Plant Physiology

The One Thing

Plant physiology explains how plants acquire, transport, transform, and use materials and energy, while coordinating growth and responses through cellular processes, environmental factors, and hormones.

Reactions, Processes and Experiments

Key Terms

• Diffusion: Movement of particles from a region of higher concentration to a region of lower concentration until equilibrium is reached.
• Osmosis: Movement of water through a selectively permeable membrane from a region of higher water potential to lower water potential.
• Water potential: The tendency of water to move from one region to another; pure water has the highest water potential, conventionally represented as zero.
• Plasmolysis: Shrinkage of the protoplast away from the cell wall when a plant cell loses water in a hypertonic solution.
• Xylem: Vascular tissue that mainly conducts water and minerals from roots to aerial parts of the plant and also provides support.
• Phloem: Vascular tissue that transports organic food, especially sucrose, from sources to sinks.
• Transpiration: Loss of water vapour from the aerial parts of a plant, mainly through stomata.
• Transpiration pull: Upward force generated by evaporation of water from leaves, helping lift a continuous water column through the xylem.
• Root pressure: Positive pressure developed in roots due to active absorption of ions and water, which may assist upward water movement.
• Stomata: Tiny pores in the leaf epidermis controlled by guard cells that regulate gas exchange and transpiration.
• Essential mineral element: An element required for normal plant growth and reproduction whose function cannot be completely replaced by another element.
• Macronutrients: Mineral elements required in relatively large amounts, including nitrogen, phosphorus, potassium, calcium, magnesium, and sulphur.
• Micronutrients: Mineral elements required in very small amounts, including iron, manganese, copper, zinc, boron, molybdenum, chlorine, and nickel.
• Nitrogen fixation: Conversion of atmospheric nitrogen into usable nitrogen compounds, commonly by nitrogen-fixing microorganisms.
• Chlorosis: Yellowing of leaves caused by reduced chlorophyll formation, often due to mineral deficiency.
• Photosynthesis: Process by which green plants use light energy to synthesize carbohydrates from carbon dioxide and water, releasing oxygen.
• Chlorophyll: Green pigment in chloroplasts that absorbs light energy for photosynthesis.
• Light reaction: Light-dependent stage of photosynthesis in thylakoid membranes, producing ATP, NADPH, and oxygen.
• Photolysis: Light-driven splitting of water during photosynthesis into protons, electrons, and oxygen.
• Calvin cycle: Light-independent pathway in the chloroplast stroma that fixes carbon dioxide and forms carbohydrates using ATP and NADPH.
• C3 pathway: Carbon-fixation pathway in which the first stable product is the three-carbon compound 3-phosphoglyceric acid.
• C4 pathway: Carbon-fixation pathway in which carbon dioxide is first fixed into a four-carbon compound, helping reduce photorespiration in hot, bright conditions.
• Photorespiration: Process in which RuBisCO uses oxygen instead of carbon dioxide, causing loss of fixed carbon and energy.
• Respiration: Controlled oxidation of organic molecules to release energy, mainly captured as ATP.
• Glycolysis: Cytoplasmic breakdown of one glucose molecule into two pyruvate molecules, producing a net gain of ATP and NADH.
• Krebs cycle: Mitochondrial cycle that oxidizes acetyl-CoA and produces carbon dioxide, NADH, FADH2, and ATP or GTP.
• Electron transport system: Series of membrane-bound carriers that transfer electrons and create a proton gradient used for ATP synthesis.
• Oxidative phosphorylation: ATP formation using energy from electron transfer and the proton gradient across the inner mitochondrial membrane.
• Fermentation: Anaerobic breakdown of sugars that regenerates NAD+ and produces much less ATP than aerobic respiration.
• Growth: Irreversible increase in size, dry mass, volume, or cell number of an organism or organ.
• Differentiation: Process by which unspecialized cells develop specialized structures and functions.
• Plant growth regulators: Chemical substances controlling plant growth and development, including auxins, gibberellins, cytokinins, abscisic acid, and ethylene.
• Photoperiodism: Plant response to the relative durations of light and darkness, especially in flowering.
• Vernalisation: Induction of flowering by exposing a plant or seed to a period of low temperature.

