Nucleus
The nucleus can be thought of as the cell’s headquarters. There is normally one nucleus per cell, but this is not always the case, skeletal muscle cells, for instance, have two. The nucleus contains the majority of the cell’s DNA (a small amount is housed in the mitochondria, see below). The nucleus sends out messages to tell the cell to grow, divide, or die.
Plasma membrane
To ensure each cell remains separate from its neighbor, it is enveloped in a special membrane known as the plasma membrane. This membrane is predominantly made of phospholipids, which prevent water-based substances from entering the cell. The plasma membrane contains a range of receptors, which carry out a number of tasks, including being:
Gatekeepers: Some receptors allow certain molecules through and stop others.
Markers: These receptors act as name badges, informing the immune system that they are part of the organism and not a foreign invader.
Communicators: Some receptors help the cell communicate with other cells and the environment.
Fasteners: Some receptors help bind the cell to its neighbors.
Cytoplasm
The cytoplasm is the interior of the cell that surrounds the nucleus and is around 80 percent water; it includes the organelles and a jelly-like fluid called the cytosol. Many of the important reactions that take place in the cell occur in the cytoplasm.
Lysosomes and peroxisomes
Both lysosomes and peroxisomes are essentially bags of enzymes. Lysosomes contain enzymes that break down large molecules, including old parts of the cells and foreign material. Peroxisomes contain enzymes that destroy toxic materials, including peroxide.
Cytoskeleton
The cytoskeleton can be considered the scaffolding of the cell. It helps it maintain the correct shape. However, unlike regular scaffolding, the cytoskeleton is flexible; it plays a role in cell division and cell motility — the ability of some cells to move, such as sperm cells, for instance.
The cytoskeleton also helps in cell signaling through its involvement in the uptake of material from outside the cell (endocytosis) and is involved in moving materials around within the cell.
Endoplasmic reticulum
The endoplasmic reticulum (ER) processes molecules within the cell and helps transport them to their final destinations. In particular, it synthesizes, folds, modifies, and transports proteins.
The ER is made up of elongated sacs, called cisternae, held together by the cytoskeleton. There are two types: rough ER and smooth ER.
Golgi apparatus
Once molecules have been processed by the ER, they travel to the Golgi apparatus. The Golgi apparatus is sometimes considered the post office of the cell, where items are packaged and labeled. Once materials leave, they may be used within the cell or taken outside of the cell for use elsewhere.
Mitochondria
Often referred to as the powerhouse of the cell, mitochondria help turn energy from the food that we eat into energy that the cell can use — adenosine triphosphate (ATP). However, mitochondria have a number of other jobs, including calcium storage and a role in cell death (apoptosis).
Ribosomes
In the nucleus, DNA is transcribed into RNA (ribonucleic acid), a molecule similar to DNA, which carries the same message. Ribosomes read the RNA and translate it into protein by sticking together amino acids in the order defined by the RNA.
