Questions and Answers
Photosynthesis: How Plants Turn Light into Food
1.
An oak tree grows from a small acorn into a giant weighing tons. Where does most of the dry mass of its wood come from?
carbon dioxide from the air
Explanation
In the 1640s, Jan van Helmont grew a willow from 5 to 169 pounds while its pot of soil lost only about 2 ounces. Most of a plant's dry mass is carbon, and photosynthesis takes that carbon from carbon dioxide in the air.
2.
Fill in the gaps.
In photosynthesis, six molecules of carbon dioxide and six molecules of water are turned into one molecule of glucoseand sixmolecules of oxygen.
Explanation
The balanced equation is 6CO2 + 6H2O → C6H12O6 + 6O2. Six carbon atoms from six CO2 molecules end up in one glucose molecule, and six O2 molecules are released. Sucrose (C12H22O11) and starch are made from glucose later.
3.
Where do the oxygen atoms in the O2 released by photosynthesis come from?
from water molecules split in the light reactions
Explanation
In 1941, Samuel Ruben and Martin Kamen gave algae water labeled with the heavy isotope oxygen-18, and the labeled oxygen appeared in the released gas. The oxygen comes from water molecules split in the light reactions; the oxygen from CO2 ends up in sugar.
Match each part of a plant with its role in photosynthesis.
4.
thylakoid membrane
holds the pigments that capture light
5.
stroma
fluid where carbon dioxide is built into sugar
6.
stomata
tiny pores through which gases enter and leave the leaf
7.
xylem
carries water up from the roots
Explanation
The thylakoid membranes hold chlorophyll, the stroma is where CO2 is built into sugar, stomata let CO2 into the leaf, and xylem brings water from the roots. Carrying sugar to other organs has no pair: that is the job of the phloem.
Which statements about leaf pigments and light are true?
8.
Chlorophyll absorbs red and blue light best and reflects much of the green light.
True
9.
A plant under pure green light photosynthesizes faster than under red light of the same brightness.
False
10.
Carotenoids absorb blue light, and their yellow and orange colors show in autumn when chlorophyll breaks down.
True
11.
Leaves look green because chlorophyll gives off green light when sunlight strikes it.
False
Explanation
Chlorophyll absorbs mostly red and blue light and reflects much of the green, which is why leaves look green. Carotenoids absorb blue light; their yellow and orange show in autumn once chlorophyll breaks down. Excited chlorophyll actually glows faint red, not green.
12.
Put the steps of the light reactions in the order they happen to one electron.
Light energy absorbed by pigments is passed to photosystem II.An excited electron leaves photosystem II and enters the electron transport chain.The electron moves along the chain to photosystem I, losing energy on the way.Photosystem I absorbs light and boosts the electron to a high energy again.A carrier molecule takes the electron to the sugar-building reactions.
Explanation
Light energy is passed to photosystem II, which releases an excited electron into the transport chain. Having lost energy on the way, the electron is boosted again by light at photosystem I, and then a carrier takes it to the sugar-building reactions in the stroma.
13.
Inside a chloroplast, the enzyme ATP synthase makes ATP. What directly powers it?
hydrogen ions flowing out of the thylakoid through it
Explanation
The electron transport chain pumps hydrogen ions into the thylakoid, and as they flow back out through ATP synthase, it turns and makes ATP. In 1966, André Jagendorf showed this by soaking chloroplasts in acid and then moving them to a basic solution: they made ATP in total darkness.
14.
In the light reactions, electrons pass through photosystem II and then photosystem I. Which molecule accepts them at the end of this path?
NADP+
Explanation
NADP+ takes up two electrons and a hydrogen ion to become NADPH, which carries this energy to the sugar-building reactions. In cellular respiration, by contrast, the final electron acceptor is oxygen.
15.
A plant is moved from bright light into darkness. Why do its light-independent reactions stop soon afterward, even though they do not use light directly?
They run out of the energy carriers made in the light reactions.
Explanation
The light-independent reactions need ATP and NADPH, which only the light reactions make. In the dark, the supply is used up within seconds to minutes, so carbon dioxide can no longer be built into sugar.
16.
The light-independent reactions of photosynthesis form a cycle named after the American chemist who mapped them in the 1950s by tracing radioactive carbon-14. What is this cycle called?
Calvin cycle
Explanation
Melvin Calvin, with Andrew Benson and James Bassham, fed carbon-14 to Chlorella algae and followed it from CO2 into sugar. Calvin received the 1961 Nobel Prize in Chemistry for this work.
Which statements about how plants photosynthesize in hot, dry climates are true?
17.
Pineapples and cacti open their stomata at night and store carbon dioxide until daytime.
True
18.
Corn and sugarcane first trap carbon dioxide in a four-carbon compound, which helps them in hot weather.
True
19.
Cacti keep their stomata wide open at midday to cool down by evaporation.
False
20.
On a hot day, closing the stomata saves water but also cuts the supply of carbon dioxide.
True
Explanation
Cacti and pineapples (CAM plants) open their stomata only at night and store CO2 for the day. Corn and sugarcane (C4 plants) first trap CO2 in a four-carbon compound, which keeps them efficient in heat. Any plant that shuts its stomata to save water also cuts off its CO2 supply.
21.
Tomatoes in a winter greenhouse are well watered and kept at an ideal temperature. Adding more lamps no longer makes them grow faster. What is most likely limiting their photosynthesis now?
the amount of carbon dioxide in the air
Explanation
When more light no longer helps, another factor has become limiting, and in a closed greenhouse it is usually CO2. Growers often raise it to about 1,000 ppm, more than twice the outdoor level, by burning propane or releasing bottled CO2.
Which statements about plants and cellular respiration are true?
22.
Plant cells carry out cellular respiration both day and night.
True
23.
At night, a plant gives off carbon dioxide instead of taking it in.
True
24.
Plants respire only in their roots, because their leaves make their own sugar.
False
25.
Photosynthesis and cellular respiration both use an electron transport chain to make ATP.
True
Explanation
Every living plant cell respires day and night. In the dark, with no photosynthesis to use it, the CO2 from respiration is released. Chloroplasts and mitochondria both make ATP with an electron transport chain and ATP synthase.
26.
Roughly half of all photosynthesis on Earth happens in the oceans. Which organisms do most of it?
phytoplankton drifting near the surface
Explanation
Phytoplankton, tiny single-celled organisms drifting near the surface, are so numerous that they match all land plants combined. Kelp, seagrasses, and mangroves are very productive locally but cover only a thin strip along the coasts.
27.
Chloroplasts have their own circular DNA and ribosomes, and they divide on their own inside the cell. What does this suggest about their origin?
They descend from bacteria swallowed by an ancient cell.
Explanation
This is the endosymbiotic theory: over a billion years ago, a eukaryotic cell engulfed a photosynthetic cyanobacterium and kept it alive. Its circular DNA and bacteria-like ribosomes are what remains of that bacterium.
28.
A potato plant makes sugar in its leaves, yet its tubers are packed with starch. Why does the plant store starch rather than glucose?
Starch does not dissolve, so it does not draw water into cells.
Explanation
Starch does not dissolve in water, so it can be packed tightly without drawing water into the cells by osmosis. When the tuber sprouts, enzymes break the starch back down into glucose.