1. Membrane Structure and Function
Chapter 7
Membrane Structure and Function

2. Introduction
The plasma membrane separates the living cell from its nonliving surroundings.
Like other membranes, the plasma membrane is selectively permeable, allowing some substances to cross more easily than others.

3. Concept 7.1 Cell Membrane Structure
The main macromolecules in membranes are lipids and proteins, but include some carbohydrates.
The most abundant lipids are phospholipids.
Phospholipids and most other membrane constituents are amphipathic molecules.
Amphipathic molecules have both hydrophobic regions and hydrophilic regions.

5. A Mosaic of Structures
A membrane is a collage of different proteins embedded in the fluid matrix of the lipid bilayer.
The fluid mosaic model states that a membrane is a fluid structure with a “mosaic” of various proteins embedded in it

7. As temperatures cool, membranes switch from a fluid state to a solid state
The temperature at which a membrane solidifies depends on the types of lipids
Membranes rich in unsaturated fatty acids are more fluid than those rich in saturated fatty acids
Membranes must be fluid to work properly; they are usually about as fluid as salad oil

8. The steroid cholesterol has different effects on membrane fluidity at different temperatures
At warm temperatures (such as 37°C), cholesterol restrains movement of phospholipids
At cool temperatures, it maintains fluidity by preventing tight packing

9. Unsaturated hydrocarbon tails Saturated hydrocarbon tails
Figure 7.8
Fluid
Viscous
Unsaturated hydrocarbon tails
Saturated hydrocarbon tails
(a) Unsaturated versus saturated hydrocarbon tails
(b) Cholesterol within the animal cell membrane
Figure 7.8 Factors that affect membrane fluidity.
Cholesterol 9

10. Proteins determine most of the membrane’s specific functions.
The plasma membrane and the membranes of the various organelles each have unique collections of proteins.
There are two populations of membrane proteins.
Peripheral proteins are not embedded in the lipid bilayer at all.
Instead, they are loosely bound to the surface of the membrane.

11. Integral proteins penetrate the hydrophobic core of the lipid bilayer, often completely spanning the membrane (a transmembrane protein).
Where they contact the core, they have hydrophobic regions with nonpolar amino acids, often coiled into alpha helices.
Where they are in contact with the aqueous environment, they have hydrophilic regions of amino acids.

12. Six major functions of membrane proteins
Transport
Enzymatic activity
Signal transduction
Cell-cell recognition
Intercellular joining
Attachment to the cytoskeleton and extracellular matrix (ECM)

13. The proteins in the plasma membrane may provide a variety of major cell functions.

14. Membrane Carbohydrates
The membrane plays the key role in cell-cell recognition.
Cell-cell recognition is the ability of a cell to distinguish one type of neighboring cell from another.
This attribute is important in cell sorting and organization as tissues and organs in development.
It is also the basis for rejection of foreign cells by the immune system.
The four human blood groups (A, B, AB, and O) differ in the external carbohydrates on red blood cells.

15. Synthesis and Sidedness of Membranes
Membranes have distinct inside and outside faces
The asymmetrical distribution of proteins, lipids, and associated carbohydrates in the plasma membrane is determined when the membrane is built by the ER and Golgi apparatus

16. Transmembrane glycoproteins
Figure 7.12
Secretory protein
Transmembrane glycoproteins
Golgi apparatus
Vesicle ER
ER lumen
Glycolipid
Figure 7.12 Synthesis of membrane components and their orientation in the membrane.
Plasma membrane:
Cytoplasmic face
Transmembrane glycoprotein
Extracellular face
Secreted protein
Membrane glycolipid 16

17. Concept 7.2 Membrane structure results in selective permeability
A cell must exchange materials with its surroundings, a process controlled by the plasma membrane
Plasma membranes are selectively permeable, regulating the cell’s molecular traffic
Small molecules and ions move across the plasma membrane in both directions.
For example, sugars, amino acids, and other nutrients enter a muscle cell and metabolic waste products leave.
The cell absorbs oxygen and expels carbon dioxide.
It also regulates concentrations of inorganic ions, like Na+, K+, Ca2+, and Cl-, by shuttling them across the membrane.
However, substances do not move across the barrier indiscriminately; membranes are selectively permeable.

18. Permeability of a molecule through a membrane depends on the interaction of that molecule with the hydrophobic core of the membrane.
Hydrophobic molecules, like hydrocarbons, CO2, and O2, can dissolve in the lipid bilayer and cross easily.
Ions and polar molecules pass through with difficulty.
This includes small molecules, like water, and larger critical molecules, like glucose and other sugars.
Proteins can assist and regulate the transport of ions and polar molecules.

19. Specific ions and polar molecules can cross the lipid bilayer by passing through transport proteins that span the membrane.
Some transport proteins have a hydrophilic channel that certain molecules or ions can use as a tunnel through the membrane. Channel proteins called aquaporins facilitate the passage of water
Others bind to these molecules and carry their passengers across the membrane physically.
Each transport protein is specific as to the substances that it will translocate (move).

20. Concept 7.3 Passive Transport - Diffusion
Diffusion is the tendency of molecules of any substance to spread out in the available space
Movements of individual molecules are random.
However, movement of a population of molecules may be directional.

21. A cube of sugar will diffuse from a concentrated area into a more dilute region, until an equilibrium is reached.
Fig. 7.5 Diffusion of molecules in aqueous solutions

22. For example, if we start with a permeable membrane separating a solution with dye molecules from pure water, dye molecules will cross the barrier randomly.
The dye will cross the membrane until both solutions have equal concentrations of the dye.
Equilibrium is meet as roughly half the molecules pass one way across the membrane.

