Photosynthesis: quick revision, 12 bites

What is Photosynthesis?

Life's energy factory.

**Photosynthesis** is the process by which green plants, algae, and some bacteria convert light energy into chemical energy. This chemical energy is stored in organic compounds, primarily glucose. It's fundamental for almost all life on Earth, as it produces the oxygen we breathe and the food we eat.

Takeaway. Photosynthesis converts light energy into chemical energy, producing food and oxygen.

Autotrophs vs. Heterotrophs

Who makes their own food?

**Autotrophs** are organisms that can produce their own food, typically through photosynthesis (photoautotrophs) or chemosynthesis. Plants are classic examples. **Heterotrophs**, on the other hand, obtain food by consuming other organisms. Animals and fungi are heterotrophs.

Takeaway. Autotrophs make their own food, while heterotrophs consume others.

The Photosynthesis Equation

The core chemical reaction.

This balanced equation summarizes the overall process, showing the reactants (carbon dioxide, water, light energy) and the products (glucose, oxygen).

Takeaway. Carbon dioxide and water, with light, yield glucose and oxygen.

Where it Happens: Chloroplasts

The plant's solar panels.

In plants, photosynthesis primarily occurs in specialized organelles called **chloroplasts**, found mainly in the cells of leaves. Chloroplasts contain the pigment **chlorophyll**, which absorbs light energy. Each chloroplast has internal structures like thylakoids and stroma.

Takeaway. Chloroplasts, containing chlorophyll, are the sites of photosynthesis in plants.

Chlorophyll: The Green Pigment

Why plants are green.

**Chlorophyll** is the primary pigment responsible for absorbing light energy during photosynthesis. It absorbs most wavelengths of light in the blue and red parts of the electromagnetic spectrum, but reflects green light, which is why plants appear green to our eyes.

Takeaway. Chlorophyll absorbs light for photosynthesis and reflects green light.

Two Main Stages: Light-Dependent Reactions

Capturing light energy.

The **light-dependent reactions** occur in the **thylakoid membranes** of the chloroplast. Here, light energy is absorbed by chlorophyll and converted into chemical energy in the form of **ATP** (adenosine triphosphate) and **NADPH** (nicotinamide adenine dinucleotide phosphate). Water is split, releasing oxygen as a b…

Takeaway. Light reactions convert light into ATP and NADPH, releasing oxygen, in thylakoids.

Two Main Stages: Light-Independent Reactions (Calvin Cycle)

Making sugar from CO2.

The **light-independent reactions**, also known as the **Calvin Cycle**, occur in the **stroma** (fluid-filled space) of the chloroplast. They use the ATP and NADPH generated during the light-dependent reactions to convert carbon dioxide ($CO_2$) into glucose ($C_6H_{12}O_6$). This process is also called carbon fixatio…

Takeaway. The Calvin Cycle uses ATP and NADPH to fix CO2 into glucose in the stroma.

Role of Stomata

Gas exchange in leaves.

**Stomata** are small pores, primarily on the underside of leaves, regulated by guard cells. They allow for the exchange of gases: carbon dioxide enters the leaf for photosynthesis, and oxygen and water vapor (transpiration) exit. Their opening and closing are crucial for balancing gas exchange with water conservation.

Takeaway. Stomata regulate CO2 intake and O2 release, balancing photosynthesis and water loss.

Factors Affecting Photosynthesis: Light Intensity

More light, more sugar?

As **light intensity** increases, the rate of photosynthesis generally increases, up to a certain saturation point. Beyond this point, other factors become limiting. Too little light means insufficient energy to drive the light-dependent reactions.

Takeaway. Photosynthesis rate increases with light intensity until saturation.

Factors Affecting Photosynthesis: Carbon Dioxide Concentration

The CO2 fuel.

An increase in **carbon dioxide concentration** generally leads to an increased rate of photosynthesis, assuming other factors are not limiting. CO2 is a key reactant in the Calvin Cycle, so its availability directly impacts glucose production.

Takeaway. Higher CO2 concentration generally boosts photosynthesis rate.

Factors Affecting Photosynthesis: Temperature

Enzymes at work.

**Temperature** affects the enzyme-catalyzed reactions within photosynthesis, particularly the Calvin Cycle. The rate increases with temperature up to an optimal point, after which enzymes can denature, causing the rate to sharply decline.

Takeaway. Temperature affects photosynthesis, with an optimal range for enzyme activity.

Why Photosynthesis Matters

The foundation of ecosystems.

Takeaway. Photosynthesis provides food, oxygen, and regulates atmospheric carbon dioxide.