Photosynthesis is the process by which plants, algae, and some bacteria convert light energy into chemical energy. It is the main biological pathway by which energy from the Sun enters ecosystems and is responsible for producing much of Earth’s oxygen.

## Basic idea

In oxygen-producing photosynthesis, organisms use light energy to convert carbon dioxide and water into sugars and oxygen.

A simplified overall equation is:

\[
6CO_2 + 6H_2O + light \rightarrow C_6H_{12}O_6 + 6O_2
\]

This equation summarizes the process, but the actual chemistry happens through many enzyme-controlled steps.

## Where photosynthesis happens

In plants and algae, photosynthesis occurs in **chloroplasts**, especially in leaf cells.

A chloroplast has several important parts:

- **Thylakoids**: Flattened membrane sacs where light-dependent reactions happen.
- **Grana**: Stacks of thylakoids.
- **Stroma**: Fluid-filled space surrounding thylakoids; where the Calvin cycle happens.
- **Chlorophyll**: Green pigment that absorbs light.

In photosynthetic bacteria, the process occurs in specialized internal membranes or membrane systems rather than chloroplasts.

## Two main stages of photosynthesis

Photosynthesis has two major stages:

1. **Light-dependent reactions**
2. **Light-independent reactions**, also called the **Calvin cycle**

Despite the name “light-independent,” the Calvin cycle usually occurs during the day because it depends on products made by the light reactions.

---

# 1. Light-dependent reactions

These occur in the **thylakoid membranes**.

Their job is to capture light energy and convert it into chemical energy in the form of:

- **ATP**
- **NADPH**

They also produce oxygen as a byproduct.

## Key events

### Light absorption

Pigments such as **chlorophyll a**, **chlorophyll b**, and carotenoids absorb photons of light.

Chlorophyll absorbs mostly blue and red light and reflects green light, which is why plants appear green.

### Photosystem II

Light energy excites electrons in **Photosystem II**. These electrons are passed into an electron transport chain.

To replace the lost electrons, water molecules are split:

\[
2H_2O \rightarrow 4H^+ + 4e^- + O_2
\]

This is why oxygen is released during photosynthesis.

### Electron transport chain

Electrons move through a series of proteins in the thylakoid membrane. Their movement helps pump hydrogen ions into the thylakoid interior, creating a proton gradient.

### ATP production

Hydrogen ions flow back through an enzyme called **ATP synthase**, which produces ATP from ADP and phosphate.

This process is called **photophosphorylation**.

### Photosystem I

Electrons are re-energized by light in **Photosystem I** and eventually used to reduce NADP⁺ into NADPH.

\[
NADP^+ + H^+ + 2e^- \rightarrow NADPH
\]

## Products of light reactions

The light-dependent reactions produce:

- ATP
- NADPH
- Oxygen

ATP and NADPH are used in the Calvin cycle.

---

# 2. Calvin cycle

The Calvin cycle occurs in the **stroma** of the chloroplast.

Its job is to use ATP and NADPH to convert carbon dioxide into carbohydrate molecules.

The Calvin cycle has three main phases:

## 1. Carbon fixation

Carbon dioxide is attached to a five-carbon molecule called **RuBP**, or ribulose-1,5-bisphosphate.

This reaction is catalyzed by the enzyme **Rubisco**.

The resulting six-carbon compound immediately splits into two three-carbon molecules called **3-PGA**.

## 2. Reduction

ATP and NADPH are used to convert 3-PGA into **G3P**, or glyceraldehyde-3-phosphate.

G3P is a three-carbon sugar that can be used to make glucose, sucrose, starch, cellulose, and other organic molecules.

## 3. Regeneration of RuBP

Some G3P molecules are used to regenerate RuBP so the cycle can continue.

## Calvin cycle summary

For every 3 molecules of CO₂ fixed, the Calvin cycle produces one net G3P molecule.

To produce one G3P, the cycle uses:

- 3 CO₂
- 9 ATP
- 6 NADPH

To make one glucose molecule, the cycle needs roughly:

- 6 CO₂
- 18 ATP
- 12 NADPH

---

# Photosynthetic pigments

Photosynthetic organisms use pigments to absorb light.

## Chlorophyll a

The main pigment in oxygenic photosynthesis. It participates directly in the light reactions.

## Chlorophyll b

An accessory pigment in plants and green algae. It expands the range of light wavelengths that can be absorbed.

## Carotenoids

Yellow, orange, and red pigments. They help absorb light and protect chlorophyll from damage caused by excess light.

## Phycobilins

Pigments found in cyanobacteria and red algae. They are useful for absorbing wavelengths that penetrate deeper into water.

---

# Types of photosynthesis

## Oxygenic photosynthesis

This is the type used by plants, algae, and cyanobacteria.

It uses water as an electron donor and releases oxygen.

\[
CO_2 + H_2O + light \rightarrow carbohydrate + O_2
\]

## Anoxygenic photosynthesis

Some bacteria perform photosynthesis without producing oxygen.

