transport across cell membranes

• What are some roles of the membrane inside the cell?

  - isolate harmful enzymes in the lysosome

  - surround vesicles that transport molecules between parts of cells

  - divide cell into compartments where functions can occur more efficiently

  - provide a large SA for holding enzymes and co-enzymes
• What are some roles of the membrane outside the cell?

  - permit recognition by other cells

  - provide receptors for signalling molecules

  - may extend microvilli - large SA for absorption

  - helps cells attach to each other

  - controls movement of substances in and out of cells
• what is the name of the small unit that makes up the membranes?

  phospholipid
• what is the structure of cholesterol

  lots of rings with fatty acid tails coming off
• what are the two types of protein in cell membranes

  intrinsic and extrinsic
• what is the name of a protein with a carbohydrate on it?

  glycoprotein
• Describe the fluid-mosaic model of membrane structure.

  fluid: PHOSPHOLIPID BILAYER in which individual phospholipids an move - membrane has a flexible shape

  Mosaic: EXTRINSIC& 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 & reduces fluidity - bilayer is more stable

  glycolipids: cell signalling & cell recognition
• explain the functions of extrinsic proteins in membranes


  - binding sites/receptors

  - antigens

  - bind cell together

  - involved in cell signalling
• explain the function of intrinsic 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.

  ● Provide reaction surface.

  ● Isolate organelles from cytoplasm for specific metabolic reactions.
• name and explain 3 factors that affect membrane permeability

  - temperature: high temp denatures membrane proteins/phospholipid molecules have more kE - move further apart

  -pH: changes tertiary structure of membrane proteins

  - using a solvent (e.g. alcohol) : may dissolve membrane
• Outline how colorimetry could be used to investigate membrane permeability.

  1. use plant tissue with soluble pigment in vacuole (e.g. beetroot) Tonoplast & cell-surface membrane disrupted = higher permeability so pigment can diffuse into solution

  2. Select colorimeter filter with complementary colour

  3. use distilled water to set colorimeter to 0. Measure absorbance/% transmission value of solution

  4. high absorbance/low transmission = more pigment in solution
• define osmosis


  Diffusion of water through a semi permeable membrane from an area of high water potential to an area of lower water potential
• what is water potential?

  - Pressure created by water molecules measured in kPa

  - more solute = water potential more negative
• how does osmosis affect plant cells?

  Osmosis INTO cell: protoplast swells = cell turgid

  Osmosis OUT of cell:

  protoplast shrinks = cell flaccid
• how does osmosis affect animal cells?

  osmosis into cell - lysis

  osmosis out of cell - 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 proteins work

  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; in facilitated diffusion, passive process; in active transport, requires energy from ATP hydrolysis
• Name 5 factors that affect the rate of diffusion.

  ● Temperature

  ● Diffusion distance

  ● Surface area

  ● Size of molecule

  ● Difference in concentration (how steep the concentration gradient is)
• how are cells adapted to maximise the rate of transport across their membranes

  - many carrier/channel proteins

  - folded membrane 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/ ions from area of low concentration to area of higher concentration (i.e. against concentration gradient).
• compare and contrast active transport and facilitated diffusion

  - both may involve carrier proteins

  - active transport requires energy from ATP hydrolysis but facilitated is a passive process

  - facilitated may also involve channel proteins
• Define co-transport


  Movement of a substance against its concentration gradient is coupled with the movement of another substance down its concentration/ 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 the absorption of glucose / amino acids in the small intestine.

  1. Na+ actively transported out of epithelial cells into bloodstream

  2. Na+ conc. lower in epithelial cells than lumen of gut

  3. Transport of glucose/amino acids from lumen to epithelial cells is 'coupled' to facilitated diffusion of Na+ down electrochemical gradient