L2 Flashcards
What does the plasma membrane do?
Coordinates interactions with the surrounding environment.
What are the five key functions of membranes?
Define boundaries and act as permeability barriers.
Sites of specific biochemical functions
Regulate transport of solutes
Detect and transmit electrical and chemicals signals
Mediate cell to cell communication and adhesion.
Plasma membranes - how do they act as boundaries and permeability barriers?
Control which substances are inside and outside of the cell. Compartmentalises specific functions.
Plasma membranes - site of what specific biochemical functions?
Hosting of essential cellular functions which include - ETC, photosynthesis, translation of specific proteins. Functional proteins are embedded in the membranes. Localise activity. Exploit gradients to drive processes.
Plasma membranes - how do they regulate transport of solutes?
Passive transport - does not require energy - simple diffusion, facilitated diffusion, filtration, osmosis. Active transport - requires energy. Endocytosis/exocytosis.
Plasma membranes - how do they detect and transmit electrical and chemical signals?
Signal transduction is where an extracellular signal is transmitted to the interior of the cell. Cell signalling is divided into three steps. Receptor binds a molecule, receptor binding generates secondary messengers, the secondary messengers alter gene expression and cell function.
Plasma membranes - how do they mediate cell to cell communication and adhesion?
Join cells together of similar function. Cells connected and components exchanged via : gap junctions (animals), plasmodesmata (plants), septa (fungi).
Why do cellular requirements impose a limit on size?
Cell size is often limited to maintain a favourable SA:V ratio. As cell volume increases, SA increases to maintain a sufficient nutrient flux. A cell that contains multiple small structures greatly increases the surface area. This allows for more sufficient solute exchange, allows biochemical processes to occur more efficiently, many cells have a modified surface area.
What types of membrane proteins are there?
Integral and peripheral
What are integral proteins?
Include transmembrane proteins that span the membrane. Membrane spanning regions consist of hydrophobic amino acids. Usually arranged into alpha helices. Cytoplasmic or extracellular parts are hydrophobic
What are peripheral proteins?
Contact the membrane temporarily, easily removed allows them to be involved in cell signalling.
What are the key structural features of membranes?
Very thin 8nm, a physical barrier - keeping contents in and external materials out, barrier must be insoluble in water. But cannot be completely impermeable as they must allow certain molecules and water to permeate through
What are the main components of membranes?
Phospholipids, sphingolipids, glycolipids, membrane proteins, sterols.
What can phospholipids be?
Amphipathic (hydrophilic and hydrophobic, self assemble in water into bilayers. Placed in water form into micelles.
What is the composition of phospholipids?
Glycerol molecule, two fatty acids which are uncharged nonpolar tails, which are hydrophobic. Phosphate group, negatively charged polar head, which is hydrophilic. That is modified by an headgroup.
What are membrane proteins?
Proteins in the plasma membrane - they can be partially inserted into the membrane, exposed on only one surface, span the entire membrane.
What are properties of biological membranes?
Lateral movement is frequent. Increased phospholipid movement leads to increased membrane fluidity. Composition of bilayer influences the movement of phospholipids. High levels of unsaturated fatty acids tails prevent tight molecular packing. Cholesterol reduces membrane fluidity.
Where is cholesterol inserted into in the plasma membrane?
Lipophilic interior
What is the fluid mosaic model?
Describes the cell membrane as a tapestry of several types of molecules that are constantly moving.
What happens in membrane synthesis?
Phospholipids are synthesised in the smooth ER. Membrane proteins have N-terminal sequences that direct ribosomes to the ER membrane. Attachment of the signal recognition particle (SRP). Ribosomes dock on a protein complex (translocon). Polypeptide synthesis starts. Signal peptide is cleaved. Full synthesis. Folding.
What is a signal recognition particle (SRP)?
Abundant, cytosolic, universally conserved ribonucleoprotein (protein-RNA complex) that recognises and targets specific proteins to the endoplasmic reticulum in eukaryotes and the plasma membrane in prokaryotes.
What happens with membrane transport?
The plasma membrane regulates the traffic of molecules into and out of the cell. Membranes are selectively permeable. With the exception of gases and small hydrophobic molecules, most molecules cannot diffuse across the phospholipid bilayer at rates sufficient to cellular needs.
What are the three classes of transmembrane proteins that mediate transport of ions, sugars, amino acids, and other metabolites across cell membranes?
ATP-powered pumps
Protein (ion) channels
Protein transporters
What are ATP-powered pumps and what do they do?
Pumps which utilise the energy released by ATP hydrolysis to power the movement of specific ions or small molecules against chemical concentration gradient. This process is active transport.
What are ABC transporters and what do they do?
A large family of active transporters. E.g. peroxisomal ABC transporter that transports fatty acids into the peroxisomes for degradation. When this gene is mutated in humans it causes a buildup of very long chain fatty acids damaging the myelin sheath of the neurons of the brain. In plants it causes buildup of fatty acids in the lead and prevents the germination of seedlings.
What are protein channels and what do they do?
Channels permit movement of specific ions down their electrochemical gradient. This process is referred to as facilitated diffusion. Most ion channels open only in response to specific chemical or electrical signals (gated channels). Some ion channels are open much of the time (non-gated).
How do you study the function of voltage gated channels?
Electrophysiologists use a technique called patch clamping. E.g. voltage-clamp - voltage across the cell membrane is controlled by the experimenter. Voltage-clamp enables researchers to investigate the opening, closing, regulation, and ion conductance of a single ion. 1. Glass electrode, filled with a current-conducting saline solution, is applied, with a slight suction, to the plasma membrane. 2. The tip covers a region that contains only one or a few ion channels. A second electrode is inserted through the membrane into the cytosol. A recording device measures current flow only through the channels in the patch of plasma membrane.
What types of protein transporters are there?
Uniporters, symporters, antiporters.
What do uniporters do?
Transport a single type of molecule down its concentration gradient. Binding of glucose to the outward facing site, triggers a conformational change in the transporter that results in the binding site facing inward toward the cytosol. Glucose then is released to the inside of the cell. Finally, the transporter undergoes the reverse conformational change, regenerating the outward-facing binding site.
What do symporters do?
Transport molecules and ions in the same direction. Simultaneous binding of Na+ and glucose to the outward facing binding sites. Generate a conformational change resulting in inward-facing sites. Dissociation of the bound Na+ and glucose into the cytosol. Allows the protein to revert to its original outward-facing conformation ready to transport additional substrates.
What do antiporters do?
Transport molecule and ions in the opposite direction. Protons bind to the protein carrier in the outward-facing state causing it to switch to the inward-facing state. On the inside, protons are exchanged for Na+ and the protein carrier moves back to the outward-facing state.
What are symporters and antiporters?
Cotransporters
What are vesicles?
Self-contained structures consisting of fluid or gas surrounded and enclosed by an outer membrane.
What is vesicle transport and what two types are there?
Control the transport of large substances into and out of cells. Exo - sends things out, endo - bring things in.
What happens in endocytosis and what are the two types?
Pinching off of the PM to form vesicles, there is phagocytosis - cell eating, where large particles are internalised. Pinocytosis - cell drinking, takes in substances from extracellular fluid.
What happens in exocytosis?
Fusion of secretory vesicles with the plasma membrane, discharges the vesicle contents into extracellular space.
