Diffusion, Osmosis, active transport

Explaining the importance of substances move in and out of cells

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  • Movement of molecules or ions from an area of high concentration- area of low concentration-Diffuse down a concentration gradient.
  • Process used in oxygen entering a cell, and carbon dioxide leaving a cell. 
  • Requiresno energy as always down the concentration gradient. (Natural kinetic energy causes then to move around randomly)

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  • Movement of water.
  • from a high water potential(less concentrated solution) to a low water potential (more concentrated solution)
  • needs only the kinetic energy of the particles themselves. 
  • move through a semi-permeable membrane-only allows certain molecules through ie water molecules= they will cross until an equilibration is reached. 

In cells

if water potential higher= gain water swell and bursts.

If lower-will  lose water shrivel up and burst. 


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Facilated Diffusion

Movement of specific molecules across cell membranes through protein channels.

Channel Protein:

  • Line a water-filled pore in the membrane so water soluble molecules can easily pass through. 
  • particular shapes so only particular molecules move through. 
  • e.g chloride ions.

Carrier Protein:

  • Can change its shape to make diffusion possible. 
  • has binding sites for particular solute molecules, these can enter and leave the site randomly. 
  • moving down the conc. gradient so no energy required. 
  • eg glucose, sodium ions
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Active Transport

  • Active transport is the energy-demanding transfer of a substance across a cell membrane against its concentration gradient eg low to high. .
  • Special proteins within the cell membrane act as specific protein ‘carriers’. 
  • The energy for active transport comes from ATP generated by respiration (in mitochondria).
  • eg Na ions diffuse from the gut lumen in small intestine >cell (taking glucose with it)> concentration. of gradient inside cell increases and glucose diffuses out of the cell by facilitated diffusion. 
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Bulk Transport


Into the cells

Involves engulfing of the material by the cell surface membrane. 


Out of cells

Process by which materials are removed from the cells. 


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