HHMI BioInteractive Photosynthesis — Questions and Answers

Question 1: Which pigment absorbs light primarily in the blue-violet range and is found in antenna complexes?

Beta-carotene
Chlorophyll b (Correct answer)
Chlorophyll a
Phycoerythrin

Correct answer: Chlorophyll b

Chlorophyll b absorbs light mainly in the blue-violet range (~453 nm) and transfers energy to chlorophyll a reaction centers.

Question 2: What are grana in chloroplasts?

Single thylakoid discs
The site of glucose synthesis
The space between membranes
Stacks of thylakoids (Correct answer)

Correct answer: Stacks of thylakoids

Grana (singular: granum) are stacks of thylakoids within the chloroplast. This stacked arrangement increases the surface area of the thylakoid membranes, which is crucial for maximizing the efficiency of the light-dependent reactions of photosynthesis.

Question 3: In C3 plants, what is the first stable product of carbon fixation?

Glucose
Pyruvate
Oxaloacetate
3-phosphoglycerate (3-PGA) (Correct answer)

Correct answer: 3-phosphoglycerate (3-PGA)

In C3 plants, RuBisCO fixes CO2 onto RuBP to produce two molecules of 3-phosphoglycerate, a 3-carbon compound.

Question 4: Why is RuBisCO considered an inefficient enzyme?

It cannot function above 20°C
It only works in bundle sheath cells
It requires light to function
It has a slow catalytic rate, a dual affinity for both CO2 and O2, and is easily inhibited (Correct answer)

Correct answer: It has a slow catalytic rate, a dual affinity for both CO2 and O2, and is easily inhibited

RuBisCO is slow (3–10 reactions/second), can react with O2 (causing photorespiration), and requires large amounts of protein to compensate, making it metabolically expensive.

Question 5: What is the role of RuBisCO activase in photosynthesis?

It transfers electrons to RuBisCO
It phosphorylates RuBP for use in the Calvin cycle
It reactivates RuBisCO by removing inhibitory sugar phosphates from its active site (Correct answer)
It synthesizes new RuBisCO protein

Correct answer: It reactivates RuBisCO by removing inhibitory sugar phosphates from its active site

RuBisCO activase uses ATP to remove inhibitors (like RuBP and other sugar phosphates) from the active site of RuBisCO, keeping it functional in the light.

Question 6: What is CAM photosynthesis?

A strategy where CO2 is fixed at night and stored as organic acids, then released for the Calvin cycle during the day (Correct answer)
Photosynthesis that occurs only in cold climates
Carbon fixation that bypasses RuBisCO
A form of cyclic electron flow

Correct answer: A strategy where CO2 is fixed at night and stored as organic acids, then released for the Calvin cycle during the day

CAM (Crassulacean Acid Metabolism) plants open stomata at night to fix CO2 into malate, then close stomata during the day and release CO2 for the Calvin cycle.

Question 7: How does photosynthesis influence the global carbon cycle?

It has no significant effect on atmospheric CO2 on human timescales
It converts methane to CO2
It removes CO2 from the atmosphere and stores carbon in organic molecules, partially offsetting emissions (Correct answer)
It releases CO2 into the atmosphere

Correct answer: It removes CO2 from the atmosphere and stores carbon in organic molecules, partially offsetting emissions

Globally, photosynthesis fixes roughly 120 Gt of carbon per year, making it the dominant mechanism for removing atmospheric CO2 and forming the base of the carbon cycle.

Question 8: Why is photosynthesis important for pharmaceutical and industrial applications?

Plants can only produce glucose, which is not useful industrially
Plants produce valuable secondary metabolites (medicines, oils, fibers) using carbon fixed by photosynthesis (Correct answer)
Pharmaceutical compounds are all synthetically produced
Photosynthesis has no industrial applications

Correct answer: Plants produce valuable secondary metabolites (medicines, oils, fibers) using carbon fixed by photosynthesis

Photosynthetically derived carbon skeletons are used by plants to make alkaloids, terpenes, phenolics, and other compounds used in medicine (aspirin, morphine), textiles (cotton), and biofuels.

Question 9: What is the concept of limiting factors in photosynthesis?

