5 Biological Membranes

Lecture Outline

I. Biological membranes are lipid bilayers with associated proteins

A. Phospholipids form bilayers in water

1. Phospholipids have two fatty acid chains linked to a glycerol molecule

2. The hydrophilic portion of the molecule is the phosphate bonded to the glycerol

3. The fatty acids make up the hydrophobic portion of the molecule

4. The molecule is amphipathic

a) The bilayer forms spontaneously because of the amphipathic characteristic

b) Many common detergents are also amphipathic, and so are able to A solubilize@ oil

B. Current data support a fluid mosaic model of membrane structure

1. Davson and Danielli (1935)C phospholipids form a membrane two molecules thick: the lipid bilayer

a) Singer and Nicholson (1972)C the fluid mosaic model

b) This membrane is only 10 nanometers thick

c) Various types of proteins move around like icebergs in the lipid sea

C. Biological membranes are two-dimensional fluids

1. Phospholipids act as liquid crystals, and the molecules move laterally

a) Fluidity is affected by temperature, among other factors

2. Molecules rarely move from one side of the membrane to the other

3. Frye and Ediden (1970) demonstrated movement of proteins in the membrane

4. Various bonds may occur between hydrocarbon chains

a) Saturated fats lack double bonds in the fatty acid chains, causing the membrane to be less fluid

b) Organisms may alter the proportion of saturation in response to temperature

5. In animal cells, cholesterol, which is slightly amphipathic, stabilizes the membrane at higher temperatures

6. In plant cells, other steroids function in a similar manner

D. Biological membranes fuse and form closed vesicles

1. Membrane fusion is due to the liquid crystalline state

2. When vesicles and another membranes fuse, their bilayers and lumens become continuous

3. Endocytosis and exocytosis are products of membrane fusion.

E. Membrane proteins include integral and peripheral proteins

1. Integral proteins are firmly bound to the membrane, or are bound to other lipids that are part of the membrane

2. Transmembrane proteins span the entire bilayer

a) Some transmembrane proteins span the membrane only once, but many weave back and forth a number of times

b) Transmembrane proteins are amphipathic

c) The most common type is an alpha helix

- 1 -

3. Peripheral proteins are bound to the hydrophilic ends of the integral proteins

F. Proteins are oriented asymmetrically across the bilayer

1. Evidence comes from freeze-fracture electron microscopy

2. Proteins are produced in the ER, then pass to the Golgi, then pass via a vesicle to the plasma membrane

3. Various chemical modifications occur along the way

G. Membrane proteins function in transport, information transfer, and as enzymes

1. Membrane proteins are formed by free ribosomes

2. Proteins are involved in transport of small molecules

3. Enzymes may be embedded in the membrane

4. Receptor proteins are embedded in the exterior surface of the membrane

a) Signal molecules convert an extracellular signal into an intracellular signal via signal transduction

5. Membrane proteins can serve as identification tags functioning in cell-to-cell recognition; others form junctions between adjacent cells

Passage of materials through cell membranes: Cell membranes are selectively permeable

H. Most biological membranes are permeable to small or lipid-soluble molecules

1. Water molecules may pass the lipid bilayer

2. Gases, small polar molecules, and large hydrophobic substances may also pass

3. Other molecules move through special channels, primarily through membrane transport proteins.

Passive transport does not require the cell to expend energy.

I. Diffusion: Random motion of particles leads to diffusion

1. Atoms and molecules above absolute zero exhibit motion

2. Due to random motion, particles move from an area of higher concentration to one of lower concentration, ultimately reaching equilibrium

3. The rate of diffusion depends on temperature, the size of the molecules, electrical charges, and the concentration gradient

4. Osmosis is the diffusion of water (solvent) across a selectively permeable membrane

a) The osmotic pressure is the tendency of water to move into that solution

b) Two solutions may be isotonic to each other, or one may be relatively hypertonic and the other relatively hypotonic

(1) Human cells are isotonic, with a 0.9% sodium chloride solution

(2) Solutions that are hypertonic have a higher osmotic pressure than the cells; solutions that are hypotonic have a lower osmotic pressure than the cells

(a) Animal cells placed in a hypertonic solution tend to shrivel and die

(b) Plant cells, and others with cell walls, placed in a hypertonic solution tend to plasmolyze

(c) Animal cells placed in a hypotonic solution tend to swell and burst

(d) Plant cells placed in a hypotonic solution tend to become turgid

 

- 2 -

(3) Turgor pressure is the internal hydrostatic pressure usually present in walled cells

