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3.2.3 Transport Across Cell Membranes

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3.2.3 Transport Across Cell Membranes

Eukaryotic cells contain many membrane bound organelles and all cells including prokaryotes are surrounded by a plasma membrane

Plasma Membrane - flexible and self sealing

As a result they can maintain all their own conditions - pH, enzymes and other substances can be different in each organelle - therefore many different reactions can take place

There may be a single phospholipid bilayer (cell membrane and vesicles, vacuoles and some organelles) or there may be a double bilayer (nucleus, mitochondria & chloroplasts)

Plasma Membrane - 3 functions :

  • To control the transport of substances into and out of the cell or organelles - allow certain molecules to pass through (they are partially permeable) - plants and prokaryotes = cell wall as well which is thicker and freely permeable - large pores

  • To act as a receptor site to recognise chemicals that need to the enter the cell or organelle

  • To separate off the cell from the environment and the different reactions of the cell from each other by forming the organelles - different concentrations can be maintained on either side of the membrane

Cell Membranes

Membranes and made up of phospholipids

In water they form a two-layered structure called a bilayer where hydrophobic tails point inwards and the hydrophilic heads point out into the water

Membranes are partially permeable - smaller and less polar = faster diffusion rate across a cell membrane

  • Small, non-polar molecules - oxygen, co2 rapidly diffuse across a phospholipid bilayer

  • Small, polar molecules such as water also diffuse across but much slower

  • Charged particles such as Na+ and K+ are unlikely to diffuse across even if they are small

Large, polar, water-soluble molecules are therefore highly unlikely to be able to pass through the phospholipids

Intrinsic Proteins

span across the whole membrane and are used for transport

  • Protein carriers

  • Hydrophilic protein channels

Extrinsic Proteins

Sit on the inner or outer surface and do not go the whole way through the membrane

  • Provide structural support, binding cells together

  • Form recognition sites (also glycoproteins)

  • Act as receptor sites on cell surface for chemicals (hormones, etc) to bind to - how cells detect chemicals from other cells

  • Enzymes

Glycoproteins

  • Short, branching carbohydrate chains are attached to proteins in the membrane

  • These glycoproteins act as receptors for hormones and neurotransmitters or antigens

Glycolipids

  • A carbohydrate bonded to a phospholipid

  • They act as recognition sites

  • They also form hydrogen bonds with the water surrounding the membrane which helps to maintain the stability of the membrane and help cells the attach to one another, so forming tissues

Cholesterol

  • Cholesterol molecules sit between the hydrophobic tails in the phospholipid membrane

  • They restrict the movement of molecules in the membrane

  • The more cholesterol the less fluid and more rigid the membrane is. This is especially important at body temperature, when a membrane would become too fluid and break apart without cholesterol.

Fluid-Mosaic Model

  • Fluid refers to the fact that all the different molecules can move around

  • Mosaic because there are different types of molecules - the proteins move sideways within the phospholipid bilayer

The fluidity means that the cells are able to change their shape. The ease with which they do this is dependent on the number of phospholipids with unsaturated fatty acids in the phospholipids.

Membrane and Temperature