Campbell Biology Chapter 10 Review: Photosynthesis

10.1 Where photosynthesis happens and what it does

pp. 186–189

On the AP exam? Yes

Topic 3.4 covers this: know the overall equation, the chloroplast's parts and which stage runs where. The chloroplast's bacterial origin is Topic 2.10, and producers feeding ecosystems is Topic 8.2. You won't be asked who discovered what.

In the course: Topic 3.4 Photosynthesis, Topic 2.1 Cell Structure and Function, Topic 2.10 Origins of Cell Compartmentalization, Topic 8.2 Energy Flow Through Ecosystems (notes, videos and more questions)

Key points

Autotrophs build their own organic molecules from CO₂, and photoautotrophs (plants, algae, cyanobacteria) power this with light. Heterotrophs, like you, rely on molecules autotrophs made, so nearly every food chain starts with photosynthesis.
Leaves do most of a plant's photosynthesis, mainly in the mesophyll cells inside them. Stomata handle gas exchange (CO₂ in, O₂ out), and the veins run both ways: water arrives from the roots, and sugar is shipped out to parts that can't make their own.
Think of a chloroplast as three membrane systems: two wrapped around the outside, and a third folded into flat sacs called thylakoids that sit in a thick fluid, the stroma. Thylakoids pile up into stacks called grana, and their membranes are where chlorophyll is found.
The net equation is 6 CO₂ + 6 H₂O + light energy → C₆H₁₂O₆ + 6 O₂. Glucose stands in for the real direct product, a three-carbon sugar, and the O₂ released comes from splitting water, not from CO₂.
Photosynthesis is a redox process run uphill: water is oxidized (it loses electrons) and CO₂ is reduced (it gains them). The electrons end up with more potential energy than they started with, so light has to supply that energy.
Stage one, the light reactions, happens in the thylakoid membranes: water is split, O₂ is released, and ATP and NADPH are made, but no sugar. Stage two, the Calvin cycle, happens in the stroma and spends that ATP and NADPH to turn CO₂ into sugar.
Photosynthesis first evolved in prokaryotes. Chloroplasts are descendants of a cyanobacterium-like cell that a eukaryotic ancestor took in, and cyanobacteria's oxygen output is what gave Earth an oxygen-rich atmosphere.

Key terms (13)

autotroph: An organism that makes all its own organic molecules out of CO₂, water and minerals, so it never has to eat anything.
heterotroph: An organism that has to get its organic molecules from other organisms, by eating them or absorbing what they made. Animals and fungi are heterotrophs.
photoautotroph: An autotroph whose energy source is light, such as a plant, an alga or a cyanobacterium.
mesophyll: The tissue in the middle of a leaf, between its top and bottom surfaces. Its cells are crowded with chloroplasts and do most of the leaf's photosynthesis.
stomata: Small pores in the leaf surface that open and close. They let CO₂ in and let O₂ and water vapor out. A single pore is a stoma.
chloroplast: The organelle where plants and algae do photosynthesis. It has two outer membranes plus an inner system of thylakoids.
stroma: The fluid that fills a chloroplast around the thylakoids. The Calvin cycle's enzymes work here.
thylakoid: A flattened sac of membrane inside a chloroplast. Its membrane holds the pigments and proteins of the light reactions.
granum: A neat stack of thylakoids, a bit like a stack of coins. More than one are called grana.
chlorophyll: The green pigment in thylakoid membranes that soaks up light energy and starts photosynthesis.
light reactions: The first stage of photosynthesis. In the thylakoid membranes, light energy is used to split water, give off O₂ and make ATP and NADPH.
Calvin cycle: The second stage of photosynthesis. In the stroma, it spends ATP and NADPH to build sugar from CO₂.
NADPH: An electron carrier. The light reactions load it with high-energy electrons, and the Calvin cycle uses those electrons to make sugar. Its empty form is NADP⁺.

Check yourself: 10.1 Where photosynthesis happens and what it does

4 questions on 10.1 Where photosynthesis happens and what it does. Pick an answer to see if you got it, and why.

