Cell Membranes

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Cell Membrane Structure

Phospholipids molecules form a continuous double layer (bilayer)

Fluid: Phospholipid molecules are constantly moving

Mosaic: Proteins are scattered through the bilayer

Receptor proteins: Allow the cell to detect chemicals released from other cells.

Glycoproteins: Proteins with a carbohydrate chain attached

Glycolipids: Lipids with a polysaccharide chain

Phospholipids form a barrier to dissolved substances, the centre is phydophobic so the membrane doesn't allow water-soluble substances through it.

Cholesterol: Type of lipid which fits between phospholipids and bind to hydrophobic tails which causes them to fit more closely together. This makes the membrane less fluid and more rigid. It helps to maintain the shape of animal cells.

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Diffusion

  • The net movement of particels from an area of higher concentrtion to an area of lower concentration.
  • Continues until particles are evenly distributed
  • Particles diffuse down a concentration gradient
  • Passive process-no energy needed for it to happen
  • Particles can diffuse across cell membranes, as long as they can move freely through the membrane.
  • When molecules diffuse directly through a cell membrane, it's known as simple diffusion.

Simple diffusion depends on:

  • Concentration gradient: the higher it is, the faster the rate of diffusion.
  • Thickness of exchange surface- the thinner the exchange surface the (shorter the distance the particles have to travel) the faster the rate of diffusion.
  • The surface area- the larger the surface are the faster the rate of diffusion.
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Facilitated Diffusion

  • To speed things up, large or charged particles diffuse through carrier proteins or channel proteins in the membrane instead.
  • Facilitated diffusion moves particles down a concentration gradient from higher to lower.
  • Passive process.

Carrier proteins: 

Move large particles across membranes across membranes, down their concentration gradient.

  • 1. A large molecule attaches to a carrier protein in the membrane.
  • 2. Then the protein changes shape
  • 3. This releases the molecule on the opposite side of the membrane.

Channel proteins form pores in the membrane for charged particles to diffuse through. Different channel proteins faciltate the diffusion of different charged particles.

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Facilitated Diffusion depends on:

The concentration gradient- the higher the concentration gradient, the faster the rate of facilitated. As equilibrum is reached, the rate of facilitated diffusion will level off.

The number of channel or carrier proteins- once all the proteins in a membrane are in use, facilitated diffusion can't happen any faster, even if you increase the concentration gradient.

The greater the number of channel or carrier proteins in the cell membrane, the faster the rate of facilitated diffusion.

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Osmosis

Diffusion of water molecules across a partially permeable membrane, from an area of higher water potential to and area of lower water potential.

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

Pure water has the highest water potential, all solutions have a ower water potential tht pure water.

If two solutions have the same water potential, they are said to be isotonic.

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Active transport

Uses energy to move molecues and ions across membranes usually against a concentration gradient.

Carrier proteins: a molecule attaches to the carrier protein, the protein changes shape and this moves the molecule across the membrance, releasing it on the other side.

Differences between AT and FD:

AT: moves solutes from low to high conc. requires energy

FD: moves solutes from high to low conc. doesn't require energy

ATP undergos a hydroysis reaction, splitting it into ADP and Pi, this releases energy so that solutes can be transported.

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Carrier proteins: co-transporters

Co-transporters are a type of carrier protein

They bind to two molecules at a time

The concentration gradient of one of the molecules is used to move the other molecules against its own concentration gradient

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Co-transport

Glucose is absorbed into the bloodstream in the small intestine.

In the ileum the concentration of glucose is too low for glucose to diffuse out into the blood. So glucose is absorbed from the lumen of the ileum by co-transport.

1. Sodium ions are actively transported out of the ileum epithelial cells, into the blood, by the sodium-potassium pump. This creates a concentration gradient- there is now a higher concentration of sodium ions in the lumen of the ileum than inside the cell.

2. This causes sodium ions to diffuse from the lumen of the ileum into the epithilial cell, down their concentration gradient. They do this via the sodium-glucose co-transporter proteins.

3. The co-transporter carries glucose into the cell with the sodium. The concentration of glucose inside the cell increases.

4. Glucose diffuses out of the cell, into the blood, down its concentration gradient, through a protein channel by facilitated diffusion.

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