Chapter 7: Photosynthesis

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Photosynthesis: Using Light to Make Food

Introduction: The Central Role of the Sun

Nearly all life on Earth, including humans, can trace its source of energy back to the sun. The process of photosynthesis enables living organisms to capture solar energy and convert it into chemical energy, forming the foundation of most food webs.

Key Point: The sun is the ultimate energy source for most life forms.
Example: Plants use sunlight to produce sugars, which are then consumed by animals.

Biology and Society: A Solar Revolution

Solar energy can be harnessed by both technological and biological means. Photovoltaic (PV) solar panels convert sunlight into electricity, while living organisms use photosynthesis to store solar energy in organic molecules.

Key Point: Plant matter can be burned or converted into biofuels, providing renewable energy sources.
Example: Ethanol produced from corn is a biofuel derived from photosynthetic energy.

The Basics of Photosynthesis

Definition and Overview

Photosynthesis is a process used by plants, algae (protists), and certain bacteria to transform light energy into chemical energy. The process uses carbon dioxide () and water () as starting materials and releases oxygen gas () as a by-product.

Key Point: The chemical energy produced is stored in the bonds of sugar molecules.
Equation:

Autotrophs and Photoautotrophs

Organisms that generate their own organic matter from inorganic ingredients are called autotrophs. Those that use photosynthesis are specifically termed photoautotrophs and serve as producers in most ecosystems.

Key Point: Photoautotrophs include plants, algae, and some bacteria.
Example: Cyanobacteria are aquatic photoautotrophs.

Diversity of Photoautotrophs

Photosynthetic organisms are found in various environments:

Type	Example
Plants (land)	Forest plants
Photosynthetic Protists (aquatic)	Kelp, a large multicellular alga
Photosynthetic Bacteria (aquatic)	Cyanobacteria

Chloroplasts: Sites of Photosynthesis

Structure and Function

Chloroplasts are light-absorbing organelles found mostly in the interior cells of leaves. Their green color is due to chlorophyll, a pigment that plays a central role in converting solar energy to chemical energy.

Key Point: Chloroplasts have a double-membrane envelope, enclosing a compartment filled with stroma (thick fluid).
Key Point: Thylakoids are interconnected membranous sacs suspended in the stroma, concentrated in stacks called grana (singular: granum).
Key Point: Stomata are tiny pores in leaves where enters and exits.

An Overview of Photosynthesis

General Equation and Stages

The overall process of photosynthesis can be divided into two main stages:

Light Reactions: Occur in the thylakoid membranes, where chlorophyll absorbs solar energy and converts it to the chemical energy of ATP and NADPH.
Calvin Cycle: Uses ATP and NADPH to power the production of sugar from carbon dioxide.

Key Point: The reactants of photosynthesis ( and ) are the same as the waste products of cellular respiration.
Key Point: Photosynthesis produces glucose () and oxygen (), which are used in cellular respiration.

Carbon Fixation

Carbon fixation is the initial incorporation of carbon from the atmosphere into organic compounds. This process helps reduce atmospheric and is crucial for global climate regulation.

Key Point: Carbon fixation links biological processes to global climate.

The Light Reactions: Converting Solar Energy to Chemical Energy

The Nature of Sunlight

Sunlight travels as rhythmic waves, and the distance between the crests of two adjacent waves is called a wavelength. The full range of radiation is known as the electromagnetic spectrum.

Type of Radiation	Wavelength (nm)	Energy
Gamma rays	<1	Highest
Visible light	380-750	Medium
Radio waves	>106	Lowest

Chloroplast Pigments

Chloroplasts contain several pigments that absorb light of different wavelengths:

Chlorophyll a: Participates directly in light reactions; absorbs mainly blue-violet and red light.
Chlorophyll b: Conveys absorbed energy to chlorophyll a; absorbs blue and orange light.
Carotenoids: Yellow-orange pigments; absorb mainly blue-green light.

Leaves appear green because green light is poorly absorbed and thus reflected or transmitted.

How Photosystems Harvest Light Energy

Light behaves as both waves and discrete packets of energy called photons. When a pigment molecule absorbs a photon, one of its electrons becomes "excited" and gains energy.

Key Point: Excited electrons usually lose excess energy and return to their ground state, often releasing heat or light (fluorescence).
Example: Fluorescent dyes emit light when their electrons return to the ground state.

Photosystems and Light-Gathering Antennae

In the thylakoid membrane, chlorophyll molecules are organized into photosystems, each consisting of a cluster of pigment molecules that function as a light-gathering antenna. Energy is focused onto a reaction center, where electron transfer occurs.

How the Light Reactions Generate ATP and NADPH

Mechanism and Steps

Two photosystems cooperate in the light reactions:

Photons excite electrons in the chlorophyll of the first photosystem.
Energized electrons pass down an electron transport chain to the second photosystem, releasing energy used to make ATP.
The second photosystem transfers its light-excited electrons to NADP+, reducing it to NADPH.

Key Point: The light reactions are located in the thylakoid membrane.
Key Point: Electron transport chains pump hydrogen ions () across the membrane, and ATP synthase uses the gradient to produce ATP.
Comparison: In cellular respiration, food provides high-energy electrons; in photosynthesis, light-excited electrons flow down the transport chain.

The Calvin Cycle: Making Sugar from Carbon Dioxide

Process and Products

The Calvin cycle functions like a sugar factory within a chloroplast. It constructs an energy-rich sugar molecule called glyceraldehyde 3-phosphate (G3P) using carbon from , energy from ATP, and high-energy electrons from NADPH.

Key Point: G3P can be used to make glucose and other organic compounds.
Equation (simplified):

Summary Table: Key Components of Photosynthesis

Component	Function
Chloroplast	Site of photosynthesis
Chlorophyll	Absorbs light energy
Thylakoid	Location of light reactions
Stroma	Location of Calvin cycle
Photosystem	Light-gathering antenna
ATP & NADPH	Energy carriers for Calvin cycle
G3P	Product of Calvin cycle; precursor to glucose

Additional info: These notes expand on the provided slides by including definitions, equations, and comparisons to cellular respiration, ensuring a self-contained study guide for exam preparation.