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Unlock This CourseImagine if you could make your own food just by standing in sunlight! That's exactly what plants do through photosynthesis - one of the most important processes on our planet. Without photosynthesis, there would be no oxygen for us to breathe, no food for us to eat and no life as we know it.
Photosynthesis is the process where plants use sunlight, carbon dioxide and water to make glucose (sugar) and oxygen. It's like nature's own solar panel system, converting light energy into chemical energy that can be stored and used later.
Key Definitions:
Word Equation:
Carbon dioxide + Water → Glucose + Oxygen
Chemical Equation:
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
This equation tells us that six molecules of carbon dioxide plus six molecules of water make one molecule of glucose and six molecules of oxygen.
Photosynthesis happens in two main stages, both occurring in the chloroplasts of plant cells. Think of it like a two-step recipe for making plant food!
This stage happens in the thylakoids (disc-like structures) inside chloroplasts. Here's what happens:
Chlorophyll absorbs light energy, mainly red and blue light. This is why plants look green - they reflect green light instead of absorbing it!
The absorbed light energy splits water molecules (H₂O) into hydrogen and oxygen. The oxygen is released as a waste product - lucky for us!
The energy from splitting water is captured in special molecules called ATP and NADPH, which act like rechargeable batteries for the next stage.
This stage happens in the stroma (the fluid-filled space) of chloroplasts. It's also called the Calvin Cycle, named after scientist Melvin Calvin who figured out how it works.
Carbon dioxide from the air enters through tiny pores called stomata. Using the ATP and NADPH energy from Stage 1, the plant combines CO₂ with other molecules to build glucose. It's like using batteries (ATP and NADPH) to power a factory that assembles sugar from raw materials!
Chloroplasts are perfectly designed for photosynthesis. Let's explore their amazing structure:
Outer and Inner Membranes: Control what enters and leaves the chloroplast.
Thylakoids: Disc-shaped structures containing chlorophyll where light reactions occur.
Grana: Stacks of thylakoids (like stacks of coins).
Stroma: The fluid surrounding thylakoids where the Calvin Cycle happens.
Several factors can speed up or slow down photosynthesis. Understanding these helps farmers and gardeners grow better plants!
More light generally means faster photosynthesis, but only up to a point. Too much light can actually damage the plant. It's like having the perfect brightness for reading - too dim and you can't see, too bright and it hurts!
Plants need CO₂ as a raw material. More CO₂ in the air can increase photosynthesis rates. This is why some greenhouses pump in extra CO₂.
Photosynthesis involves enzymes, which work best at optimal temperatures (usually 25-35°C for most plants). Too cold and they work slowly, too hot and they stop working altogether.
Modern greenhouses control all three limiting factors. They use artificial lights when it's cloudy, pump in extra CO₂ and maintain perfect temperatures. This allows farmers to grow crops year-round and increase yields by up to 300% compared to outdoor growing. Dutch tomato growers are world leaders in this technology, producing 500 tonnes of tomatoes per hectare!
Photosynthesis isn't just important for plants - it's essential for all life on Earth. Here's why:
Every year, plants produce about 100 billion tonnes of glucose through photosynthesis. This glucose becomes the foundation of nearly all food chains on Earth.
Photosynthesis produces all the oxygen we breathe. About 70% comes from ocean phytoplankton (tiny floating plants) and 30% from land plants. Without photosynthesis, Earth's atmosphere would have no oxygen!
Plants remove CO₂ from the atmosphere during photosynthesis, helping to reduce greenhouse gases. A large tree can absorb 22kg of CO₂ per year - equivalent to the emissions from driving a car for 90 kilometres!
Plants have adapted photosynthesis to work in various environments, from scorching deserts to dim forest floors.
Cacti and other desert plants use a special type of photosynthesis called CAM (Crassulacean Acid Metabolism). They open their stomata at night to collect CO₂ when it's cooler and more humid, then use this stored CO₂ for photosynthesis during the day when their stomata are closed. This prevents water loss in the hot, dry desert air. It's like doing your shopping at night and eating during the day!
Scientists measure photosynthesis rates by counting oxygen bubbles produced by aquatic plants or measuring CO₂ uptake. In school labs, you might use pondweed (Elodea) to see photosynthesis in action - the faster the bubbles, the faster the photosynthesis!
Let's clear up some common misunderstandings about photosynthesis:
Myth: Plants only photosynthesise during the day.
Reality: Most plants do, but some desert plants photosynthesise at night too!
Myth: Plants don't need to respire because they photosynthesise.
Reality: Plants do both! They photosynthesise to make food and respire to release energy from that food.