cellular organization

Cellular Organisation

You know that in order to build a house, bricks are arranged in a certain pattern. Similarly, our body is made up of similar structures called  cells,  which assemble to form our body. Hence, cells are the structural units of our body.

But who discovered and coined the term ‘cells’? Let us explore.

The discovery of cells was first made by Robert Hooke. While examining a section of a cork tree under the microscope, he observed small compartment-like structures and named them cell.

With the discovery of advanced microscopes (like electron microscopes), the study of the structure of cells and various cell organelles was made possible.

Microscope

What is a microscope? 

A microscope is an instrument used to see the objects that are not visible to the naked eye. It magnifies the object several hundred times and makes it clearly visible.

 

Parts of Microscope

 

The important parts of the microscope are:

 

• Eye piece: It is the lens through which we see and is located at the top. It is usually 10x and 15x.

 

• Objective lens:It is the lens that is facing the object. They are usually 10x, 40x or 100x.

 

• Mechanical stage: It is the flat platform where slides are kept. It has clips for holding slide and moving it left, right and up or down.

 

• Mirror: It is located near the base and directs the light through the microscope. The light makes the objects visible.

 

 

 

Magnification of Compound Microscope:

Do you know how to find the magnification of a compound microscope ? Let us find out with the help of an example

In order to find the total magnification while observing image with the help of compound microscope, the power of eye piece which is typically 10x or 15x is multiplied by the power of objective lens which is at 10x, 40x or 100x.
 

Therefore, total magnification = Power of eye piece used  X  Power of the Objective lens used.

Cells: An Overview

Living organisms are made up of tiny structures called cells. Cells are the building blocks of life. They are the basic structural and functional units from which life takes shape. A cell is the smallest living entity in a living organism.

How cells are formed

In 1838 & 1839, the two German scientists Matthias Schleiden (1838) and Theodor Schwann (1839) formulated that all plant and animal tissues are made up of cells. They, however, were unsuccessful in explaining how new cells are formed. Later, in 1855, Rudolf Virchow proposed a cell theory which stated that: 

• All living organisms are composed of cells and products of cells.

• Cells are the basic units of structure and function in an organism.

• All cells arise from pre-existing cells.

Properties of Living Cells 

Some important properties of living cells are as follows:

• A cell is so small that it is not visible to the naked eye. 

• Cell shape and size vary both within an organism and between different organisms. The shape and size of a cell is related to the specific function it performs.

• All living cells exhibit certain basic properties like respiration, growth and metabolism. 

• Nerve cells are some of the longest cells.

• Cells originate from a pre-existing cell. A mother cell divides to produce daughter cells. Hence, cells exhibit cell division.

Examples of cells with different shapes and sizes

• The smallest unicellular organism we know is the Mycoplasma, a type of bacteria. Its diameter is 0.1 µm.

• There are more red blood cells in our body than any other type of cell.

Example 1: Illustrate how the shape and size of a cell is related to the specific function it performs.

Solution: Different types of cells with different shapes and sizes are present in our body. A cell’s shape and size are relevant to the specific function it performs. The irregularly shaped white blood cell is a case in point. A white blood cell protects the body by killing harmful foreign bodies. Whenever it encounters any antigen, it changes its shape accordingly and engulfs the antigen. Thus, the shape of the white blood cell is directly related to the function it performs.

Classification of Cells

Based on the number of cells: unicellular and multicellular

Organisms that are made up of only a single cell; are known as unicellular organisms. Examples of unicellular organisms include Amoeba and yeast. All other organisms (i.e. those made up of more than one cell) are known as multicellular organisms. Examples of multicellular organisms include humans, plants and animals. 

• Multicellular organisms can perform a variety of tasks efficiently due to division of labour. This gives the organisms a wide range of adaptabilities to survive. 

• In multicellular organisms, dead cells play an important role. For example, the dead epidermal cells in animal skin protect the underlying cells.

Division of labour 

Division of labour refers to the specialized roles of the different organs present in a multicellular organism. All organs, tissues or cells of a multicellular organism are evolved to carry out a specific set of tasks minimizing the load of carrying out all the functions and, consequently, increasing efficiency. 

For example, the digestive system is assigned to carry out digestion, while the excretory system is assigned to carry out excretion. This is division of labour. Ultimately, a healthy body is one in which the different organ systems perform their respective functions properly.