Question 1 of 4

In a plant mesophyll cell, where is chlorophyll located?

Question 2 of 4

A student's notes say: "In photosynthesis, CO₂ is oxidized to make sugar, and the electrons give off energy as they move." Which correction is accurate?

Question 3 of 4

Nitrosomonas bacteria live in dark soil. They build all of their organic molecules from CO₂, using energy they get by oxidizing ammonia (NH₃). How are these bacteria best classified?

Question 4 of 4

A student coats only the lower surface of several leaves on a bean plant with petroleum jelly, which blocks gas exchange. In this species most stomata are on the lower surface. After a day in bright light, the coated leaves have made much less starch than uncoated leaves on the same plant. Which input to photosynthesis most likely became limiting?

0 of 4 answered

10.2 The light reactions: from photons to ATP and NADPH

pp. 189–197

On the AP exam? Yes

This is the core of Topic 3.4: know how light boosts electrons in photosystems II and I, how splitting water replaces them, and how the H⁺ gradient drives ATP synthase. You don't need the names of the electron carriers or P680 and P700, and absorption spectra show up as data to read.

In the course: Topic 3.4 Photosynthesis, Topic 3.3 Cellular Energy, Topic 3.5 Cellular Respiration (notes, videos and more questions)

Key points

Light travels as waves but arrives in packets of energy called photons. The shorter the wavelength, the more energy each photon carries, so a blue photon carries more energy than a red one. Visible light, about 380–750 nm, is what drives photosynthesis.
A pigment absorbs some wavelengths and reflects or transmits the rest, and the rest is the color you see. Chlorophyll a absorbs blue-violet and red light best and absorbs little green, which is why leaves look green.
An absorption spectrum shows which wavelengths a pigment soaks up; an action spectrum shows which wavelengths actually drive photosynthesis. They don't match exactly because accessory pigments (chlorophyll b and carotenoids) absorb other wavelengths and pass the energy along. Carotenoids also protect the chloroplast by getting rid of excess light energy.
When a pigment absorbs a photon, one of its electrons jumps to an unstable excited state. Isolated chlorophyll just gives the energy back as heat and a red glow (fluorescence). In a photosystem, light-harvesting complexes relay the energy pigment to pigment until it reaches two linked chlorophyll a molecules at the reaction center, which hand the excited electron to a primary electron acceptor.
In linear electron flow, photosystem II works first: it loses an excited electron, and splitting water replaces it, releasing O₂ and H⁺. The electron passes down an electron transport chain to photosystem I, gets boosted again by light, and ends up reducing NADP⁺ to NADPH.
As electrons move between the photosystems, the chain moves H⁺ out of the stroma and into the thylakoids, so the inside of each sac becomes acidic. H⁺ flows back out through ATP synthase, making ATP in the stroma. This is chemiosmosis, the same mechanism mitochondria use, except here the electrons come from water and the energy comes from light.
In cyclic electron flow, electrons from photosystem I loop back into the chain instead of reaching NADP⁺. It makes extra ATP but no NADPH and no O₂, which helps match the Calvin cycle's need for more ATP than NADPH.

Key terms (15)