The minimum ATP needed for carbon fixation
The enzyme that limits electron transport
The factor in shortest supply relative to the plant's needs that controls the overall rate of photosynthesis (Correct answer)
The maximum CO2 a leaf can absorb

Correct answer: The factor in shortest supply relative to the plant's needs that controls the overall rate of photosynthesis

Blackman's law of limiting factors states that the rate of photosynthesis is controlled by whichever factor — light, CO2, or temperature — is present at the least favorable level.

Question 10: What is the ratio of ATP to NADPH consumed per three CO2 molecules fixed in the Calvin cycle?

2 ATP : 1 NADPH
3 ATP : 2 NADPH (Correct answer)
1 ATP : 2 NADPH
1 ATP : 1 NADPH

Correct answer: 3 ATP : 2 NADPH

Fixing 3 CO2 in the Calvin cycle consumes 9 ATP and 6 NADPH, a ratio of 3:2, which partly explains why cyclic flow is needed to supplement ATP beyond what non-cyclic flow alone produces.

Question 11: Which gas is primarily measured to determine the rate of photosynthesis?

Nitrogen (N₂)
Oxygen (O₂)
Carbon dioxide (CO₂) (Correct answer)
Methane (CH₄)

Correct answer: Carbon dioxide (CO₂)

Carbon dioxide (CO₂) is a key reactant in photosynthesis, being absorbed from the atmosphere and fixed into organic compounds during the Calvin Cycle. Therefore, measuring the rate of CO₂ uptake by a plant is a direct and common method used to determine its photosynthetic activity and efficiency.

Question 12: How do C4 plants reduce photorespiration?

They lack RuBisCO entirely
They concentrate CO2 around RuBisCO in bundle sheath cells using a CO2 pump (Correct answer)
They use a different electron transport chain
They fix carbon only at night

Correct answer: They concentrate CO2 around RuBisCO in bundle sheath cells using a CO2 pump

C4 plants initially fix CO2 into 4-carbon acids in mesophyll cells and transport them to bundle sheath cells, where CO2 is released in high concentrations for RuBisCO.

Question 13: Which molecule captures light energy during the light reactions?

ATP
Chlorophyll (Correct answer)
NADPH
RuBisCO

Correct answer: Chlorophyll

Chlorophyll absorbs light energy and initiates the light-dependent reactions.

Question 14: Compared to non-cyclic electron flow, what does cyclic electron flow produce?

ATP without NADPH or O2 (Correct answer)
Additional O2 only
Both ATP and NADPH
More NADPH but no ATP

Correct answer: ATP without NADPH or O2

Cyclic electron flow generates a proton gradient for ATP synthesis but does not produce NADPH (electrons return to PSI) and does not split water, so no O2 is released.

Question 15: How does temperature affect the rate of photosynthesis?

Higher temperatures always increase photosynthesis
It affects enzyme activity; rates increase with temperature up to an optimum, then fall sharply (Correct answer)
Temperature has no effect on photosynthesis
Low temperatures increase photosynthesis indefinitely

Correct answer: It affects enzyme activity; rates increase with temperature up to an optimum, then fall sharply

Photosynthesis increases with temperature up to the enzyme's optimal temperature, above which enzymes denature and the rate falls rapidly.

Question 16: What is the 4-carbon compound first produced when CO2 is fixed in C4 plants?

Pyruvate
Malate
Oxaloacetate (Correct answer)
3-phosphoglycerate

Correct answer: Oxaloacetate

PEP carboxylase combines CO2 with phosphoenolpyruvate (PEP) to form oxaloacetate, a 4-carbon compound, in the mesophyll cells.

Question 17: Why is the thylakoid membrane's extensive surface area important for the light reactions?

It houses the Calvin cycle enzymes
It absorbs CO2
It maximizes the number of photosystems and electron carriers available (Correct answer)
It stores glucose

Correct answer: It maximizes the number of photosystems and electron carriers available

The large membrane surface area allows many photosystems and electron transport complexes to be embedded, increasing the rate of light capture and ATP/NADPH production.

Question 18: How does photosynthesis affect local and global climate?

