Photosynthesis

What is photosynthesis?

Photosynthesis is a complex reaction by which plants generate organic matter (sugars) with the energy from sunlight from inorganic matter, namely water and CO₂. Therefore, photosynthesis is a light-driven, oxidation-reduction reaction.

The glucose formed in photosynthesis provides energy for the plant and carbon molecules to make a wide array of biomolecules.

There are two stages of photosynthesis: the light-dependent reaction and the light-independent reaction. We sometimes call the light-independent reaction the ‘dark reaction’ or the ‘Calvin cycle.’

Where does photosynthesis occur in the plant?

Photosynthesis takes place in the leaves, specifically in the chloroplasts from the leaves. Chloroplasts are membranous organelles specialised in photosynthetic reactions. Like mitochondria, they contain their own DNA and are thought to have evolved into organelles following the endosymbiotic theory.

Leaves have several structural adaptations that allow them to perform photosynthesis efficiently. These include:

• A wide and flat structure, creating a large surface area that absorbs a high amount of sunlight and allows for more gas exchange.
• They are organised in thin layers with minimal overlapping between the leaves. This minimises the chance of one leaf shadowing another, and the thinness allows for the diffusion of gases to be kept short.
• The cuticle and epidermis are transparent, allowing sunlight to penetrate through to the mesophyll cells underneath.

Fig. 1. Plant leaf structure. Note all the adaptations we are mentioning in this article. The plant leaf is truly optimised to photosynthesize!

As you will see from Figure 1, leaves also have multiple cellular adaptations that allow for photosynthesis to occur. These include:

• Elongated mesophyll cells. This allows more chloroplasts to be packed inside them. Chloroplasts are responsible for collecting light energy from the sun.
• Multiple stomata that allow for gaseous exchange, so there is a short diffusion pathway between the mesophyll cells and the stomata. Stomata will also open and close in response to changes in light intensity.
• Networks of xylem and phloem that respectively bring water to the leaf cells and carry away the products of photosynthesis - specifically glucose.
• Multiple air spaces in the lower mesophyll. These allow for more efficient diffusion of carbon dioxide and oxygen.

Where does photosynthesis occur in the leaf cell?

Most of the photosynthesis reaction occurs in the plant's chloroplasts. Chloroplasts contain chlorophyll, a green pigment that can ‘capture’ sunlight. Chlorophyll is found in the membrane of the thylakoid discs, which are small compartments inside the structure of the chloroplast. The light-dependent reaction takes place along this thylakoid membrane. The light-independent reaction takes place in the stroma, fluid inside the chloroplast that surrounds stacks of thylakoid discs (collectively called ‘grana’).

Below, Figure 2 outlines the general structure of a chloroplast:

Fig. 2. Chloroplast structure.

Photosystems and photosynthesis

Photosystems are multi-protein complexes found in the thylakoid membranes of chloroplasts in plants and some algae. They are responsible for absorbing light energy and converting it into chemical energy through the process of photosynthesis.

There are two types of photosystems:

• Photosystem I (PSI). Counterintuitively, PSI functions second in the light-dependent reactions of photosynthesis and absorbs light with a peak wavelength of 700 nm.
• Photosystem II (PSII). PSII functions first and absorbs light with a peak wavelength of 680 nm.

Together, these two photosystems work in concert during the photosynthetic reaction to produce ATP and NADPH, which are necessary for the Calvin cycle or dark phase of photosynthesis. I.e. they are responsible for producing the energy that is required to produce glucose at the end of the process, which is the main goal of photosynthesis for plants.

What is the equation for photosynthesis?

The balanced equation for photosynthesis in plants is the following:

\(6CO_2 + 6H_2O \xrightarrow {\text{Solar energy}} C_6H_{12}O_6 + 6O_2\)

As you can see, each photosynthesis reaction needs 6 molecules of carbon dioxide (CO₂) and 6 water (H₂O) molecules because each glucose molecule, the sugar (i.e. organic molecule) that is produced through photosynthesis, has 6 carbon and 12 hydrogen atoms.

