Photosynthesis: Light Reactions, Carbon Fixation, and Photorespiration

Comprehensive flashcards covering light-dependent reactions, photosystem organization, photophosphorylation, the Calvin cycle, photorespiration, and C4/CAM plant adaptations.

What is the primary difference between oxygenic and anoxygenic photosynthesis?

Oxygenic photosynthesis produces oxygen ($O_2$) and is carried out by cyanobacteria, seven groups of algae, and all land plants, whereas anoxygenic photosynthesis does not produce oxygen and is carried out by four bacterial groups.

What did Jan Baptista van Helmont (1580–1644) demonstrate regarding plant growth?

He showed that the substance of the plant was produced from soil and air ($CO_2$).

What conclusion did F.F. Blackman (1866–1947) reach regarding photosynthesis?

He concluded that photosynthesis is a multistage process, only one portion of which uses light directly.

What were the key contributions of C.B. Van Niel and Robin Hill to the understanding of photosynthesis?

C.B. Van Niel proposed the general formula for photosynthesis, and Robin Hill demonstrated that light energy could be harvested and used in a reduction reaction.

What are the two major sets of chemical reactions in photosynthesis?

The light-dependent reactions (which capture sunlight to make $\text{ATP}$ and $\text{NADPH}$ while oxidizing water to produce $O_2$) and carbon fixation / light-independent reactions (which use $\text{ATP}$ and $\text{NADPH}$ to synthesize organic molecules from $CO_2$).

What is the overall balanced chemical equation for photosynthesis?

$$6CO_2 + 12H_2O + \text{Light} \rightarrow C_6H_{12}O_6 + 6H_2O + 6O_2$$

Where in the leaf are chloroplasts found, and how are thylakoids arranged inside them?

Chloroplasts are located inside the mesophyll cell layer; their inner membrane forms flattened structures called thylakoid disks, which are stacked into columns called grana.

What is the stroma in a chloroplast?

The fluid-filled compartment located outside the thylakoid membrane.

What is a photon, and what is the photoelectric effect?

A photon is a discrete particle or bundle of light energy; the photoelectric effect occurs when light removes electrons from certain molecules, producing an electrical current.

What wavelength range of the electromagnetic spectrum corresponds to visible light?

Visible light spans from approximately $400\,\text{nm}$ to $740\,\text{nm}$.

What happens when a photon strikes a photosynthetic pigment molecule?

Its energy is either lost as heat or absorbed by the molecule's electrons, which raises an electron to a higher energy level.

How is an absorption spectrum defined?

The range and efficiency of photons a molecule is capable of absorbing.

Which photosynthetic pigments are represented by the absorption curves in this spectrum?

Chlorophyll $a$ (peaks in blue and red), chlorophyll $b$ (peaks in blue-green and red-orange), and carotenoids (absorb in the blue-green range of $450\text{--}500\,\text{nm}$).

Why do plant leaves appear green to human eyes?

Chlorophylls preferentially absorb violet-blue and red light while transmitting and reflecting green light.

How do chlorophyll a and chlorophyll b differ functionally?

Chlorophyll $a$ is the main pigment that directly converts light energy to chemical energy, whereas chlorophyll $b$ is an accessory pigment that complements and expands the light-absorption spectrum of chlorophyll $a$.

What central ion is held in the porphyrin head of chlorophyll, and what chemical group distinguishes chlorophyll a from chlorophyll b at the R position?

A magnesium ion ($Mg$) sits at the center of the porphyrin head; chlorophyll $a$ has a methyl group ($-\text{CH}_3$), whereas chlorophyll $b$ has an aldehyde group ($-\text{CHO}$).

What dual biological roles do carotenoids fulfill in photosynthetic organisms?

They absorb photons across a broad energy spectrum and act as antioxidants by scavenging harmful free radicals, serving a protective function.

What are phycobiliproteins, and where are they biologically important?

Accessory pigments found in cyanobacteria and some algae that are important for capturing light in low-light ocean areas.

Why do deciduous leaves turn yellow and orange in the fall?

Cool temperatures cause plants to cease chlorophyll production; as green chlorophyll degrades, the underlying carotenoids and accessory pigments reflecting yellow and orange light are unmasked.

What are the two structural components of a photosystem, and what does each do?

The antenna complex (hundreds of accessory pigment molecules that gather photons and channel energy to the center) and the reaction center (one or more chlorophyll $a$ molecules in a protein matrix that pass excited electrons out).

What are the four sequential stages of the light-dependent reactions?

1. Primary photoevent, 2. Charge separation, 3. Electron transport, and 4. Chemiosmosis.

What are the absorption peaks and primary roles of Photosystem II and Photosystem I?

Photosystem II ($P_{680}$) generates sufficient oxidizing potential to oxidize water; Photosystem I ($P_{700}$) passes electrons to reduce $\text{NADP}^+$ to $\text{NADPH}$.

What is the function of the cytochrome b6-f complex?

It connects Photosystem II and Photosystem I by transferring electrons and pumping protons across the thylakoid membrane to establish a proton gradient.

What electron pathway is depicted in this Z diagram of noncyclic photophosphorylation?

Electrons from water oxidation pass from Photosystem II to plastoquinone ($\text{PQ}$), through the $b_6\text{-}f$ complex to plastocyanin ($\text{PC}$), to Photosystem I, then through ferredoxin ($\text{Fd}$) and $\text{NADP}$ reductase to reduce $\text{NADP}^+$ to $\text{NADPH}$.

How does cyclic photophosphorylation differ from linear photophosphorylation?

