Lecture Eight - Membranes Flashcards
What is the structure of the phospholipid by layer?
Hydrophilic (charged) head.
Hydrophobic fatty acid (uncharged) tail.
Water cannot form ionic bonds with the non-polar lipid tails.
What components can be found in a membrane?
Glycolipids: have polysaccharides attached - for signal perception and cell-cell recognition.
Cholesterol: reduces fluidity, also prevents membrane ‘freezing’ at colder temperatures.
Proteins: are an essential component of all biological membranes, they maintain membrane structure (support) and define membrane function (transport).
What are membrane proteins used for?
Transport, enzymes, signal transduction, cell-cell recognition, joining cells together and attachment to structures outside the cell.
Define intrinsic, extrinsic and membrane anchored proteins.
Intrinsic proteins are embedded in the membrane and have cytoplasmic and external components protruding.
Extrinsic or peripheral proteins associate with the surface of the membrane or with other membrane proteins.
Membrane-anchored proteins are attached by a short fatty acid tail protruding into the by layer.
How is membrane protein activity regulated?
Ligand gated/regulated membrane proteins - only active when a specific ligand (molecule) is bound to the protein.
Voltage gated/regulated membrane proteins - ative when the voltage drop across the membrane is above a threshold.
Energy dependant membrane proteins - operate when sufficient energy from ATP is available.
Physical stretching membrane proteins - open when the membrane is stretched, E.g. by cell movement or tissue stretching.
How does transport across the membrane work?
Small, non-polar molecules can cross by simple diffusion, E.g. CO2 and O2.
Large or charged (polar) molecules cannot cross unless transported by membrane proteins, I.e. are not lipid soluble.
Transport proteins - allow large or larger molecules to be transported across a membrane.
Facilitated diffusion - molecules move down a concentration gradient via transport proteins.
Active transport - Molecules are pumped across a membrane using energy (ATP) against a concentration gradient.
Diffusion - movement of solutes from region of high to low concentration. Net diffusion ceases when equilibrium is reached.
Osmosis - movement of water molecules from a higher to lower concentration of water molecules, across a membrane, through the lipid by layer and aquaporins.
What are electrogenic pumps?
They generate a voltage gradient and concentration gradient across a membrane. E.g. Na+/K+ pump.
What are proton pumps?
Pumps that generate an electrochemical gradient (in addition to the concentration gradient).
The proton pump:
The electrogenic pump of plants, fungi and bacteria.
Pumps protons (H+) out of the cell.
The electrochemical gradient is a new store of potential energy.
What is cotransport?
Transporting two or more molecules together.
Symports - in the same direction.
Antiports - in opposite directions.
Uniport - one molecule transported one way.
What does the sodium potassium pump do?
It is active.
Moves excess sodium ions out of the cell and bring in potassium ions.
What is exocytosis and endocytosis?
Bulk transport of whole sections of the membrane itself.
Section of the membrane are bent by proteins.
Exocytosis - Fusion of vesicles with the plasma membrane to release their contents outside the cell. Secretes proteins (extracellular matrix proteins, neurotransmitters, hormones, signals to neighbouring cells).
Endocytosis - Brings things into the cell. Proteins in the membrane interact with protons in the extracellular space - this changes the membrane.
