Photosynthesis
51 flashcards covering Photosynthesis for the MCAT Biology & Biochemistry section.
Photosynthesis is the process by which plants, algae, and some bacteria harness sunlight to convert carbon dioxide and water into glucose and oxygen. This essential biochemical reaction occurs in chloroplasts, where light energy drives the synthesis of organic compounds, releasing oxygen as a byproduct. It's the foundation of life on Earth, powering ecosystems by producing food and maintaining atmospheric balance.
On the MCAT, photosynthesis questions in the Biology and Biochemistry section often involve multiple-choice items testing the light-dependent and light-independent reactions, the role of pigments like chlorophyll, and factors such as light intensity or CO2 levels. Common traps include confusing photosynthesis with cellular respiration or overlooking how environmental stresses affect efficiency, so pay close attention to the overall equation and key pathways like the Calvin cycle. Focus on applying concepts to experimental scenarios or data interpretation.
A helpful tip: Practice sketching the photosynthesis equation to quickly recall inputs and outputs.
Terms (51)
- 01
Photosynthesis
Photosynthesis is the process by which green plants, algae, and some bacteria convert light energy into chemical energy, using carbon dioxide and water to produce glucose and oxygen.
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Chloroplast
The chloroplast is the organelle in plant cells and eukaryotic algae where photosynthesis occurs, containing thylakoids and stroma that house the necessary pigments and enzymes.
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Chlorophyll
Chlorophyll is the primary pigment in photosynthesis that absorbs light energy, primarily in the blue and red wavelengths, and is essential for capturing photons to drive the reaction.
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Light-dependent reactions
These are the initial phase of photosynthesis that occurs in the thylakoid membranes, where light energy is converted into ATP and NADPH through electron transport.
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Light-independent reactions
Also known as the Calvin cycle, these reactions use ATP and NADPH from the light-dependent phase to fix carbon dioxide into organic molecules like glucose in the stroma.
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Calvin cycle
A series of biochemical reactions in photosynthesis that incorporates CO2 into organic compounds, producing sugars through a cycle of carbon fixation, reduction, and regeneration.
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Photosystems I and II
Photosystem II absorbs light to split water and generate electrons, while Photosystem I uses light to produce NADPH; both are protein complexes in the thylakoid membrane.
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Reaction center
The reaction center is the core of a photosystem where absorbed light energy excites an electron, initiating electron transfer in the photosynthetic electron transport chain.
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Electron transport chain
In photosynthesis, this is a series of protein complexes in the thylakoid membrane that transfers electrons from water to NADP+, creating a proton gradient for ATP synthesis.
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ATP synthase
ATP synthase is an enzyme complex in the thylakoid membrane that uses the proton gradient generated during photosynthesis to produce ATP from ADP and inorganic phosphate.
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Chemiosmosis
Chemiosmosis in photosynthesis is the process where a proton gradient across the thylakoid membrane drives ATP production as protons flow back through ATP synthase.
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Oxygen evolution
Oxygen evolution is the release of O2 gas as a byproduct of photosynthesis, resulting from the photolysis of water in Photosystem II.
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Carbon fixation
Carbon fixation is the initial step in the Calvin cycle where CO2 is attached to ribulose bisphosphate by the enzyme Rubisco, forming an unstable six-carbon compound.
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Rubisco
Rubisco is the enzyme that catalyzes the first major step of carbon fixation in the Calvin cycle, combining CO2 with ribulose bisphosphate to produce 3-phosphoglycerate.
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C3 plants
C3 plants are those that use the standard Calvin cycle for photosynthesis, fixing CO2 directly into 3-phosphoglycerate, but they are less efficient in hot, dry conditions due to photorespiration.
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C4 plants
C4 plants minimize photorespiration by initially fixing CO2 into a four-carbon compound in mesophyll cells, then transporting it to bundle sheath cells for the Calvin cycle.
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CAM plants
CAM plants, like cacti, open their stomata at night to fix CO2 into organic acids, which are then used during the day for the Calvin cycle, conserving water.
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Photorespiration
Photorespiration occurs when Rubisco reacts with O2 instead of CO2, leading to a wasteful cycle that reduces the efficiency of photosynthesis in C3 plants.
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Stomata
Stomata are pores in plant leaves that regulate gas exchange, allowing CO2 entry for photosynthesis while also permitting water vapor loss, which can limit the process.
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Thylakoid membrane
The thylakoid membrane is the site of light-dependent reactions in chloroplasts, containing embedded photosystems and electron carriers that facilitate energy conversion.
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Stroma
Stroma is the fluid-filled space in chloroplasts surrounding the thylakoids, where the Calvin cycle enzymes are located and carbon fixation occurs.
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Absorption spectrum
The absorption spectrum of a pigment like chlorophyll shows the wavelengths of light it absorbs, with peaks in the blue and red regions, enabling photosynthesis.
