Microscopy:

The use of the microscope is known as microscopy. The first compound Microscope was developed by Zacharias Janssen, in Holland in 1595. It was simply a tube with lenses at each end and its magnification ranged from 3X to 9X.

Important Terms used in Microscopy:

Two important terms are used in microscopy i.e. magnification and resolving power.

1. Magnification:

Magnification is the increase in the apparent size of an object and it is an important factor in Microscopy.

1. Resolving Power or Resolution:

Resolving Power or Resolution is the measure of the clarity of an image. It is the minimum distance at which two objects can be seen as separate objects.

Resolution of human Eye:

The Human Naked eye can differentiate between two points, which are at least 0.1mm apart. This is known as the resolution of the Human Eye. If we place two objects 0.05mm apart, the human eye would not be able to differentiate them as two separate objects. The Resolution can be increased with the help of lenses.

Light Microscope (LM):

A light Microscope works by passing visible light through a specimen.

Structure of Light Microscope:

It uses two glass lenses. One lens produces an enlarged image of the specimen and the second lens magnifies the image and projects it into the viewer's eye or onto photographic film.

Micrograph:

A photograph taken through a microscope is called a Micrograph.

Function Of light Microscope:

The LM cannot resolve(distinguish) objects smaller than 0.2µm. It is about the size of the smallest bacteria. The image of a Bacterium can be magnified many times, but the Light Microscope cannot show the details of its internal structure.

When we use a micrograph on the page of a book, we see some words like “LM 109X” printed along the edge of the Micrograph. It tells us that the photomicrograph was taken through a Light Microscope and that the image has been magnified 109 times. 

Electron Microscope:

It is the most advanced form of Microscope

Structure of Electron Microscope:

In EM, the object and the lens are placed in a vacuum chamber and a beam of electrons is passed through the object. Electrons passed through or are reflected from the object and make an image. Electromagnetic lenses in the large end focus the image onto a screen or photographic film.

Resolving Power of Electron Microscope:

The EM has much more resolving Power than the LM. The most modern EM can distinguish objects as small as 0.2nm (nm) and 1nm=1/1000mm. It is a 1000-fold improvement over the LM.

Uses of Electron Microscope:

Under special conditions, EM can detect individual atoms. Cells, organelles, and even molecules like DNA and proteins are much larger than single atoms.

Types of Electron Microscope:

Biologists use two types of Electron microscopes:

1. Transmission Electron Microscope (TEM)
2. Scanning Electron Microscope (SEM)

Transmission Electron Microscope (TEM):

A transmission Electron Microscope (TEM) is used to study the details of the internal cell structure. The TEM can magnify objects about 250,000 times. In this microscopy, the specimen is cut into extremely thin sections. When the Electron beam is directed through the chamber, the electrons hit the specimen and are transmitted. The lenses focus the transmitted electron beam onto a screen or photographic film for viewing.

Scanning Electron Microscope (SEM):

Scanning Electron Microscope (SEM) is used to study the detailed architecture of cell surfaces. It uses an electron beam to scan the surface that has been coated with metal. When the beam hits the metal, it is not absorbed or transmitted. The electrons are reflected from the metal and are collected and used to produce an image. SEM doesn’t have great magnifying Power. It is about 10,000 to 100,000 times.

EM has revolutionized the study of cells and organelles. One problem with EM is that it cannot be used to study life processes because the specimen must be held in a vacuum chamber; i.e. all the air must be removed. To study the life process e.g., the movement of Amoeba, a light microscope is better.

(a) LM (Light Microscope)

(b) Transmission Electron Microscope (TEM)

(c) Scanning Electron Microscope (SEM)

History of the formulation of cell theory:

In the history of biological Sciences, ancient Greeks were the first to make comprehensive attempts to organize the data of the natural world. Aristotle presented organized observations to support the idea that all animals and plants are somehow related. Later this idea gave rise to questions like “Is there a fundamental unit or structure shared by all organisms?” however, before microscopes were first used in the 17^th century, No one knew with certainty that living organisms do share a fundamental unit i.e. cell.

Cells were the first described by a British scientist,

Robert Hooke:

In 1665. He used his self-made light microscope to examine a thin slice of cork. Hooke observed a “Honeycomb” of tiny empty compartments. He called the compartments in the cork “cellular”. His term has come to as cells. The First living cells were observed a few years later by the Dutch naturalist Antonie van Leeuwenhoek he observed tiny organisms (from Pond water) under his microscope and called them “animalcules”

Robert Hooke:

He was a chemist, mathematician, and physicist. His remarkable engineering abilities enabled him to invent and improve many mechanical devices, including timepieces, the quadrant, and the Gregorian Telescope. His observation about the section of cork is also illustrated here.

