Active Transport:

• Active transport is the energy-consuming transport of molecules or ions across a membrane against a concentration gradient.
• Energy is required because the substance must be moved against its natural tendency to diffuse in the opposite direction.
• Movement is usually in one direction only, unlike diffusion which is reversible.
• Active transport is achieved by carrier proteins situated in the cell surface membrane.
• The carrier proteins involved in active transport need a supply of energy to keep changing shape which is provided by ATP from respiration.

Sodium potassium Exchange Pump (Na⁺ - K⁺ pump)

• The pump is a carrier protein which spans the membrane from one side to the other.
• On the inside it accepts sodium and ATP, while on the outside it accepts potassium. For every 2K⁺ taken into the cell, 3Na⁺ are removed.

Bulk Transport:

• Endocytosis and exocytosis are active processes involving the bulk transport of materials through membrane, either into cells (indocytosis) or out of cells (exocytosis).
• Endocytosis occurs by an infolding or extension of the cell surface membrane to form a vesicle or vacuole. It is of two types:

• Phagocytosis (cell eating) Material taken up is in solid form. Cells specializing in the process are called phagocytes and are said to be phagocytic. e.g., some white blood cells take up bacteria by phagocytosis. The sec formed during uptake is called a phagocytic vacuole.
• Pinocytosis (cell drinking) Material taken up is in liquid form. Vesicles formed are often extremely small. Pinocytosis is used by the human egg cell to take up nutrients from the surrounding follicle cells.

Exocytosis is the reverse process of endocytosis. Waste materials may be removed from cells, such as solid, undigested remains from phagocytic vacuoles. Or useful materials may be secreted. Secretion of enzymes from the pancreas is achieved in this way. Plant cells use exocytosis to export the materials needed to form cell walls.