Outline the subsequent use and storage of the carbohydrates made in photosynthesis.

Objective

Students will be able to:

• State the word equation for photosynthesis and the role of chlorophyll.
• Identify the main parts of a leaf that are involved in photosynthesis.
• Explain how the three key factors (light, carbon dioxide and temperature) affect the rate of photosynthesis.
• Describe how the carbohydrates produced in the chloroplast are used, transported and stored in a plant.
• Link the essential mineral ions nitrate (NO₃^‑) and magnesium (Mg²⁺) to carbohydrate utilisation.
• Plan a simple investigation of a factor affecting the rate of photosynthesis.

1. Photosynthesis – core concepts required by the syllabus

1.1 Word equation and role of chlorophyll

• Word equation: Carbon dioxide + water → glucose + oxygen
• Chlorophyll a (and b) in the thylakoid membranes absorbs light energy (mainly blue and red wavelengths) and transfers it to the reaction centres of photosystem II and photosystem I.
• Light energy drives the light‑dependent reactions (splitting H₂O, producing O₂, ATP and NADPH) and the light‑independent (Calvin‑cycle) reactions that fix CO₂ into glucose.

1.2 Factors that affect the rate of photosynthesis (syllabus 6.3)

Factor	Effect on rate (up to a limit)	Typical observation
Light intensity	Rate ↑ with ↑ intensity until the photosystems become saturated.	More O2 bubbles from a leaf disc in bright light; plateau at very high intensity.
Carbon‑dioxide concentration	Rate ↑ with ↑ CO2 until the enzyme Rubisco is saturated.	Higher O2 production in a sealed container when CO2 is added.
Temperature	Rate ↑ with temperature (up to an optimum, usually 25‑30 °C) then ↓ because enzymes denature.	Maximum O2 evolution at the optimum temperature; rapid decline above 35 °C.

2. Leaf structure – parts involved in photosynthesis (syllabus 6.2)

• Epidermis – protects the leaf; contains stomata for gas exchange.
• Guard cells – regulate opening of stomata, controlling CO₂ entry and water loss.
• Palisade mesophyll – columnar cells rich in chloroplasts; main site of light absorption and carbon fixation.
• Spongy mesophyll – loosely packed cells with many intercellular air spaces; facilitates diffusion of gases.
• Vascular bundles – xylem transports water and minerals upward; phloem transports the products of photosynthesis (mainly sucrose) away from the leaf.

3. Immediate use of the carbohydrate formed in the chloroplast

• Glucose (or the triose‑phosphates that leave the chloroplast) enters the cytosol and is oxidised in mitochondria by cellular respiration.
• Respiration produces ATP, which powers:
  • Growth and cell division
  • Repair and maintenance of tissues
  • Active transport of ions and nutrients
  • Synthesis of other organic molecules (amino acids, lipids, nucleic acids)
• The carbon skeletons of glucose also provide the backbone for nitrogen‑containing compounds after nitrate is reduced to NH₄⁺ (see section 5).

4. Transport of soluble carbohydrates (sucrose) – pressure‑flow mechanism

1. Loading (source) – In mature leaves excess glucose is converted to sucrose. Sucrose is actively transported into the sieve‑tube elements of the phloem (often with a H⁺‑symporter).
2. Long‑distance movement – The accumulation of sucrose lowers the water potential in the phloem, causing water to enter by osmosis. The resulting turgor pressure drives bulk flow of the sap toward sink tissues (roots, developing fruits, tubers, growing shoots).
3. Unloading (sink) – Sucrose is removed from the sieve tubes either by active transport (in growing tissues) or by diffusion (in storage organs). Once inside the sink cell it can be:
   • Hydrolysed to glucose for immediate metabolism, or
   • Used for synthesis of storage compounds (starch, fructans, vacuolar sucrose).

