Cell Theory, Structure, and Functions

A theory is a set of propositions describing the operation and causes of natural phenomena. Cell theory describes where the term “cell” and the other basic information about the cell came from. Cell is the basic unit of life. It is the building block of all organisms. There are organisms that are made up of only one cell called unicellular organisms. Others are composed of many cells known as multicellular organisms. But, even the life of a very huge multicellular organism begins with a single cell – a sperm cell and an egg cell.

Cell membrane separates the internal contents of the cell from its environment. It is a phospholipid bilayer with proteins embedded in it. As a selectively permeable membrane, some substances can readily pass through it while others cannot. The three major functions performed by the cell membrane are as follows: (1) separates the contents of the cell from its environment, (2) regulates the passage of materials in and out of the cell, and (3) permits communication with other cells.

The whole region within the cell membrane is the cytoplasm. Organelles are found and perform their functions in the jelly-like substance called cytosol. Various metabolic reactions like protein synthesis and breakdown of sugar into ATP take place in the cytoplasm. Other organic materials are found in the cytoplasm such as glucose and other simple sugars, polysaccharides, amino acids, nucleic acids, fatty acids, etc. Sodium, potassium, and calcium ions are also dissolved in the cytoplasm. Centrioles, cytoskeleton, and plastids are the specialized organelles in the cytoplasm.

Nucleus is the third main part of the cell. It is generally oval in shape, covered with a nuclear membrane or envelope. It is the control center of the cell that regulates and coordinates all activities of the cell. Chromosomes are found in the nucleus. Within the chromosomes are DNA that make up the gene that bears the hereditary traits.

The cell theory states that the cell is the basic unit of life. To be able to perform its function, the cell contains cytoplasm structures called organelles.

The following are the basic organelles found in the cytoplasm.

• Mitochondria are the sites for the breakdown of sugar molecules into adenosine triphosphate or ATP, which is the main source of energy in the body.

• Ribosomes are the sites of protein synthesis. The coded message from DNA as to what kind of protein is to be synthesized is carried by the mRNA from the nucleus to the ribosome.

• Endoplasmic reticulum (ER) consists of flattened sheets, sacs, and tubes of membranes that extend throughout the cytoplasm of eukaryotic cells. It is structurally continuous with the nuclear membrane, and it specializes in the transport of lipids and membrane proteins.

• A system of flattened, membrane-bound sacs that looks like stack of pancakes is the Golgi apparatus. Its function is to modify, sort, and pack macromolecules for secretion or for transport to other organelles. Rounded vesicles are associated to the Golgi apparatus that pinched off from its flattened sacs and carry the materials to other parts of the cell.

• Lysosome is also produced by the Golgi apparatus. It is a membrane-bound organelle that comes in various sizes and shapes. Lysosomes contain numerous hydrolytic or digestive enzymes for the breakdown of carbohydrates, proteins, and fats within the cell. Thus, they are known as “digestive sacs.” Destruction or repair of defective parts of the cell is another function of lysosomes.

• Another membrane-bound vesicles that contain oxidative enzymes are the peroxisomes. Enzymes responsible for oxidizing certain molecules to form hydrogen peroxide are found in peroxisomes. They are found abundantly in liver cells, where fats and lipids are metabolized. In plants, they convert fatty acids into sugars needed by growing seedlings during seed germination.

• Vacuoles are large storage sacs in cells. The smaller ones are called vesicles. In animals, some vacuoles function for storage of water or food, while others are for excretion of waste materials. Vacuoles in plants occupy up to 95% of the cell volume. Intracellular digestion, space filling, and control of cell turgor are some of the functions of vacuoles in plants. They contain water, 6 sugar, salts, and anthocyanin pigment. Toxic molecules are present that protect plants from herbivorous animals.

• Outside the nucleus of animal cells is found the centrosome. It consists of two rod-shaped centrioles which are at right angle to each other. They replicate before cell division resulting into two pairs of centrioles that later move apart becoming part of the newly formed cells. They are believed to function in the formation of spindle fibers.

• A network of interconnected protein filaments that extends throughout the cytoplasm is the cytoskeleton that provides the cell’s structural support. It is responsible for cell shape and motility.

• Plastids are large membrane-bound organelle found in plant cells. They have three classifications. The first is the chloroplast, the green-colored plastid which is caused by the green pigment called chlorophyll and is the site for photosynthesis. The second are chromoplasts, colored plastids other than green. These are specialized to synthesize and store carotenoid pigments such as red, orange, and yellow. The third are leucoplasts, the colorless plastid that contain stored food.

• Another organelle found in plant cells only is the cell wall. It protects and supports the cells of the plants.

An average adult has around 30 trillion cells in the body.

Thousands of new cells are replicated (copied) from old ones every day.

Old cells that are worn-out or damaged are replaced by the new cells.

To sustain the cellular and overall health is to provide raw materials which are obtained from the nutrients in the food eaten for the creation of new cells.

Certain nutrients also protect the cells from damage, and nutrients in foods support the body's energy production machinery.

Plant Cell

Animal Cell