Cells As The Basic Unit Of Living Organisms


Cells As The Basic Unit Of Living Organisms

Cells As The Basic Unit Of Living Organisms


The objective of this unit is to have a deep knowledge of the cell and its organelles.

Electron Micrographs Of Cells

The development of an electron microscope leads to a better understanding of the cell and its organelle. Electron microscope gives a better resolution than a light microscope. It uses a beam of electrons to create the image. As electrons have smaller wavelength the resolution is better.

Plant Cells In Electron Microscope

The electron micrograph of a plant cell shows the detailed structural organization of its organelles.

A typical plant cell has a large membrane-bound vacuole in the center of it. It stores the excess amount of water and solutes.

Due to the vacuole, the nucleus is pushed to the periphery of the cell. The nucleus is membrane-bound and has a nucleolus in the center. The rest of the cytoplasm contains an extensive network of membranes called the endoplasmic reticulum. It serves as a transport system of the cell. Two types of Endoplasmic reticulum can be seen. One has a granular surface due to the presence of ribosomes on its surface. The other one is smooth as ribosomes are absent.

Other organelles mitochondria and chloroplast are also present in the cytoplasm. These organelles are the center of ATP generation and photosynthesis.

Animal Cells In Electron Microscope

Animal cells have a similar structure although few things are different from plant cells. This is due to the difference in their mode of habitat. Animal cells lack chloroplast and cell walls. The outer most boundary is the cell membrane.

The cytoplasm has an internal membrane system as well. This comprises of endoplasmic reticulum, mitochondria, Golgi apparatus. Ribosomes are present on the endoplasmic reticulum. The nucleus is present in the center as the vacuole is absent. Although glycogen in the form of droplets is present in the cytoplasm.

Cell Organelles

Organelles are the structures that are present inside the cell. They perform different functions inside the cell. There is very little difference between animal and plant cells otherwise they have structural organization.

1)     Endoplasmic Reticulum


The endoplasmic reticulum is the membrane-bound organelles. It comprises of tubules of the membrane which extends from nuclear membrane to cell membrane.

Types Of Endoplasmic Reticulum

There are two types of the endoplasmic reticulum. This classification is made based on the presence of ribosomes.

i)     Smooth Endoplasmic Reticulum

Smooth endoplasmic reticulum has a smooth appearance due to the absence of ribosomes.
Its functions are lipid synthesis, detoxification, and storage of calcium ions.

ii)     Rough Endoplasmic Reticulum

Rough endoplasmic reticulum appears granular due to the presence of ribosomes.
Their function is mostly associated with protein synthesis.


The main function of the endoplasmic reticulum is the transport of substances from the nucleus to the cytoplasm.
It also helps in maintaining the shape of the cell.

2)     Cell Membrane


The cell membrane is the outermost boundary of the cell. It is made of phospholipid molecules. A phospholipid molecule has two ends hydrophobic and hydrophilic. The membrane has a fluid nature.

The cell membrane is semi-permeable. That is it allows a certain substance to pass through. This helps in maintaining the internal environment of the cell.

There are protein molecules that are attached to the membrane. These proteins help in the transport of a substance.

All the membrane inside the cell has the same structure.


The cell membrane serves as the outer boundary of the cell. Thus, it separates the intracellular environment from extracellular.

3)     Nucleus


The nucleus is a double membrane-bound organelle. It is the control center of the cell.

1)     Nuclear Membrane

It is a double membrane that forms the outer boundary. It separates the nuclear contents from the cytoplasm. The nuclear membrane has pores that allow exit and entry of substances.

2)     Nucleolus

The nucleolus is present in the center of the nucleus. It consists of RNA and proteins.

3)     Nucleoplasm

Nucleoplasm is a gelatinous substance present inside the nucleus. It consists of salts, enzymes and other substances dissolved within it. Nucleoplasm act as a medium for the transport of substances and helps the nucleus in maintaining its shape.


The nucleus is the control unit of the cell. It contains all the hereditary information of the cell.

4)     Golgi apparatus


Golgi apparatus consists of a stack of membranes.


Its main function is the packaging and transport of substances outside the cell.

5)     Lysosomes


Lysosomes are membrane-bound vesicles. It contains digestive enzymes that are required for the degradation of various substances.

Lysosomes originate from the Golgi apparatus itself.


They help in the degradation and digestion of various substances. Lysosomes are also called suicide sac as they release enzymes that destroy its cells.

6)     Cell wall


The cell wall is the outermost boundary of the plant cell.

It is mostly made of cellulose. The cell wall can be divided into three parts. Primary cell wall, secondary cell wall, and middle lamella.

