1. About Cell and Cell Theory

The Fundamental Unit of Life

Introduction of cell

The introduction to cell began back in the year 1655 when a revolutionary observation was made by an English scientist Robert Hooke. 
In all the living beings, cells are the basic structural units. We can compare the presence of cells in our body to the bricks in a building. All the bricks are assembled to make a building. Similarly, all the cells are assembled to make the body of an organism. 
Thus, it is the basic structural and functional unit of life and all the organisms are made up of cells. The subcellular structures of the cell comprise of the plasma membrane, organelles and in some cases a nucleus as well. As for the size of the cell, it is variable and maybe anything from 1 to 100 micrometre.
Robert Hooke was examining a dried section of the cork tree using a crude light microscope. In this analysis, he observed multiple small chambers which he named the cells. 
The first theory was proposed by the German botanist Matthias Jacob Schleiden and the German physiologist Theodore Schwann in 1838. This theory was formalized in the year 1858 by the German researcher Rudolf Virchow by suggesting that all cells arise from pre-existing cells.

 

Cell Theory

•    The cell is the basic functional and structural unit of life. All the living organisms are composed of cells.
•    All cells are formed by the division of the already existing cells which in terms of biology means reproduction.       Every cell of our body comprises of genetic material which is passed down during the process.
•    All the basic physiological and chemical functions i.e. the growth, repair, movement, communication, immunity and digestions are performed inside the cells.
•    All the activities of the cell depend mainly on the activities of the subcellular structures that lie within the cell. These subcellular structures comprise of the plasma membrane, organelles and if present, the nucleus.

Types of Cells

Broadly, there are two key types of cells i.e., the Prokaryotic Cell and the Eukaryotic Cell. The difference between the two is defined mainly by the presence or the absence of the nuclear membrane.

  1. Prokaryotic Cell

 If a cell has a nuclear material without a nuclear membrane, then it is known as the prokaryotic cell, where ‘pro’ stands for primitive and ‘karyon’ stands for the nucleus. Some of the organisms that have prokaryotic cells include bacteria and the blue-green algae.

  1. Eukaryotic Cell

If a cell has a nuclear material with a nuclear membrane, then it is known as the Eukaryotic Cell, where ‘EU’ stands for true and ‘karyon’ stands for the nucleus. All the living organisms except bacteria and blue-green algae have Eukaryotic Cells.

2. Cell Membrane and Cell Wall

Cell Size

The cell size is variable. In the living organisms, the cell size may be as small as a millionth of a metre or may be as large as a few centimetres. Usually, all the cells are microscopic in size and aren’t visible to the naked eye. Thus, they need to be enlarged by a microscope for seeing.

Cell Shape

  • Usually, the cells are round, elongated or spherical.
  • There are also some cells which are long and pointed on both the ends. Such cells exhibit spindle shape.
  • In some cases, the cells are very long.
  • Some may be branched like the neuron or the nerve cell. The nerve cell transfers and receives messages.

It, therefore, helps in coordinating and controlling the working of the different parts of the body. The components of the cell are enclosed in a membrane. This membrane provides shape to the cells of animals and plants. There is a cell wall surrounding them. The cell wall is an additional covering over the cell membrane in the plant cells. It offers rigidity and shape to the cells.

Depending on the purpose, the cell takes on a number of forms. Cells come in a variety of forms, including:

Skin cells

  1. These skin cells have a flat surface.
  2. They are a protective body cover with a flat form that allows them to cover a large area.

Muscle cells

            i. Muscle cells are long, thin, and elongated.
            ii. Muscle cells expand and contract, and their long and thin structure aids in this process.

Nerve cells

            i. These nerve cells have a lengthy fibrous structure.
            ii. Long, thin extensions (axons and dendrites) connect these nerve cells to other nerve cells.
            iii. The extensions aid the neuron in swiftly transmitting chemical and electrical information throughout                    the body.
            iv. Nerve cells can contain fibres that are over a metre long.
             v. Messages are transmitted down these fibres from one nerve cell to the next.

 Blood cells

            i. White blood cells are disc-shaped but have the ability to alter form.
            ii. WBC in the blood alters their form to kill bacteria or any pathogens that are disease-causing                                organisms.                                                                                                                                                           
iii. The structure of red blood cells (erythrocytes) allows them to pass through capillaries with ease.

