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 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 function of extrinsic proteins in membranes
- Binding sites/ receptors e.g. for hormones and drugs
- Antigens (glycoproteins)
- Bind cells together
- Involved in cell signalling
Explain the function 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 permeate 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 the transport of substances
- Involved in cell signalling / 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 in tertiary structure of membrane proteins
- Use of any solvent: may dissolve membranes
Outline how colorimetry could be used to investigate membrane permeability
- Use plant tissue with soluble pigment in vacuole. Tonoplast and cell surface membrane disrupted = increased 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
The movement of water across a partially permeable membrane from an area of higher water potential to an area of lower water potential until a dynamic equilibrium is set up
What is water potential (ψ)
- Pressure created by water molecules measured in kPa
- ψ of pure water at 25*C and 100kPa: 0
- More solute = ψ more negative
How does osmosis affect plant or animal cells?
Osmosis INTO cell:
Plant: protoplast swells = cell turgid
Animal: lysis
Osmosis OUT of cell
Plant: protoplast shrinks = cell flaccid
Animal: Crenation
Diffuse 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 low 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 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 bind to carrier protein causing it to change shape
Specific carrier protein transports molecules and ions from area of low concentration to area of higher concentration (i.e. against the concentration gradient)
