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Life Science Basics

Geneva Lake - DSC 0196
(Geneva Lake, Switzerland - Alvin Wei-Cheng Wong)



Cell - A Structural and Functional Unit of Life


Cell is called structural unit because body of all the organisms is made up of cells. It is functional unit of life because all the functions of body (physiological, biochemical. genetic and other functions) are carried out by cells.

- Cells as the Basic Unit of Life

A cell is the basic unit of a living organism, consisting of a quantity of protoplasm surrounded by a cell membrane, which is able to synthesize proteins and replicate itself.

A cell is the basic building block of all organisms. A living thing can be composed of either one cell or many cells. There are two broad categories of cells: prokaryotic and eukaryotic cells. Cells can be highly specialized with specific functions and characteristics.

  • Prokaryotic cells do not contain a nucleus; lack structure; usually found in single-cell organisms; Prokaryotic cells in the domains Bacteria and Archaea that do not contain a membrane-bound nucleus or other membrane-bound organelles.
  • Eukaryotic cells contain a nucleus as well as other parts that work together; usually compose multi-cellular organisms. Eukaryotic cells have complex cells in which the genetic material is contained within membrane-bound nuclei.

- Cells as Building Blocks

A cell is the smallest unit of a living thing. A living thing, whether made of one cell (like bacteria) or many cells (like a human), is called an organism. Thus, cells are the basic building blocks of all organisms. Several cells of one kind that interconnect with each other and perform a shared function form tissues; several tissues combine to form an organ (your stomach, heart, or brain); and several organs make up an organ system (such as the digestive system, circulatory system, or nervous system). Several systems that function together form an organism (like a human being). There are many types of cells all grouped into one of two broad categories: prokaryotic and eukaryotic. For example, both animal and plant cells are classified as eukaryotic cells, whereas bacterial cells are classified as prokaryotic.

- Types of Specialized Cells

Our body has many kinds of cells, each specialized for a specific purpose. Just as a home is made from a variety of building materials, the human body is constructed from many cell types. For example, epithelial cells protect the surface of the body and cover the organs and body cavities within. Bone cells help to support and protect the body. Cells of the immune system fight invading bacteria. Additionally, blood and blood cells carry nutrients and oxygen throughout the body while removing carbon dioxide. Each of these cell types plays a vital role during the growth, development, and day-to-day maintenance of the body. In spite of their enormous variety, however, cells from all organisms - even ones as diverse as bacteria, onion, and human - share certain fundamental characteristics.

Parts of a Cell


(Simple Animal and Plant Cells - Wikipedia Commons)

Each cell has a nucleus, which contains its DNA— the genetic information for the organism that the cell is a part of. DNA is contained in units called chromosomes. 

Besides the nucleus, most cells have other parts called organelles that perform different functions within the cell. Which organelles a cell has depends on what type of organism it is part of. We’re going to focus specifically on the parts of animal and plant cells. 

Animal and plant cells have these major parts in common: 

  • Nucleus: control center of the cell; contains genetic code (DNA). The cell nucleus is a membrane-bound structure that contains a cell's hereditary information and controls its growth and reproduction. It is the command center of a eukaryotic cell and is usually the most notable cell organelle in both size and function.
  • Cytoplasm: liquid between contained within the cell membrane in which all the other parts of the cell float. Cytoplasm is a thick solution that fills each cell and is enclosed by the cell membrane. It is mainly composed of water, salts, and proteins. In eukaryotic cells, the cytoplasm includes all of the material inside the cell and outside of the nucleus. Most of the important activities of the cell occur in the cytoplasm. Cytoplasm contains molecules such as enzymes which are responsible for breaking down waste and also aid in metabolic activity. Cytoplasm is responsible for giving a cell its shape. It helps to fill out the cell and keeps organelles in their place.
  • Cell membrane: permeable barrier on the outside of the cell. The cell membrane (plasma membrane) is a thin semi-permeable membrane that surrounds the cytoplasm of a cell. Its function is to protect the integrity of the interior of the cell by allowing certain substances into the cell while keeping other substances out. It also serves as a base of attachment for the cytoskeleton in some organisms and the cell wall in others. Thus the cell membrane also serves to help support the cell and help maintain its shape.
  • Mitochondria: powerhouse of the cell; makes energy (ATP). In a cell, a long or round piece found in the cytoplasm that produces energy for the cell by breaking down food. Mitochondria are rod-shaped organelles that can be considered the power generators of the cell, converting oxygen and nutrients into adenosine triphosphate (ATP). ATP is the chemical energy "currency" of the cell that powers the cell's metabolic activities. This process is called aerobic respiration and is the reason animals breathe oxygen. Without mitochondria (singular, mitochondrion), higher animals would likely not exist because their cells would only be able to obtain energy from anaerobic respiration (in the absence of oxygen), a process much less efficient than aerobic respiration. In fact, mitochondria enable cells to produce 15 times more ATP than they could otherwise, and complex animals, like humans, need large amounts of energy in order to survive.



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