Partners in Energy

Taskinas Creek is a small marsh creek off the York River in Virginia. A sensor there checks the oxygen in the water every 15 minutes. On one dry summer day, the water held far more oxygen at 6 p.m. than in the early morning. Marsh grasses grow along this creek, and animals live in its water. Something adds oxygen to the water. Something takes it away. Before you find out what, decide what you think now.

Explore: run a day and a night in a tank of Bay water

In this virtual tank you have Bay water, underwater grass, algae and fish. A clock runs through 24 hours. Bars show the oxygen and carbon dioxide in the water. A line graph plots the oxygen hour by hour. You can change the amount of plants and algae, the number of animals, and the sky (sunny or cloudy).

Plants and algae 3 Animals 2

Sky Sunny Cloudy

Run 24 hours Reset

Clock6:00 a.m.

Oxygen

50

Carbon dioxide

50

Photosynthesis adds0.0 units per hour

Respiration uses0.0 units per hour

this run previous run

Highest oxygen

–

Lowest oxygen

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Oxygen at the next dawn

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Press Run 24 hours to start a day in the tank. Predict the oxygen line first.

In this sketch: amounts are relative units, not mg/L. There is no gas exchange with the air, the temperature stays the same, daylight lasts from 6 a.m. to 6 p.m., and plants and animals are drawn as blocks. A cloudy sky lets through 60 out of 100 parts of the light, a plant block adds up to 3 units an hour and uses 0.4, an animal block uses 0.8, and a gas below 15 units slows its process (all made-up numbers for this sketch). Carbon dioxide is drawn as 100 minus oxygen (a sketch simplification). “Very low” here means under 20 units.

Rule to keep: photosynthesis adds oxygen only in light, but respiration uses oxygen day and night. A real creek is bumpier, because tides, air and warm water also change oxygen.

Try this.

1. Predict what the oxygen line will do between sunset and sunrise. Then run one day and check.
2. Predict how a cloudy day will change the oxygen at noon. Then switch the sky and test it.
3. Add many fish and remove most of the grass. Predict the oxygen at dawn before you run it.

Explain: two processes in one leaf cell

You met both processes in Photosynthesis Feeds the World and Releasing Energy from Food. Now put them in the same cell.

Photosynthesis uses radiant energy to make sugar (glucose) and oxygen from carbon dioxide and water. It happens in the chloroplast, a green part of the cell that takes in light.

carbon dioxide + water + radiant energy → sugar (glucose) + oxygen

Cellular respiration joins sugar with oxygen and releases energy the cell can use. Most of it happens in the mitochondria, small parts found in almost all plant and animal cells.

sugar (glucose) + oxygen → carbon dioxide + water + energy for the cell

A green leaf cell has both organelles. An animal cell has mitochondria but no chloroplasts. Not every plant cell is green, though. Root cells usually have no chloroplasts. They get sugar sent down from the leaves.

	Photosynthesis	Cellular respiration
Where	Chloroplast	Mostly in mitochondria
Goes in	Carbon dioxide, water, radiant energy	Sugar (glucose), oxygen
Comes out	Sugar (glucose), oxygen	Carbon dioxide, water, usable energy
When	Only in light	Day and night

Explain: what one makes, the other uses

A reactant is a substance that goes into a chemical process. A product is a substance that comes out. Look at the two word equations again. The products of photosynthesis, sugar and oxygen, are the reactants of respiration. The products of respiration, carbon dioxide and water, are the reactants of photosynthesis. Processes that depend on each other like this are interdependent. Each one needs the other.

Matter and energy move differently through this pair:

• Matter is used again. The same carbon, oxygen and hydrogen go around and around.
• Energy flows one way. Light from the sun becomes chemical energy in sugar. Respiration releases that energy for life processes. Some of it leaves as heat. Energy is never made or used up. It changes form.

Producers, like the Bay’s algae and underwater grasses, make the food and oxygen that almost every food web needs.