Photosynthesis warms the environment by releasing heat
Photosynthesis only affects climate in tropical regions
Through evapotranspiration (cooling), CO2 uptake (reducing greenhouse effect), and albedo effects of vegetation cover (Correct answer)
Photosynthesis has no measurable effect on climate

Correct answer: Through evapotranspiration (cooling), CO2 uptake (reducing greenhouse effect), and albedo effects of vegetation cover

Forests and other vegetation cool local climates through transpiration, reduce global warming by sequestering CO2, and alter Earth's reflectivity (albedo) compared to bare ground.

Question 19: What is the light compensation point in photosynthesis?

The point at which a plant needs the most water
The light intensity at which photosynthesis exactly equals cellular respiration (Correct answer)
The maximum light a plant can absorb
The intensity at which chlorophyll is produced

Correct answer: The light intensity at which photosynthesis exactly equals cellular respiration

At the light compensation point, the rate of CO2 fixed by photosynthesis equals the rate of CO2 released by cellular respiration, resulting in no net gas exchange.

Question 20: What is carbon sequestration and how does photosynthesis contribute to it?

Long-term storage of CO2 in organic matter or soils; photosynthesis fixes atmospheric CO2 into plant biomass that may be stored for decades to millennia (Correct answer)
Sequestration is an industrial process unrelated to biology
Sequestration refers only to ocean absorption of CO2
Photosynthesis releases carbon into the soil immediately

Correct answer: Long-term storage of CO2 in organic matter or soils; photosynthesis fixes atmospheric CO2 into plant biomass that may be stored for decades to millennia

When photosynthetically fixed carbon is stored in wood, peat, or soil organic matter for long periods, it is effectively removed from the active carbon cycle, acting as a carbon sink.

Question 21: What are the overall reactants and products of the photosynthesis equation?

Reactants: glucose + light; Products: CO2 + H2O
Reactants: CO2 + H2O + light; Products: glucose + O2 (Correct answer)
Reactants: CO2 + O2; Products: glucose + H2O
Reactants: O2 + glucose; Products: CO2 + H2O

Correct answer: Reactants: CO2 + H2O + light; Products: glucose + O2

The overall equation of photosynthesis is: 6CO2 + 6H2O + light energy → C6H12O6 + 6O2.

Question 22: Which wavelengths of light does chlorophyll a absorb most strongly?

Infrared (> 700 nm)
UV (< 400 nm)
Green (500–560 nm)
Blue (~430 nm) and red (~662 nm) (Correct answer)

Correct answer: Blue (~430 nm) and red (~662 nm)

Chlorophyll a has strong absorption peaks in the blue and red portions of the visible spectrum.

Question 23: What is the overall efficiency of photosynthesis in converting solar energy to glucose?

About 50% of all sunlight absorbed
Typically 1–3% under natural conditions (Correct answer)
100% in C4 plants
Exactly 10% in all plants

Correct answer: Typically 1–3% under natural conditions

Plants typically convert only 1–3% of incident solar energy into chemical energy in glucose under field conditions, due to reflection, heat loss, and metabolic inefficiencies.

Question 24: What technique is commonly used to separate photosynthetic pigments in a laboratory?

PCR
Western blotting
Paper or thin-layer chromatography (Correct answer)
Gel electrophoresis

Correct answer: Paper or thin-layer chromatography

Chromatography separates pigments based on their solubility and interaction with the stationary phase, revealing distinct colored bands.

Question 25: What is the connection between photosynthesis and fossil fuels?

Fossil fuels are formed from inorganic minerals unrelated to photosynthesis
Fossil fuels release energy that was generated by animal metabolism
Fossil fuels are ancient organic carbon originally fixed by photosynthesis millions of years ago (Correct answer)
Photosynthesis produces fossil fuels directly

Correct answer: Fossil fuels are ancient organic carbon originally fixed by photosynthesis millions of years ago

Coal, oil, and natural gas are the remains of ancient photosynthetic organisms; combusting them releases CO2 that was fixed from the atmosphere millions of years ago.

Question 26: What is the primary pigment in Photosystem II reaction centers?

Beta-carotene
Chlorophyll a (P680) (Correct answer)
Xanthophyll
Chlorophyll b

Correct answer: Chlorophyll a (P680)

The reaction center of Photosystem II contains chlorophyll a molecules with peak absorption at 680 nm (P680).

