only.)
b. This is very important in helping cells of plants and animals to be able to take up glucose or
sucrose by active transport.
V. Large Molecule Transport (These molecules are TOO big for proteins to transport.)
A. Exocytosis – This is the process of moving materials out of a cell. (Exo means “out”; cyto means “cell”; sis
means “process of”)
1. This would be like Pancreas cells releasing the hormone Insulin into the blood stream to help
regulate blood glucose levels.
B. Endocytosis – This is the process of moving materials into a cell. (Endo means “in”)
1. Phagocytosis – This process is “cell eating”. (Phage means “to eat”)
2. Pinocytosis – This process is “cell drinking”. (Pino means “ to drink”)
C. Receptor-Mediated Endocytosis
1. Ligand (This refers to the molecule to be transported.) attaches to a receptor protein (like the blind
man’s hands) on the cell membrane… (The cell can tell what the molecule is by its shape basically.)
(“ mediated” means the ligand must be present to make the process occur)
2. The molecule in the cell’s “hand” (receptor protein) triggers endocytosis to occur.
3. Familial Hypercholesterolemia is an example. (A Genetic Disease that is inherited.)
a. No LDL cholesterol receptor proteins on the cell membranes of the individual.
b. LDL stays in the blood to clog the arteries.