23. In the absence of other forces, a substance will diffuse from where it is more concentrated to where it is less concentrated, down its concentration gradient.
Each substance diffuses down its own concentration gradient, independent of the concentration gradients of other substances.

24. The diffusion of a substance across a biological membrane is passive transport because it requires no energy.
The concentration gradient represents potential energy and drives diffusion.
However, because membranes are selectively permeable, the interactions of the molecules with the membrane play a role in the diffusion rate.
Diffusion of molecules with limited permeability through the lipid bilayer may be assisted by transport proteins.

25. Passive Transport - Osmosis
Water diffuses across a membrane from the region of lower solute concentration to the region of higher solute concentration.
The solution with the higher concentration of solutes is hypertonic.
The solution with the lower concentration of solutes is hypotonic.
These are comparative terms.
Tap water is hypertonic compared to distilled water but hypotonic when compared to sea water.
Solutions with equal solute concentrations are isotonic.

26. Imagine that two sugar solutions differing in concentration are separated by a membrane that will allow water through, but not sugar.
The hypertonic solution has a lower water concentration than the hypotonic solution.
More of the water molecules in the hypertonic solution are bound up in hydration shells around the sugar molecules, leaving fewer unbound water molecules.

27. Unbound water molecules will move from the hypotonic solution where they are abundant to the hypertonic solution where they are rarer.
This diffusion of water across a selectively permeable membrane is a special case of passive transport called osmosis.
Osmosis continues until the solutions are isotonic.

29. Osmosis in Cells
Tonicity is the ability of a surrounding solution to cause a cell to gain or lose water
An animal cell immersed in an isotonic environment experiences no net movement of water across its plasma membrane.
The same cell in a hypertonic environment will lose water, shrivel, and probably die.
A cell in a hypotonic solution will gain water, swell, and may burst.

31. Hypertonic or hypotonic environments create osmotic problems for organisms
Osmoregulation, the control of solute concentrations and water balance, is a necessary adaptation for life in such environments
The protist Paramecium, which is hypertonic to its pond water environment, has a contractile vacuole that acts as a pump

32. Passive Transport with Proteins
Many molecules and ions that are normally impeded by the lipid bilayer of the membrane diffuse passively with the help of transport proteins that span the membrane.
The passive movement of molecules down its concentration gradient via a transport protein is called facilitated diffusion.

33. Transport proteins have much in common with enzymes.
They may have specific binding sites for the solute.
Transport proteins can become saturated when they are translocating passengers as fast as they can.
Transport proteins can be inhibited by molecules that resemble the normal “substrate.”

34. So How Does it Work?
Many transport proteins simply provide corridors allowing a specific molecule or ion to cross the membrane.
These channel proteins allow fast transport.
Water channel proteins, aquaporins, facilitate massive amounts of diffusion.

35. Concept 7.4 Active Transport
Some facilitated transport proteins can move solutes against their concentration gradient, from the side where they are less concentrated to the side where they are more concentrated.
This active transport requires the cell to expend its own metabolic energy.
Active transport is performed by specific proteins embedded in the membranes.
ATP supplies the energy for most active transport.

36. The sodium-potassium pump is one type of active transport system
Active transport allows cells to maintain concentration gradients that differ from their surroundings
The sodium-potassium pump is one type of
active transport system
For the Cell Biology Video Na+/K+ATPase Cycle, go to Animation and Video Files.

37. EXTRACELLULAR FLUID [Na] high Na Na [K] low Na Na Na Na Na
Figure
EXTRACELLULAR FLUID
[Na] high
Na
Na
[K] low
Na
Na
Na
Na
Na
Na
CYTOPLASM
[Na] low
ATP
Na P P 1
[K] high 2
ADP 3 K
Figure 7.18 The sodium-potassium pump: a specific case of active transport. K K K K P 6 K 5 4
P i 37

39. Concept 7.5 Bulk Transport
Large molecules, such as polysaccharides and proteins, cross the membrane via vesicles.
Bulk transport requires energy
During exocytosis, a transport vesicle budded from the Golgi apparatus is moved by the cytoskeleton to the plasma membrane.
When the two membranes come in contact, the bilayers fuse and spill the contents to the outside.

40. Endocytosis is a reversal of exocytosis.
During endocytosis, a cell brings in macromolecules and particulate matter by forming new vesicles from the plasma membrane.
Endocytosis is a reversal of exocytosis.
A small area of the palsma membrane sinks inward to form a pocket
As the pocket into the plasma membrane deepens, it pinches in, forming a vesicle containing the material that had been outside the cell

41. One type of endocytosis is phagocytosis, “cellular eating”.
In phagocytosis, the cell engulfs a particle by extending pseudopodia around it and packaging it in a large vacuole.
The contents of the vacuole are digested when the vacuole fuses with a lysosome.

42. In pinocytosis, “cellular drinking”, a cell creates a vesicle around a droplet of extracellular fluid.
This is a non-specific process.

43. Receptor-Mediated Endocytosis
Figure 7.22
Phagocytosis
Pinocytosis
Receptor-Mediated Endocytosis
EXTRACELLULAR FLUID
Solutes
Pseudopodium
Receptor
Plasma membrane
Ligand
Coat proteins
Coated pit
“Food” or other particle
Coated vesicle
Figure 7.22 Exploring: Endocytosis in Animal Cells
Vesicle
Food vacuole
CYTOPLASM 43

44. Receptor-Mediated Endocytosis
Figure 7.22c
Receptor-Mediated Endocytosis
Receptor
Plasma membrane
Coat proteins
Ligand
Coat proteins
Coated pit
0.25 m
Figure 7.22 Exploring: Endocytosis in Animal Cells
Coated vesicle
Top: A coated pit. Bottom: A coated vesicle forming during receptor-mediated endocytosis (TEMs). 44