🧬 Biology · Energy
Photosynthesis: true or false
Decide fast, then read the reason. Wrong answers are the best teachers, so every statement explains itself.
All statements and answers
The oxygen gas released in photosynthesis comes from the water that is split in the light-dependent reactions.
✓ True. Stage 1 splits water, and its oxygen atoms leave as O2. Tagging the oxygen in water with a heavy isotope showed this.
Plants carry out cellular respiration only at night.
✗ False. Plant cells respire day and night. In the light, a leaf does photosynthesis and respiration at the same time.
Most of a tree’s dry mass comes from the soil.
✗ False. Most of the dry mass is carbon, hydrogen and oxygen that came from carbon dioxide and water. Soil supplies only a small share, in the form of minerals.
Leaves look green because chlorophyll absorbs red and blue light better than green light, so more green light is reflected.
✓ True. The color we see is the light that is sent back to our eyes. Chlorophyll absorbs red and blue light best, so more of the green light comes back.
The Calvin cycle takes place in the thylakoid membranes.
✗ False. The Calvin cycle takes place in the stroma. The thylakoid membranes are where the light-dependent reactions happen.
Only plants can carry out photosynthesis.
✗ False. Algae and cyanobacteria photosynthesize too. Algae have chloroplasts, and cyanobacteria have no chloroplasts.
The Calvin cycle uses ATP and NADPH that were made in the light-dependent reactions.
✓ True. The two stages are linked. Stage 1 makes the energy carriers, and stage 2 spends them to build sugar.
In the light-dependent reactions, light energy is used to make ATP and NADPH.
✓ True. The energy captured by chlorophyll is used to make these two energy carriers, which the Calvin cycle then uses.
Every cell in a leafy plant contains chloroplasts.
✗ False. Many cells do not. Cells in the underground part of a root usually have no chloroplasts, because no light reaches them.
If light is the limiting factor, adding more carbon dioxide will raise the rate of photosynthesis.
✗ False. By definition, the limiting factor is the one in shortest supply. Until the light is increased, adding carbon dioxide will not raise the rate.
In a plant, the chemical energy stored in glucose came originally from light absorbed by chlorophyll and other pigments.
✓ True. Photosynthesis transforms absorbed light energy into chemical energy stored in sugar.
Cyanobacteria carry out photosynthesis even though they have no chloroplasts.
✓ True. Cyanobacteria are prokaryotes. Their chlorophyll sits in membranes inside the cell instead of in chloroplasts.
In a land plant, carbon dioxide enters the leaf through the stomata.
✓ True. Stomata are the tiny pores in the leaf surface. Carbon dioxide goes in through them, and oxygen comes out.
Photosynthesis creates new energy out of nothing.
✗ False. Energy is transformed, not created. Light energy becomes chemical energy stored in sugar, and some ends up as heat.