Photosynthesis and the Calvin Cycle: Key Concepts and Processes End of CH 8 need to know

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Photosynthesis: Inputs, Outputs, and Key Processes

Overview of Photosynthesis

Photosynthesis is a fundamental biological process by which autotrophic organisms, such as plants, convert light energy into chemical energy, producing carbohydrates from carbon dioxide and water. This process occurs in the chloroplasts and involves two main stages: the light reactions and the Calvin Cycle.

Autotrophs are organisms that produce their own food from inorganic substances. Heterotrophs rely on consuming other organisms for energy.
Producers are autotrophs that form the base of the food chain, while consumers are heterotrophs that feed on producers or other consumers.
Chloroplast structure includes the inner and outer membranes, thylakoid membranes (site of light reactions), granum (stack of thylakoids), and stroma (fluid surrounding thylakoids).

Inputs and Outputs of Photosynthesis

Photosynthesis requires specific inputs and produces essential outputs for cellular function and growth.

Inputs: Light energy, water (H2O), and carbon dioxide (CO2).
Outputs: Glucose (C6H12O6), oxygen (O2).
General equation:

Light Reactions

The light reactions are the first stage of photosynthesis, occurring in the thylakoid membranes.

Convert light energy into chemical energy in the form of ATP and NADPH.
Inputs: Light, water, NADP+, ADP.
Outputs: Oxygen (O2), ATP, NADPH.
Water is split to provide electrons, releasing oxygen as a byproduct.

The Calvin Cycle: Anabolic Pathway for Carbohydrate Synthesis

Overview of the Calvin Cycle

The Calvin Cycle is the second stage of photosynthesis, taking place in the stroma of the chloroplast. It is an anabolic process that uses ATP and NADPH from the light reactions to build carbohydrates from CO2.

Uses ATP and NADPH to convert CO2 into glucose and other carbohydrates.
Does not require light directly, but depends on products of the light reactions.

Phases of the Calvin Cycle

The Calvin Cycle consists of three main phases:

Carbon Fixation: CO2 is attached to a five-carbon sugar, ribulose bisphosphate (RuBP), by the enzyme Rubisco, forming two molecules of 3-phosphoglycerate (3-PGA).
Reduction: ATP and NADPH are used to convert 3-PGA into glyceraldehyde-3-phosphate (G3P), a three-carbon sugar.
Regeneration of RuBP: Some G3P molecules are used to regenerate RuBP, enabling the cycle to continue.

Stoichiometry of the Calvin Cycle

Each turn of the Calvin Cycle incorporates one carbon atom from CO2.
Three turns are required to produce one G3P molecule (a three-carbon sugar).
Six turns are needed to produce enough G3P to synthesize one glucose molecule (which has six carbons).

Key Terms and Definitions

Rubisco: The enzyme that catalyzes the first step of carbon fixation in the Calvin Cycle.
RuBP (Ribulose bisphosphate): The five-carbon sugar that accepts CO2 in the Calvin Cycle.
G3P (Glyceraldehyde-3-phosphate): The three-carbon sugar produced by the Calvin Cycle, which can be used to form glucose and other carbohydrates.

Summary Table: Photosynthesis Inputs and Outputs

Stage	Inputs	Outputs
Light Reactions	Light, H2O, NADP+, ADP	O2, ATP, NADPH
Calvin Cycle	CO2, ATP, NADPH	G3P (used to make glucose), ADP, NADP+

Example Application

For a plant to produce one molecule of glucose, the Calvin Cycle must turn six times, each time fixing one molecule of CO2 and ultimately generating two G3P molecules that combine to form glucose.