In this lesson, you will learn about the 12 structural components that make up a living cell as well as the function each one of them performs.
Cell organization is the components that make up the cell and how they are arranged inside it. Each component is called an organelle and performs a specific function that is vital for the cell.
Cells, the basic unit of life, are of two types:
Prokaryotic cells, do not contain a nucleus. They include bacteria. These cells are small and therefore have a larger surface area to volume ratio. Therefore, nutrients can reach any part of the cell more easily.
Eukaryotic cells contain a nucleus. They include those of fungi, protozoa, algae, plants, and animals. These cells are larger and therefore have a smaller surface area to volume ratio. The diffusion of nutrients in cells is therefore not easy. Therefore, eukaryotic cells require specialized internal organelles to perform metabolism, transport chemicals, and generate energy in the cell.
Cells have different shapes and sizes. A cell may be divided into two parts: the cell wall and the cell membrane and the protoplasm. The protoplasm is further divided into the cytoplasm (all the protoplasm apart from the nucleus) and the nucleoplasm (nucleus, DNA and RNA).
It is made up of a phospholipid bilayer that creates a rigid barrier between the inside of the cell and its outside environment. The phospholipid bilayer is composed of two layers of phospholipids with one layer having hydrophilic (water-loving) heads on its outer side and hydrophobic (water-hating) tail interior side. The proteins found in the bilipid layer performs selective transport of molecules and recognition of cells.
Not all living things have cell walls.
The cell wall is located outside the plasma membrane. Plasmodesmata are connections through which cells communicate chemically with each other through their thick walls. Fungi and many protists have cell walls although they do not contain cellulose, rather a variety of chemicals like chitin for fungi.
In eukaryotic organisms, the nucleus is known to be the control center of the cell. It houses the cell’s genetic material and it is where the processes such as DNA replication, transcription, and the processing of RNA take place. In prokaryotes, there is no nucleus but instead, they have a single chromosome: a piece of circular, double-stranded DNA located in an area of the cell called the nucleoid.
The nucleus is the largest organelle in the cell and contains all the cell’s genetic information in the form of DNA. The presence of a nucleus is the primary factor that distinguishes eukaryotes from prokaryotes. The structure of the nucleus is described below:
It is the gel-like, water-based fluid that occupies the majority of the volume of the cell. The cytoplasm is composed mainly of water but also contains enzymes, salts, organelles, and various organic molecules. The cytoplasm is the site of almost all of the chemical activity occurring in a eukaryotic cell. In eukaryotic cells, cytoplasm refers to the contents of the cell with the exception of the nucleus. The part of the cytoplasm that does not contain any organelles is referred to as the 'cytosol'. The cytoplasm is responsible for giving the cell its shape.
They are autonomous bodies containing their own DNA. It is the double-membrane organelle where the process of cellular respiration takes place. They function as the sites of energy discharge and ATP formation. They act like a digestive system that takes in nutrients, breaks them down, and creates energy-rich molecules for the cell. Mitochondria have been labeled as the powerhouse of the cell. In animals, as the muscle cells need much energy for locomotion, they contain the greatest number of mitochondria.
The endoplasmic reticulum (ER) is an organelle found in eukaryotic cells only. The ER has a double membrane consisting of a network of hollow tubes, flattened sheets, and round sacs. These flattened, hollow folds and sacs are called cisternae. The ER is located in the cytoplasm and is connected to the nuclear envelope. There are two types of endoplasmic reticulum:
Ribosomes are composed of RNA and protein. They occur in the cytoplasm and are the sites where protein synthesis occurs. Ribosomes may occur singly in the cytoplasm or in groups or may be attached to the endoplasmic reticulum thus forming the rough endoplasmic. Ribosomes are important for protein production. Together with a structure known as messenger RNA (a type of nucleic acid) ribosomes form a structure known as polyribosome which is important in protein synthesis.
The Golgi body is also known as the Golgi apparatus or Golgi complex. The Golgi body is found in all plants and animal cells and is the term given to groups of flattened disc-like structures located close to the endoplasmic reticulum. The number of ‘Golgi apparatus’ within a cell is variable. Animal cells tend to have fewer and larger Golgi apparatus. Plant cells contain as many as several hundred smaller versions. The Golgi apparatus receives proteins and lipids (fats) from the rough endoplasmic reticulum. It modifies some of them and sorts, concentrates and packs them into sealed droplets called vesicles. Depending on the contents, these are dispatched to one of three destinations:
For this reason, the Golgi body can be considered to be the ‘post office’ of the cell.
These are single-membrane organelles that are essentially part of the outside that is located within the cell. The single membrane is known in plant cells as a 'tonoplast'. Many organisms will use vacuoles as storage areas. Vesicles are much smaller than vacuoles and function in transporting materials both within and to the outside of the cell.
Plastids are organelles found only in plants. There are three different types:
Animal cells contain a special organelle called a centriole. It is a cylindrical tube-like structure that is composed of microtubules arranged in a very particular pattern. Two centrioles arranged perpendicular to each other are referred to as a centrosome. The centrosome plays a very important role in cell division. The centrioles are responsible for organizing the microtubules that position the chromosomes in the correct location during cell division.
| Plant cell | Animal cell |
| Have plastids | Do not have plastids |
| Have a cell wall (made of cellulose) | Do not have a cell wall |
| Have a large, central vacuole | Have small, temporary vacuoles |
| May have plasmodesmata | Do not have plasmodesmata |
| Do not have centrioles | Have paired centrioles within the centrosome |
| Do not have cholesterol in the cell membrane | Have cholesterol in the cell membrane |
| Generally have fixed, regular shape | Generally, have an amorphous shape |
| Stores excess glucose as starch | Stores excess glucose as glycogen |