5. Storage forms of carbohydrates (syllabus 6.4)

Storage form (as named in the syllabus)	Where it is stored	Main function
Starch (polymer of glucose)	Chloroplasts of mature leaves; amyloplasts in roots, tubers and seeds	Long‑term energy reserve for germination, regrowth after dormancy or periods of low light.
Fructans (e.g. inulin)	Roots of some dicotyledons (e.g. chicory, dandelion, Jerusalem artichoke)	Osmotic regulation and winter storage of carbohydrate energy.
Sucrose (as a storage sugar)	Vacuoles of young leaves, fruits and some herbaceous stems	Rapid mobilisation when the plant needs a quick energy supply or to cope with stress.
Nectar (sucrose‑rich solution)	Flower nectaries	Short‑term carbohydrate store that is secreted to attract pollinators.

6. Use of carbohydrate carbon skeletons for structural and secondary compounds

• Cellulose – linear chains of β‑D‑glucose polymerise to form the principal fibre of cell walls, providing rigidity.
• Hemicellulose and pectin – branched polysaccharides that bind cellulose fibres into a matrix.
• Lignin – although not a carbohydrate, its phenylpropanoid precursors are derived from glucose‑derived aromatic compounds; lignin strengthens woody tissues.
• Secondary metabolites – carbon skeletons from glycolysis and the pentose‑phosphate pathway are precursors for alkaloids, flavonoids, tannins and other phenolics.
• Storage proteins in seeds – e.g. legumin and vicilin are built from amino acids whose carbon backbones originate from carbohydrate metabolism.

7. Essential mineral ions linked to carbohydrate use (syllabus 6.5)

• Nitrate ions (NO₃^‑) – taken up by roots, reduced to nitrite and then to ammonium (NH₄⁺) in the leaf. The ammonium is incorporated into amino acids using carbon skeletons derived from glucose (e.g., α‑ketoglutarate → glutamate).
• Magnesium ions (Mg²⁺) – sit at the centre of the chlorophyll molecule; without Mg²⁺ light absorption and the light‑dependent reactions cannot occur, so carbohydrate synthesis would cease.

8. Overall flow of carbohydrates in a plant (summary diagram description)

1. Photosynthesis in chloroplasts of palisade mesophyll → glucose (or triose‑phosphates).
2. Immediate use: glucose enters mitochondria → cellular respiration → ATP for growth, repair, active transport and biosynthesis.
3. Excess glucose → conversion to sucrose.
4. Sucrose loading into phloem sieve tubes (source).
5. Pressure‑flow transport to sink tissues (roots, tubers, developing fruits, young shoots).
6. At the sink:
   • Hydrolysis to glucose for metabolism.
   • Polymerisation to starch (chloroplasts or amyloplasts) or fructans (roots).
   • Storage as vacuolar sucrose (young leaves, fruits, herbaceous stems).
   • Incorporation into structural polymers (cellulose, hemicellulose, pectin, lignin).
   • Use in secondary metabolism and seed‑protein synthesis.
   • Conversion to nectar in flower nectaries.

9. Key equation (photosynthesis)

Word form (required by the syllabus):

Carbon dioxide + water → glucose + oxygen

Balanced chemical form (useful for reference):

$$ 6\text{CO}_2 + 6\text{H}_2\text{O} \xrightarrow{\text{light}} \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 $$

10. Suggested practical investigation (syllabus 6.6)

Title: Effect of light intensity on the rate of photosynthesis (leaf‑disc assay).

Materials: Fresh spinach leaves, hole‑punch, sodium bicarbonate solution (0.02 M), syringes, light source with variable intensity, lux meter, timer.

Method (brief):

1. Punch out equal‑sized leaf discs and place 5 discs in a syringe filled with bicarbonate solution.
2. Seal the syringe, invert it and expose it to a chosen light intensity measured with the lux meter.
3. Record the time taken for the discs to rise (indicating O₂ production).
4. Repeat for at least five different light intensities (e.g., 0, 2000, 4000, 6000, 8000 lux).
5. Plot “time to rise” (or rate = 1/time) against light intensity to illustrate the relationship and identify the saturation point.

Link to syllabus: Demonstrates the effect of one of the three key factors (light intensity) on the rate of photosynthesis, fulfilling the practical requirement of Topic 6.

11. Suggested diagram for the hand‑out