Biological Membranes Flashcards
Describe the fluid mosaic model of membranes.
Fluid: phospholipid bilayer in which individual phospholipids can move = membrane has flexible shape.
Mosaic: extrinsic & intrinsic proteins of different sizes and shapes are embedded.
Explain the role of cholesterol & glycolipids in membranes.
Cholesterol: steroid molecule in some plasma membranes; connects phospholipids & reduces fluidity to make bilayer more stable.
Glycolipids: cell signalling & cell recognition.
Explain the functions of extrinsic and transmembrane proteins in membranes.
● Binding sites/ receptors e.g. for hormones & 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 & 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.
- Use plant tissue with soluble pigment in vacuole. Tonoplast & cell-surface membrane disrupted = ↑ permeability = pigment diffuses into solution.
- Select colorimeter filter with complementary colour.
- Use distilled water to set colorimeter to 0. Measure absorbance /
% transmission value of solution. - 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 (ψ)?
● Pressure created by water molecules measured in kPa
● Ψ of pure water at 25°C & 100 kPa: 0 ● More solute = ψ 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 (i.e. down a concentration gradient).
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 & 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 (i.e. against concentration gradient).
