Chapter 7: Photosynthesis – Using Light to Make Food

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Photosynthesis: Using Light to Make Food

Overview of Photosynthesis

Photosynthesis is the process by which plants, algae, and some bacteria convert solar energy into chemical energy, producing organic molecules and oxygen essential for life on Earth.

Photoautotrophs: Organisms that use light energy to synthesize organic compounds from inorganic substances. Examples include plants, algae, and cyanobacteria.
Heterotrophs: Organisms that obtain organic food molecules by eating other organisms or their by-products.

Key Equation for Photosynthesis:

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

Structure of Chloroplasts

Chloroplasts are the organelles where photosynthesis occurs, surrounded by a double membrane.
Inside are stacks of thylakoids (membranous sacs) and a dense fluid called stroma.
Chlorophyll is the main pigment that absorbs light energy for photosynthesis.

Tracing Photosynthesis: Isotope Experiments

Scientists used isotopes to determine the path of atoms during photosynthesis.

Experiment 1: $6\ \mathrm{CO}_2 + 12\ \mathrm{H}_2\mathrm{O} \rightarrow \mathrm{C}_6\mathrm{H}_{12}\mathrm{O}_6 + 6\ \mathrm{H}_2\mathrm{O} + 6\ \mathrm{O}_2$
Experiment 2: $6\ \mathrm{CO}_2 + 12\ \mathrm{H}_2^{18}\mathrm{O} \rightarrow \mathrm{C}_6\mathrm{H}_{12}\mathrm{O}_6 + 6\ \mathrm{H}_2\mathrm{O} + 6\ ^{18}\mathrm{O}_2$

Conclusion: The oxygen produced in photosynthesis comes from water, not carbon dioxide.

Photosynthesis as a Redox Process

Photosynthesis involves oxidation-reduction (redox) reactions, similar to cellular respiration.

In photosynthesis, water is oxidized (loses electrons) and carbon dioxide is reduced (gains electrons).
Redox reactions transfer energy by moving electrons from one molecule to another.

The Light Reactions: Converting Solar Energy to Chemical Energy

Nature of Light

Sunlight is a form of electromagnetic energy (radiation).
Visible light ranges from about 380 nm (violet) to 750 nm (red).
Shorter wavelengths have higher energy; longer wavelengths have lower energy.

Photosynthetic Pigments

Chlorophyll a: Absorbs blue-violet and red light; reflects green.
Chlorophyll b: Absorbs blue and orange light; reflects green.
Carotenoids: Absorb excess light energy and protect chlorophyll; reflect yellow and orange.

Absorption spectra show which wavelengths are absorbed by each pigment.

Photosystems and Light Harvesting

Photosystems are complexes of chlorophyll and proteins in the thylakoid membrane.
They capture light energy and transfer excited electrons to a primary electron acceptor.
There are two types: Photosystem II (PSII) and Photosystem I (PSI).

Electron Transport Chain and Chemiosmosis

Excited electrons move through an electron transport chain, releasing energy used to pump H+ ions into the thylakoid space.
ATP synthase uses the flow of H+ back into the stroma to synthesize ATP from ADP and Pi.
This process is called photophosphorylation.
NADP+ is reduced to NADPH, which carries high-energy electrons to the Calvin cycle.

$\text{ADP} + \text{P}_i \xrightarrow{\text{ATP synthase}} \text{ATP}$ $\text{NADP}^+ + 2e^- + 2H^+ \rightarrow \text{NADPH} + H^+$

Summary Table: Light Reactions

Input	Output	Location
H2O, Light, NADP+, ADP + Pi	O2, NADPH, ATP	Thylakoid membrane

The Calvin Cycle: Reducing CO2 to Sugar

Overview of the Calvin Cycle

The Calvin cycle uses ATP and NADPH from the light reactions to convert CO2 into G3P, a three-carbon sugar that can be used to form glucose and other organic molecules.

Occurs in the stroma of the chloroplast.
Three main phases: carbon fixation, reduction, and regeneration of RuBP.
Rubisco is the enzyme that catalyzes the first step (carbon fixation).

$3\ \mathrm{CO}_2 + 9\ \mathrm{ATP} + 6\ \mathrm{NADPH} \rightarrow 1\ \mathrm{G3P} + 9\ \mathrm{ADP} + 8\ \mathrm{P}_i + 6\ \mathrm{NADP}^+ + 6\ \mathrm{H}_2\mathrm{O}$

The Global Significance of Photosynthesis

Photosynthesis provides food and oxygen for almost all living organisms.
Sugars produced are used to make other organic molecules (proteins, lipids, cellulose).
Many glucose molecules are linked to form cellulose, the main component of plant cell walls.

Photosynthesis and Climate Change

CO2 and other greenhouse gases trap heat in the atmosphere (greenhouse effect).
Reducing fossil fuel use and deforestation can help moderate climate change.
Global warming is a long-term increase in Earth's average temperature, largely due to increased greenhouse gases.

Evolutionary Adaptations in Photosynthesis

C3, C4, and CAM Pathways

C3 plants: Use the Calvin cycle directly; most common but less efficient in hot, dry climates due to photorespiration.
Photorespiration: Occurs when Rubisco binds O2 instead of CO2, wasting energy and reducing sugar output.
C4 plants: Use spatial separation; CO2 is first fixed into a 4-carbon compound in mesophyll cells, then transported to bundle sheath cells for the Calvin cycle.
CAM plants: Use temporal separation; CO2 is fixed at night into organic acids and released for the Calvin cycle during the day (e.g., cacti, pineapples).

Type	Adaptation	Example
C3	Direct Calvin cycle	Wheat, rice
C4	Spatial separation of steps	Corn, sugarcane
CAM	Temporal separation of steps	Cacti, pineapple

Key Terms and Definitions

Autotroph: An organism that makes its own food from inorganic substances.
Photoautotroph: An autotroph that uses light as an energy source.
Heterotroph: An organism that obtains food by consuming other organisms.
Chloroplast: Organelle where photosynthesis occurs.
Thylakoid: Membranous sac inside chloroplasts; site of light reactions.
Stroma: Fluid inside chloroplasts surrounding thylakoids; site of Calvin cycle.
Chlorophyll: Green pigment that absorbs light energy.
Photosystem: Cluster of pigments and proteins that capture light energy.
ATP synthase: Enzyme that synthesizes ATP using a proton gradient.
Photophosphorylation: Production of ATP using light energy.
Calvin cycle: Series of reactions that use ATP and NADPH to convert CO2 into sugars.
Rubisco: Enzyme that catalyzes the first step of the Calvin cycle.
Photorespiration: Process where Rubisco binds O2 instead of CO2, reducing photosynthetic efficiency.

Sample Checkpoint Questions

How do the reactant molecules of photosynthesis reach the chloroplasts in leaves?
What color of light is least effective at driving photosynthesis? (Answer: Green, because it is reflected, not absorbed.)
What are the reactants and products of the light-dependent reaction?
How is water used in the light-dependent reaction?
What is the purpose of photosystems?

Summary Table: Photosynthesis Overview

Stage	Location	Inputs	Outputs
Light Reactions	Thylakoid membrane	Light, H2O, NADP+, ADP + Pi	O2, ATP, NADPH
Calvin Cycle	Stroma	CO2, ATP, NADPH	G3P (sugar), ADP, NADP+

Additional info: These notes synthesize and expand upon the provided slides and images, filling in standard academic context for a comprehensive understanding of photosynthesis as covered in a General Biology course.