Plant cells

            i. Depending on their purpose, plant cells might be rectangular, circular, oval, or elongated. Plant cells                   on the outside of the stem give the plant its strength.
            ii. For support, these cells have strong cell walls. Food is stored in specific cells in plants, and these                       cells are bigger than other cells.

Cell Wall

           The cell wall is the outer covering of a cell, present adjacent to the cell membrane, which is also called the plasma membrane. The cell wall is normally present in all plant cells, fungi, bacteria, algae, and some archaea. Animal cells are not regular in their shape and this is mainly due to the lack of cell walls in their cells. Cell wall composition usually varies along with organisms.

The Function of the Cell Wall

There are various functions of cell wall like:

•    The cell wall of the plants provides definite shape, strength, and rigidity.
•    The cell wall also protects against mechanical stress and physical shocks.
•    Cell wall helps to control cell expansion due to the intake of water.
•    The cell wall also helps in preventing water loss from the cell.
•    The cell wall is responsible for transporting substances between and across the cell.
•    The cell wall acts as a barrier between the interior cellular components and the external environment.

Cell Membrane

•    The cell membrane is also called the plasma membrane. 
•    Cell membrane is found in all cells and separates the interior of the cell from the outside environment.
•     The cell membrane also consists of a lipid bilayer that is semipermeable. 
•    The cell membrane is also responsible for the transportation of materials entering and exiting the cell.
•     The thin membrane surrounds every living cell, fixing the cell from the environment around it. 
•    The cell membranes are the cell’s constituents, often large, water-soluble, highly charged molecules such as proteins, nucleic acid, carbohydrates, and substances involved in cellular metabolism. 
•    The surrounding of the cell membrane is a water-based environment, containing ions, acids, and alkalis that are toxic to the cell, as well as nutrients that the cell must absorb to live and grow. 

Functions of the Cell Membrane

•    There are various functions of cell membrane-like:
•    The cell membrane protects the integrity of the interior cell.
•    The cell membrane provides support and maintains the shape of the cell.
•    The cell membrane helps in regulating cell growth through the balance of endocytosis and exocytosis.
•    It also plays an important role in cell signalling and communication.
•    The cell membrane acts as a selectively permeable membrane by allowing the entry of only selected substances into the cell.


 

 

 

3. Cell Organelles and their Functions

Cytoplasm

Cytoplasm Structure

  • Cytoplasm is a jelly-like substance found between cell membrane and nucleus.
  •  All the cell organelles are embedded in the cytoplasm. It is composed of water, organic and inorganic compounds. 
  • Cytoplasm is one of the essential components of the cell that is present in both plant and animal cells. 
  • Cytoplasm functions by controlling all the metabolic activities taking place within the cell and most of the chemical reactions are carried within it.

 

Endoplasmic reticulum

Endoplasmic reticulum is a network of tiny tubular structures scattered in the cytoplasm. It divides the intracellular space into two distinct compartments, one is luminal (inside ER) and another is extra luminal (outside ER). They function as the transport system of a cell, involved in transporting materials throughout the cell.

Endoplasmic reticulum is divided into two types based on presence and absence of ribosomes.

  • Rough endoplasmic reticulum: The endoplasmic reticulum bearing ribosomes on their surface is called rough endoplasmic reticulum (RER). It is involved in the protein synthesis and secretion. They are extensive and continuous with the outer membrane of the nucleus.
  • Smooth endoplasmic reticulum: The endoplasmic reticulum with the absence of ribosomes on its surface is called smooth endoplasmic reticulum (SER). They are responsible for lipid syntheses. In animal cells, lipid like steroid hormones is synthesized in SER. They are also responsible for detoxifying the cell.

 

Mitochondria

  • Mitochondrion is a double membrane bound cell organelle that are not normally visible under the microscope.
  • This cylindrical or sausage shaped structure is also called the power house of the cell as they are the sites for aerobic respiration of the cell and produce cellular energy in the form of ATP. 
  • The outer membrane and the inner membrane divide its lumen into two compartments i.e., inner compartment called matrix possessing single circular DNA molecule and a few RNA molecules. forming a number of infoldings called cristae and the outer membrane forming the continuous limiting boundary of the organelle.
  • Depending on the shape, size and physiological activity of the cell, the number of mitochondria per cell are variable. 
  • The structure has a diameter of 0.2 to 1µm (average 0.5 µm) and length 1.0 to 4.1 µm.

