Photosynthesis is the process by which certain organisms use light energy to convert carbon dioxide and water into chemical energy, usually in the form of sugars, while releasing oxygen in oxygenic photosynthesis. It is one of the most important processes on Earth because it underpins most food chains and has shaped the planet’s atmosphere.

## Big picture
Photosynthesis is carried out by:
- **Plants**
- **Algae**
- **Cyanobacteria**

There are also **anoxygenic photosynthetic bacteria** that use light but do not produce oxygen.

At a simple level, oxygenic photosynthesis can be summarized as:

**carbon dioxide + water + light → sugar + oxygen**

A more specific overall equation is often written as:

**6 CO₂ + 6 H₂O + light energy → C₆H₁₂O₆ + 6 O₂**

That formula is a simplification, but it captures the main idea.

## Why it matters
Photosynthesis:
- Produces most of the **oxygen** in Earth’s atmosphere
- Forms the base of most **ecosystems**
- Converts **solar energy** into chemical energy
- Removes **carbon dioxide** from the atmosphere
- Supports agriculture, forestry, and nearly all animal life indirectly

## Where it happens
### In plants
Photosynthesis occurs mainly in **chloroplasts**, especially in leaf cells.

Key parts of a chloroplast:
- **Outer and inner membranes**
- **Stroma**: fluid-filled interior
- **Thylakoids**: flattened membrane sacs
- **Grana**: stacks of thylakoids

### In algae
Photosynthesis occurs in chloroplasts similar to plants.

### In cyanobacteria
Photosynthesis occurs on internal membrane systems, since they do not have chloroplasts.

## Pigments
Pigments absorb light energy.

### Main pigment
- **Chlorophyll a**: the primary pigment in oxygenic photosynthesis

### Accessory pigments
- **Chlorophyll b** in plants
- **Carotenoids**: absorb additional wavelengths and help protect from damage
- **Phycobilins** in cyanobacteria and red algae

These pigments broaden the range of light that can be used.

## Two major stages of photosynthesis
Photosynthesis is often divided into:

1. **Light-dependent reactions**
2. **Carbon fixation reactions** often called the **Calvin cycle**

These are linked but distinct.

---

# 1) Light-dependent reactions
These occur in the **thylakoid membranes**.

Their main goals are to:
- Capture light energy
- Split water
- Produce **ATP**
- Produce **NADPH**
- Release **O₂**

## How they work
### Step 1: Light absorption
Light excites electrons in chlorophyll molecules, especially in two major photosystems:
- **Photosystem II (PSII)**
- **Photosystem I (PSI)**

### Step 2: Water splitting
At PSII, water is split in a process called **photolysis**:

**2 H₂O → 4 H⁺ + 4 e⁻ + O₂**

This provides:
- Electrons to replace those lost from chlorophyll
- Protons that help build a gradient
- Oxygen as a byproduct

### Step 3: Electron transport chain
Excited electrons move through an electron transport chain. As they move, energy is used to pump protons into the thylakoid interior, creating a **proton gradient**.

### Step 4: ATP synthesis
Protons flow back through **ATP synthase**, which uses that gradient to make **ATP** from ADP + phosphate.

This is called **chemiosmosis**.

### Step 5: NADPH formation
At PSI, electrons are re-excited by light and eventually used to reduce **NADP⁺** to **NADPH**.

## Outputs of the light reactions
- ATP
- NADPH
- O₂

ATP and NADPH are then used in the next stage.

---

# 2) Calvin cycle
This occurs in the **stroma** of chloroplasts.

Its purpose is to use ATP and NADPH to convert **CO₂ into carbohydrates**.

## Main phases
### A. Carbon fixation
The enzyme **RuBisCO** attaches CO₂ to a 5-carbon molecule called **RuBP**.

This creates an unstable 6-carbon compound that quickly splits into two 3-carbon molecules.

### B. Reduction
ATP and NADPH are used to convert these molecules into **G3P** (glyceraldehyde-3-phosphate), a 3-carbon sugar.

### C. Regeneration
Most of the G3P is used to regenerate RuBP so the cycle can continue.

## Net result
The Calvin cycle does not directly make glucose in one step. Instead, it produces G3P, which can be used to build:
- Glucose
- Sucrose
- Starch
- Cellulose
- Other organic molecules

## Important point
The Calvin cycle is sometimes called the “dark reactions,” but that term is misleading because these reactions do not require darkness; they just do not directly use light.

---

# Photosynthetic efficiency and limitations
Photosynthesis is not perfectly efficient.

## Limiting factors
- **Light intensity**
- **Carbon dioxide concentration**
- **Temperature**
- **Water availability**
- **Nutrient availability**
- **Leaf structure and chlorophyll content**

## Why temperature matters
Enzymes involved in photosynthesis are temperature-sensitive. Too cold slows reactions; too hot can damage enzymes and increase water loss.