For another century and a half, the general importance of cells was not appreciated by biologists. In 1809, Jean Baptist De-Lamarck proposed that “Nobody can have life if its constituent parts are not cellular tissues or are not formed by cellular tissues”.

In 1831, a British Botanist Robert Brown discovered a nucleus in a cell. In 1838, a German Botanist Matthias Schleiden studied plant tissues and made the first statement of the first theory. He stated that all plants are “aggregates of individual cells which are fully independent and separate beings”. One year later in 1839, a German Zoologist Theodor Schwann reported that all animal tissues are also composed of individual cells. Thus, Schleiden and Schwann proposed cell theory in its initial form i.e. “all living things are composed of living cells”.

Twenty years later in 1855, Rudolph Virchow, A German Physician, proposed an important extension of cell theory. He proposed that all living cells arise from pre-existing cells (“Omnis cellula e cellula”). In 1862, Louis Pasteur presented experimental evidence for this idea.

Cell theory:

The cell theory, in its modern form, includes the following principles.

1. All organisms are composed of one or more cells, within which all life processes occur
2. Cells are the smallest living things, the basic unit of organization of all organisms Cells arise only by divisions in previously existing cells.

Sub-cellular and Acellular particles:

According to the first principle of the cell theory, all organisms are composed of one or more cells. The Discovery of viruses, prions, and Viroids claims that the statement is not so universal. They are not composed of cells, rather they are sub-cellular or acellular particles that do not run any mechanism inside them, but they show some characteristi9cs of living organisms i.e., they can increase in number and can transmit characters to the next generations.

     The ultra-structure of an animal cells.

Cell wall:

The cell wall is a non-living and strong component of the cell, located outside the plasma membrane. It provides shape, strength, protection, and support to the inner living matter (protoplasm) of the cell.

Structure of Cell wall:

Plant cells have a variety of chemicals in their cell walls. The outer layer of the plant cell wall is known as the primary wall and cellulose is the most common chemical in it. Some plant cells, for example, xylem cells, also have secondary walls on the inner side of the primary wall. It is much thicker and lignin and other chemicals are embedded in it in the walls of neighboring cells there are cytoplasmic connections called Plasmodesmata. Through these connections, cells transfer chemicals among each other.

Presence of Chitin in the Cell wall of fungi:

Fungi and many protists have cell walls although they do not contain cellulose. Their cell walls are made of a variety of chemicals. For example, chitin is present in the cell wall of fungi. Prokaryotes have a cell wall composed of peptidoglycan that is a single large polymer of amino acids and sugar.

There are two general classes of cells: prokaryotic and Eukaryotic. The Evolution of Prokaryotic cells preceded that of Eukaryotic cells by 200 billion years.

• Streptococcus pyogenes, the bacterium that causes infection in the upper respiratory tract (e.g. sore throat) is an example of prokaryotes.
• Yeast, an organism that carries out fermentation and makes our bread rise is an example of unicellular eukaryotes.

Humans, of course, are an example of multicellular Eukaryotes. 

The function of Cell Membrane:

All prokaryotic and eukaryotic cells have a thin and elastic cell membrane, covering the cytoplasm. The cell membrane functions as a semi-permeable barrier, allowing very few molecules across it while fencing the majority of chemicals inside the cell. In this way, the membrane maintains the internal composition of the cell to a constant or nearly constant level. In addition to this vital role, the cell membrane can also sense chemical messages and can identify materials and other cells.

1. Another lipid, cholesterol is an important component of cell membranes embedded in the inner region of the lipid bilayer. Most bacterial cell membranes do not contain cholesterol.
2. In Eukaryotic cells most organelles e.g. mitochondria, chloroplast, Golgi Apparatus, and Endoplasmic Reticulum are also bounded by cell membranes.

Chemical analysis reveals that the cell membrane is mainly composed of proteins and lipids with small quantities of carbohydrates. Electron microscopic examinations of cell membranes have led to the development of the fluid-mosaic model of cell membranes.

The fluid-mosaic model of cell membrane:

1. Lipids are aligned in such a way that they make a bilayer. It gives fluidity and elasticity to the cell membrane.
2. Proteins may be fully submerged in the lipid bilayer or some of them may "stick out into the interior and outside of the cell. These proteins function as gateways that allow certain molecules to cross into and out of the cell.
3. Small amounts of carbohydrates are also found in cell membranes. These are joined with proteins (in the form of glycoproteins) or with lipids in the form of glycolipids). Both these forms act as fingerprints of the cell.

Formation of Cytoplasm:

The cytoplasm is defined as the material between the plasma membrane (cell membrane) and the nuclear envelope. It is a semi-viscous and semi-transparent substance. The chemical analysis of cytoplasm reveals that it contains water in which many organic (proteins, lipids, and carbohydrates) and inorganic salts are completely or partially dissolved.

