It is the basic building block of all living things: the cell. An adult human being, for example, has around 100 trillion cells, which is 100,000,000,000,000 individual cells. In the human body there are more than 200 different cell types with the most diverse forms and tasks: red blood cells transport oxygen, neurons pass on information and skin cells build up the skin.
The size of a cell is related to its function
In order for the cell to fulfil its tasks, it needs energy. It can obtain this energy itself from nutrients with the help of oxygen. To do this, it must be supplied with these substances. The material exchange takes place at the cell membrane and depends on the surface of the cell.
The surface/volume ratio of the cell plays an important role. If a cell grows, its volume increases significantly more than its surface. Metabolically active cells are therefore usually small – and thus have a more favourable surface/volume ratio.
Within the mammalian family, the blueprints of the individual cell types are very similar because they usually perform the same tasks.
However, mammals sometimes differ considerably in size. The Etruscan shrew, for example, grows only about five centimetres, while the sperm whale can grow up to 20 metres long. Are the cells of the sperm whale significantly larger than those of the mouse?
Red blood cells must be small
Generally speaking, this cannot be said. We have chosen the red blood cells as an example because their cell size is particularly important for their function.
The red blood cells are responsible for the gas exchange in our body. In the lungs, oxygen docks with haemoglobin, a special protein in the red blood cells. The oxygen is then transported by the red blood cells via the bloodstream to all the cells of the body.
To be able to squeeze through the narrow capillaries, the red blood cells must be small and deformable. The red blood cells of mammals therefore have a special feature: Compared to the red blood cells of other vertebrates, they lack a nucleus.
To enable them to absorb oxygen efficiently, they are shaped as a round, biconcave disc. This increases their surface area in relation to their volume. As a result, they can better absorb oxygen and also release it . Due to the lack of a cell nucleus, they also consist of 90% haemoglobin.
If you compare the red blood cells of the sperm whale with those of the Etruscan shrew, you will see that their size does not differ significantly. The red blood cells of the sperm whale, which can grow to 20 metres long and weigh 70 tonnes, have a diameter of four to twelve micrometres.
Rodents, like the one to two centimetre tall Etruscan shrew, have red blood cells four to seven micrometres in size. By way of comparison, the human red blood cells have a diameter of about seven to eight micrometers.
The size of cells depends on adaptation
Sperm whales can dive more than 2,500 metres deep and for more than an hour without taking a breath. For this they need an enormous supply of oxygen. Nevertheless, the sperm whale – just because it is big itself – does not need huge red blood cells for this. It has developed other strategies to reach peak performance during its dives.
For example, sperm whales not only have twice as much haemoglobin in the same amount of blood as humans. They can also absorb significantly more oxygen by storing it in myoglobin, a hemoglobin-like protein in the muscles.
Sperm whales have about ten times as much myoglobin in their muscles as humans. In order to consume as little oxygen as possible when diving, the whales also slow down their entire metabolism. Their heart beats only four to six times per minute. At the same time, it supplies only the brain, heart and spinal cord with blood during the dive.
Small animals have faster metabolism
In stark contrast to this is the Etruscan shrew. It shares the title of the smallest mammal with the bumblebee bat. The smaller an animal is, the larger its surface area in relation to its volume and the greater the loss of heat.
In order to maintain its body temperature nevertheless, the Etruscan shrew has an extremely high metabolism: the mouse, which weighs just under two grams, consumes twice its body weight every day.
The body of the mouse is specially adapted to its high metabolism: In order for its blood to absorb a lot of oxygen, it has a higher haemoglobin content than that of other mammals.
Their heart is also twice as large as that of other animals in relation to their body weight. It beats up to 1,500 times per minute. For comparison: the drum and bass beat is 160 to 190 bpm, the heart rate is 50 to 100 beats per minute.
So the cells of large animals do not necessarily have to be larger. It is much more important that biological functions such as metabolism, respiration and body heat are optimally adapted to the animals’ different lifestyles.