membranes

• roles of membranes

  • for compartmentalisation - keeps the contents of the cell separate to its environment allowing optimum conditions ,such as chemical gradients, for cellular reactions to be maintained
  • can be sites for specific chemical reactions
  • sites of cell communication e.g. cell signalling
• Fluid mosaic model

  • fluid: phospholipids and the various proteins and lipids are able to move within the layer laterally, giving the membrane flexibility
  • mosaic: the random distribution of proteins and lipids of different shapes and sizes throughout the membrane like tiles of a mosaic
• The phospholipids are arranged in a bilayer because the phospholipid heads are hydrophilic thus point outwards to interact with the surrounding aqueous environment, whereas the tails are hydrophobic so they are repel water, thus point inwards away from water.
• • extrinsic proteins : present in only one side of the bilayer. They can provide mechanic support or they can be glycoproteins/glycolipids.
  • intrinsic proteins : embedded through both sides of the bilayer e.g carrier proteins and channel proteins that are involved in the transport of molecules across the membrane.
• Glycoproteins:

  • play a role in cell recognition and adhesion so similar cells are grouped together to form tissues
  • act as receptors for chemical signals to bind to which can cause a direct response or a cascade of events inside the cell. This allows cells inside an organism to communicate and coordinate the activities of the organism. Allows chemicals such as hormones and neurotransmitters to bind to due to their complementary shape to these molecules
  • receptors for medicinal drugs to bind to e.g salbutamol- relaxes smooth muscle in the airways
• glycoproteins/ glycolipids - proteins/lipids with a carbohydrate chain attached onto them
• glycolipids:

  • act as markers so the immune system can recognise it as self or non-self(antigen)
  • act as receptors for chemical to bind to which can cause a direct response or a cascade of events inside the cell. This allows cells inside an organism to communicate and coordinate the activities of the organism. Allows chemicals such as hormones and neurotransmitters to bind to due to their complementary shape to these molecules
  • allows medicinal drugs to bind to them e.g salbutamol relaxes smooth muscles in the airways
• • fat soluble i.e non polar molecules can use simple diffusion to get through the pores in between the phospholipids as they wont be repelled.
  • water soluble i.e polar molecules have to use proteins in facilitated diffusion as they will be repelled by the hydrophobic interior if they moved in using simple diffusion.
• • cholesterol sits inbetween the phospholipids in the bilayer with the hydrophilic end interacting with the hydrophilic heads vice versa.
  • This pulls the phospholipids close together so they arent too far apart whilst also preventing them from grouping too closely and becoming rigid.
  • It also restricts the lateral movement of other molecules in the membrane adding stability without making it too rigid.
  • These all contribute in maintaining the correct fluidity in membranes especially in high/low temperature.
• factors affecting membrane structure: Temperature

  • As temperature increases, phospholipids gain more KE so move about more, making membranes more fluid
  • If temperatures get too high, cell membranes will loose there structure completely and will become increasingly permeable to certain molecules.
  • As the phospholipids moving about too much will create temporary gaps that will allow other small molecules to move across the membrane
• Factors affecting membrane structure: Temperature pt 2

  • the proteins in the membrane might denature, therefore the change in the shape of the active site, or the slight shifts in the enzyme's position can affect the rate at which it catalyses their reactions.
  • changes in the protein also contribute in the creation of holes
  • However, cholesterol lowers the effects of increasing heat as it prevents excessive membrane fluidity
• Factor affecting membrane structure - Cold temperatures

  • at colder temperatures, the proportion of saturated and unsaturated fatty acid chains determine membrane permeability
  • At cold temperatures, the saturated fatty acid chains of phospholipids become more packed together, whereas the unsaturated fatty acids contain kinks so this will push adjacent phospholipid molecules away, keeping membranes fluid.
• factors affecting membrane structure - solvents

  • solvents such as alcohol can dissolve the lipids in the membrane, disrupting membranes and increasing its fluidity and permeability.
  • This can happen to the neurones in the brain, explaining the changes in people's behaviour when they drink.
• Passive transport is the movement of molecules that doesn't use metabolic energy, instead it uses the existing kinetic energy of particles as they move down the concentration gradient.