photon: A single packet of light energy. Its energy depends on wavelength (the distance between wave crests): shorter wavelengths mean more energetic photons.
pigment: A molecule that absorbs some colors of light. The colors it reflects or lets through are the ones you see.
absorption spectrum: A graph of how much light a pigment absorbs at each wavelength. It’s measured with a spectrophotometer.
action spectrum: A graph of how fast photosynthesis runs under each wavelength of light, measured by O₂ released or CO₂ taken up.
chlorophyll a: The main photosynthetic pigment. Reaction centers are built around it, and it absorbs blue-violet and red light best.
accessory pigment: A pigment such as chlorophyll b or a carotenoid that absorbs wavelengths chlorophyll a misses and passes the energy on to it.
carotenoid: A yellow-orange accessory pigment. Besides widening the range of usable light, it protects the chloroplast by shedding extra energy safely.
excited state: The unstable, high-energy state of a pigment after a photon bumps up one of its electrons. On its own the electron drops back fast, giving off heat and fluorescence.
photosystem: A cluster in the thylakoid membrane made of light-harvesting complexes around a reaction center. Plants have two kinds, photosystem II and photosystem I.
reaction-center complex: The core of a photosystem: a protein framework holding two chlorophyll a molecules, called the special pair. They're the only chlorophylls in the photosystem that actually give away an excited electron.
primary electron acceptor: The molecule beside the special pair that catches its excited electron and becomes reduced. This light-powered transfer is where the light reactions begin.
linear electron flow: The main path in the light reactions: electrons travel from water through photosystem II, an electron transport chain and photosystem I to NADPH, making ATP and O₂ along the way.
cyclic electron flow: A side loop where electrons from photosystem I return to the electron transport chain. It makes ATP but no NADPH and no O₂.
ATP synthase: A membrane protein that lets H⁺ flow down its gradient and uses that flow to attach phosphate to ADP, making ATP.
photophosphorylation: Making ATP with energy that started as light: the light reactions build an H⁺ gradient across the thylakoid membrane, and ATP synthase uses it.

Check yourself: 10.2 The light reactions: from photons to ATP and NADPH

4 questions on 10.2 The light reactions: from photons to ATP and NADPH. Pick an answer to see if you got it, and why.

Question 1 of 4

Students cut leaf disks, pull the air out of them so they sink in a bicarbonate solution, and then light them with LED lamps of a single color, all giving the same number of photons per second. As photosynthesis makes O₂, disks float. The table shows the time for half the disks to float (invented data). Light | Time for half the disks to float (min) Blue (450 nm) | 9 Green (530 nm) | 24 Red (660 nm) | 8 Darkness | No disks floated in 40 min Which conclusion is best supported?

Question 2 of 4

An alga has a mutation that makes the light-harvesting complexes around its photosystems much smaller, while its reaction centers stay normal. Its photosynthetic rate per reaction center is measured in dim light and in very bright light. Compared with the normal alga, which result is most likely?

Question 3 of 4

A herbicide stops photosystem II from passing electrons on to the electron transport chain. When treated leaves are lit with blue light, they give off much more red fluorescence than untreated leaves. Which explanation best accounts for this?

Question 4 of 4

In an illuminated chloroplast, the thylakoid space measures pH 5.2 and the stroma measures pH 7.7. Which statement correctly describes H⁺ movement in this chloroplast?

0 of 4 answered

10.3 The Calvin cycle: building sugar from CO₂

pp. 198–199

On the AP exam? Yes

Topic 3.4 expects the big picture: CO₂, ATP and NADPH go in, sugar comes out, ADP and NADP⁺ go back to the light reactions, all in the stroma. The exam won't ask you to memorize the cycle's steps, its intermediates, the ATP and NADPH counts, or enzyme names like rubisco.

In the course: Topic 3.4 Photosynthesis, Topic 3.3 Cellular Energy (notes, videos and more questions)

Key points

The Calvin cycle runs in the stroma and builds sugar from CO₂. ATP supplies the energy and NADPH supplies the high-energy electrons. It is anabolic (it builds molecules), the opposite job from the Krebs cycle, though both regenerate their own starting molecule.
Phase 1, carbon fixation: the enzyme rubisco attaches CO₂ to a five-carbon acceptor called RuBP. The six-carbon product splits at once into two three-carbon acids.
Phase 2, reduction: ATP and NADPH convert those acids into a three-carbon sugar, G3P. Phase 3, regeneration: more ATP is spent rearranging most of the G3P back into RuBP so the cycle can keep going.
The carbon bookkeeping: each turn fixes one CO₂, so three turns are needed for one G3P to leave the cycle as profit. Making that one G3P costs 9 ATP and 6 NADPH.
The G3P that leaves becomes glucose and other sugars. Plants ship sugar to other parts of the plant as sucrose, store it as starch, and build cellulose walls, amino acids and fats from it.
Although it doesn't use light directly, the cycle depends on the light reactions for ATP and NADPH, so it normally runs in daylight. The light reactions depend on it too: it returns ADP, phosphate and NADP⁺ for reuse.

