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Photosynthesis - lecture 6a-chapter 10(exam 2)

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  • What is photosynthesis?

    Photosynthesis is the process that converts solar energy into chemical energy within chloroplasts, nourishing almost the entire living world directly or indirectly.

  • What are autotrophs?

    Autotrophs are self-feeders that sustain themselves without eating other organisms; they produce organic molecules from CO2 and inorganic molecules.

  • What are photoautotrophs?

    Photoautotrophs use sunlight energy to make organic molecules; most plants, multicellular algae, and some prokaryotes are photoautotrophs.

  • What role do heterotrophs play in the biosphere?

    Heterotrophs obtain organic material from other organisms and act as consumers or decomposers in the biosphere.

  • Where does most photosynthesis occur in plants?

    Most photosynthesis occurs in the leaves, specifically in the mesophyll cells where chloroplasts are abundant.

  • Describe the structure of a chloroplast.

    A chloroplast has a double membrane envelope, a dense fluid called stroma, and a third membrane system of connected sacs called thylakoids, which may be stacked into grana.

  • What is the overall chemical equation for photosynthesis?

    6 CO2 + 12 H2O + light energy → C6H12O6 + 6 O2 + 6 H2O

  • What happens during the splitting of water in photosynthesis?

    Chloroplasts split H2O into hydrogen and oxygen, incorporating electrons into sugar molecules and releasing O2 as a by-product.

  • How is photosynthesis a redox process?

    Photosynthesis oxidizes H2O and reduces CO2, reversing electron flow compared to respiration and requiring light energy.

  • What are the two main stages of photosynthesis?

    The light reactions in the thylakoids convert light energy to ATP and NADPH, and the Calvin cycle in the stroma uses these to fix CO2 into sugar.

  • What occurs during the light reactions?

    Light reactions split water, release O2, reduce NADP+ to NADPH, and generate ATP by photophosphorylation.

  • What is the role of the Calvin cycle?

    The Calvin cycle fixes CO2 into organic molecules, reduces fixed carbon to carbohydrate using NADPH, and regenerates its CO2 acceptor.

  • What is the nature of sunlight relevant to photosynthesis?

    Sunlight is electromagnetic radiation with wavelengths; visible light (380–740 nm) drives photosynthesis and consists of photons with energy inversely related to wavelength.

  • What are photosynthetic pigments?

    Photosynthetic pigments absorb visible light; chlorophyll a is the key pigment, chlorophyll b and carotenoids are accessory pigments that broaden the spectrum.

  • How do chlorophyll a and b differ?

    They differ in one functional group: chlorophyll a has a CH3 group, chlorophyll b has a CHO group; both have a porphyrin ring with magnesium at the center.

  • What happens when chlorophyll absorbs light?

    An electron moves from ground to excited state; if isolated, it falls back releasing heat or fluorescence.

  • What is a photosystem?

    A photosystem consists of a reaction-center complex with chlorophyll a and a primary electron acceptor, surrounded by light-harvesting complexes that transfer photon energy.

  • What are the two types of photosystems?

    Photosystem II (P680) absorbs light best at 680 nm; Photosystem I (P700) absorbs best at 700 nm; both participate in linear electron flow.

  • Describe linear electron flow in photosynthesis.

    Electrons excited in PSII are replaced by splitting water, move through electron transport chain generating ATP, then re-energized in PSI to reduce NADP+ to NADPH.

  • What is cyclic electron flow?

    Cyclic electron flow involves only PSI, electrons cycle back to PSI, generating ATP without producing NADPH or oxygen.

  • How does chemiosmosis in chloroplasts compare to mitochondria?

    Both use electron transport chains to pump protons and ATP synthase to make ATP, but chloroplasts use light energy and pump protons into thylakoid space, mitochondria use chemical energy and pump into intermembrane space.

  • What are the three phases of the Calvin cycle?

    1. Carbon fixation by rubisco binding CO2 to RuBP
    2. Reduction of 3-phosphoglycerate to G3P using ATP and NADPH
    3. Regeneration of RuBP using ATP

  • What is the net output of the Calvin cycle per 3 CO2 fixed?

    One molecule of glyceraldehyde 3-phosphate (G3P) sugar is produced as net gain; six G3P molecules form but five are used to regenerate RuBP.

  • What is photorespiration?

    Photorespiration occurs when rubisco binds O2 instead of CO2, producing a two-carbon compound and reducing photosynthetic efficiency, especially in hot, dry conditions.

  • How do C4 plants minimize photorespiration?

    C4 plants fix CO2 into a four-carbon compound in mesophyll cells using PEP carboxylase, then release CO2 in bundle-sheath cells for the Calvin cycle, reducing photorespiration.

  • What is the role of PEP carboxylase in C4 plants?

    PEP carboxylase has a higher affinity for CO2 than rubisco and fixes CO2 into four-carbon compounds even at low CO2 concentrations.

  • How do CAM plants conserve water?

    CAM plants open stomata at night to fix CO2 into organic acids stored in vacuoles; stomata close during day and CO2 is released for the Calvin cycle, separating steps temporally.

  • What is the main difference between C4 and CAM photosynthesis?

    C4 plants separate carbon fixation and Calvin cycle spatially in different cells; CAM plants separate these steps temporally in the same cell.