Unlike cell membrane it is permeable. There are pores in the cell wall for entry and exit of substances. These pores are called plasmodesmata.


The main function of the cell wall Is to provide support to the cell and maintain its shape.

7)     Mitochondria

Mitochondria is also a double membrane-bound organelle. It consists of an outer membrane and an inner folded membrane. Inside the inner mitochondrial membrane is the matrix.


Mitochondria is the powerhouse of the cell. It produces ATP by cellular respiration.
Mitochondria contain their DNA. Thus, it has a semi-autonomous existence.

8)     Chloroplast


The chloroplast is also a double membrane-bounded organelle. The inner membrane is in the form of stacks called the thylakoid membrane. This thylakoid membrane has a light-harvesting system called the photosystem. The photosystem consists of chlorophyll and other pigments.


The chloroplast is the site of photosynthesis.
It also consists of a small circular DNA. It encodes genes necessary for the function of the chloroplast.

9)     Ribosomes


Ribosomes are the protein factories present freely in the cytosol or attached to the endoplasmic reticulum. It consists of two subunits. The large subunit is called the 60s and the small subunit is 40s. both of these together are called the 80s.

Ribosomes In Chloroplast And Mitochondria

The ribosomes are also present in mitochondria and chloroplast. It consists of 70s subunits. These ribosomes resemble the one which is present in bacteria. Thus, it is believed that these organelles are originated from bacteria.

10)    Centrioles And Microtubules

Centrioles are present in animal cells only. They are located adjacent to the nucleus. It consists of an array of nine microtubules. Its function is to produce spindles during cell division.

Microtubules are polymers of tubulin protein. They play an important role in cell division and the transportation of substances within cells.

11)    Vacuoles

Vacuole may be present in animal cells but mostly a large vacuole is present inside plant cells. They are membrane-bound organelles whose function is to store substances. The membrane of the vacuole is called tonoplast.


To store nutrients, waters and other substances.

Mitochondria and atp production:

Mitochondria is the powerhouse of the cell. It is the site where energy is produced by aerobic respiration. This energy is then used by plants for various purposes.

During aerobic respiration, glucose enters mitochondria and is broken down in the presence of oxygen to release energy. This energy is used to form a molecule called ATP.

ATP is the energy currency of cells. Whenever a cell needs energy a molecule of ATP is broken down by hydrolysis. The energy released is then used to drive the cellular processes.

ATP consist of an adenosine molecule bonded with 3 molecules of phosphate. These phosphates are bounded by a high energy bond. When this bond is broken energy is released.
ATP+H20-------à ADP+Pi +Energy

In chloroplast, the light energy trapped is first converted to ATP. This ATP is then used to produce glucose.

Prokaryotic Cell

Prokaryotic cell means Before the nucleus, these cells do not have a true membrane-bound nucleus. The nuclear membrane is absent thus, the nucleus is freely dispersed in the cytoplasm. Bacteria are an example of prokaryotic cells.

Besides this, prokaryotic cells also lack other membrane-bound organelles such as mitochondria, endoplasmic reticulum, etc.


Bacteria are the smallest organisms. They range in size from 1-5 micrometers.

It consists of a cell wall that is made of peptidoglycans. The function cell wall is to protect bacteria from osmotic lysis.

Inside the cell wall is the cell membrane

Bacteria also have a polysaccharide capsule around their cell wall.

There are no membrane-bound organelles. But, they have ribosomes 70s and glycogen droplets.

There is no true nucleus and the genetic material is freely dispersed in the cytoplasm.

They have mesosomes which are cell membrane invaginations. Their function is the transport of substances. Mesosomes also contain enzymes that produce energy.

Bacteria have flagella which help them in transportation.

They also contain pilli. It is a hollow tube and is associated with conjugation.

An extra genetic material called plasmid is also present.

Difference Between Prokaryotic And Eukaryotic Cell

Prokaryotic Cell
Eukaryotic Cell
Nuclear membrane absent
Nuclear membrane present
Mitochondria, golgi bodies, endoplasmic reticulum and mitochondria absent

Mitochondria and other organelles present
70s ribosome
80s ribosome
Cell wall made of peptidoglycan
Cell wall made of cellulose


Viruses are intracellular obligate parasites. They reproduce inside a host cell as they lack the cellular components to make their proteins. They range from 200 to 250 nm.

Viruses are referred to as smallest living particles but in many ways they are placed on a borderline between living and non living substance.

It consist of a nucleic acid molecule which is surrounded a protein coat. The protein coat is made up of capsomere subunits which is called capsids.

Some viruses also have an accessory envelope. 

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