Plant Nutrition

The One Thing

Photosynthesis uses light energy absorbed by chlorophyll to convert carbon dioxide and water into glucose, releasing oxygen. Leaf structure, mineral ions and environmental conditions determine how efficiently this process occurs and how the products are used.

Reactions, Processes and Experiments

Key Terms

• Photosynthesis: The process by which green plants use light energy to make glucose from carbon dioxide and water, releasing oxygen.
• Word equation: Carbon dioxide + water → glucose + oxygen, with light energy absorbed by chlorophyll.
• Balanced symbol equation: 6CO2 + 6H2O → C6H12O6 + 6O2, with light and chlorophyll required.
• Chlorophyll: A green pigment in chloroplasts that absorbs light energy for photosynthesis.
• Chloroplast: An organelle containing chlorophyll and the structures where photosynthesis takes place.
• Glucose: The sugar produced by photosynthesis; it can be used in respiration or converted into other substances.
• Starch: An insoluble storage carbohydrate formed from glucose; its presence in a leaf can be tested with iodine solution.
• Leaf adaptation: A structural feature that helps a leaf carry out photosynthesis efficiently.
• Waxy cuticle: A waterproof layer that reduces water loss from the leaf surface.
• Upper epidermis: A protective cell layer that is usually transparent, allowing light to reach the palisade cells.
• Palisade mesophyll: A layer of elongated cells containing many chloroplasts, adapted to absorb light and carry out photosynthesis.
• Spongy mesophyll: A loosely arranged tissue with air spaces that allow carbon dioxide and oxygen to diffuse through the leaf.
• Stoma: A pore, usually in the lower epidermis, through which gases enter and leave the leaf.
• Guard cells: Cells surrounding a stoma that change shape to open or close the pore and control gas exchange and water loss.
• Vascular bundle: A group of transport tissues in a leaf: xylem carries water and mineral ions, while phloem transports dissolved sugars.
• Xylem: Tissue that transports water and mineral ions upward from the roots to the leaves and also helps support the plant.
• Phloem: Tissue that transports sucrose and other dissolved organic substances from sources to sinks by translocation.
• Limiting factor: A factor that is in shortest supply and restricts the rate of photosynthesis.
• Light intensity: The amount of light available; increasing it usually increases photosynthesis until another factor becomes limiting.
• Carbon dioxide concentration: The amount of carbon dioxide available for photosynthesis; increasing it can raise the rate until another factor limits the process.
• Temperature: Affects photosynthesis because it changes the activity of enzymes; very high temperatures can damage enzymes and reduce the rate.
• Nitrate ions: Mineral ions needed to make amino acids and proteins; deficiency causes poor growth and yellowing of older leaves.
• Magnesium ions: Mineral ions needed to make chlorophyll; deficiency causes yellow leaves, often beginning with older leaves.
• Mineral ion deficiency: A condition caused by insufficient mineral ions, leading to characteristic symptoms such as stunted growth or chlorosis.
• Chlorosis: Yellowing of leaves caused by a shortage of chlorophyll, commonly due to magnesium or nitrate deficiency.
• Iodine test for starch: A leaf is heated in boiling water, decolorized in ethanol, rinsed and tested with iodine; a blue-black color indicates starch.
• Destarching: Keeping a plant in darkness for a period of time so stored starch is used up before testing whether photosynthesis has occurred.

Easily Confused

• Chlorophyll and chloroplast: Chlorophyll is the green light-absorbing pigment; a chloroplast is the organelle that contains chlorophyll and is the site of photosynthesis.
• Glucose and starch: Glucose is soluble and can be used in respiration or converted into other substances; starch is an insoluble storage carbohydrate formed from glucose.
• Stoma and guard cells: A stoma is the pore through which gases move; guard cells control whether the pore is open or closed.
• Xylem and phloem: Xylem carries water and mineral ions upward from the roots; phloem transports dissolved sugars and other organic substances from sources to sinks.
• Nitrate deficiency and magnesium deficiency: Nitrate deficiency causes stunted growth and yellowing of older leaves; magnesium deficiency causes yellowing between the veins of older leaves because magnesium is required for chlorophyll production.
• Light intensity and carbon dioxide concentration: Both can limit photosynthesis, but light intensity concerns available light whereas carbon dioxide concentration concerns the supply of carbon dioxide.
• Photosynthesis and respiration: Photosynthesis stores light energy in glucose and releases oxygen; respiration uses glucose to release energy.
• Destarching and the iodine test for starch: Destarching removes stored starch before the investigation; the iodine test detects whether starch is present afterward.

What Gets Asked

• Writing the photosynthesis equations: Questions may require the word equation, Carbon dioxide + water → glucose + oxygen, with light energy absorbed by chlorophyll, or the balanced symbol equation, 6CO2 + 6H2O → C6H12O6 + 6O2, with light and chlorophyll required. Marks are lost by omitting light or chlorophyll, or by failing to balance the symbol equation.
• Explaining leaf adaptations: Questions may ask how the waxy cuticle, transparent upper epidermis, palisade mesophyll, spongy mesophyll, stomata, guard cells and vascular bundles support photosynthesis. Marks are lost by stating a structure without linking it to light absorption, gas exchange, transport or water loss.
• Interpreting limiting-factor investigations: Questions may describe changes in light intensity, carbon dioxide concentration or temperature. Marks are lost by failing to state that the rate eventually levels off when another factor becomes limiting, or by omitting enzyme denaturation above the optimum temperature.
• Describing the iodine test for starch: Questions may require the sequence of boiling the leaf, decolorizing it in ethanol, rinsing it and adding iodine solution. Marks are lost by omitting destarching, the water bath for heating ethanol, or the blue-black positive result.
• Identifying mineral deficiencies: Questions may distinguish nitrate deficiency from magnesium deficiency. Marks are lost by confusing nitrate-related stunted growth and yellowing of older leaves with magnesium-related yellowing between the veins of older leaves.
• Planning or evaluating an investigation: Questions may require control of leaf area, temperature, light distance and carbon dioxide availability, with measurement of oxygen production, carbon dioxide uptake or changes in starch. Marks are lost when variables are not controlled or when the measured outcome is not suitable for determining the rate of photosynthesis.