Photosynthesis (Junior Cert Science): Revision Notes

Photosynthesis

Why is photosynthesis important?

Life on Earth depends on photosynthesis in many ways. Most living organisms need oxygen to survive, and the oxygen in our atmosphere comes from photosynthesis. Animals, including humans, would die if oxygen levels dropped too low - most need at least $10\%$ oxygen to live.

But photosynthesis does much more than just produce oxygen. It also removes carbon dioxide from the air, helps provide raw materials for cloth and paper, gives us wood for building, provides ingredients for medicines, and most importantly, produces the food we eat. Plants create fruits, vegetables, grains and spices through photosynthesis. Even the animals we get meat from depend on plants for their food.

What is photosynthesis?

Photosynthesis is the process by which plants make their own food using light energy. The word comes from two parts: 'photo' meaning light, and 'synthesis' meaning to produce or make.

During photosynthesis, plants take in carbon dioxide and water, and use light energy to transform these into glucose (a type of sugar) and oxygen.

The word equation for photosynthesis

We can write photosynthesis as a simple word equation:

$\text{carbon dioxide} + \text{water} + \text{light energy (with chlorophyll)} \rightarrow \text{glucose} + \text{oxygen}$

The chemical equation for photosynthesis

We can also write this as a balanced chemical equation using symbols:

$6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2$

This equation shows that six molecules of carbon dioxide combine with six molecules of water to produce one molecule of glucose and six molecules of oxygen. The chlorophyll acts as a catalyst and light provides the energy needed.

The connection between photosynthesis and respiration

Photosynthesis and respiration are opposite processes that work together in nature. Plants use photosynthesis to produce glucose and oxygen. Animals (and plants themselves) then use respiration to break down glucose using oxygen, which releases energy and produces carbon dioxide and water. This creates a natural cycle where the products of one process become the starting materials for the other.

How do plants photosynthesise?

For photosynthesis to happen in a plant, several things must occur:

Step 1: Plants absorb water from the soil through their roots. This water travels up through the plant to reach the leaves.

Step 2: Sunlight is absorbed by the leaves. Inside each leaf are special structures called chloroplasts that contain a green pigment called chlorophyll.

Step 3: Carbon dioxide from the air enters the leaf through tiny pores on the underside of leaves called stomata (singular: stoma). These stomata can open and close to control gas exchange.

Step 4: Inside the chloroplasts, the chlorophyll traps light energy from the sun. This energy allows carbon dioxide and water molecules to combine and react.

Step 5: The reaction produces glucose, which the plant can use for energy or store for later. Oxygen is released as a waste product.

Step 6: The oxygen leaves the leaf through the stomata and is released into the air.

Photosynthesis as a chemical process

We can describe photosynthesis as a chemical process because it involves reactants (carbon dioxide and water) being converted into new substances (glucose and oxygen). During this process, chemical bonds are broken and new ones are formed, creating different molecules from the starting materials.

Photosynthesis as a biological process

Photosynthesis is also a biological process because it takes place in living cells and involves multiple steps happening inside plant tissues. The process depends on living structures like chloroplasts and is controlled by biological factors.

Oxygen production by aquatic plants

Plants that live underwater also carry out photosynthesis. Aquatic plants absorb dissolved carbon dioxide from the water and release oxygen bubbles. This oxygen is essential for fish and other aquatic animals to breathe. You can often see these oxygen bubbles rising from healthy aquatic plants in ponds or aquariums.

Factors affecting photosynthesis

Several environmental factors can affect how quickly photosynthesis occurs. A factor is anything (living or non-living) that influences something else. The three main factors that affect the rate of photosynthesis are:

• Light intensity
• Carbon dioxide concentration
• Temperature

Light intensity

As light intensity increases, the rate of photosynthesis also increases. This is because more light energy is available to power the chemical reactions in the chloroplasts. However, this relationship only continues up to a certain point. After a particular light level is reached, making the light even brighter will not increase the photosynthesis rate any further. At this point, light is no longer the limiting factor - something else (such as carbon dioxide concentration or temperature) is restricting the rate instead.

Carbon dioxide concentration

When carbon dioxide levels increase, the rate of photosynthesis also rises. This happens because more carbon dioxide molecules are available to react with water in the chloroplasts. However, similar to light intensity, this only works up to a certain level. If carbon dioxide levels fall too low, there will not be enough gas available for the photosynthesis reactions to proceed quickly, and this will limit the overall rate.

Temperature

Temperature affects photosynthesis because the process is controlled by enzymes - special protein molecules that speed up chemical reactions in living cells. Enzymes work best at particular temperatures.

At low temperatures, photosynthesis happens slowly. As temperature increases, so does the rate of photosynthesis, because the enzymes and molecules move faster and react more readily.

However, if the temperature gets too high (generally above $40°C$), the enzymes begin to break down and lose their shape. This is called denaturation. When enzymes are denatured, they stop working, and photosynthesis will slow down dramatically or stop altogether.

Investigating photosynthesis

Scientists can investigate photosynthesis through carefully designed experiments. These experiments help us understand which factors affect the process and how they work as limiting factors.

Knowledge of photosynthesis and plant yields

Understanding photosynthesis helps farmers and growers increase plant yield - the amount of food or other products a plant produces for a grower. By controlling the factors that affect photosynthesis, we can help plants grow bigger crops more efficiently.

Greenhouses

Farmers use greenhouses to create ideal conditions for photosynthesis. Inside a greenhouse, growers can control the temperature to keep it at the optimum level for plant growth. The glass or plastic covering traps heat, allowing plants to photosynthesise more efficiently even in cooler weather. This leads to larger crops and faster growth rates.

Lighting adjustments

Artificial lights can be used to extend the hours of light that plants receive, or to increase light intensity in shaded areas. For example, lighting rigs are used in some stadiums to help grass grow in areas that receive limited natural sunlight. This ensures the grass stays healthy and green by maintaining a good rate of photosynthesis.

Burning natural gas

Some greenhouse operators burn natural gas inside the greenhouse. This might seem unusual, but it serves an important purpose. Burning the gas releases carbon dioxide, which increases the carbon dioxide concentration in the air. Higher carbon dioxide levels lead to increased rates of photosynthesis, helping plants grow faster and produce larger yields. The burning also generates heat, which helps maintain the optimum temperature for growth.

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