Structure of cells:

The best way to describe a cell is by way of comparison with something familiar. This can be tricky because cells are somewhat unique. The redeeming thing here is once you familiarize yourself with a cell, then it won’t matter that a human is made of billions of cells because although they might be different in what they do, they will have the same basic components (like how human beings are; we all have the same basic structure but have different personalities and jobs).

The below are a list of the basic structures inside a cell and have been briefly described but discussed with further detail in subsequent chapters. And that is almost all that you need to know in cell biology.

A Typical Animal Cell

1-    Plasma membrane: Similar to the skin, it protects the cell from the outside environment. Most importantly, it regulates movements of water, nutrients, and wastes in and out of the cell. It is also called Cell Membrane.


2-    Cytosol: Semi-fluid medium that is made of water with micromolecules dissolved in it. Parts of the cytosol are made by the cell and partly have defused in from outside. Think of curry soup which was originally water but when mixed with chicken, vegetables and spices and got transformed. Even if we take away the chicken and the other ingredients, the water will no longer be just water, but a concoction of various ingredients dissolved in it. The cytosol along with the organelles of the cell is referred to as the cytoplasm.



3-    Nucleus: It is usually at the center of the cell and plays an important role. It contains chromatin (DNA chain) which is the code for synthesizing protein.  (What are proteins and why are they important will be discussed in the next chapter. For now, proteins are responsible for most of the functions in the cell). Moreover, nucleolus is concentrated chromatin. Every single cell has all the genetic information (genome) and therefore the ability to produce all the proteins in the body. However, cells usually specialize and only produce a particular set of proteins depending on their location and the requirements. Example; when the nucleus of an undifferentiated cell starts synthesizing muscle protein (because of certain signals it has received), the cell becomes a specialized muscle cell and remains that way.


4-    Ribosome: Are proteins on which proteins are made. Similar to an anvil (iron block used to mold metal).

5-    Rough Endoplasmic Reticulum (RER): An organelle that looks like an interconnected network of membranes contains many ribosomes (giving it a rough and studded appearance) on which large and complicated proteins can be synthesized.

6-    Smooth Endoplasmic Reticulum (SER): Similar structure to an RER but does not contain ribosome and are responsible for making lipids (another micromolecule in which we will talk about in the next chapter).


7-    Golgi Apparatus: Also known as Golgi complex, is a group of membranous sacks that are responsible for modifying, storing and transporting products from the endoplasmic reticulum (both proteins and lipids), to other parts and organelles of the cell or and even outside it- similar to how workers in a factory regulate their products and ship them.


8-    Mitochondrion: Is the energy power house, the electricity generator (more on it in chapter 5). Many mitochondrion is referred to as mitochondria.


9-    Lysosome: Are vesicles that contain enzymes (proteins whose function is to catalyze reactions) that digest nutrient molecules and wastes. It is basically the stomach of the cell.

10-  Peroxisome: Also vesicles that contain enzymes but whose main function is to produce and hydrolyze the H2O2 (hydrogen peroxide) of the cell.

11-  Cytoskeleton: The mechanical frame-work of the cell, similar to the skeleton of the human body. It is made of different proteins (microtubules, microfilaments and intermediate filaments).

12-  Centrioles: Are proteins responsible for synthesizing the spindles (made of microtubules) which pull DNA apart during cell division. It is usually located close to the nucleus.

13-  Endosomes: Are transport vesicles made by the inner folding of the plasma membrane (invagination until ultimately a separate internal compartment is created, the process is called endocytosis) to accommodate molecules for transferring to lysosomes or to different parts of the cell.

Again, the entire course on cell biology will be spent discussing the structure and functions of these organelles in detail so you do not need to memorize anything now, just read and understand. Excluded are occasional structures such as cilia, flagella and other secretory vesicles which are sometimes found and will be discussed later when the need arises.