What is a cell made up of? What is the structural organization of a cell?

A cell is made up of three basic units, which provide all its needs:

1. Plasma membrane
2. Nucleus
3. Cytoplasm

Let us see some major components of a cell in detail.

Plasma Membrane

 

• All cells are surrounded by an outer covering called the plasma membrane, or cell membrane. it acts as a barrier between the internal cell and its external environment.
• It is a selectively permeable membrane because the plasma membrane allows or permits the entry and exit of some materials in and out of the cell and prevents the movement of some other materials.
• It is made up of organic molecules such as lipids and proteins.
• Its structure can be observed only through an electron microscope, as its very minute.

Movement of substances into and out of the cell:

The plasma membrane allows the particles to move into and out of the cell through the following modes:

1. Diffusion
2. Osmosis
3. Active transport
4. Endocytosis

Diffusion

Diffusion is the process of the spontaneous movement of a substance from a region of high concentration to a region of low concentration. It helps the movement of and across the cell membrane. Thus, diffusion helps during the cellular respiration of plants and animals.

• Moves out of the cell due to its high concentration inside the cell.
• Enters the cell as its concentration is low inside the cell.

Osmosis

The movement of water across the cell membrane depends on the amount of substance dissolved in it. Osmosis is the passage of water from a region of high water concentration to a region of low water concentration through a semi-permeable membrane. Based on the concentration of water present in the solution around the cell, there are three types of solutions:

1. Hypotonic solution
2. Isotonic solution 
3. Hypertonic solution

 

Hypotonic	Isotonic	Hypertonic
If the medium has a greater concentration of water than the cell, more water moves into the cell by osmosis. Such a solution is called a hypotonic solution.	If the medium has the same concentration of water as the cell, no net movement of water takes place across the membrane. Such a solution is called an isotonic solution	If the medium has a lower concentration of water than the cell, more water flows out of the cell by osmosis. Such a solution is called a hypertonic solution.
Water crosses the cell membrane in both directions.	Water crosses the cell membrane in both directions.	Water crosses the cell membrane in both directions.
More water will come into the cell than leave it.	The amount of water going in is the same as the amount going out.	More water leaves the cell than enters it.
The overall result is that water enters the cell.	There is no overall movement of water.	The overall result is water loss to the cell.
The cell is likely to swell up.	The cell will stay the same size.	The cell will shrink.

Below are some of the experiments demonstrating the process of osmosis.

 

Activity

Put an egg in dilute hydrochloric acid to remove the shell. The eggshell (calcium carbonate) is dissolved in dil.HCL. Now the egg is enclosed with a thin outer skin. Put the egg in pure water and wait for 5 minutes. What do you see? The egg swells because water passes into it by osmosis. Put a similar de-shelled egg in a concentrated salt solution and observe for 5 minutes. The egg shrinks. Why? The egg shrinks because water passes out of the egg solution into the salt solution by osmosis. Here, the concentration of water is low in salt solution.

 

Activity

Put dried raisins or apricots in plain water and leave them for some time. Observation: We can see that each of them swells because they gain water by osmosis. Now, place them into a concentrated solution of sugar or salt. Observation: They shrink due to the loss of water

Other examples of osmosis are:

• Plants and unicellular freshwater organisms gain water by osmosis.
• Absorption of water by plant roots

Active Transport

• In order to move molecules across a cell membrane from a region of lower concentration to a region of higher concentration, i.e., against the concentration gradient, energy is required. The process of taking molecules into the cell using energy in the form of ATP ( Adenosine Triphosphate) is called active transport.
• Different molecules, such as minerals, move in and out of the cell by means of active transport.

Endocytosis

• The process of the cell engulfing food and other materials from its external environment is known as endocytosis.
• It is the flexibility of the cell membrane that enables endocytosis.
• Amoeba acquires its food through the process of endocytosis.

Electron Microscope

It is a type of microscope used to view the image of the very fine details of the specimen by passing a beam of electrons through it. The first of its kind was developed by a German physicist, Ernst Ruska, in 1931. He won the Nobel Prize in Physics in 1986 for his contribution to electron optics.

 

Features: It has high resolving power and higher magnification as compared to the microscopes that use photons to illuminate the specimen.

Uses: It is used by researchers to examine the individual components of the cell, microorganisms, viruses, samples of tissues, crystalline structures, etc.

