Module 2: Section 5 - Biological Membranes

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Function of membranes

Plasma membranes

  • Barrier between the cell and its environment - controls which substances enter and leave the cell (partially permeable)
  • Allow recognition by other cells eg. cells of the immune system
  • Allow cell communication (cell signalling)

Membranes within cells

  • Membranes around organelles divide the cell into different compartments - act as a barrier between the organelle and the cytoplasm, making functions more efficient (eg. the sustances needed for respiration, like enzymes, are kept inside the mitochondria)
  • Can form vesicles to transport substances between different areas of the cell
  • Control which substances enter and leave the organelle - partially permeable
  • Membranes within organelles act as barriers between the membranes contents and the rest of the organlelle eg. thylakoid membranes in chloroplasts
  • Membranes within cells can be the site of chemical reactions eg. the inner mitochondrial membrane for respiration
  • Create concentration gradients 
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The structure of cell membranes

Fluid mosaic model:

  • Phospholipid molecules form a continuous, double layer (bilayer) - fluid because the phospholipids are constantly moving
  • Cholesterol molecules are present within the bilayer
  • Protein molecules are scattered through the bilayer, like tiles in a mosaic
  • Some proteins have a polysaccharide chain attached - glycoprotein
  • Some lipids have a polysaccharide chain attached - glycolipids 
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The structure of cell membranes

Proteins control what enters and leaves the cell:

  • Some proteins form channels in the membrane - allow small or charged particles through
  • Carrier proteins transport molecules and ions across the membrane by active transport and facilitated diffusion
  • Proteins also act as receptors for molecules in cell signalling - when a molecule binds to the protein, a chemical reaction is triggered inside the cell

Glycolipids and glycoproteins act as receptors for messenger molecules:

  • Stabilise the membrane by forming hydrogen bonds with surrounding water molecules
  • Sites where drugs, hormones and antibodies bind
  • Receptors for cell signalling
  • Antigens 
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Cell signalling

Cells need to communicate to controll processes in the body and respond to changes in the environment

1. Cell releases messenger molecule

2. Messenger molecule travels (eg. in the blood) to another cell

3. Messenger molecule binds to a receptor on the target cell membrane 

Proteins in the cell membrane act as receptors - 'membrane-bound receptors'

Receptor proteins have specific shapes - only messenger molecules which have a complementary shape can bind

Many drugs work by binding to receptors - they either trigger a response in the cell or block the receptor to prevent it from working

eg. antihistamines block histamine (released due to cell damage) receptors - stops inflammation

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Effect of temp. on membrane permeability

Below 0°C

  • The phospholipids dont have much energy, so they can't move very much - packed closely together and the membrane is rigid
  • Channel proteins and carrier proteins deform, increasing the permeability of the membrane
  • Ice crystals may form and pierce the membrane, making it highly permeable when it thaws

Between 0 and 45°C

  • Phospholipids can move and aren't packed as tightly - membrane is partially permeable
  • As temperature increases the phospholipids move more because they have more energy - increases the permeability of the membrane

Above 45°C

  • Phospholipid bilayer starts to melt (break down) and the membrane becomes more permeable
  • Water inside the cell expands, putting pressure on the membrane
  • Channel and carrier protiens deform so they can't control what enters or leaves the cell - increases permeability 
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Effect of solvent on membrane permeability

Surrounding cells in a solvent eg. ethanol increases the permeability of cell membranes

Solvents dissolves the lipids in a cell membrane so the membrane loses its structure

Some solvents increase membrane permeability more than others

Increaing the concentration of the solvent willl also increase membrane permeability 

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Diffusion

Diffusion - the net movement of particles from an area of higher concentration to an area of lower concentration (down a concentration gradient)

Molecules will diffuse both ways, but the net movement will be to the area of lower concentration

Diffusion is a passive process - no energy is needed for it to happen

Rate of diffusion depends on:

  • Concentration gradient - higher gradient = faster the rate of diffusion
  • Thickness of the exchange surface - thinner surface = faster rate of diffusion
  • Surface area - larger surface area = faster the rate of diffusion
  • Temperature - the warmer it is, the faster the rate of diffusion as the particles have more kinetic energy
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Facilitated diffusion

Some larger molecules (eg. amino acids, glucose), ions and polar molecules don't diffuse directly through the phospholipid bilayer of the cell membrane

Instead they diffuse through carrier or channel proteins in the cell membrane - facilitated diffusion

Occurs down a concentration gradient

Passive process

Carrier proteins:

  • Large molecule attaches to the carrier protein, changing the shape of the protein
  • This releases the molecule on the opposite side of the membrane

Channel proteins:

  • Form pores in the membrane for charged particles to diffuse through (down their concentration gradient)
  • Different channel proteins facilitate the diffusion of different charged particles
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Active transport

Uses energy to move molecules and ions across plasma membranes against a concentration gradient

Involves carrier proteins

Similar to facilitated diffusion - molecule attaches to the carrier protein, the protein changes shape and this moves the molecule across the membrane, releasing it on the other side

Only difference is that energy is used from ATP to move the solute against its concentration gradient

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Osmosis

Osmosis - the movement of water molecules across a partially permeable membrane down a water potential gradient 

Water potential is the potential of water molecules to diffuse out of or into a solution

Pure water has the highest water potential - all solutions have a lower water potential than pure water 

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Cells and water potential

Animal cells

  • Hypotonic solution - net movement of water molecules into the cell, cell bursts (lysed)
  • Isotonic solution - water molecules move into and out of the cell in equal amounts so the cell stays the same
  • Hypertonic solution - net movement of water molecules is out of the cell, cell shrinks (crenated)

Plant cells

  • Hypotonic solution - net movement of water molecules into the cell, vacuole swells and the vacuole and the cytoplasm push against the cell wall - cell becomes turgid
  • Isotonic solution - water molecules move into and out of the cell in equal amounts so the cell stays the same
  • Hypertonic solution - net movement of water molecules is out of the cell, cell becomes flaccid, the cytoplasm and the membrane pull away from the cell wall - plasomolysis
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