PHOTOSYNTHESIS REVIEW -- ANSWERS

Photosynthesis Review Questions

1. The primary function of photosynthesis is to capture light energy, convert some of this light energy into chemical energy, and store this chemical energy in molecules of carbohydrates (such as phosphoglyceraldehyde, glucose, or starch).

2. (A) Oxygen is released as a by-product (= “waste material”) of photosynthesis. (B) The oxygen atoms are removed from water as the water is broken down during the light reactions. (C) Water is broken down to provide hydrogen atoms (which, in turn, provide electrons to replace electrons that are lost from chlorophyll P680). At this point, oxygen is of no value to the system and, consequently, is released from the chloroplast and from the leaf into the surrounding air.

3. Two high-energy molecules that are produced during the light reactions are adenosine triphosphate (ATP) and nicotinamide adenine dinucleotide phosphate (NADP-H). These two molecules temporarily store energy; they transfer energy into carbohydrates being produced during the light independent reactions (= Calvin cycle). The carbohydrates (PGAL, glucose, starch) can store energy for much longer periods of time.

4. When a molecule loses an electron, the molecule has been oxidized. When a molecule gains an electron, the molecule has been reduced (and has gained energy that the electron brought.) OIL RIG

5. When chlorophyll P680 absorbs energy, an electron gains the energy. (Actually a series of electrons, one after the other, gains energy; however, if you understand what happens to one electron, then you understand what happens to each of the millions of other electrons in this series.)

6. When chlorophyll P680 loses an electron, the P680 molecule replaces this electron with an electron taken from an atom of hydrogen which has come out of a molecule of water. (Review: a hydrogen atom consists of one proton and one electron. If the electron is removed, only the proton remains. The proton will be used later, in Photosystem I, to join with an electron that is picked up by NADP. The result is the formation of NADP-H, one of products of the light-dependent reactions.)

7. A molecule of NADP-H contains more energy than a molecule of NADP. The NADP picks up a high-energy electron and combines the electron with a proton to form the hydrogen atom that is the “H” of the NADP-H. Compare NADP to an empty Wells Fargo armored delivery truck while NADP-H is the Wells Fargo truck loaded with money (or, in the case of NADP-H, loaded with energy).

8. The name of the 3-carbon product of the Calvin cycle is phosphoglyceraldehyde (which is also called glyceraldehyde phosphate) and is commonly symbolized as PGAL.

9. Carbon dioxide, in the form of a gas, is removed from the environment of a plant (either from the surrounding air or water, depending on whether the plant lives in air or submerged in water), taken into leaves and chloroplasts, and used as a carbon source during the manufacture of carbohydrates (such as PGAL, glucose, and starch) in the Calvin cycle.

10. The shortest wavelengths (such as violet) have more energy than do the longer wavelengths of light such as red. Of the three colors of light listed here (blue, green, red), blue has the shortest wavelength and, consequently, the most energy. If we compare the wavelengths longer than light (such as radio waves) with wavelengths shorter than light (such as X-rays), the differences in energy level are easier to appreciate. X-rays (short wavelengths with high energy levels) are dangerous to living cells because the X-rays can damage DNA in the chromosomes. Conversely, the long wavelengths of radio waves (with their low energy levels) are apparently harmless to cells.

11. Chlorophyll is green because it reflects green wavelengths of light and these are what we see.

12. Photosynthesis occurs in cell parts called chloroplasts that occur primarily in plant cells that are exposed to light. (Would the root cells of a plant be able to use chloroplasts?)

13. The term phosphorylation refers to the addition of a phosphate group to an atom or molecule. The addition of the phosphate involves the addition of large amounts of energy. This energy may come from light (as in the light reactions of photosynthesis) or from the breakdown of sugars (as in cellular respiration).

14. True. The Calvin cycle requires the input of carbon dioxide (as a source of carbon for building carbohydrates) and the availability of ATP and NADP-H to provide energy to reduce the carbon dioxides and construct high-energy carbohydrates. If these materials (carbon dioxide, ATP, and NADP-H) are available, the Calvin cycle can occur, even in darkness. Normally, however, what would happen to the quantities of ATP and NADP-H shortly after the plant is placed in darkness? (The light dependent reactions would shut down and no more ATP and NADP-H would be produced. When the available supplies of ATP and NADP-H are used up, the Calvin cycle would stop.)

15. Packets or “bundles” of light energy are called photons.

16. Light energy is absorbed by electrons that supply energy for the formation of molecules of ATP and NADP-H which are used as temporary energy storing molecules.

17. Molecules of carbon dioxide enter the Calvin cycle where the carbons are reduced by acquiring energy from ATP and NADP-H (from the light dependent reactions) and high energy molecules of PGAL are constructed.

18. The electron gains so much energy (by absorbing photons) that the electron leaves the chlorophyll molecule. The electron is now described as a “high-energy electron” and can provide energy for the formation of molecules of ATP.

19. High energy electrons in Photosystem I are used to manufacture molecules of NADP-H.

20. High energy electrons in Photosystem II are used to manufacture molecules of ATP.