ch.13

Question 1

Rubisco’s Inefficiency and Non-Specificity

Answer 1

Inefficiency: Rubisco is inefficient because it can bind to both CO₂ and O₂, often binding O₂ instead of CO₂, especially under certain conditions.
Possible Explanation for Non-Specificity: Rubisco evolved in ancient atmospheric conditions with low oxygen, so there was less selective pressure for it to differentiate between O₂ and CO₂. As oxygen levels increased over time, Rubisco’s dual affinity became less advantageous.

Question 2

First Reaction of Photorespiration with O₂

Answer 2

Reaction: When O₂ binds to Rubisco, the enzyme catalyzes a reaction with RuBP (ribulose-1,5-bisphosphate) to form one molecule of 3-phosphoglycerate (used in the Calvin cycle) and one molecule of 2-phosphoglycolate (which cannot be used in the Calvin cycle).

Question 3

Processing 2-Phosphoglycolate in Photorespiration

Answer 3

Process: 2-Phosphoglycolate is transported out of the chloroplast and undergoes multiple steps in the peroxisome and mitochondria to eventually form a usable molecule (3-phosphoglycerate).
Energy Carriers Used: The process consumes ATP and NADPH, both of which could otherwise be used in photosynthesis.
Byproducts: Releases CO₂, which reduces the efficiency of carbon fixation.

Question 4

Why Photorespiration is Considered Inefficient

Answer 4

Reason: Photorespiration consumes energy (ATP and NADPH) without producing sugars, and it releases fixed CO₂, wasting energy that could have been used to build carbohydrates in the Calvin cycle.

Question 5

Conditions Under Which Photorespiration Occurs

Answer 5

When: Occurs when O₂ concentration is higher than CO₂ concentration in the chloroplast.
Importance of CO₂
₂ Ratio: A high CO₂
₂ ratio favors photosynthesis, while a low ratio (high O₂ concentration) increases the likelihood of photorespiration

Question 6

Photorespiration in Hot and Dry Climates

Answer 6

Cellular Conditions: In hot and dry climates, plants close their stomata to conserve water, reducing CO₂ intake and increasing O₂ concentration inside leaf tissues. This leads to a low CO₂
₂ ratio in the chloroplast, favoring photorespiration.

Question 7

C4 Pathway Adaptation to Limit Photorespiration

Answer 7

Overview: C4 plants spatially separate the initial carbon fixation from the Calvin cycle to limit photorespiration.
Key Components:
PEP Carboxylase: An enzyme in mesophyll cells that specifically binds CO₂ to form a 4-carbon organic acid (oxaloacetate), reducing the chances of O₂ interference.
4-Carbon Organic Acids: The 4-carbon acids (e.g., malate) transport CO₂ from mesophyll cells to bundle-sheath cells, where the Calvin cycle occurs.
Bundle-Sheath Cells: In these cells, CO₂ is released from the 4-carbon acid, creating a high local concentration of CO₂ for Rubisco to use, minimizing photorespiration.

Question 8

CAM Pathway Adaptation to Limit Photorespiration

Answer 8

Overview: CAM plants temporally separate carbon fixation and the Calvin cycle.
Key Components:
Temporal Separation: CAM plants open their stomata at night to take in CO₂ and close them during the day to conserve water.
PEP Carboxylase: Fixes CO₂ at night into a 4-carbon organic acid, stored in vacuoles.
4-Carbon Organic Acid: During the day, the acid releases CO₂ for the Calvin cycle, maintaining a high concentration of CO₂ around Rubisco and reducing photorespiration.