Despite the many differences, human body cells have several similar features: a cell membrane, cytoplasm, cell organelles, a nucleus. Mature RBCs (red blood cells) are the exception with no nuclei.
- Cell membrane:
- Also called a plasma membrane. a cell membrane is selectively permeable and made up of phospholipids, cholesterol, and proteins.
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The proteins have several functions:
- Some form pores or openings to permit passage of materials.
- Some act as enzymes to help substances enter the cell.
- Others with oligosaccharides on their surface act as antigen markers to identify the cell as “self.”
- Others serve as receptor sites for hormones.
- Cytoplasm:
- It is a watery solution of minerals, gasses, and organic molecules that is found between the cell membrane and the nucleus. It is the site of chemical reactions.
- Cell organelles:
- These are intracellular structures that are bound by their own membranes, with each having a specific duty to perform.
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Cell organelles consist of:
- Endoplasmic reticulum (ER): a passageway for transport of materials within the cell. Rough ER has ribosomes attached to its membranes in order to synthesize secretory proteins. Smooth ER has no ribosomes attached and synthesizes lipids and carbohydrates.
- Ribosomes: tiny structures of protein and rRNA that are the sites of protein synthesis.
- Golgi apparatus: a series of flat membranous sacs that synthesize carbohydrates . They package material for secretion from the cell by breaking off some of their small sacs and fusing with the cell membrane in order to release the contained substance to the exterior of the cell.
- Mitochondria: organelles within the cytoplasm bound by a double membrane. The inner layer has folds called cristae. It is the site of ATP production (energy).
- Lysosomes: single-membraned structures within the cytoplasm that contain digestive enzymes that destroy engulfed bacteria and other cellular debris.
- Centrioles: a pair of rod-shaped structures that lie perpendicular to one another and located just outside the nucleus. Their function is to organize the spindle fibers during cell division.
- Celia and flagella: mobile thread-like projections through the cell membrane. Cilia are shorter than flagella. Their function is to sweep materials across the cell surface. The only human cell with a flagellum is the sperm cell, which provides it with motility or movement.
- Nucleus:
- A nucleolus is a small sphere made of DNA, RNA, and protein which forms a type of RNA called ribosomal RNA, which becomes part of the ribosomes (a cell organelle) involved in protein synthesis. It is found within the cytoplasm and is bound by a double-layered porous nuclear membrane and contains one or more nucleoli and the chromosomes of the cell. The nucleus is the control center of the cell because it contains the chromosomes. The forty-six chromosomes of the human cell are long threads called chromatin and made of DNA and protein.
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Updated October 2012
