Photosynthesis: What Is It & How Does It Work?

What is Photosynthesis?

Photosynthesis is a process where light energy is used to convert carbon dioxide and water into glucose and oxygen. This reaction is catalysed by enzymes in the chlorophyll of plant cells. 

What gas is produced by plants as a waste product of photosynthesis?

Oxygen is produced as a waste product. Some of it is used in the leaf for respiration, but some of it is not needed and diffuses out of the stomata of the leaf after the reaction is complete. 

Photosynthesis Equations

Photosynthesis word equation

Carbon dioxide + water → glucose + oxygen

Chemical equation for photosynthesis

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Why is photosynthesis important?

Photosynthesis is important because it is the only reaction that exists in living things that takes carbon from the atmosphere (in the form of carbon dioxide) and converts it into a form that can be used by living organisms, glucose. Glucose is needed for an important reaction called respiration, which provides energy for all processes in metabolism. 

The carbon in glucose can also be converted into all the different types of biological molecules that are used in the plant for growth and metabolism, such as carbohydrates, lipids and proteins. These can then be passed on to all other types of organisms through feeding relationships. Plants are the producers at the start of every food chain and photosynthesis is essential to allow them to carry out this role. 

This diagram shows the uses of glucose within a plant, remembering that glucose is made as part of the photosynthesis reaction

What product from photosynthesis is used to make cellulose?

Glucose is also used to make cellulose, which is an important structural carbohydrate found in plant cell walls. Cellulose differs from starch not only in its structure, but also in the type of glucose it is made from. Starch is made from an isomer of glucose called alpha glucose, whereas cellulose is made from beta glucose. 

In cellulose the beta glucose units, called monomers, are positioned so that every other glucose is flipped 180° from the previous one in the chain. This allows the glucose monomers to form cross-linkages of hydrogen bonds between the chains, which increases the strength of the cellulose fibres; this is very important for its function of protecting and supporting the cell. 

This diagram shows the structure of cellulose where each beta glucose molecule is flipped 180o

More information: Cellulose (AQA A Level Biology)

What type of reaction is photosynthesis? Is photosynthesis endothermic or exothermic?

Photosynthesis is an endothermic reaction because it takes in energy from its surroundings in the form of light energy. There is a common misconception that endothermic reactions only take in heat energy and feel cold to the touch but any kind of energy can be used in an endothermic reaction. 

Factors affecting photosynthesis

The three main factors that affect the rate of photosynthesis are:

• temperature,

• light intensity, and

• carbon dioxide concentration. 

How does temperature affect the rate of photosynthesis?

Temperature affects the rate of photosynthesis because the reaction is catalysed by enzymes. The rate of enzyme action increases with temperature because the reactants gain kinetic energy and so collide more frequently with the active site of the enzymes, forming more enzyme-substrate complexes. If the temperature rises too high it can cause the enzymes to become denatured and they are no longer able to function.

This graph shows the effect of temperature on rate of photosynthesis

How does light intensity affect the rate of photosynthesis?

Light intensity affects the rate of photosynthesis because light energy is a requirement for the reaction of photosynthesis. As the light intensity increases so does the rate of photosynthesis. However, this does not continue forever and eventually the rate of photosynthesis levels out because another factor limits the rate. 

This graph shows the effect of light intensity on the rate of photosynthesis

How does carbon dioxide concentration affect the rate of photosynthesis?

This graph shows the effect of carbon dioxide concentration on the rate of photosynthesis

What name is given to a factor which is preventing any increase in photosynthesis?

A factor which prevents an increase in photosynthesis is called a "limiting factor of photosynthesis".

Limiting factors of photosynthesis: A limiting factor of photosynthesis is a factor, such as carbon dioxide concentration or light intensity, that prevents the rate of photosynthesis from increasing because it is in short supply. 

Let’s use an analogy: let’s say you’re trying to bake as many cupcakes as possible for a cake stall at a local fundraising event. You go into your cupboard and search for the ingredients but you find you only have six eggs. In this scenario the maximum amount of cupcakes you can make depends on the amount of eggs you have. The number of eggs is the limiting factor. If you go out and buy more eggs you may then find that you only have enough sugar to make a certain amount of cupcakes, or perhaps a limited supply of flour. Each of these factors could act as a limiting factor in this scenario. 

Instead of cupcakes we can apply this to photosynthesis. If there isn’t enough carbon dioxide in the environment, the rate of photosynthesis can’t increase any further than the maximum rate dictated by the carbon dioxide. This is the limiting factor. 

Where does photosynthesis take place? 

Photosynthesis takes place in the leaves of plants. Within the leaves there is a specialised layer of cells called the palisade mesophyll layer, containing palisade cells. Although the palisade cells are specialised for photosynthesis, there are other cells in the leaf that can photosynthesise as well, such as the spongy mesophyll cells. It is important to have as many chloroplasts as possible in the leaf cells to reduce the amount of light that passes through the leaf without being absorbed by a chloroplast. 

This diagram shows the structure of a leaf and a typical plant cell

Which part of a plant cell absorbs light for photosynthesis?

These leaf cells are packed full of chloroplasts which contain the green pigment chlorophyll. The chlorophyll is where the enzymes that catalyse the reaction of photosynthesis are found. 

How is a leaf adapted for photosynthesis? 

Leaves are adapted for photosynthesis in many ways, such as:

• Large surface area for absorbing light and carbon dioxide

• Veins in the leaf support the cells to be held up to the light, they also contain vascular bundles with xylem vessels that carry water to the leaf for the reaction of photosynthesis

• They are very thin to reduce the diffusion distance for carbon dioxide, which enters through the stomata on the base of the leaf, but must diffuse through to reach the palisade mesophyll at the top of the leaf

• Large numbers of chlorophyll in the specialised cells of the leaf

• The upper and lower epidermis of the leaf are thin and transparents to allow as much light as possible into the leaf

• The spongy layer contains air spaces to allow the gasses carbon dioxide,oxygen and water vapour, to diffuse through the leaf space easily, whereas the palisade cells are packed tightly together at the top of the leaf to ensure very little light passes through the leaf without going through a palisade cell

• The stomata are positioned on the bottom of the leaf (for most species of plant) to ensure the carbon dioxide can enter the leaf whilst also protecting against excess evaporation / water loss from the leaf

More information: Plant Tissues (AQA GCSE Biology)

What substance is tested for to see if photosynthesis has occurred in a leaf?

Starch is tested for to see if photosynthesis has occurred in a leaf. If the leaf contains starch it indicates that photosynthesis is happening and if it is not found then it indicates that photosynthesis is not happening. 

The reason for this is because glucose from photosynthesis is stored as starch after photosynthesis has taken place. Glucose must be stored as starch in the leaf cells because otherwise the glucose would lower the water potential of the cell and cause water to move into the cell by osmosis. Starch is insoluble and so does not affect the water potential of the cells. If the leaf is unable to photosynthesise (for example, if it is being kept in a dark space) the starch in the leaf is broken down and the glucose is released and used in the cell for respiration. 

The practical investigation steps shown in the image below describes how this test is carried out:

Learn More About Photosynthesis

• Photosynthetic Reaction (AQA GCSE Biology)

• Chloroplast Structures & their Functions (AQA A Level Biology)

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