A. Sherman
Chapter 7 – Membrane Structure and Function
I.
II.
Life at the Edge
A. Plasma membrane
1. separates the living cell from its surroundings
2. Boundary between two aqueous compartments
3. Controls traffic in and out of its cell
4. Selective Permeability – allows some substances to cross more easily than others
5. Ability of a cell to discriminate in its chemical exchanges with its environment is
fundamental
Cellular Membranes are fluid mosaics of lipids and proteins
A. Phospholipids
1. Most abundant lipids in most membranes
2. Are amphipathic – have both a hydrophilic region and a hydrophobic region
3. Most membrane proteins and lipids tend to be amphipathic
B. Fluid Moasic Model
1. Membrane is fluid with a mosaic of various proteins in a double layer (bilayer) of
phospholipids
C. Membrane Models – How the fluid mosaic model has evolved
1. Gorter and Grendel reasoned that cell membranes must be phospholipid bilayers
2. Davson and Danielli proposed that membrane is coated on both sides with hydrophilic
proteins – this was wrong because membranes with different functions differ in
structure
3. Singer and Nicolson proposed that membrane proteins are dispersed in the bilayer with
their hydrophilic regions protruding. This maximizes contact of hydrophilic regions of
the membrane with water.
4. Freeze-fracture has demonstrated that there are proteins embedded in the bilayer.
5. Recent findings show that membranes may be more mosaic than fluid, proteins can
associate in patches, and membranes may have more proteins than originally thought.
D. The Fluidity of Membranes
1. Membranes are held together by hydrophobic interactions
a. Much weaker than covalent bonds
2. Movement
a. Most lipids and some proteins can shift laterally – side to side
b. Rapid - Adjacent phospholipids switch positions about 10^7 times per second
c. Some proteins move in a highly directed manner, some seem immobile
d. It is rare for molecules to flip flop transversely across the membrane
3. Temperature
a. Membrane remains fluid as temperature decreases until the phospholipids
settle into a close arrangement and the membrane solidifies
b. Solidifying temperature depends on the types of lipids in the membrane
i. Lower temperature – rich in phospholipids with unsaturated
hydrocarbon tails
i. The tails have kinks which prevents them from packing as closely
as unkinky tails, and this makes the membrane more fluid
c. Cholesterol – a temperature buffer
i. Wedged between phospholipids in membranes of animal cells
ii. At high temperatures, cholesterol makes the membrane less fluid by
confining phospholipid movement
iii. Lowers the solidifying temperature by hindering close packing of
phospholipids.
4. Membranes must be fluid to work properly – like salad oil
a. Solidified membrane has a changed permeability – this can mess up the cell
i. Enzymatic proteins and other molecules that require lateral movement
in the membrane will not be able to do so
b. Lipid composition can change in response to temperature change – ie: more
unsaturated fatty acids in a membrane in the winter prevents packing
together and solidification
E. Membrane Proteins and Their Functions (SEE FIGURE 7.9)
1. Proteins determine most of a membrane’s functions while phospholipids form the
fabric
2. Integral proteins
a. Can penetrate the hydrophobic core of the lipid layer
b. Transmembrane proteins – span the membrane
c. Hydrophobic regions of an integral protein contain stretches of nonpolar
amino acids usually coiled into alpha helices.
d. Hydrophilic regions are exposed to the aqueous areas on either side of the
membrane
e. Some proteins have a hydrophilic channel through their center that allows
passage of hydrophilic substances
3. Peripheral Proteins
a. Not embedded in the lipid bilayer
b. Appendages loosely bound to the surface of the membrane
4. On the cytoplasmic side, some proteins are held in place by attachment to the
cytoskeleton
5. Extracellular side – proteins attached to fibers of the extracellular matrix
F. The Role of Membrane Carbohydrates in Cell-Cell Recognition
1. Cell-cell recognition – a cell’s ability to distinguish one type of cell from another
2. Important in
a. Cell sorting into tissues and organs in embryos
b. Rejection of foreign cells by the immune system
3. Cells recognize others by binding to surface molecules, often carbohydrates, on the cell
membrane
4. Membrane carbohydrates – short, branched chains of <15 sugar units
5. Glycolipids – Membrane carbohydrates covalently bonded to lipids
6. Glycoproteins – Membrane carbohydrates covalently bonded to proteins
7. Human blood type variation reflects variation in membrane carbohydrates
G. Synthesis and Sidedness of Membranes – See figure 7.10
1. Note: this section is pretty much copied word for word
2. When a vesicle fuses with a membrane, the outside layer of the vesicle becomes
continuous with the inner layer of the membrane
3. Synthesis of membrane proteins and lipids: Carbs added to proteins become
glycoproteins
4. Glycoproteins change more, and lipids acquire carbs, becoming glycolipids
III.
IV.
5. Transmembrane proteins, membrane glycolipids, and secretory proteins are
transported in vesicles to the plasma membrane
6. Vesicles fuse with the membrane, releasing secretory proteins from the cell. Fusion
positions the carbs of membrane glycoproteins and glycolipids on the outside of the
membrane.
