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Educerie · AP · Biology

AP Biology 3.4 Photosynthesis

In one breath

  • Photosynthesis uses light energy to make carbohydrate and oxygen from carbon dioxide and water, and it is done by plants, algae and cyanobacteria.
  • It first evolved in prokaryotes; cyanobacteria oxygenated the atmosphere, and the chloroplast is a descendant of one.
  • In the thylakoid membranes of the grana, light boosts electrons in photosystems II and I.
  • Water is split to replace electrons lost from photosystem II, releasing oxygen.
  • Electrons pass along a chain of redox reactions that pumps protons into the lumen; protons flow back to the stroma through ATP synthase, which makes ATP by photophosphorylation; photosystem I re-energises the electrons to reduce NADP⁺ to NADPH.
  • In the stroma, the Calvin cycle spends ATP and NADPH to build carbohydrate from carbon dioxide, and sends ADP, phosphate and NADP⁺ back to be recharged.

What you must be able to do

  • State the inputs and outputs of photosynthesis and where the energy comes fromMCQ: which molecules are used and which are made; where the oxygen comes from
  • Describe how photosynthesis evolved in prokaryotes and changed the atmosphereQ4: "Describe the evidence that cyanobacteria produced the oxygen in Earth's atmosphere"
  • Describe the stroma, thylakoids and grana, and what happens in eachQ5: label a chloroplast model and say where each stage happens
  • Explain how the light reactions use light to make ATP and NADPHA thylakoid diagram: trace electrons from water to NADP⁺
  • Explain how a proton gradient across the thylakoid membrane drives ATP synthasePredict the effect of a chemical that makes the membrane leaky to H⁺
  • Explain how ATP and NADPH from the light reactions power the Calvin cycle in the stromaPredict what happens to the Calvin cycle when the light is switched off

Once you have read the note, tick off each one you can do in Check your understanding.

In the exam. Section I multiple choice on a model of the thylakoid membrane (where protons are concentrated, what ATP synthase does, where water is split), on data from a photosynthesis experiment (oxygen released or carbon dioxide taken up at different light intensities), and on predicting what happens when one part of the process is blocked. In Section II it is a regular setting for Interpreting and evaluating experimental results (Q1) and the graphing version (Q2), Analyze model or visual representation (Q5) with a diagram of the light reactions, and Conceptual analysis (Q4), for example on the evolution of photosynthesis and the oxygen in the atmosphere.

1What goes in and what comes out

The overall process, summed up:

6CO2+6H2O→lightC6H12O6+6O26\text{CO}_2 + 6\text{H}_2\text{O} \xrightarrow{\text{light}} \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2
  • Inputs: carbon dioxide from the air, water from the soil (or the surrounding water), and light energy.
  • Outputs: carbohydrate, shown here as glucose, and oxygen.

The sugar holds the captured energy in its chemical bonds. The organism can use it at once in respiration, build it into other molecules such as cellulose or proteins, or store it, for example as starch.

Two points that catch students out. First, the oxygen released comes from water, split in the light reactions, not from carbon dioxide. The carbon and the oxygen atoms in carbon dioxide end up in the sugar. Second, photosynthesis is not only a plant process: algae and cyanobacteria do it too, and in the oceans they carry out a large share of all the photosynthesis on Earth.

2Where photosynthesis came from

Photosynthesis first evolved in prokaryotes, long before there were plants. Ancient cyanobacteria were the first organisms to split water and release oxygen as a by-product. Early Earth's atmosphere had almost no free oxygen: rocks older than about 2.4 billion years show almost no sign of it, and after that, iron minerals that form only when oxygen is present appear in the record. At that time all life was prokaryotic, and cyanobacteria are the only prokaryotes that release oxygen by photosynthesis. This evidence supports the claim that cyanobacterial photosynthesis produced the oxygenated atmosphere.

Eukaryotes did not invent photosynthesis for themselves. A eukaryotic cell took in a cyanobacterium, which became the chloroplast (Topic 2.10). That is why the light reactions of a chloroplast still work like those of a cyanobacterium: prokaryotic pathways were the foundation of eukaryotic photosynthesis.

3The chloroplast

A chloroplast has a double outer membrane, and inside it two regions that matter for this course.

  • Thylakoids are flattened sacs of membrane, stacked into piles called grana (one stack is a granum). The thylakoid membranes hold the chlorophyll, arranged in photosystem I and photosystem II, along with the proteins of the electron transport chain and ATP synthase. The light reactions happen in the thylakoid membranes of the grana. The space inside each thylakoid is the lumen.
  • The stroma is the fluid inside the inner membrane but outside the thylakoids. The Calvin cycle happens in the stroma.

Stacking many thylakoids gives a very large area of membrane in a small chloroplast, so there is room for many photosystems and many ATP synthase molecules.

