Photosynthesis is a biological process that lets plants, algae and some bacteria convert light from the sun into chemical energy as glucose. It is crucial to the growth and health of plants. It also creates the oxygen that we breathe and removes carbon dioxide from the atmosphere. Put simply, the world would be a very different and far more hostile climate without photosynthesis. It is also an important topic in the GCSE science curriculum.
Photosynthesis happens in the chloroplasts of plant cells. This contains the pigment chlorophyll, which absorbs light and uses it to drive the chemical reactions that convert carbon dioxide and water into glucose and oxygen. This is the equation:
Carbon dioxide + Water + Light energy -> Glucose + Oxygen
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What is Photosynthesis?
Photosynthesis is how plants, algae and some bacteria turn light into chemical energy stored in glucose, which is a type of sugar. The process creates oxygen and is the basis of the food chain.
Chloroplast is an organelle you find in the cells of green plants and algae. Inside chloroplasts are pigments like chlorophyll that capture light energy. Chlorophyll absorbs light best in the blue-violet and red parts of the electromagnetic spectrum. This reflects green light, which is why plants look green to us.
This is a simplified form of the photosynthesis equation:
6 CO2 + 6 H2O + Light energy -> C6H12O6+O2
We can break this down into the following:
- Carbon dioxide (CO2): This gas enters the plant through tiny pores (stomata).
- Water (H2O): This is absorbed by the roots from the soil.
- Light energy: It is captured by chlorophyll in the chloroplasts.
- Glucose (C6H12O6): Used by the plant for energy and growth.
- Oxygen (O2): Released as a by-product into the atmosphere.
As this equation shows, the photosynthesis process encourages the plant's growth and as a helpful byproduct contributes to the oxygen we breathe in the atmosphere.
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Why is chlorophyll important in photosynthesis?
The Photosynthetic reaction
There are two stages in photosynthesis: the light-dependent reactions and the light-independent reactions. The independent reactions are known as the Calvin Cycle. Both stages are important in turning light into chemical energy the plant can use.
Light-dependent
This reaction requires energy from direct light. It occurs in the thylakoid membranes of the chloroplasts:
- Light absorption: Chlorophyll and other pigments in the thylakoid membranes capture sunlight. This energy excites electrons and puts them in a higher energy state.
- Water splitting: The absorbed sunlight is used to split water molecules into oxygen, protons and electrons. This process is called photolysis and it creates oxygen as a byproduct.
- ATP and NADPH formation: The electrons in a high energy state are moved through a series of proteins called the electron transport chain. This movement releases energy that creates Adenosine Triphosphate and Nicotinamide Adenine Dinucleotide Phosphate (known as ATP and NADPH for short). ATP and NADPH carry energy and are important in the next stage of photosynthesis.
Follow the link to see the Khan Academy's guide exploring how light-dependent reactions work.
Light-independent reactions (Calvin Cycle)
The Calvin Cycle happens in the stroma of the chloroplasts - it doesn't need direct sunlight. However, they do need the products created from the light-dependent reactions: ATP and NADPH. There are three steps to this cycle:
- Carbon fixation: Carbon dioxide molecules are attached to Ribulose Bisphosphate (RuBP) by the enzyme RuBisCO. RuBP is a 5-carbon sugar and this attachment creates a 6-carbon compound, which splits into 3 three-carbon molecules.
- Reduction phase: The ATP and NADPH molecules convert the 3-carbon molecules into a sugar called Glyceraldehyde-3-Phosphate (G3P). G3P can create glucose and other carbohydrates.
- Regeneration of RuBP: Some of the G3P molecules leave the Calvin cycle and are used in glucose synthesis. The remaining molecules are recycled to regenerate RuBP. This allows the cycle to continue.
How plants use glucose
The glucose created from photosynthesis is important for plant growth and survival. Glucose is a type of sugar and therefore gives the plant energy. This is how the plant uses glucose:
- Energy production: Glucose is broken down through cellular respiration to release energy for the plant cells to grow.
