Transport Systems

Movement of substances across a membrane §

For cells to stay alive, they:

1. Require nutrients and water to enter the cell
2. need oxygen for cellular respiration and produce energy
3. get rid of waste matter

Structure of membrane:

• Selectively permeable

Function:

• Separates cell contents from external environment
• Controls movement of substances into and out of cell

What can pass through the cell surface membrane?

• Small soluble substances

Diffusion §

Net movement of particles from a region where they are of higher concentration to a region where they are of lower concentration.

!!! DIFFUSION CAN OCCUR THROUGH A MEMBRANE BUT DOES NOT REQUIRE MEMBRANES !!!

Factors affecting rate of diffusion §

• Temperature
  • Temp goes up, diffusion goes up
• Surface area to Volume ratio
  • SA:V goes down, diffusion goes down
  • Cells have increased SA:V to have better absorption
• Concentration gradient
  • As difference in concentration goes up, rate of diffusion goes up
• Mass of particles (not tested)

E.g. shorter distance between cytoplasm and external environment -> faster diffusion

DIFFUSION CAN ALSO OCCUR IN SOLIDS!!!!

Osmosis §

Net movement of water molecules through a partially permeable membrane from a region of higher water potential to lower water potential.

• Water potential -> Tendency of water to leave
  • More dilute = higher water potential

Effects on cells §

• Plant Cells
  • Water enters
    • Cell becomes turgid
    • Cell wall holds cell together
  • Water leaves
    • Cell becomes pasmolysed
• Animal Cells
  • Water enters
    • Cell expands and bursts
    • lack of cell wall
  • Water leaves
    • cell becomes crenated

Active Transport §

Factors: §

• mitochondria
• oxygen