Releasing Energy from Food

Pack sprouting peas into one thermos and boiled peas into another. Over a day or two, the thermometer in the sprouting peas can climb above room temperature, while the boiled peas stay close to it.

Now stir some yeast into warm sugar water. It foams, and a balloon on top of the bottle slowly fills with gas. In ice-cold water, you would expect the same mix to fill the balloon more slowly. Peas have no muscles, and yeast has no lungs. So where do the warmth and the gas come from?

Explore: warm up a flask of living cells

In the simulation you set up a flask of sprouting seeds, or yeast in sugar water. A balloon on top collects the carbon dioxide, and a thermometer shows the temperature. You can change the temperature of the water bath, open or seal the flask to change the oxygen, and change the amount of sugar or the number of seeds.

Try this.

1. Predict what will happen to the balloon when you warm the water bath from 10 °C to 25 °C. Write down your guess, then test it.
2. Seal the flask so no fresh air can get in. Predict how fast the balloon fills after a while, then watch.
3. Keep raising the temperature in small steps. Predict whether the rate keeps rising forever, then check the graph.

Explain: how cells release energy from sugar

A peanut, a slice of bread and a pea seed all store energy in their molecules. Your cells must break the food down to use it.

The main fuel is glucose, a sugar that stores energy in its molecules. Plants make sugar (glucose) by photosynthesis, as you saw in that lesson.

Cellular respiration is the process in which cells break down sugar (glucose) with oxygen and release energy. Oxygen is a gas in the air that cells use in this process. Carbon dioxide is a gas that cells give off when they respire.

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

If you like chemistry, the same idea in symbols is C6H12O6 + 6O2 → 6CO2 + 6H2O + energy. You do not have to learn it.

Inside a cell this happens in many small steps, not one big burst. The energy comes out in a form the cell can easily use.

Most of this work happens in the mitochondria (one is a mitochondrion). These are small organelles where cells release energy from food. Plant cells and animal cells both have them. Chloroplasts let a plant make food. Mitochondria let a cell use food for energy. Almost all plant, animal and fungus cells have mitochondria. A few do not, such as mature red blood cells in mammals.

Breathing is moving air into and out of your lungs. It is not the same thing as cellular respiration.

Yeast is a fungus. It breaks down sugar and gives off carbon dioxide, the gas in the balloon. Yeast can also release energy from sugar without oxygen, which you will study in Biology. In a closed bottle with little air, some of the gas in the balloon may come from that process. Photosynthesis and respiration work as partners, as you will see in that lesson.

Explain: where the energy goes and what changes the speed

Cells spend the released energy on work, such as contracting muscle cells or building new cell parts.

Some energy always leaves as thermal energy, the energy of moving particles that you feel as warmth. That is why the sprouting peas warmed their thermos. The thermos kept the thermal energy from escaping quickly.

Your body loses thermal energy to the air all the time. Respiration in your cells keeps releasing more. This is a big reason your body stays near 37 °C, even on a cold day.

Food does not turn into energy, and energy is never used up. It changes form. In respiration, energy stored in sugar becomes energy for cell work, plus thermal energy.

The rate of respiration is how fast cells carry out respiration. You can measure it as how much carbon dioxide is given off, or how much oxygen is used, in one hour. Three things change the rate:

• Temperature. Within a suitable range, warmer cells usually respire faster. Too much heat damages cells, and respiration slows or stops. Boiled peas are the extreme case: their cells are dead.
• Oxygen. With less oxygen, respiration that uses oxygen slows down.
• Sugar (glucose). With too little sugar, respiration slows. Adding more sugar helps only up to a point.

Elaborate: read real respiration data

We could not find a full data set for sprouting seeds at many temperatures. The closest real set is for fresh garden peas, picked and stored as food. Their cells are still alive, so they still respire.

Source: USDA Agriculture Handbook 66 (2016 edition), pea chapter by Morris and Jobling. It gathers earlier measurements (ASHRAE 1985; Suslow and Cantwell 1998). Each row is a range. The handbook does not say where or in which year the peas were measured.

A second study tested oxygen. Scientists grew baker’s yeast at 30 °C. They changed only how much oxygen was in the gas bubbled into the yeast.

Source: Jouhten and others, BMC Systems Biology, 2008. Each value is a mean from repeated lab cultures of one yeast strain.

1. Describe how carbon dioxide given off by the peas changes from 0 °C to 25 °C.
2. Compare 0 °C and 20 °C. Is the lowest value at 20 °C bigger than the highest value at 0 °C?
3. How many times as much oxygen did the yeast use with 20.9% oxygen as with 0.5%?

Design problem

Plan an investigation to test whether sprouting peas release thermal energy. You have dried peas, water, four or more thermoses, thermometers that read in °C, and a teacher who can boil water.

Evaluate: check your thinking

The sprouting peas are alive. Their cells break down stored sugar with oxygen, and some of the released energy leaves as thermal energy, which the thermos holds in. The boiled peas are dead, so they do not respire. Yeast cells break down sugar and give off carbon dioxide, which fills the balloon, and in ice-cold water they should work more slowly.

Science practices you used: predicting and testing with a model, reading data tables and comparing values, making a claim from evidence and naming its limits, and planning a fair test with a control and repeated trials.