Membrane Transport

Passive Transport and Membrane Transport

Cells can predominantly control the movement of substances in and out due to the selectively permeable nature of membranes.

Key Definitions

• Solute: Ion or molecule being transported across the membrane.

1. Passive Transport

• Definition: Passive transport does not require energy during the transport process.

  • The cell does not expend energy to execute the transport.

  • Solutes move from areas of high concentration to areas of lower concentration, described as moving "down the concentration gradient."

A. Simple Diffusion

• Concept: Movement of solute through the phospholipid bilayer.

  • Characteristics of Solutes for Simple Diffusion:

  • Hydrophobic

  • Non-Polar

  • Lipid Soluble (small molecules)

• Examples of Molecules:

  • Steroid hormones, O$2$, CO$2$, urea.

B. Facilitated Diffusion

• Concept: Allows solutes to cross the membrane with the help of proteins specific to the solute.

• Types of Transport Proteins:

  1. Channel Proteins:

  • Function: Act as tunnels for solutes.

  • Characteristics: Do not change shape during transport.

  • Examples: Sodium (Na$^+$) channels, Potassium (K$^+$) channels, Calcium (Ca$^{2+}$) channels, Chlorine (Cl$^-$) channels, aquaporins (water channels).

  1. Carrier Proteins:

  • Function: Act as revolving doors for transport.

  • Characteristics: Change shape during transport to facilitate movement.

  • Examples: Transport glucose, amino acids, and other small to medium-sized molecules.

C. Osmosis Diffusion

• Concept: The movement of water across a membrane.

• Mechanism: Water tends to move towards regions of higher solute concentration.

• Methods of Water Transport:

  • Movement through the lipid bilayer, via aquaporins, or through processes like endocytosis and exocytosis.

Key Percentages of Osmosis:

• Represents movement and concentration: 90% - 70% - 10% - 30% across membranes (specific contexts needed).

2. Active Transport

• Definition: Active transport requires energy at the time of transport.

  • The cell must perform work, commonly utilizing ATP as the energy source.

  • Solutes generally move from areas of low concentration to areas of high concentration, described as moving "up the concentration gradient."

A. Primary Active Transport

• Example: Sodium-Potassium Pump (Na$^+$-K$^+$ Pump)

  • Function: Moves Sodium ions (Na$^+$) outside the cell and Potassium ions (K$^+$) inside the cell.

  • Characteristics: Proteins that act as pumps, thus also known as ATPases.

  • Direction of Movement: Both ions are moved against their respective concentration gradients using ATP.

B. Secondary Active Transport (Indirect Active Transport)

• Concept: Utilizes the energy from the concentration gradient of a different solute to drive the transport of solute A.

C. Bulk or Vesicular Transport

• Overview: Involves the transport of large molecules across the membrane, which requires energy.

• Key Processes in Bulk Transport:

  1. Exocytosis:

  • Process where a vesicle moves to the plasma membrane and fuses with it.

  • Outcome: Allows increased surface area of the membrane and the release of vesicle contents outside the cell (e.g., waste solutes).

  1. Endocytosis:

  • Process where a vesicle forms from the plasma membrane and brings material into the cytosol, reducing the surface area of the membrane.

  • Types of Endocytosis: a. Pinocytosis:

    • Concept of random uptake of solutes that are trapped within the vesicle.

    • Note: This process is performed by all cells.
      b. Receptor-Mediated Endocytosis:

    • A more deliberate uptake of specific solutes using specific membrane protein receptors.

    • Also performed by all cells.
      c. Phagocytosis:

    • Known colloquially as "cell eating."

    • Occurs in specialized cells (e.g., amoebas, human macrophages, neutrophils).

    • Mechanism: Phagosomes are formed (food vesicles) and fuse with lysosomes for digestion.