Chapter 7: Membrane Structure
Campbell Text 7th Ed. PGS: 124 – 138
I.
Selectively Permeable
A. The cell “selects” what materials enter or exit the cell through the membrane.
II.
Membrane Structure (Fig. 7.7)
A. Phospholipids make up the majority of the membrane.
1. These are Amphipathic Molecule. (It means there is a hydrophilic and hydrophobic component.)
2. These molecules create the bi-layer and the structure is held intact by the presence of water
outside and inside the cell. The negatively charged phosphorus line up to make a barrier
preventing water from forming hydration shells around the phospholipids and thereby dissolving
the membrane.
B. Proteins
1. These are also Amphipathic molecules. (This is due to proteins folding into a 3-D structure and
that proteins are composed of amino acids, of which some are polar and some are non-polar.)
2. Two types of proteins are present on the membrane:
a. Integral – These run completely through the bi-layer from the outside to the inside.
i. These function in the transport of molecules and foundation. (Help to maintain the
INTEGRITY of the structure.)
b. Peripheral – These are located on one side of the membrane. (They do not extend into the
bi-layer of the membrane.
i.These act as sites for attachment of the Cytoskeleton on the inside of the cell and the
attachment of the ECM (like armor for the fragile cell) on the outside of the cell.
3. The proteins of the cell membrane can have several functions. (Fig.7.9)
a. Molecule transport (Helps move food, water, or something across the membrane.)
b. Act as enzymes (To control metabolic processes.)
c. Cell to cell communication and recognition (So that cells can work together in tissues.)
d. Signal Receptors (To catch hormones or other molecules circulating in the blood.)
e. Intercellular junctions (For “stiching” cells together to make tissues.)
f. Attachment points for the cytoskeleton and ECM
C. Cholesterol
1. This molecule helps keeps the membrane of all cells flexible.
2. It also helps to keep the cell membrane of plant cells from freezing solid in very cold
temperatures, like the Tundra.
D. The membrane is described as a Fluid-Mosaic model because it looks like a moving (Fluid) puzzle (mosaic).
All the pieces can move laterally, like students moving from seat to seat. The proteins moving in this sea of
phospholipids would be like the teacher moving around the student desks. Imagine the ceiling and floor are
water molecules. They keep you from moving up and down to some extent by their presence.
III.
Charles Overton (1895)
A. He stated the membrane must be made of lipids based on lipid diffusion rates. (Like begets like- The lipid
substances went into the cells faster than the other substances.)
IV.
Gorter and Grendel (1925)
A. They stated the membrane must be a phospholipid bi-layer since water and “oil” does not mix.
B. Their reasoning was that water is inside a cell as well as outside a cell.
V.
Singer and Nicholson (1972)
A. They propose the Fluid-Mosaic model.
B. This is confirmed by the electron microscope (Fig. 7.4) using freeze fracture.
VI.
Agre and MacKinnon (2003)
A. Awarded the Nobel Prize for discovering the structure and function of aquaporins. (These move water.)
VII.
Scientific Model
A. These are used to represent what is difficult to actually see. (Like a model of the solar system. or the model
of DNA or a cell membrane.)
VIII.
Cell-to-Cell Recognition
A. This is vital to tissue formation, Red Blood Cells (RBCs), and White Blood Cells (WBCs).
B. Glycolipids (sugar lipids) and Glycoproteins (sugar proteins) that function in this process act as hands on a
blind person. Imagine cells are like blind, deaf, and mute individuals… how can they communicate with the
environment around them...by using their hands to identify molecules and other cells.
C. Species recognition is unique. (Each type of cell in your body has different “hands” basically and WBC’s can
recognize them all.)
IX.
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.)
1. Some proteins are Tunnels and some are Grabbers. (Fig. 7.15)
X.
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 Hypo to Hyper until Iso. (Fig. 7.12)(Leave pressure out)
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.
2. This process is crucial for all cells to control.
a. Osmoregulation (Water control)
b. Pure water vs. normal water. Pure water is ALWAYS the HYPO. (Fig. 7.13)
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. Pushing is positive pressure being exerted on the cell. (+ΨP)
d. 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.)
C. Facilitated Diffusion (Fig. 7.15 (a))
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.
XI.
Active Transport (This process REQUIRES E.)
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. (Fig. 7.16)
a. E from ATP by Phosphorylation (Attaching a phosphate ion to a structure to make it
work.) activates the protein to grab and move molecules.
XII.
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; The outside of a cell is positive (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) (Fig. 7.18)
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. 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.
XIII.
Co-transport (Fig. 7.19)
a. Protons (H+) acts like party invitations. (Very little E is required to send them out of the
cell. They then bring in INVITED guests only.)
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”)
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 (Fig. 7.20)
1. Ligand (This refers to the item 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.