Movement of Solutes and Water Across Cell Membranes

Focus on the transport functions of membranes, with emphasis on the plasma membrane.
Explains the transport of solutes (ions, glucose, gases) and water across membranes.
Discusses how transport mechanisms help maintain cell size, shape, and energy balance and facilitate communication via electrical/chemical signals.
Introduction of key concepts like diffusion, osmotic balance, and mediated transport systems.

Molecules of substances (solid, liquid, gas) are in continuous movement due to thermal energy.
Higher temperature increases molecular movement speed.
Collisions among molecules cause random movements and eventual uniform distribution throughout a space.
Simple diffusion is defined as the movement of substances from an area of higher concentration to one of lower concentration due solely to random thermal motion.

Simple diffusion: Uncontrolled redistribution of solute molecules until equilibrium is reached.
Diffusion equilibrium: No net flux of molecules occurs when concentrations are equal in two compartments.
Net flux: The difference between one-way fluxes occurring in opposite directions.

Concentration Gradient: Greater differences lead to greater net flux. Substances generally move downhill in concentration.
Temperature: Higher temperatures increase molecular movement.
Mass of the Molecule: Larger molecules move more slowly; hence, have a slower net flux.
Surface Area: More area allows for greater diffusion.
Medium of Diffusion: Molecules diffuse more quickly in gases than liquids due to fewer collisions.
Distance: Diffusion time increases with the square of the distance: Time ext{ } ext{[proportional to]} ext{ } Distance^2.

Example of glucose diffusion between two compartments (one with higher concentration). Demonstrates one-way flux relationship and equilibrium outcomes.
At time A, concentration in Compartment 1 is 20 ext{ mmol/L}, and Compartment 2 is 0 ext{ mmol/L}.
After reaching equilibrium, concentrations are identical (e.g., 10 ext{ mmol/L} each).

Facilitated diffusion: Uses specific membrane proteins to help transport polar substances across membranes from areas of high to low concentration without energy expenditure.
Transport reaches a maximum when binding sites on transporters are saturated.

Facilitated diffusion is slow compared to ion channels due to conformational changes of transporters.

Requires energy to move substances against their concentration gradient (uphill movement).

Primary Active Transport: Utilizes ATP to pump ions against the concentration gradient (e.g., Na+/K+ ATPase).
- Transports 3 ext{ Na}^+ out and 2 ext{ K}^+ into cells for each ATP hydrolyzed.
Secondary Active Transport: Does not use ATP directly but relies on the electrochemical gradient created by primary active transport.

Distinct transport occurrence in the same transporter, allowing influx of Na+ and efflux of substance X (like glucose) in cotransport and countertransport mechanisms.

Osmosis: net diffusion of water across a selectively permeable membrane from an area of higher water concentration (lower solute concentration) to an area of lower water concentration (higher solute concentration).

Osmolarity affects the solute concentration and, therefore, direct water concentrations across membranes.
Examples of Tonic Solutions:
- Isotonic: Solutions with the same nonpenetrating solute concentration as cells, resulting in no net change in volume.
- Hypotonic: Solutions with lower nonpenetrating solute concentration will cause cell swelling.
- Hypertonic: Solutions with higher solute concentration than cells will cause cell shrinkage.

Endocytosis: Cellular process where substances are brought into the cell within vesicles.

Pinocytosis: Engulfs extracellular fluid and solutes generally in a non-specific manner.
Phagocytosis: Engulfs larger particles and debris via extensions called phagopodia.
Receptor-Mediated Endocytosis: Highly selective uptake of specific molecules that bind to receptors before internalization.

Opposite of endocytosis, allowing the release of substances synthesized in the cell by vesicle fusion to the plasma membrane.
Important in material secretion, especially in neuron neurotransmitter release and hormone secretion.

Transcellular Pathway: Involves movement through the cytoplasm of the epithelial cells.
Paracellular Pathway: Movement occurs between adjacent cells.

Special characteristics of epithelial cells facilitate the absorption of substances (e.g., in the gut and kidney).
Different transport dynamics and mechanisms depending on membrane specialization on the apical and basolateral surfaces.

The transport processes covered are vital to maintain cellular homeostasis, enabling nutrient uptake, waste removal, and effective communication within bodily systems.
Overall transport depends on diffusion principles, mediated transport mechanisms, osmosis, and specific cellular dynamics governing endocytosis and exocytosis.