Lecture: Photosynthesis
Lecture script · about 15–20 minutes spoken · Week 7
Opening
A redwood starts as a seed small enough to pinch between two fingers. It becomes a towering tree. So here's my question for you: where did all that mass come from? Most people will say the soil, since roots go into the soil. By the end of this lecture, you'll be able to explain why that guess is wrong, and what the tree is really made of.
A nursing student needs to know where the oxygen in every breath first came from. A gardener picks a sunny spot for a houseplant, and a farmer decides how to light a greenhouse. Underneath all three is one process running in every green leaf.
Here's where we are. In Week 4 you met the chloroplast as the organelle where a plant cell does photosynthesis. In Week 5 you met ATP, the energy currency, and enzymes. In Week 6 you followed cellular respiration, as cells took sugar apart to recharge ATP. In this lecture we run the story in the other direction. A plant spends light to build sugar. I'll take you through the equation, the chloroplast, the two stages in order, where the atoms come from, and how all of this fits with respiration. Then we'll read the leaf disk experiment from your lab. Next week, in Week 8, we pull the first seven weeks together for the midterm.
1. The equation: what goes in and what comes out
Picture a houseplant on a sunny kitchen windowsill. You water it, it sits in the light, and it puts out new leaves. The plant's recipe has only three ingredients.
The ingredients are carbon dioxide from the air, water from the roots, and light. Carbon dioxide gets into the leaf through tiny pores. Those pores are called stomata: tiny pores in the surface of a leaf through which carbon dioxide enters for photosynthesis. One pore is a stoma.
And what comes out? Two things. Glucose, the sugar you met in Week 3, and oxygen gas, which leaves the plant and enters the air you breathe. That whole process has a name. Photosynthesis is the process in which a plant, alga, or other autotroph uses light energy to build sugar from carbon dioxide and water, releasing oxygen.
Let me say the equation in words, since a formula doesn't read aloud well. Six carbon dioxide molecules, plus six water molecules, plus light energy, become one glucose molecule and six oxygen molecules. If you count the atoms, it balances. Carbon is 6 on each side. Hydrogen is 12 on each side. Oxygen is 18 on each side. Nothing is created and nothing is destroyed; the atoms are only rearranged.
That word "autotroph" needs a gloss. An autotroph is a living thing that makes its own food from simple inorganic materials, as plants and algae do by photosynthesis. A heterotroph is a living thing that cannot make its own food and must eat food that other organisms made, as animals and fungi do. Your houseplant is an autotroph. You're a heterotroph. Nearly every bite of food you eat traces back to an autotroph that caught light.
Pause and try this. A classmate writes the photosynthesis equation with glucose and oxygen on the left side, as the ingredients. What went wrong?
Think about what the plant takes in.
Here's the answer. Glucose and oxygen are products, so they belong on the right. The ingredients are carbon dioxide, water, and light; glucose and oxygen go in for cellular respiration, which is why the two get mixed up.
2. The chloroplast: discs, stacks, and fluid
Imagine a sealed plastic bag full of water, with several short stacks of coins floating inside. Shrink it until it fits inside one leaf cell. That's the shape of a chloroplast, and I'll describe it from the outside in.
First, two membranes, an outer one and an inner one, wrap the whole thing. That's the bag. Inside is a fluid. The stroma is the fluid that fills a chloroplast around the thylakoids, where the Calvin cycle builds sugar. That's the water in the bag.
Floating in that fluid are the coins. A thylakoid is a flat, disc-shaped membrane sac inside a chloroplast; its membrane holds chlorophyll and runs the light-dependent reactions. And the coins come in stacks. A granum is a stack of thylakoids inside a chloroplast, piled like coins; the plural is grana.
Last, the pigment. Chlorophyll is the green pigment in the thylakoid membrane of a chloroplast; it absorbs red and blue light and reflects green, which is why leaves look green. Here's the twist people miss. The green you see is the light the leaf is not using. Chlorophyll absorbs the red and the blue and puts that energy to work. It bounces the green back to your eyes.
Structure determines function here too. Chlorophyll sits in the thylakoid membrane, so that's where light is caught; RuBisCO in the stroma starts the sugar-building. Two places, two jobs.
Pause and try this. Someone tells you chlorophyll is green because it absorbs green light. Is that right?
Think about which light reaches your eyes.
Here's the answer. No, it's the reverse. Chlorophyll reflects green, which is why green reaches your eyes, and it absorbs the red and blue light it actually uses.
A memory line: the thylakoid is the disc, and the stroma is the sea around it.
3. Stage one: the light-dependent reactions
Think of a solar panel charging a battery. The panel builds nothing; it stores energy so something else can work later. Stage one does the same job.
