THE FUNDAMENTAL UNIT OF LIFE.

INTRODUCTION TO

FUNDAMENTAL LIFE OF CELL

All About Cells

Cells are the basic structural and functional unit of life. Cell was discovered by Robert Hooke. 

A number of cells can work together to form tissues and organs.

Facts that Matter

•  The smallest functional unit of life is cell, discovered by Robert Hooke in 1665. A cell can independently perform all necessary activities to sustain life. Hence cell is the basic unit of life.

There are two types of cells → plant cell and animal cell. The different cell organelles and their functions are as follows:

Cellular respiration

Cellular respiration is the process by which the food releases energy in the

 mitochondria. Cells absorb glucose from the food and burn it to produce energy.

Structural Organization of Cells

Plasma/Cell membrane: 

This is the outermost covering of the cell that separates the contents of the cell from its external environment. The plasma membrane allows or permits the entry and exit of some materials in and out of the cell so the cell membrane is called a selectively permeable  membrane. Some substances like CO2 or O2gases can move across the cell membrane by a process called diffusion. The movement of water molecules (liquid) through such a selectively permeable 

membrane is called osmosis. Osmosis is the passage ot water from a region of high water  concentration through a semi-permeable membrane to a region of low water concentration. If the medium surrounding the cell has a higher water concentration than the cell, the cell will gain water by osmosis. Such a solution is known as hypotonic solution. If the medium has exactly the same water concentration as the cell, there will be no net movement of water across the cell membrane. Such a solution is known as isotonic solution. If the medium has 

a lower water concentration then the cell will lose water by osmosis. Such a solution is known as hypertonic solution.

The plasma membrane is flexible and is made up of organic molecules called lipids and proteins The flexibility of cell membrane also enables the cell to engulf in food and other material from its external environment. Such process is known as endocytosis. It is observed in Amoeba.

Prokaryotic & Eukaryotic cells

Two types of cell; Prokaryotic and Eukaryotic cells. Prokaryotic cells are primitive and lack well defined nucleus. Eukaryotic cells are more advanced and have well defined nucleus.

Prokaryotic Cells: 

In some organisms like bacteria, the nuclear material is not enclosed by nuclear membrane and membrane bound cell organelle are absent. Such nucleus is called

nucleoid and such cells are known as prokaryotic cell. Such cells have single chromosome.

 Eukaryotic Cells: 

Cells having well defined nucleus and having membrane bound cell organelle is termed as eukaryotic cell. Such cells have more than one chromosomes.

Cell structure in Eukaryotic cells

Eukaryotic cells have the most well defined structure. These cells have cell membrane, membrane bound cell organelles and a well defined nucleus. The nucleus has its own membrane called nuclear membrane.

Cell membrane

• Cell membrane is the outer covering of a cell.
• It is made up of phospho-lipid bilayer membrane.
• It is selectively permeable in nature.
• The structure of cell membrane is best described by the fluid mosaic model.

2.  Cell wall (Protective wall): Plants cells, in addition to the plasma membrane have another rigid outer covering called cell wall. The cell wall lies outside the plasma membrane. The plant cell wall is mainly composed of cellulose. It is a complex substance and provides structural strength to plant  cells. When a living plant loses water through osmosis there is shrinkage or contraction of contents of the cell away from cell wall. This phenomenon is known as plasmolysis.

Diffusion

The movement of molecules from a region of their high concentration to a region of their lower concentration is known as diffusion.

Osmosis in selectively permeable membrane

Osmosis is the movement of water across a semi-permeable membrane. Osmosis is a selective process since the membrane does not allow all molecules to pass through it. Water is usually the only free flowing molecule across this membrane.

Isotonic, hypotonic solutions, hypertonic solutions

• Isotonic solutions are those which have the same solute and pH concentration as the surrounding body fluid or the cytoplasm.
• Hypotonic solutions contain lesser amount of solute concentration compared to the 
• surrounding fluid and can force the cell to rupture due to excess input of water into the cell.
• Hypertonic solutions contain higher concentration of solute compared to the surrounding
•  fluid and thus push water out of cell, shrinking it.

Cell walls in plants

Plant cells are different from animals cells due to the presence of cell wall. Cell wall is made of cellulose and gives a rigid structure to the plant cell.

Cell Organelles

Endocytosis

Endocytosis is the invagination of cell membrane, followed by pinching off forming a membrane bound vesicle. This is commonly seen in Amoeba.

Nucleus in cells

Nucleus is the processing unit of the cell. It is a double membrane bound organelle which contains the genetic material for inheritance.

Nucleus (Brain of a cell): 

The nucleus has a double-layered covering called nuclear membrane. The nuclear membrane has pores which allow the transfer of material from insidethe nucleus to

its outside, i.e., to the cytoplasm. The nucleus contains chromosomes, which are visible as rod-shaped structures only when the cell is about to divide. Chromosomes contain information for inheritance of features from parents to next generation in form of DNA [Deoxyribo Nucleic Acid) molecules. Chromosomes are composed Of

DNA and protein. Functional segments of DNA are called genes. The nucleus plays a central role in

cellular reproduction.

Chromosomes

During the growth phase of the cell, the chromatin condenses into much thicker structure called chromosome.  

Chromatin

Chromatin is a thread like structure which serves as the genetic material present inside the nucleus of the cell. It is made up of DNA and protein molecules. The DNA contains the hereditary information needed for the structure and function of the organism.