Key terms (10)

carbon fixation: Taking carbon from CO₂ and building it into an organic molecule. In the Calvin cycle this is the first step.
RuBP: The five-carbon sugar that accepts CO₂ at the start of the Calvin cycle. The cycle has to rebuild it to keep running. The full name, ribulose bisphosphate, isn't needed for the AP exam.
rubisco: The enzyme that attaches CO₂ to RuBP. There is probably more of it on the planet than any other protein, and it can also mistakenly grab O₂.
G3P: Glyceraldehyde 3-phosphate, the three-carbon sugar the Calvin cycle makes. The plant uses it to build glucose, sucrose, starch and much more.
reduction phase: The part of the Calvin cycle where ATP and NADPH turn the newly fixed three-carbon acids into the sugar G3P. The electrons come from NADPH.
regeneration phase: The last part of the Calvin cycle, where ATP is spent to rebuild RuBP from G3P so more CO₂ can be fixed.
anabolic pathway: A series of reactions that builds larger molecules from smaller ones and uses up energy. The Calvin cycle is one.
light-independent reactions: Another name for the Calvin cycle, because none of its steps uses light directly. It still stops in the dark once ATP and NADPH run out.
sucrose: A two-part sugar (glucose + fructose) that plants make from Calvin cycle sugar and send through their veins to roots, fruits and other parts.
starch: A glucose polymer plants use to store extra sugar, in chloroplasts and in roots, seeds and tubers.

Check yourself: 10.3 The Calvin cycle: building sugar from CO₂

4 questions on 10.3 The Calvin cycle: building sugar from CO₂. Pick an answer to see if you got it, and why.

Question 1 of 4

Algae are given CO₂ containing radioactive ¹⁴C for different lengths of time, then killed instantly, and the labeled compounds are identified (invented data). Time exposed to ¹⁴CO₂ | Where most of the ¹⁴C is found 2 seconds | A three-carbon acid 10 seconds | The three-carbon acid and three-carbon sugars 60 seconds | Six-carbon sugars, sucrose and some amino acids Which conclusion is best supported?

Question 2 of 4Calculator allowed

For every 3 CO₂ the Calvin cycle fixes, it forms 6 molecules of G3P, a three-carbon sugar. Normally 1 G3P leaves as product, and the other 5 are used to rebuild 3 molecules of the five-carbon CO₂ acceptor. Suppose a cell instead pulled 2 G3P out of the cycle every time. What would most likely happen?

Question 3 of 4

Question 4 of 4

Which statement correctly compares the Calvin cycle with the Krebs (citric acid) cycle?

0 of 4 answered

10.4 Photorespiration and the C₄ and CAM workarounds

pp. 199–202

On the AP exam? Background

Photorespiration, C₄ and CAM aren't in the current course, but an exam scenario might describe them. You'd reason with Topic 3.4 (the Calvin cycle needs CO₂), Topic 3.2 (two molecules competing for one active site) and Topic 7.2 (traits that fit an environment).

In the course: Topic 3.4 Photosynthesis, Topic 3.2 Environmental Impacts on Enzyme Function, Topic 7.2 Natural Selection (notes, videos and more questions)