The light-dependent reactions are the first stage of photosynthesis, in the thylakoid membrane, where chlorophyll absorbs light, water is split to release oxygen, and ATP and NADPH are made.
Let me walk you through it in order. Chlorophyll absorbs light. That energy is used to split water. Each water molecule gives up electrons, hydrogen ions, and oxygen. The oxygen leaves the plant as oxygen gas, and that's the oxygen in the air you breathe. The captured energy is stored in two loaded molecules: ATP, which you know from Week 5, and NADPH.
NADPH is a loaded electron carrier that the light-dependent reactions make and the Calvin cycle spends, together with ATP, to build sugar. Last week you met NADH, a carrier that picks up electrons in one step and drops them off later. NADPH does the same kind of job here.
Now notice what stage one doesn't make. It makes no sugar at all. It lets the oxygen go, and it charges up ATP and NADPH, the batteries the next stage will spend.
Pause and try this. Stage one takes in light and water. Name its outputs, and say which one leaves the plant.
Count the products as you name them.
Here's the answer. Stage one makes oxygen, ATP, and NADPH. The oxygen leaves the plant, while the ATP and NADPH carry the energy forward to stage two.
4. Stage two: the Calvin cycle builds sugar
Picture a bakery that runs its ovens on batteries a rooftop solar panel charged during the day. Flour comes in, loaves go out, and if the batteries run flat, the ovens stop. Stage two is that bakery.
The Calvin cycle is the second stage of photosynthesis, in the stroma, where carbon dioxide is fixed and ATP and NADPH from the light-dependent reactions are spent to build the three-carbon sugar G3P.
Three words come with it. Carbon fixation is the step that attaches carbon from carbon dioxide in the air to a molecule already in the Calvin cycle, turning a gas into part of a sugar. RuBisCO is the enzyme in the stroma that fixes carbon dioxide at the start of the Calvin cycle. You need its name and its job, not its chemistry. And G3P is the three-carbon sugar the Calvin cycle builds; three turns of the cycle export one G3P, and glucose and other sugars are assembled from it.
Let's count turns. Three turns of the cycle fix three carbon dioxide molecules and send out one G3P with three carbons. Glucose has six carbons. So it takes 2 G3P to make one glucose, and 2 times 3 is 6 turns. Those 6 turns fix 6 carbon dioxide molecules, exactly the six in the equation. The order of the two stages is a fact you can be graded on. The light-dependent reactions, in the thylakoid membrane, come first. Then the Calvin cycle, in the stroma.
Now the trap. People sometimes say the Calvin cycle doesn't need light, because no step in it absorbs light. But remember the bakery. The cycle spends ATP and NADPH, and only stage one makes them. Turn off the light, and the supply runs down. The Calvin cycle stops soon after. It needs light, one step removed.
Pause and try this. A plant is moved into a dark closet. Does its Calvin cycle keep building sugar all night?
Follow the batteries.
Here's the answer. No, it stops soon after the light goes out. With no light, stage one stops making ATP and NADPH, and the Calvin cycle can't build sugar without them.
5. Tracking the atoms
Back to the redwood. You can answer it the way a detective follows a suspect. Pick one kind of atom and follow it from where it starts to where it ends up. That habit has a name. Atom tracking is following one kind of atom from a reactant into a product to learn where it goes, as in showing that the oxygen plants release comes from water and the carbon in sugar comes from carbon dioxide.
Start with oxygen. Oxygen atoms appear in both reactants, in carbon dioxide and in water. So which one supplies the oxygen gas a plant gives off? Reading the equation can't tell you. You have to tag one source and follow it. A tracer is a detectable form of an atom, such as a heavy form of oxygen, that scientists follow through a process to see where that atom ends up. Ruben, Randall, Kamen, and Hyde, in 1941, used a heavy form of oxygen as a tracer. Following that tagged oxygen showed that the oxygen plants release comes from water. That fits stage one: water is split, and its oxygen leaves.
Now follow the carbon. The only reactant that carries carbon is carbon dioxide. The Calvin cycle fixes that carbon into G3P, and G3P is assembled into glucose. So every carbon atom in a plant's sugar came out of the air. And a plant builds its body from that sugar, including cellulose, the structural fiber you met in Week 3.
So here's the answer to the redwood puzzle. A tree's mass is mostly carbon pulled from the air, plus water. The soil supplies minerals, not bulk. A whole tree, built largely out of thin air.
Pause and try this. A friend says a plant's oxygen comes from the carbon dioxide it takes in, since carbon dioxide has oxygen in its name. What would you tell them?
Think about which molecule gets split in stage one.
Here's the answer. The oxygen comes from water, not carbon dioxide. Stage one splits water and releases its oxygen, and the 1941 tracer work followed that oxygen from water into the gas.
