Cells
make up the smallest level of a living organism such as yourself and other
living things. The cellular level of an organism is where the metabolic
processes occur that keep the organism alive. That is why the cell is
called the fundamental unit of life.
What
exactly are cells?
Cells are sacs of fluid surrounded by membranes. Inside the fluid
float chemicals and organelles. An organism contains parts that are
smaller than a cell, but the cell is the smallest part of the organism that
retains characteristics of the entire organism. For example, a cell can take in
fuel, convert it to energy, and eliminate wastes, just like the organism as a
whole can. But, the structures inside the cell cannot perform these functions
on their own, so the cell is considered the lowest level.
Each
cell is capable of converting fuel to useable energy. Therefore, cells not only
make up living things; they are living things. Cells are found in all
plants, animals, and bacteria. Many of the basic structures found inside all
types of cells, as well as the way those structures work, fundamentally are
very similar, so the cell is said to be the fundamental unit of life.
The
most important characteristic of a cell is that it can reproduce by dividing.
If cells did not reproduce, you or any other living thing would not continue to
live. Cell division is the process by which cells duplicate and
replace themselves. If you did not replace your red blood cells, for example,
you would have a life span only as long as that of red blood cells — a mere 120
days.
Increasingly
more complex organisms are made up of increasingly more groups of cells (for
example, in humans, groups of cells make up each organ and muscle tissue), and
the organisms survive based on products that the cells make. For example, cells
in the pancreas make insulin, which is necessary to ensure that the blood
glucose level doesn’t skyrocket. Without insulin, the blood glucose can reach a
level that is lethal. So, without that cellular product, you would die.
Examining
eukaryotes and prokaryotes:
Cells
fall into two major categories: eukaryotes and prokaryotes.
Eukaryotes are organisms that contain chromosomes, including plants
and animals, as well as fungi (like mushrooms), protozoa, and most algae.
Eukaryotes have the following characteristics:
- They have a nucleus that stores their genetic information.
- Animal cells have an organelle called a mitochondria that effectively combines oxygen and food to convert energy to a useable form.
- Plant cells have chloroplasts, which use energy from sunlight to create food for the plant.
- Eukaryotic cells have internal membranes, which create compartments inside the cells that have different functions.
- Plants cells have a cell membrane and a cell wall, which is rigid; animal cells have only a cell membrane, which is soft.
- The cytoskeleton, which reinforces the cytoplasm of the cell, controls cellular movements.
Prokaryotes are cellular organisms that do not have a “true” nucleus. A
nucleus is the control center of a cell. A nucleus contains the genetic
material packed into chromosomes, and it is associated with other organelles
that function in the production of amino acids and proteins based on what the
genetic material dictates. Prokaryotes have some genetic material, but it is
not as well organized as it is in eukaryotes. Still, prokaryotes are able to
reproduce. Examples of these organisms include bacteria and blue-green algae.
Source:
Dummies.com