The Basic Unit of Animate Matter
Cells have been the basic unit of animate matter for billions of years. Many forms of life can be found in nature, from plants to fish to insects. While they may look different from one another, they all share some common characteristics. For example, each cell has a membrane that separates it from the outside world. It is also studded with proteins, which regulate the flow of substances in and out. This allows a cell to engulf food and permit other substances to leave.
The cell contains a nucleus, which is an organelle that holds the genetic information necessary to replicate the cell. The nucleus is also responsible for cellular reproduction. When a cell divides, its chromosomes become visible as rod-shaped structures. These chromosomes are made of protein and DNA. Nucleic acids are comprised of two major classes: ribonucleic acid and deoxyribonucleic acid. Each class of nucleic acid has a specific function. Some of these functions include catalytic, cataphoric, and informational activities.
Axons are special regions of the cell body that transmit signals to and from the central nervous system. They are usually branched, and their ends are wrapped around the axon fiber. In neurons, these branches are known as dendrites. Dendrites are often highly branched processes that provide informational locations for other neurons to communicate with the cell body. Information passes from the dendrites through the axon to the central nervous system.
Nerve cells have thin extensions that transmit signals quickly. The axon can branch repeatedly to communicate with many different target cells. The axon hillock, or initial segment, is a special region of the cell body where the axon is projected.
There are many different types of cells, including single-celled organisms, bacteria, and multicellular organisms. Most cells are either prokaryotes or eukaryotes. Prokaryotes have a cytoplasmic genome, while eukaryotes have a nuclear-encased genome. All of the organisms on Earth evolved from the cell.
Cells also contain many structural elements, such as lipids and nucleic acids. These molecules are located within the cell and are important in relaying messages, energy storage, and other functions. Plasma is the outer covering of the cell, and it is made up of proteins and lipids. Lipids act as a natural barrier that blocks water-loving substances from entering the cell. Other materials, such as oxygen and gases, can pass through the plasma membrane in a process called selective permeability.
Cytoplasm is the inner portion of the cell, and it is characterized by several major classes of intracellular organic molecules, including carbohydrates, proteins, and nucleic acids. Organelles are organs that exist within the cell, such as the lysosome. However, there are some organelles that are not located inside the cell, such as the chloroplast. The mitochondria are a well-known organelle in eukaryotes, and are believed to be present in the cell bodies of modern eukaryotes.
Chromoplasts are organelles that are present in the cell bodies of plant and animal cells. They are known to retain their own genomes, and are therefore part of the multicellular organisms.