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Action spectrum
The action spectrum indicates the rates of photosynthesis at different wavelengths, correlating with the absorption spectrum of chlorophyll for effective energy capture.
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Photons
Photons are particles of light energy that excite electrons in chlorophyll during photosynthesis, with their energy determined by wavelength.
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NADPH
NADPH is a high-energy electron carrier produced in the light-dependent reactions of photosynthesis, used in the Calvin cycle to reduce carbon compounds.
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Photolysis
Photolysis is the light-driven splitting of water molecules in Photosystem II, releasing electrons, protons, and oxygen for use in photosynthesis.
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Proton gradient
The proton gradient is established across the thylakoid membrane during electron transport in photosynthesis, driving ATP synthesis through chemiosmosis.
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Cyclic photophosphorylation
Cyclic photophosphorylation is a process in Photosystem I that generates ATP by cycling electrons back through the electron transport chain without producing NADPH or oxygen.
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Non-cyclic photophosphorylation
Non-cyclic photophosphorylation involves both photosystems to produce ATP, NADPH, and oxygen by passing electrons from water to NADP+ in a linear pathway.
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Carotenoids
Carotenoids are accessory pigments in photosynthesis that absorb light in the blue-green range and protect chlorophyll from photo damage by dissipating excess energy.
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Quantum yield
Quantum yield measures the efficiency of photosynthesis as the number of moles of product formed per mole of photons absorbed, typically around 0.1 for plants.
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Hill reaction
The Hill reaction demonstrates that isolated chloroplasts can evolve oxygen when provided with an artificial electron acceptor, proving the light-dependent splitting of water.
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G3P
G3P, or glyceraldehyde-3-phosphate, is a three-carbon sugar produced in the Calvin cycle that serves as a precursor for glucose and other carbohydrates in photosynthesis.
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RuBP
RuBP, or ribulose bisphosphate, is the five-carbon molecule in the Calvin cycle that reacts with CO2 via Rubisco to initiate carbon fixation.
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PGA
PGA, or 3-phosphoglycerate, is the first stable product of carbon fixation in the Calvin cycle, formed when CO2 is added to RuBP.
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Factors affecting photosynthesis
Factors such as light intensity, CO2 concentration, and temperature influence photosynthesis rates, with optimal levels maximizing enzyme activity and energy capture.
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Photosynthesis equation
The overall equation for photosynthesis is 6CO2 + 6H2O + light energy → C6H12O6 + 6O2, summarizing the conversion of inorganic to organic molecules.
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Z-scheme
The Z-scheme illustrates the energy changes in the photosynthetic electron transport chain, showing electrons moving from water to NADP+ with a zigzag pattern of redox potentials.
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Photoinhibition
Photoinhibition is the damage to photosynthetic apparatus from excessive light, reducing efficiency by overwhelming the electron transport chain.
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Compensation point
The compensation point is the light intensity at which the rate of photosynthesis equals the rate of respiration, resulting in no net carbon fixation.
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Redox reactions in photosynthesis
Photosynthesis involves redox reactions where water is oxidized to O2, losing electrons, and NADP+ is reduced to NADPH, gaining electrons for carbon reduction.
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Electrons from water
In photosynthesis, electrons are sourced from water molecules, which are split in Photosystem II to replace those lost from chlorophyll and drive the process.
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Light harvesting complexes
Light harvesting complexes are antenna pigments surrounding the reaction center that absorb photons and transfer energy to chlorophyll for use in photosynthesis.
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Phycobilins
Phycobilins are accessory pigments in cyanobacteria and red algae that absorb green light, extending the range of wavelengths usable for photosynthesis.
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Photoautotrophs
Photoautotrophs are organisms that perform photosynthesis to produce their own food from light, CO2, and water, including plants and cyanobacteria.
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Strategy for maximizing efficiency
To maximize photosynthetic efficiency, plants adapt through mechanisms like C4 or CAM pathways, which minimize water loss and photorespiration in varying environments.
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Common trap: Confusing with respiration
A common error is confusing photosynthesis, which produces glucose and oxygen, with cellular respiration, which consumes them, as they are opposite processes.
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Worked example: ATP yield
In non-cyclic photophosphorylation, each pair of electrons from water produces about 1.5 ATP molecules via chemiosmosis, though actual yields vary based on conditions.
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Granum
A granum is a stack of thylakoids in chloroplasts that increases the surface area for light absorption and houses the photosystems for efficient photosynthesis.
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Wavelength and photosynthesis
Wavelengths of light between 400-700 nm (visible spectrum) are most effective for photosynthesis, as they match the absorption peaks of chlorophyll.
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Dark reactions misconception
Though called dark reactions, the Calvin cycle can occur in light or dark if ATP and NADPH are available, but it depends on products from light-dependent reactions.