Plasma membranes Flashcards
Does the effect of cholesterol depend on temperature?
Yes
At low temperatures what affect does cholesterol have on the fluidity of the membrane?
increases fluidity
At high temperatures what affect does cholesterol have on the fluidity of the membrane?
decreases fluidity
Describe the fluid mosaic model of membranes
Fluid: phospholipid bilayer in which individual phospholipids can move = membrane has flexible shape
Mosaic: extrinsic and intrinsic proteins of different sizes and shapes are embedded
Explain the role of cholesterol and glycolipids in membranes
Cholesterol: steroid molecule in some plasma membranes; connects phospholipids and reduces fluidity to make bilayer more stable
Glycolipids: cell signalling and cell recognition
Explain the functions of extrinsic and transmembrane proteins in membranes
- binding sites/receptors e.g. for hormones and drugs
- antigens (glycoproteins)
- bind cells together
- involved in cell signalling
Explain the functions of intrinsic transmembrane proteins in membranes
- electron carriers (respiration/photosynthesis)
- channel proteins (facilitated diffusion)
- carrier proteins (facilitated diffusion/active transport)
Explain the functions of membranes within cells
- provide internal transport system
- selectively permeable to regulate passage of molecules into/out of organelles or within organelles
- provide reaction surface
- isolate organelles from cytoplasm for specific metabolic reactions
Explain the functions of the cell-surface membrane
- isolates cytoplasm from extracellular environment
- selectively permeable to regulate transport of substances
- involved in cell signalling/cell recognition
Name and explain 3 factors that affect membrane permeability
- temperature: high temperature denatures membrane proteins/phospholipid molecules have more kinetic energy and move further apart
- pH: changes tertiary structure of membrane proteins
- use of a solvent: may dissolve membrane
Outline how colorimetry could be used to investigate membrane permeability
1, Use plant tissue with soluble pigment in vacuole. Tonoplast and cell-surface membrane disrupted = increased permeability = pigment diffuses into solution
2, Select colorimeter filter with complementary colour
3, Use distilled water to set colorimeter to 0. Measure absorbance/ % transmission value of solution
4, High absorbance/low transmission = more pigment in solution
Define osmosis
Water diffuses across semi-permeable membranes from an area of higher water potential to an area of lower water potential until a dynamic equilibrium is established
What is water potential?
- pressure created by water molecules measured in kPa
- water potential of pure water at 25 degrees C and 100kPa is 0
- more solute = water potential more negative
How does osmosis affect plant and animal cells?
osmosis INTO cell:
plant: protoplast swells = cell turgid
animal: lysis
osmosis OUT of cell:
plant: protoplast shrinks = cell flaccid
animal: crenation
Define simple diffusion
Passive process requires no energy from ATP hydrolysis.
Net movement of small, lipid-soluble molecules directly through the bilayer from an area of high concentration to an area of lower concentration.
Define facilitated diffusion
Passive process. Specific channel or carrier proteins with complementary binding sites transport large and/or polar molecules/ions (not soluble in hydrophobic phospholipid tail) down concentration gradient
Explain how channel and carrier proteins work
Channel: hydrophilic channels bind to specific ions = one side of the protein closes and the other opens
Carrier: binds to complementary molecule = conformational change releases molecule on other side of membrane; in facilitated diffusion, passive process; in active transport, requires energy from ATP hydrolysis
Define active transport
Active process: ATP hydrolysis releases phosphate group that binds to carrier protein, causing it to change shape.
Specific carrier protein transports molecules/ions from area of low concentration to area of higher concentration
Define exocytosis and endocytosis
- active process
- involved in bulk transport and transporting large particles
- vesicles fuse with cell surface phospholipid membrane
Name 5 factors that affect the rate of diffusion
- temperature
- diffusion distance
- surface area
- size of molecule
- difference in concentration (how steep the concentration gradient is)
