You have organs and are made up of cells. Your organ systems perform certain functions in you, the entire organism. Cells have organelles that perform certain functions in the cell. Although it takes millions and millions of cells to create you, each cell functions on its own and metabolizes individually.
Holding it all together: The plasma membrane
The fluid inside a cell (intracellular fluid) is called plasma or cytoplasm (cyto- means cell). The membrane holding the fluid in the cell is called a plasma membrane, also called the cell membrane.
The cells themselves are floating in a type of fluid, called a matrix. The matrix is insoluble — substances do not dissolve in its fluid. The matrix just supports the cells. The fluid that squeezes in between each and every cell is called extracellular fluid because it is outside of the cell.
The job of the plasma membrane is to separate the chemical reactions occurring inside the cell from the chemicals that are floating in the extracellular fluid. If the plasma membrane didn’t separate the inside and the outside of the cell, waste products excreted from the inside of the cell to the outside could flow back inside.
Controlling the show: The nucleus
Every cell of every living thing has a nucleus, and every nucleus in every living thing contains genetic material. The genetic material directs the production of proteins that make the entire organism function; the nucleus makes the entire cell function.
In the nucleus of cells that are not currently dividing, clumps of thread-like genetic material called chromatin appear. Right before a cell divides, the chromatin bunches up into chromosomes, which contain DNA (deoxyribonucleic acid).
The DNA has two strands, each of which has sequences of nitrogenous bases that form the genetic code. The genetic code, which is derived from the nucleotide bases in the genes on strands of DNA, is “interpreted,” and then a ribonucleic acid (RNA) molecule called messenger RNA (mRNA) is produced from the DNA template. The mRNA uses the information from the genetic code for certain amino acids — the building blocks of protein — in the cell. The amino acids are then taken by transfer RNA (tRNA) to an organelle called a ribosome, where the final proteins are made.
Proteins either contribute to the structure of the cell, or they contribute to the function of the cell, meaning that they are used as enzymes in metabolic processes. Either way, it is the genetic material housed in the nucleus that ultimately controls the structure and function of each and every cell.
Each nucleus has a round mass inside it called a nucleolus. The nucleolus produces the third type of RNA molecule — ribosomal RNA (rRNA). This type of RNA helps to make ribosomes, which get transferred from the nucleus to the cytoplasm to help in making proteins.
Surrounding each nucleus is a double layer formed from proteins and lipids that separates the nucleus from the cytoplasm. This two-layered structure is called the nuclear envelope or nuclear membrane.
The factory of the cell: The endoplasmic reticulum
The endoplasmic reticulum (ER) is a series of canals that connects the nucleus to the cytoplasm of the cell. The part of the ER that is dotted with ribosomes is called rough ER; the part of the ER that has no ribosomes is called smooth ER.
Ribosomes on the rough ER serve as the place for the synthesis of proteins that are directed by the genes to be put together in the ER. (Other proteins are put together on ribosomes attached to other organelles or floating free in the cytoplasm.) The smooth ER contains transport vesicles that shuttle cellular products from cytoplasm to organelle, from organelle to organelle, or from organelle to plasma membrane. In addition to protein synthesis, the ER is involved in the metabolism of lipids (fats).
The main function of ER is to make and transport proteins. The ER is essentially the “womb” for new protein chains. Protein synthesis, or production, begins in the nucleus, with the mRNA molecule carrying the genetic information as to what amino acids (proteins) should be produced.
The tRNA molecules bring the amino acids from the cytoplasm to the ribosomes, which are produced by rRNA. At the ribosomes, the amino acids are joined together to form a protein, and the protein is stored in the ER until it can be moved to the Golgi apparatus.
Preparing for distribution: The Golgi apparatus
In biology, as well as other sciences, structures usually are named for the person who found them. In this case, the Italian scientist Camillo Golgi finds fame. The Golgi apparatus is very close to the ER; in the figure above, it looks like a maze with water droplets splashing off of it. The “water droplets” are transport vesicles bringing material from the ER to the Golgi apparatus.
Inside the Golgi apparatus, products produced by the cell, such as hormones or enzymes, are packaged for export to other organelles or to the outside of the cell. The Golgi apparatus surrounds the product to be secreted with a sac called a vesicle. The vesicle finds its way to the plasma membrane, where certain proteins allow a channel to be produced so that the products inside the vesicle can be secreted to the outside of the cell. Once outside the cell, the products can enter the bloodstream and be transported through the body to where they are needed.
Lysosomes really clean up
Lysosomes are special vesicles formed by the Golgi apparatus to “clean up” the cell. They are the garbage men (or sanitation engineers) of the cell. Lysosomes contain digestive enzymes, which are used to break down products that may be harmful to the cell and “spit” them back out into the extracellular fluid. Lysosomes also remove dead organelles by surrounding the dead organelle, breaking down the proteins of the dead organelle, and releasing them to reconstruct a new organelle. Because the lysosome acts upon its own cell, the process is called autodigestion.
Peroxisomes break down hydrogen peroxide
Peroxisomes are little sacs of enzymes produced by smooth ER to help protect the cell from toxic products. You know how hydrogen peroxide is helpful when you use it to clean out a wound because it kills bacteria? Well, too much hydrogen peroxide inside you could kill you. Hydrogen peroxide is normally produced in some metabolic reactions, so it is inside you. However, hydrogen peroxide becomes harmful to the cells of the body if too much accumulates, so the key is to keep breaking it down to keep it from accumulating.
The powerhouses of the cell: The mitochondria
The ER supplies the products, the Golgi apparatus distributes the products, and the mitochondria supply the energy for all of those processes to take place.
When you get a bill for electricity, the amount of electricity your household used in the past month is measured in kilowatt hours. Inside an organism, the amount of energy a cell uses is measured in molecules of adenosine triphosphate (ATP). The mitochondria produce the ATP, and to do it, mitochondria use products of glucose metabolism as fuel.
