What is a membrane?/ Membrane transport

1. Selectively permeable- controls the entry and exit of nutrients and waste/ secretory products.
2. Maintains osmotic ion concentration
3. Joins cells to form tissues and organs
4. Enables cell response to stimuli

It primarily consists of lipids and proteins.

1. Head - negatively charged, polar, hydrophilic
2. Tails - uncharged, nonpolar, hydrophobic
3. Form a bilayer in an aqueous solution
   • Head groups oriented towards the water
   • Tails oriented away from the water

The phospholipids can move laterally, rotate or flex and vibrate around their own half of the membrane.

1. The lipid bilayer is ideally suited to separate two aqueous compartments.
2. Pure phospholipid bilayer membranes are extremely impermeable to almost any water-soluble substance-E.g. ions, proteins, and sugars are insoluble in the hydrophobic membrane core.
3. In contrast, small uncharged polar molecules can cross fairly freely- E.g. O2, CO2, NH3 and water itself

1. Fluid - Phospholipids constantly moving, temperature dependent

• Cholesterol provides some stability

2. Impermeable to charged molecules- Water soluble substances can’t diffuse through the membrane e.g. ions, proteins

• Small, uncharged polar molecules can diffuse freely e.g. oxygen, CO2
• Membrane is selectively permeable

1. Hydrophobic interior acts as a barrier- Cell can maintain differences in solute composition/concentrations inside and outside the cell
2. Responsible for fluidity of membrane- Enables cells to change shape e.g. RBC

1. Peripherally associated proteins: adhere to the cytoplasmic or extracellular surfaces of plasma membrane (not embedded)
2. Integral membrane proteins (a) Transmembrane proteins: span lipid bilayer (once or several times). (b) Some are embedded but don’t cross bilayer (c) Some are linked to a lipid component that intercalates the membrane.

Adhere to the cytoplasmic or extracellular surfaces of plasma membrane (not embedded)

(a) Transmembrane proteins: span lipid bilayer (once or several times) (b) Embedded but don’t cross bilayer (c) Linked to a lipid component that intercalates the membrane

1. Ligand binding receptors e.g. hormone receptors
2. Adhesion molecules - form physical contacts with the extracellular matrix or with cellular neighbours e.g. integrins, CAMs
3. Transmembrane movement- through pores/ channels (water or specific ions), carriers (facilitated transport), pumps (active transport)
4. Can be enzymes
5. Intracellular signaling - associated with cytoplasmic surface (GTP binding proteins, kinases)
6. Docking-marker acceptors - exocytosis through secretory vesicles
   • Located on inner surface membrane

1. A small amount of membrane carbohydrate is located on the outer surface of cells.
2. Short carbohydrate chains are often bound to membrane proteins and to a lesser extent lipids.
3. Glycoproteins and glycolipids on the outer layer of the plasma membrane form the glycocalyx.

Together form a layer called the glycocalyx

1. Self-identity markers- important for cell to cell interaction
2. Tissue growth - cells do not overgrow their own territory. Cancer cells have abnormal markers

1. Tight junctions
2. Desmosomes
3. Gap junctions

Join lateral edges of epithelial cells near to their luminal (apical) membranes, can be tight or leaky.

Adhering junctions that anchor cells together, especially in tissues subject to stretching (e.g. skin, heart)

Gap junctions/ communicating junctions allow the movement of charge carrying ions and small molecules between two adjacent cells.

Two properties that influence whether a particle can permeate the plasma membrane without assistance:

1. Solubility of the particle in lipid
2. Size of the particle

Driving forces:-

1. Passive
2. Active- requires the cell to expend energy (ATP) to produce movement

Molecules and ions that can penetrate the membrane are passively driven across the membrane by two forces:

1. Diffusion down a concentration gradient
2. Movement along an electrical gradient

1. If a substance can permeate a membrane, the substance will move from higher concentration to lower concentration through the membrane.
2. If the membrane is impermeable to the substance, no diffusion will occur.

Factors which influence the rate of net diffusion:- (a) ↑ concentration gradient (𝝙C) = ↑ rate of diffusion (Q). (b) ↑ surface area of membrane (A) = ↑ rate of diffusion. (c) ↑ lipid solubility = ↑ rate of diffusion. (d) ↑ molecular weight of substance = ↓ rate of diffusion. (e) ↑ distance (thickness) = ↓ rate of diffusion.

Q ∝ 𝝙C · A · P, when Q= rate of diffusion, C= concentration gradient, A= surface area of membrane, P= permeability.

Difference in charge between two adjacent areas, promotes the movement of ions to the area of opposite charge

Electrical and concentration (chemical) gradient acting on an ion simultaneously

The net diffusion of water down its own concentration gradient through a selectively permeable membrane.

The concentration of osmotically active particles in a solution (Osm/l). Osmolarity of body fluids- 300 mOsm/l

Osmoles (Osm) of solute per litre - (Osm/l or osmol/l)

The effect a solution has on cell volume- solute and water concentration in comparison to body fluids.

Same solute concentration and water concentration in comparison to body fluids. No net movement of water, no change in cell volume

Lower solute concentration and higher water concentration in comparison to body fluids. Water diffuses into cells, cells swell.

Higher concentration of solute and lower concentration of water in the solution. Water diffuses out of cells, cells shrink.

Substance binds onto a carrier which undergoes a conformational change, which transports the substance.

1. Specificity
2. Saturation - transport maximum, Tm
3. Competition - if a carrier can transport two substances, the presence of both diminishes the rate of transfer for either.

Uses a carrier to facilitate the transfer of a substance across the membrane from high to low concentration.

Requires the carrier to expend energy to transfer a substance against a concentration gradient.

Energy (ATP) is directly required to move a substance against its concentration gradient - e.g. Na+-K+ ATPase (Na+/K+ pump) - helps establish concentration gradient across the plasma membrane, helps regulate cell volume and provides energy for secondary active transport (creates the ion concentration gradient - 3 Na+ out for every 2 K+ in)

Carrier moves molecule against concentration gradient by using secondhand energy stored as an ion concentration gradient (usually Na+) (a) Symport: solute and Na+ move in the same direction (b) Antiport: solute and Na+ move in opposite directions (Na+ in, solute out)

Requires energy for vesicle formation and movement within the cell (active). (a)Endocytosis- ‘Pinching off’ materials from the surface of the membrane to engulf substance. (b) Exocytosis- A vesicle fuses with the plasma membrane, releasing its contents to the ECF. Secretion of enzymes, protein hormones Way of adding carriers, channels or receptors to the plasma membrane (E.g. GLUT4 & insulin)