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examBoard: Pearson Edexcel
examType: IGCSE
lessonTitle: Chloroplasts and Photosynthesis
Plants are amazing living factories that can make their own food using sunlight! This process, called photosynthesis, takes place in special structures called chloroplasts. Without photosynthesis, life as we know it wouldn't exist on Earth.
Key Definitions:
Chloroplasts are oval-shaped organelles with a double membrane. Inside, they contain a fluid called stroma and a system of membrane discs called thylakoids. These thylakoids are stacked into columns called grana. The green pigment chlorophyll is found in the thylakoid membranes.
The main function of chloroplasts is to carry out photosynthesis. They capture light energy and convert it into chemical energy stored in glucose molecules. This process provides energy for the plant and produces oxygen as a waste product, which most living organisms need to survive.
Photosynthesis can be summarised by this equation:
Carbon dioxide + Water → Glucose + Oxygen
6CO2 + 6H2O → C6H12O6 + 6O2
This process happens in two main stages:
These occur in the thylakoid membranes and require light energy. Chlorophyll absorbs light, which excites electrons. These electrons are used to split water molecules (photolysis), releasing oxygen. The process also produces ATP (energy) and reduced NADP.
Also known as the Calvin cycle, these reactions take place in the stroma. They don't directly need light but use the products of the light-dependent reactions. Carbon dioxide is combined with a 5-carbon compound to eventually produce glucose.
The glucose produced can be used for respiration to release energy, converted to starch for storage, used to make cellulose for cell walls, or combined with other nutrients to make proteins and lipids for growth.
The rate of photosynthesis can be affected by several factors:
A limiting factor is something that restricts the rate of photosynthesis when in short supply. The main limiting factors are:
More light generally means faster photosynthesis, up to a point. Beyond this point, another factor becomes limiting. Plants in shady areas often have adaptations to capture more light.
Photosynthesis involves enzymes that work best at optimum temperatures (around 25°C for most plants). Too cold: enzymes work slowly. Too hot: enzymes denature and stop working.
Higher CO2 levels can increase photosynthesis rates. This is why some commercial greenhouses add extra CO2 to boost plant growth.
Commercial greenhouses manipulate all the factors affecting photosynthesis to maximise crop yields. They maintain optimal temperatures, use artificial lighting to extend growing hours and sometimes add extra CO2. Some high-tech greenhouses in the Netherlands can produce up to 10 times more tomatoes per square metre than open field farming!
Leaves are like solar panels for plants, designed to maximise photosynthesis:
Plants have evolved amazing adaptations to photosynthesise in different conditions:
Scientists can measure photosynthesis rates by:
Photosynthesis is vital for life on Earth for several reasons:
Despite its importance, photosynthesis is actually not very efficient! Most plants convert only about 1-2% of the light energy they receive into chemical energy stored in glucose. Scientists are researching ways to improve this efficiency to help grow more food for our growing population and to develop better biofuels.
Chloroplasts are the plant cell's photosynthesis factories, containing chlorophyll that captures light energy. Photosynthesis converts carbon dioxide and water into glucose and oxygen through light-dependent and light-independent reactions. The process is affected by light intensity, temperature and carbon dioxide concentration. Plants have evolved various adaptations to optimise photosynthesis in different environments. This remarkable process sustains most life on Earth by producing food and oxygen while removing carbon dioxide from the atmosphere.
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