Working: A very thin slice of the specimen should be prepared and placed inside the vacuum chamber. An electron beam is passed through the specimen. The beam undergoes bending when it passes through the electromagnets (used instead of lenses in normal microscopes). The magnified image can be viewed on a digital screen.

Advantages:

• Finer details of microscopic objects such as tissues, cells, etc. can be observed.
• Higher resolution.
• High magnification.
• It has a wide range of applications in industry and scientific research.

Disadvantages:

• As the penetrating power of the electron beam is very low, a very thin slice of the specimen should be prepared for observation.
• It cannot be used for observing a live specimen.
• It is very expensive to build.
• It requires more space as the size of the microscope is very large. 

Cell Wall

• The cell wall is a rigid outermost layer found in the cells of plants, fungi, and bacteria.

• It lies outside the plasma membrane.

• It is a selectively permeable layer.

• The plant cell wall is made up of cellulose, which is a complex substance. It provides strength and rigidity to plants.

• The bacterial cell wall is made up of a material called peptidoglycan, and the fungal cell wall is made up of chitin.

• Advantages of having a cell wall: Cell walls permit the cells of plants, fungi, and bacteria to withstand very dilute external media without bursting. In hypotonic media, the cells tend to take up water by osmosis. As a result, the cell swells, building up pressure against the swollen cell. So, the cells of plants, bacteria, and fungi can withstand much greater fluctuations in the surrounding medium than animal cells.

Plasmolysis

Plasmolysis is a phenomenon in which living plant cells lose water by osmosis when placed in a hypertonic medium. When the cell is placed in a hypertonic solution (a solution with a low water concentration), water flows out of the cell due to osmotic pressure. Thus, the cell loses water, and cell components start to shrink away from the cell wall. So, the gap between the cell wall and the cell membrane increases, and the cell wall loses its shape. Thus, the rigidity of the cell decreases.

 

Only living cells, not dead cells, can absorb water by osmosis. It is because the cell membrane becomes denatured when it is dead.

Let us do an activity to illustrate that osmosis takes place only in living cells, not dead cells.

Activity

Take a green leaf and observe cells under the high power of a microscope. You can see the green granules called Chloroplasts, which contain a green pigment called Chlorophyll. Put the leaf peel into a salt solution and wait for a minute. What do you observe? The leaf starts to shrink due to plasmolysis. Put the leaf peel into the boiling water, which kills the cells. Then mount it on a slide and observe it under the microscope. What do you see? Pour some salt solution onto the slide and note down the observation after some time. Boiling the leaf damages the cell wall and cell membrane. This washed out the green pigments in the cell. The leaf cells do not look green. When we pour the salt solution onto this boiled leaf, nothing will happen. The salt solution cannot enter it. Osmosis does not take place here. Because the cell is dead.

 

Nucleus

The nucleus is a spherical, dot-like structure found near the center of the cell. It can be clearly seen by staining the cells.

Observe the pictures below:

                      

Without staining, we cannot see the parts of the cells clearly. Different regions of cells get colored differently according to their chemical composition. Some regions appear darker than others. We could use an iodine solution, safranin stain, or methylene blue solution to stain the cells.

You can see darkly colored, spherical, or oval, dot-like structures near the center of each cell. You have seen similar structures in onion peel cells. These structures are called nuclei (plural).

The nucleus is acidic in nature. To see the nucleus under a microscope. a basic stain is needed.

Structure of Nucleus

Observe the picture and become familiar with the major components of a cell nucleus.

What have you learned about the structure and functions of the nucleus?

The nucleus has the following components:

Parts of Nucleus	Features
1. Nuclear membrane	It is the outer covering of the nucleus. It is double layered  The nuclear membrane has numerous pores on it.
2.Endoplasmic Reticulum	It is a network of membranes found around the nucleus. It helps in the transportation of proteins.
3. Nucleolus	A spherical body called a nucleolus is seen inside the nuclear material. The nucleolus is related to the production of proteins.
4, Chromatin	It is seen as an entangled mass of thread-like structure. During cell division, chromatin is organized to form chromosomes.
5. Nucleoplasm	It is the fluid content inside the nucleus that surrounds the nucleolus and chromatin.
6. Chromosomes	It is a rod-shaped structure found inside the nucleus. It is composed of DNA (Deoxyribose Nucleic Acid) and protein. It is visible only during cell division. It contains information for the inheritance of characters from parents to the next generation in the form of DNA molecules. It carries genes, the genetic factors. Genes are the functional segments of DNA. DNA contains genetic information.