Simplified to be written in plain words, it is as follows:

\(\text{Carbon dioxide + Water + Solar energy} \longrightarrow \text{Glucose + Oxygen}\)

However, the equation in plain text is not totally correct, as it is not stating how many molecules of each reagent and product are needed for the reaction. The word equation is an easy way to explain the key concepts of photosynthesis: carbon dioxide and water are used, together with the energy from sunlight, to produce organic matter (glucose) and oxygen as a byproduct.

Fig. 3. The basic diagram of photosynthesis.

What are the stages of photosynthesis?

There are two main stages to photosynthesis: the light-dependent phase and the dark phase or light-independent reaction. The light-dependent phase can be further divided into 4 stages, whilst the dark phase consists of only 1 step, meaning that in total photosynthesis has 5 steps.

Light-dependent phase reactions

Step 1: Absorption of light

The first step involves the chlorophyll in the photosystem II complex (PSII) of chloroplasts absorbing light. By absorbing light the chlorophyll is absorbing energy, which ionises the chlorophyll as electrons leave it and are carried down an electron transfer chain down the thylakoid membrane.

Step 2: Oxidation

Using the light energy absorbed by chlorophyll, the light-dependent reaction occurs. This occurs in two photosystems, which are located along the thylakoid membrane. Water splits into oxygen (O₂), protons (H⁺) ions and electrons (e^-). The electrons are then carried by plastocyanin (a copper-containing protein that mediates electron transfer) from PSII to PSI for the next part of the light reaction.

The equation for the first light-dependent reaction is:

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

Step 3: Reduction

The electrons produced in the last stage pass through PSI and are used to make NADPH (reduced NADP). NADPH is a molecule that is essential for the light-independent reaction, as it provides it with energy.

The equation for this reaction is:

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

Fig. 4. The light-dependent reactions in the thylakoid membrane. Note that this diagram gives an extra level of complexity for those interested.

Step 4: Generation of ATP

In the final stage of the light-dependent reaction, ATP is generated in the thylakoid membrane of the chloroplasts. ATP is also known as adenosine 5-triphosphate and is often referred to as the energy currency of a cell. Like NADPH, it is essential for the light-independent reaction.

The equation for this reaction is:

\[ADP + P_i \longrightarrow ATP\]

Dark phase reaction

Step 5: Carbon Fixation

This occurs in the stroma of the chloroplast. Through a series of reactions, ATP and NADPH are used to convert carbon dioxide into glucose. You can find these reactions explained in the light-independent reaction article.

The overall equation for this is:

\[6CO_2 + 12NADPH + 18ATP \longrightarrow C_6H_{12}O_6 + 12 NADP^+ + 18 ADP + 18 P_i\]

What are the products of photosynthesis?

The products of photosynthesis are glucose (C₆H₁₂O₆) and oxygen (O₂).

We can further split the process of photosynthesis and the products of each stage into the products for the light-dependent and the light-independent stages:

• Light-dependent reactions products: ATP, NADPH, O₂, and H⁺ ions.
• Light-independent reaction products: glyceraldehyde 3-phosphate (which is used to make glucose) and H⁺ ions.

What are the limiting factors of photosynthesis?

A limiting factor inhibits or slows the rate of a process when it is in short supply. In photosynthesis, a limiting factor would be something needed to fuel the light-dependent or light-independent reaction, so that when it is missing, the rate of photosynthesis decreases.

When all limiting factors are at optimal levels, the rate of photosynthesis will increase steadily up to a certain point before plateauing (a state of little or no change). The plateau will happen because one of these three factors will be in short supply, causing the rate of photosynthesis to stop increasing or decrease.

The rate of photosynthesis is affected by a number of factors, including:

• Light intensity
• Carbon dioxide concentration
• Temperature

To learn more about how these factors impact the rate of photosynthesis, check out our article Rate of Photosynthesis.