Plant Physiology

The One Thing

Plant survival depends on the coordinated functioning of roots, stems and leaves: roots absorb water and minerals, vascular tissues transport materials, leaves regulate water loss and perform photosynthesis, while respiration and responses to stimuli support growth and maintenance.

Reactions, Processes and Experiments

Key Terms

• Absorption: The uptake of water and dissolved mineral salts from the soil by the roots.
• Root hair: A thin, elongated outgrowth of an epidermal cell that increases surface area for absorbing water and minerals.
• Osmosis: The movement of water through a selectively permeable membrane from a region of higher water concentration to a region of lower water concentration.
• Diffusion: The movement of particles from a region of higher concentration to a region of lower concentration until they are more evenly distributed.
• Imbibition: The absorption of water by substances such as seeds, wood or cell walls, causing them to swell.
• Active transport: The movement of mineral ions into root cells against their concentration gradient using energy from respiration.
• Turgidity: The firm condition of a plant cell when it contains sufficient water and presses against its cell wall.
• Plasmolysis: The shrinking of the cell contents away from the cell wall when a plant cell loses water in a concentrated solution.
• Root pressure: The pressure developed in the root that can help push water upward through the xylem.
• Xylem: A conducting tissue that carries water and mineral salts mainly from the roots to the stems and leaves.
• Phloem: A conducting tissue that transports prepared food from leaves to other parts of the plant.
• Transpiration: The loss of water vapour from the exposed aerial parts of a plant, chiefly through stomata.
• Stoma: A small pore, usually present in the epidermis of leaves, through which gases are exchanged and water vapour is lost.
• Guard cells: Specialized cells surrounding a stoma that control its opening and closing.
• Transpiration pull: The upward pulling force created by evaporation of water from leaves, helping water rise through the xylem.
• Guttation: The release of liquid water droplets from leaf margins or tips through special structures called hydathodes.
• Photosynthesis: The process by which green plants make glucose from carbon dioxide and water using light energy trapped by chlorophyll.
• Chlorophyll: The green pigment in chloroplasts that absorbs light energy for photosynthesis.
• Chloroplast: A cell organelle containing chlorophyll and serving as the main site of photosynthesis.
• Starch test: A test for photosynthesis in which iodine solution turns blue-black when starch is present in a leaf.
• Limiting factor: A condition such as light intensity, carbon dioxide concentration or temperature that restricts the rate of photosynthesis when it is insufficient or unsuitable.
• Respiration: The chemical process by which cells release energy from food, usually using oxygen.
• Aerobic respiration: The release of energy from glucose in the presence of oxygen, producing carbon dioxide and water.
• Tropism: A directional growth response of a plant part to an external stimulus.
• Phototropism: The growth response of a plant part to light; shoots usually grow towards light.
• Geotropism: The growth response of a plant part to gravity; roots generally show positive geotropism and shoots negative geotropism.
• Plant hormones: Chemical substances produced in small quantities that coordinate plant growth and development.

Easily Confused

• Osmosis and diffusion: Osmosis concerns the movement of water through a selectively permeable membrane, whereas diffusion concerns the movement of particles from higher to lower concentration.
• Osmosis and imbibition: Osmosis involves water crossing a selectively permeable membrane, whereas imbibition is the absorption of water by substances such as seeds, wood or cell walls.
• Active transport and osmosis: Active transport moves mineral ions against a concentration gradient using energy from respiration; osmosis moves water down a water-concentration gradient.
• Transpiration and guttation: Transpiration is the loss of water vapour, chiefly through stomata; guttation is the release of liquid water droplets through hydathodes.
• Xylem and phloem: Xylem carries water and mineral salts mainly upwards from roots, whereas phloem transports prepared food from sources such as leaves to sinks such as roots, fruits, seeds and growing regions.
• Photosynthesis and respiration: Photosynthesis occurs only when suitable light is available and makes glucose, whereas respiration occurs in all living plant cells during the day and at night and releases energy from food.
• Transpiration pull and root pressure: Transpiration pull draws water upwards because of evaporation from leaves; root pressure is pressure developed in the root that can push water upwards.
• Phototropism and geotropism: Phototropism is growth in response to light; geotropism is growth in response to gravity.
• Potometer and direct measurement of transpiration: A potometer estimates water uptake by a cut shoot, not transpiration directly, because some absorbed water may be used or stored.
• Stoma and guard cells: A stoma is the pore through which gases and water vapour pass; guard cells regulate the opening and closing of the pore.

What Gets Asked

• Root-hair adaptations: Questions may ask why root hairs are effective in absorption. Marks depend on identifying that they are numerous, thin-walled, elongated and closely surrounded by soil particles, increasing the absorbing surface.
• Water-absorption mechanisms: Questions may distinguish osmosis from active transport. The key slip is describing mineral-ion uptake as osmosis or failing to state that active transport requires energy from respiration and can occur against a concentration gradient.
• Water transport through plants: Questions may require the pathway from soil water to the stomata. The specific sequence is soil water -> root hair -> cortex -> endodermis -> xylem -> stem -> leaf veins -> mesophyll cells -> stomata.
• Transpiration investigations: Questions may ask about environmental factors or a potometer. The main error is claiming that a potometer measures transpiration directly; it estimates water uptake by a cut shoot.
• Starch testing and photosynthesis: Questions may require the sequence of boiling in water, heating in alcohol, rinsing and adding iodine. A blue-black colour indicates starch, and the essential conditions are light, chlorophyll, carbon dioxide, water and a suitable temperature.
• Plant transport and respiration: Questions may compare xylem with phloem or photosynthesis with respiration. Marks are lost by reversing the transported materials, omitting that respiration occurs during both day and night, or using an incorrect equation.