4The light reactions, step by step

Figure 1 shows a short stretch of thylakoid membrane, with the stroma above and the lumen below. Work through it one step at a time.

Figure 1. The thylakoid membrane, stroma above and lumen below: photosystem II splits water, electrons pass along carriers that pump protons into the lumen, protons return through ATP synthase making ATP, and photosystem I reduces NADP+ to NADPH stroma thylakoid lumen PSII PSI ATP synthase light 2H2O O2 + 4H+ e- H+ electron carriers H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ ADP + Pi ATP light NADP+ + H+ NADPH
Figure 1 · The light reactions in a thylakoid membrane: electrons from water reach NADP⁺, and a proton gradient drives ATP synthase
1

Light excites photosystem II, and water is split. Chlorophyll in photosystem II absorbs light, and the energy boosts electrons to a higher energy level. Those excited electrons leave the photosystem, and it replaces them by taking electrons from water. Splitting water on the lumen side releases oxygen, which diffuses out of the leaf, and protons (H⁺), which stay in the lumen.

2

Electrons pass along the electron transport chain, and protons are pumped. The electrons are passed from one carrier to the next in a series of oxidation–reduction (redox) reactions: each carrier is reduced when it accepts electrons and oxidised when it passes them on. At each step the electrons lose a little energy, and some of that energy is used to pump protons from the stroma into the lumen. The lumen now has a high proton concentration and the stroma a low one: an electrochemical gradient across the thylakoid membrane.

3

Protons flow back through ATP synthase, which makes ATP. The membrane is impermeable to protons except through the channel in ATP synthase. Protons diffuse down their gradient, from the lumen back to the stroma, through that channel, and the flow drives the enzyme to join ADP and inorganic phosphate into ATP. Making ATP this way, powered by a proton gradient, is chemiosmosis; in photosynthesis it is called photophosphorylation. The ATP is made on the stroma side, where the Calvin cycle will use it.

4

Light excites photosystem I, and NADP⁺ is reduced. The electrons arrive at photosystem I having lost some energy. Light absorbed by photosystem I boosts them to a high energy level again, and they are passed to NADP⁺, which picks up a proton and becomes NADPH. NADPH carries the high-energy electrons into the stroma.

Why does a plant need both photosystems? Photosystem II can take electrons from water and power the proton pumping, but by the end of the chain its electrons do not have enough energy left to reduce NADP⁺. Photosystem I gives them a second boost so they can. One photosystem alone could not both split water and make NADPH.

You do not need the names of the carriers in the chain; only ATP synthase is named in this course.

The light reactions make ATP and NADPH; the Calvin cycle spends them to turn carbon dioxide into sugar.

5The Calvin cycle

In the stroma, the Calvin cycle takes carbon dioxide from the air and builds it into carbohydrate. It is a cycle of enzyme-catalysed reactions; you do not need the steps, the intermediates or the enzymes. You need what goes in and what comes out:

  • In: carbon dioxide, ATP and NADPH.
  • Out: carbohydrate, plus ADP, inorganic phosphate and NADP⁺.

The ATP provides energy and the NADPH provides high-energy electrons for building sugar. The ADP, phosphate and NADP⁺ go back to the thylakoids to be recharged. The two stages depend on each other, and that lets you predict:

  • Light switched off: the light reactions stop, so the stores of ATP and NADPH run out within a minute or so, and the Calvin cycle slows and stops, even though carbon dioxide is still present.
  • Carbon dioxide removed: the Calvin cycle stops, so ADP and NADP⁺ are no longer returned, and the light reactions slow down too, because they run out of NADP⁺ to reduce.

6Measuring photosynthesis

The rate of photosynthesis is measured by something that changes as it goes on: oxygen released, carbon dioxide taken up, or sugar made.

Worked example. Invented data from an oxygen sensor in a sealed chamber with an aquatic plant, at three light intensities. Each reading is the increase in dissolved oxygen over 10 minutes.

Light intensity (relative units)2050100
Increase in oxygen in 10 min (µmol)184147

The rate at intensity 50:

rate=4110micromoles of oxygen divided by minutes=4.1 μmol O2 min−1\begin{aligned}\text{rate} &= \frac{41}{10} && \textcolor{#838c91}{\text{micromoles of oxygen divided by minutes}} \\[6pt] &= 4.1\ \mu\text{mol O}_2\ \text{min}^{-1}\end{aligned}

Doubling the light from 50 to 100 raised the oxygen only from 41 to 47, so at high intensity something other than light, such as carbon dioxide supply or the speed of the Calvin cycle's enzymes, is limiting the rate. A good answer names the trend, quotes the numbers, and gives the reason.

Note that these are net figures: the plant is also respiring and using oxygen at the same time (Topic 3.5).

Practice questions

11 questions on this note, from easy to hard. Each one you get right lights a star of this topic's constellation.