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.

<ol> <li>The lipid bilayer is ideally suited to separate two aqueous compartments.</li> <li>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.</li> <li>In contrast, small uncharged polar molecules can cross fairly freely- E.g. O2, CO2, NH3 and water itself</li> </ol> Signup and view all the answers

<ol> <li>Fluid - Phospholipids constantly moving, temperature dependent</li> </ol> <ul> <li>Cholesterol provides some stability</li> </ul> <ol start="2"> <li>Impermeable to charged molecules- Water soluble substances can’t diffuse through the membrane e.g. ions, proteins</li> </ol> <ul> <li>Small, uncharged polar molecules can diffuse freely e.g. oxygen, CO2</li> <li>Membrane is selectively permeable</li> </ul> Signup and view all the answers

<ol> <li>Hydrophobic interior acts as a barrier- Cell can maintain differences in solute composition/concentrations inside and outside the cell</li> <li>Responsible for fluidity of membrane- Enables cells to change shape e.g. RBC</li> </ol> Signup and view all the answers

<ol> <li>Peripherally associated proteins: adhere to the cytoplasmic or extracellular surfaces of plasma membrane (not embedded)</li> <li>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.</li> </ol> Signup and view all the answers

<p>Adhere to the cytoplasmic or extracellular surfaces of plasma membrane (not embedded)</p> Signup and view all the answers

<p>(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</p> Signup and view all the answers

<ol> <li>Ligand binding receptors e.g. hormone receptors</li> <li>Adhesion molecules - form physical contacts with the extracellular matrix or with cellular neighbours e.g. integrins, CAMs</li> <li>Transmembrane movement- through pores/ channels (water or specific ions), carriers (facilitated transport), pumps (active transport)</li> <li>Can be enzymes</li> <li>Intracellular signaling - associated with cytoplasmic surface (GTP binding proteins, kinases)</li> <li>Docking-marker acceptors - exocytosis through secretory vesicles <ul> <li>Located on inner surface membrane</li> </ul> </li> </ol> Signup and view all the answers

<ol> <li>A small amount of membrane carbohydrate is located on the outer surface of cells.</li> <li>Short carbohydrate chains are often bound to membrane proteins and to a lesser extent lipids.</li> <li>Glycoproteins and glycolipids on the outer layer of the plasma membrane form the glycocalyx.</li> </ol> <p>Together form a layer called the glycocalyx</p> Signup and view all the answers

<ol> <li>Self-identity markers- important for cell to cell interaction</li> <li>Tissue growth - cells do not overgrow their own territory. Cancer cells have abnormal markers</li> </ol> Signup and view all the answers

<ol> <li>Tight junctions</li> <li>Desmosomes</li> <li>Gap junctions</li> </ol> Signup and view all the answers

<p>Join lateral edges of epithelial cells near to their luminal (apical) membranes, can be tight or leaky.</p> Signup and view all the answers

<p>Adhering junctions that anchor cells together, especially in tissues subject to stretching (e.g. skin, heart)</p> Signup and view all the answers

<p>Gap junctions/ communicating junctions allow the movement of charge carrying ions and small molecules between two adjacent cells.</p> Signup and view all the answers

<p>Two properties that influence whether a particle can permeate the plasma membrane without assistance:</p> <ol> <li>Solubility of the particle in lipid</li> <li>Size of the particle</li> </ol> Signup and view all the answers

<p>Driving forces:-</p> <ol> <li>Passive</li> <li>Active- requires the cell to expend energy (ATP) to produce movement</li> </ol> Signup and view all the answers

<p>Molecules and ions that can penetrate the membrane are passively driven across the membrane by two forces:</p> <ol> <li>Diffusion down a concentration gradient</li> <li>Movement along an electrical gradient</li> </ol> Signup and view all the answers

<ol> <li>If a substance can permeate a membrane, the substance will move from higher concentration to lower concentration through the membrane.</li> <li>If the membrane is impermeable to the substance, no diffusion will occur.</li> </ol> Signup and view all the answers

<p>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.</p> Signup and view all the answers

<p>Q ∝ 𝝙C · A · P, when Q= rate of diffusion, C= concentration gradient, A= surface area of membrane, P= permeability.</p> Signup and view all the answers

<p>Difference in charge between two adjacent areas, promotes the movement of ions to the area of opposite charge</p> Signup and view all the answers

<p>Electrical and concentration (chemical) gradient acting on an ion simultaneously</p> Signup and view all the answers

<p>The net diffusion of water down its own concentration gradient through a selectively permeable membrane.</p> Signup and view all the answers

<p>The concentration of osmotically active particles in a solution (Osm/l). Osmolarity of body fluids- 300 mOsm/l</p> Signup and view all the answers

<p>Osmoles (Osm) of solute per litre - (Osm/l or osmol/l)</p> Signup and view all the answers

<p>The effect a solution has on cell volume- solute and water concentration in comparison to body fluids.</p> Signup and view all the answers

<p>Same solute concentration and water concentration in comparison to body fluids. No net movement of water, no change in cell volume</p> Signup and view all the answers

<p>Lower solute concentration and higher water concentration in comparison to body fluids. Water diffuses into cells, cells swell.</p> Signup and view all the answers

<p>Higher concentration of solute and lower concentration of water in the solution. Water diffuses out of cells, cells shrink.</p> Signup and view all the answers

<p>Substance binds onto a carrier which undergoes a conformational change, which transports the substance.</p> Signup and view all the answers

<ol> <li>Specificity</li> <li>Saturation - transport maximum, Tm</li> <li>Competition - if a carrier can transport two substances, the presence of both diminishes the rate of transfer for either.</li> </ol> Signup and view all the answers

<p>Uses a carrier to facilitate the transfer of a substance across the membrane from high to low concentration.</p> Signup and view all the answers

<p>Requires the carrier to expend energy to transfer a substance against a concentration gradient.</p> Signup and view all the answers

<p>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)</p> Signup and view all the answers

<p>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)</p> Signup and view all the answers

<p>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 &amp; insulin)</p> Signup and view all the answers