cell membranes B Flashcards
describe the fluid-mosaic model of membrane structure
● Molecules free to move laterally in phospholipid bilayer
● Many components - phospholipids, proteins, glycoproteins and glycolipids
The structure of a cell membrane
-Phospholipid bilayer=fatty acid tails face inwards, phosphate heads face outwards
- Embedded proteins= Channel and carrier proteins
- Glycolipids (lipids and attached polysaccharide chain) and glycoproteins (proteins with polysaccharide chain attached)
- Cholesterol which binds to phospholipid hydrophobic fatty acid tails
what are the membrane components and how do they affect movement of substances across cell membranes
*PHOSPHOLIPID BILAYER
-Allows diffusion of non-polar small/lipid-soluble molecules e.g. oxygen or water, down a concentration gradient
- Restricts the movement of larger/polar molecules
*CHANNEL PROTEIN
-allow diffusion of water soluble substances (facilitated diffusion)
*CARRIER PROTEINS
-allow diffusion of slightly larger substances (facilitated diffusion)
-allow active transport of substances against a concentration gradient
*CHOLESTEROL
-restricts movement of other molecules making up membrane -> decreases fluidity and permeability
+Phospholipid bilayer is fluid - membrane can bend for vesicle formation / phagocytosis
+Glycoproteins / glycolipids act as receptors / antigens -involved in cell signalling / recognition
+Cholesterol - Regulates fluidity / increases stability
Adaptations of cells for transport across their internal or external membranes
+membrane folded e.g. microvilli in ileum/inner membrane of mitochondria - increases in surface area
+ more protein channels/carriers- for facilitated diffusion (or active transport-carrier proteins only)
what is simple diffusion
- Net movement of small, non-polar molecules e.g. oxygen or carbon dioxide, across a selectively permeable membrane, down a concentration gradient
- Passive / no ATP / energy required
factors affecting the rate of simple diffusion
Factors affecting rate – surface area, concentration gradient, thickness of surface / diffusion distance
what is facilitated diffusion
- Net movement of larger/polar (water soluble) molecules e.g. glucose, across a selectively permeable membrane, down a concentration gradient
- Through a channel/carrier protein
- Passive /no ATP/energy required
factors affecting the rate of facilitated diffusion
Factors affecting rate – surface area, concentration gradients (until the number of proteins is the limiting factor as all are in use / saturated), number of channel/carrier proteins
what is the role of carrier and channel proteins in facilitated diffusion
● Channel proteins facilitate diffusion of water-soluble substances
○ Hydrophilic pore filled with water
○ May be gated - can open / close
● Carrier proteins facilitate diffusion of (slightly larger) substances
○ Complementary substance attaches to binding site
○ Protein changes shape to transport substance
what is active transport
- Net movement of molecules/ions against a concentration gradient
- Using carrier proteins
- Using energy from the hydrolysis of ATP to change the shape of the tertiary structure and push the substances through
what are the steps of active transport
1.complementary substance binds to a specific carrier protein(on side of lower conc.)
2.ATP binds and is hydrolysed into ADP+Pi,releasing energy
3.this causes carrier proteins to change shape -> release substance on side of higher conc.
4. Pi released -> protein returns to original shape
factors affecting the rate of active transport
Factors affecting rate – pH/temp (tertiary structure of carrier protein), speed of carrier protein, number of carrier proteins, rate of respiration (ATP production)
Describe an example that illustrates co-transport (Absorption of sodium ions and glucose (or amino acids) by cells lining the mammalian ileum)
- ● Na+ actively transported from epithelial cells to blood (by Na+/K+ pump)
● Establishing a conc. gradient of Na+ (higher in lumen than epithelial cell) - ● Na+ enters epithelial cell down its concentration gradient with glucose against its concentration gradient
● Via a co-transporter protein - ● Glucose moves down a conc.
gradient into blood via
facilitated diffusion
describe what happens in exocytosis
1) Ribosomes associated with the RER synthesise a polypeptide chain in
translation.
2) The polypeptide chain is folded and processed in the RER.
3) The folded protein is transported to the Golgi apparatus in a vesicle.
4) The protein is modified in the Golgi apparatus and packaged into a Golgi vesicle.
5) The vesicle is transported to the cell surface membrane.
6) The vesicle fuses with the cell-surface membrane and the protein is secreted.
