Photosynthesis — light-dependent and Calvin cycle (KCSE Biology Form 3)

TL;DR

Photosynthesis is how plants make their own food using sunlight, water, and carbon dioxide. It happens in two main stages: the light-dependent reactions capture light energy, and the Calvin cycle uses that energy to build sugars. Together, these processes convert light into chemical energy stored in glucose.

1. The Mental Model

Think of photosynthesis like a tiny factory inside a plant's leaves. The first part, the light-dependent reactions, is like the power generator, capturing sunlight. The second part, the Calvin cycle, is like the assembly line, using that power to build sugar molecules from carbon dioxide.

2. The Core Material

What is Photosynthesis?

Photosynthesis is the process by which green plants, algae, and some bacteria convert light energy into chemical energy, in the form of glucose (sugar). This process is vital for almost all life on Earth, as it produces the oxygen we breathe and the food we eat.

The overall equation for photosynthesis is:
6CO₂ (Carbon Dioxide) + 6H₂O (Water) + Light Energy → C₆H₁₂O₆ (Glucose) + 6O₂ (Oxygen)

This happens in specialized organelles called chloroplasts, mainly found in the leaves. Inside chloroplasts, there are stacks of disc-like structures called grana (singular: granum), which are made of individual thylakoids. The fluid-filled space surrounding the grana is called the stroma.

Photosynthesis is divided into two main stages:
1. Light-Dependent Reactions: These reactions require light and occur in the thylakoid membranes.
2. Light-Independent Reactions (Calvin Cycle): These reactions do not directly require light but use the products of the light-dependent reactions. They occur in the stroma.

Light-Dependent Reactions

These reactions are all about capturing light energy and converting it into chemical energy in two forms: ATP (Adenosine Triphosphate) and NADPH (Nicotinamide Adenine Dinucleotide Phosphate, reduced form). Think of ATP as the plant's immediate energy currency and NADPH as a carrier of high-energy electrons.

Here's how it works:
* Light Absorption: Chlorophyll, the green pigment in plants, absorbs light energy. This energy excites electrons within the chlorophyll molecules.
* Water Splitting (Photolysis): Water molecules are split (a process called photolysis) to replace the lost electrons in chlorophyll. This splitting also releases:
* Oxygen (O₂): Released into the atmosphere as a byproduct.
* Protons (H⁺): Used to create a proton gradient.
* Electrons (e⁻): Replace those lost by chlorophyll.
* Electron Transport Chain: The excited electrons move through a series of protein complexes embedded in the thylakoid membrane. As they move, they release energy.
* ATP Formation (Photophosphorylation): The energy released by electrons is used to pump protons (H⁺) into the thylakoid lumen, creating a high concentration of protons. These protons then flow back out through an enzyme called ATP synthase, which uses their movement to produce ATP from ADP and inorganic phosphate.
* NADPH Formation: At the end of the electron transport chain, the electrons, along with protons, are used to reduce NADP⁺ to NADPH.

So, the light-dependent reactions take light energy and water, and produce ATP, NADPH, and oxygen.

Light-Independent Reactions (Calvin Cycle)

Also known as the Calvin-Benson cycle, this stage uses the ATP and NADPH generated in the light-dependent reactions to convert carbon dioxide into glucose. This happens in the stroma of the chloroplast.

The Calvin cycle has three main phases:

1. Carbon Fixation:

   • A molecule of CO₂ from the atmosphere combines with a five-carbon sugar called RuBP (Ribulose-1,5-bisphosphate).
   • This reaction is catalyzed by the enzyme RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase).
   • The resulting six-carbon compound is unstable and immediately splits into two molecules of a three-carbon compound called 3-PGA (3-Phosphoglycerate).

2. Reduction:

   • The 3-PGA molecules are then converted into G3P (Glyceraldehyde-3-phosphate).
   • This conversion requires energy from ATP and reducing power from NADPH (both supplied by the light-dependent reactions).
   • For every six G3P molecules produced, one molecule leaves the cycle to be used to build glucose or other organic compounds.

3. Regeneration of RuBP:

   • The remaining five G3P molecules are rearranged and combined to regenerate three molecules of RuBP.
   • This regeneration step also requires energy from ATP.
   • This allows the cycle to continue, fixing more CO₂.

