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Life Processes: Cellular vs. Organism Level

Instructor: Bridgett Payseur

Bridgett has a PhD in microbiology and immunology and teaches college biology.

Living things are made of cells. How does a cell's life compare with a whole organism's life? This lesson will compare and contrast cells and whole organisms, and how these two different levels meet the requirements of being alive.

Living Things

You've probably heard that cells are the building blocks of life. Every living organism on Earth is made of one or more cells. This might seem strange, though, when you compare a small cell to a larger organism like an animal or plant. How do cells fit into the mold of living things?

There are several criteria that scientists use to define 'life', including the following:

  • Organization
  • Homeostasis
  • Response to stimuli
  • Energy use
  • Growth and reproduction
  • Evolutionary history

Let's look at how individual cells and whole organisms perform these different processes.

Organization

All living things are organized. Does that mean your cells have bookshelves with all their books lined up alphabetically? Of course not.

Organization in living things refers to how they're laid out. A living organism, like a person, has all their body parts in the same place. Your arms are at the side of your body, your head is at the top, and your legs at the bottom. This organization is similar in every human.

organelles

Cells are also organized, but in a different way. Your cells have organelles, small pieces that perform specific jobs. These organelles are a form of organization for the cell.

Homeostasis and Response to Stimuli

Homeostasis is a big word that means 'maintaining a constant internal environment.' Basically, your insides want to stay the same, even when the outside world changes. Homeostasis is very closely related to responding to stimuli. For example, if your body temperature goes up, you start sweating to lower it. This is both maintaining homeostasis and responding to the stimulus of a high temperature.

It's very easy to think of ways a whole organism can maintain homeostasis. Besides the temperature example above, humans need to maintain blood pressure, pH balance, and blood glucose levels. Plants can also respond to stimuli by bending toward the source of light as they grow.

What about individual cells? How can a single cell respond to a stimulus? The white blood cells in your body can follow chemical signals to find invading pathogens and destroy them. This is an important part of their job of keeping you healthy.

Energy and Metabolism

You know that you need to eat to get energy to do all the things you love doing, including playing games, hanging out with friends, and studying biology. You also get rid of the leftover waste products of your food.

Cells have no mouth, so how can they possibly eat? Well, cells can take in nutrients through a process called endocytosis (endo=into; cyto=cell). Cells convert the energy in food into a molecule called ATP, which they then use for all their cellular functions. Plant cells have an organelle called a chloroplast that can take energy from sunlight and convert it into sugar, a process called photosynthesis.

Growth and Reproduction

Animals and plants can reproduce. You've seen baby humans, puppies, and maybe even lion cubs at the zoo. Plants make seeds, which you can then plant and have whole new plants grow. Reproduction means making new individuals, and that's something you can see easily in organisms.

You know that you grow. You're not the same size you were as a baby. Your cells have grown as well, and reproduced. You have more cells in you now than when you were first born. As you get bigger, you add more cells, which have come into being by copying themselves through a process called mitosis. Cellular reproduction is definitely different than how humans reproduce, but the end result for both is having more living things.

Individual cells in your body divide to make more cells, so that you can grow bigger.
mitosis

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