Cell Membrane Transport Mechanisms Quiz

Study Notes

Cell membranes are semi-permeable. They allow the passage of lipid-soluble substances but prevent water- and water-soluble ones.
Proteins embedded in the membrane (integral or peripheral) act as channels, carriers, pumps, enzymes, receptors, etc. and facilitate the passage of water and water-soluble substances.
Transport mechanisms are classified as passive or active.

Passive transport does not require energy.
Mechanisms within passive transport include:
- Simple diffusion
- Facilitated diffusion
- Others

Simple diffusion is the movement of a substance from a region of higher concentration to a region of lower concentration (down its concentration gradient or electrical gradient).
Energy is not needed.
Protein carriers are not needed.
It follows Fick's law of diffusion. This law describes the rate of diffusion of a substance across a membrane
- J = -DA*(ΔC/ΔX). Where:
  - J = net flux of solute particles (or diffusing tendency)
  - D = Diffusion coefficient
  - A = surface area available for diffusion
  - ΔC = difference in concentration
  - ΔX = thickness
Examples include diffusion of lipid-soluble substances, gases like O2 and CO2, and other non-polar substances.
Certain substances diffuse directly or through protein channels passively. This is sometimes considered facilitated diffusion.

Facilitated diffusion differs from simple diffusion in that protein carriers are needed.
It has a maximum rate of transport (Tmax) that depends on the number of carriers.
The maximum rate is reached when all the carriers are saturated.
Examples include the transport of glucose and amino acids in the kidney and intestine through the basolateral membrane using GLUT and SGLT protein carriers.

Active transport requires energy.
Mechanisms within active transport include:
- Primary active transport
- Secondary active transport
- Exocytosis
- Endocytosis

A substance is transported against its chemical or electrical gradient, with energy consumption and with a carrier protein.
Examples include the sodium-potassium pump with the use of ATP.

A substance is transported against its gradient in association with another substance transported down its gradient.
The transport may occur in the same direction as the driving substance (symport or co-transport) or in the opposite direction (antiport).
Examples include co-transport (e.g., glucose) and antiport (e.g., hydrogen).

Exocytosis is the process of exporting substances from the cell. A granule or vesicle moves to the cell membrane, its membrane fuses with the cell membrane, the area of fusion breaks down, and the contents of the granule or vesicle are released outside.
Important for secretion of proteins synthesized within the cell

Endocytosis is the process of importing substances into the cell, which is the reverse of exocytosis.
Two main types
- Phagocytosis: For solid substances, like bacteria..
- Pinocytosis: For soluble substances.