Transport across cell membranes

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  • Created by: Laelle
  • Created on: 24-03-16 20:23

Diffusion

The net movement of molecules/ions from a region of high to low concentration, until the particles are evenly distributed.

This a passive process
Small and non-polar molecules can diffuse easily by simple diffusion (e.g. Carbon dioxide and oxygen)- they're non polar so they're soluble in lipids: can dissolve in the phospholipid bilayer.

Larger molecules (e.g. Amino acids and glucose) diffuse slowly as they're so big. Charged particles- ions and polar molecules also diffuse slowly- because they're water soluble, and the centre of the bilayer is hydrophobic.
So large and charged molecules diffuse through facilitated diffusion: carrier proteins + protein channels.

Carrier proteins- large molecules specific to the protein bind to it, cause it's shape to change. The molecule is then released on the other side of the membrane.

Protein channels- form water-filled hydrophilic pores. The channel opens in the presence of a specific ion. The ion diffuses to the other side of the membrane.

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Factors affecting the rate of simple diffusion

1) CONCENTRATION GRADIENT: the higher it is, the faster the rate of diffusion.
As diffusion occurs the gradient (difference between concentrations of both sides) decreases until equilibrium is reached, so it slows over time.

2) THICKNESS IF EXCHANGE SURFACE: the thinner it is, the faster the rate of diffusion.
As the particles have less of a distance to travel.

3) SURFACE AREA: the larger it is, the faster the rate of diffusion.
As more particles can be exchanged in a given time.

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Factors affecting the rate of facilitated diffusio

1) CONCENTRATION GRADIENT: the larger it is, the faster the rate of facilitated diffusion up to a certain point. As equilibrium is reached, diffusion levels off (slows).

2) NUMBER OF CARRIER / CHANNEL PROTEINS: the more, the faster the rate of F.diffusion.

Once they are all in use, facilitated diffusion cannot occur any faster (even with a large concentration gradient).

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Osmosis

The passage of water molecules from a region of high to low water potential, across a selectively permeable membrane.

Water potential= the pressure created by water molecules (measured in kilo pascals)

Pure water has the highest water potential of 0kPa.

A very concentrated solution (little water, lots of solute) has a low water potential of -1000kPa.

So the MORE NEGATIVE the value, the MORE CONCENTRATED the solution and LOWER the water potential.

2 solutions with the same water potential are said to be ISOTONIC.

To test the water potential of cells/tissues; place them in a series of solutions with different water potentials. Where there is no net gain/loss, the water potential of the cells/tissues must be the same as that of the solution.

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Factors affecting the rate of osmosis

1) CONCENTRATION GRADIENT: the higher it is, the faster the rate of osmosis.
As osmosis occurs, the gradient decreases until equilibrium is reached. So the rate levels off (slows)

2) THICKNESS OF THE EXCHANGE SURFACE: the thinner it is, the faster the rate of osmosis as the water molecules have a shorter diffusion pathway.

3) SURFACE AREA OF EXCHANGE SURFACE: the larger it is, the faster the rate of osmosis.

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Active transport

An active process: requires metabolic energy, in the form of ATP

The movement of molecules/ions from a region of low to high concentration, using ATP and carrier proteins.

  • AGANIST a concenrptration gradient (low to high)
  • a very SELECTIVELY process, only specific substances are transported.

1) the molecule/ion to be transported binds with receptor sites on carrier proteins that span the plasma membrane

2) on the inside of the cell/organelle ATP binds to the carrier protein, causing it to split into ADP and a phosphate molecule, also releasing energy.

3) the phosphate molecule causes the carrier protein to change shape and opens to open opposite side of the membrane, transporting the molecule/ion across

4) the phosphate is then released from the protein, causing it revert back to its original shape so the process can be repeated.

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Osmosis in Plant cells

CENTRAL VACUOLE: contains a solution of salts, sugars and organic acids in water
PROTOPLAST: consists of the outer cell surface membrane, nucleus, cytoplasm and inner vacuole membrane
CELLULOSE CELL WALL: tough, inelastic covering- permeable even to large molecules

Plant cells consist of a variety of so lutes dissolved in the central vacuole.

When placed in PURE WATER; water enters the vacuole by osmosis due to its lower water potential (water is also constantly absorbed by the plants roots)
This causes the PROTOPLAST to swell and press against the cell wall. The cell wall is inelastic so it's limited to expansion. A pressure therefore builds up on it that resists any further entry of water.
The cell is turgid as the PROTOPLAST in pushed against the cell wall.

If placed in a solution with a lower water potential, then the cells water leaves by osmosis the volume of the cell decreases and the protoplast no longer pushes against the cell wall- INCIPENT PLASMOLYSIS.
Further water loss causes the cell to shrink and the PROTOPLAST pulls away from the cell wall. The cell is PLASMOLYSED.

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Osmosis in Animal cells

If cells are placed in a solution with LOWER WATER POTENTIAL than the cell solution: water leaves the cell by osmosis and it shrinks and shrivels

If cells are placed in a solution with the SAME WATER POTENTIAL as the cell solution: there's no movement of water.

If cells are placed in a solution with HIGHER WATER POTENTIAL than the cell solution: water enters the cell by omosis resulting in the cell swelling and bursting- becoming loses.

& ...
If red blood cells are placed in a higher water potential solution, water will enter by osmosis. Since the cell surface membrane is very thin, although flexible they can't stretch to a great extent. The CSM will break, bursting the cell and releasing its contents (= haemolysis).

To prevent this, animal cells usually live in solutions with the same water potential as the cells solution, e.g. Blood plasma.

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Factors affecting the rate of active transport

1) CONCENTRATION GRADIENT: increasing this, doesn't affect the rate of active transport

2) AMOUNT OF CARRIER PROTEINS: the more carrier proteins, the faster the rate of active transport as more molecules/ions can be transported in a given time

3) SPEED OF CARRIER PROTEINS: the faster they are, the faster the rate of active transport as they transport the molecules/ions quicker

4) RATE OF RESPIRATION: the faster the rate of respiration the faster the rate of active transport as respiration produces ATP which hydrolyses into ADP and an inorganic phosphate molecule, releasing the energy needed to active transport.

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