Botany Basics
The science of plants — how they are built, how they function, how they grow
Chapter 8: Photosynthesis and Respiration
Why this matters: Photosynthesis is where all food comes from — every calorie you eat traces back to a plant capturing sunlight. Understanding it explains why plants need light, CO₂, and water; why shade kills some plants and scorches others; why plants grow slower in heat; and why healthy soil biology makes plants more productive. Respiration explains why plants need oxygen at the roots, why waterlogging kills, and why compost heats up.
8.1 The Big Picture
PHOTOSYNTHESIS and RESPIRATION are opposites
From: 8.1 The Big Picture
Image file: ../../../images/s01-foundation/c01-botany-basics/ch08/c01-botany-basics_ch08_photosynthesis_and_respiration_fig01.png
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8.2 Photosynthesis in Detail
The Two Stages
PHOTOSYNTHESIS has two main stages
From: 8.2 Photosynthesis in Detail
Image file: ../../../images/s01-foundation/c01-botany-basics/ch08/c01-botany-basics_ch08_photosynthesis_and_respiration_fig02.png
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Light and Chlorophyll
VISIBLE LIGHT SPECTRUM
From: 8.2 Photosynthesis in Detail
Image file: ../../../images/s01-foundation/c01-botany-basics/ch08/c01-botany-basics_ch08_photosynthesis_and_respiration_fig03.png
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8.3 The Three Photosynthetic Pathways
Plants have evolved three different ways to fix carbon from CO₂. This affects which plants thrive in Texas heat.
C3 Photosynthesis (the original)
C3 PHOTOSYNTHESIS
From: 8.3 The Three Photosynthetic Pathways
Image file: ../../../images/s01-foundation/c01-botany-basics/ch08/c01-botany-basics_ch08_photosynthesis_and_respiration_fig04.png
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C4 Photosynthesis (evolved for heat)
C4 PHOTOSYNTHESIS
From: 8.3 The Three Photosynthetic Pathways
Image file: ../../../images/s01-foundation/c01-botany-basics/ch08/c01-botany-basics_ch08_photosynthesis_and_respiration_fig05.png
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CAM Photosynthesis (for extreme drought)
CAM PHOTOSYNTHESIS
From: 8.3 The Three Photosynthetic Pathways
Image file: ../../../images/s01-foundation/c01-botany-basics/ch08/c01-botany-basics_ch08_photosynthesis_and_respiration_fig06.png
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8.4 Factors Affecting Photosynthesis Rate
LIGHT INTENSITY
From: 8.4 Factors Affecting Photosynthesis Rate
Image file: ../../../images/s01-foundation/c01-botany-basics/ch08/c01-botany-basics_ch08_photosynthesis_and_respiration_fig07.png
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8.5 Cellular Respiration
Respiration happens in every living cell, every hour, plant or animal. It is the process of extracting usable energy (ATP) from glucose.
CELLULAR RESPIRATION overview
From: 8.5 Cellular Respiration
Image file: ../../../images/s01-foundation/c01-botany-basics/ch08/c01-botany-basics_ch08_photosynthesis_and_respiration_fig08.png
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8.6 The Carbon Cycle and Plants
CARBON CYCLE (simplified)
From: 8.6 The Carbon Cycle and Plants
Image file: ../../../images/s01-foundation/c01-botany-basics/ch08/c01-botany-basics_ch08_photosynthesis_and_respiration_fig09.png
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8.7 Practical Applications
GROW LIGHTS and light management
From: 8.7 Practical Applications
Image file: ../../../images/s01-foundation/c01-botany-basics/ch08/c01-botany-basics_ch08_photosynthesis_and_respiration_fig10.png
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Chapter Summary
PHOTOSYNTHESIS = building sugar using light
From: Chapter Summary
Image file: ../../../images/s01-foundation/c01-botany-basics/ch08/c01-botany-basics_ch08_photosynthesis_and_respiration_fig11.png
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1. Photosynthesis converts light energy into chemical energy stored in glucose.