(a) Turgor pressure provides structural support in nonwoody plants

J. Carrier-mediated transport of solutes requires special integral membrane proteins

1. Aquiporins are integral membrane proteins that function as gated water channels

2. Impermeability of the cell membrane is advantageous so cells do not lose valuable polar molecules

3. Two forms of carrier-mediated transport are facilitated diffusion and carrier-mediated active transport

K. Facilitated diffusion occurs down a concentration gradient

1. A membrane may become permeable by a protein that combines with the material to be transported

2. Glucose transport across erythrocyte membranes is an example

3. Liposomes are artificial vesicles that have been used to study facilitated diffusion

II. Active Transport:

A. Some carrier-mediated active transport systems A pump@ substances against their concentration gradients

1. Materials are transported from a region of low concentration to high by carrier mediated active transport mechanisms

2. ATP is required

3. The sodium-potassium pump is an example

4. Other mechanisms involve the pumping of ions in photosynthesis and aerobic respiration, which results in a membrane potential and electrochemical gradient

B. Linked cotransport systems indirectly provide energy for active transport

1. These transport systems cotransport molecules against their concentration gradient

2. ATP is a requirement for linked cotransport systems

3. Example: glucose transport across microvilli of the intestine

4. More than one transport system may transport one substance

C. Facilitated diffusion is powered by a concentration gradient; active transport requires another energy source. The energy of ATP is used directly or indirectly in transport of materials

D. The patch-clamp technique has revolutionized the study of ion channels

1. This was first developed in the mid 1970s at the Max Planck Institute in Germany

2. With this technique, scientists can study single ion channels of even very small cells over time

III. In exocytosis and endocytosis, large particles are transported by vesicles or vacuoles

1. In exocytosis, the cell expels wastes or a secretory product

a) Exocytosis also results in growth of the cell membrane

2. In endocytosis, a cell takes up materials

a) Phagocytosis involves taking in solid materials

b) Pinocytosis involves taking in liquid droplets

3. Receptor-mediated endocytosis involves the bonding of receptor proteins to the material to be engulfed

- 3 -

a) Molecules bind to receptors, called ligands, concentrated in coated pits

(1) Coated pits are formed by a protein, clathrin

b) The coated pits move inwards, via endocytosis, and are then termed coated vesicles

c) The coated vesicle ultimately becomes an endosome when the coating is lost

d) Cholesterol is taken into cells by this pathway

(1) Brown and GoldsteinC 1986 Nobel Prize

IV. Cell signaling is the mechanism by which cells communicate with one another

A. An example of a molecule that is a cell signal is cyclic adenosine monophosphate (cAMP). This is clearly illustrated when studying slime molds

B. Cell signaling involves six steps

1. Synthesis and release of the signaling molecules

a. Signaling molecules may be neurotransmitters, hormones, or other types of molecules

b. Signaling molecules are ligands (often called the first messengers), which specifically bind to receptors on the target cells

c. Some ligand-receptor complexes bind to and activate specific integral membrane proteins: the G proteins

2. Transport to target cells

3. Reception of the information by target cells

a. This is typically composed of a transmembrane protein with a functional portion on the extracellular surface

b. Typically there are three domains: an external domain for docking by the signaling molecule, a second domain extending through the plasma membrane, and a third domain resembling a A tail@ that extends into the cytoplasm

4. Signal transduction

a. In this process, cells convert and amplify an extracellular signal into an intracellular one

5. Response by the cell

a. The tail undergoes a conformational change, which then activates a sequence of proteins that initiate changes within the cytoplasm

b. The G proteins may pass the message to the second messenger, often cyclic AMP

c. The second messengers typically activate protein kinases

6. Termination of signaling

V. Cell Junctions are specialized contacts between cells

a. Anchoring junctions connect epithelial cells

i. A common form of anchoring junction, desmosomes, are points of attachment between some animal cells

1. Desmosomes hold cells subject to mechanical stresses together

2. Desmosomes are composed of intermediate filaments which span the gap between two cells

b. Adhering junctions cement cells together with proteins

- 4 -

i. Tight junctions seal off intercellular spaces between some animal cells

ii. Tight junctions seal cells tightly with protein links

iii. Tight junctions are important in epithelia

c. Gap junctions permit transfer of small molecules and ions

i. Gap junctions contain pores that connect cells

ii. Proteins form the pores

iii. Gap junctions allow rapid chemical and electrical communication between cells

iv. The apertures of gap junctions may be controlled

d. Plasmodesmata allow movement of certain molecules and ions between plant cells

i. Plasmodesmata are connections between plant cells

ii. Plasma membranes are continuous through the plasmodesmata

iii. A desmotubule may connect the ER of adjacent cells

iv. Typically, molecules, but not organelles, pass through plasmodesmata