Osmosis and Transport Mechanisms in Cells

Effects of Osmosis on Water Balance

Osmosis: The diffusion of water across a selectively permeable membrane.
- Water moves from regions of higher water concentration to lower water concentration through the membrane.
Selectively Permeable Membrane: A barrier that allows some substances to pass while blocking others.

Tonicity: The ability of a surrounding solution to influence a cell's water content.
- Isotonic Solution:
- Definition: Solute concentration is equal to that inside the cell.
- Result: No net water movement across the plasma membrane.
- Hypertonic Solution:
- Definition: Solute concentration is greater than that inside the cell.
- Result: Cell loses water.
- Hypotonic Solution:
- Definition: Solute concentration is less than that inside the cell.
- Result: Cell gains water.
Figure 7.15 illustrates the effects on cells:
- Animal Cell:
- Hypertonic: Shriveled
- Isotonic: Normal
- Hypotonic: Lysed
- Plant Cell:
- Hypotonic: Turgid (normal)
- Isotonic: Flaccid (limp)
- Hypertonic: Plasmolyzed.

Hypertonic or hypotonic environments may create osmotic problems for organisms.
Osmoregulation: The process of controlling solute concentrations and water balance as an adaptation to survive in varying environments.
- Example: The protist Paramecium uses a contractile vacuole to pump out excess water.

Cell Walls: Structures that help maintain water balance in cells (especially in plants).
- In a hypotonic solution, plant cells swell until the wall opposes further uptake, resulting in a turgid state.
- In isotonic conditions, plant cells become flaccid and may lead to wilting.
- In hypertonic solutions, plant cells lose water, leading to plasmolysis where the membrane detaches from the cell wall, often resulting in cell death.

Facilitated Diffusion: A process by which transport proteins assist the movement of molecules across the plasma membrane without using energy.
- Channel Proteins: Create corridors for specific molecules or ions to cross the membrane.
- Aquaporins: Channel proteins that facilitate water diffusion.
- Ion Channels: Open or close in response to stimuli (gated channels).
- Carrier Proteins: Change shape to translocate the solute-binding site across the membrane.
Diseases: Some diseases, such as cystinuria, result from malfunctions in specific transport systems.

Active Transport: Moves substances against their concentration gradients, requiring energy (usually ATP).
- Facilitated diffusion is passive since it requires no energy and moves substances down their concentration gradients.

Active transport is necessary for cells to maintain concentration gradients that differ from their environment.
Sodium-Potassium Pump: A key example of an active transport system that exchanges sodium ions ( ext{Na}^+ ) for potassium ions ( ext{K}^+ ).

Membrane Potential: The voltage difference across a membrane caused by the distribution of ions.
- Two forces create the electrochemical gradient:
- Chemical Force: Based on the ion's concentration gradient.
- Electrical Force: Related to the membrane potential's influence on ion movement.
Electrogenic Pumps: Transport proteins that generate voltage across membranes (ex. sodium-potassium pump in animal cells).
- Proton Pumps: Major electrogenic pumps in plants, fungi, and bacteria, helping to store energy.

Cotransport: Refers to the process in which active transport of one solute indirectly drives the transport of another.
- Example: Plants use hydrogen ion ( ext{H}^+ ) gradients created by proton pumps to facilitate nutrient transport.

Bulk Transport: Larger molecules, such as polysaccharides and proteins, cross membranes via vesicles, requiring energy.

Exocytosis: Process in which transport vesicles migrate to the plasma membrane, fuse with it, and release their contents outside the cell.
- Commonly utilized by secretory cells to export products.

Endocytosis: The process by which a cell takes in macromolecules by forming vesicles from the plasma membrane.
- Types of Endocytosis:
- Phagocytosis: "Cellular eating"; the cell engulfs a particle in a vacuole which then fuses with a lysosome for digestion.
- Pinocytosis: "Cellular drinking"; extracellular fluid is absorbed into tiny vesicles.
- Receptor-mediated Endocytosis: Triggered by ligand binding to receptors, leading to the formation of vesicles containing the bound ligands.

Figures (7.14 to 7.22) depict the processes and mechanisms of osmosis, diffusion, and transport through various types of cells, incorporating both visual diagrams and descriptive labels to assist in understanding these biological processes.