Cyclic photophosphorylation short-circuits Photosystem I by routing excited electrons backward into the electron transport chain to pump protons and synthesize $\text{ATP}$ without producing $\text{NADPH}$ or oxidizing water.

What two products of the light-dependent reactions drive the Calvin cycle?

$\text{ATP}$ (which provides energy) and $\text{NADPH}$ (which provides reducing potential, energetic electrons, and protons).

Why is the Calvin cycle also termed C3 photosynthesis?

Because the first stable organic intermediate formed after carbon fixation contains three carbon atoms ($3\text{-phosphoglycerate}$ or $3\text{PG}$).

Who discovered the carbon fixation cycle, and what were his lifespan dates?

Melvin Calvin ($1911\text{ to }1997$).

What enzyme catalyzes the addition of CO2 to ribulose 1,5-bisphosphate (RuBP)?

Ribulose bisphosphate carboxylase/oxygenase (rubisco).

What are the three distinct phases of the Calvin cycle?

Phase 1: Carbon fixation, Phase 2: Reduction, and Phase 3: Regeneration of RuBP.

According to this Calvin cycle diagram, how many ATP and NADPH molecules are consumed to produce two net molecules of G3P for glucose synthesis?

A total of $18\,\text{ATP}$ ($12\,\text{ATP}$ during reduction and $6\,\text{ATP}$ during regeneration) and $12\,\text{NADPH}$ (during reduction) are consumed per $6\,\text{molecules of }CO_2$ fixed.

How many turns of the Calvin cycle are needed to synthesize one molecule of G3P and one molecule of glucose?

Three turns incorporate enough carbon to produce one net molecule of $\text{G3P}$, and six turns incorporate enough carbon to produce one molecule of glucose ($2\,\text{G3P}$).

What are the primary metabolic end products formed from G3P produced in the Calvin cycle?

Sucrose (a transport disaccharide composed of glucose and fructose) and starch (an insoluble glucose polymer stored for future energy needs).

How are photosynthesis and cellular respiration metabolically and evolutionarily linked?

Photosynthesis uses respiration's products ($CO_2$ and $H_2O$) as inputs, respiration uses photosynthesis's products (glucose and $O_2$), glucose synthesis from $\text{G3P}$ runs part of glycolysis in reverse, and their electron transport proteins are evolutionarily related.

What two competing enzymatic activities does rubisco possess?

Carboxylation (the addition of $CO_2$ to RuBP to form $3\text{PG}$) and oxidation (the incorporation of $O_2$ into RuBP leading to photorespiration).

Under normal conditions without water or heat stress, what is the relative rate of rubisco's carboxylation compared to its oxidation?

The carboxylation reaction is four times ($4\times$) faster than the oxidation reaction.

What is photorespiration, and why is it disadvantageous to plants?

It is the oxidation of RuBP by rubisco under high $O_2$ and low $CO_2$ conditions, which consumes energy and releases previously fixed $CO_2$, effectively undoing carbon fixation.

What structures shown here regulate leaf gas exchange, and how do they function?

Stomata, each composed of two guard cells that alter their shape based on internal turgor pressure to open or close the pore.

Why does hot, arid weather induce photorespiration in C3 plants?

Stomata close to conserve water, which prevents $CO_2$ from entering while trapping photosynthetic $O_2$ inside the leaf, creating high-$O_2$ and low-$CO_2$ conditions that favor rubisco oxidation.

Above what temperature threshold does photorespiration become a severe problem for C3 plants?

Above $28\,^\circ\text{C}$.

What two advantages does PEP carboxylase have over rubisco?

It has a much higher affinity for $CO_2$ and completely lacks oxygenase/oxidase activity.

How do C4 and CAM plants prevent photorespiration at the biochemical level?

They use PEP carboxylase to fix $CO_2$ into a $4\text{-carbon}$ organic acid that is later decarboxylated, concentrating $CO_2$ directly around rubisco.

How do C4 plants spatially separate carbon fixation from the Calvin cycle?

They fix carbon into a $4\text{-carbon}$ acid using PEP carboxylase in mesophyll cells, then transfer malate to bundle-sheath cells where it is decarboxylated to supply concentrated $CO_2$ to rubisco.

In the C4 carbon fixation pathway shown, what intermediate is shuttled into the bundle-sheath cell, and what is recycled to the mesophyll cell?

Malate is transported into the bundle-sheath cell to release $CO_2$, and pyruvate is returned to the mesophyll cell where it is converted back to phosphoenolpyruvate (PEP) using $\text{ATP}$.

What are four agricultural examples of C4 plants?

Corn, sugarcane, sorghum, and various other grasses.

How do CAM plants temporally solve the problem of photorespiration as illustrated here?

They open stomata at night to fix $CO_2$ into $C_4$ acids via PEP carboxylase, then close stomata during the day and decarboxylate these acids to supply $CO_2$ to the Calvin cycle.

In which cellular organelle do CAM plants store the organic acids fixed overnight?

In the vacuole.

What types of plants commonly utilize crassulacean acid metabolism (CAM)?

Succulent plants such as cacti, pineapples, and members of about two dozen other plant groups.

What is the energetic cost of the C4 pathway compared to C3 photosynthesis?

Converting pyruvate back to phosphoenolpyruvate (PEP) requires $2\,\text{ATP}$ per $CO_2$ fixed.

Under what environmental conditions is C3 photosynthesis more efficient than C4 or CAM photosynthesis?

Under optimal, cooler conditions with adequate moisture, where stomata can stay open to release excess $O_2$, because $C_3$ avoids the extra $2\,\text{ATP}$ cost per $CO_2$ required to regenerate PEP.