Biological membranes Flashcards
Describe the fluid mosaic model of membranes?
fluid = phospholipid bilayer in which individual phospholipids can move (membrane has a flexible shape)
mosaic = extrinsic and intrinsic proteins of different sizes and shapes are embedded
Explain the role of cholesterol in membranes?
- steroid molecules in some plasma membranes
- connects phospholipids and reduces fluidity to make bilayer more stable
Explain the role of glycolipids in membranes?
- cell signalling and cell recognition
Explain the role of extrinsic proteins in membranes?
- binding sites/ receptors (e.g for hormones)
- antigens (glycoproteins)
- bind cells together
- involved in cell signalling
Explain the role of intrinsic proteins in membranes?
- electron carriers
(respiration/ photosynthesis) - channel proteins (facilitated diffusion)
- carrier proteins (facilitated diffusion/ active transport)
Explain the function of membranes within the cell?
- provide internal transport system
- selectively permeable to to regulate passage of molecules in/out/within organelles
- provide reaction surface
- isolate organelle from cytoplasm for specific metabolic reactions
Explain the function 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 temp denatures membrane proteins/ phospholipid molecules have more Ke and move further apart
PH
- changes tertiary structure of membrane proteins
Solvent
- may dissolve membrane
Explain how colorimetry could be used to investigate membrane permeability?
1) use plant tissue with soluble pigment in vacuole
2) a disrupted tonoplast and cell surface membrane increases membrane permeability and pigment diffuses into solution
3) select colorimeter filter with complementary colour
4) use distilled water to set colorimeter to 0 then measure absorbance of solution
5) high absorbance/ low transmission = more pigment in solution
Define osmosis?
the passive diffusion of water across a semi-permeable membrane from an area of higher water potential to an area of lower water potential
What is water potential?
- pressure created by a water molecules measured in kPa
- more solute results in a more negative water potential
How does osmosis affect plant and animal cells?
into cell:
animal = lysis
plant = cell turgidity
out of cell:
animal = crenation
plant = cell flaccidity
Define diffusion?
the passive net movement of molecules down a concentration gradient from an area of high to low concentration
Define facilitated diffusion?
process where specific channel and carrier proteins transport large polar molecules/ ions down a concentration gradient
Explain how channel proteins work?
- carry small polar molecules
- hydrophilic channels bind to specific ions
- one side of the protein closes and the other opens
Explain how carrier proteins work?
- binds to complementary molecule
- conformational change releases molecule on other side of membrane
Define active transport?
active process where ions and molecules move from an area of lower concentration to an area of higher concentration (against a concentration gradient) using energy from respiration
Define endocytosis?
- active process
- process where the cell surface membrane engulfs material forming a sac/ vesicle round it
Define exocytosis?
- active process
- process which materials are removed/ transported out of the cell
- substances to be released are packaged into secretory vesicles formed by the golgi body
- vesicles travel to the cell surface membrane
- they fuse with the membrane and release their contents out of the cell
Name 5 factors affecting rate of diffusion?
- temperature
- diffusion distance
- surface area
- size of molecule
- concentration gradient steepness