## Why water matters
Water is needed as an electron source, and water stress causes stomata to close, limiting CO₂ entry.

---

# Stomata and gas exchange
In plants, gas exchange occurs through tiny openings called **stomata**, mostly on leaf surfaces.

- CO₂ enters for photosynthesis
- O₂ exits
- Water vapor can also escape, causing **transpiration**

Guard cells regulate stomatal opening and closing.

---

# Photorespiration
A major inefficiency in many plants is **photorespiration**.

## What causes it?
RuBisCO can bind:
- **CO₂** normally
- **O₂** mistakenly

When O₂ is used instead of CO₂, the plant wastes energy and releases previously fixed carbon.

## Why it matters
Photorespiration is more common:
- At high temperatures
- When CO₂ is low
- When stomata are closed to conserve water

---

# C3, C4, and CAM plants
Plants have evolved different ways to reduce photorespiration.

## C3 plants
- Most plants
- First stable product has 3 carbons
- Efficient in cool, moist, moderate-light conditions

## C4 plants
Examples: maize, sugarcane

- First fix CO₂ into a 4-carbon compound
- Separate initial fixation and Calvin cycle between different cells
- Reduce photorespiration
- Better in hot, bright environments

## CAM plants
Examples: cacti, pineapple

- Open stomata at night
- Fix CO₂ at night and store it
- Use stored CO₂ during the day
- Excellent water conservation strategy

---

# Photosynthesis in non-plant organisms
## Algae
Algae are major global photosynthesizers, especially in oceans. Phytoplankton produce a huge fraction of Earth’s oxygen.

## Cyanobacteria
These ancient organisms were crucial in oxygenating Earth’s atmosphere billions of years ago.

## Anoxygenic photosynthesis
Some bacteria use light but do not split water and do not release oxygen. They may use substances like:
- Hydrogen sulfide
- Hydrogen
- Iron compounds

This is thought to resemble earlier forms of photosynthesis on early Earth.

---

# Evolutionary significance
Photosynthesis likely evolved very early in life’s history.

## Key milestones
- Early anoxygenic photosynthesis in bacteria
- Evolution of oxygenic photosynthesis in cyanobacteria
- Rise of atmospheric oxygen in the **Great Oxidation Event** about 2.4 billion years ago
- Endosymbiosis leading to chloroplasts in algae and plants

## Endosymbiotic theory
Chloroplasts are believed to have originated when a eukaryotic cell engulfed a cyanobacterium-like ancestor that became a permanent symbiont.

Evidence includes:
- Chloroplast DNA
- Double membranes
- Bacterial-like ribosomes
- Replication by binary fission

---

# The role of light
Photosynthesis depends on **light quality** and **light quantity**.

## Action spectrum
Shows which wavelengths are most effective for photosynthesis.

## Absorption spectrum
Shows which wavelengths are absorbed by pigments.

Chlorophyll absorbs mainly:
- Blue light
- Red light

It reflects much of the green light, which is why plants look green.

---

# Chemical energy and carbon flow
Photosynthesis is the primary route by which inorganic carbon enters living systems.

It supports:
- Biomass formation
- Food webs
- Fuel and biomaterial production
- Long-term carbon storage in biomass, soils, and fossil fuels

Over geological time, photosynthesis has also influenced:
- Atmospheric composition
- Climate
- Ocean chemistry
- Evolution of aerobic life

---

# Artificial and applied photosynthesis
Scientists study photosynthesis to:
- Improve crop yields
- Engineer drought- and heat-tolerant plants
- Develop **artificial photosynthesis** systems
- Produce solar fuels such as hydrogen or carbon-based fuels
- Capture and store atmospheric CO₂

Artificial photosynthesis aims to mimic natural light harvesting and energy conversion.

---

# Common misconceptions
## “Plants get their mass from soil”
Mostly false. Most plant biomass comes from **CO₂ in the air**, not from soil. Soil provides minerals and water, but carbon in dry plant mass largely comes from atmospheric carbon dioxide.

## “Photosynthesis only happens in daylight”
The light reactions need light, but the Calvin cycle can run when ATP and NADPH are available. In practice, it is usually linked to light because those energy carriers are produced by the light reactions.

## “Plants breathe in CO₂ and breathe out O₂”
This is a simplification. Plants do both **photosynthesis** and **cellular respiration**. In daylight, photosynthesis usually dominates gas exchange; at night, respiration dominates.

---

# A compact summary
Photosynthesis is the biological process that uses light energy to turn CO₂ and water into sugars, with oxygen as a byproduct in oxygenic photosynthesis. It happens in plants, algae, and cyanobacteria, uses chlorophyll and other pigments, involves light reactions and the Calvin cycle, and is essential for life on Earth.

If you want, I can also give you:
1. a **very simple explanation for kids**,  
2. a **high-school level breakdown**, or  
3. a **deep biochemistry version** with all the reactions and enzymes.
