2. There are two types of plants, C3 and C4. C3 plants use the Calvin Cycle to g

Question

2. There are two types of plants, C3 and C4. C3 plants use the Calvin Cycle to generate sugar. C4 plants use phosphoenol pyruvate to make malate and oxaloacetate which will eventually be used to make sugar. Both types of plants have stomatas (openings on leaves that can be opened and closed) for CO2 entry. In addition, water is also lost through the stomata. C3 plants are only able to synthesize sugar at relatively high CO2 levels because photosynthesis will stop when CO2 production from the mitochondria equals the use of CO2 in the chloroplasts. C4 plants are able to continue photosynthesis even at low CO2 levels. (1) Why are C4 plants able to survive in hot, dry conditions when C3 plants are not? (2) Plants can develop cancer but it is usually from an infection not from the ionizing rays of the sun. What part of the photosynthesis pathway do you think protects the plant form sun damage?

Explanation / Answer

(1) Why are C4 plants able to survive in hot, dry conditions when C3 plants are not?

C3 plants doesn't have photosynthetic adaptations to reduce photorespiration . Whereas in C4 plants, alternative forms of photosynthesis are used to alleviate problems of photorespiration and excess water loss. The C4 plants  photosynthetic pathway is attached to a unique leaf structure known as Kranz anatomy where the leaf cells that surround the vascular system are packed very tightly together and are called bundle sheath cells. Around the bundle sheath a densely packed layer of mesophyll cells exist. The mesophyll cells are usually in contact with air spaces in the leaf and the initial fixation of CO2 from the atmosphere takes place in the mesophyll cells. Once the CO2 is fixed into a four-carbon compound, the malate is transferred to the bundle sheath cells. Inside the bundle sheath cells, the malate is broken down into a small organic molecule, and CO2 is released . After, the CO2 enters the chloroplast of the bundle sheath cell, it is getting fixed second time with the enzyme Rubisco and continues the C3 pathway. The double-carbon fixation pathway confers a photosynthetic efficiency on C4 plants over C3 plants.

However, Rubisco is inefficient in the presence of elevated levels of O2 and in order to operate Rubisco, CO2 must first attach to the enzyme at a particular location known as the active site. In fact, O2 is also able to attach to its active site and prevent carbon dioxide from attaching, a process known as photorespiration. Generally, the relative concentrations of these CO2 and O2 determines the attachment at the active site of Rubisco. When plant opens its stomata, the air will be at equilibrium with the atmosphere, which is 21 percent oxygen and 0.04 percent carbon dioxide. During hot, dry weather conditions the pores close to avoid water loss, eventually the concentration of gases change. When the concentration of CO2 drops below 0.01 percent, (photosynthesis) O2 will outcompete CO2 at the active site, and no net photosynthesis occurs. C4 plants are able to prevent photorespiration, because the PEP carboxylase enzyme in the C4 plant is not inhibited by oxygen. Accordingly, when the stomata are closed, PEP continues to fix CO2 the leaf until it is consumed. As, the bundle sheath is isolated from the leaf’s air spaces, it is not influenced by the rising O2 levels, and the C3 cycle functions without interference. Evenually, C4 photosynthesis have a better adaptation to hot, dry environments than C3 plants.

2) Plants can develop cancer but it is usually from an infection not from the ionizing rays of the sun. What part of the photosynthesis pathway do you think protects the plant form sun damage?

Plants have a 'photoreceptor' for UV-B wavelengths in sunlight. Studies have shown that expression of UVR8 could facilitate plant from sun damage. UVR8 occur throughout a plant and it respond immediately to sunlight. Normally two molecules of UVR8 form a molecule, known as dimer. The UV-B light converts the dimer into single molecules of UVR8. This conversion has a direct effect on the protein and ultimately expression of gene which leads to the production of the plant’s sunscreen. UVR8 interacts with a protein COP1, stimulates the synthesis of sunscreen compounds that are deposited in the outer tissues and minimize any harmful transmittance to the cells below in plants.

Related Questions

Navigate

• Browse (All)
• Browse 2
• Subjects

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.