Biology is the study of life occurring at many different levels. In the broadest of terms, life occurs in an ecosystem made up of all organisms living in a certain area or environment, such as on a savanna or in a rain forest. All organisms within that ecosystem are a community, with different groups interacting with one another. These communities are made up of particular species, such as zebras or bees. Collectively, they are called populations.
Populations are further broken down into organisms, like one zebra, or a human being. Then within that organism, life occurs at the organ system level, such as the cardiovascular system. Systems are further broken down into individual organs, organs into tissues and finally, tissues into individual cells. Within the individual cell, life occurs at the chemical level.
What Is a Cell?
Cells are the basic units of all life on Earth. They are responsible for the intake and conversion of nutrients, repair and reproduction. More importantly, cells house genetic instructions for the entire organism. Some organisms are unicellular, consisting of a single cell, while others are multicellular and made up of many cells, such as the human body.
Significance
On the planet Earth, there are two major types of cells: prokaryotic and eukaryotic. The cells of bacteria and archea (blue-green algae) are prokaryotic. All the rest of Earth's organisms such as plants, fungi and animals (including humans) are made up of eukaryotic cells. Protists such as the amoeba and protozoans also belong to the eukaryotic cell category.
The chief difference between the two categories of cells is that prokaryotic cells lack internal structures that are enclosed by a plasma membrane, and they do not have a true nucleus. A true nucleus contains the genetic information (DNA) for the cell and is surrounded by a membrane, separating it from the rest of the cell. Prokaryotic cells have a "nucleoid region" only.
Cells of eukaryotic organisms are 10 times larger than those of prokaryotes, and are capable of containing 1,000 times the volume.
Cell Structure
Organelles, the major structures found within human cells, include the nucleus, endoplasmic reticulum (ER), mitochondrion, Golgi apparatus, plasma membrane and centrioles. Organelles are suspended within cytosol, a thin transparent fluid, and combine to make up the cell's "cytoplasm."
The plasma membrane is the cell's outer covering, a very thin film serving as the protective outer boundary separating and enclosing inner cell parts from the exterior environment. Its principal function is to regulate and control movement of substances into and out of the cell. Small hair-like structures may be attached to the plasma membrane; cilia or flagella are responsible for the cell's movement. The nucleus, protected by a double-layered membrane, is the largest organelle within the cell. It is spherical in shape and contains deoxyribonucleic acid, or DNA. The nucleus also contains the nucleolus and chromosomes. Commonly called "the powerhouse of the cell," the mitochondrion is where nutrients are converted into energy the cell can use.
The endoplasmic reticulum, or ER, is a membranous network extending through a large portion of the cytoplasm. ER's role is in the synthesis of lipids (fats) and modifying proteins. Some parts of ER have tiny, spherical organelles attached to its outer surface, called ribosomes, which are the sites of protein synthesis. ER with ribosomes is called rough ER. Playing a significant role in cell division, centrioles are cylindrical in shape and located near the nucleus.
Cell Types
In the human body, cells are grouped into four basic categories: epithelial, muscular, nerve and connective.
Epithelial cells are the protective covering surrounding all organs, glands and skin. They line hollow organs, such as the esophagus, colon and lung passageways.
Muscle cells group together to become the individual muscles of the body. They contain an abundance of microfilaments which themselves contain actin and myosin. Actin and myosin make it possible for muscle cells to contract in order to produce muscle movement.
Nerve cells, or neurons, are highly excitable as they have axons and dendrites. Axons and dendrites are specialized appendages extending from the cell and are responsible for carrying electrochemical messages both to and from each cell. Neurons differ from other cells in that they are not replaced when they die.
The most widely distributed kind of cell, connective tissue cells vary in shape, size and structure. They are found in cartilage, bone, fat, blood and lymph fluid.
Cancer
Cancer refers to a category of some 100 diseases in which abnormal cells grow uncontrollably, moving into other tissues and organs. The type of cancer is typically determined by the kind of cell in which growth initiates. Cancer beginning in epithelial cell tissues are referred to as carcinomas, and those affecting connective tissue cells are sarcomas. When abnormal cell growth begins in neurons (brain or spinal cord), it is a central nervous system cancer.
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