Key points

Leaves face a trade-off: open stomata let CO₂ in but also let water evaporate out (transpiration). On hot, dry days, many plants close their stomata to save water, so CO₂ inside the leaf drops while O₂ made by the light reactions accumulates.
Rubisco sometimes grabs O₂ instead of CO₂. When it does, the product is broken down in other organelles and CO₂ is released, with no sugar made and ATP used up. This photorespiration wastes some of the carbon the plant has already fixed, and it gets worse in hot weather.
C₃ plants, whose first fixed product is a three-carbon acid, rely on rubisco alone and lose the most to photorespiration. Rubisco probably evolved when the air had almost no O₂, so telling O₂ and CO₂ apart didn't matter. Photorespiration may also help protect leaves when light is strong but CO₂ is scarce.
C₄ plants split the job between two kinds of cells. In mesophyll cells, PEP carboxylase, which grabs CO₂ well even at low levels and ignores O₂, fixes CO₂ into four-carbon acids. These move into bundle-sheath cells around the veins and release CO₂ there, keeping CO₂ high around rubisco. It costs extra ATP.
CAM plants split the job between night and day. Their stomata open after dark, when they fix CO₂ into organic acids and store the acids in vacuoles. By day, with stomata shut, the acids release CO₂ to the Calvin cycle in the same cells.
All three types still rely on the Calvin cycle for making sugar. C₄ and CAM are adaptations that pay off in hot, sunny, dry places; in cool, moist conditions, C₃ plants avoid their extra costs and often do as well or better.

Key terms (8)

transpiration: Water evaporating from a plant, mostly through open stomata. It pulls water up from the roots but can dry a plant out.
C₃ plant: A plant that fixes CO₂ only with rubisco, so its first product is a three-carbon acid. Most plants are C₃, including oats, spinach and most trees.
photorespiration: What happens when rubisco grabs O₂ instead of CO₂: fixed carbon is lost as CO₂, ATP is used, and no sugar is made. It is worst when it’s hot and stomata are closed.
C₄ plant: A plant that first fixes CO₂ into a four-carbon acid in mesophyll cells, then releases it around rubisco in bundle-sheath cells. Sorghum and millet are examples.
PEP carboxylase: The enzyme C₄ and CAM plants use to fix CO₂ first. It binds CO₂ tightly even at low levels and doesn’t react with O₂.
bundle-sheath cell: A cell in the tight ring around a leaf vein. In C₄ plants, these cells run the Calvin cycle with CO₂ delivered by the mesophyll.
mesophyll cell (in C₄ plants): In a C₄ leaf, the outer photosynthetic cells that capture CO₂ with PEP carboxylase and pass four-carbon acids inward.
CAM plant: A plant that keeps its stomata open in the dark hours, stores CO₂ in organic acids, and runs the Calvin cycle by day with stomata closed. CAM stands for crassulacean acid metabolism. Agave and aloe are examples.

Check yourself: 10.4 Photorespiration and the C₄ and CAM workarounds

4 questions on 10.4 Photorespiration and the C₄ and CAM workarounds. Pick an answer to see if you got it, and why.

Question 1 of 4

Researchers track an agave plant in a desert over 24 hours (invented data). Time | Stomata | CO₂ uptake by leaves (µmol/m²/s) | Acid level in leaf cells (relative) 10 p.m. | Open | 8 | 40 4 a.m. | Open | 6 | 95 10 a.m. | Closed | 0 | 70 4 p.m. | Closed | 0 | 20 Which explanation best fits these data?

Question 2 of 4Calculator allowed

Net photosynthesis of leaves from a C₃ plant is measured in normal air (21% O₂) and in air with only 2% O₂, at two temperatures (invented data, µmol CO₂/m²/s). Temperature | 21% O₂ | 2% O₂ 25 °C | 20 | 26 38 °C | 12 | 21 Which conclusion is best supported?

Question 3 of 4

Two plants, a spinach plant and a sorghum plant, are each sealed in their own lit chamber. As they photosynthesize, the CO₂ in each chamber falls and then levels off. In one chamber it levels off at 45 ppm; in the other it drops to 4 ppm. Which plant most likely drew the CO₂ down to 4 ppm, and why?

Question 4 of 4

A leaf from a C₄ grass is treated with a chemical that blocks PEP carboxylase but doesn’t affect rubisco. The leaf is kept in hot, bright conditions with its stomata partly closed. Which outcome is most likely?

0 of 4 answered