B2.1 Membranes and membrane transport Flashcards
B2.1.1 Role of plasma membranes
Plasma membrane forms the border between a cell and its environment. Membranes inside eukaryotic cells compartmentalize the cytoplasm. A phospholipid bilayer controls the passage of substances.
B2.1.2 Lipid bilayers as barriers
Membrane core has low permeability to H-philic particles, as the tails are more attracted to each other.
Molecular size also influences membrane permeability: larger molecule = lower permeability.
B2.1.3 Simple diffusion across membranes - definition
Diffusion is the passive net movement of molecules from a region of high conc. to a region of low conc.
Simple diffusion across membranes is due to particles passing between phospholipids in the membrane.
B2.1.3 What particles can simply diffusion across membranes due to their permeability?
Non-polar particles = easily.
Polar molecules = diffuse at low rates.
Ions/large polar mol. =cannot easily diffuse through.
B2.1.4 Integral proteins in membranes
Penetrates the bilayer, permanently attached to the membrane. Hyrdophilic parts project out with the heads, hydrophobic parts are with the tails.
B2.1.4 Peripherial proteins in membranes
Associated (temporarily) with one side of a membrane (can be removed by polar solvents).
Either attached to integral proteins, anchored by a hydrocarbon chain, or held in place by the cytoskeleton or extracellular matrix.
B2.1.5 Movement of water molecules
across membranes by osmosis
Osmosis is the net movement of water molecules across a semi-permeable membrane from a region of low solute concentration to a region of high solute concentration (until equilibrium).
B2.1.5 The role of aquaporins
Aquaporins are water channels that increase membrane permeability to water. Its narrowest point is slightly wider than water mol. Positive charges at this point prevent protons (H+) passing through.
B2.1.6 Channel proteins definition
An integral, transmembrane protein with a pore connecting otusdie + inside. The pore’s diameter and chem. properties of its sides ensure only one type of particle passes through (e.g. Na+).
B2.1.6 Channel proteins for facilitated diffusion
Helps ions and polar molecules diffuse. Facilitated diffusion = channel proteins are required.
Particles can pass in either direction, but there is a net movement from the higher conc. to the lower.
Cells control diffusion of h-philic substances by the channels synthesized. Some can open/close, to alter permeability.
B2.1.7 Pump proteins for active transport
Pump proteins use energy so they carry out AS. They can only move particles across the membrane in one direction, (typically) against the conc. gradient.
B2.1.7 Pump proteins process
- Particle enters pump from either side to reach a central chamber/binding site.
- Pump protein changes conformation, ion/molecule is translocated to the other side.
- Pump protien returns to its original conformation.
The hydrolysis of ATP (to ADP + Pi) causes a conformational change in the protein pump
B2.1.7 Pump proteins benefits
Pump proteins transfer a specific particle. This allows the cell to control the content of its cytoplasm.
Allows specific solutes to be absorbed even when in very low concentrations in the environment.
B2.1.7 Indirect active transport
Movement of one solute down its conc. gradient drives the movement of a second solute against its.
The conc. gradient stores potential energy. When ions move down this gradient through FD energy is released. which can be captured by a transporter to move other molecules against their gradient.
B2.1.8 Selectivity in membrane permeability
PMS regulate the transport of materials entering and exiting the cell.
simple diffusion = not selective
channel + pump proteins = selective
B2.1.9 Structure of glycoproteins and glycolipids
A protein w/ a carbohydrate, making it conjugated. The protein is embedded and the carbohydrate projects out to the exterior environment.
Glycolipids are carbohydrates linked to lipids (same).
B2.1.9 Function of glycoproteins and glycolipids
By displaying a distincive GP, cells allow other cells to recognise them via the receptor on the surface of another cell. Helps organise tissues.
(GL?) Helps immune system to distinguish between self/non-self cells, they are recognized + destroyed.
B2.1.9 Function of glycoproteins and glycolipids structurally
GP and GL form a carbohydrate-rich layer, with an aqueous solution in the gaps. This layer = glycocalyx.
The glycocalyx of adjacent cells can fuse, binding cells together and preventing the tissue from falling apart.
This maintains the structural integrity of the extracellular matrix.
B2.1.10 Fluid mosaic model of membrane structure
Fluid – the phospholipid bilayer is viscous and individual phospholipids can move position
Mosaic – the phospholipid bilayer is embedded with proteins, resulting in a mosaic of components
B2.1.11 Relationships between fatty acid composition of lipid bilayers and their fluidity (HL) - structure
Longer phospholipid tails means more interactions between the tails, resulting in a less fluid membrane.
Saturated fatty acid tails can press closely together = dense and fairly viscous membrane.
Unsaturated fatty acid tails have “kinks” preventing close packing = increases the membrane fluidity.
