Cell membranes

• Transport across the membrane

  The cell membrane is selectively permeable to water and some solutes.

  Lipid soluble substances move easier than water soluble
• Lipid soluble substances

  Small uncharged molecules, such as oxygen and CO2, dissolve in the hydrophobic tails of the phospholipids and diffuse across the cell membrane.

  Non-polar substances, such as Vitamin A, can also dissolve in phospholipids and diffuse through cell membranes.
• Water soluble substances

  Polar molecules, such as glucose and amino acids, as well as charged ions, e.g. Nat, cannot easily diffuse through the phospholipids. They pass through intrinsic protein molecules (channels or carrier proteins).
• Permeability of the membrane is affected by:

  TEMPERATURE -increased temperature increases membrane permeability as the molecules diffusing will have more kinetic energy so will diffuse quicker

  ORGANIC SOLVENTS -e.g ethanol, it will dissolve / emulsify the phospholipids and may denature the proteins , this leaves gaps /holes in the cell membrane , it becomes more porous
• methods of transport across the membrane

  .

  • diffusion
  • facilitated diffusion
  • co-transport
  • active transport
  • Bulk transport (endo/exocytosis)
  • osmosis
• Diffusion 


  The passive movement of a molecule or ion down a concentration gradient, from a region of high concentration to a region of low concentration
• Factors affecting diffusion

  Concentration gradient -greater the difference in conc , the more molecules will diffusion in a given time

  Thickness of a surface -shorter distance =more molecules diffuse in a give time

  Surface area of the membrane-higher the area the more molecules will diffuse
• Factors affecting diffusion

  size of the diffusing molecules-smaller molecules diffuse faster (less kinetic energy ). nature of diffusing molecule -if it is non polar (soluble in phospholipid) it will diffuse faster than polar (water soluble)as they will diffuse anywhere in the membrane

  temperature - increased temp increases rate of diffusion as the molecule or ion has more kinetic energy
• Rate of diffusion

  = surface area x difference in concentration/length of diffusion pathway

  
• What is facilitated diffusion

  Passive transfer of polar molecules or charge ions (relatively insoluble in phospholipid) down a conc gradient , across a membrane by channel or carrier proteins.

  it is more rapid than simple diffusion as going through the protien is quicker than through the bilayer
• Rate of facilitated diffusion is affected by

  Number of channel/carrier proteins present in the membrane ,once filled rate will plateau

  The steepness of the concentration gradient
• Channel proteins

  Protien molecules with water-filled hydrophilic pores ,

  ions being water soluble can pass through as the pores are hydrophilic ,

  each channel is specific for one type of ion

  channels open and close depending on the needs of the cell
• Carrier proteins

  Allow diffusion of larger polar molecules like glucose or amino acids

  polar molecules attaches to a binding site on the carrier protein , the protein changes shape and releases the molecule on the other side
• Gated protien Chanel

  Open and closed to allow ions through
• Co-transport

  type of facilitated diffusion that brings molecules and ions into cells together on the same protein transport molecule , it is a passive process

  
• Example of co transport e,g sodium - glucose


  .

  1. there is a conc gradient for sodium ions to move into the cell
  2. sodium ions and glucose bind to the carrier proteins
  3. carrier protein changes shape, sodium ion and glucose are transported to the other side of the membrane
  4. the process can move glucose against its concentration gradient without the use of ATP( known as secondary active transport )
• What is active transport

  Transport of ions and molecules against the conc gradient using ATP. If respiration is inhibited then active transport is as well , requires an intrinsic carrier protien however it is against the conc gradient so requires energy and the carrier protien acts as a pump

  only charge particles that are insoluble in lipids can be actively transported
• Stages of active transport

  Active transport

  1. the molecule or ion binds to the carrier protien on the outside of Cell membrane
  2. ATP transfers a phosphate group to the carrier protein
  3. carrier protein changes shape and carries the ion or molecule across the membrane
  4. molecule or ion is released into the cytoplasm
  5. carrier protein returns to original shape
• What is bulk transport

  Where the cell transports materials in bulk into or out of the cell . ATP is required so it is an active process
• Two types of bulk transport

  Exocytosis (secretion)

  endocytosis (uptake)
• Describe exocytosis

  Secretion

  • substance leave the cell , being transported through the cytoplasm in a vesicle , which fuses with the cell membrane
  • the vesicle is produced after budding of one end of the Golgi body
  • the vesicle migrates to plasma membrane, fuses with it and secrete its content to outside if the cell

  
• Describe endocytosis(uptake)

  Cell membrane folds around a particle, the folding closes off the link to the outside of the cell, now the particle is fully trapped inside the cell in a vesicle/vacuole

  two types if endocytosis :

  phagocytosis - Upatake of solids

  pinocytosis - uptake of liquids

  
• Osmosis

  Diffusion of water from an are of high water potential to an area of lower water potential across a selectively permeable membrane, passive process
• What is water potential

  The tendency for water to leave a soloution or cell by osmosis, measures in kPa, the greater the number of free molecules in a solution the higher the water potential
• Pure water

  Has the greatest potential energy to move so therefore is given the highest value of 0kPa
• describe water potential

  As you add a solute (sugar or salt) the number of free water molecules is decreased , the potential energy of the water is decreased so the water potential becomes more negative.

  
• hypotonic

  if the water potential of the external solution is higher than then solution inside the cell, water flows INTO the cell
• Hypertonic

  If the water potential outside of the cell is lower than the inside of the cell, water flows OUT of the cell
• Isotonic

  If the water potential of the external solution is the same as the solution inside , no net movement of water
• Equation for water potential

  Water potential =solute potential + pressure potential

  
• What is solute potential

  Due to solutes in the vacuole and cytoplasm, it is the lowering of water potential due to presence of solute molecules
• What is pressure potential

  The pressure exerted by the cell contents on the cell wall, this force increases the tendency of water to move out
• Hypotonic solution

  If a plant cell is placed in a hypotonic solution then water will move into the cell by osmosis. This causes the plant cell to become turgid
• Hypertonic solution

  If a plant cell is placed in a hypertonic solution water will move out of the cell into the surrounding solution by osmosis, the vacoule will shrink and the cytoplasm and cell membrane will withdraw from the cell wall . This causes the cel to become plasmolysed
• Incipient plasmolysis

  The point where the cell membrane is just about to come away from the cell wall. Experimentally, 50% of the Cells in the sample are plasmolysed