- Storage: Glucose can be stored as starch, which the plant uses when photosynthesis isn't possible (such as the night or in winter).
- Building material: Glucose creates cellulose, which strengthens plant cell walls and other organic compounds needed for the plant's structure and function.
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Why is the Calvin Cycle important in photosynthesis?
Factors Affecting Photosynthesis
There are a variety of environmental factors that impact the rate of reaction in photosynthesis, an obvious example being winter. Understanding these factors explains why plants grow better under certain conditions and how they adapt to their environment.
Three main factors affect photosynthesis: light intensity, carbon dioxide concentration and temperature.
Light intensity
It isn't surprising that more light leads to greater energy to drive the reactions in photosynthesis to create glucose and oxygen. That said, the relationship is not always linear:
- Increasing light intensity: The rate of photosynthesis rises proportionally when more light is available. This is because more photons are available to energise the chlorophyll molecules.
- Saturation point: However, the rate no longer increases after a certain point. This is because of limiting factors, including the availability of carbon dioxide or temperature, which is called the light saturation point.
Carbon Dioxide concentration
Carbon dioxide (CO2) is a key reactant in the photosynthesis process. The amount of CO2 in the environment can significantly affect the rate of photosynthesis:
- Increasing CO2 levels: The rate of photosynthesis increases with the concentration of carbon dioxide. This is because there's more CO2 available to be turned into glucose.
- Plateau effect: There is a point beyond which additional CO2 won't increase the rate of photosynthesis. As above, this is because of other limiting factors.
Temperature
Temperature affects the enzymes that enable the chemical reactions involved in photosynthesis:
- Optimum temperature range: Photosynthesis normally increases as the temperature gets hotter and the enzymes become more active. The optimum temperature varies depending on the plants - it's normally around 25-30°C.
- Enzyme denaturation: If the temperature exceeds the optimum, the enzymes can become denatured and they'll lose their shape and functionality. This drastically reduces the rate of photosynthesis. In extreme cold/heat, it can stop photosynthesis altogether.
How plants adapt
Plants have the remarkable ability to adapt their optimal photosynthesis according to their environment. As an example, plants in dry climates evolve thicker leaves to reduce water loss, while other plants open their stomata at night to reduce water loss in hot conditions. Understanding these adaptations helps us appreciate the diversity of plant life and its resilience, especially in a time of climate change.
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What decreases the rate of photosynthesis when the light intensity and carbon dioxide levels are ideal?
The importance of Photosynthesis
Photosynthesis is a biochemical reaction that is essential for the survival of nearly all ecosystems. It provides oxygen to the climate and forms the foundation of the food chain. This section will explore how photosynthesis impacts the environment and living organisms.
Oxygen production
Oxygen is produced as a by-product of photosynthesis when plants convert water and carbon dioxide into glucose. All aerobic organisms use oxygen for breathing, including humans.
- Contribution to Earth's atmosphere: Photosynthesis helps maintain the oxygen levels in the atmosphere. Approximately 50-80% of the oxygen produced yearly is made by phytoplankton, which is a microscopic plant found in the sea. The rest comes from plants on land.
- Supporting life: The concentration of oxygen in the atmosphere would deplete without the gas produced by photosynthesis. Put simply, life wouldn't survive, showing the interconnectedness of life forms and their dependence on plants and photosynthetic organisms.
Foundation of the food chain
Photosynthesis is at the heart of the food chain. It's one of the main producers of food for the majority of organisms:
- Producers: Plants, algae and some bacteria are the main producers in most ecosystems. They turn light into chemical energy stored in glucose.
- Consumers: Herbivores typically eat plants and other greenery as their food source. These herbivores are then eaten by carnivores and so on. This transfer of energy from producers to consumers is the basis of the food chain.
- Decomposers: Plants contribute to the ecosystem even after they have died. Decomposers break down dead organic matter and recycle nutrients back into the soil. These nutrients are used by plants to grow and the cycle continues.
