What is a living thing made up of one or more cells and able to carry on the activities of life?

Written by: Shyamala Iyer
Illustrated by: Dr. Biology

How Many Cells Are in the Human Body?

All living beings are made up of cells. Some of them are made up of only one cell and others have many cells. The average adult human body has around 37.2 trillion cells. WOW, that's a lot of cells. So many, in fact, that it's hard to picture. But let's try to imagine it: If we lined up all the cells in a human body end to end, could the line reach around the Earth? If so, how many times?

An adult human body is made up of about 37.2 trillion cells. If we were able to put all of these cells end to end, how many times do you think they would circle the Earth? Click to find the answer.

Cells got their name from an Englishman named Robert Hooke in the year 1665. He first saw and named "cells" while he was experimenting with a new instrument we now call a "microscope."

A drawing of cork seen through the microscope by Robert Hooke.

For his experiment, he cut very thin slices from cork. He looked at these slices under a microscope. He saw tiny box-like shapes. These tiny boxes reminded him of the plain small rooms that monks lived in called "cells".

If We Are Made up of Cells, What Are Cells Made From?

Look around at your house and nearby houses. They are made from smaller building materials such as wood, bricks and cement. So are the cars in the street and bike you ride. In fact, everything is made from building blocks including living things. 

What Are the Building Blocks of a Cell Like?

If you take a look at your home you will notice it is enclosed by outer walls. All cells are enclosed within something called a plasma membrane. The plasma membrane is not exactly the same thing as the wall in your house, but it does hold parts of a cell inside. These parts of the cell are what biologists call "organelles." That is a Latin name for little organs.

Sometimes people think of cells as a balloon filled with fluid. That is not really true because a balloon does not let things move in and out like the membrane of a cell. It is important for cells to be able to move materials in and out of the cell.

The plasma membrane in cells have special structures that allow water and other food materials to pass in and out of the cell. At thousands of places across its surface, the plasma membrane holds gatekeeper structures- called channels and pores. These channels allow things to move in and out of the cell. Not everything can freely pass in and out of the cell. The cells allow only those things which are necessary for them to function.

Cells are amazing. They are all made of similar building blocks, but they do many different things depending on how they are programmed. Some cells carry oxygen to parts of our body. Other cells defend against invading bacteria and viruses. There are cells that transmit signals through out the body like the signals from your eyes to your brain while reading this article. Some cells can even convert the sun's energy into food. This is called photosynthesis. There are hundreds of jobs that cells can do. Cells also make other cells in a process called cell division. That is something other building blocks cannot do.

References:

(Number of cells) Bianconi E, Piovesan A, Facchin F, Beraudi A, Casadei R, Frabetti F, Vitale L, Pelleri MC, Tassani S, Piva F, Perez-Amodio S, Strippoli P, Canaider S. Ann. An estimation of the number of cells in the human body. Retrieved March 14, 2014 from https://www.ncbi.nlm.nih.gov/pubmed/23829164.

(Diameter of cells) Freitas, Robert A., Jr.1999. Nanomedicine, Volume 1: Basic Capabilities. Section 8.5.1. Cytometrics.

Cells function differently in unicellular and multicellular organisms, but in every organism, each cell has specialized cell structures, or organelles, of which there are many. These organelles are responsible for a variety of cellular functions, such as obtaining nutrients, producing energy, and making proteins. Unicellular organisms are made up of only one cell that carries out all of the functions needed by the organism, while multicellular organisms use many different cells to function.

Unicellular organisms include bacteria, protists, and yeast. For example, a paramecium is a slipper-shaped, unicellular organism found in pond water. It takes in food from the water and digests it in organelles known as food vacuoles. Nutrients from the food travel through the cytoplasm to the surrounding organelles, helping to keep the cell, and thus the organism, functioning.

Multicellular organisms are composed of more than one cell, with groups of cells differentiating to take on specialized functions. In humans, cells differentiate early in development to become nerve cells, skin cells, muscle cells, blood cells, and other types of cells. One can easily observe the differences in these cells under a microscope. Their structure is related to their function, meaning each type of cell takes on a particular form in order to best serve its purpose. Nerve cells have appendages called dendrites and axons that connect with other nerve cells to move muscles, send signals to glands, or register sensory stimuli. Outer skin cells form flattened stacks that protect the body from the environment. Muscle cells are slender fibers that bundle together for muscle contraction.

The cells of multicellular organisms may also look different according to the organelles needed inside of the cell. For example, muscle cells have more mitochondria than most other cells so that they can readily produce energy for movement; cells of the pancreas need to produce many proteins and have more ribosomes and rough endoplasmic reticula to meet this demand. Although all cells have organelles in common, the number and types of organelles present reveal how the cell functions.

What is a living thing made up of one or more cells?

What is a living cell made up of?

What is a living thing called?

What is a living thing that can carry out life processes?