Plants obtain energy from sunlight and make starch themselves. Let us study photosynthesis, the foundation of food chains on Earth.
Basic Knowledge
Photosynthesis occurs in chloroplasts and can be represented by the following reaction:Carbon dioxide (CO₂) + water (H₂O) → starch (organic matter) + oxygen (O₂)
Starting materials: carbon dioxide (through stomata) and water (from roots)
Required energy: light energy (sunlight)
Products: starch (= organic matter = nutrients) and oxygen
Experiments to confirm photosynthesis:
① Use BTB solution to confirm that CO₂ is absorbed (it returns to blue)
② Use iodine solution to confirm that starch is stored in the leaf (blue-purple)
③ The covered part does not react → we learn that light is necessary
④ In a variegated leaf, only the green parts react → we learn that chloroplasts are necessary
Photosynthesis (photosynthesis)（A reaction that uses light energy to make organic matter and oxygen from CO₂ and water）
Chloroplasts and chlorophyll（The site of photosynthesis and the green pigment. Absorbs light）
Starch (starch)（The first organic matter produced by photosynthesis. Turns blue-purple with iodine solution）
Iodine solution（A reagent that turns blue-purple when it reacts with starch）
BTB solution（An indicator for observing CO₂ concentration. Neutral = green, acidic = yellow, basic = blue）
Destarching（Removing color from a leaf with hot water + ethanol before applying iodine solution makes the reaction easier to observe）
Further Study (Background and Significance)
Photosynthesis is the starting point of all food chains on Earth. Organic matter made by photosynthesis enables plants to grow; herbivores eat the plants, and carnivores eat the herbivores. The energy in the rice, wheat, vegetables, and fruit we eat all comes from sunlight.
Photosynthesis is thought to have begun 2.7 billion years ago with cyanobacteria. Before that, Earth’s atmosphere contained almost no oxygen. As photosynthetic organisms continued releasing oxygen, they formed today’s atmosphere with an oxygen concentration of about 21%. This is called the “Great Oxidation Event.”
The reverse reaction of photosynthesis is “respiration”: Organic matter + oxygen → CO₂ + water + energy. Plants carry out photosynthesis and respiration at the same time. During the day, photosynthesis > respiration, so they absorb CO₂ overall; at night, they carry out only respiration and release CO₂.
- Photosynthesis equation: CO₂ + water → starch + oxygen
- Required: light, chloroplasts, water, and CO₂
- Iodine solution confirms starch (blue-purple)
- BTB solution confirms CO₂ absorption (returns to blue)
- Chloroplasts absorb light with green chlorophyll
- Plants carry out respiration and photosynthesis at the same time
- Earth’s oxygen originated from photosynthesis
Points to Note (Easily Confused)
① The raw materials for photosynthesis are CO₂ and water; the products are starch and oxygen. Do not reverse them. ② Chloroplasts (organelles inside cells) and chlorophyll (the pigment inside chloroplasts) are different. ③ Photosynthesis, transpiration (water evaporation), and respiration (oxygen uptake) are separate phenomena, although they occur simultaneously in plants. ④ Iodine solution is for starch, while BTB is for CO₂. Do not confuse their roles.
Practice
- Name two substances that are raw materials for photosynthesis.
- What gas is produced by photosynthesis?
- Name the reagent used to confirm that starch has formed in a leaf.