Mod 2 Chap 5: Plasma Membranes Flashcards
Describe the roles of membranes within cells.
- may house pigments essential for chemical reactions e.g. Chlorophyll on thylakoid membranes of chloroplasts
- can form vesicles to transport substances e.g proteins from RER to Golgi
- compartmentalisation (all components needed for a specific function held in one place)
- folded membranes increases surface area
- contain enzymes for a specific chemical reaction
Describe the roles of membranes on the surface of cells.
- contain antigens so the organism’s immune system recognises the cell as ‘self’, so does not attack it
- may release chemicals to signal to other cells
- separate cell’s components from extracellular environment
- regulates transport of materials in and out of cells
- contains receptors for chemical signals, released by other cells
- may contain enzymes involved in metabolic pathways
What are cell membranes PARTIALLY permeable to?
- v small molecules that can diffuse easily across by moving between structural components of membrane e.g. O2
- lipid-soluble molecules can dissolve in lipid bi-layer and pass through
- large polar molecules pass proteins in membrane e.g. Glucose, water
Name the components of the Fluid-Mosaic Model of the plasma membrane and describe their roles.
Intrinsic / Integral proteins: are transmembrane (embedded in both layers of membrane), have amino acids w/ hydrophobic R-groups on their external surfaces, which interact w/ hydrophobic core of membrane, holding them in place. Examples:
- Channel proteins: provide hydrophilic channel allowing passive movement of polar molecules + ions down conc gradient through membranes.
- Carrier proteins: have role in both passive transport (down a conc gradient) + active transport (against a conc gradient) into cells, this often involves shape of protein changing
- Glycoproteins: embedded in surface membrane w/ attached carb (sugar) chains, have role in cell adhesion + as receptors for chemical signals (= process of cell communication / cell signalling).
Extrinsic / Peripheral proteins: present in one side of bilayer (can be either, some move between two), normally have hydrophilic R-groups on outer surfaces + interact w/ polar heads of phospholipids / w/ intrinsic proteins.
Cholesterol: a lipid w/ a hydrophilic end + a hydrophobic end, like a phospholipid, regulates fluidity of membranes, positioned between phospholipids in a membrane bilayer so hydrophobic-hydrophilic interactions pull them together, makes cholesterol add stability to membrane without being too rigid, stop phospholipid molecules grouping + crystallising, which stops membrane being too solid.
Glycolipids: similar to glycoproteins, lipids w/ attached carb (sugar) chains, called cell markers / antigens, can be recognised by immune system as self (part of organism) or non self (belonging to another organism).
Describe the process of cell communication / cell signalling, carried out by glycoproteins.
- when chemical binds to receptor, a response is elicited from cell
- causes a direct response / sets off cascade of events in cell
- (some drugs act by binding to cell receptors)
Example of this process:
- receptors for peptide hormones such as Insulin + glucagon, which affect uptake + storage of glucose by cells
Describe diffusion as a method of movement for molecules.
The net movement of molecules or ions from a region of high concentration to a region of low concentration until they are spread out evenly.
Simple Diffusion:
- a passive process, no energy required
- particles able to diffuse across plasma membrane: small non polar molecules that are small enough to fit between phospholipids e.g. O2 + CO2, or water small enough to fit through too, despite being polar
Describe the factors affecting the rate of diffusion.
- concentration gradient: steeper the conc gradient = faster rate of diffusion
- size of particles: larger particles = slower rate
- thickness of membrane: thicker membrane = slower rate
- surface area: larger SA:V ratio = faster rate
- temperature: higher temp = more KE molecules have = faster their movement = faster rate
Describe the difference between simple diffusion and facilitated diffusion.
Simple: diffusion in the absence of a barrier or membrane.
Facilitated: diffusion across a membrane through protein channels.
Describe facilitated diffusion as a method of movement for molecules across membranes.
- large polar molecules can only enter cell by facilitated diffusion, down their conc gradient w/ aid of proteins
- uses proteins to facilitate (allow) diffusion
The two types of proteins used:
- channel proteins
- carrier proteins
Membranes with channel proteins are selectively permeable.
Describe how channel proteins work in facilitated diffusion.
- charged ions / polar molecules like water cannot diffuse through non polar centre of phospholipid bilayer
- channel proteins form pores to allow them to pass through
- each is specific to one type of ion (so membrane is selectively permeable here)
- some can open and close pores, acting like gates
Describe how carrier proteins work in facilitated diffusion.
- allow diffusion of larger polar molecules e.g. Sugar and amino acids
- molecule attaches to specific binding site on carrier protein
- this causes carrier protein to change shape to ‘deliver’ molecule through membrane
Describe a practical investigation into the factors affecting diffusion rates in model cells.
Investigating factors of: temp and concentration
- dialysis tubing used as a membrane
- one end of tubing tied, fill with a solution, then tie other end = model cell
- place into another solution/s (of various concentrations / sizes of solute molecules)
- measure changes over time in conc of solute molecules, inside and out of model cell
- calculate rate of diffusion across tubing (changes in conc divided by time)
Describe active transport as a method of movement for molecules.
The movement of molecules or ions into or out of a cell from a region of lower concentration to a region of higher concentration.
- a process requiring energy and carrier proteins
- energy needed as particles are being moved up a conc gradient (opposite direction to diffusion)
- metabolic energy supplied by ATP
- carrier proteins span membranes + act as ‘pumps’
- a selective process, specific substances transported by specific carrier proteins
Describe HOW the process of diffusion occurs for movement of molecules.
- particles in a gas / liquid have KE, so are moving randomly
- so unequal distribution of particles eventually becomes even, equilibrium is reached so movement is equal in both directions
- particles are moving at high speeds so are constantly colliding, slowing down their overall movement
- means that over short distance, diffusion is fast, but as diffusion distance increases, rate of diffusion slows as more collisions have taken place
Describe HOW the process of active transport occurs for movement of molecules into or out of a cell.
- molecule / ion to be transported binds to receptors in channel of carrier protein
- ATP binds to carrier protein + is hydrated into ADP and phosphate
- binding of phosphate to carrier protein causes protein to change shape, opening up to other side of cell
- molecule or ion is released to other side of cell
- phosphate molecule released from carrier protein + recombined w/ ADP to form ATP
- carrier protein returns to original shape