Based on the cellular complexity: prokaryotes and eukaryotes

This type of classification is based on the sub-cellular organisation of a cell.

The given table lists the characteristic features of prokaryotes and eukaryotes. 

Characteristics	Prokaryotes	Eukaryotes
Size of the cell	Cells are small in size.	Cells vary in size and are generally larger than those in prokaryotes.
Nucleus	No nucleus with a nuclear membrane is present.	There is a well-defined nucleus with a nuclear membrane.
Membrane-enclosed organelles	Organelles having a membrane around them (e.g., mitochondria, plastids) are absent.	Membrane-enclosed organelles are present.
Cell wall	Cell wall is usually present. It is composed of peptidoglycan.	Cell wall is usually present in plant cells. It is composed of cellulose.
Genetic material	The genetic material is present as nucleoid, i.e., a properly defined nucleus is absent.	The genetic material is present inside the well-defined nucleus.

Example 2: 

Distinguish between bacteria and yeast.

Solution:

Bacterium	Yeast
It is a unicellular prokaryote.	It is a unicellular eukaryote.
It lacks a well-defined nucleus.	It has a well-defined nucleus
It has no cellular organelles such as mitochondria and endoplasmic reticulum.	It has cellular organelles such as mitochondria and endoplasmic reticul

Structure of Eukaryotic and Prokaryotic Cell

 

 

Basic Components of a Cell

The two basic components of a cell are the protoplasm and plasma/cell membrane. The protoplasm consists of the cytoplasm and nucleus. The cell membrane is the outermost covering in animal cells, and is next to the cell wall in plant cells.

The term ‘protoplasm’ refers to the living contents of a cell, i.e., the nucleus and cytoplasm enclosed by a membrane. The cytoplasm is a jelly-like matrix surrounding the nucleus.

 

Plasma Membrane

In animal cells (which lack the cell wall), the cell membrane is the outermost covering. It acts as a barrier between the internal cell machinery and the harsh external environment. Hence, it functions as a protecting agent.

Basic structure of the plasma membrane

Conceived by Singer and Nicolson in 1972, the fluid mosaic model is used for describing the structure of the plasma membrane. According to this model, the major components of the plasma membrane are lipids and proteins. A small amount of carbohydrates can also be found in it. The plasma membrane is flexible in nature and allows the entry and exit of selective molecules.

To understand the cell membrane better, imagine it to be like a big plastic bag with many pores on its surface. This bag contains all the cellular contents, including the organelles, and keeps them separate from the outside environment. The pores present on the surface allow the entry and exit of only some molecules/substances.The cell membrane is extremely delicate, thin and elastic. It is the living membrane of the cell. It surrounds the cytoplasm and regulates the movement of substances into and out of the cell. This means that the cell membrane allows only certain substances to enter and exit. For this reason it is known as a selectively permeable membrane. 

Functions of the plasma membrane

• It protects the cellular organelles from the outside environment.
• It selectively allows molecules to move into and out of the cell.
• It can engulf substances within a cell through endocytosis. It can also expel substances out of the cell through exocytosis.
• It establishes communication between cells.

What makes the plasma membrane selectively permeable?

                          

   

The plasma membrane is made up of several organic molecules, but the major component is phospholipid. A phospholipid is a lipid molecule containing phosphorus. It has two parts—the head region and the tail region. The former is hydrophilic in nature, i.e., it has a strong affinity for water. The latter is hydrophobic in nature, i.e., it lacks the affinity for water. 

The phospholipids arrange themselves in such a way that the polar heads face toward the outside (i.e., toward water) and the fatty-acid tails face toward the inner side of the bilayer. Consequently, the hydrophobic region remains protected from the aqueous environment (as shown in the figure). 

Due to this special arrangement, all molecules cannot pass through the membrane. Polar molecules like glucose and water can easily pass through the membrane, but non-polar molecules like oils cannot pass through. This structural arrangement that allows only selective molecules to pass through the plasma membrane is what makes it a selectively permeable membrane.

 

Cell Wall

As mentioned before, in animal cells, the plasma membrane is the outermost covering. In plant cells, however, the outermost covering is the cell wall. 

Now, you might wonder why the cell wall is present only in plant cells. Let us first understand what the cell wall is and then we will ascertain as to why it is found solely in plant cells.

