2.1.5 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 shapes and sizes 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)
- Cells bind 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
- Provides internal transport system
- Selectively permeable to regulate passage of molecules into/ out of organelles or within organelles
- Provide reaction surface
- Isolates 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
- Use plant tissue with soluble pigment in vacuole. Tonoplast and cell surface membrane disrupted so pigment diffuses into solution
- Select colorimeter filter with contemporary 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 partially 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℃ and 100 kPa: 0
More solute = Ψ more negative
How does osmosis affect plant and animal cells
Osmosis into cells:
- Plant: protoplast swells = cell turgid
- Animals: lysis
Osmosis out of cells:
- 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 low concentration (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 and the other opens
Carrier: binds to complementary molecule = conformational change releases molecules on other side of membrane; facilitated diffusion; passive process that requires ATP
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 (against concentration gradient)
Name 5 factors that affect the rate of diffusion
- Temperature
- Diffusion distance
- Surface area
- Size of molecule
- Difference in concentration/ steepness of concentration gradient
