Active Transport & Bulk Transport (HL) (HL IB Biology)

• The processes of diffusion, osmosis and active transport are responsible for the transport of individual molecules or ions across cell membranes
• However, the bulk transport of larger quantities of materials into or out of cells is also possible
• Examples of these larger quantities of materials that might need to cross the membrane include:
  • Bulk transport into cells = endocytosis
  • Bulk transport out of cells = exocytosis
• Bulk transport processes require energy and are therefore forms of active transport
• They also require the formation of vesicles, which is dependent on the fluidity of membranes
  • Vesicles are small spherical sacs of plasma membrane that containing substances for transport, e.g. enzymes
  • The formation of vesicles is an active process and involves a small region of the plasma membrane being pinched off
  • Vesicles can also fuse with cell membranes, at which point they are re-incorporated into a larger membrane
  • In order to form from or fuse with membranes, vesicles need membranes to flex and bed, so fluidity is essential

Endocytosis

• Endocytosis transports material into cells
• During endocytosis the plasma membrane engulfs material, forming a small sac around it
• There are two forms of endocytosis:
  • Phagocytosis:
    • This is the bulk intake of solid material by a cell
    • Cells that specialise in this process are called phagocytes
    • The vacuoles formed are called phagocytic vacuoles
    • An example is the engulfing of bacteria by phagocytic white blood cells
  • Pinocytosis:
    • This is the bulk intake of liquids

Endocytosis diagram 

Phagocytosis is an example of endocytosis

Exocytosis

• Exocytosis is the process by which materials are removed from, or transported out of, cells
  • It is the reverse of endocytosis
• The substances to be released are packaged into secretory vesicles
• These vesicles then travel to the cell surface membrane
• Here they fuse with the cell membrane and release their contents outside the cell
• An example is the secretion of digestive enzymes from pancreatic cells

Exocytosis diagram

Exocytosis involves the fusion of a vesicle with the cell surface membrane

• Specialised ion channels, called gated ion channels, are present in some cell membranes
  • These channels operate in response to chemical or electrical stimuli

Nicotinic acetylcholine receptors

• Nicotinic acetylcholine receptors are an example of a gated ion channel, more specifically a neurotransmitter-gated ion channel
• The neurotransmitter acetylcholine can bind to nicotinic acetylcholine receptors which triggers the ion channel to open allowing certain ions, such as calcium (Ca²⁺) or sodium (Na⁺), to pass through
• The influx of ions causes the membrane potential to change; this can generate an action potential in neurones
• Nicotinic acetylcholine receptors are found specifically at the neuromuscular junction; the point at which nerve cells connect to muscles

Nicotinic acetylcholine receptor diagram

Nicotinic acetylcholine receptors are an example of a gated ion channel

• Sodium-potassium pump proteins are integral proteins that generate an electrochemical gradient between the inside and outside of a nerve cell
• Sodium-potassium pumps are an example of an exchange transporter
  • The sodium-potassium pumps move three sodium ions out of the cell and two potassium ions into the cell using one ATP molecule
  • The pumps are always moving the ions against their concentration gradient via active transport
• The steps that occur during the pumping process are:
  1. Three sodium ions from the inside of the axon bind to the pump
  2. ATP attaches to the pump and transfers a phosphate to the pump (phosphorylation), causing it to change shape and resulting in the pump opening to the outside of the axon
  3. The three sodium ions are released out of the axon
  4. Two potassium ions from outside the axon enter and bind to their sites
  5. The attached phosphate is released altering the shape of the pump again
  6. The change in shape causes the potassium ions to be released inside the axon
• This process is essential to the function of nerve cells
  • The sodium-potassium pumps transport more positively charged sodium ions to the outside of the cell than positively charged potassium ions to the inside; the inside of the cell is therefore negatively charged in comparison to the outside
  • When nerve cells are stimulated, sodium ion channels open and sodium ions rush in down the electrochemical gradient, reversing the charge across the membrane
  • This can lead to the generation of a nerve impulse

Sodium-potassium pump diagram

Sodium-potassium pumps use ATP to transport sodium and potassium ions across cell membranes

Cotransport & indirect active transport

• Co-transport is the coupled movement of substances across a cell membrane via a carrier protein
  • Coupled processes occur at the same time and do not occur independently of each other
• Cotransport involves a combination of facilitated diffusion and indirect active transport
  • Indirect active transport uses the energy released by the movement of one molecule down its concentration gradient to move another against its concentration gradient
  • ATP is used to set up the initial gradient

Sodium-dependent glucose co-transport

• A well-known example of a co-transporter protein can be found on the cell surface membrane of the epithelial cells lining the mammalian ileum
• This specific sodium-dependent glucose co-transporter protein is involved in the absorption of glucose into the blood 
  1. Sodium-potassium pumps actively transport sodium ions into the blood, reducing the concentration of sodium ions in the cell
  2. Sodium ions move down their concentration gradient into the cell via a cotransporter protein
  3. Glucose is drawn into the cell along with sodium ions via the same cotransporter protein
     • Glucose moves against its concentration gradient
  4. Glucose then moves down its concentration gradient into the blood
• The active part of the process is the generation of the initial sodium ion gradient; the transport of glucose itself does not require energy; this is why the process is described as indirect active transport

Co-transport in the small intestine diagram

Both facilitated diffusion and active transport occur during co-transport. Glucose molecules can only enter the epithelial cell when sodium ions are present.

• This process also takes place in the kidney
  • Reabsorption of glucose back into the blood is under the control of sodium-dependent glucose cotransporter proteins
  • Glucose is co-transported with sodium ions in the way described above