Biology | Answers to Review Questions

Author Dr. George B. Johnson, Washington University
Author Dr. Peter H. Raven, Missouri Botanical Gardens & Washington University
Contributor Dr. Susan Singer, Carleton College
Contributor Dr. Jonathan Losos, Washington University

Answers to Review Questions

Chapter 10 (p. 204)

1. The final oxygen comes from the lysis of water.

2. Van Helmont grew a tree over a period of years in soil to which he added only water. When the tree was weighed after five years, it had gained 74.4 kilograms, while the soil had only lost a few grams in weight. The weight of the tree clearly came from some place other than the soil.

3. Pigments absorb light energy when their electrons become excited by photons. Green is not absorbed at all by the plant, so green light is reflected back to our eyes.

4. Three main stages in the light-dependent reactions in photosynthesis are 1) capture of photon by a pigment, 2) shuttling of an electron through a series of electron carriers to an acceptor, and 3) proton-pump driven chemiosmosis. Reaction center chlorophyll molecules pass excitation energy to the primary electron acceptor. The primary electron acceptor serves as an energy-channeling site for excitation energy, in the form of electrons.

5. Light causes ejection of the high-energy electron; it travels through the photosynthetic unit and drives the proton pump to generate ATP; the electron is then returned to the pigment. The ejected electron is high-energy and the returned one is not, so it is not a precise cycle. Three electrons yield one ATP.

6. A photon is absorbed by P₆₈₀ of PS II, light excites an electron, the high-energy electron drives the proton pump to make ATP, the electron returns to its normal energy state and is passed to PS I where another photon re-excites it, and it is channeled to ferredoxin, causing NADP⁺ to become NADPH. The PS II system generates ATP, and the PS I system NADPH.

7. In a C₃ plant, thylakoid membranes within the chloroplasts are the sites of the light-dependent reactions, which generate the ATP and NADPH that fuel the light-dependent reactions. The stroma contains the Calvin cycle enzymes that carry out the carbon-fixing light-independent reactions.

8. Photorespiration is the oxidation of ribulose 1,5-bisphosphate, releasing CO₂ without producing ATP or NADPH. The advantage of C₄ photosynthesis is the fact that bundle sheath cells concentrate CO₂ and inhibit photorespiration. C₄ photosynthesis requires use of ATP to concentrate the "expenditure" because it conserves glucose molecules that would otherwise be converted back into CO₂ by photorespiration in the consistently higher temperatures in warmer climates.