They may use molecules such as hydrogen sulfide instead of water.

A simplified example:

\[
CO_2 + H_2S + light \rightarrow carbohydrate + sulfur
\]

Anoxygenic photosynthesis is found in organisms such as purple sulfur bacteria and green sulfur bacteria.

---

# C3, C4, and CAM photosynthesis

Plants have different adaptations for capturing carbon dioxide.

## C3 photosynthesis

Most plants use C3 photosynthesis.

In C3 plants, CO₂ is fixed directly by Rubisco into a three-carbon compound.

Examples include:

- Wheat
- Rice
- Soybeans
- Most trees

C3 plants work well in cool, moist environments but can suffer from photorespiration in hot or dry conditions.

## C4 photosynthesis

C4 plants first fix CO₂ into a four-carbon compound. This allows them to concentrate CO₂ around Rubisco and reduce photorespiration.

Examples include:

- Corn/maize
- Sugarcane
- Sorghum

C4 photosynthesis is efficient in hot, sunny environments.

## CAM photosynthesis

CAM plants open their stomata at night to take in CO₂ and store it as organic acids. During the day, they close their stomata to conserve water and use the stored CO₂ for the Calvin cycle.

Examples include:

- Cacti
- Pineapple
- Agave
- Many succulents

CAM photosynthesis is especially useful in deserts and dry habitats.

---

# Photorespiration

Photorespiration is a process where Rubisco binds oxygen instead of carbon dioxide.

This wastes energy and reduces sugar production.

Photorespiration becomes more common when:

- Temperatures are high
- CO₂ levels are low
- Stomata are closed to conserve water
- Oxygen levels are high

C4 and CAM plants have evolved ways to reduce photorespiration.

---

# Factors affecting photosynthesis

Several environmental factors influence photosynthetic rate.

## Light intensity

As light increases, photosynthesis usually increases until it reaches a saturation point.

Too much light can damage photosystems, causing photoinhibition.

## Light wavelength

Red and blue light are especially effective for photosynthesis. Green light is less absorbed, though it can still contribute.

## Carbon dioxide concentration

More CO₂ generally increases photosynthesis up to a limit.

## Temperature

Photosynthesis depends on enzymes, so temperature matters. Too cold slows reactions; too hot can denature enzymes or increase photorespiration.

## Water availability

Water is needed as an electron source, but it also affects stomata. When water is scarce, plants close stomata, reducing CO₂ intake.

## Nutrients

Elements such as nitrogen, magnesium, phosphorus, and iron are important for chlorophyll, ATP, enzymes, and electron transport proteins.

---

# Why photosynthesis is important

Photosynthesis is one of the most important processes on Earth.

It:

- Produces oxygen for aerobic life.
- Forms the base of most food webs.
- Removes carbon dioxide from the atmosphere.
- Produces sugars and biomass.
- Supports agriculture and forestry.
- Supplies fossil fuel energy indirectly, because coal, oil, and gas originated from ancient photosynthetic organisms.
- Helps regulate Earth’s climate.

---

# Evolution of photosynthesis

Early photosynthesis likely evolved in bacteria more than 3 billion years ago.

The first photosynthetic organisms probably performed **anoxygenic photosynthesis**, which did not release oxygen.

Later, cyanobacteria evolved oxygenic photosynthesis using water as an electron donor. This led to the **Great Oxidation Event** around 2.4 billion years ago, when oxygen began accumulating in Earth’s atmosphere.

This dramatically changed life on Earth and allowed the evolution of aerobic respiration and eventually complex multicellular organisms.

Chloroplasts in plants and algae originated through **endosymbiosis**, when an ancestral eukaryotic cell engulfed a photosynthetic cyanobacterium that became a chloroplast.

---

# Photosynthesis and cellular respiration

Photosynthesis and cellular respiration are complementary processes.

Photosynthesis stores energy:

\[
CO_2 + H_2O + light \rightarrow sugar + O_2
\]

Cellular respiration releases energy:

\[
sugar + O_2 \rightarrow CO_2 + H_2O + ATP
\]

Plants do both. They photosynthesize in the light and perform cellular respiration continuously.

---

# Common misconceptions

## “Plants get their food from soil”

Plants get minerals and water from soil, but most of their biomass comes from carbon dioxide in the air.

## “Photosynthesis only happens in leaves”

Leaves are the main site, but green stems and other green tissues can also photosynthesize.

## “Plants only release oxygen”

Plants release oxygen during photosynthesis, but they also consume oxygen during cellular respiration.

## “The Calvin cycle does not need light at all”

It does not directly use light, but it depends on ATP and NADPH from the light reactions, so it is usually active when light is available.

---

# In short

Photosynthesis is the biological process that converts sunlight, carbon dioxide, and water into chemical energy stored in sugars. It occurs mainly in chloroplasts, uses chlorophyll and other pigments to capture light, produces oxygen, and supports nearly all life on Earth either directly or indirectly.