Climate regulation
Photosynthesis has a significant impact on the planet's climate. This natural process mitigates the effects of climate change by absorbing carbon dioxide:
- Carbon sequestration: Plants absorb carbon dioxide during photosynthesis, which is a major greenhouse gas. They store it in their biomass, such as their leaves, stems and roots. Forests and oceanic phytoplankton are significant carbon sinks (anything that absorbs more carbon from the atmosphere than it releases).
- Impact on global warming: Photosynthesis reduces the greenhouse effect of CO2 by lowering its atmospheric levels. This helps to lower the risk of global warming and stabilise climate patterns.
Supporting biodiversity
Photosynthesis provides a source of food and oxygen for life forms on our planet. Photosynthetically active ecosystems are home to a wide variety of species, such as rainforests and coral reefs.
- Habitat creation: Plants create habitats where many species can thrive. As an example, trees give shelter and food to animals and insects.
- Ecosystem services: Photosynthetic plants contribute to ecosystem services, including pollination, nutrient cycling and soil formation. These are important for preserving biodiversity.
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Why is photosynthesis important to the global carbon cycle?
Common mistakes on the Photosynthesis topic
There are some common misconceptions regarding photosynthesis, especially at the GCSE level. We have covered some of the main misunderstandings.
Mistake: Photosynthesis only happens during the day
It's often assumed that photosynthesis only takes place inside daylight hours. The first stage of light-dependent reactions indeed needs light energy. However, the second stage of the Calvin Cycle can happen at any time as long as the products of the light-dependent reactions (ATP and NADPH) are available. This means plants can continue to produce glucose and absorb carbon dioxide at night-time, using energy stored in the day.
Mistake: Green light is the best for Photosynthesis
A common belief is green light is the most effective light energy for photosynthesis because plants are green. The reason plants are green is that the pigment Chlorophyll absorbs light best in the blue-violet and red parts of the spectrum. This reflects a green light, which is why plants appear green to the eye. Ironically, green is the least effective lighting for photosynthesis - blue and red wavelengths are much more effective.
Mistake: Plants get their energy directly from the sun
Another misunderstanding is that plants get their energy directly from sunlight in the same way humans get energy from food and drink. Plants use light energy to create glucose, which in turn produces ATP through cellular respiration. ATP is the actual energy used by the plant's cells so it is a layered process for the plant to get the energy it can use.
Mistake: Photosynthesis occurs equally in all parts of the plant
Some people believe photosynthesis takes place everywhere in a plant. In truth, it happens largely in the leaves where the chloroplasts are concentrated. The process can happen in green stems at a much smaller level. A leaf is also ideal for photosynthesis because it has a large surface area to maximise light absorption.
Mistake: Photosynthesis and cellular respiration are completely separate processes
A common misunderstanding is that photosynthesis and cellular respiration are completely different and unrelated. They are interconnected processes, where the products of photosynthesis are reactants for cellular respiration and vice versa:
- Photosynthesis converts carbon dioxide and water into glucose and oxygen.
- Cellular respiration found in plants takes glucose and oxygen to create carbon dioxide, water and energy (ATP).
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Which statement describes the relationship the relationship between photosynthesis and cellular respiration?
Thoughts on Photosynthesis - GCSE Biology revision
Photosynthesis is a remarkable process that underpins all life on our planet. It turns light into chemical energy to fuel plants and in doing so, ensures the life of all living organisms. We've seen there are two stages to the process: the light-dependent reactions and the Calvin Cycle. The reaction turns captured sunlight into chemical energy, while the cycle uses this energy to synthesise glucose from carbon dioxide and water.
Photosynthesis helps to regulate the Earth's atmosphere and climate. Plants help to balance the gases in the atmosphere so that life can flourish and survive. It also mitigates climate change by action as a natural carbon sink, which is why we often see people in positions of power advocating for the planting of new trees.
If you need more help, follow the link to see TeachTutti's list of verified GCSE Science tutors who can give you tailored support to understand this and prepare revision notes. If you want to learn more about the broader environmental cycles, explore topics like the carbon cycle and understand how plants play a pivotal role in sustaining life on Earth.
