In which part of the chloroplast do the light-dependent reactions of photosynthesis take place?
Thylakoid membrane
✓
Stroma
Outer membrane
Intermembrane space
Correct Answer
Thylakoid membrane
The light-dependent reactions happen in the thylakoid membrane, where chlorophyll and the photosystems are embedded. The stroma is the fluid around the thylakoids where the Calvin cycle runs. The outer membrane is part of the chloroplast envelope and holds no photosystems. The intermembrane space lies between the two envelope membranes and does not capture light.
Category: Light-Dependent Reactions
Question 2
The oxygen gas that plants release during photosynthesis comes from the splitting of which molecule?
Carbon dioxide
Water
✓
Glucose
RuBP
Correct Answer
Water
In the light-dependent reactions, water molecules are split to replace electrons lost by chlorophyll, and their oxygen atoms are released as O2. Carbon dioxide supplies carbon to the Calvin cycle, not the released oxygen. Glucose is a product of photosynthesis, not a molecule split to make oxygen. RuBP is the Calvin cycle molecule that accepts CO2.
Category: Light-Dependent Reactions
Question 3
In the linear electron pathway of photosynthesis, which complex receives the electrons released when H2O molecules are split?
Photosystem I
ATP synthase
Photosystem II
✓
Cytochrome complex
Correct Answer
Photosystem II
Photosystem II acts first: when light excites its chlorophyll and an electron leaves, electrons from split water replace it. Photosystem I acts later and receives electrons that have already passed down the electron transport chain. ATP synthase makes ATP from a hydrogen ion gradient and does not accept electrons. The cytochrome complex sits in the transport chain between the photosystems and gets its electrons from photosystem II, not from water.
Category: Light-Dependent Reactions
Question 4
Which transmembrane enzyme uses the flow of hydrogen ions to add a phosphate group to ADP in chloroplasts?
RuBisCO
Hexokinase
Amylase
ATP synthase
✓
Correct Answer
ATP synthase
ATP synthase spans the thylakoid membrane and lets hydrogen ions flow from the thylakoid space back into the stroma, using that energy to attach a phosphate to ADP. RuBisCO is the Calvin cycle enzyme that fixes CO2. Hexokinase spends ATP to add a phosphate to glucose in glycolysis. Amylase is a digestive enzyme that breaks down starch.
Category: Light-Dependent Reactions
Question 5
Which pair of energy carriers do the light-dependent reactions supply to the Calvin cycle?
ATP and NADPH
✓
ADP and NADP+
ATP and NADH
GTP and FADH2
Correct Answer
ATP and NADPH
The light-dependent reactions convert light energy into ATP and NADPH, which power sugar building in the Calvin cycle. ADP and NADP+ are the spent forms that return from the Calvin cycle to be recharged. ATP and NADH is the wrong pair because NADH is the carrier of cellular respiration. GTP and FADH2 are made during cellular respiration, not photosynthesis.
Category: Calvin Cycle
Question 6
Which portion of the electromagnetic spectrum do plants use to power photosynthesis?
Radio waves
Visible light
✓
X-rays
Microwaves
Correct Answer
Visible light
Photosynthetic pigments absorb wavelengths within the visible range, especially violet-blue and red. Radio waves carry far too little energy per photon to excite pigment electrons. X-rays carry so much energy that they damage molecules instead of driving photosynthesis. Microwaves also have too little energy per photon to excite pigments.
Category: Light and Pigments
Question 7
Leaves look green because chlorophyll does what with most of the green light that strikes it?
Absorbs it
Converts it to heat
Reflects it
✓
Stores it as ATP
Correct Answer
Reflects it
Chlorophyll absorbs blue and red light strongly but absorbs green poorly, so most green light is reflected or passes through, and that is the color we see. Absorbs it describes what happens to red and blue light. Converts it to heat would require the green light to be absorbed first. Stores it as ATP is wrong because light energy is not stored directly as ATP, and green light is mostly not absorbed.
Category: Light and Pigments
Question 8
Which five-carbon molecule accepts CO2 at the start of the Calvin cycle?
3-PGA
G3P
Glucose
RuBP
✓
Correct Answer
RuBP
RuBP (ribulose bisphosphate) is a five-carbon molecule that RuBisCO joins to CO2 in carbon fixation. 3-PGA is the three-carbon product formed after fixation. G3P is the three-carbon sugar made in the reduction stage. Glucose is a six-carbon sugar built later from G3P.
Category: Calvin Cycle
Question 9
When RuBisCO joins CO2 to its five-carbon acceptor, the unstable six-carbon product immediately splits into two molecules of which compound?
3-PGA
✓
G3P
Pyruvate
Glucose
Correct Answer
3-PGA
The six-carbon intermediate made in carbon fixation breaks at once into two three-carbon molecules of 3-PGA. G3P forms only after ATP and NADPH act on 3-PGA in the reduction stage. Pyruvate is the end product of glycolysis in cellular respiration. Glucose has six carbons and is assembled later, not formed by this split.
Category: Calvin Cycle
Question 10
Which Calvin cycle stage uses some G3P and ATP to rebuild the starting acceptor molecule so the cycle can continue?
Carbon fixation
Regeneration
✓
Reduction
Photolysis
Correct Answer
Regeneration
In regeneration, most of the G3P is rearranged, with ATP supplying energy, to remake RuBP so more CO2 can be fixed. Carbon fixation is the step where RuBisCO attaches CO2 to RuBP. Reduction is the step where ATP and NADPH convert 3-PGA into G3P. Photolysis is the splitting of water in the light-dependent reactions.
