The Calvin Cycle

Photosynthesis Overview

Photosynthesis occurs in two stages: the Light Reactions and the Calvin Cycle. The Light Reactions use light energy to form ATP and NADPH, while the Calvin Cycle uses those products plus CO₂ to form sugars.

Role of Chlorophyll

Chlorophyll absorbs photons of light to power the Light Reactions, enabling the production of ATP and NADPH used in the Calvin Cycle.

Two Stages of Photosynthesis

Light Reactions (light-dependent, in thylakoid membranes) and the Calvin Cycle (light-independent, in the stroma).

Light Requirement

The Light Reactions cannot occur without light because their photosystems harvest energy directly from photons.

Purpose of the Calvin Cycle

The Calvin Cycle uses ATP and NADPH from the Light Reactions along with CO₂ from the atmosphere to produce sugars.

Nickname for the Calvin Cycle

The Calvin Cycle is sometimes called a “sugar factory” because it synthesizes glucose and other carbohydrates.

Why It’s Called a Cycle

The Calvin Cycle regenerates its starting molecule (RuBP) at the end of each round, allowing it to continue repeatedly.

Calvin Cycle Location

The Calvin Cycle takes place in the stroma of the chloroplast, the fluid outside the thylakoid membranes.

Inputs of the Calvin Cycle

ATP, NADPH, and CO₂.

Outputs of the Calvin Cycle

G3P (a 3-carbon sugar), ADP + P, and NADP⁺, which return to the Light Reactions.

Main Product of the Calvin Cycle

G3P (glyceraldehyde-3-phosphate), a high-energy 3-carbon sugar used to build glucose and other organic molecules.

Step 1 – Carbon Fixation

The enzyme Rubisco attaches CO₂ to a 5-carbon sugar called RuBP, forming an unstable 6-carbon compound that splits into two 3-carbon molecules of 3-PGA.

Rubisco

The enzyme responsible for fixing carbon by combining CO₂ with RuBP during the first step of the Calvin Cycle.

RuBP

Ribulose bisphosphate, a 5-carbon sugar that binds with CO₂ during carbon fixation and is regenerated at the end of the cycle.

3-PGA

3-phosphoglycerate, the 3-carbon compound formed immediately after carbon fixation when the 6-carbon intermediate breaks down.

Step 2 – Reduction Phase

ATP and NADPH from the Light Reactions are used to convert 3-PGA into G3P, a higher-energy 3-carbon compound.

ATP and NADPH in Reduction

6 ATP molecules convert to 6 ADP + P, and 6 NADPH convert to 6 NADP⁺ during the reduction phase.

G3P Formation

The reduction reactions produce G3P, a 3-carbon sugar that stores chemical energy and can be used to form glucose.

Step 3 – Release of G3P

One G3P molecule exits the cycle to be used in sugar synthesis, while the other five G3P molecules remain to regenerate RuBP.

Step 4 – Regeneration of RuBP

Using 3 ATP molecules from the Light Reactions, the remaining 5 G3P molecules are rearranged to regenerate 3 RuBP molecules, restarting the cycle.

ATP in Regeneration

ATP provides the energy to rearrange G3P molecules back into RuBP, maintaining the cyclical process.

Calvin Cycle Summary Equation

3 CO₂ + 9 ATP + 6 NADPH → 1 G3P + 9 ADP + 8 P + 6 NADP⁺.

Number of Turns to Make Glucose

Two full turns of the Calvin Cycle are required to produce enough G3P to make one glucose molecule (C₆H₁₂O₆).

Relationship Between Light Reactions and Calvin Cycle

The Light Reactions provide ATP and NADPH for the Calvin Cycle, and the Calvin Cycle regenerates ADP, P, and NADP⁺ for the Light Reactions.

Dependence of the Calvin Cycle on Light Reactions

Though it can occur in the dark, the Calvin Cycle depends indirectly on light because it needs ATP and NADPH, which are only made when light is present.

Factor: Light Intensity

Higher light intensity increases the rate of photosynthesis up to a point, after which it levels off because other factors become limiting.

Factor: Carbon Dioxide Concentration

Increased CO₂ concentration speeds up photosynthesis until Rubisco is saturated and the rate levels off.

Factor: Temperature

Photosynthesis occurs most efficiently at an optimal temperature range; extreme heat or cold slows enzyme activity.

The Greenhouse Analogy

A greenhouse allows sunlight in but traps heat inside; Earth’s atmosphere behaves similarly, trapping heat near the surface.

Greenhouse Effect

The process by which gases in Earth’s atmosphere trap heat, preventing it from escaping into space.

Main Greenhouse Gas

Carbon dioxide (CO₂) is one of the most important greenhouse gases contributing to the greenhouse effect.

Photosynthesis and Climate Change

Increased photosynthesis could help offset CO₂ emissions, but deforestation reduces the number of plants available to absorb CO₂.

Human Impact on Photosynthesis

Deforestation and pollution reduce the rate of global photosynthesis, worsening the greenhouse effect and climate change.

Hope for Balance

Restoring vegetation and protecting ecosystems could increase photosynthetic CO₂ absorption, helping stabilize Earth’s climate.

Overall Role of the Calvin Cycle

Converts inorganic CO₂ into organic sugars using energy from ATP and reducing power from NADPH.

Why ATP and NADPH Are Needed

ATP provides the energy, and NADPH provides the electrons and hydrogen required to form G3P from CO₂.

Connection to the Light Reactions

The ATP and NADPH used in the Calvin Cycle are generated during the Light Reactions, linking the two stages of photosynthesis.

End Products Used By Plants

G3P molecules produced in the Calvin Cycle are used to synthesize glucose, starch, cellulose, and other essential biomolecules.