Each process also has things that change its speed:

• Photosynthesis: light, carbon dioxide and temperature.
• Respiration: temperature, oxygen and sugar (glucose).

Explain: why creek oxygen rises and falls

Dissolved oxygen is oxygen gas mixed into water. Fish, crabs, plants and microbes can use it there. Scientists measure it in milligrams per liter (mg/L).

In daylight, algae and grasses photosynthesize and add oxygen to the water. At the same time, nearly every living thing in the creek respires and takes oxygen out. On a bright day, photosynthesis often adds oxygen faster than respiration takes it. The level climbs. After dark, photosynthesis stops, but respiration does not. The level falls through the night.

Think of a bathtub with the tap running and the drain open. Sunlight turns the tap on. The drain never closes. The comparison breaks because a real creek is open. Tides bring in new water. Air at the surface can add oxygen, or take some away when the water holds extra. Warm water holds less oxygen than cold water.

When oxygen drops very low over a large area, many animals can die. You will study these dead zones in Big Changes (a later lesson).

Elaborate: a real day in Taskinas Creek

Here are real readings from Taskinas Creek. It is a shallow creek, and the tide moves water in and out from the York River. The light readings come from a weather station about 200 meters away. Times are standard time, one hour behind summer clock time.

Time	Dissolved oxygen (mg/L)	Water temperature (°C)	Light in the 15 minutes before (mmol photons per m²; higher means more light)
July 30, midnight	5.7	32.6	0.0
3 a.m.	4.2	30.6	0.0
6 a.m.	4.0	29.3	90.8
9 a.m.	5.3	29.1	1,170.8
Noon	5.7	30.3	1,155.6
3 p.m.	4.6	31.3	1,396.5
6 p.m.	10.7	32.6	240.6
9 p.m.	9.0	31.2	0.0
July 31, midnight	6.4	30.3	0.0

Source: Chesapeake Bay National Estuarine Research Reserve in Virginia (run by VIMS), NOAA monitoring program, stations cbvtcwq (water) and cbvtcmet (weather), July 30, 2023. No rain fell that day. The table shows one reading every 3 hours. In the full record, the lowest oxygen was 3.4 mg/L at 2:45 a.m., and the highest was 10.8 mg/L at 5:45 p.m. The last midnight reading carries a rain-event note in the record.

1. How much did oxygen fall between 6 p.m. and midnight? What was the light during most of that time?
2. Find the lowest and highest oxygen in the table. Was it light or dark at each of those times?
3. Oxygen dropped between noon and 3 p.m., even in bright light. Name one thing besides living things that could explain that.

Design problem

Plan an experiment to test whether a water plant uses oxygen in the dark and adds it in the light. You can buy water plants and a dissolved oxygen probe for school use.

One way to do it

• Question: Does light change whether a water plant adds or removes dissolved oxygen?
• Independent variable: light or dark (a lamp or a closed cabinet).
• Dependent variable: change in dissolved oxygen over 4 hours, in mg/L.
• Constants: same jar size, 500 mL of the same water, about 2 grams of plant per jar, same room temperature.
• Control: jars with water and no plant, in light and in dark. They show how much oxygen changes without the plant.
• Trials: three jars of each kind. Report the mean change.
• Safety: keep lamps and cords away from water, wash your hands after, and never release store-bought plants into a creek or pond.

If oxygen rises in the lit jars and falls in the dark jars, while the plain water barely changes, the plant added oxygen in light and used it in darkness.

Evaluate: check your thinking

In Taskinas Creek, algae and other producers in the water probably added oxygen by photosynthesis while the sun was up. Nearly every living thing in the creek, plants included, used oxygen for respiration day and night. When light faded, only the using went on. Oxygen fell until the early hours of the morning. Tides and temperature moved the curve too. That is why the real record is bumpier than the tank.

Science practices you used: making and testing predictions with a model, reading a real data table, finding a pattern and other possible causes, and making a claim backed by evidence and reasoning.