Movement Into & Out of Cell – Diffusion, Osmosis, Active Transport (Notes)

Definition: net movement of particles from a region of higher concentration to a region of lower concentration (i.e. down a concentration gradient) due to random movement.
Energy source: kinetic energy of random movement of molecules and ions.
Through membranes: some substances move into and out of cells by diffusion through the cell membrane.
Concentration gradient: a difference in concentration; the gradient slopes from high to low. \text{gradient} = C{high} - C{low}
Key concepts:
- Random motion drives diffusion, not purposeful movement.
- Occurs in gases, solutions, and mixtures of liquids.
Factors influencing diffusion (restricted to): surface area, temperature, concentration gradient, and distance.
Diffusion in daily life:
- Gas spread from a high-concentration area to fill available space (e.g., odour from rotten egg).
Diffusion in living organisms:
- Gases: CO₂ diffuses into leaves for photosynthesis; O₂ diffuses out as a waste product.
- Diffusion of solutes: solutes dissolved in water (the solvent is water) diffuse through cytoplasm.
- In cells, solutes like glucose or sodium ions diffuse within the cytoplasm.
Diffusion of water and solutes in plant and animal contexts:
- Water movement and solute diffusion depend on concentration gradients and membrane permeability.
Water potential gradient (concept): a gradient analogous to a slope for water movement, moving from high to low water potential. \Psiw^{high} \to \Psiw^{low}

Definition: diffusion of water through a partially permeable (semipermeable) membrane.
Role of water as solvent: water moves to balance concentrations of dissolved substances (solutes) across membranes.
Water potential gradient: water moves from higher water potential to lower water potential (like a slope). \Psiw^{high} \to \Psiw^{low}
Key terms to know:
- Solute: substance dissolved in a solvent.
- Solvent: the liquid in which solutes are dissolved (in organisms, mostly water).
Osmosis in animal cells:
- Pure water outside -> water moves in -> cell swells and may burst (lysis).
- Concentrated solution outside -> water moves out -> cell shrinks (crenation in some contexts).
Osmosis in plant cells:
- Cell wall prevents bursting; movement of water changes turgor.
- Pure water outside makes cells turgid (firm) but not bursting due to cell wall.
- Concentrated external solution causes water to move out -> cytoplasm shrinks -> plasma membrane may pull away (plasmolysis).
Plant cell states:
- Turgid: firm plant cell due to water uptake.
- Flaccid: soft plant cell when water uptake is reduced.
- Plasmolysed: cell membrane pulls away from cell wall due to water loss.

Definition: movement of particles through a cell membrane from a region of lower concentration to a region of higher concentration (against a concentration gradient) using energy from respiration.
Active vs passive transport:
- Active transport: against gradient; requires energy.
- Passive transport: with gradient; does not require energy.
- Differs by direction of movement and energy use.
How active transport happens (example in plants): root hair cells take in nitrate ions from soil where soil nitrate is low and inside the cell it is higher.
Mechanism (carrier-mediated):
1) Nitrate ion binds to a carrier protein.
2) Carrier protein changes shape; energy from respiration drives this change.
3) Carrier protein pushes the nitrate ion into the cell.
Key example: uptake of nutrients (e.g., nitrates) by plant root hairs against their concentration gradient.

Diffusion: movement from high to low concentration; no energy required; down a gradient; includes gases and solutes.
Osmosis: diffusion of water through partially permeable membranes; driven by water potential differences.
Active Transport: movement against a gradient (low to high concentration); requires energy; uses carrier proteins.
Semipermeable membrane: essential for osmosis and many diffusion processes across cell boundaries.