                                                                                                                            Plastids

Plastids are large and membrane bound organelles that are found in all plants and in euglenoids. They contain some specific pigments that are responsible for imparting specific colours to the plants. On the basis of the type of pigment, plastids can be classified into three types.

  • Chloroplasts: Chloroplasts are double membrane bound cell organelles that contain chlorophyll and carotenoid pigments that are responsible for trapping light energy and this energy is used for the purpose of photosynthesis.

                                                                                                                      Chloroplasts

The inner membrane surrounds a space called stroma. Chlorophyll containing structures called thylakoids are arranged as piles of coins and each pile is called a granum. Thylakoids are connected by flat membranous tubules known as stromal lamella.

  • Chromoplasts: These include carotenoid, fat soluble pigments like carotene, xanthophyll etc. providing characteristic colours like yellow, orange, red, etc. to the plants.
  • Leucoplasts: These types of plastids store nutrients and are colourless. They include amyloplasts storing carbohydrates, leucoplasts storing proteins, and leucoplasts storing oils and fats.

The Golgi Apparatus

  • It is responsible for sorting, modifying, and shipping off the cellular products that come from the rough endoplasmic reticulum (ER), just like a post office.
  • The Golgi apparatus is similar to stacked flattened discs, almost like stacks of oddly shaped pancakes.
  • The Golgi apparatus has two distinct sides with each having a different role. 

Lysosomes

  • An organelle that contains enzymes that break down and digest inessential cellular components, such as a damaged organelle is called the lysosome.
  • Protein products packaged by the Golgi such as digestive enzymes are meant to stay inside the cell in order to break down certain materials.
  • The enzyme-containing vesicles are released by the Golgi, and they may form new lysosomes or fuse with existing lysosomes.

 Vacuoles

  • Vacuoles are generally characterized as storage bubbles of unpredictable shapes which are found in cells.
  • They are liquid-filled organelles enclosed by a film.
  • The vacuole stores the food or a spread of nutrients that a cell might need to survive.
  • In addition, it also stores waste products.
  • The by products are at last tossed out by vacuoles. Thus, the remainder of the cell is shielded from contamination.
  • The animal and plant cells have different sizes and numbers of vacuoles. Compared to animals, plant cells have larger vacuoles.

4. Diffusion and Osmosis

Diffusion

Diffusion is the net movement of molecules of a substance from a region of their higher concentration to a region of their lower concentration. Net movement means there are more molecules moving in one direction than in the opposite direction.

Example: Opening a bottle of perfume in a room will result in the gradual diffusion of the perfume from the region of higher concentration (the bottle) out into the room. Diffusion will continue until the perfume has a more or less uniform concentration throughout the bottle and room.

Osmosis

It can be defined as the movement of water molecules from a higher water concentration area to the area of less water concentration through a semipermeable membrane. For example, water in the roots of plants is transported through osmosis.

Difference between diffusion and osmosis:

Osmosis

Diffusion

It happens only in the liquid state.

It occurs in all states of matter i.e., solids, liquids or gases.

It should be movement of only water or solvent through semipermeable membrane from lower concentration to higher concentration.

Any type of substance that moves from higher concentration area to lower concentration area.

It is applied only for the solvent part of the solution.

Diffusion is applied to all states of matter.

 

It requires semipermeable membrane.

This phenomenon does not require semipermeable membrane.

Osmotic Solutions

There are three different types of solutions:

Isotonic Solution
Hypertonic Solution
Hypotonic Solution

An isotonic solution is one that has the same concentration of solutes both inside and outside the cell.

A hypertonic solution is one that has a higher solute concentration outside the cell than inside.

A hypotonic solution is one that has a higher solute concentration inside the cell than outside.

 

Types of Osmosis

Osmosis is of two types:

Endosmosis– When a substance is placed in a hypotonic solution, the solvent molecules move inside the cell and the cell becomes turgid or undergoes de plasmolysis. This is known as endosmosis.
Exosmosis– When a substance is placed in a hypertonic solution, the solvent molecules move outside the cell and the cell becomes flaccid or undergoes plasmolysis. This is known as exosmosis.