The main parts of human cell structure can be divided into the outer plasma membrane, the usually central nucleus, the cytoplasm, and the different organelles that are found in the cytoplasm. The plasma membrane is the outer boundary of the cell and controls what enters and leaves it. The nucleus has a special membrane of its own and contains the cell's deoxyribonucleic acid (DNA). The cytoplasm contains supporting structures and fluid as well as organelles that lie between the nucleus and plasma membrane. The organelles are different structures that have unique functions in the cell.
Mostly made of lipids, the plasma membrane gives human cell structure its basic shape and contains the other parts of the cell. It separates the intracellular parts inside the cell from the extracellular space outside of it. The plasma membrane regulates what enters and exits the cell through different chemical processes. It has sites on the outside, called receptors, that receive chemical messages from other cells and allow cells to communicate with each other. Human cell membranes also have unique markers that allow other cells to tell the difference between them and foreign cells like bacteria.
The cytoplasm forms the rest of human cell structure, excluding the nucleus. The fluid part of the cytoplasm contains enzymes as well as other proteins and molecules. The cytoplasm also contains a framework that gives human cell structure its basic shape. Also found in the cytoplasm are the cytoplasmic organelles, some of which are involved in making and transporting proteins or in providing energy for the cell.
The cytoplasmic organelles are responsible for performing a number of special cell functions. One example is the ribosome, which is one site where proteins are made. The mitochondria are often thought of as cellular power plants because they provide energy for the cell. The Golgi apparatus changes proteins and lipids and transports them to other parts of the cell. A lysosome is a structure that can break down lipids, proteins, and other molecules. Lysosomes are sometimes called the cell's digestive system.
In human cell structure, the nucleus is considered an organelle but has unique features and functions of its own. One of these features is a double membrane that more tightly regulates what can enter and leave the nucleus. The nucleus contains the cell's DNA, which it uses in a modified form to direct the production of proteins and all of the cell's enzymes. It also contains the parts of DNA that form chromosomes when a cell divides.