Cytoplasm

Cytoplasm is the fluid found inside the cell. It gives the structure to the cell and houses different organelles of the cell.

Cytoplasm: 

The cytoplasm is the fluid content inside the plasma membrane. It also contains many specialised cell organelles. Each of these organelles performs a specific function for the cell.

Cell Organelles: 

Every cell has a membrane around it to keep its content separate from the external environment. The different components of cell perform different function and these components are called cell organelles.

Organelles

Organelles are structures present in the cytoplasm of the cell that help in several functions of the cell.

Endoplasmic Reticulum

Endoplasmic reticulum is membrane like cell organelle that plays an integral role in the interpretation of the genetic information present in the nucleus.

Rough ER

Rough ER are the ones that have ribosomes on it. The ribosome is made up of nucleic acids and proteins. They are the site of protein synthesis. The Rough ER is also involved in the modification and folding of protein.

Smooth ER

Smooth ER do not have ribosomes and thus are not involved in protein synthesis. They are however involved in the lipid metabolism and detoxifying poisonous molecules.

Endoplasmic Reticulum (ER) (Channels, Network for transport): 

The ER is a large network of membrane-bound tubes and sheets. It looks like long tubules or round or oblong bags.There are two types of ER-Rough endoplasmic reticulum [RER] and smooth endoplasmic reticulum [SER]. RER has particles called ribosomes attached to its surface. The ribosomes are the sites of protein manufacture.

            

The SER helps in the manufacture of fat molecules, or lipids, important for cell function. Some of these proteins and lipids help in building the cell membrane. This process is known as membrane biogenesis. Some other proteins and lipids function as enzymes and hormones.

           

The one function of ER is to serve as channels for the transport of materials between various regions of the cytoplasm or between the cytoplasm and the nucleus. The ER also functions as a cytoplasmic framework providing a surface for some of the biochemical activities of the cell.

 (ii)   Golgi Apparatus (Packaging): The golgi apparatus, first described by Camillo    Golgi, consists 

of a system of membrane-bound vesicles arranged approximately, parallel to each other in stacks called cisterns.

The material synthesised near the ER is packaged and dispatched to various targets inside andoutside the cell through the Golgi apparatus. It's function include the storage, modification and packages of products in vesicles. In some cases complex sugar may be made from simple sugar in the Golgi apparatus. It is also involved in the formation of lysosomes.

Golgi Apparatus

Golgi Apparatus is also called the post office of the cell. They package and transport the proteins across the cytoplasm.

Lysosomes

They are referred to as suicide bags of the cell as they contain potent enzymes that can digest a cell. Lysosome also help in defense by attacking a foreign object.

Lysosomes [Suicide bags] (Cleanliness of cell): 

Lysosomes are a kind of waste dispatch and disposal system of the cell. Lysosome help to keep the cell clean by digesting any foreign material as well as worn-out cell organelles. Foreign materials entering the cells such as bacteria or food, aswell as old organelles, end up in the lysosome, which break them up into small pieces. They are able to do this because they contain powerful digestive enzymes capable of breaking down all organic material. Under abnormal condition, when the cell gets damaged, lysosomes may burst and the enzymes digest their own cell. Therefore they are also  known as “suicide bags”

Mitochondria

Mitochondria are also called power plant of the cell. They generate ATP via the electron transport chain. They also have a DNA called mtDNA, which makes them semi-autonomous organelle.

 Mitochondria (Powerhouse, Energy provider):

 Mitochondria are known as powerhouses of the cell. The energy required for various chemical activities needed for life is released by mitochondria in the form of ATP [Adenosine Triphosphate] molecules. ATP is known as energy currency of the cell. Mitochondria have two membrane coverings instead of just one. The outer membrane is very porous while the inner membrane is deeply folded. They are able to make some of their own protein.

Plastids

There are various types of plastids in different cells based on the pigment they contain. The chloroplast is the plastid where the photosynthesis occurs. Some of the other plastids are leucoplast and chromoplast.

Plastids: Plastids are present only in plant cells. There are two types of plastids chromoplasts and leucoplasts. Chromoplasts are the coloured plastids present in leaves, flowers and fruits. Plastids containing the pigment chlorophyll are known as chloroplasts. They are important for photosynthesis 

in plants. Chloroplasts also contain various yellow or orange pigments in addition to chlorophyll. Leucoplasts are found primarily in organelles in which materials such as starch, oils and protein 

granules are stored. The internal organisation of the plastids consists of numerous membrane layers embedded in a material called stroma. Plastids are similar to mitochondria in external structure. 

Plastids have their own DNA and ribosomes.

Vacuoles

Vacuoles are large vesicles that hold water or air in them and give structural rigidity to the cell. Vacuoles are common in plant cells. In animals the vacuoles are either very small or absent.

Vacuoles (Storage): 

Vacuoles are storage sacs for solid or liquid contents. Vacuoles are small-sized in animal cells while plant cells have very large vacuoles [50% to 90% cell volume].In plant cells, vacuoles are full of cell sap and provide turgidity and rigidity to the cell. In 

Amoeba, the food vacuole contain the food items that is consumed by it and contractile 

vacuoles expels excess water and some wastes from the cell.

Comparison between plant & animal cells

Plants cells are different from animals cells structurally. Plant cells have cell wall and chloroplast which are missing in animals cells. Plants cells also have large vacuoles which are either very small or missing in animal cells. The nucleus is present at the center of the cell in animal cells and at the periphery in plant cells.