Photosynthesis: How Plants Power Life
Foundations of Life’s Energy
Life on Earth fundamentally relies on two critical elements: the sun and the process of photosynthesis.
Capturing Solar Energy
For photosynthesis to occur, autotrophs (organisms that produce their own food) must capture light energy directly from the sun.
How Sunlight Reaches Earth
Energy travels from the sun to Earth primarily in the form of light.
The Colors of Sunlight
Although the sun’s light appears white, it is composed of various wavelengths, visible as the colors of the rainbow:
- Red
- Orange
- Yellow
- Green
- Blue
- Indigo
- Violet
Plants’ Light-Gathering Tools
Plants gather the sun’s energy through specialized molecules called pigments.
What Are Pigments?
Pigments are molecules that absorb light. Their primary role for living things that carry out photosynthesis is to capture energy from sunlight.
Chlorophyll: The Essential Plant Pigment
The most important pigment in plants is chlorophyll.
Chlorophyll is a green pigment found in the chloroplasts of plants, algae, and some bacteria. It absorbs light energy, which is then used to carry out photosynthesis.
Chlorophyll’s Absorption Spectrum
Chlorophyll in plants absorbs blue-violet and red light most effectively. It does not absorb green light very well.
Why Leaves Appear Green
Leaves look green because chlorophyll reflects green light rather than absorbing it.
Where Photosynthesis Occurs
Photosynthesis takes place within a specialized organelle called the chloroplast.
Chloroplasts: The Photosynthesis Organelle
A chloroplast is the organelle where photosynthesis occurs.
Key Structures within Chloroplasts
Inside the chloroplast, several structures play vital roles:
- Thylakoids
- Flattened membrane sacs within the chloroplast, used to convert light energy into chemical energy. They are connected to one another and arranged in stacks called grana (singular: granum).
- Thylakoid Membrane
- The photosynthetic membrane within a chloroplast that contains light-gathering pigment molecules (like chlorophyll) and electron transport chains.
- Stroma
- In plants, this is the fluid-filled space that surrounds the thylakoids within a chloroplast.
Electron Carriers and Energy Transfer
During photosynthesis, energy is transferred via electron carrier molecules.
NADP+ and NADPH
NADP+ (nicotinamide adenine dinucleotide phosphate) is a carrier molecule that accepts high-energy electrons from chlorophyll and transfers them to other molecules during photosynthesis.
NADPH is the reduced form of NADP+, an electron carrier that provides the high-energy electrons needed for the light-independent reactions of photosynthesis.
High-Energy Electrons
High-energy electrons are crucial for transferring energy. In photosynthesis, these electrons are energized by light and passed along electron transport chains, where their energy is used to produce ATP and NADPH.
The Two Stages of Photosynthesis
Photosynthesis proceeds in two main stages:
- Light-Dependent Reactions: This is the first stage of photosynthesis. These reactions use energy from light to produce ATP and NADPH. They absorb energy from sunlight and transfer it to the light-independent reactions. These reactions occur in the thylakoid membranes.
- Light-Independent Reactions (Calvin Cycle): This is the second stage of photosynthesis. Taking place in the stroma, ATP and NADPH (which carries hydrogen and high-energy electrons) are used as forms of chemical energy to convert carbon dioxide and water into useful organic molecules (sugars).
The Overall Photosynthesis Equation
The entire process of photosynthesis can be summarized by a chemical equation:
Chemical Equation
6CO₂ + 6H₂O + Light Energy → C₆H₁₂O₆ + 6O₂
Word Equation
In words, the equation is:
Carbon Dioxide + Water + Light Energy → Sugars + Oxygen