The function of cytoplasm:

The cytoplasm of a cell provides space for the proper functioning of organelles and also acts as a site for various biochemical (metabolic) reactions for example Glycolysis (breakdown of glucose during cellular respiration).

Structure of Cytoskeleton:

The Cytoskeleton is an important, complex, and dynamic cell component. It is invisible under a light Microscope.

Functions of Cytoskeleton:

The cytoskeleton makes the cell shapes, anchors organelles, and moves parts of cells in growth and motility.

Types of Filaments:

There are many types of filaments that make up the cytoskeleton but the most important types are microtubules and microfilaments.

1. Microtubules:

   Microtubules are made up of tubulin subunits and are often used by cells to hold their shape. Microtubules are also the component of cilia and Flagella.

2. Microfilaments:

   Microfilaments are made up of actin subunits. These microfilaments are approximately one-third of the diameter of the microtubule and are often used by cells to change their shapes to hold structures.

Cell organelles: Organelles are small structures within cells that perform dedicated functions. There are about a dozen organelles found in eukaryotic cells.

Ribosomes:

Ribosomes are tiny granular structures that are either floating freely in the cytoplasm or are bound to the Endoplasmic reticulum.

Structure of Ribosomes:

Each ribosome is made up of an almost equal amount of proteins and ribosomal RNA (rRNA). Ribosomes are not bound by membranes and so are found in Prokaryotes.

The function of Ribosomes:

Eukaryotic ribosomes are slightly larger than Eukaryotic ones. Ribosomes are the sites for protein synthesis where the message carried by mRNA is translated into proteins. Protein synthesis is extremely important to cells, and so a large number of ribosomes are found throughout cells (often numbering in hundreds and thousands). Ribosomes disassemble in two subunits when not involved in protein synthesis.

Nucleus:

A prominent nucleus occurs in eukaryotic cells. In animal cells, it is present in the center while in mature plant cells, due to the formation of a large central vacuole, it is pushed to the side.

Structure of Nucleus:

The nucleus is bounded by a double membrane known as a nuclear envelope.

Nuclear Envelope:

The nuclear envelope contains many small pores that enable it to act as a differentially-permeable membrane.

Nucleoplasm:

Inside the nuclear envelope a granular matrix, the nucleoplasm, is present. The nucleoplasm contains one or two nucleoli (singular, nucleolus) and chromosomes.

Visibility of Nucleolus:

The nucleolus is usually visible as a dark spot and it is the site where ribosomal RNA is formed and assembled as ribosomes.

Chromosomes: One of the rods shaped bodies found in the nucleus of cells that contain genetic information (DNA) Chromosomes are visible only during cell division while during the interphase (non-dividing phase) of the cell they are in the form of fine thread-like structures known as chromatin Structure of DNA: Chromosomes are composed of Deoxyribonucleic acid (DNA) and histone proteins provide structural support to DNA for making the structure of the chromosome.

Messenger Ribonucleic Acid (mRNA):

DNA contains the message for the synthesis of specific proteins. According to this message, a molecule of messenger Ribonucleic Acid (mRNA) is synthesized. In this way, the message is handed over to mRNA, which carries it to ribosomes. Ribosomes manufacture specific proteins according to the message present on mRNA.

The function of DNA:

In this way, DNA controls all the activities of the cell and is also responsible for the transmission of characteristics to the next generation. That is why it is honored as hereditary material.

Main Function of Nucleus:

The nucleus is called the brain of the cell. The nucleus controls all the activities of the cell.

Note:

The prokaryotic cells do not contain a prominent nucleus rather their chromosome is made of DNA only and is submerged in the cytoplasm.

Plastids: Plastids are also membrane-bound organelles that only occur in plants and photosynthetic protists.

Types of Plastids:

There are three types of Plastids:

1. Chloroplasts
2. Leucoplasts
3. Chromoplasts

Like mitochondria, the chloroplast is also bound by a double membrane. The outer membrane is smooth while the inner one gives rise to membranous sacs called thylakoids (the stack of thylakoids is known as a granum (plural = grana]) floating in a fluid termed the stroma.

The function of chloroplast:

Chloroplasts are the sites of photosynthesis in eukaryotes. They contain chlorophyll, the green pigment necessary for photosynthesis, and associated accessory pigments. These pigments are present in thylakoids of the grana of chloroplasts.

Chromoplasts:

The second type of Plastids in Plants is chromoplasts. They contain pigments associated with bright colors and are present in the cells of Flower petals and Fruits. Their function is to give color to the part and thus help in pollination and dispersal of the Fruit.

