Chat with AIExam 20: Carbohydrate Biosynthesis in Plants and Bacteria
Plants carrying out photosynthesis produce O2. Describe the source of this O2, and explain, with chemical equations or schematic diagrams, why O2 production occurs only during daylight hours.
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Answered by Examlex AI CopilotThe O2 produced by plants during photosynthesis is glucose. Glucose is a simple sugar that serves as the primary source of energy for the plant and is also used as a building block for other organic molecules.
During photosynthesis, plants use sunlight, carbon dioxide, and water to produce glucose and oxygen. The chemical equation for photosynthesis is:
6CO2 + 6H2O + light energy → C6H12O6 + 6O2
This process occurs in the chloroplasts of plant cells, where the pigment chlorophyll absorbs light energy and converts it into chemical energy. This energy is used to convert carbon dioxide and water into glucose and oxygen.
The production of glucose only occurs during daylight hours because it requires light energy. Without sunlight, the chlorophyll cannot absorb the necessary energy to drive the photosynthetic reactions. As a result, plants only produce glucose and oxygen when they are exposed to light. During the night, plants rely on stored glucose for energy and other metabolic processes.
Which compound is NOT directly involved in the Calvin cycle?
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Explain the utility to plants in using sucrose as the transport form of carbon.
(77) Answered by Examlex AI CopilotPlants primarily produce glucose through the process of photosynthesis. However, glucose is not the main form in which they transport sugars from the leaves (where photosynthesis occurs) to other parts of the plant. Instead, plants convert glucose into sucrose for transport, and there are several reasons why sucrose is the preferred molecule for this purpose:
1. Energy Efficiency: Sucrose is a disaccharide composed of one molecule of glucose and one molecule of fructose. This configuration allows plants to transport twice the amount of carbon per molecule compared to glucose alone, making it a more energy-efficient transport form.
2. Reduced Osmotic Pressure: Sucrose is less reactive and less osmotically active than glucose. If glucose were transported in high concentrations, it would cause water to enter the phloem (the plant's vascular tissue for transporting nutrients) by osmosis, potentially leading to issues with water balance. Sucrose allows for the transport of a high concentration of carbon without significantly affecting the osmotic balance.
3. Stability: Sucrose is a non-reducing sugar, which means it does not readily participate in reduction-oxidation reactions that can break down sugars. This stability is beneficial during transport as it reduces the risk of the sugar being metabolized or degraded before it reaches its destination.
4. Storage and Mobilization: Sucrose can be easily converted back into glucose and fructose, or into other forms like starch for storage. This flexibility allows plants to efficiently move carbon around to where it is needed, whether for immediate energy, growth, or storage for later use.
5. Phloem Loading and Unloading: The process of moving sucrose into and out of the phloem (phloem loading and unloading) is well-regulated by the plant. Sucrose can be actively transported against its concentration gradient into the phloem, and then later removed at the site where it is needed. This targeted delivery system is crucial for the plant's overall resource management.
6. Reduced Risk of Pathogen Attraction: Simple sugars like glucose can be signals for pathogens, attracting them to the site of sugar availability. By transporting sucrose instead of glucose, plants may reduce the risk of pathogen attack during the transport process.
In summary, the use of sucrose as the transport form of carbon in plants is a result of its energy efficiency, reduced osmotic effects, stability, ease of conversion for storage or use, efficient phloem loading and unloading, and potentially reduced attraction of pathogens. These advantages make sucrose an ideal carbohydrate for long-distance transport within the plant's vascular system.
Diagram the pathway by which sucrose is synthesized from glucose 6-phosphate; indicate how any required cofactors are involved.
(72) The glycine decarboxylase complex in the leaves of pea or spinach plants is localized mainly in the:
(81) Explain why both ATP and NADPH are required for the operation of the Calvin cycle, and why these two reactants are required in different amounts.
(77) Describe how plants and some microorganisms can, unlike animals, convert acetyl-CoA derived from fatty acids into glucose or sucrose.
(83) The known mechanisms of activation of rubisco or of other enzymes of the Calvin cycle during illumination do NOT include:
(72) Which sugar phosphate is NOT part of the pool of readily interconvertible metabolites used by the plant cell?
(81) Which compound is produced by sucrose synthase and used by cellulose synthase to make cellulose?
(81) What is an action spectrum, and what do peaks in an action spectrum signify? Show a typical action spectrum plot for photosynthesis.
(79) Show the reaction in which 3-phosphoglycerate is converted into glyceraldehyde 3-phosphate. Show all required cofactors, and circle the carbon atom(s) in glyceraldehyde 3-phosphate that is/are derived from CO2 during the photosynthetic fixation of CO2.
(67) Quinone A (QA) and quinone B (QB) differ from each other in that:
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