Structures of the Cell Cytoplasm


Microtubles are long, straight, hollow structures formed by lots of protein subunits called tubulin proteins. They function as sort of a railroad track system within cells. This allows for the transport of various structures from one part of the cytoplasm to another. During cell division, two sets of chromosomes ride along this railroad track system to reach opposite ends of the cell.

Rough Endoplasmic Reticulum (rER)

Microtubules pass through numerous other structures in the cytoplasm. One structure—a mass of flattened membranes called the rough endoplasmic reticulum (rER)—is specialized to make proteins. Tiny, dark-staining machines called ribosomes are embedded in the membranes of the rER. These ribosomes use the information on messenger RNA to make proteins. Once the proteins have been made, they are packaged in hollow bubbles called vesicles. These protein-filled vesicles also ride along microtubules to reach another stack of membranes called the Golgi apparatus. Proteins are modified within the Golgi apparatus to achieve their final form, and then ride in another vesicle to their final destinations. They can be inserted into the cell membrane, or else can be exported into the watery environment around the cell.


Mitochondria are small, oval-shaped structures that generate energy for the cell. They do this by slowly combining nutrient molecules (fats, sugars) with oxygen; in other words, they “burn” these molecules. When a portion of the cell runs out of energy, mitochondria travel on microtubules to this part of a cell. To provide energy, mitochondria produce an energy-rich molecule called ATP, which powers many of the processes carried out by cell machinery.

Cell Membrane

The cell membrane is a thin film of oil and proteins. While it is an excellent water-proof barrier, it is not very strong and is always in danger of being punctured, like a soap bubble. To prevent this, filaments of a protein called actin are placed just beneath the cell membrane. Actin filaments form a thick, rubbery mat called a gel that reinforces the cell membrane. Also, other proteins called spectrin filaments form an interconnected meshwork on the inside of the cell membrane. If the function of these filaments is altered, this can produce dramatic changes in the overall shape of the cell.

Proteins in the cell membrane can have numerous functions. Some of these proteins form hollow, barrel-shaped structures called transporters. These allow for the entry or exit of small molecules like sugars or water from the cell. Other proteins called receptor proteins react to the presence of signaling molecules like hormones that attach to the outside of the cell membrane.

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Written by John Young

John K. Young is a retired professor of Cell Biology. He worked in the Department of Anatomy at Howard University College of Medicine in Washington, DC for 35 years, teaching his students about cells. During his career, Dr. Young published scientific articles about a part of the brain called the hypothalamus and also wrote a number of books about cells and about the brain.

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