Photosynthesis
AdvancedPlants use light energy to turn carbon dioxide and water into glucose and oxygen — inside chloroplasts.
- Inputs: sunlight + carbon dioxide + water
- Site: chloroplasts (chlorophyll absorbs light)
- Products: glucose + oxygen
§Text Passage
1A potted plant stands in plain soil, yet with water and sunlight it keeps growing taller. Where does the food come from? The answer lies inside chloroplasts: using light energy, green plants combine carbon dioxide taken in through the stomata with water carried up from the roots, producing glucose and releasing oxygen. This process is called photosynthesis.
2Photosynthesis needs four conditions at once — carbon dioxide, water, light and chlorophyll; without any one of them the reaction cannot go on. Part of the glucose made is used in cell respiration to release energy for life, while the surplus is converted into starch and stored. In this way the Sun's energy is turned into chemical energy that living things can use.
3Examples surround us: on a morning walk in the park, trees are quietly making food and oxygen; phytoplankton in the oceans photosynthesize too and supply a large share of the Earth's oxygen; the rice and vegetables we eat every day are all products of photosynthesis. In the laboratory, iodine solution stains starch blue, providing a classic test for whether a leaf has made food.
4Photosynthesis underpins almost every food chain and is a key link in the carbon cycle. Energy stored by ancient organisms through photosynthesis slowly became the coal and oil we burn today. Large-scale planting of trees absorbs carbon dioxide and eases the greenhouse effect, so protecting plants means maintaining the planet's carbon-oxygen balance and responding to climate change.
1Summary 重點摘要
Plants use light energy to turn carbon dioxide and water into glucose and oxygen — inside chloroplasts.
2Vocabulary cards Vocabulary
Making food from carbon dioxide and water using light energy.
The green pigment that absorbs light.
A gas taken in from the air through stomata — a raw material of photosynthesis.
A product of photosynthesis, released into the air.
The food made, used for growth or stored as starch.
The storage form of glucose; turns blue-black in the iodine test.
The structure inside plant cells where photosynthesis takes place; it contains chlorophyll.
Tiny pores on the leaf surface through which carbon dioxide enters and oxygen and water vapour escape.
3Step by step Steps
- Raw materials inThe raw materials of photosynthesis arrive from two routes: roots absorb water from the soil and pipe it up to the leaves, while the leaves take in carbon dioxide from the air through the stomata; sunlight supplies the energy for the reaction. All three are indispensable — a shortage of water or carbon dioxide slows the reaction or even stops it, which is why watering and good air circulation directly affect how fast a plant makes its food.
- The factory worksPhotosynthesis takes place inside the chloroplasts: chlorophyll captures light energy and gradually builds glucose from carbon dioxide and water, storing the energy in the glucose. Found only in chloroplasts, chlorophyll makes leaves green, and the chloroplast serves as the workshop of the reaction. The process requires light: with no sunshine at night the workshop pauses, and the plant falls back on the food it stored during the day.
- Products outThe products then leave the factory: part of the glucose is broken down in respiration to release energy for life, while the surplus is converted into starch and stored in leaves or roots for long-term keeping. Oxygen, the by-product, escapes through the stomata back into the air — exactly what animals need to breathe. Starch turns blue-black with iodine solution, the classic test for whether a leaf has made food.
- Why it mattersPhotosynthesis feeds the whole planet: it converts the energy of sunlight into chemical energy that living things can use, supporting the first link of almost every food chain. Tiny drifting plants in the oceans also produce vast amounts of oxygen, keeping the atmosphere in balance. The rice and vegetables we eat every day are direct products of photosynthesis; even coal and petroleum come from the photosynthetic products stored by organisms long ago — its influence reaches remarkably far.
- Write the word equationThe whole reaction can be summarised in a word equation: carbon dioxide plus water, in the presence of light energy and chlorophyll, produces glucose and oxygen. Always state the conditions — without light and chlorophyll the reaction simply cannot happen. The equation is the exact reverse of respiration: one stores energy while the other releases it, and this contrast is a favourite comparison question in exams, so make sure you understand it thoroughly.
- Testing starch with iodineTo test whether a leaf has made food, use the starch–iodine experiment: first keep the plant in the dark for one or two days so it uses up its stored starch; then soften the leaf in hot water, decolourise it by heating in alcohol over a hot-water bath, and finally add iodine solution. Blue-black shows that starch is present, while a shaded control patch stays unchanged — proving that photosynthesis needs light. The control in the design makes the conclusion far more reliable.
4Diagrams Diagrams
5Worked example Example
Example: a plant is kept in a dark cupboard for two weeks. What happens and why?
- No light → no photosynthesis
- No new food made
- Respiration continues, using up reserves
- Stored food runs out → wilting
- Answer: It wilts and dies. Photosynthesis needs light as its energy source: in a pitch-dark cupboard chlorophyll can build no new food from carbon dioxide and water and the leaves slowly yellow, yet respiration keeps burning reserves day and night. Once they run out the plant wilts and dies — light is indispensable. Compare the two processes side by side in your answer.
6Common mistakes Mistakes
7Quick check Quick check
Q1What three inputs does photosynthesis need?
Q2Where does photosynthesis happen, and what absorbs the light?
Q3Name the two products of photosynthesis.
Q4Write the word equation for photosynthesis, including the conditions.
Q5How can an experiment show that a leaf has made starch?
8Recap Recap
- Inputs: sunlight + carbon dioxide + water
- Site: chloroplasts (chlorophyll absorbs light)
- Products: glucose + oxygen
- Glucose can be stored as starch for the night or unfavourable conditions
- Plants respire day and night, constantly using oxygen