Question 27: What is the primary function of the antenna complex (light-harvesting complex) in photosystems?

Capture light energy and transfer it to the reaction center (Correct answer)
Synthesize NADPH directly
Produce ATP via chemiosmosis
Split water molecules

Correct answer: Capture light energy and transfer it to the reaction center

Antenna complexes contain arrays of chlorophyll and accessory pigment molecules that absorb photons and funnel excitation energy to the reaction center via resonance energy transfer.

Question 28: Which plants use CAM photosynthesis?

Wheat and rice
Mosses and liverworts
Cacti and succulents (Correct answer)
Corn and sugarcane

Correct answer: Cacti and succulents

CAM photosynthesis is common in succulent plants like cacti, agaves, and pineapples that live in arid environments.

Question 29: What are the three main products/byproducts of the complete non-cyclic electron flow pathway?

O2, glucose, and ATP
ATP, NADPH, and O2 (Correct answer)
CO2, glucose, and water
NADPH, glucose, and CO2

Correct answer: ATP, NADPH, and O2

Non-cyclic electron flow uses light energy to transfer electrons from water through PSII and PSI, producing ATP via chemiosmosis, NADPH via NADP+ reductase, and releasing O2 as a byproduct of water splitting.

Question 30: What is sink-source regulation in the context of photosynthesis?

Light is the source and CO2 is the sink
Photosynthate export from source leaves to sink tissues (roots, fruits) feedback-regulates photosynthetic rate (Correct answer)
Sinks are leaves and sources are roots
ATP is the sink for NADPH produced in the light reactions

Correct answer: Photosynthate export from source leaves to sink tissues (roots, fruits) feedback-regulates photosynthetic rate

When sinks (growing tissues) actively import sugars, source leaves maintain high photosynthesis; when sinks are full, sugar accumulates in leaves and photosynthesis can be downregulated.

Question 31: What is bioremediation through photosynthesis?

Using photosynthesis to manufacture synthetic chemicals
Burning plant biomass to remove pollution
Replacing soil bacteria with algae
Using photosynthetic organisms to absorb pollutants (heavy metals, CO2, nitrates) and clean contaminated environments (Correct answer)

Correct answer: Using photosynthetic organisms to absorb pollutants (heavy metals, CO2, nitrates) and clean contaminated environments

Algae and plants can absorb heavy metals into their tissues, remove excess nitrates from water, and fix CO2 from industrial emissions, providing low-cost environmental cleanup.

Question 32: How many photons are required to produce one O2 molecule from water splitting?

2
8 (Correct answer)
4
6

Correct answer: 8

Eight photons (4 per photosystem × 2 photosystems) drive the reactions needed to split two water molecules and release one O2.

Question 33: How do stomata regulate photosynthesis?

By absorbing light energy
By controlling CO2 entry and water vapor exit, balancing gas exchange with water conservation (Correct answer)
By storing glucose produced in the Calvin cycle
By producing chlorophyll on demand

Correct answer: By controlling CO2 entry and water vapor exit, balancing gas exchange with water conservation

Stomata open to allow CO2 in for photosynthesis but simultaneously allow water to escape; plants regulate stomatal aperture to balance carbon gain against water loss.

Question 34: What is the first step of the Calvin Cycle?

Reduction phase
Carbon fixation (Correct answer)
ATP synthesis
Regeneration of RuBP

Correct answer: Carbon fixation

The first step of the Calvin Cycle is carbon fixation, where the enzyme RuBisCO catalyzes the attachment of a carbon dioxide molecule to a five-carbon sugar, ribulose-1,5-bisphosphate (RuBP). This initiates the conversion of inorganic carbon into organic compounds.

Question 35: Why do leaves appear green to human eyes?

Leaves contain mostly carotenoids
Green light has the most energy
Chlorophyll absorbs green light only
Chlorophyll reflects green wavelengths while absorbing red and blue (Correct answer)

Correct answer: Chlorophyll reflects green wavelengths while absorbing red and blue

Chlorophyll absorbs red and blue light for photosynthesis but reflects green light, which is what we see.

Question 36: Which type of plants are best adapted to hot, sunny, arid environments due to reduced photorespiration?