Membrane structure results in selective permeability
A. Permeability of the Lipid Bilayer
1. Nonpolar Molecules are hydrophobic and can dissolve in the lipid bilayer of the
membrane and cross it easily
a. Hydrocarbons, carbon dioxide, oxygen
2. Hydrophobic core of the membrane prevents passage of ions and polar molecules
a. Glucose, sugars, water
3. Proteins are key in regulating transport
B. Transport Proteins
1. Cell membranes are permeable to certain ions and polar molecules. These pass through
transport proteins.
2. Channel proteins – have a hydrophilic channel that certain molecules can use as a
tunnel to enter the cell.
a. Aquaporins – facilitate water transport through membranes
3. Carrier proteins – hold onto their passengers and change shape in a way that shuttles
them across the membrane.
a. Substance specific
b. IE: glucose transporter allows glucose to pass 50000 times faster through a
membrane, but the transporter will reject fructose, a structural isomer of
glucose
Passive Transport is diffusion of a substance across a membrane with no energy investment
A. Molecules have a type of energy called thermal motion (heat)
1. One result of this is diffusion
a. The movement of molecules of any substance so that they spread out evenly
into the available space
b. Each molecule moves randomly
c. Diffusion of a population of molecules may be directional
d. In the absence of other forces, a substance will diffuse from where it is more
concentrated to where it is less concentrated
i. Concentration gradient – region along which the density of a chemical
substance decreases
ii. All substances diffuse in their own concentration gradient, regardless of
other substances
e. Happens spontaneously, does not need an energy input
2. Much of a membrane’s traffic occurs with diffusion
a. Uptake of oxygen by a cell performing cellular respiration
b. Dissolved O2 diffuses into the cell across the plasma membrane. As
respiration consumes O2 as it enters, diffusion into the cell will continue
because the concentration gradient favors movement in that direction since
O2 there keeps getting depleted.
3. Passive Transport
a. The diffusion of a substance across a biological membrane
b. Cell does not have to expend energy to make it happen
B. Effects of Osmosis on Water Balance
1. Osmosis – the diffusion of water across a selectively permeable membrane
2. In a U shaped tube of sugar-water solution separates with a membrane that allows
water through but not sugar…
a. The side with more solutes will have more water
b. The water will cluster around the hydrophilic sugar
c. The free water will go to the side with more sugars until the solute
concentration on both sides is equal.
3. Water Balance of Cells Without Walls
a. Tonicity
i. the ability of a solution to cause a cell to gain or lose water
ii. Depends on the solution’s concentration of solutes that cannot cross the
membrane relative to that inside the cell
b. Isotonic environment
i. same tonicity as the cell
ii. there will be no net movement of water across the plasma membrane
iii. Animal cells do best in this sort of environment
c. Hypertonic
i. The cell loses water to its environment and probably dies
ii. Increase in lake salinity makes the lake take water from its inhabitants
cells and kills the animals.
d. Hypotonic
i. Water enters the cell faster than it leaves
ii. Cell can swell and lyse/burst
iii. Plant cells do well in this environment due to their stiff walls because the
wall pushes back on the cell
e. A cell without rigid walls cannot tolerate excessive uptake nor excessive loss of
water
f. Osmoregulation –the control of water balance
i. i.e: Paramecium lives in hypotonic water. Its membranes let through
water much slower than other cells, and its filling vacuole pumps water
out
4. Water Balance of Cells with Walls
a. Plants, prokaryotes, fungi, some protists have cell walls
b. When their cells are immersed in a hypotonic solution, the wall helps maintain
the cell’s water balance
c. In a hypotonic environment, these cells will swell, but at a point, their walls will
exert back a pressure that prevents further water uptake. At this point, the
cells are turgid, or very firm.
d. If a plant cell’s surroundings are isotonic, there is no tendency for water to
enter, and the cell becomes flaccid, or limp.
e. Plasmolysis – Plasma membrane pulls away from the cell wall. Happens when
the cell is immersed in a hypertonic environment and it loses water to its
surroundings and shrinks.
C. Facilitated Diffusion: Passive Transport Aided by Proteins
1. Facilitated Diffusion – What happens when polar molecules and ions passively get
through the lipid bilayer with the help of transport proteins.
2. Channel proteins provide corridors for certain things to enter the membrane
a. Aquaporins allow water to enter
i. Kidneys reclaim water from urine before you pee it out with the help of
aquaporins...If they didn’t do this, you would sit around all day and drink
50 gallons of water
b. Ion channels – function as gated channels – open or close in response to a
stimulus
i. Stimulus may be electrical or chemical
ii. Chemical stimuli can open gated channels and allow certain ions into the
cell.
c. Carrier proteins seem to undergo shape changes that translocates their
passengers across the membrane.
d. In certain diseases, specific transport systems are either defective or missing
altogether.