2.1.7 Active Transport

• Active transport is the movement of molecules and ions through a cell membrane from a region of lower concentration to a region of higher concentration
  • Active transport requires energy in the form of ATP from respiration to move substances against their concentration gradient; hence this is an active process
• Active transport requires carrier proteins
  • Each carrier protein is specific to a particular type of molecule or ion
• Energy is required to allow the carrier protein to change shape; this transfers the molecules or ions across the cell membrane
  • The energy required is provided by ATP (adenosine triphosphate) produced during respiration
  • The ATP is hydrolysed to release energy

Active transport moves substances across a membrane from low to high concentration. Note that ATP is required for carrier proteins to change shape.

• Examples of active transport include
  • Reabsorption of useful molecules and ions into the blood after filtration into the kidney tubules
  • Absorption of some products of digestion from the digestive tract into the blood
  • Loading sugar from the photosynthesising cells of leaves into the phloem tissue for transport around the plant
  • Loading inorganic ions from the soil into root hair cells

Endocytosis

• Some molecules are too large to travel via membrane proteins, e.g.
  • Proteins
  • Lipids
  • Some carbohydrates
• In such cases a cell can surround a substance with a section of the cell surface membrane
  • The membrane engulfs the substance and pinches off inside the cell to form a temporary vacuole with the ingested substance contained inside
  • This is endocytosis
    • Phagocytosis is an example of endocytosis
• Endocytosis is an active process and requires a source of energy

Phagocytosis is an example of endocytosis; the cell surface membrane extends around a pathogen, engulfing it and enclosing it within a temporary vacuole inside the cell

Exocytosis

• Some substances produced by the cell need to be secreted, such as hormones, some enzymes, and lipids
• Vesicles containing the substance pinch off from sacs of the Golgi apparatus 
• These vesicles are moved toward the cell surface and fuse with the cell surface membrane to be released outside the cell
  • This is exocytosis
• Exocytosis is an active process and requires a source of energy

The active processes of endocytosis and exocytosis

Exocytosis allows the bulk secretion of substances from cells