- Define and explain How Photosynthesis Works in your own words
- Use key terms such as chlorophyll accurately
- Apply what you have learned to new examples and questions
- Avoid the common mistakes learners make with this topic
Life runs on energy, and nearly all of it is captured from sunlight by photosynthesis, then released by respiration. In this chapter you will master both equations, investigate what limits the rate of photosynthesis, compare aerobic and anaerobic respiration, and calculate how efficiently energy moves through ecosystems.
This lesson focuses on How Photosynthesis Works: master the word and symbol equations and how leaves are adapted for the job.
Master the word and symbol equations and how leaves are adapted for the job.
Key ideas
Photosynthesis
Photosynthesis takes place in the chloroplasts of green cells: carbon dioxide + water → glucose + oxygen, using light energy trapped by chlorophyll. Leaves are adapted with a large surface area, thin structure for a short diffusion distance, and stomata that let gases in and out. Glucose is used for respiration, converted to starch for storage, or built into cellulose for cell walls.
Limiting factors
The rate of photosynthesis is limited by whichever factor is in shortest supply: light intensity, carbon dioxide concentration or temperature. On a graph of rate against light intensity, the rate rises steeply at first, then levels off when another factor, such as carbon dioxide, becomes limiting. Greenhouse growers exploit this by adding extra light, carbon dioxide and warmth — but only raising the limiting factor helps.
Key term — chlorophyll: The green pigment in chloroplasts that absorbs light energy for photosynthesis.
Give two ways a leaf is adapted for efficient photosynthesis.
Any two of: large surface area to catch light; thin for a short diffusion distance; chlorophyll in chloroplasts to trap light energy; stomata for gas exchange; veins to transport water and glucose.
Answer: Any two of: large surface area to catch light; thin for a short diffusion distance; chlorophyll in chloroplasts to trap light energy; stomata for gas exchange; veins to transport water and glucose.
- Writing the photosynthesis equation backwards Photosynthesis builds glucose from carbon dioxide and water; respiration breaks glucose down. Mixing them up reverses the direction of energy flow.
- Saying plants only respire at night Plants respire all the time to release energy. At night photosynthesis stops but respiration continues.
Practice
Carbon dioxide + water → glucose + oxygen (using light energy).
Light is no longer the limiting factor; something else, such as carbon dioxide concentration or temperature, is now in shortest supply, so extra light cannot raise the rate further.
Glucose + oxygen → carbon dioxide + water (energy is released).
In human muscle, anaerobic respiration converts glucose to lactic acid. In yeast, it converts glucose to ethanol and carbon dioxide.
Quick check
Which of these best defines "chlorophyll"?
Grass at one trophic level contains 20 000 kJ of energy, and the rabbits feeding on it contain 2 400 kJ. Calculate the percentage efficiency of energy transfer.
- How Photosynthesis Works: master the word and symbol equations and how leaves are adapted for the job.
- Photosynthesis: Photosynthesis takes place in the chloroplasts of green cells: carbon dioxide + water → glucose + oxygen, using light energy trapped by chlorophyll.
- limiting factor: The factor in shortest supply that restricts the rate of a process; raising it increases the rate until another factor takes over.
- Watch out for: writing the photosynthesis equation backwards