Prokaryotes and Eukaryotes

• The nuclear region of the cell may be poorly defined in some organisms, like bacteria. It is because of the absence of the nuclear membrane. Such an undefined nuclear region containing only nucleic acids is called a nucleoid.
• Based on the presence of the nuclear membrane, organisms are classified into two categories.

                 1. Prokaryotes: organisms whose cells lack a nuclear membrane.

                 2. Eukaryotes: organisms with cells having a nuclear membrane

The difference between prokaryotes and eukaryotes are listed below:

Prokaryotes	Eukaryotes
Nuclear Region: Undefined nuclear region that contains only nucleic acids and is known as the nucleoid	Nuclear Region: Well-defined and surrounded by a nuclear membrane.
Cells are of small size (1 - 10 micrometers)	Cells are of larger size (5 - 100 micrometers)
The chlorophyll in photosynthetic prokaryotic bacteria is associated with membranous vesicles	The chlorophyll in photosynthetic eukaryotic cells is found in plastids.
Single circular chromosome.	It has more than one chromosome.
Membrane-bound organelles are absent.	Membrane-bound organelles such as endoplasmic reticulum, mitochondria, etc present.
Examples: Bacteria, blue-green algae, archaea, etc	Examples: Amoeba, fungi, protozoa, animals, plants, etc.

Cytoplasm

The cytoplasm is the fluid content inside the plasma membrane and contains many cell organelles. Membranes enclose organelles. Each of these organelles is specialized to perform a specific function for the cell. It is the cytoplasm that separates the cell membrane from the nucleus.

                 

 

Significance of membrane in terms of virus

Viruses need a host cell to show the characteristics of life. They used the host cell machinery to multiply. The reason is that viruses have no membrane at all. Hence, they do not show any features of life until they enter a living body.

Cell Organelles

Large and complex cells need a lot of chemical activity to carry out their functions and maintain their structure. These chemical activities take place in the organelles. membrane help keep the different activities in different organelles separate from each other.

Some important cell organelles are:

• Nucleus
• Endoplasmic Reticulum
• Golgi Apparatus
• Lysosomes
• Mitochondria
• Plastids
• Vacuoles

Observe the following pictures and become familiar with the different cell organelles.

1. Endoplasmic Reticulum (ER)

The endoplasmic reticulum is a network of complicated tubular pathways connecting the cell membrane and nuclear membrane. Its structure is similar to the plasma membrane. Its appearance varies in different cells.

Ribosomes: Ribosomes are the sites of protein manufacture present in all active cells.

There are two types of ER based on their appearance under a microscope:

      1. Rough Endoplasmic reticulum (RER)

      2. Smooth Endoplasmic Reticulum (SER)

RER	SER
ER with ribosomes attached to its surface is called RER.	ER without ribosomes is called SER
It is rough and looks like sheets.	It is smooth and looks like a tube.
It helps in the synthesis of proteins using ribosomes.	It helps in the manufacture of lipids or fat molecules.
It helps in the transportation of synthesized proteins to various places in the cell based on need.	It contains enzymes to detoxify many poisons and drugs.

Membrane Biogenesis

Some of the proteins and lipids synthesized by ER help build the cell membrane. This process is known as membrane biogenesis.

 Functions of the ER

• It acts as a channel for the transport of materials (mainly proteins) between various regions of the cytoplasm or between the cytoplasm and the nucleus.
• It functions as a cytoplasmic framework, providing a surface for some of the biochemical activities of the cell.

Golgi apparatus

Camillo Golgi first described the Golgi apparatus.

• It constitutes another portion of a complex cellular membrane system.
• Consists of a system of membrane-bound vesicles.
• The vesicles lay approximately parallel to each other in stacks called cisterns.
• It often has connections with the membranes of the ER.
• It was discovered by Camillo Golgi, an Italian biologist, in 1898.

Functions

• The Golgi apparatus helps in packaging and dispatching the material synthesized near the ER to various targets inside and outside the cell.
• It helps in the storage, modification, and packaging of products in vesicles.
• It helps in the formation of lysosomes.

Lysosomes

• Lysosomes are membrane-bound sacs filled with digestive enzymes.

• The Rough Endoplasmic Reticulum (RER) makes these enzymes.

