Biology- Photosynthesis- Light independent reactions (A01)

They take place in the stroma of the chloroplasts.

Where precisely do the light independent reactions take place?

CO₂ , ATP and reduced NADP.

What are the three required reactants for these reactions?

Carbon dioxide (CO₂) – This is taken from the atmosphere. ATP – Produced during the light-dependent reactions. NADPH – Also produced in the light-dependent reactions.

Where are these reactants from?

RUBISCO catalyzes the reaction between ribulose-1,5-bisphosphate (RuBP), a 5-carbon sugar, and carbon dioxide (CO₂).

What does the enzyme RUBISCO catalyse?

The 6-carbon compound breaks down almost immediately into two molecules of 3-phosphoglycerate (3-PGA), each containing 3 carbon atoms.

What happens to the unstable 6 carbon compound formed?

Energy Source: ATP provides the energy necessary for the various steps in the Calvin cycle.

Use: ATP is used to phosphorylate 3-phosphoglycerate (3-PGA), converting it into a higher-energy molecule called 1,3-bisphosphoglycerate (1,3-BPG). This step requires ATP to add a phosphate group. ATP is also used later in the cycle to regenerate ribulose-1,5-bisphosphate (RuBP) from glyceraldehyde-3-phosphate (G3P).

Use: NADPH is used to reduce 1,3-bisphosphoglycerate (1,3-BPG) into glyceraldehyde-3-phosphate (G3P), a 3-carbon sugar.

What exactly are the ATP and NADPH used for?

Regeneration of RuBP for the continuation of the Calvin cycle.

Formation of glucose, sucrose, and starch for energy and storage.

Synthesis of amino acids and fatty acids for growth and development.

Outline the fates of triose phosphate formed.

The Calvin cycle is called a "cycle" because it is a series of reactions that repeat over and over. The cycle continues because RuBP is constantly recycled and used again.

Why are these reactions referred to as a cycle?

Steps:

1. Prepare the Solution:

   • Mix sodium bicarbonate in water to release CO₂.

   • Add a few drops of bicarbonate indicator to the solution. The indicator will change color depending on the amount of CO₂.

2. Set Up the Algae:

   • Put the immobilised algae into the test tube or container, making sure they are evenly distributed.

3. Add the Bicarbonate Solution:

   • Pour the bicarbonate solution with the indicator into the container so the algae are covered.

4. Expose to Light:

   • Place the container under a light source to start the photosynthesis process.

5. Observe the Color Change:

   • As the algae use CO₂ for photosynthesis, the solution will become more alkaline (less CO₂), and the indicator will change color.

   • Bromothymol blue will shift from yellow (high CO₂) to blue (low CO₂). Phenolphthalein will turn from colorless to pink.

6. Measure CO₂ Use:

   • The speed of the color change indicates how fast the algae are using CO₂. Faster color change means more CO₂ is being used by the algae during the Calvin cycle.

Conclusion: By observing the rate of color change in the indicator, you can measure how much CO₂ is being used by the algae during photosynthesis over time.

Outline a method by which immobilised algae and bicarbonate indicator can be used to measure carbon dioxide use in the Calvin Cycle.

Herbicides stop electrons from moving through the chain, preventing the formation of ATP and NADPH, which are essential for the next stages of photosynthesis.

So, the plant cannot power the Calvin cycle to produce sugars (like glucose), which are the plant's food.

As a result, the plant cannot make the energy it needs to survive and grow, causing it to starve and eventually die.

Some herbicides work by inhibiting the electron transfer chain in the thylakoid membrane; explain how this leads to the death of the plant.