Transport.

Big organisms can't rely on substances simply diffusing where they're needed — the distances are too great. Plants and animals both evolved transport systems to move water, food, gases and wastes around the body.

1 · Osmosis — the engine behind water movement

Osmosis is the movement of water molecules from a region of higher water potential (more dilute) to a region of lower water potential (more concentrated) across a partially (selectively) permeable membrane. It's how water enters root cells and how cells stay firm.

• Plant cell in pure water: water enters, the vacuole swells, the cell becomes turgid (firm) — the wall stops it bursting. Turgor supports soft tissues.
• Plant cell in concentrated solution: water leaves, the cell becomes flaccid, then the membrane pulls away from the wall — plasmolysis.
• Animal cell (no wall): in pure water it swells and bursts (haemolysis); in concentrated solution it shrinks (crenation).

2 · Transport in plants

Plants have two pipe systems running through the roots, stem and leaves:

Xylem — water & minerals, upward

Water is absorbed from the soil by root hair cells (by osmosis) and travels up the xylem — hollow, dead, reinforced tubes — all the way to the leaves. The pull comes from transpiration: water evaporates from the leaves and the cohesive water column is dragged up behind it (the transpiration stream). Transpiration speeds up in bright light, heat, low humidity and wind.

Phloem — sugars, both ways

Sugars (mainly sucrose) made in the leaves are carried in the living phloem to wherever they're needed — growing tips, roots, fruits. This is translocation, and it can run up or down, from a source (where sugar is made) to a sink (where it's used or stored).

3 · Transport in animals — the circulatory system

Humans have a double circulation: blood passes through the heart twice for each complete loop — once to the lungs (pulmonary circuit) and once to the rest of the body (systemic circuit).

The heart

Four chambers: two atria (receive blood) on top, two ventricles (pump blood out) below. The right side pumps deoxygenated blood to the lungs; the left side pumps oxygenated blood to the body (its wall is thicker — further to push). Valves keep blood flowing one way only.

Blood vessels

Blood

• Plasma — straw-coloured fluid carrying dissolved food, CO₂, wastes, hormones.
• Red blood cells — carry oxygen using haemoglobin; biconcave, no nucleus.
• White blood cells — defence (engulf microbes, make antibodies).
• Platelets — help blood clot at wounds.