Transport across membrane Flashcards
Cells revision
Describe 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 & shapes are embedded
Explain 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 functions of extrinsic proteins in membranes
- binding sites/ receptors e.g. for hormones
- antigens (glycoproteins)
- bind cells together
- involved in cell signalling
Explain functions of intrinsic proteins in membranes
- electron carriers (respiration/ photosynthesis)
- channel proteins (facilitated diffusion)
- carrier proteins (facilitated diffusion/ active transport)
Explain functions of membranes within cells
- Provide internal transport system
- Selectivley permeable to regulate passage of molecules into/ out of organelles
- Provide reaction surface
- Isolate organelles from cytoplasm for specific metabolic reactions
Explain functions of cell-surface membrane
- Isolates cytoplasm from extracellular environment
- Selecively permeable to regulate transport of substances
- Involved in cell signalling/ cell recognition
Name & explain 3 factors that affect membrane permeability
- Temp: high temp. denatures membrane proteins/ phospholipid molecules have more kinetic energy & move further apart.
- pH: changes tertiary structure of membrane proteins
- Use of solvent: may dissolve membrane
Outline how colorimetry could be used to investigate membrane permeability
- Use plant tissue with soluble in vacuole. Tonoplast & cell-surface membrane disrupted = high permeability = pigment diffuses into solution
- Select colorimetry 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 area of lower water potential
What is water potential (Ψ) ?
- pressure created by water molecules measured in kPa
- Ψ of pure water at 25℃ & 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
Suggest how student could produce desired concentration of solution from stock solution
- Volume of stock solution = required conc. x final volume needed/ conc. of stock solution
- Volume of distilled water = final volume needed - volume of stock solution
Define Simple Diffusion
- Passive process requires no energy from ATP hydrolysis
- Net movement of small, lipid-soluble molecules directly through bilayer from an area of high conc. to area of low conc. (down conc. 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 conc. gradient
Explain how channel & carrier proteins work
Channel: hydrophilic channels bind to specific ions = one side of protein closes & 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 ATP (energy)
5 Factors that affect rate of diffusion
- Temperature
- Diffusion distance
- Surface area
- Size of molecule
- Difference in conc. (how steep conc. gradient is)
State Fick’s Law
Surface area x difference in conc. / diffusion distance
How are cells adapted to maximise rate of transport across membranes?
- many carrier/ channel proteins
- folded membranes increases surface area
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 from area of low conc. to higher conc. (against conc. gradient)
Compare and contrast active transport and facilitated diffusion
- Both may involve carrier proteins
- Active transport requires energy from ATP hydrolysis; facilitated diffusion is passive process
- Facilitated diffusion also involve channel proteins
Define co-transport
Movement of substance against conc. gradient is coupled with movement of another substance down conc./ electrochemical gradient
Substances bind to complementary intrinsic protein:
- symport: transports substances in same direction
- antiport: transports substances in opposite direction (e.g. sodium-potassium pump)
Explain how co-transport is involved in absorption of glucose/ amino acids in small intestines
- Na+ actively transported out of epithelial cells & into bloodstream
- Na+ concentration lower in epithelial cells than lumen of gut
- Transport of glucose/ amino acids from lumen to epithelial cells is ‘coupled’ to facilitated diffusion of Na+ down gradient