Category: Calvin Cycle
Question 11
How many ATP molecules does the Calvin cycle use in total to build one glucose molecule?
6
12
18
✓
36
Correct Answer
18
Making one glucose takes six turns of the cycle, which use 12 ATP in the reduction stage and 6 ATP in regeneration, for 18 in all. 6 is only the regeneration share, or the number of turns. 12 is only the reduction share, which also matches the NADPH count. 36 is close to the ATP yield often quoted for respiring one glucose, not the Calvin cycle's cost.
Category: Calvin Cycle
Question 12
What is the name for the flow of hydrogen ions down their concentration gradient across a membrane, which drives ATP production?
Photolysis
Carbon fixation
Active transport
Chemiosmosis
✓
Correct Answer
Chemiosmosis
Chemiosmosis is the movement of hydrogen ions down their gradient through ATP synthase, and the energy released is used to make ATP. Photolysis is the light-driven splitting of water. Carbon fixation is the incorporation of CO2 into organic molecules. Active transport moves substances against their gradient and costs energy rather than releasing it.
Category: Light-Dependent Reactions
Question 13
Which organism is a photoautotroph?
Oak tree
✓
Mushroom
Deer
Yeast
Correct Answer
Oak tree
A photoautotroph uses sunlight for energy and carbon from CO2 to make its own sugar, as an oak tree does. A mushroom is a fungus that absorbs nutrients from other organisms. A deer is a heterotroph that eats plants. Yeast is a fungus that breaks down sugars it takes in.
Category: Photosynthesis Overview
Question 14
On a hot, dry day a plant closes its stomata. Which raw material for photosynthesis does this directly restrict?
Sunlight
Carbon dioxide
✓
Soil minerals
Nitrogen gas
Correct Answer
Carbon dioxide
Carbon dioxide reaches the chloroplasts by diffusing in through open stomata, so closing them cuts off the carbon supply for the Calvin cycle. Sunlight passes through the leaf surface whether or not stomata are open. Soil minerals come in through the roots. Nitrogen gas is not a raw material of photosynthesis.
Category: Leaf and Chloroplast
Question 15
How does the overall equation for photosynthesis compare with the overall equation for cellular respiration?
It is identical
It shares no molecules
It is the reverse
✓
It uses the same reactants
Correct Answer
It is the reverse
Photosynthesis turns CO2 and water into glucose and oxygen, while respiration turns glucose and oxygen into CO2 and water. It is identical is wrong because the reactants and products trade places. It shares no molecules is wrong because the same four molecules appear in both. It uses the same reactants is wrong because the reactants of one are the products of the other.
Category: Energy Cycle
Question 16
Which organelles do plant cells use to carry out cellular respiration?
Chloroplasts
Ribosomes
Vacuoles
Mitochondria
✓
Correct Answer
Mitochondria
Plant cells have mitochondria as well as chloroplasts, and they break down sugar for ATP by cellular respiration just as animal cells do. Chloroplasts make sugar by photosynthesis rather than break it down. Ribosomes build proteins. Vacuoles store water and other substances.
Category: Energy Cycle
Question 17
What advantage does an organism gain from having several kinds of photosynthetic pigments?
It absorbs a wider range of wavelengths
✓
It fixes carbon without any enzymes
It no longer needs to take in CO2
It can photosynthesize in total darkness
Correct Answer
It absorbs a wider range of wavelengths
Each pigment absorbs its own set of wavelengths, so a mix of pigments captures more of the available light, which helps shade plants and underwater organisms. It fixes carbon without any enzymes is wrong because carbon fixation still requires RuBisCO. It no longer needs to take in CO2 is wrong because pigments capture light, not carbon. It can photosynthesize in total darkness is wrong because every pigment still needs light to absorb.
Category: Light and Pigments
Question 18
The reduction stage of the Calvin cycle is named for a chemical change. What does reduction mean?
Loss of electrons
Gain of electrons
✓
Loss of a carbon atom
Gain of an oxygen atom
Correct Answer
Gain of electrons
Reduction is the gain of electrons; in this stage NADPH donates electrons, with ATP supplying energy, to turn 3-PGA into G3P. Loss of electrons is oxidation, the opposite process. Loss of a carbon atom does not happen in this step, since both molecules have three carbons. Gain of an oxygen atom is a form of oxidation, not reduction.
Category: Calvin Cycle
Question 19
Photosystems embedded in the thylakoids are built from which two kinds of molecules?
Lipids and starch
DNA and RNA
Pigments and proteins
✓
Cellulose and sugars
Correct Answer
Pigments and proteins
A photosystem is a cluster of light-absorbing pigments, such as chlorophyll, held in place by proteins. Lipids and starch is wrong because lipids form the membrane around photosystems and starch is a storage carbohydrate. DNA and RNA carry genetic information and are not parts of a photosystem. Cellulose and sugars make up cell walls and fuel, not light-capturing complexes.
Category: Light-Dependent Reactions
Question 20
What happens to an electron in chlorophyll when the molecule absorbs a photon?
It becomes a proton
It joins with oxygen
It bonds to CO2
It is boosted to higher energy
✓
Correct Answer
It is boosted to higher energy
Absorbed light energy excites an electron in chlorophyll to a higher energy level, and the excited electron can then leave and enter the electron transport chain. It becomes a proton is impossible because electrons and protons are different particles. It joins with oxygen is wrong because the oxygen is released as O2 gas. It bonds to CO2 is wrong because CO2 is handled in the Calvin cycle, not at the photosystem.
Category: Light and Pigments