Cell Definition

Cells are the basic unit of life. In the modern world, they are the smallest known world that performs all of lifes functions. All living organisms are either single cells, or are multicellular organisms composed of many cells working together.

Cells are the smallest known unit that can accomplish all of these functions. Defining characteristics that allow a cell to perform these functions include:

Below we will discuss the functions that cells must fulfill in order to facilitate life, and how they fulfill these functions.

Scientists define seven functions that must be fulfilled by a living organism. These are:

It is the biology of cells which enables living things to perform all of these functions. Below, we discuss how they make the functions of life possible.

In order to accomplish them, they must have:

The different cell types we will discuss below have different ways of accomplishing these functions.

Because of the millions of diverse species of life on Earth, which grow and change gradually over time, there are countless differences between the countless extant types of cells.

However, here we will look at the two major types of cells, and two important sub-categories of each.

Prokaryotes are the simpler and older of the two major types of cells. Prokaryotes are single-celled organisms. Bacteria and archaebacteria are examples of prokaryotic cells.

Prokaryotic cells have a cell membrane, and one or more layers of additional protection from the outside environment. Many prokaryotes have a cell membrane made of phospholipids, enclosed by a cell wall made of a rigid sugar. The cell wall may be enclosed by another thick capsule made of sugars.

Many prokaryotic cells also have cilia, tails, or other ways in which the cell can control its movement.

Prokaryote cell

These characteristics, as well as the cell wall and capsule, reflect the fact that prokaryotic cells are going it alone in the environment. They are not part of a multicellular organism, which might have whole layers of cells devoted to protecting other cells from the environment, or to creating motion.

Prokaryotic cells have a single chromosome which contains all of the cells essential hereditary material and operating instructions. This single chromosome is usually round. There is no nucleus, or any other internal membranes or organelles. The chromosome just floats in the cells cytoplasm.

Additional genetic traits and information might be contained in other gene units within the cytoplasm, called plasmids, but these are usually genes that are passed back and forth by prokaryotes though the process of horizontal gene transfer, which is when one cell gives genetic material to another. Plasmids contain non-essential DNA that the cell can live without, and which is not necessarily passed on to offspring.

When a prokaryotic cell is ready to reproduce, it makes a copy of its single chromosome. Then the cell splits in half, apportioning one copy of its chromosome and a random assortment of plasmids to each daughter cell.

There are two major types of prokaryotes known to scientists to date: archaebacteria, which are a very old lineage of life with some biochemical differences from bacteria and eukaryotes, and bacteria, sometimes called eubacteria, or true bacteria to differentiate them from archaebacteria.

Bacteria are thought to be more modern descendants of archaebacteria.

Both families have bacteria in the name because the differences between them were not understood prior to the invention of modern biochemical and genetic analysis techniques.

When scientists began to examine the biochemistry and genetics of prokaryotes in detail, they discovered these two very different groups, who probably have different relationships to eukaryotes and different evolutionary histories!

Some scientists think that eukaryotes like humans are more closely related to bacteria, since eukaryotes have similar cell membrane chemistry to bacteria. Others think that archaebacteria are more closely related to us eukaryotes, since they use similar proteins to reproduce their chromosomes.

Still others think that we might be descended from both that eukaryotic cells might have come into existence when archaebacteria started living inside of a bacterial cell, or vice versa! This would explain how we have important genetic and chemical attributes of both, and why we have multiple internal compartments such as the nucleus, chloroplasts, and mitochondria!

Eukaryotic cells are thought to be the most modern major cell type. All multicellular organisms, including you, your cat, and your houseplants, are eukaryotes. Eukaryotic cells seem to have learned to work together to create multicellular organisms, while prokaryotes seem unable to do this.

Eukaryotic cells usually have more than one chromosome, which contains large amounts of genetic information. Within the body of a multicellular organism, different genes within these chromosomes may be switched on and off, allowing for cells that have different traits and perform different functions within the same organism.

Eukaryotic cells also have one or more internal membranes, which has led scientists to the conclusion that eukaryotic cells likely evolved when one or more types of prokaryote began living in symbiotic relationships inside of other cells.

Organelles with interior membranes found in eukaryotic cells typically include:

As mentioned above, archaebacteria are a very old form of prokaryotic cells. Biologists actually put them in their own domain of life, separate from other bacteria.

Key ways in which archaebacteria differ from other bacteria include:

Archaebacterias unique chemical attributes allow them to live in extreme environments, such as superheated water, extremely salty water, and some environments which are toxic to all other life forms.

Scientists became very excited in recent years at the discovery of Lokiarchaeota a type of archaebacteria which shares many genes with eukaryotes that had never before been found in prokaryotic cells!

It is now thought that Lokiarchaeota may be our closest living relative in the prokaryotic world.

You are most likely familiar with the type of bacteria that can make you sick. Indeed, common pathogens like Streptococcus and Staphylococcus are prokaryotic bacterial cells.

But there are also many types of helpful bacteria including those that break down dead waste to turn useless materials into fertile soil, and bacteria that live in our own digestive tract and help us digest food.

Bacterial cells can commonly be found living in symbiotic relationships with multicellular organisms like ourselves, in the soil, and anywhere else thats not too extreme for them to live!

Plant cells are eukaryotic cells that are part of multicellular, photosynthetic organisms.

Plants cells have chloroplast organelles, which contain pigments that absorb photons of light and harvest the energy of those photons.

Chloroplasts have the remarkable ability to turn light energy into cellular fuel, and use this energy to take carbon dioxide from the air and turn it into sugars that can be used by living things as fuel or building material.

In addition to having chloroplasts, plant cells also typically have a cell wall made of a rigid sugars, to enable plant tissues to maintain their upright structures such as leaves, stems, and tree trunks.

Plant cells also have the usual eukaryotic organelles including a nucleus, endoplasmic reticulum, and Golgi apparatus.

For this exercise, lets look at a type of animal cell that is of great importance to you: your own liver cell.

Like all animal cells, it has mitochondria which perform cellular respiration, turning oxygen and sugar into large amounts of ATP to power cellular functions.

It also has the same organelles as most animal cells: a nucleus, endoplasmic reticulum, Golgi apparatus, etc..

But as part of a multicellular organism, your liver cell also expresses unique genes, which give it unique traits and abilities.

Liver cells in particular contain enzymes that break down many toxins, which is what allows the liver to purify your blood and break down dangerous bodily waste.

The liver cell is an excellent example of how multicellular organisms can be more efficient by having different cell types work together.

Your body could not survive without liver cells to break down certain toxins and waste products, but the liver cell itself could not survive without nerve and muscle cells that help you find food, and a digestive tract to break down that food into easily digestible sugars.

And all of these cell types contain the information to make all the other cell types! Its simply a matter of which genes are switched on or off during development.

1. Which of the following is NOT an essential function that all living things must perform?A. A living thing must reproduce.B. A living thing must be able to maintain its internal environment, regardless of external changes.C. A living thing must respond to changes in its environment.D. None of the above.

Answer to Question #1

D is correct. All of the above are essential functions of life!

2. Which of the following is NOT a type of prokaryotic cell?A. ArchaebacteriaB. Staphylococcus bacteriaC. Streptococcus bacteriaD. Liver cell

Answer to Question #2

D is correct. Liver cells are eukaryotic cells, like all cells from multicellular organisms!

3. Which of the following is NOT a eukaryotic cell organelle?A. PlasmidB. NucleusC. MitochondriaD. Chloroplast

Answer to Question #3

B is correct. Plasmids are pieces of DNA that are passed between prokaryotic cells. They are not organelles.

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Cell - Definition, Functions, Types and Examples | Biology ...