It takes six turns of the Calvin cycle (fixing six CO₂ molecules) to produce one molecule of glucose.

Here's a simplified flow of the Calvin Cycle:

graph TD
    A[CO2 from atmosphere] --> B{Carbon Fixation};
    B --> C[RuBP (5-carbon sugar)];
    C -- RuBisCO enzyme --> D[Unstable 6-carbon intermediate];
    D --> E[2 x 3-PGA (3-carbon compound)];
    E -- ATP & NADPH from Light Reactions --> F[2 x G3P (Glyceraldehyde-3-phosphate)];
    F --> G{One G3P leaves cycle to make Glucose};
    F --> H[Remaining G3P molecules];
    H -- ATP from Light Reactions --> I[Regeneration of RuBP];
    I --> C;

Interdependence of the Stages

It's crucial to understand that these two stages are tightly linked. The light-dependent reactions produce ATP and NADPH, which are essential for the Calvin cycle. In turn, the Calvin cycle uses up ATP and NADPH, converting them back into ADP, inorganic phosphate, and NADP⁺, which are then recycled back to the light-dependent reactions. This continuous cycle ensures that photosynthesis can proceed efficiently.

3. Worked Example

Let's trace the path of carbon and energy through photosynthesis.

Imagine a plant absorbs 12 molecules of water and 6 molecules of carbon dioxide.

1. Light-Dependent Reactions (Thylakoid):

   • The 12 molecules of water (H₂O) are split (photolysis).
   • This releases 12 pairs of electrons, 12 protons (H⁺), and 6 molecules of oxygen (O₂). The oxygen is released into the atmosphere.
   • The energy from sunlight, along with the electrons and protons, is used to produce approximately 18 molecules of ATP and 12 molecules of NADPH. These are the energy carriers.

2. Calvin Cycle (Stroma):

   • The 6 molecules of carbon dioxide (CO₂) enter the cycle.
   • Each CO₂ molecule combines with RuBP, eventually forming 2 molecules of 3-PGA, so 6 CO₂ molecules yield 12 molecules of 3-PGA.
   • Using the 18 ATP and 12 NADPH from the light reactions, these 12 molecules of 3-PGA are converted into 12 molecules of G3P.
   • Out of these 12 G3P molecules, 2 molecules leave the cycle. These two G3P molecules combine to form one molecule of glucose (C₆H₁₂O₆).
   • The remaining 10 G3P molecules are used, along with more ATP, to regenerate the 6 molecules of RuBP needed to continue the cycle.

So, from 6 CO₂ and 12 H₂O, the plant produces 1 glucose molecule and releases 6 O₂ molecules, using light energy.

4. Key Takeaways

• Photosynthesis converts light energy into chemical energy (glucose) in two main stages.
• The light-dependent reactions occur in the thylakoid membranes and produce ATP, NADPH, and oxygen.
• Water is split during the light-dependent reactions, releasing oxygen as a byproduct.
• The Calvin cycle (light-independent reactions) occurs in the stroma and uses ATP and NADPH to fix carbon dioxide into glucose.
• RuBisCO is the key enzyme that fixes carbon dioxide in the Calvin cycle.
• ATP and NADPH are crucial energy carriers linking the two stages of photosynthesis.
• Chloroplasts are the organelles where photosynthesis takes place, with specific parts for each stage.

Common Mistakes to Avoid:
- Don't confuse the location of the two stages: Light-dependent in thylakoids, Calvin cycle in stroma.
- Remember that the Calvin cycle doesn't directly need light, but it depends on the products (ATP, NADPH) from the light reactions.
- Don't forget that oxygen released during photosynthesis comes from the splitting of water, not carbon dioxide.
- Don't mix up ATP and NADPH; ATP is immediate energy, NADPH carries high-energy electrons.

5. Now Try It

Draw a detailed diagram of a chloroplast and label the thylakoids, grana, and stroma. Then, on your diagram, indicate where the light-dependent reactions occur and where the Calvin cycle occurs, and draw arrows showing the flow of ATP, ADP, NADPH, and NADP⁺ between these two locations.

Success looks like: A clear, labelled diagram showing the chloroplast structures, correct locations for each stage, and accurate arrows demonstrating the recycling of ATP/ADP and NADPH/NADP⁺ between the thylakoid membrane and the stroma.