Molecular & Cell Biology For Dummies
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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.

Defining cells of living creatures

What exactly are cells? Cells are sacs of fluid surrounded by cell 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.

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