Life Processes · Lesson 3 of 13
Autotrophic Nutrition
“Leaves turn sunlight, air and water into stored food.”
• Describe the raw materials and conditions required for photosynthesis. • Explain the major events of photosynthesis. • Relate chloroplast structure to light absorption. • Explain how stomata regulate gas exchange and water loss. • Interpret experiments testing the requirements for photosynthesis. • Describe the uptake of water, minerals and nitrogen.
A plant cannot walk toward food, yet it steadily increases in mass. Much of that new material is built from carbon dioxide entering the leaves and water entering the roots. Light supplies the energy that allows these simple substances to be rearranged into energy-rich carbohydrates.
Photosynthesis is the process by which chlorophyll-containing organisms use light energy to convert carbon dioxide and water into carbohydrates, releasing oxygen.
Events During Photosynthesis
First, chlorophyll absorbs light energy. That energy is converted into chemical energy, and water molecules are split so hydrogen becomes available and oxygen is released. Carbon dioxide is then reduced to form carbohydrates. These events are linked but need not occur immediately one after another. Many desert plants take in carbon dioxide at night, store it in an intermediate form, and use captured light energy during the day.
| Event | What occurs | Why it matters |
|---|---|---|
| Light absorption | Chlorophyll captures light energy | Provides energy for the process |
| Energy conversion and water splitting | Light energy becomes chemical energy; water supplies hydrogen | Makes reduction reactions possible and releases oxygen |
| Carbon dioxide reduction | Carbon dioxide is converted into carbohydrate | Stores energy in an organic molecule |
Chlorophyll And Chloroplasts
In a leaf cross-section, many cells contain green chloroplasts. Chlorophyll within these organelles absorbs light. A variegated leaf has green and non-green regions, so it can test whether chlorophyll is required. If a plant is first kept in darkness, stored starch is used up. After exposure to light, only chlorophyll-containing regions produce starch and turn blue-black with iodine.
Observing The Need For Chlorophyll
Purpose: determine whether chlorophyll is necessary for photosynthesis. Setup: use a destarched plant with variegated leaves. Procedure: expose it to sunlight, trace the green regions on paper, boil a leaf in water, decolourise it safely in alcohol heated through a water bath, rinse it and add dilute iodine. Observation: green regions exposed to light turn blue-black, while non-green regions do not. Conclusion: starch forms only where chlorophyll is present.
Alcohol is flammable and must never be heated directly over a flame. It should be heated using a water bath under adult supervision.
Carbon Dioxide And Stomata
Carbon dioxide enters a leaf mainly through stomata, tiny pores controlled by guard cells. When water enters the guard cells, they swell and the pore opens. When they lose water, they shrink and the pore closes. This control allows the plant to obtain carbon dioxide while limiting water loss. Gas exchange can also occur across the surfaces of stems and roots.
Stomata are microscopic pores in the epidermis of leaves and some young stems that permit gas exchange and water-vapour loss.
Problem
Two destarched plants are placed under sealed bell jars in sunlight. Potassium hydroxide is kept beside one plant but not the other. What result is expected after an iodine test?
- 1.Potassium hydroxide absorbs carbon dioxide inside its jar.
- 2.Both plants receive light, and both contain chlorophyll.
- 3.The plant without available carbon dioxide cannot manufacture appreciable starch.
- 4.Its leaf therefore does not turn blue-black with iodine.
- 5.The control plant has carbon dioxide and forms starch, so its leaf turns blue-black.
- 6.The comparison shows that carbon dioxide is necessary for photosynthesis.
Water, Minerals And Nitrogen
Roots absorb water from the soil for photosynthesis. They also take up minerals such as phosphorus, iron and magnesium. Nitrogen is essential for proteins and other compounds, but plants generally cannot use atmospheric nitrogen directly. They absorb it mainly as nitrates or nitrites, or as organic nitrogen compounds prepared through the activity of microorganisms.
Carbohydrate not used immediately may be stored as starch in plants. Animals store a related carbohydrate called glycogen, especially in the liver and muscles.
Quiz
Which substance absorbs light energy for photosynthesis?
Why is a plant kept in darkness before a starch test?
What happens when guard cells gain water?
What is the role of potassium hydroxide in the bell-jar investigation?
Which observation best supports the need for chlorophyll?
Practice Problems
- List the raw materials and conditions needed for photosynthesis and state where each comes from.
- Explain the three major events of photosynthesis in a logical sequence.
- Predict the starch-test result for a leaf whose middle portion was covered with opaque paper before exposure to sunlight.
- Explain how guard cells balance carbon dioxide uptake against water conservation.
- A plant has adequate light, water and chlorophyll but receives no usable nitrogen. Explain which process may continue initially and why long-term growth will still be poor.
Key Takeaways
• Photosynthesis stores light energy in carbohydrates. • Carbon dioxide and water provide the matter used to form carbohydrate. • Chlorophyll absorbs light energy. • Water splitting and carbon dioxide reduction are major events of photosynthesis. • Starch tests can reveal where photosynthesis occurred. • Guard cells regulate stomatal opening and closing. • Roots supply water and mineral nutrients including usable nitrogen compounds.