• The cell wall is the hard, protective outermost covering of plants, fungi and bacteria. 

• It is a rigid structure that gives support to a plant cell. 

• It allows a plant to stand upright and maintain the shape of the cell when placed in a hypotonic or hypertonic solution. 

• Its constituent compounds are different in case of different organisms. The cell wall of plants, fungi and bacteria is made up of cellulose, chitin and peptidoglycan respectively. 

• The cell wall connects the internal environment of a cell to the external environment. 

• It can withstand dilute hypotonic media and prevent bursting of cells. For this reason plant cells can withstand changes in environmental concentration better than animal cells.

Example 1:  Why is the cell wall not necessary in animal cells?

Solution: 

The cell wall is the tough and rigid layer around the cells of plants and bacteria. It provides both structural support and protection to the cells. Animal cells do not require the cell wall for the following reasons. 

1) Animal cells have other forms of support such as the exoskeleton and endoskeleton. 

2) Animal cells can regulate osmotic pressure by pumping ions and salts across the cell membrane. So, they do not require the cell wall to protect themselves from bursting due to endosmosis. 

3) Animal cells require flexibility for function and movement, which would not have been possible if the cells were surrounded by the cell wall. This is because the cell wall restricts flexibility.

Cytoplasm

Cytoplasm is the inner content of the cell membrane which separates the cell membrane from the nucleus. Some important features of cytoplasm are as follows:

• It is composed of cytosol, organelles and inclusions. 

• Cytosol is the soft, sticky and semi-transparent fluid in which various cell organelles are suspended.

• Cytoplasm is not a simple clear fluid. Rather, it is a complex viscous fluid that contains 70% water. The remaining portion is made up of proteins, carbohydrates and lipids.

• Cytoplasm is one of the most active parts of a cell. While it does not take part in the cellular processes, it does host most of the metabolic reactions. 

• It helps a cell to perform several vital functions by transporting essential nutrients to the required destinations. 

• One of the important components of the cytoplasm is the cytoskeleton. Cytoskeleton is a network of proteins (microtubules and microfilaments) which together form the skeleton of the cytoplasm. The cytoskeleton is responsible for the shape and movement of a cell.

Example 1:  Differentiate between cytoplasm, cytosol, protoplasm and protoplast.

Solution: 

Cytoplasm is the inner content of the cell membrane. It separates the cell membrane from the nucleus. It comprises cytosol, organelles and inclusions.

∴ Cytoplasm = Cytosol + Cell organelles + Inclusions

A constituent of cytoplasm, cytosol is a soft, sticky and semi-transparent fluid. In it the various cell organelles remain suspended.

Along with the nucleus, the cytoplasm makes up the protoplasm—the living part of a cell. It does not include the cell membrane or the cell wall

∴ Protoplasm = Cytoplasm + Nucleus

Protoplast is the protoplasm of a living plant or bacterial cell whose cell wall has been removed but has its cell membrane intact.

Have you ever heard of a cybrid?

The word ‘cybrid’ is the combination of the words ‘cytoplasmic’ and ‘hybrid’. The plasma membranes of cells (having different origins) are broken down to obtain cytoplasms. These naked cells are then fused to obtain a hybrid cell called a cytoplasmic hybrid or cybrid. Cybrids are often known as heterokaryons as they contain multiple and genetically-different nuclei. Cybrids are important for research purposes.

Cytoplasmic Streaming

Cytoplasmic streaming refers to the movement of the viscous fluid of a cell (i.e., cytoplasm) in order to perform a particular function. This phenomenon can be easily understood with the help of the unicellular eukaryotic organism Amoeba.

Amoeba can constantly change its shape. It forms finger-like projections called pseudopodia (meaning ‘false feet’) as shown in figures 1 and 2. These pseudopodia are nothing but the flowing projections of cytoplasm. 

     

When Amoeba senses its prey, it uses its pseudopodia to engulf the prey. These cytoplasmic projections not only help Amoeba in engulfing food but also in locomotion. Thus, they exemplify the phenomenon of cytoplasmic streaming.

• Cells can be grown outside the body through a process known as cell culture.

• Cell culture media is always isotonic in nature. The salt concentration of an isotonic solution is 0.9% of NaCl, which is equivalent to the intracellular salt concentration.

• Viruses do not have the plasma membrane and cytoplasm.