Capturing Sunlight

In the 1600s, a man planted a 5-pound willow stem in a pot of 200 pounds of dried soil. Five years later he reported a tree of about 169 pounds, and the soil was only about 2 ounces lighter. So where did more than 160 pounds of tree come from? The same question matters today in Chesapeake Bay, where underwater grasses spread in some years and shrink in others. Before you start, decide what you think now.

Explore: race leaf disks under a lamp

In this virtual lab you punch small disks out of a spinach leaf, pull the air out of them so they sink, and drop them into water with baking soda. Baking soda adds dissolved carbon dioxide. As the disks photosynthesize, oxygen gas collects inside them, and they float back up; the sooner they float, the faster photosynthesis is running. You can change the lamp distance, the amount of baking soda and the water temperature, and the time to float is plotted for you. The simulation is a sketch, not real measurements; the shape of each graph is what to look at.

Try this.

1. Predict how the time to float will change as you move the lamp from 40 cm to 10 cm. Then test it.
2. Set the lamp close and try no baking soda, then a little, then a lot. Predict first: will more always be faster?
3. Keep light and baking soda fixed and change the temperature from cold to warm to hot. Write your prediction, then run it.
4. Find a setting where changing one factor stops making a difference. Which factor is holding the rate back there?

Explain: what goes in and what comes out

Photosynthesis is the process in which plants, algae and cyanobacteria use light energy to turn carbon dioxide and water into sugar, releasing oxygen. The sugar is glucose, a small sugar molecule with 6 carbon atoms. In words:

carbon dioxide + water + light energy → glucose + oxygen

With chemical formulas:

6CO2 + 6H2O + light energy → C6H12O6 + 6O2

This equation is a summary. Inside the cell it takes many steps, but you only need the start and the end.

Count the atoms. On the left there are 6 carbon, 12 hydrogen and 18 oxygen atoms. On the right there are exactly the same. This is conservation of matter: in a chemical change, atoms are rearranged, never created or destroyed.

Now the willow makes sense. The atoms in its wood had to come from somewhere. The carbon and much of the oxygen in its sugar came from carbon dioxide in the air. The hydrogen came from water. Soil gave water and a small amount of minerals such as nitrogen, which is why the dried soil barely changed. Most of a living plant’s fresh mass is water, often about 80 to 90 percent in textbook accounts, but most of its dry mass is built from carbon taken in as carbon dioxide.

Energy is conserved too. Light energy is not used up or made. It is transformed into chemical energy, energy held in the arrangement of atoms in molecules. Glucose and oxygen together hold more chemical energy than the carbon dioxide and water they came from.

Explain: where it happens and where the sugar goes

In plants and algae, photosynthesis happens in the chloroplast, a green organelle in leaf cells and other green parts. Inside it, stacks of membranes hold chlorophyll, a green pigment that absorbs light, mostly red and blue. That captured energy powers the reactions that split water and build glucose. The oxygen released comes from the split water. It is a by-product, and it leaves the leaf through tiny pores.

Not every photosynthetic cell has chloroplasts. A prokaryote is a cell with no nucleus and no membrane-bound organelles, such as a bacterium. Cyanobacteria are prokaryotes that photosynthesize and release oxygen. Their chlorophyll sits on folded membranes inside the cell instead of in chloroplasts.

Once a plant has glucose, it has three main uses for it:

• Used right away. Plant cells carry out cellular respiration, which breaks glucose down and releases its energy in a form cells can use. Plants respire all the time, in light and dark. Respiration and ATP follows that path.
• Stored. Glucose molecules are linked into starch, a long chain the plant stores in seeds, roots and leaves.
• Rebuilt. Glucose molecules are linked a different way into cellulose, the tough fiber of plant cell walls. Wood is mostly cell walls. Plants also rebuild glucose, with minerals from soil, into proteins, fats and DNA. What Am I Made Of? covers those molecules.

Explain: what limits the rate

A limiting factor is the condition that holds a process back, such as the resource in shortest supply or a temperature that is too low or too high. Photosynthesis has three main limits:

• Light intensity. More light usually means faster photosynthesis, up to a point.
• Carbon dioxide. More carbon dioxide also speeds it up, up to a point.
• Temperature. The reactions are run by enzymes. Warmth speeds them up to a point, and too much heat slows them as the enzymes lose their shape. Enzymes at Work explains why.

Think of a sandwich line with one worker making bread and one adding fillings. Speeding up the bread maker helps only until the fillings worker can’t keep up. Then the line runs at the slower worker’s pace. The analogy breaks because a plant’s factors also affect each other: heat can change how much water a leaf loses, for example.

That is why a graph of rate against light usually rises and then levels off. At the flat part, more light does nothing, because carbon dioxide or temperature is now the limit.

Elaborate: underwater grass in the Chesapeake

Underwater grasses, which scientists call submerged aquatic vegetation (SAV), live on the bottom of shallow parts of the Bay. Every year the Virginia Institute of Marine Science (VIMS) maps them from the air.

Source: VIMS annual SAV survey executive summaries, 2018 to 2023, for Chesapeake Bay and its tidal tributaries (all mapped beds, not a single station). For areas mapped in both 2018 and 2019, VIMS compared 98,828 acres with 62,334 acres, a 37% drop.

How much light do these plants need? A Chesapeake Bay Program technical synthesis (2000) set habitat targets for the growing season. In fresher upper reaches, more than 13% of the light at the surface should reach the grass through the water. In the saltier middle and lower Bay, the target is more than 22%. Most eelgrass lives in the lower Bay. Mud and floating algae block light, and so does material growing on the leaves themselves.

1. Which year had the smallest mapped area? By how many acres did the area grow from that year to 2023?
2. Why is it fairer to compare 2018 and 2019 using the matched areas than the two totals?
3. The table has no water-clarity numbers. What would you need to add before claiming that clearer water caused the increase after 2020?

Design problem

Plan a real leaf-disk experiment to test how light intensity affects the rate of photosynthesis. Name your variables, what you keep the same, your control, and how many trials you would run.

Evaluate: check your thinking

The willow gained mass because its leaves took in carbon dioxide and, using light energy, combined it with water to build glucose and then wood. The soil gave water and a little mineral matter, which is why it barely changed. Bay grasses face the same rule underwater: when murky water cuts the light, light becomes the limiting factor.

Science practices you used: making predictions and testing them, planning a fair test with a control and repeated trials, reading a data table and its coverage notes, separating a pattern from a cause, and using a model and naming its limits.