7. Biochemical Pathways-Photosynthesis


7.5. Interrelationships Between Autotrophs and Heterotrophs

The differences between autotrophs and heterotrophs were described in chapter 6. Autotrophs are able to capture energy to manufacture new organic molecules from inorganic molecules. Heterotrophs must have organic molecules as starting points. However, it is important for you to recognize that all organisms must do some form of respiration. Plants and other autotrophs obtain energy from food molecules, in the same manner as animals and other heterotrophs—by processing organic molecules through the respiratory pathways. This means that plants, like animals, require oxygen for the ETS portion of aerobic cellular respiration.

Many people believe that plants only give off oxygen and never require it. Actually, plants do give off oxygen in the light-dependent reactions of photosynthesis, but in aerobic cellular respiration they use oxygen, as does any other organism that uses aerobic respiration. During their life spans, green plants give off more oxygen to the atmosphere than they take in for use in respiration. The surplus oxygen given off is the source of oxygen for aerobic cellular respiration in both plants and animals. Animals are dependent on plants not only for oxygen but ultimately for the organic molecules necessary to construct their bodies and maintain their metabolism (figure 7.11).

FIGURE 7.11. The Interdependence of Photosynthesis and Aerobic Cellular Respiration

Although both autotrophs and heterotrophs carry out cellular respiration, the photosynthetic process that is unique to photosynthetic autotrophs provides essential nutrients for both processes. Photosynthesis captures light energy, which is ultimately transferred to heterotrophs in the form of carbohydrates and other organic compounds. Photosynthesis also generates O2, which is used in aerobic cellular respiration. The ATP generated by cellular respiration in both heterotrophs (e.g., animals) and autotrophs (e.g., plants) is used to power their many metabolic processes. In return, cellular respiration supplies two of the most important basic ingredients of photosynthesis, CO2 and H2O.

Thus, animals supply the raw materials—CO2, H2O, and nitrogen—needed by plants, and plants supply the raw materials—sugar, oxygen, amino acids, fats, and vitamins— needed by animals. This constant cycling is essential to life on Earth. As long as the Sun shines and plants and animals remain in balance, the food cycles of all living organisms will continue to work properly.


12. Even though animals do not photosynthesize, they rely on the Sun for their energy. Why is this so?

13. What is an autotroph? Give an example.

14. Photosynthetic organisms are responsible for producing what kinds of materials?

15. Draw your own simple diagram that illustrates how photosynthesis and respiration are interrelated.


Sunlight supplies the essential initial energy for making the large organic molecules necessary to maintain the forms of life we know. Photosynthesis is the process by which plants, algae, and some bacteria use the energy from sunlight to produce organic compounds. In the light-capturing events of photosynthesis, plants use chemicals, such as chlorophyll, to trap the energy of sunlight using photosystems. During the light-dependent reactions, they manufacture a source of chemical energy, ATP, and a source of hydrogen, NADPH. Atmospheric oxygen is released in this stage. In the light-independent reactions of photosynthesis, the ATP energy is used in a series of reactions (the Calvin cycle) to join the hydrogen from the NADPH to a molecule of carbon dioxide and form a simple carbohydrate, glyceraldehyde-3-phosphate. In subsequent reactions, plants use the glyceraldehyde-3-phosphate as a source of energy and raw materials to make complex carbohydrates, fats, and other organic molecules. Table 7.2 summarizes the process of photosynthesis.

TABLE 7.2. Summary of Photosynthesis


Where in the Chloroplast It Occurs



Light-energy trapping events

In the chlorophyll molecules and accessory pigments of the thylakoids


Excited electrons

Light-dependent reactions

In the thylakoids of the grana

Water, ADP, NADP+

Oxygen, ATP, NADPH




Carbon dioxide, ribulose, ATP, NADPH

Glyceraldehyde-3-phosphate, ribulose, ADP, NADP+

Basic Review

1. Which of the following is not able to carry out photosynthesis?

a. algae

b. cyanobacteria

c. frogs

d. broccoli

2. A _____ consists of stacks of membranous sacs containing chlorophyll.

a. granum

b. stroma

c. mitochondrion

d. cell wall

3. During the _____ reactions, ATP and NADPH are used to help combine carbon dioxide with a 5-carbon molecule, so that ultimately organic molecules, such as glucose, are produced.

a. light-independent

b. light-dependent

c. Watson cycle

d. Krebs cycle

4. Pigments other than the green chlorophylls that are commonly found in plants are collectively known as _____. These include the carotenoids.

a. chlorophylls

b. hemoglobins

c. accessory pigments

d. thylakoids

5. This enzyme speeds the combining of CO2 with an already present 5-carbon ribulose.

a. DNAase

b. ribose

c. Ribulose-1,5-bisphosphate carboxylase oxygenase (RuBisCO)

d. phosphorylase

6. Carbon dioxide undergoes carbon fixation, which occurs in the

a. Calvin cycle.

b. Krebs cycle.

c. light-dependent reactions.

d. photosystem I.

7. The chlorophylls and other pigments involved in trapping sunlight energy and storing it are arranged into clusters called

a. chloroplasts.

b. photosystems.

c. cristae.

d. thylakoids.

8. Light energy comes in discrete packages called

a. NADP+.

b. lumina.

c. photons.

d. brilliance units.

9. The electrons released from photosystem _____ end up in the chlorophyll molecules of photosystem _____.

a. I, II

b. A, B

c. B, A

d. II, I

10. _____ are sacs containing chlorophylls, accessory pigments, electron-transport molecules, and enzymes.

a. Thylakoids

b. Mitochondria

c. Photosystems

d. Ribosomes

11. Which kind of organisms use respiration to generate ATP?

a. plants

b. animals

c. algae

d. all of the above

12. Plants, like animals, require _____ for the ETS portion of aerobic cellular respiration.

a. silicone

b. hydrogen

c. nitrogen

d. oxygen

13. _____ are an important group of organic molecules derived from plants. These are organic molecules that we cannot manufacture but must have in small amounts.

a. Accessory pigments

b. Vitamins

c. Nitrogenous compounds

d. Minerals

14. These prokaryotic organisms are capable of manufacturing organic compounds using light energy.

a. algae

b. protozoa

c. cyanobacteria

d. tomatoes

15. Chlorophyll-containing organisms look green because they reflect _____ - colored light.

a. green

b. red

c. yellow

d. white


1. c 2. a 3. a 4. c 5. c 6. a 7. b 8. c 9. d 10. a 11. d 12. d 13. b 14. c 15. A

Thinking Critically

From a Metabolic Point of View

Both plants and animals carry on metabolism. From a metabolic point of view, which of the two is the more complex organism? Include in your answer the following topics:

1. Cell structure

2. Biochemical pathways

3. Enzymes

4. Organic molecules

5. Photosynthetic autotrophy and heterotrophy