Plant Physiology

The One Thing

Plant physiology depends on coordinated transport, water regulation, photosynthesis and responses to stimuli. Roots absorb water and minerals, xylem and phloem distribute materials, leaves regulate gas exchange and water loss, and hormones coordinate growth and movement.

Reactions, Processes and Experiments

Key Terms

• Root hair: A thin, delicate extension of an epidermal cell that increases the surface area for absorbing water and mineral salts.
• 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.
• Active transport: The movement of mineral ions from a region of lower concentration to a region of higher concentration using energy from respiration.
• Turgidity: The firm condition of a plant cell when it absorbs water and the cell contents press against the cell wall.
• Plasmolysis: The shrinking of the cell contents away from the cell wall when a plant cell loses water in a concentrated solution.
• Xylem: The conducting tissue that carries water and dissolved mineral salts mainly from roots to stems and leaves.
• Phloem: The conducting tissue that transports manufactured food from leaves to other parts of the plant.
• Transpiration: The loss of water vapour from the aerial parts of a plant, mainly through stomata.
• Stoma: A small pore in the epidermis of a leaf, usually controlled by a pair of guard cells, through which gases and water vapour pass.
• Transpiration pull: The suction force created by transpiration that helps draw water upward through the xylem.
• 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: The cell organelle in which photosynthesis takes place.
• Starch test: A test for photosynthesis in which iodine solution turns blue-black when starch is present.
• Limiting factor: A condition that restricts the rate of photosynthesis when it is present in insufficient quantity.
• Plant hormone: A chemical substance produced in small amounts that controls growth, development and responses in plants.
• Auxin: A plant hormone that promotes cell elongation and is involved in phototropism and geotropism.
• Phototropism: The growth response of a plant part to light; shoots usually show positive phototropism by growing towards light.
• Geotropism: The growth response of a plant part to gravity; roots are generally positively geotropic and shoots negatively geotropic.
• Hydrotropism: The growth response of roots towards water or moisture.
• Nastic movement: A non-directional movement in response to a stimulus, such as the folding of leaves when touched.

Easily Confused

• Osmosis and diffusion: Osmosis concerns the movement of water through a selectively permeable membrane; diffusion concerns the movement of particles down a concentration gradient.
• Diffusion and active transport: Diffusion moves particles from higher to lower concentration without respiratory energy; active transport moves mineral ions from lower to higher concentration using respiratory energy.
• Xylem and phloem: Xylem carries water and dissolved mineral salts mainly upwards; phloem transports manufactured food from leaves to other parts of the plant.
• Transpiration and transpiration pull: Transpiration is the loss of water vapour; transpiration pull is the suction force produced by that loss.
• Stomatal, cuticular and lenticular transpiration: Stomatal transpiration occurs through stomata, cuticular transpiration through the cuticle, and lenticular transpiration through lenticels.
• Turgidity and plasmolysis: Turgidity occurs when a cell gains water and its contents press against the cell wall; plasmolysis occurs when a cell loses water and its contents shrink away from the wall.
• Photosynthesis and the starch test: Photosynthesis produces glucose; the starch test detects starch formed from stored glucose.
• Chlorophyll and chloroplast: Chlorophyll is the light-absorbing pigment; the chloroplast is the organelle containing chlorophyll and the site of photosynthesis.
• Phototropism and geotropism: Phototropism is a directional growth response to light; geotropism is a directional growth response to gravity.
• Tropisms and nastic movements: Tropisms are directional growth responses; nastic movements are non-directional responses and may not involve growth.
• Positive and negative tropism: Positive tropism is growth towards the stimulus, whereas negative tropism is growth away from it.
• Root pressure and transpiration pull: Root pressure originates in the roots, whereas transpiration pull results from water loss from aerial parts.

What Gets Asked

• Define or explain absorption: Questions may require osmosis, diffusion or active transport to be identified. Marks are lost by stating that all mineral ions enter by osmosis or by omitting the energy requirement for active transport.
• Describe water movement through a plant: The required sequence is soil water → root hair → cortex → endodermis → pericycle → xylem, followed by upward movement in the transpiration stream. Replacing this with a vague statement about water moving through the plant loses specificity.
• Explain factors affecting transpiration: High temperature, strong wind and bright light generally increase transpiration, whereas high humidity generally decreases it. The direction of each effect must be stated accurately.
• Write or interpret the photosynthesis equations: Both carbon dioxide + water → glucose + oxygen, in the presence of light and chlorophyll. and 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂, in the presence of light and chlorophyll. may be required. Light and chlorophyll are conditions, not raw materials.
• Describe photosynthesis experiments: A variegated-leaf experiment tests the necessity of chlorophyll, a covered-leaf experiment tests the necessity of light, and a limewater or potassium hydroxide setup tests the necessity of carbon dioxide. Confusing the purpose of these experiments costs marks.
• Explain plant responses: Questions may ask about auxin in phototropism, positive and negative geotropism, hydrotropism or nastic movement. The key distinction is whether the response is directional and whether it involves growth.