Module 1 - Cells

Exchanges across membranes

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  • Created by: Nick
  • Created on: 08-05-10 16:34

Diffusion - Passive process

Diffusion: The net movement of molecules or ions in a gas or liquid from an area of high concentration to an area of low concentration to an area where they are less concentrated.

  • Occurs passively i.e. no ATP is required by the cell to move substances.
  • Substances move down a cells concentration gradient.
  • Molecules such as oxygen and carbon dioxide diffuse across membranes as long as a concentration gradient exists. (As both molecules are uncharged, they diffuse through the phospholipid bilayer very easily).
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Osmosis - Passive process

Osmosis: The movement of water molecules from a region of higher water potential to a region of lower water potential across a partially permeable membrane.

The movement of water into and out of cells is influenced by:

  • The amount of water present in the cytoplasm and in the exterior environment.
  • The concentration of solutes, such as ions and sugars, on either side of the cell surface membrane.
  • The presence of aquaporins in membranes.
  • (Aquaporins - are special, highly selective channel proteins for water which make cells very permeable to water. e.g. red blood cell)
  • (In plants) the pressure exerted on cell contents by the cell wall, which is rigid and resists expansion ans thus the uptake of water.
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Water potential

Water potential - A measure of the ability of water molecules to move freely in solution. Measures the potential for a solution to lose water - water moves from a solution with high water potential to one of lower water potential. Water potential is decreased by the presence of solutes.

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Facilitated diffusion (channel proteins & carrier

Facilitated diffusion: The passive movement of molecules across membranes down their concentration gradient, which is aided by transport (carrier) protein molecules, No metabolic energy is required.

Channel proteins:

  • Are the span of the membrane.
  • They form tunnels (or pores) through the bilayer for water-soluble molecules.
  • Some channels are open all the time, others need to be triggered by cell signalling.
  • The lining of he pore allows water and polar substances to pass through.

Carrier proteins:

  • Also the span of the membrane.
  • Change shape to help move molecules into or out of the cell.
  • The molecules bind to the protein, which stimulates the protein to change its overall shape, so allowing the molecules to pass through the membrane.
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Active transport - Active process

Active transport: Movement of substances across membranes against their concentration gradient, requiring the use of energy in the for of ATP. Active transport usually involves the use of transport proteins.

  • Carrier proteins use energy from ATP made in respiration to move substances against their concentration gradient.
  • Root hair cells absorb nutrients, such as potassium ions, from the water in the soil. Active transport is also used to pump molecules and ions out of cells.
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Endocytosis & Exocytosis (bulk transport) - Active

Endocytosis:

  • The process of taking materials into a cell by surrounding them with part of the plasma membrane, which then pinches off to form a vesicle inside the cell.
  • Some cells take up large molecule e.g. proteins or larger objects e.g. bacteria.

Exocytosis:

  • The process of removing materials from the cell by fusing vesicles containing the material with the plasma membrane (cell surface membrane).
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How various water potentials affects animal and pl

Animal cells

  • Distilled water (higher water potential than cells)
  • = Water moves into cells by osmosis.
  • = Cells swell. Cells burst as there is no cell wall and the cell membrane is not strong enough to withstand the pressure. It is haemolysed (burst open)
  • Dilute solution of salt or sugar (same water potential as cells)
  • = No net movement of water into or out of cells.
  • = Cells remain the same shape and volume.
  • Concentrated solution of salt or sugar (lower water potential than cells)
  • = Water moves out of cells by osmosis.
  • = Cells decrease in volume and shrink. Becomes crenated (shrunken and wrinkled)
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How various water potentials affects animal and pl

Plant cells

  • Distilled water (higher water potential than cells)
  • = Water moves into cells by osmosis.
  • = Cells become turgid. Cell wall prevents any more water entering.
  • Dilute solution of salt or sugar (same water potential as cells)
  • = No net movement of water into or out of cells.
  • = Cells remain the same shape and volume.
  • Concentrated solution of salt or sugar (lower water potential than cells)
  • = Water moves out of cells by osmosis.
  • = Plasmolysis - vacuole shrinks, pulling cytoplasm and cell membrane away from cell wall. Cell becomes plasmolysed.
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