The cell is the basic unit of life. The average human body contains over 75 trillion cells, but many life forms exist as single cells that perform all the functions necessary for independent existence. There are two types of cells, one that has no membrane-bound structures inside the cell (prokaryotic), and one that does have a membrane-bound structure inside the cell (eukaryotic). The following table illustrates the difference

Cells are the basic building blocks of all organisms. Plants and animals are made up of one or more cells. Some organisms, such as bacteria, are made up of a single cell. Other organisms are far more complex, such as your pet dog or even you.

Cells are so small that you need magnification to see them. Microscopes are generally used to observe and study cells. Using a powerful microscope, you would be able to see that cells are made up of parts that have special functions.

Cells are made up of several different parts, including an outer membrane (sometimes called the plasma membrane or cell membrane), organelles, and a large amount of mass called cytoplasm. The cell membrane is the dictator of the cell. It determines what goes into or out of the cell. Cytoplasm is a gelatin-like material that fills the cell. Cell functions are accomplished by structures called organelles. Organelles are specialized parts that move around the cell and perform functions that are necessary for life. Examples of organelles are the cell nucleus, vacuoles, and mitochondria.

The nucleus is a membrane that contains the cell's hereditary information and controls the cell's growth and reproduction. It is generally the most prominent organelle in the cell. The nucleus contains chromosomes that are made up of DNA. DNA determines the characteristics and traits of the organism, such as the color of your hair, the shape of a leaf, and so on.

Vacuoles are the storage containers of the cell. They may store waste until it is eliminated or food until it is needed. In plant cells, there are large vacuoles that hold water.

Energy for the cell is produced by mitochondria through a process called respiration. Respiration is a series of chemical reactions that combine food and oxygen to create energy and a waste by-product, carbon dioxide.

Plant cells have some additional components. They have a cell wall that gives the cell a firmer shape and support. The cell wall is made up of cellulose, which is not digestible by humans, but provides fiber for our good health. Plant cells also have organelles called chloroplasts; these contain chlorophyll, which uses the process of photosynthesis to make food for the plant cells. The chloroplasts are green; they are what give plants their green color. During photosynthesis, chloroplasts interact with light energy, combining carbon dioxide from the air with water to make food. With light, the green parts of plants produce a sugar called glucose (C6H12O6) and oxygen (O2) from carbon dioxide (CO2) and water (H2O).


Cell and Species Reproduction Species of plants and animals cannot continue without reproduction. All cells have hereditary material that is passed on to the next generation. This hereditary material is called DNA, or deoxyribonucleic acid. DNA determines how one looks and how one functions. DNA is the instruction manual or blueprint for life.

The DNA molecule consists of two long strands that form a double helix. It spirals around like a twisted ladder. The DNA molecule has a sugar component, a phosphate component, and four different basesadenine (A), thymine (T), cytosine (C), and guanine (G). Adenine is always paired with thymine, and cytosine with guanine. These pairs are held together to form the rungs of the ladder.


If a cell has a nucleus, the DNA is found in chromosomes. Chromosomes are duplicated before cell division takes place. The DNA unzips and separates. New bases are formed, and they link up in the proper order to form two new DNA helixes that are exactly alike. The instructions for the appearance and function of the new organism are contained in units called genes, which are parts of the DNA.

There are two reasons for cell division. One is to replace old or worn-out cells, and the other is for sexual reproduction. The process of mitosis takes place when cells are replacing themselves because they are old or worn-out. The process called meiosis takes place with sex cell formation. In sex cell formation, cells with only 23 chromosomes are formed so that when an egg and a sperm join up, they have the full set of 46 chromosomes and possess the genetic make-up of both parents.

The chart indicates the difference between the processes.

Organisms reproduce in different ways: sexual reproduction between a sperm and an egg or asexual reproduction.

Some plants and animals reproduce asexually. Some examples are bacteria, the hydra, and the eye of a potato, which can grow new potatoes.

Some plants reproduce sexually. Examples are flowers, which have male and female parts. When the sperm and egg join, they produce seeds, which, when planted, reproduce the species.