Photosynthesis

Photosynthesis occurs when organisms use light energy from the sun to convert inorganic molecules (namely, carbon dioxide and water) into organic molecules (glucose) and oxygen. It 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 on the leaf. 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.
• 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.

Figure 1. Leaf structure. Source: Zephyris, CC BY-SA 3.0, commons.wikimedia.org

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.

Photosynthesis 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:

Figure 2. Chloroplast structure. Source: Charles Molnar and Jane Gair, OpenStax, CC BY-SA 4.0, commons.wikimedia.org

What is the equation/formula for photosynthesis?

In plants, photosynthesis occurs as follows:

Carbon dioxide + Water + solar energy → Glucose + Oxygen

As a balanced equation, this is:

What are the stages of photosynthesis?

Photosynthesis has five stages.

Step 1: Absorption of light

The first step involves the chlorophyll attached to photosystem II in the thylakoids of chloroplasts absorbing light. The chlorophyll is ionised as electrons leave the chlorophyll molecule in photosystem II and are carried down an electron transfer chain down the thylakoid membrane.

Step 2: Light reaction: 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, H+ ions and electrons. The electrons are then carried by plastocyanin (copper-containing protein that mediates electron-transfer) from photosystem II to photosystem I for the next part of the light reaction.

The equation for this reaction is:

Step 3: Light reaction: Reduction

The electrons produced in the last stage pass through photosystem I and are used to make NADPH (reduced NADP). NADPH is a molecule that is essential for the light-independent reaction.

The equation for this reaction is:

Figure 3. The overall light reaction.

Step 4: Light reaction: 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:

Step 5: Dark reaction: 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 ligh-independent reaction article.

The overall equation for this is:

Figure 3. An overview of light-independent reaction.

What are the products of photosynthesis?

The products of the light-dependent reactions of photosynthesis are ATP, NADPH, , and H+ ions.

The products of the light-independent reactions are glyceraldehyde 3-phosphate (which is used to make glucose) and H+ ions.

The overall products of photosynthesis are glucose and oxygen.

What is a limiting factor?

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, such as carbon dioxide, temperature, or light energy. When all three of these factors are at optimal levels, the rate of photosynthesis will increase steadily up to a certain point before plateauing (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 law of limiting factors was proposed in 1905. Formulated by Frederick Blackman, it states that "the rate of a physiological process will be limited by whatever factor is in the shortest supply".

Any change in the level of a limiting factor will affect the rate of reaction.

What are the limiting factors for photosynthesis?

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

Light

Figure 4. Effects of light intensity on the rate of photosynthesis.

As the intensity of light increases, so does the rate of light-dependent reactions of photosynthesis. Therefore, increasing the intensity of light increases the overall rate of photosynthesis. This is because more photons (basic unit of all light) will fall on the leaf, allowing the water to be oxidised faster. Therefore, the production of ATP and NADPH increases and more cycles of the light-independent reaction will occur.

However, after a certain point, the rate of photosynthesis remains constant even if the light intensity increases, as there is not enough heat energy, enzymes or carbon dioxide available to increase the rate of photosynthesis further.

Carbon Dioxide Concentration

Figure 5. Effects of carbon dioxide concentration on the rate of photosynthesis.

As Figure 5 demonstrates, increasing the concentration of carbon dioxide will increase the rate of photosynthesis up to a certain point (similarly to the effects of light intensity). If more carbon dioxide molecules are available, more cycles of the light-independent reaction will occur at a higher rate. This means that more glucose molecules are produced, more NADPH and ATP are used up, more RuBP (an organic subtance involved in photosynthesis) is produced, and the overall rate of photosynthesis will increase.

However, at a certain level, the rate of photosynthesis will be limited by other factors. There may not be enough light energy to produce enough NADPH and ATP to fuel more cycles of the light-independent reaction. Therefore, the rate of photosynthesis will not increase even if the concentration of carbon dioxide increases.

Alternatively, there may not be enough heat energy available to catalyse the enzyme-controlled reactions in the light-independent reaction at a higher rate, so the rate of photosynthesis will not be able to increase with the concentration of carbon dioxide.

Temperature

Figure 6. Effects of temperature on the rate of photosynthesis.

Since enzymes control photosynthesis, temperature is an important limiting factor for the rate of photosynthesis. As you will be able to see from Figure 6, the rate of photosynthesis increases with the temperature. However, unlike with carbon dioxide concentration and light intensity, the rate of photosynthesis reaches an optimum point before drastically declining. At around 35 - 40, the enzymes that control photosynthesis work at their best. However, if the temperature increases past this optimum point, the enzymes start to denature. The enzyme’s active site (where the substrate binds) shape is altered, and the substrate (substance that enzyme acts on) no longer fits. This explains the sharp decrease in photosynthesis rate at higher temperatures.

Figure 7. A normal versus a denatured enzyme.