B2.1.11 Relationships between fatty acid composition of lipid bilayers and their fluidity (HL) - temperature
Closer packing in saturated means stronger intermolecular forces = higher melting points (remain more solid at inc. temps).
Unsaturated kinks means intermolecular forces are weaker = lower melting points (stay more fluid at decreasing temperatures).
B2.1.11 Relationships between fatty acid composition of lipid bilayers and their fluidity (HL) - significance of fluidity
Enables molecules to diffuse through the membrane.
Facilitates the interaction between proteins, crucial for cell signaling.
Enables membranes to fuse with one another during vesicle formation, endocytosis and exocytosis.
B2.1.12 Cholesterol and membrane fluidity in animal cells (HL) -placement
An amphipathic steriod molecule that forms hydrogen bonds with the phospholipid head and is positioned between the (sat.) hydrocarbon chains.
B2.1.12 Cholesterol and membrane fluidity in animal cells (HL) - role as a modulator
Higher temps: stabilizes membranes by restraining fatty acid chains movement, reduces permeability to small hydrophilic particles.
Low temps: prevents solidifying/stifling by preventing tight packing of the chains, maintaining fluidity.
B2.1.13 Endocytosis (HL)
- The plasma membrane folds inward forming a cavity that fills with extracellular fluid, dissolved molecules, food particles, foreign matter etc.
- The vesicle is pinched off as the ends of the in-folded membrane fuse together.
- The vesicle breaks away and moves into the cytoplasm. The cell membrane has gotten smaller.
B2.1.13 Endocytosis example (HL)
Placenta: proteins from the mother’s blood (e.g. antibodies), are absorbed into the foetus.
Unicellular (e.g. Amoeba): take in large undigested food particles by endo.
B2.1.13 Exocytosis (HL)
- Vesicles containing molecules are transported from within the cell to the cell membrane.
- The vesicle membrane attaches to the cell membrane.
- Fusion of the membranes releases the vesicle contents into into the extracellular space. Cell membrane has grown larger.
B2.1.13 Exocytosis example (HL)
Waste/unwanted products. E.g. unicellular organisms removing excess water in a contractile vacuole.
Polypeptides processed in the Golgi are secreted. E.g. Digestive enzymes (pepsin)
Proteins bound to vesicle membrane which become part of cell membrane e.g. channels, pumps etc.
B2.1.14 How does a stimulus trigger an action potential in a voltage-gated sodium channel (HL)?
A stimulus causes the cell’s membrane potential to change, making inside of the axon less negative. If this reaches the threshold potential, the channels open, Na⁺ moves down conc. gradient. Cell inside becomes more positive. (depolarization)
B2.1.14 What happens to voltage-gated potassium channels during an action potential (HL)?
When the membrane potential reaches +40 mV, voltage-gated K⁺ channels open, allowing K⁺ to leave the cell. This repolarizes the membrane, bringing it back toward its resting state.
B2.1.14 What is the role of the ball protein in voltage-gated potassium channels (HL)?
After the K⁺ channel opens, a ball-shaped protein enters the open pore, partially blocking it. This slows K⁺ from leaving. As the membrane potential returns to its resting state, the K⁺ channel fully closes, and the ball protein is released.
B2.1.14 Ligand-gated channels - Nicotinic acetylcholine receptors (HL)
In muscles, acetylcholine (neurot.) binds to NAR, which has 5 transmembrane subunits with a binding site between 2.
This opens a pore, Na+ diffuses, changing the postsynaptic neuron’s voltage and activating v-g sodium channels, leading to muscle contraction.
B2.1.15 Sodium–potassium pumps as an example of exchange transporters (HL)
- 3 Na+ bind inside the axon.
- ATP phosphorylates the pump -> conformational change, pump opens outside of the axon.
- Na+ is released; 2 K+ from outside enter & bind.
- Phosphate is relseased, restoring the pumps shape, and K+ is released inside.
- The pump is always moving ions against their conc. gradient via active transport
B2.1.15 Sodium–potassium pumps significance (HL)
The pump moves more Na+ out than K+ in, creating an electrochemical gradient.
B2.1.16 Sodium-dependent glucose cotransporters as an example of indirect active transport (HL)
Transfers Na+ and glucose into a cell. Glucose moves against its gradient as Na+ moves down its own. Na+’s energy from movement powers glucose transport.
This relies on a higher Na+ concentration outside, maintained by sodium-potassium pumps actively transporting Na+ out toward capillaries.
B2.1.16 Sodium-dependent glucose cotransporters in the nephron (HL)
- Helps reclaim glucose from the urine.
As the kidneys filter blood, glucose enters the urine but is mostly reabsorbed into the blood by transporters, retaining the energy source.
B2.1.17 Cell-adhesion molecules (HL)
Cells in a tissue form cell junctions which utilise CAM’s, proteins embedded within the plasma membrane They provide for:
- Adhesion between neighboring cells
- Anchoring of a cell to the extracellular matrix
