Cell Organelles- Structure

A cell is the building block of all that sustains here on earth. The Latin meaning of cell is ‘a small room’. It is the basic functional and structural unit of life. The cells make up the tissues that make other organs and muscles of the body. Usually, the plant and animal cells are visible only under a microscope. However, there are the egg cells that are big as the yolk of the Ostrich egg (largest cell).

Every cell is made up of many  cell organelles. Like in the body, the various systems that run the body, comprise of organs. Similarly, the cell is composed of many small organs called the cell organelles. These organelles are suspended in the cell cytoplasm, which is enveloped within a cell membrane. There are other components, such as nucleic acids and biomolecules. 

The cell organelle- structure is one of the basic functions that a cell performs; it provides a structure to the body. The cell organelles also perform various functions. There are many organelles that are common for both plant and animal cells. These organelles are the nucleus, the cell membrane, and the cytoplasm. Understanding the cell organelle- structure becomes all the more important in order to identify these organelles and finally analyse their function.

Structure of Cell Organelles

Cell membrane

The cell membrane is common for both plants and animals. It varies in shapes for the different types of animal cells. The cell membrane is composed of two types of protein called the peripheral proteins and the embedded proteins and Bilayer of phospholipids, which have their hydrophobic fatty acid tails in contact with each other. It is selectively permeable.

Cell Wall

Plants have an additional cell wall that helps keep the structure of the cell intact. The cell wall is made up of cellulose and hemicellulose. It is rigid, semi permeable and non living. It has the following parts:

•     Primary cell wall : It is the first formed cell wall and is situated closest to the inside of the cell. As it is made up of cellulose so it can stretch to accommodate growth.
•     Middle lamella : It is the outermost layer and it holds the neighbouring cells together. It is primarily made of pectins.
•     Secondary cell wall : It is formed inside the primary wall after the cell has fully grown. It is the thickest layer of a cell and provides its characteristic shape.

Centriole

Centriole has a tube-like structure, with the walls that are made of nine triple microtubes (one tubule is made of three microtubules). Tubules form a Y-shape to help the centriole keep its balance.  Another structure called a cartwheel is present inside a centriole. The cartwheel has nine spokes that are connected together at the ends, called the pinhead.

Cilia and Flagella

Cilia and flagella are tiny projections that are hair-like in structure. Two single microtubules are surrounded by nine microtubules on the outside. Their structure comprises of three parts

• Basal body
• Hook
• Axoneme

Chloroplast

The chloroplast is a unique organelle with its very own DNA and 70s ribosomes. This structure is round, oval or disc-like and it has a different cytoplasm, like a matrix called the stroma. Inside the stroma are present disc like thylakoids. These thylakoids stack one upon another and form grana. It is enveloped inside a double membrane, which is porous in structure.

Cytoplasm

The cytoplasm is the colourless matric inside the cell wall. It houses everything other than the nucleus. It is 80 percent water, and the remaining is composed of nutrients. 

Cytoskeleton

The cytoskeleton is a network of three types of filaments that give the cell its shape. These filaments are made up of protein of the three types.

Endoplasmic Reticulum (ER)

The endoplasmic reticulum is present near the nucleus. On the basis of the attachment of ribosomes on the surface of the endoplasmic reticulum, these are categorised into two categories, the rough ER and the smooth ER.  They are in three forms – vesicles, tubules and cisternae. ER is responsible for formation of proteins( rough ER), lipids and steroids(smooth ER).

Plastids

There are three types of plastids present in plants and algae. These help in the pigmentation of the cell. These pigments help in the process of photosynthesis. The three main types of plastids are leucoplasts and chloroplasts and chromoplasts.

Ribosomes

The ribosomal protein and ribosomal RNA are the components of ribosomes. They are mainly made up of RNA. Each ribosome consists of large and small subunits. Each subunit had more RNA molecules and ribosomal proteins attached to it. This is a cell organelle without a cell membrane. It is also called protein factory of cell as it is responsible for protein synthesis.

Golgi apparatus

The structure of the Golgi apparatus is made up of

•     Cisternae: Parallel bundles of flat sac-like structures, these are the smallest part of the Golgi apparatus.
•     Tubules: These are tubular structures growing from the cisternae
•     Vesicles: Spherical bodies. 

Endosomes

These organelles originate from the Golgi bodies.

•     Early endosomes: have a tubular-vascular structure
•     Late endosomes: not tubular but are made up of closely packed vascular structures.

Lysozyme

Mostly spherical or granular in structure but can be irregular in shape. Surrounded by a membrane that encases the enzymes of lysosomes.

Microfilaments

Made up of filaments that are either constituent of some network or bundled together. 

Mitochondria

Made up of double membranes. The outer membrane is a smooth case, and the inner membrane is folded into finger-like projections. Inside the membranes is a mesh-like structure that contains enzymes required for metabolic processes. Single or double-stranded DNA is also present as a component of mitochondria.

Nucleus

Three main structures constitute the nucleus.

•     Nuclear envelope: Much the same as a cell membrane. Keeps the nuclear matter and nucleolus inside the nucleus. It lets proteins and RNA move in and out of the nucleus.
•     Chromatin material: Made up of compared DNA strands.
•     Nucleolus: It is like a nucleus in the structure; that is, it is spherical. But it does not have a membrane. So the organelle without a cell membrane is a nucleolus.

It controls and regulates activities of cell.

Vacuole

A vacuole is part of a plant cell and functions as an organelle. It is made up of a membrane called tonoplast, inside which are substances like water and nutrients. They resemble vesicles structurally. 

Vesicles

These are organelles bound by a lipid bilayer that encloses a liquid or cytosol. The membrane is very similar to the plasma membrane since it is also made up of lipids.

Microtubules

These are strands of long, hollow beads. Their diameter is about 24nm. One of their ends is positively charged, while the other is negatively charged.

Microvilli

Microvilli are tiny finger-like growths on the surface of the cell. The main component of these structures is actin and fibrin, epsin, and villin bind these structures.  The plasma membrane surrounds them.

Plasmodesmata

These are the passages that allow the exchange of material in and out of the cell. As such, their plasma membrane extends into the plasma membrane of the cell, and it too has phospholipids; the desmotubules are an extension of the endoplasmic reticulum and allow the exchange of particular molecules. The cytosol extends into the cytoplasmic sleeve. This, too, allows for the exchange of material. 

Conclusion

The body functions because the different organs perform various functions in the body. From metabolism to growth to the regulation of each tissue, every part of the body has been made in such a way that the whole body can function as one unit. Similarly, cell organelles play the same role inside cells. They perform various life functions because of which the cell can work.