The role of oxygen in biology

Oxygen is essential to life. It is used by animals to break down food and provide energy. It is

involved in a very important chemical reaction called photosynthesis, which occurs in the leaves of green plants.

In human beings and other animals, oxygen is breathed into the lungs. There, it reacts with a complex molecule called hemoglobin. An unstable compound is formed between hemo­globin and oxygen, known as oxyhemoglobin. It is this compound that gives the blood in the arteries its bright red color. Oxyhemoglobin flows through the arteries to tissues in the body. There, it releases its oxygen, which is used in the breakdown of food and waste products in the cells. Complex organic mole­cules (enzymes) help to catalyze (speed up) these reactions at body temperatures.

One of the products formed in tissue cells after these reactions is the gas carbon dioxide. The blood carries this gas back to the lungs, where it is exhaled. When oxyhemoglobin has lost its oxygen, it becomes deep purple. This accounts for the color of the blood in the veins.

In green plants, oxygen is an important by­product of photosynthesis. In photosynthesis, carbon dioxide reacts with water in the pres­ence of chlorophyll (the green coloring matter) and light. This chemical reaction produces the sugars, starches, and cellulose that plants need to grow. During this process, plants ab­sorb carbon dioxide from the air and replace it with fresh oxygen.

Water contains about 3 per cent oxygen by volume. Aquatic animals (such as fish) and plants (such as seaweed) are able to make use of the oxygen in much the same way that ani­mals and plants on land use air. Dissolved oxy­gen also helps to break down sewage and other wastes in natural water, leaving behind

The earth's atmosphere

contains about 21 per cent (by volume) of oxygen at sea level, at the bottom of the troposphere. The amount of oxygen decreases rapidly with altitude. At orbital heights, there is virtually no oxygen at all. Astronauts have to carry their own oxy­gen supply in tanks strapped to their backs (far right). Within the strato­sphere, ultraviolet radiation (right) from the sun converts some oxygen into its allo-trope ozone (mechanism below). Ozone acts as a filter that blocks most ultraviolet radiation.

Altitude miles kilometers

Stratosphere Ozone layer

10 1

Troposphere

0 0

Earth

Oxygen

atoms O Oxygen 0₂

Most UV blocked by ozone layer

Ozone 0₃

Major groups of elements: Oxygen 51

harmless substances.

Date: 2015-12-11; view: 1882