Easily Confused

• Diffusion and osmosis: Diffusion concerns particles generally, whereas osmosis specifically concerns water crossing a selectively permeable membrane.
• Water potential and concentration gradient: Water potential describes the tendency of water to move; a concentration gradient describes differences in particle concentration.
• Xylem and phloem: Xylem mainly transports water and minerals, whereas phloem transports organic food, especially sucrose.
• Transpiration pull and root pressure: Transpiration pull is a tension generated by evaporation from leaves, whereas root pressure is positive pressure generated in roots.
• Macronutrients and micronutrients: Macronutrients are required in relatively large amounts, whereas micronutrients are required in very small amounts.
• Light reactions and the Calvin cycle: Light reactions occur mainly in thylakoid membranes and produce ATP, NADPH, and oxygen; the Calvin cycle occurs in the stroma and uses ATP and NADPH to fix carbon dioxide.
• Non-cyclic and cyclic photophosphorylation: Non-cyclic photophosphorylation produces ATP, NADPH, and oxygen, whereas cyclic photophosphorylation produces ATP but not NADPH or oxygen.
• C3 and C4 pathways: C3 plants form 3-phosphoglyceric acid as the first stable product, whereas C4 plants first form a four-carbon compound and reduce photorespiration in hot, bright conditions.
• Aerobic respiration and fermentation: Aerobic respiration uses oxygen and produces much more ATP, whereas fermentation occurs without oxygen and produces much less ATP.
• Glycolysis and the Krebs cycle: Glycolysis occurs in the cytoplasm and forms pyruvate; the Krebs cycle occurs in the mitochondrial matrix and oxidizes acetyl-CoA.
• Arithmetic and geometric growth: Arithmetic growth shows a constant increase, whereas geometric growth shows an exponential increase under ideal conditions.
• Photoperiodism and vernalisation: Photoperiodism involves responses to the relative durations of light and darkness, whereas vernalisation involves induction of flowering by low temperature.

What Gets Asked

• Define or distinguish transport processes: Questions may require diffusion, osmosis, water potential, plasmolysis, transpiration pull, or root pressure. Marks are lost by treating osmosis as general particle movement or by confusing transpiration-generated tension with root-generated positive pressure.
• Use the water-potential equation: Students may be asked to state or apply Ψw = Ψs + Ψp. Marks are lost by omitting or misidentifying solute potential and pressure potential.
• Compare xylem and phloem transport: Questions may ask for the tissue, material transported, and direction or mechanism of movement. Marks are lost by assigning sucrose transport to xylem or describing phloem movement without the source–sink pressure-flow sequence.
• Explain mineral nutrition and deficiency: Questions may require functions of nitrogen, phosphorus, magnesium, iron, calcium, or potassium, or the meaning of chlorosis. Marks are lost by confusing macronutrients with micronutrients or assigning one element another element’s function.
• Complete or explain photosynthesis: Questions may use 6CO2 + 6H2O + light energy → C6H12O6 + 6O2 and may test thylakoid versus stroma, photolysis, Photosystem II before Photosystem I, or the Calvin cycle. Marks are lost by stating that cyclic photophosphorylation produces NADPH or oxygen.
• Compare respiration pathways and calculate or interpret energy yield: Questions may test glycolysis, the Krebs cycle, electron transport, fermentation, the respiration equation, ATP yield, or RQ = volume of CO2 evolved ÷ volume of O2 consumed. Marks are lost by placing glycolysis in mitochondria, treating fermentation as aerobic respiration, or assigning incorrect RQ patterns to carbohydrates, fats, and organic acids.
• Explain growth regulation and environmental responses: Questions may ask about cell division, enlargement, differentiation, plant growth regulators, tropisms, photoperiodism, or vernalisation. Marks are lost by confusing hormone functions or by treating photoperiodism as a temperature response rather than a response mainly to uninterrupted darkness.