Transport across cell membranes

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Transport across cell membranes
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Reminder

• Cell walls are non living, slightly elastic and
  permeable.
• Cell membrane (plasma) is semi-permeable. It is
  made of fats and lipids. The fats are arranged
  hydrophilic head out. Embedded in the fluid
  layer are proteins some are tube like, some
  act as receptors, some actively pump molecules,
  some are structural.

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• PASSIVE TRANSPORT

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Diffusion

• Particles are always in motion. The tendency of
  particles is to move to fill up a space. In
  biology terms, diffusion is the net movement of
  molecules from an area of high concentration to
  an area of low concentration.

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• The difference in concentration in the two areas
  is the concentration gradient. A large
  difference gives a steep gradient and fast
  diffusion.
• No energy is used to move the molecules.

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• Rate varies depending on
• size of molecules (smaller faster)
• temperature of liquid or gas (warmerfaster)
• state being diffused (gas faster than liquid)
• concentration of chemical

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Facilitated Diffusion

• Some proteins in cell membrane help the diffusion
  of certain chemicals across the membrane faster
  than others glucose is carried faster than many
  others. The transport proteins are specialized
  to the type of chemical they can carry.

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Osmosis

• Net movement of water from hi to low
  concentration through a semi-permeable membrane.
  (only water!)
• A weak solution is hypotonic, a strong solution
  is hypertonic and if the same they are isotonic.

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Osmoregulation

• The control of water in cells is very important
  as water passes rapidly across cell membranes.

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• In plants the vacuole acts as a fluid skeleton
  too little and the plant becomes floppy , too
  much and it becomes crisp.
• Single celled organisms constantly have water
  flooding them so use a contractile vacuole to
  pump water out.
• Freshwater fish can drown due to water flooding
  their cells so produce copious amounts of urine.
• Saltwater fish either keep their bodies at the
  same saltiness as the sea or drink the seawater
  and extract salt.
• Humans have water-resistant skin, so dont need
  to worry!

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Active transport

• Movement against a concentration gradient from
  low to high. This process uses energy we can
  tell when energy is used if heat is given off, O2
  is used, CO2 is made or glucose is used. If
  cells have a large number of mitochondria then it
  is likely to have a high energy requirement.

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Endocytosis and exocytosis

• Some cells can engulf large particles
  (endocytosis). If the particle is solid we call
  it phagocytosis and if liquid pinocytosis.
• Exocytosis is
• the reverse process
• getting rid of waste
• particles or transporting
• materials out of the cell.

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Cell size

• Cell size is important in diffusion the larger
  a cell is the more difficult it is to diffuse
  substances in and out. Cells normally grow until
  they double their size and then divide. To have
  efficient diffusion you need cells to have a high
  SA/Vol ratio. This is why cells divide the
  volume drops but the surface area increases so
  the ratio goes up.

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