MOVEMENT ACROSS THE PLASMA MEMBRANE

Diffusion, Osmosis, Active Transport

There are two ways in which substances can enter or leave a cell:

1) Passive
a) Simple Diffusion
b) Facilitated Diffusion
c) Osmosis (water only)

2) Active
a) Molecules
b) Particles

Diffusion

Diffusion is the net passive movement of particles (atoms, ions or molecules) from a region in which they are in higher concentration to regions of lower concentration. It continues until the concentration of substances is uniform throughout. Some major examples of diffusion in biology:

• Gas exchange at the alveoli — oxygen from air to blood, carbon dioxide from blood to air.
• Gas exchange for photosynthesis — carbon dioxide from air to leaf, oxygen from leaf to air.
• Gas exchange for respiration — oxygen from blood to tissue cells, carbon dioxide in opposite

direction.
• Transfer of transmitter substance — acetylcholine from presynaptic to postsynaptic membrane

at a synapse.
• Osmosis — diffusion of water through a semipermeable membrane.

High Diffusion Rate: short distance, large surface area, big concentration difference (Fick’s Law).
High temperatures increase diffusion; large molecules slow diffusion.

Facilitated Diffusion

This is the movement of specific molecules down a concentration gradient, passing through the membrane via a specific carrier protein. Thus, rather like enzymes, each carrier has its own shape and only allows one molecule (or one group of closely related molecules) to pass through.
Selection is by size; shape; charge.
Common molecules entering/leaving cells this way include glucose and amino-acids.
It is passive and requires no energy from the cell.
If the molecule is changed on entering the cell (glucose + ATP → glucose phosphate + ADP), then the concentration gradient of glucose will be kept high, and there will a steady one-way traffic.

Osmosis

Osmosis is a special example of diffusion. It is the diffusion of water through a partially permeable membrane from a more dilute solution to a more concentrated solution – down the water potential gradient)
Note: diffusion and osmosis are both passive, i.e. energy from ATP is not used.
A partially permeable membrane is a barrier that permits the passage of some substances but not others; it allows the passage of the solvent molecules but not some of the larger solute molecules.
Cell membranes are described as selectively permeable because not only do they allow the passage of water but also allow the passage of certain solutes. The presence of particular solutes stimulates the membrane to open specific channels or trigger active transport mechanisms to allow the passage of those chemicals across the membrane.

Some major examples of osmosis
• Absorption of water by plant roots.
• Re-absorption of water by the proximal and distal convoluted tubules of the nephron.
• Re-absorption of tissue fluid into the venule ends of the blood capillaries.
• Absorption of water by the alimentary canal — stomach, small intestine and the colon.

Osmoregulation

Osmoregulation is keeping the concentration of cell cytoplasm or blood at a suitable concentration.
(a) Amoeba, living in freshwater, uses a contractile vacuole to expel the excess water from its cytoplasm (thus need more respiration/O2/ATP than isotonic (marine) Amoebae).

(b) The kidneys maintain the blood (thus, whole body) at the correct concentration.

Osmosis and Plant Cells

(a) Plant Cells in a hypotonic (= weaker) solution – cells have lower water potential
• the plant cells gain water by osmosis.
• the vacuole and cytoplasm increase in volume.
• the cell membrane is pushed harder against the cell wall causing it to stretch a little.
• the plant tissue becomes stiffer (= turgid).

(b) Plant Cells in a hypertonic (=stronger) solution – cells have higher water potential
• the plant cells lose water by osmosis.
• the vacuole and cytoplasm decrease in volume.
• the cell shrinks away from the cell wall.
• shrinkage stops when the cell sap is at the same concentration as the external solution.
• the plant tissue becomes flaccid, it has shrunk slightly
• may go on to become plasmolysed.

Turgor

Turgor is the pressure of the swollen cell contents against the cell wall when the external solution more dilute than the cell sap of the vacuole.
Role of Turgor in Plants
• Mechanical support for soft non-woody tissue, e.g., leaves.
• Change in shape of guard cells forming the stomatal opening between them.
• Enlargement of young immature plant cells to mature size.