Shade-adapted ferns
Bryophytes
C3 plants
C4 plants (Correct answer)

Correct answer: C4 plants

C4 plants minimize photorespiration in hot, sunny conditions by concentrating CO2 around RuBisCO, making them highly efficient in such environments.

Question 37: What is gross primary productivity (GPP) in an ecosystem?

The net gain of biomass by the entire food web
The total rate of carbon fixation by photosynthesis before subtracting respiration losses (Correct answer)
The amount of carbon consumed by herbivores
The carbon remaining after plant respiration

Correct answer: The total rate of carbon fixation by photosynthesis before subtracting respiration losses

GPP is the total rate at which photosynthetic organisms fix CO2 into organic compounds, before any energy is consumed by their own cellular respiration.

Question 38: How does leaf age affect photosynthetic capacity?

Photosynthetic rate is constant throughout leaf life
Young leaves have no chlorophyll and cannot photosynthesize
Young mature leaves photosynthesize most rapidly; very young and old leaves have lower rates (Correct answer)
Old leaves always photosynthesize fastest

Correct answer: Young mature leaves photosynthesize most rapidly; very young and old leaves have lower rates

Photosynthetic capacity increases as leaves develop and chloroplasts mature, peaks in fully expanded leaves, then declines as leaves age and chloroplasts break down.

Question 39: What is the light-harvesting complex (antenna complex) in a photosystem?

The ATP synthase complex
A cluster of pigment molecules that absorbs light and funnels energy to the reaction center (Correct answer)
The site where water is split
The enzyme that fixes CO2

Correct answer: A cluster of pigment molecules that absorbs light and funnels energy to the reaction center

The antenna complex consists of many chlorophyll and accessory pigment molecules that capture photons and transfer the energy via resonance to the reaction center.

Question 40: Under what conditions is photorespiration most likely to occur?

High humidity and low light
Low O2, high CO2, and low temperature
High O2, low CO2, and high temperature (Correct answer)
Darkness and cool temperatures

Correct answer: High O2, low CO2, and high temperature

Photorespiration is favored when O2 concentrations are high relative to CO2 and temperatures are elevated, causing RuBisCO to act as an oxygenase.

Question 41: What is the key advantage of CAM over C3 photosynthesis in desert environments?

CAM plants grow faster
CAM plants require less sunlight
CAM plants fix more CO2 per day
Stomata open only at night, dramatically reducing water loss (Correct answer)

Correct answer: Stomata open only at night, dramatically reducing water loss

By opening stomata at night when temperatures are cooler and humidity is higher, CAM plants lose far less water through transpiration than C3 plants.

Question 42: What enzyme initially fixes CO2 in C4 plants?

PEP carboxylase (Correct answer)
Carbonic anhydrase
Rubisco activase
RuBisCO

Correct answer: PEP carboxylase

PEP carboxylase (PEPC) fixes CO2 into oxaloacetate in the mesophyll cells of C4 plants; it has a much higher affinity for CO2 than RuBisCO.

Question 43: What are bundle sheath cells in C4 plants?

Cells located in the epidermis
Cells that only absorb water
Cells surrounding vascular bundles where the Calvin cycle and high CO2 concentrations are maintained (Correct answer)
Cells that contain no chloroplasts

Correct answer: Cells surrounding vascular bundles where the Calvin cycle and high CO2 concentrations are maintained

Bundle sheath cells in C4 plants are tightly packed around the vascular bundles and receive concentrated CO2 from 4-carbon acids delivered by mesophyll cells.

Question 44: What evolutionary advantage do C4 plants have over C3 plants during drought?

C4 plants achieve greater water-use efficiency because less CO2 loss occurs per water molecule transpired (Correct answer)
C4 plants have deeper roots that access groundwater
C4 plants open stomata wider to absorb more CO2
C4 plants photosynthesize faster in low light

Correct answer: C4 plants achieve greater water-use efficiency because less CO2 loss occurs per water molecule transpired

The CO2-concentrating mechanism of C4 plants allows them to fix more carbon per water molecule transpired, giving them superior water-use efficiency in dry conditions.

Question 45: How does photosynthesis contribute to the oxygen content of Earth's atmosphere?

