Anything that is alive is called an organism. Some organisms are only one cell. Other living things - like humans - are made of trillions of cells. How is this possible? Millions of years ago, most organisms were just one cell. Over time, cells joined together to make bigger and bigger organisms like plants and animals.
Millions of years ago, Life consisted mostly of one-celled organisms.
All living things need to find food and use it for energy. Very small organisms like bacteria live very close to their food because they can't move around very far. Bigger organisms - like humans - need more energy but can move farther to get it.
But, when an organism takes food into itself, it puts itself in danger of taking in germs, poisons or other harmful things. So, organisms are in a constant balancing act of getting food for energy and protecting themselves from things that might hurt them. They balance these things using metabolism to get food and the immune system to keep them safe!
Metabolism
All organisms, no matter how big or small, need energy to live. Metabolism is how we get energy from food. Metabolism means eating food, breaking it down into nutrients and using these nutrients to build new cell parts. Breaking down food gives us energy. Every cell needs to be able to generate energy to live.
Immune System
The immune system is not a simple organ with a single function like the heart, which pumps blood. It is a combination of cells and tissues that work together to identify what belongs to the body (the self) and things that may be harmful - like old, dead cells or outsider particles such as bacteria and viruses. Our immune systems have to be able deal with all of the types of germs in the world.
Metabolism and the immune system have something in common: they both swallow other particles! Metabolism swallows things for energy, and the immune system swallows particles to keep us safe! Discover the role of the swallowing cells in both >>
The simplest life forms are made of a single cell, and the most complex life forms are made of trillions of cells. Millions of years ago, life consisted mostly of one-celled organisms, but over time a myriad of bigger and more complex organisms emerged. No matter how large an organism is, it needs to be able to find food and use it for energy. Single-celled organisms like bacteria usually live very close to their food source. Though they can move around, they can't move very far. More complex organisms - like humans - need more energy but are also more able to move and interact with their environment. They have evolved larger muscles and limbs that allow them to move greater distances with greater control, which makes it easier for them to seek food and comfort.
Millions of years ago, Life consisted mostly of one-celled organisms.
But, when an organism takes food into itself, it puts itself in danger of taking in germs, poisons or other harmful things. As a result, organisms are in a constant balancing act of getting food for energy and protecting themselves from things that might disrupt or infect their cells or bodies. These two interdependent needs gave rise to the most fundamental processes in our bodies: metabolism and the immune system!
Metabolism
All organisms, no matter how big or small, and all the cells that compose them, need energy to survive. Metabolism is the process through which a life form gets energy from its environment and uses it to live. Metabolism includes eating food, breaking it down into nutrients and using these nutrients to build new cell parts. Breaking down food gives us energy. Every cell needs to be able to generate energy to live.
The mitochondria (animals) or chloroplasts (plants) are the parts of the cell that generate energy from food. Mitchondria generate energy from sugars that we eat, while chloroplasts generate energy from sunlight to make sugars. In humans, cells absorb sugars, proteins and fats that have been digested in the stomach and intestine and break them down to produce energy. For a cell, energy is in the form of a molecule called ATP. ATP can travel through the cell and provide energy anywhere the cell needs it.
Immune System
The immune system is not a simple organ with a single function like the heart, which pumps blood. It is a combination of cells and tissues that work together to identify what belongs to the body (the self) and things that may be harmful - like old, dead cells, or outsider particles such as bacteria and viruses. Our immune systems have to be both sensitive and diverse to be able deal with the magnificent variety of unknowns in the world.
Metabolism and the immune system have something in common: somehow they must swallow other particles - one for energy, one to stop a harmful invader. Discover the role of the swallowing cells in both >>
In fact, scientists have found evidence that mitochondria and chloroplasts used to be single celled organisms that teamed up with larger organisms in a symbiotic relationship! The single-celled organisms were provided safety and a constant food supply, while the larger organism gained a nearly endless supply of ATP.
