Cell Transport

• Diffusion:
• Net movement of particles from high concentration to low concentration down the concentration gradient.
• Passive process — no ATP required.
• Occurs due to random movement of particles.
• Factors increasing rate of diffusion:
• Steeper concentration gradient
• Higher temperature
• Larger surface area
• Shorter diffusion distance
• Examples: oxygen into cells, CO₂ out of leaf cells, glucose in small intestine.

• Movement of substances down their concentration gradient
• (from high to low concentration).
• No energy (ATP) required because it is a passive process.
• Transport occurs through channel proteins or carrier proteins embedded in the membrane.
• Channel proteins form hydrophilic pores allowing specific ions or polar molecules to pass.
• Carrier proteins undergo a shape change when the correct molecule binds, helping it cross the membrane.
• Transport is highly selective — each protein transports only certain molecules (e. g., glucose, amino acids, ions).
• Rate of facilitated diffusion shows saturation: once all carrier proteins are occupied, increasing concentration does not increase rate.
• Allows cells to take in substances that cannot diffuse through the phospholipid bilayer (large, polar, or charged molecules).
• Temperature affects rate because protein movement and molecular kinetic energy change.
• Important in processes like glucose uptake in muscle cells via GLUT transporters.

• TONICITY: — the relative solute concentration of a solution compared to the inside of a cell, which determines the direction of water movement during osmosis.
• Hypertonic — higher solute concentration outside the cell → water moves out of the cell.
• Hypotonic — lower solute concentration outside the cell → water moves into the cell.
• Isotonic — equal solute concentration inside and outside → no net water movement.
• These terms explain how cells behave in different solutions — swelling, shrinking, or staying the same — because osmosis always moves water toward the side with more solute.

• Osmosis is defined as net movement of water molecules from a dilute solution to a concentrated solution through a partially permeable membrane.
• It is a passive process.
• Only water moves through a partially permeable membrane.
• Water moves from high water potential to low water potential.
• Examples:
• Water entering root hair cells
• Water entering red blood cells in dilute solutions
• Plant cell responses:
• Turgid: Water enters; membrane presses against wall when placed in dilute (hyptonic) solution.
• Flaccid: Water leaves; cell softens when placed in concentrated (hypertonic) solution.

Plasmolysis: In very concentrated solution; membrane pulls away from wall.

• Animal cell responses:
• Haemolysis: water diffuses into red blood cells and cause swelling and bursting when placed in a dilute (hypotonic).
• Crenation: water is drawn out, causing the cells to shrivel and shrink when the cells are placed in a concentrated (hypertonic) solution.

• Definition: Movement of particles against the concentration gradient (low → high) using energy (ATP) from respiration.
• Requires carrier proteins in the membrane.
• Requires ATP from mitochondria.
• Cells that do lots of active transport have many mitochondria.
• Examples: Root hair cells absorbing mineral ions from dilute soil
• Gut lining absorbing remaining glucose into blood that already has higher concentration

• Smaller organisms or cells with large SA: V exchange substances faster.
• Larger organisms need specialised exchange surfaces (lungs, gills, villi).
• Thin membranes + good blood supply + ventilation increase transport efficiency.
• Required practical (osmosis in potato tissues)
• Measure mass before and after placing potato cylinders in solutions of different concentrations.
• % change in mass = (Final − Initial) ÷ Initial × 100
• Positive % = water gained; negative % = water lost.