2.3 transport across cell membranes

• purpose of a cell membrane

  • to control the entry and exit of molecules
  • to separate organelles within the cytoplasm
  • to provide an internal transport system
  • to provide a surface for reactions to take place
• cell membranes are in a fluid mosaic model

  fluid = phospholipid bilayer makes it flexible

  mosaic = many different proteins and carbohydrates embedded
• phospholipid bilayer structure

  • phospholipids form a double layer
  • hydrophilic heads point outwards as they are attracted to water within cytoplasm and tissue fluid
  • hydrophobic tails point inwards away from water
• phospholipid bilayer function

  • allows lipid soluble substances to enter the cell
  • prevents water soluble substances entering and leaving the cell
  • makes membrane flexible and self-sealing
• channel protein structure

  • intrinsic proteins
  • have hydrophilic channel through the center
• channel proteins function

  • allow small, polar molecules to diffuse across the membrane
  • aquaporins = channel proteins for water
• carrier protein structure

  • intrinsic proteins
  • binding sites for ions etc
  • specific to particular molecules
  • change shape when molecules bind to them or when ATP is hydrolysed
• carrier protein function

  • to move large + charged molecules across the membrane
  • when used with ATP, can move molecules against a concentration gradient
• glycoproteins structure

  • protein with a carbohydrate group attached,
  • may be intrinsic or extrinsic
• glycolipid structure

  • phospholipid with carbohydrate group attached
• glycolipid function

  • cell recognition
  • cell adhesion
  • helps to maintain stability of the membrane
• cholesterol structure

  • a steroid
  • has a hydrophilic head and a hydrophobic tail so it orientates alongside phospholipids
• cholesterol function

  • reduces lateral movement of membrane components, so controls fluidity of the membrane
  • makes membrane less fluid at high temperatures
  • reduces leakage of water and ions from the cell
• the composition of membranes varies according to its function
• types of transport

  • simple diffusion
  • facilitated diffusion
  • active transport
  • co-transport
  • osmosis
• simple diffusion

  • small, lipid-soluble, non-polar molecules move down their concentration gradients
  • fluid nature of the membrane enables the phospholipids to move apart to allow the passage of the molecules between them
• facilitated diffusion

  • channel proteins provide a pore through which large or charged molecules can pass
  • carrier proteins can change shape when specific molecules bind to them
  • movement of molecules is down the concentration gradient
• active transport

  • enables molecules to move against their concentration gradient
  • requires energy provided by hydrolysis of ATP to make the carrier protein change shape
• co-transport

  • two different types of molecule can be transported as the protein changes shape
  • ATP is sometimes, but not always, required to make the transport protein change shape
• osmosis

  • the diffusion of water molecules from a high water potential to a low water potential across a partially permeable membrane, until dynamic equilibrium is reached
  • high water potential = dilute solution
  • pure water = 0 kPa, concentrated solution = -500 kPa
• % change = (change / original) x 100
• identifying water potential through a calibration curve

  • plot a line graph of concentration of solution against % change in mass
  • draw a line of best fit
  • use this to predict the concentration of potato cells
• when water potnetial of solution > water potential inside potato cells

  then water enters cells by osmosis - increase in length / mass
• when water potential of solution < water potential inside potato cells

  then water leaves cells by osmosis - decreases in length / mass
• calculating % error

  • error (uncertainty) is plus or minus half the smallest division of apparatus
  • when measuring length, two errors should be included
  • so error in measurement of length = plus of minus 1mm
  • % error = (error / length) x 100
• control variables in RP3: identifying water potential of plant tissue

  • same type of potato --> so cells have same water potential, so no genetic differences
  • all potato cylinders the same length --> similar surface area otherwise affects rate of water uptake via osmosis
• RP4

  effects of washing sample on the permeability of cell-surface membranes

  • pigments from damaged cells removed
  • pigment molecules too large to cross intact membranes
• RP4

  effects of change in pH on the permeability of cell-surface membranes

  • ionic bonds in tertiary structure are broken
  • denaturation of proteins creates large gaps in the membrane, increasing membrane permeability, allowing release of pigment
• RP4

  effect of high temperature on the permeability of cell-surface membranes

  • breaks hydrogen bonds in tertiary structure of proteins
  • progressive increase in temperature causes more bonds to break
  • progressive denaturation of proteins causes increasing membrane permeability