Transport Across the Plasma Membane

Transport across the plasma membranes of plant and Animal cell is Important to cells for

• obtaining oxygen and nutrients (to supply energy and raw materials for synthesis);

• excreting toxic substances (result of metabolic activities); and

• maintaining a suitable pH and solute concentration (for enzymes activity).

    There are four methods of entry into or exit from cells. These are diffusion, osmosis, active transport and endocytosis or exocytosis.

Diffusion and osmosis

These are passive processes and do not require energy. They only depend on a concentration gradient.

Active transport

Sodium ions are in low concentration in human red blood cell and in high concentration in the plasma. Yet, sodium ions are transported out of the cell into the plasma, against the existing concentration gradient. In a similar manner, pottssium ions are transported from the plasma into the cell. This process is known as active transport and requires energy. It oftens occurs against a concentration gradient as in the above example. Active transport is particularly important in cells lining the gut, where absorption of digested food lining the gut, where reabsorption occurs. Cells varying out active transport have

• numerous Mitochondria

• a high concentration of ATP (immediate energy store), and 

• a high cellular respiratory rate.

Endocytosis and exocytosis

 In these processes, bulk materjad enter and leave the cell. In endocytosis, an infolding of the as.a membrane takes up material into the CE kna vesicle or vacuole. In exocytosis, materials are transported out of the cells by an extension of the plasma membrane. These processes require energy.

   The term phagocytosis is used when solid materials are taken into the cell by engulfing them. This is how

• an Ameoba captures it's food; and

• a white blood cell engulfs bacteria that invade the body.

     The term pinocytosis is used when liquid materials are taken into the cell. Both phagocytosis and pinocytosis are endocytotic processes.

    Hormones and enzymes are secreted from the cells in which they are made by reverse pinocytosis (exocytosis).