IB Biology Topic C1.3.3: Oxygen as a By-product of Photosynthesis Notes

The Oxygen You Breathe Comes from Water Split During Photosynthesis

1. Plants, algae, and cyanobacteria carry out photosynthesis, which converts light energy into chemical energy in glucose.
2. As this happens, oxygen is released as a by-product.
3. Almost all of the oxygen in Earth's atmosphere was produced this way.

Carbon dioxide + water → glucose + oxygen

The Oxygen Comes from Water, Not Carbon Dioxide

1. The oxygen released during photosynthesis comes from water molecules, not from carbon dioxide.
2. Water is split using light energy in a process called photolysis, which means "splitting by light."
3. Splitting water releases the oxygen that leaves the plant as a by-product.
4. The hydrogen from the split water is kept and used to build glucose, so it is not wasted.

Oxygen Matters Because It Made Aerobic Life Possible

1. The main goal of photosynthesis is to make glucose for energy storage.
2. Oxygen is a by-product, left over when water is split.
3. Over billions of years this oxygen built up in the atmosphere.
4. This supply of oxygen supports aerobic respiration in most living organisms.

You Can See Oxygen Being Released Underwater

1. Bubbles that form on underwater plants in the light are oxygen released by photosynthesis.
2. Counting these bubbles is a simple way to measure the rate of photosynthesis.

1. Photosynthesis produces glucose for energy storage and releases oxygen as a by-product.
2. That oxygen comes from splitting water, which is why the atmosphere became rich in oxygen.