Cell Membrane Transport: Osmosis, Diffusion & Bulk Transport

Cellular Passive Transport Mechanisms

Osmosis: Water Movement Across Membranes

Osmosis refers to the passage of water through a semi-permeable membrane from a region of higher water concentration (purer) to one of lower water concentration (less pure).

Osmosis Effects in Animal Cells

• In an isotonic solution, there is a dynamic equilibrium; water enters and leaves the cell at the same rate.
• In a hypotonic solution, the cell absorbs water and swells, potentially bursting (a process called cytolysis).
• In a hypertonic solution, the cell loses water, shrivels, and becomes dehydrated (a process called crenation), which can lead to cell death.

Osmosis Effects in Plant Cells

• In an isotonic solution, there is a dynamic equilibrium.
• In a hypotonic solution, the cell's vacuoles absorb water and fill, increasing turgor pressure against the cell wall.
• In a hypertonic solution, the cell eliminates water, and the vacuole volume decreases. This causes the plasma membrane to pull away from the cell wall, a process known as plasmolysis.

Simple Diffusion: Passive Solute Movement

Simple diffusion is the movement of atoms, molecules, or ions (such as O2, CO2, and water vapor) from a region of higher concentration to a lower concentration without requiring cellular energy expenditure.

Facilitated Diffusion: Protein-Assisted Transport

Facilitated diffusion is the movement of larger molecules that cannot easily pass through the plasma membrane. These molecules move from an area of greater concentration to one of lesser concentration with the help of channel or carrier proteins embedded in the membrane. This process does not require cellular energy.

Dialysis: Solute Separation in Kidneys

Dialysis is a specific type of passive transport primarily associated with nephrons in the kidneys. It involves the separation of small solutes from larger molecules across a semi-permeable membrane. Note: This process occurs ONLY IN THE KIDNEY's nephrons.

Nephrons: These are the functional units (composed of specialized cells and structures) that form the kidney.

Active Transport: Endocytosis and Exocytosis

Endocytosis: Cellular Intake Mechanisms

Endocytosis is the process by which the cell internalizes large molecules or bulk substances from the outside. The material is enclosed in a vesicle formed from the cell membrane before entering the cytoplasm. This process typically requires energy.

• Phagocytosis: This involves the outward bulging (evagination) of the cell membrane to engulf large particles (like prokaryotic cells or debris). The resulting vesicle is called a phagosome.
• Pinocytosis: This occurs when the cell membrane folds inward (invagination), forming small vesicles (pinocytotic vesicles) that allow the entry of extracellular fluid and dissolved solutes.
• Receptor-mediated Endocytosis: This is a specialized type of endocytosis where the invagination of the membrane is triggered by the binding of specific external molecules (ligands) to receptor proteins located on the cell surface.

Exocytosis: Cellular Expulsion Process

Exocytosis is the process by which substances (such as hormones like insulin, neurotransmitters, or waste products) are expelled from the cell. Vesicles containing these substances move to the cell surface and fuse with the plasma membrane, releasing their contents into the extracellular environment. This process typically requires energy.