Life Processes in Plants · Lesson 5 of 6
How Plants Release Energy from Food
“Discover evidence of plant respiration and explain how plants use their food to release energy.”
• Interpret a germinating-seed investigation using lime water. • Identify the inputs and products of plant respiration. • Explain why plant parts need energy from food. • Distinguish making food from releasing energy through respiration. • Recognise that green and non-green living plant parts respire.
Making food does not finish the story
A leaf can make glucose, and phloem can carry food to other parts of the plant. Those parts still need a way to use food for growth and development. Respiration is the process that releases energy from food so that living processes can continue.
Plants do not have to move around like animals to need energy. Their roots grow, their stems and leaves develop, and their living tissues keep functioning. A supply of stored or transported food is useful because it can be used in processes that release energy.
Respiration is not the same as breathing movements. A plant does not need a chest that rises and falls in order to respire. Here, respiration refers to a chemical process inside living parts in which glucose is broken down and energy is released.
Germinating seeds provide evidence
A germinating seed is beginning to grow into a young plant. It uses stored food while it develops. Because germinating seeds are living and active, they can help us investigate whether plants release carbon dioxide during respiration.
In the teacher’s demonstration, moong bean seeds are soaked overnight and placed on moist cotton in a conical flask. The cotton must stay wet enough to support germination. A two-hole cork carries two tubes, and the flask is left in darkness for 24 hours.
The teacher prepares two test tubes containing lime water. One is connected to the seed flask using the glass and rubber tube arrangement; the other is kept as a comparison. Gas from the flask can reach the connected lime water. The same observation period is used when comparing the tubes.
Lime water becomes milky when carbon dioxide reacts with it. In this investigation, the change is used as evidence of additional carbon dioxide from germinating seeds.
The connected lime water becomes milky because the flask contains additional carbon dioxide released by the seeds. Air already contains a small amount of carbon dioxide, so the important claim is not that ordinary air contains none. The comparison helps interpret the extra carbon dioxide associated with the seeds.
Keeping the seeds in darkness avoids photosynthesis using light in the setup. It helps us focus on the carbon dioxide produced as the seeds respire. Darkness does not prevent respiration; the developing seed still needs to use its food.
Problem
Why does milky lime water connected to germinating seeds support respiration in plants?
- 1.The lime-water change is evidence that carbon dioxide has reached it.
- 2.The seed flask provides additional carbon dioxide compared with the ordinary-air situation.
- 3.Germinating seeds release that carbon dioxide while using their stored food, providing evidence of respiration in plant material.
Iodine turning blue-black indicates starch. Lime water turning milky indicates carbon dioxide. A milky lime-water result does not mean starch is present, and an iodine result does not directly identify a gas.
The respiration word equation
During the respiration process studied here, glucose is broken down using oxygen. Carbon dioxide and water are produced, and energy is released. The plant uses that energy for its growth and development. These relationships can be expressed in words before writing the equation.
The process in living plant parts in which glucose is broken down using oxygen, releasing carbon dioxide, water, and energy.
The left side names glucose and oxygen because they are used in the process. The right side names carbon dioxide and water and also shows released energy. Energy is not a gas collected in the apparatus; it is a result of using the food. The lime-water investigation gives evidence about one product, carbon dioxide, rather than measuring every result of the equation.
Problem
A learner writes: X + Y → carbon dioxide + Z + energy. Which terms represent respiration?
- 1.Respiration begins with glucose and oxygen, so X is glucose and Y is oxygen in the stated order.
- 2.The other material product besides carbon dioxide is water, so Z is water.
- 3.Read the completed equation to connect the use of glucose and oxygen with the release of carbon dioxide, water, and energy.
Living green and non-green plant parts carry out respiration. Leaves respire even though they also make food. Roots, stems, and germinating seeds also need energy. A part that does not perform photosynthesis can use food supplied or stored by the plant.
Kamala Sohonie (1911–1998) earned a Ph.D. at Cambridge for research connected with respiration in plants. She later worked at Lady Hardinge Medical College in New Delhi and the Nutrition Research Laboratory in Coonoor, and eventually became Director of the Royal Institute of Science in Bombay. Her research also contributed to understanding the nutritive value of plant foods, including Neera, a drink made from coconut-palm sap.
Relating respiration to photosynthesis
Photosynthesis and respiration involve some of the same substances, but they have different purposes. Photosynthesis uses light energy to make food. Respiration uses food to release energy. A plant needs both food production and a way to use the food.
| Feature | Photosynthesis | Respiration studied here |
|---|---|---|
| Materials used | Carbon dioxide and water | Glucose and oxygen |
| Material products | Glucose and oxygen | Carbon dioxide and water |
| Energy relationship | Uses captured light energy to make food | Releases energy from food |
| Plant parts | Parts containing chlorophyll, mainly leaves | All living parts, green and non-green |
| Need for light | Requires light | Does not depend on light |
A green plant can respire while photosynthesis is occurring in light. In darkness, photosynthesis cannot continue, but respiration can. Do not imagine a switch that allows exactly one process at a time. The amount of each gas exchanged with the surroundings reflects the processes occurring in the plant.
Problem
A root receives food from leaves through phloem. Explain what happens next to make the food useful for growth.
- 1.Transport supplies the root with food, but transport itself is not the energy-releasing process.
- 2.The living root carries out respiration, using glucose and oxygen and releasing energy.
- 3.The released energy supports growth and development. Carbon dioxide and water are products of that respiration.
Oxygen release during photosynthesis does not mean that plants only release oxygen. Respiration uses oxygen and releases carbon dioxide. Making food and using food are both part of a living plant’s activity.
Quiz
What change indicates carbon dioxide in the seed investigation?
Why are germinating seeds useful for this investigation?
Which pair is used in the respiration equation studied here?
Which outcome makes respiration useful for growth?
Which plant parts carry out respiration?
What happens to respiration when a living plant is kept in darkness?
Practice Problems
- Describe the seed-and-lime-water demonstration and explain the purpose of the comparison tube.
- Explain why keeping the seeds in darkness does not stop their respiration.
- Write the respiration word equation and explain each input and result.
- Compare iodine and lime-water tests, including what their colour changes indicate.
- Explain why a living root needs food transported from leaves and also needs to respire.
- Compare photosynthesis and respiration by purpose, inputs, products, and need for light.
Key Takeaways
• Respiration releases energy from food for plant growth and development. • The process studied here uses glucose and oxygen and produces carbon dioxide and water. • Milky lime water provides evidence of carbon dioxide released by germinating seeds. • All living plant parts respire, including those that are not green. • Photosynthesis makes food using light; respiration releases energy from that food. • Respiration can continue in darkness and can occur alongside photosynthesis in light.