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Photosynthesis - General Biology

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  • What is photosynthesis?

    Photosynthesis is the process by which autotrophs use sunlight to manufacture carbohydrates from carbon dioxide and water, producing oxygen as a by-product.

  • What are autotrophs and heterotrophs?

    Autotrophs make their own food via photosynthesis. Heterotrophs must obtain sugars from other organisms.

  • What is the overall chemical equation for photosynthesis?

    6CO2 + 6H2O + light energy → C6H12O6 + 6O2

  • What are the two main sets of reactions in photosynthesis?

    1. Light-capturing reactions produce O2 from H2O and generate ATP and NADPH.
    2. Calvin cycle uses ATP and NADPH to reduce CO2 to sugar.

  • Where does photosynthesis occur in plant cells?

    Photosynthesis occurs in chloroplasts, which have outer and inner membranes, thylakoids stacked into grana, and a fluid-filled stroma.

  • What is the role of pigments in photosynthesis?

    Pigments absorb specific wavelengths of light to capture energy. Chlorophylls absorb red and blue light and reflect green, while carotenoids absorb blue and green and reflect yellow, orange, or red.

  • What is the electromagnetic spectrum's relevance to photosynthesis?

    Visible light, part of the electromagnetic spectrum, contains photons with energy inversely proportional to wavelength; shorter wavelengths have more energy to drive photosynthesis.

  • What is the structure of chlorophyll molecules?

    Chlorophyll has a head with a magnesium atom in a ring structure that absorbs light, and a long isoprenoid tail anchoring it to the thylakoid membrane.

  • What is the function of accessory pigments like carotenoids?

    Accessory pigments absorb light and transfer energy to chlorophyll, extend the range of light wavelengths absorbed, and protect chlorophyll from damage by stabilizing free radicals.

  • What is a photosystem?

    A photosystem is a complex of 200–300 chlorophyll and accessory pigment molecules in the thylakoid membrane that capture light energy and transfer it to a reaction center.

  • What happens in Photosystem II?

    Light excites electrons in Photosystem II, which are transferred to pheophytin, then down an electron transport chain, producing a proton-motive force used to make ATP. Water is split to replace lost electrons, releasing O2.

  • What happens in Photosystem I?

    Electrons from Photosystem II are re-excited in Photosystem I and passed to ferredoxin, then to NADP+ reductase, which reduces NADP+ to NADPH in the stroma.

  • What is the Z scheme in photosynthesis?

    The Z scheme describes the linear flow of electrons from water through Photosystem II and I, producing ATP and NADPH.

  • What is cyclic electron flow?

    Cyclic electron flow recycles electrons from Photosystem I back to the electron transport chain, producing additional ATP without making NADPH.

  • How does CO2 enter plant leaves?

    CO2 enters through pores called stomata, which are openings controlled by guard cells on the leaf surface.

  • What is carbon fixation in photosynthesis?

    Carbon fixation is the addition of CO2 to ribulose bisphosphate (RuBP) by the enzyme Rubisco, producing two molecules of 3-phosphoglycerate (3PGA).

  • What are the three phases of the Calvin cycle?

    1. Fixation: CO2 added to RuBP.
    2. Reduction: 3PGA is phosphorylated and reduced to G3P.
    3. Regeneration: ATP regenerates RuBP to continue the cycle.

  • What happens to the G3P produced by the Calvin cycle?

    G3P is used to synthesize glucose and fructose, which can be combined into sucrose or polymerized into starch for storage.

  • What is photorespiration and why is it a problem?

    Photorespiration occurs when Rubisco adds O2 instead of CO2 to RuBP, consuming oxygen and releasing CO2, which reduces photosynthetic efficiency.

  • How do C4 plants minimize photorespiration?

    C4 plants fix CO2 first into a 4-carbon compound in mesophyll cells using PEP carboxylase, then transport it to bundle sheath cells where CO2 is released for the Calvin cycle, reducing O2 interference.

  • How do CAM plants reduce photorespiration?

    CAM plants fix CO2 at night into 4-carbon acids stored in vacuoles, then release CO2 during the day for the Calvin cycle, temporally separating carbon fixation and sugar production.