Structure & Function of Cell Membrane

Question 1

Chemical composition of ECF + ICF

Answer 1

Difference comes about due to the presence of the cell membrane (aka plasma membrane) which separates ECF and ICF.

Question 2

Movement of materials

Answer 2

• Water can pass freely between all compartments. It passes from the plasma to the interstitial fluid compartments and it passes from interstitial fluid to the intracellular fluid.
• Through aquaporin channels in the cell membrane.
• The cell membrane is studded with aquaporin channels – channels that are permanently open to water. This allows water to move, unhindered, between the ECF and ICF.
• There are no barriers to water movement
• Gases also pass freely between all compartments
• Nutrients and waste also pass easily, but sometimes need help crossing the cell membrane
• Ions pass freely across the capillary wall so exchange readily between plasma and ISF. They do not penetrate the cell membrane.

Question 3

Structure of Membrane (Diagram)

Answer 3

Question 4

Structure of Membrane

Answer 4

1. Cell membrane is a selective barrier. It is freely permeable to some substances e.g. O2 and CO2, but the difference in composition between ECF and ICF shows that permeability is selective and not universal.
2. Permeability can vary, may ↑ or ↓ at different times, fundamentally important for various cell functions e.g. transmission of the nervous impulse.
3. Membranes are dynamic, continually being formed and maintained or dismantled and metabolized according to the needs of the cell.
4. Membranes are thin double layers of sheets of lipids = the lipid bilayer.

• Major membrane lipids are phospholipids which have a hydrophilic (water loving) phosphate head and a hydrophobic (water repellent) fatty acid tail.
• In an aqueous environment (i.e. ECF ad ICF!) they arrange themselves so that the polar (hydrophilic) part is on the outside of the membrane and the fatty acid chains are on the inside, away from H2O.
• Hydrophilic = lipophobic (lipid repellent)
• Hydrophobic = lipophilic (lipid loving)

5. Membranes are very flexible, because of fatty acids in vivo, behave like an oil, so can stretch. Although may rupture if overstretched (e.g. in hypotonic environment.
6. Membranes are excellent insulators against movement of electrical charge. This prevents the passage of electrons – important in maintaining electrical stability of cell
7. Membranes are embedded with proteins & associated with carbohydrates

• Integral proteins : go from intracellular side to extracellular side
• Peripheral proteins : associated with only one side of the membrane
• Membrane carbohydrates: carbohydrate molecules may attach to the phospholipids, or the proteins embedded in the lipid bilayer – these are then known as glycolipids & glycoproteins.
• ALL are extracellular.
• Important roles in cell to cell communication including self vs non-self recognition by the immune system.

Question 5

Functions of cell membrane

Answer 5

roles the proteins play (integral vs peripheral proteins)

integral proteins have an intracellular (may function as an enzyme or as a site that becomes phosphorylated) & extracellular domain (might bond to carbohydrates to form glycoproteins. Often acts as receptor)

Question 6

1. Receptors

Answer 6

1. integral to the membrane structure
2. penetrate the membrane from ECF to ICF
3. allow communication of an extracellular signal
4. Some receptors (“intracellular receptors”) are present inside cells, not on the membrane
   - example : neurotransmitter or hormone, to the intracellular space to create a cellular response.
   - extracellular signal or ligand binds to the receptor protein
   - causes confirmation of change in the protein
   - this signal is transmitted to intracellular side
   - can change cellular events or behaviour

• when insulin binds to it’s receptor it generates a signal that tells the cell to insert glucose transporters in the cell membrane.
• Only tissues that express receptors for that particular hormone or neurotransmitter can respond to the chemical. e.g. Gonads express receptors for Follicle Stimulating Hormone – FSH has little action elsewhere

Question 7

2. Transport

Answer 7

• integral proteins which allow movement of ions or molecules across the membrane
• come in 2 forms : Channels & Carrier mediated Transporter proteins
• Channel proteins : create a pore through the membrane through which molecules (usually water and ions) flow. Can be open (water) or gated (ions).
• Transporters : do not create a continuous pore from ECF to ICF. Open to ECF, then ICF, but never at the same time. Typically move larger molecules than channels e.g. glucose.
• Glucose transporters permanently present in cell membranes are insulin independent e.g. nerve cells. Insulin only required for glucose uptake in muscle and adipose tissue.

Question 8

3. Enzymes

Answer 8

• membrane enzymes catalyse chemical reactions on the cell membrane
• enzymatic part may be on external side of membrane
• example : those found in small intestine which break down nutrients into smaller units
• example : or internal side such as those associated with converting signals carried from receptors into an intracellular response (below)

Question 9

4. Structural Proteins

Answer 9

• anchor the cell membrane :

— to the intracellular skeleton

— to the extracellular matrix (collagen)

— to other cells

• Dysfunction or loss can cause serious debility
• example : lack of dystrophin protein in Duchenne’s Muscular Dystrophy
  
  • The loss of that structural protein ends up causing severe muscle weakness in these patients.

Question 10

5. Communication

Answer 10

• this can be between cells or within cells.
• Glycoproteins act as markers that tell the immune system whether a cell is one of our own or a foreign cell.
• Other types of proteins are involved with communicating extracellular messages carried by hormones (or drugs) to the intracellular environment
• they convert extracellular chemical signals into intracellular responses.

— receptor/enzymes complexes

— G proteins - peripheral, act to relay a signal between receptor and effector

Question 11

Members differ in their protein content

Answer 11

1. Myelin :
   - a specialised type of membrane that serves as an insulator around myelinated nerve fibres
   - has a low content of protein (18%)
   - major component is lipid so a very good insulator i.e. ideal for function.
2. Plasma membranes :
   - have much greater activity and protein content
   - typically 50%.
3. Energy transduction membranes :
   - such as the inner membranes of mitochondria
   - have highest protein content, 75%