Here's a line to keep: the oxygen comes from water; the carbon comes from the air.
6. Photosynthesis beside respiration
Hold last week's process next to this week's, and they look like mirror images. Photosynthesis takes in carbon dioxide and water and gives off sugar and oxygen. Cellular respiration takes in sugar and oxygen and gives off carbon dioxide and water.
So photosynthesis and respiration are roughly mirror processes. Photosynthesis stores light energy in sugar and releases oxygen. Respiration releases that sugar's energy as ATP and uses oxygen. I say "roughly" on purpose. They run in different organelles, with different enzymes and carriers. They're partners, not one process run backwards.
A plant does both. Its chloroplasts make sugar in the light. Its mitochondria burn sugar day and night, because the plant needs ATP every minute, not only when the sun is up. A potted fern in a dark hallway at midnight is still respiring. Plants don't photosynthesize instead of respiring; they do both, which is the Week 4 rule that a plant cell has chloroplasts and mitochondria.
Now step back for the long view, through evolution, the framework from Week 1. The oxygen in Earth's air wasn't always there. Early oxygen-producing photosynthesis is how Earth gained the oxygen in its air. And the aerobic respiration you learned last week, with oxygen as the final electron acceptor, depends on that oxygen. So these two processes are linked not only inside one leaf, but across the history of life.
Pause and try this. It's midnight, and a houseplant sits in a dark room. Which process is running in its leaf cells: photosynthesis, respiration, or both?
Ask which one needs light.
Here's the answer. Respiration only. Photosynthesis stops once the light's ATP and NADPH run out, but the mitochondria keep respiring day and night.
7. Reading the floating leaf disk test
Now your lab. Set two clear cups on a kitchen counter, both filled with the same baking-soda solution. Into each, drop 10 small disks punched from a spinach leaf, with the air drawn out so they sink. One cup goes under a lamp. The other goes into a dark cupboard. Then you count how many disks have floated to the top at 0, 3, 6, 9, and 12 minutes.
The baking soda supplies dissolved carbon dioxide, the carbon source for the Calvin cycle. Under the lamp, stage one splits water and releases oxygen. The oxygen collects inside the disk until it floats. The dark cup is the control group: the no-light baseline.
These numbers are an illustrative set built for this course, not real data. In the light, the counts are 0, 2, 5, 8, and 10. In the dark, they're 0, 0, 0, 1, and 1. Picture two lines drawn on a sheet of paper: the light line climbs steadily to the top, and the dark line stays nearly flat along the bottom.
Now the arithmetic. For the light cup, add the five readings: 0 plus 2 plus 5 plus 8 plus 10 is 25. Divide by 5 and the light average is 5.0 disks. For the dark cup, 0 plus 0 plus 0 plus 1 plus 1 is 2, and 2 divided by 5 is 0.4 disks. At 12 minutes, 10 minus 1 means 9 more disks float in the light. And half the disks, 5 of the 10, are floating at about 6 minutes under the lamp.
Here are the variables. The independent variable is light versus dark. The dependent variable is the number of disks floating. The controlled variables are the number of disks, the leaf, the solution, the cups, and the timing. And the honest verb: the evidence supports the hypothesis that light speeds oxygen production. It doesn't prove it. The things you're graded on are the rising trend in the light, the flat dark control, and where the oxygen came from, never any one disk count.
Pause and try this. The disks are sitting in baking soda, which supplies carbon dioxide. So is the gas that lifts the disks carbon dioxide?
Think about what stage one releases.
Here's the answer. No, it's oxygen. The carbon dioxide is used up in the Calvin cycle, while the oxygen released by splitting water collects in the disk and lifts it.
Closing
Let me pull it together. Photosynthesis takes in carbon dioxide, water, and light, and gives off glucose and oxygen; autotrophs make their own food this way, and heterotrophs like you eat it. The chloroplast has thylakoid discs stacked into grana, with chlorophyll in the thylakoid membrane, all floating in the fluid stroma.
Stage one, the light-dependent reactions, runs in the thylakoid membrane: it splits water, lets the oxygen go, and makes ATP and NADPH. Stage two, the Calvin cycle, runs in the stroma: RuBisCO fixes carbon dioxide, ATP and NADPH are spent, and G3P is built, and it stops soon in the dark.
Tracking the atoms answers the redwood: the oxygen comes from water, and the carbon comes from the air. Plants do both processes, and early photosynthesis gave Earth the oxygen respiration needs. Your leaf disks show it on a counter.
Next in this module come the slide deck, the Readings & Resources page, the lecture tutorial, the practice set, and the quiz, and then the Floating Leaf Disk lab. Next week, in Week 8, we review Weeks 1 through 7 for the midterm, with one new habit for any process: name each stage in order, and the place it runs.
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