• Lysosomes serve as a kind of waste disposal system for the cell. It is because the powerful digestive enzymes in it are capable of breaking down all organic materials, worn-out organelles, and foreign materials entering the cell and digesting them. Thus, it helps to keep the cell clean.

• Cells get damaged during metabolism. As a result, lysosomes burst out, and the enzymes digest their own cells. Therefore, lysosomes are known as suicidal bags in a cell.

 

As a physician at a home for incurables in Abbiategrasso, Italy (1872 - 75), Golgi devised (1873) the silver nitrate method of staining nerve tissue. This stain enabled him to demonstrate the existence of a kind of nerve (which came to be known as the Golgi cell) possessing many short, branching extensions (dendrites) and serving to connect several other nerve cells. The discovery of Golgi cells led the German anatomist Wilhelm Von Waldeyer-Hartz to postulate and Ramon Y Cajal to establish that the nerve cell is the basic structural unit of the nervous system, a critical point in the development of modern neurology. After his arrival at the University of Pavia (1875), Golgi found and described (1880) the point (now known as the Golgi, Spindle) at which sensory nerve fibers end in rich branchings encapsulated within a tendon. He also discovered (1883) the presence in nerve cells of an irregular network of fibrils (small fibers), vesicles (cavities), and granules, now known as the Golgi apparatus.

 

Mitochondria

• Mitochondria are called 'the powerhouse of the cell' as their major function is to release energy, which is in the form of ATP (adenosine triphosphate) molecules. This energy is utilized by an organism for the chemical activities needed for life. ATP is called the energy currency of the cell.
• Mitochondria have two membrane coverings. The outer membrane is very porous. The inner membrane is deeply folded, which creates a large surface area for ATP-generating chemical reactions.
• Mitochondria are different from other cell organelles. The reasons for making it strange are:

                  1. They have their own DNA and ribosomes.

                  2. They can produce some of their own proteins.

Plastids

• It is a double-membrane organelle found only in plant cells.
• It is the main site of photosynthesis.
• There are two types of plastids:

                  1. Chromoplasts (colored plastids): Green-coloured pigment-containing plastids are called chloroplasts. Chloroplasts also contain yellow pigment (xanthophyll) or orange pigment (carotene).

                   2. Leucoplasts (white or colorless plastids): Leucoplasts are organelles responsible for the storage of starch, oils, and protein granules.

• Structure: Plastids consist of numerous membrane layers that are arranged as stacks. each stack is called Granum. Each disc-like plate in the granum is called a thylakoid. Grana is surrounded by a fluid called stroma.
• Similarity with Mitochondria: The external structure of the plastids is similar to that of mitochondria. Like mitochondria, they have their own DNA and ribosomes.

Vacuoles

Vacuoles are membrane-bound sac-like bodies found in the cytoplasm of the cell. Many substances of importance to the life of the plant cell are stored in vacuoles, small in animal cells, while plant cells have very large vacuoles. The central vacuole of some plant cells may occupy 50-90% of the cell volume.

Functions of Vacuoles

• Vacuoles are storage sacs for solid or liquid contents.
• They store amino acids, sugars, various organic acids, and some proteins.
• In plant cells, vacuoles are full of cell sap and provide turgidity and rigidity to the cell.
• In single-celled organisms like Amoeba, the food vacuole contains the food items consumed.
• In some unicellular organisms, specialized vacuoles help in expelling excess water and some wastes.

 

 

Frequently Asked Question

Q1: What are the main components of a cell?

Ans: A cell consists of various components, including the cell membrane, cytoplasm, and nucleus. In eukaryotic cells, there are also organelles such as mitochondria, endoplasmic reticulum, Golgi apparatus, and more.

Q2: What is the cytoplasm?

Ans: Cytoplasm refers to the jelly-like substance that fills the cell and contains various organelles. It's the site of many cellular processes.

Q3: What are organelles?

Ans: Organelles are specialized structures within a cell that perform specific functions. Examples include mitochondria (energy production), endoplasmic reticulum (protein synthesis), and the Golgi apparatus (protein processing and packaging).

Q4: What is the function of the mitochondria?

Ans: Mitochondria are the "powerhouses" of the cell, producing energy through cellular respiration.

Q5: What is the Golgi apparatus?

Ans: The Golgi apparatus processes, sorts, and packages proteins and lipids for transport within the cell or secretion outside the cell.