Take a deep breath. The oxygen you just pulled into your lungs was released by a plant, an alga or a bacterium, possibly a tree in your neighbourhood or tiny algae floating in the ocean. Now breathe out. The carbon dioxide you just released may be taken in by a plant and built into sugar, which may one day end up in your food. Two chemical processes drive this endless swap: photosynthesis, which stores the Sun's energy in sugar, and cellular respiration, which releases that energy so cells can use it. Together they connect the energy flow you learned about in the last lesson with the cycling of matter you will study next.
Photosynthesis: storing light energy in sugar
Photosynthesis is the process producers use to make their own food. It happens in chloroplasts, small green structures inside plant and algae cells. Chloroplasts contain a green pigment called chlorophyll that absorbs light energy. Using that energy, the cell combines carbon dioxide (taken in from the air through tiny pores in leaves called stomata) with water (absorbed by the roots) to make glucose, a sugar. Oxygen gas is released as a by-product. The light energy is now stored as chemical energy in the glucose. The plant uses glucose for energy, and also to build other substances such as starch (for storage) and cellulose (for cell walls).
Cellular respiration: releasing the energy
Cellular respiration is the process that releases the chemical energy stored in glucose so a cell can use it. It happens mainly in mitochondria, structures found in the cells of plants, animals, fungi and many other organisms. Glucose reacts with oxygen to produce carbon dioxide and water, and the energy released is stored in a molecule called ATP that powers cell activities like muscle movement, growth and repair. Some energy is always released as heat, which is why your body is warm. Cellular respiration is not the same as breathing: breathing moves air in and out of your lungs, while respiration is a chemical reaction happening inside each of your cells. Breathing supplies the oxygen respiration needs and removes the carbon dioxide it makes.
| Photosynthesis | Cellular respiration | |
|---|---|---|
| Purpose | Stores energy: makes food (glucose) | Releases energy from food for the cell to use |
| Where in the cell | Chloroplasts | Mitochondria |
| Which organisms | Producers: plants, algae, some bacteria | Almost all living things, including plants |
| Reactants (inputs) | Carbon dioxide, water, light energy | Glucose, oxygen |
| Products (outputs) | Glucose, oxygen | Carbon dioxide, water, energy (ATP and heat) |
| Energy change | Light energy becomes chemical energy | Chemical energy becomes usable energy (ATP) and heat |
| When | Only when there is light | All the time, day and night |
The speed of photosynthesis depends on abiotic factors. More light, more carbon dioxide and a warmer temperature generally speed it up, but only up to a point: very high temperatures damage the cell's chemistry and slow it down again. This is why greenhouse growers in places like Leamington, Ontario, add light and sometimes extra carbon dioxide to grow vegetables year round. It also means changes in climate, such as warmer temperatures and droughts, affect how much carbon plants can take out of the air.