2. Photosynthesis occurs only in leaves.
3. The light reactions of photosynthesis occur in the stroma of the chloroplast.
4. The Calvin cycle uses ATP and NADPH produced by the light reactions.
5. C4 plants concentrate CO₂ before the Calvin cycle to reduce photorespiration.
6. CAM plants open stomata during the day to take in CO₂.
7. Cellular respiration occurs only at night in plants.
8. Mitochondria are the site of cellular respiration.
9. Glucose is the primary input for cellular respiration.
10. The net equation for photosynthesis is: 6CO₂ + 6H₂O + light → C₆H₁₂O₆ + 6O₂.
11. Photorespiration is a problem primarily in hot dry conditions.
12. C3 plants are the most common and represent most crop plants.
13. Plants in shade have higher light saturation points than sun plants.
14. Temperature increases always increase photosynthesis rate.
15. Corn and sugarcane are examples of C4 plants.
16. Cacti and succulents often use CAM photosynthesis.
17. Respiration releases the energy stored in glucose as ATP.
18. Plants produce oxygen as a byproduct of splitting water in photosynthesis.
19. At the light compensation point, photosynthesis equals respiration.
20. Chlorophyll reflects green light, which is why leaves appear green.
1. Write and explain the overall equation for photosynthesis.
2. What is the difference between the light reactions and the Calvin cycle?
3. What is photorespiration and why is it a problem for C3 plants in Texas?
4. How do C4 plants solve the photorespiration problem?
5. How does CAM photosynthesis work and which plants use it?
6. What factors limit the rate of photosynthesis?
7. What is the light compensation point and why does it matter for shade gardening?
8. How do plants use the glucose produced by photosynthesis?
9. Describe the basic process of cellular respiration.
10. Why do plants release CO₂ at night even though they produce O₂ during the day?
1. The organelle where photosynthesis occurs is the .
2. The green pigment that absorbs light energy is .
3. The light reactions occur in the of the chloroplast.
4. The Calvin cycle occurs in the of the chloroplast.
5. C4 plants pre-fix CO₂ in cells before concentrating it for the Calvin cycle.
6. CAM plants open stomata at to absorb CO₂.
7. Cellular respiration occurs in the .
8. The net products of photosynthesis are and oxygen.
9. Oxygen is released as a byproduct of splitting molecules.
Complete in the field. Check each off as you go.
1. Leaf Disk Photosynthesis: Cut small disks from a spinach leaf using a straw. Remove air by submerging in sodium bicarbonate solution and applying vacuum (syringe). Expose to light — watch disks float as they produce O₂. Compare light vs dark.
2. CO₂ Indicator: Place a plant in a sealed clear bag with CO₂ indicator solution (bromothymol blue). Place in bright light. Observe color change as CO₂ drops. Repeat in dark and observe opposite effect.
3. Temperature and Rate: Set up identical plants at 60°F, 75°F, and 95°F. Measure CO₂ uptake or O₂ production (bubble count for aquatic plants). At which temperature is photosynthesis most efficient?
4. Light Quality Test: Cover three identical seedlings with red cellophane, blue cellophane, and green cellophane. Grow for 2 weeks. Which color grows best and why?
5. C3 vs C4 Identification: In your field or garden, identify examples of C3 plants (most broadleaf plants, wheat, rice) and C4 plants (corn, sorghum, crabgrass, bermuda grass). In peak Texas summer, which look more stressed?
Practice Exercises
- Why do native Texas grasses (C4) outcompete Kentucky bluegrass (C3) in summer, but Kentucky bluegrass can establish in cool spring conditions before native grasses green up?
- A cactus is very slow growing. Is this because conditions are poor? Explain the real reason.
- You overwater your potted plant and the roots sit in water for a week. The plant wilts even though soil is wet. Explain what happened at the biochemical level.
- Why do fall leaves turn yellow and orange? Where were those colors before?
- A plant is growing in very low light. You measure that its photosynthesis rate equals its respiration rate. What is happening to the plant and what will happen if light levels drop further?
- Why would adding CO₂ to a greenhouse increase plant growth, and what limits how much benefit you get?
Next Chapter → Plant Hormones and Growth Responses
Connections to Other Topics
→ C01 Ch01: The Plant Cell — chloroplasts and mitochondria
From: Connections to Other Topics
Image file: ../../../images/s01-foundation/c01-botany-basics/ch08/c01-botany-basics_ch08_photosynthesis_and_respiration_fig12.png
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Next Chapter → Plant Hormones and Tropisms