Photosynthesis converts N2 to O2
Photosynthetic organisms have released O2 as a byproduct for billions of years, creating the modern atmosphere (Correct answer)
Photosynthesis consumes O2 from the atmosphere
O2 is produced only by deep-sea photosynthesis

Correct answer: Photosynthetic organisms have released O2 as a byproduct for billions of years, creating the modern atmosphere

The Great Oxygenation Event ~2.4 billion years ago resulted from cyanobacteria releasing O2 via photosynthesis, fundamentally changing Earth's atmosphere.

Question 46: How do plants acclimate to growth in deep shade versus full sun?

Shade plants increase antenna complex size and chlorophyll b content; sun plants have more reaction centers and Calvin cycle enzymes per leaf area (Correct answer)
Sun plants have larger leaves to capture more light
Shade plants use CAM photosynthesis to compensate
Shade plants stop photosynthesizing entirely

Correct answer: Shade plants increase antenna complex size and chlorophyll b content; sun plants have more reaction centers and Calvin cycle enzymes per leaf area

Shade-acclimated plants invest in light-harvesting complexes to maximize photon capture at low intensities, while sun-acclimated plants invest in Calvin cycle capacity to process the abundant energy.

Question 47: What are the two main products of the light reactions used in the Calvin cycle?

ADP and NADP+
ATP and NADPH (Correct answer)
CO2 and H2O
Glucose and O2

Correct answer: ATP and NADPH

The light reactions produce ATP and NADPH, which power the Calvin cycle.

Question 48: What molecule is regenerated at the end of the Calvin Cycle to allow the cycle to continue?

Ribulose bisphosphate (RuBP) (Correct answer)
Glucose
NADPH
ATP

Correct answer: Ribulose bisphosphate (RuBP)

At the end of the Calvin Cycle, after glucose has been produced, the remaining molecules are regenerated back into ribulose bisphosphate (RuBP). This regeneration is vital as it allows the cycle to continue by providing the acceptor molecule for new carbon dioxide molecules.

Question 49: What mechanism drives ATP synthesis by ATP synthase during the light reactions?

Proton gradient across the thylakoid membrane (chemiosmosis) (Correct answer)
Direct electron transfer to ferredoxin
NADPH production by ferredoxin
Oxygen release from water splitting

Correct answer: Proton gradient across the thylakoid membrane (chemiosmosis)

Proton pumping by plastoquinone and the cytochrome b6f complex creates a high proton concentration in the thylakoid lumen; these protons flow back through ATP synthase (chemiosmosis), driving ATP production.

Question 50: Which ecosystem type has the highest net primary productivity on Earth?

Arctic tundra
Temperate grasslands
Tropical rainforests (Correct answer)
Open ocean (pelagic zone)

Correct answer: Tropical rainforests

Tropical rainforests have the highest NPP due to year-round warmth, high rainfall, and intense solar radiation that support continuous, rapid photosynthesis.

Question 51: How does ocean acidification affect photosynthesis in marine organisms?

It can impair calcification in algae with calcium carbonate structures, but elevated CO2 may enhance photosynthesis in non-calcifying algae (Correct answer)
Acidification converts CO2 to O2, reducing photosynthesis
Marine photosynthesis is unaffected by ocean pH
Ocean acidification stops all marine photosynthesis

Correct answer: It can impair calcification in algae with calcium carbonate structures, but elevated CO2 may enhance photosynthesis in non-calcifying algae

Higher CO2 dissolves in seawater to form carbonic acid (lowering pH), which harms calcifying organisms (coralline algae, corals) while potentially boosting photosynthesis in non-calcifying phytoplankton.

Question 52: Which mineral nutrient is essential for chlorophyll synthesis and is often the primary limiting nutrient for photosynthesis in natural ecosystems?

Calcium
Nitrogen (Correct answer)
Phosphorus
Potassium

Correct answer: Nitrogen

Nitrogen is a key component of chlorophyll and RuBisCO; nitrogen deficiency leads to yellowing (chlorosis) and severely reduced photosynthesis.

Question 53: What is photoinhibition?

Inhibition of the Calvin cycle by low light
Enhancement of photosynthesis by very bright light
Reduction in photosynthetic efficiency caused by excess light that damages Photosystem II (Correct answer)
Blockage of CO2 entry by high humidity

Correct answer: Reduction in photosynthetic efficiency caused by excess light that damages Photosystem II

Photoinhibition occurs when absorbed light energy exceeds the capacity of the photosynthetic machinery, leading to damage of the D1 protein in Photosystem II reaction centers.

