How plants release energy from food, with and without oxygen, and how gases move in and out.
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Respiration is the chemical breakdown of food (mainly glucose) inside living cells to release energy. It goes on day and night in every living cell, in roots, stems, leaves, flowers and seeds. Do not confuse it with breathing, which is only the exchange of gases. The energy is stored in ATP, the cell's energy currency, and used for growth, absorbing minerals and other work.
Differentiate respiration from breathing.
Respiration is the chemical release of energy from food inside cells. Breathing is the physical intake of oxygen and release of carbon dioxide.
Why is respiration said to go on all the time in a plant?
Every living cell needs energy at all times, so every cell respires day and night, even though photosynthesis happens only in light.
Name the energy-storing molecule made during respiration.
ATP (adenosine triphosphate).
Aerobic respiration needs oxygen and breaks glucose down completely: glucose + oxygen → carbon dioxide + water + large energy. Anaerobic respiration happens without oxygen and breaks glucose down only partly. In yeast and some plant tissues (for example roots in waterlogged soil) this is called fermentation: glucose → alcohol + carbon dioxide + small energy. The first step, glycolysis, takes place in the cytoplasm; aerobic respiration then continues in the mitochondria.
Show that mass is conserved in aerobic respiration of glucose: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O (C = 12, H = 1, O = 16).
Left: 180 + 192 = 372 g. Right: 264 + 108 = 372 g. Equal, so mass is conserved.
For fermentation C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂, what mass of alcohol and CO₂ comes from 180 g of glucose?
Alcohol 92 g and CO₂ 88 g; together 180 g, equal to the glucose used.
Why do waterlogged roots sometimes die?
Water fills the air spaces, so oxygen is short. The roots respire anaerobically, make little energy and build up alcohol, which harms the cells.
Oxygen enters and carbon dioxide leaves by diffusion: through stomata in leaves, lenticels in woody stems and directly through the surface of young roots (root hairs). In daylight a leaf makes more oxygen by photosynthesis than it uses, so it releases oxygen overall; at night it only takes in oxygen and releases carbon dioxide. Photosynthesis stores energy and needs light and chlorophyll; respiration releases energy and happens in all living cells.
Name the pores for gas exchange in (a) leaves, (b) woody stems.
(a) Stomata. (b) Lenticels.
Give two differences between respiration and photosynthesis.
Respiration releases energy, uses oxygen and goes on all the time in all living cells. Photosynthesis stores energy, releases oxygen and needs light and chlorophyll.
Germinating seeds in a closed flask are tested with lime water. What do you observe and why?
The lime water turns milky because the seeds respire and release carbon dioxide.
1. Define respiration.
2. Where in the cell does aerobic respiration mainly take place?
3. Write the word equation for aerobic respiration.
4. Write the word equation for fermentation by yeast.
5. Why does dough rise when yeast is added?
6. Which respiration gives more energy, aerobic or anaerobic?
7. What is ATP?
8. Name the three structures through which plants exchange gases.
9. Why is more oxygen released by a leaf in daylight than at night?
10. How much water forms when 180 g of glucose is respired aerobically?