  Diffusion is an example of passive transport. Diffusion can be simple or facilitated
• Simple diffusion is the passive net movement of molecules from a region of higher concentration down a concentration gradient until equilibrium is reached.

  Simple diffusion occurs when small or fat-soluble molecules fit between the phospholipids in a membrane to get across.
• Factors affecting the rate of simple diffusion:

  • higher temperatures - particles have more kinetic energy
  • higher concentration gradient
  • size of molecules - smaller molecules move faster
  • thinner the membrane - short diffusion pathway as fewer barriers
  • greater the surface area - more molecules can move across at the same time.
• Facilitated diffusion - passive transport that requires a transport protein to help molecules to move down the concentration gradient. This protein could be a channel/ pore protein or a carrier protein.
• Facilitated diffusion - channel protein

  • Water filled channels that allow mainly ions but sometimes very small water soluble molecules to pass through
  • They are selective - they are specific to one molecule/ ion
  • some channel proteins are gated so can open and close to control exchange of ions.
• Facilitated diffusion - carrier protein:

  • specific molecule binds to its binding site
  • protein changes shape to allow molecule to pass through the protein on the other side of the membrane.
  • once the molecule has left the protein, its shape reverts
  • other carrier proteins rotate 180 degrees when its specific molecule binds to it, allowing the protein to deposit the molecule on the other side.
• active transport is the net movement of molecules against their concentration from an area of lower concentration to higher concentration using metabolic energy in the form of ATP and carrier proteins in the membrane.
• Active transport

  1. Molecule or ions binds to the receptor site on the carrier proteins. On the inside of the membrane, ATP will bind to the protein and hydrolyse into ADP and Pi.
  2. Binding of the Pi will cause the protein to change shape and open towards the inside of the membrane, allowing the molecule to be released inside the cell.
  3. The phosphate molecule is then released from the protein causing the protein to revert to its original shape.
• Active transport is selective as only a specific substances can bind to the receptor site of a specific carrier protein.
• Bulk Transport is another type of active transport. Large molevules such as enzymes, hormones and whole cells e.g bacteria are too large to move through carrier or channel proteins so are moved through a cell by bulk transport.
• Endocytosis - the bulk transport of materials into the cell because of the plasma membrane changing shape and bending inwards around the molecule, surrounding it to form a vesicle that is pinched off and can move within the cytoplasm of the cell.

  • phagocytosis - when solid particles are being taken in
  • pinocytosis - when liquid particles are being taken in
• Exocytosis - is the bulk transport of materials out of the cell where (secretory) vesicles move towards the plasma membrane and fuses with it, allowing the contents of the vesicle to be released outside the cell.
• Osmosis is the passive net movement of water molecules from a high water potential to a lower water potential.

  Pure water has a water potential of 0Pa. Any solution has a negative water potential.

  Water potential is the (hydrostatic - inside cells) pressure exerted by water molecules as the collide with a membrane or a container
• Animal cells

  • hypertonic - solution is more concentrated so water potential will be higher inside the cell so water moves out the cell via osmosis. This can cause crenation, where the cell shrivels.
  • isotonic- water potential inside and outside the cell is the same, so there is no net movement of water as equilibrium is reached.
  • hypotonic - solution is less concentrated so water potential is higher outside the cell so water moves into the cell via osmosis. This can cause cytolysis as the plasma membrane is weak and can not withstand the increased hydrostatic pressure.
• plant cells

  • hypertonic - the solution is more concentrated this means water potential will be higher inside the cell so water moves out the cell via osmosis. This can cause the cell to become plasmolysed where the cytoplasm pulls away from the cell wall. However, the cell will not crenate due to the cell wall maintaining shape.
  • isotonic - water potential inside and outside the cell is the same, so there is no net movement of water as equilibrium is reached.
• plant cells

  • hypotonic - solution is less concentrated, meaning water potential is higher outside the cell so water moves into the cell via osmosis. However, the presence of the cell wall means that the cell can withstand the increased hydrostatic pressure and will not burst instead the cell swells and becomes turgid.