Question 54: How is glucose produced in photosynthesis used by the plant itself?

For cellular respiration, growth, structural materials (cellulose), and storage (starch) (Correct answer)
Exclusively exported to animals
Only for cellular respiration
Stored only as sucrose and never broken down

Correct answer: For cellular respiration, growth, structural materials (cellulose), and storage (starch)

Plants use photosynthetically produced glucose for energy (respiration), building cell walls (cellulose), storing energy (starch), and synthesizing other organic molecules.

Question 55: How do algae and cyanobacteria contribute to global photosynthesis?

They account for roughly half of all photosynthesis on Earth, primarily in oceans and freshwater (Correct answer)
They only photosynthesize in hot springs
They perform less than 1% of global photosynthesis
They use a different mechanism that doesn't produce O2

Correct answer: They account for roughly half of all photosynthesis on Earth, primarily in oceans and freshwater

Phytoplankton (algae and cyanobacteria) in aquatic environments contribute approximately 50% of global primary production and most of the oxygen in Earth's atmosphere.

Question 56: Why is photosynthesis considered the foundation of most food webs on Earth?

It produces O2 that animals use for cellular respiration only
It converts light energy into chemical energy stored in organic molecules that all other organisms depend on (Correct answer)
It is the only way plants can survive
It directly feeds all organisms in a food web

Correct answer: It converts light energy into chemical energy stored in organic molecules that all other organisms depend on

Photosynthesis produces the organic carbon compounds that primary consumers (herbivores) eat, which in turn supports higher trophic levels in almost every ecosystem.

Question 57: What is net primary productivity (NPP)?

GPP minus the carbon lost to plant respiration, representing energy available to consumers (Correct answer)
Energy used by animals for growth
Total carbon fixed by photosynthesis
Carbon stored in soil organic matter

Correct answer: GPP minus the carbon lost to plant respiration, representing energy available to consumers

NPP = GPP − plant respiration; it represents the organic matter actually available to support herbivores and higher trophic levels.

Question 58: How does the ratio of ATP to NADPH produced by the light reactions affect Calvin cycle efficiency?

The Calvin cycle requires a 3:2 ATP:NADPH ratio; cyclic electron flow adjusts ATP supply when needed (Correct answer)
The ratio does not matter for the Calvin cycle
NADPH is always in excess
ATP and NADPH are interchangeable in the Calvin cycle

Correct answer: The Calvin cycle requires a 3:2 ATP:NADPH ratio; cyclic electron flow adjusts ATP supply when needed

For each CO2 fixed, the Calvin cycle consumes 3 ATP and 2 NADPH; cyclic electron flow supplements ATP production to maintain this ratio when linear flow alone is insufficient.

Question 59: Which mobile electron carrier transfers electrons from Photosystem II to the cytochrome b6f complex?

NADPH
Ferredoxin
Plastoquinone (Correct answer)
Plastocyanin

Correct answer: Plastoquinone

Plastoquinone (PQ) is a lipid-soluble molecule embedded in the thylakoid membrane that picks up electrons and protons from PSII and shuttles them to the cytochrome b6f complex.

Question 60: Which photosystem absorbs light at 700 nm and is involved in producing NADPH?

Both equally
Photosystem II
Photosystem I (Correct answer)
Neither

Correct answer: Photosystem I

Photosystem I (P700) absorbs 700 nm light and passes electrons to NADP+ reductase to form NADPH.

HHMI BioInteractive Photosynthesis

A knowledge assessment based on HHMI BioInteractive's seven-part photosynthesis animation series, covering light reactions, the Calvin cycle, photorespiration, and the ecological roles of photosynthesis.

Questions

75 min

Time Limit

70%

To Pass

Checking for saved progress...

Exam Rules

You can skip questions and return to them later
Flag questions for review before submitting
No feedback shown until you submit the entire exam
Unanswered questions count as wrong — answer everything
10 pretest questions are mixed in and don't affect your score
Timer auto-submits when time runs out
Your progress is auto-saved every 30 seconds