Endoplasmic Reticulum (ER):

The endoplasmic reticulum is a network of interconnected channels that extends from the cell membrane to the nuclear envelope. This network exists in two forms;

1. Rough Endoplasmic Reticulum (RER):

Rough Endoplasmic Reticulum (RER) is so-named because of its rough appearance due to the numerous ribosomes that are attached to it.

The function of Rough Endoplasmic Reticulum (RER):

It connects to the nuclear envelope through which the messenger RNA (mRNA) travels to the ribosomes. Due to the presence of ribosomes, RER serves a function in protein synthesis.

1. Smooth Endoplasmic Reticulum (SER):

The Smooth Endoplasmic Reticulum (SER) lacks ribosomes and is involved in lipid metabolism and the transport of materials from one part of the cell to the other. It also detoxifies the harmful chemicals that have entered the cell.

Golgi Apparatus:

An Italian physician named Camillo Golgi discovered a set of flattened sacs (cisternae) that are stacked over each other. Golgi named this set of cisternae as Golgi apparatus. It is also called the Golgi body or Golgi complex and is found in both plant and animal cells.

Functions of Golgi Apparatus:

It modifies molecules coming from rough ER and packs them into small membrane-bound sacs called Golgi vesicles. These sacs can be transported to various locations in the cell or its exterior, in the form of secretions.

Leucoplasts:

They are the third type of plastids. They are colorless and store starch, proteins, and lipids. They are present in those cell parts where food is stored.

Lysosomes:

In the mid-twentieth century, the Belgian scientist Christian Rene de Duve discovered lysosomes.

Formation of lysosomes:

These are single-membrane-bound organelles. Lysosomes contain strong digestive enzymes and work for the breakdown (digestion) of food and waste materials within the cell.

The function of lysosomes:

During its function, the lysosome fuses with the vacuole that contains the targeted material, and its enzymes break down the material.

We can see the advantage of compartmentalization of Eukaryotic cell; the cell could not support such destructive enzymes if they were not contained in membrane bound lysosome.

Mitochondria: Mitochondria (singular: Mitochondrion) are double membrane-bounded structures found only in Eukaryotes. They are the sites for Aerobic respiration and are the major energy production centers.

Structure of Mitochondria:

The outer membrane of mitochondria is smooth but the inner membrane forms many infoldings called cristae (singular: crista) in the inner mitochondrial matrix. They serve to increase the surface area of the inner membrane on which membrane-bound reactions can take place.

Functions of Mitochondria:

Mitochondria have their DNA and ribosomes, and those ribosomes are more similar to bacterial ribosomes than eukaryotic ribosomes.

Ribosomes, because they are not membrane-bounded.

The existence of double membranes has led many biologists to theorize that mitochondria are descendants of some bacteria that were engulfed by a larger prokaryotic cell billions of years ago. This fascinating theory of Symbiosis (living together) explains the evolution of eukaryotic cells.

The cell could not support such destructive enzymes if they weren’t contained in a membrane-bound lysosome.

Animals and many unicellular organisms have hollow and cylindrical organelles known as centrioles.

Formation of centrioles:

They are made up of nine triplets of microtubules that are composed of an important protein known as, tubulin. Animal cells have two centrioles located near the exterior surface of the nucleus. The two centrioles are collectively called centrosomes.

The function of centrioles:

Their function is to help in the formation of spindle fibers during cell division. In cells that contain cilia and flagella, centrioles are involved in the formation of cilia and flagella.

Vacuoles:

Vacuoles are fluid-filled single membrane-bound organelles. Cells have many vacuoles in their cytoplasm.

Function and working of vacuoles:

When a plant cell matures its small vacuoles absorb water and fuse to form a single large vacuole in the center. The cell in this state becomes turgid. Many cells take in materials from the outside in the form of a food vacuole and then digest the material with the help of a lysosome. Some unicellular organisms use contractile vacuoles for the elimination of wastes from their bodies.

Similarities between Prokaryotic and Eukaryotic cells:

1. They both have DNA in their genetic material.
2. They are both membranes bound.
3. They both have ribosomes.
4. They have similar metabolism.
5. They are both amazingly diverse in forms.

Difference Between Prokaryotic and Eukaryotic Cells:

1. The significant difference between prokaryotic and eukaryotic cell is that eukaryotic cell has a prominent nucleus and many membrane-bound organelles, which are not present in prokaryotic cell
2. The DNA of a prokaryotic cell floats in the cytoplasm near the center (this region is called nucleoid); the DNA of a eukaryotic cell is held within the nucleus.
3. There is a much higher level of intracellular division of labor in eukaryotic cells than in prokaryotic.

Some more differences between prokaryotic and eukaryotic cells: 

The human body is made up of about 200 different types of cells.

(a) Size and shape:

The function of Nerve cells:

Nerve cells long for the transmission of nerve impulses.

The function of Xylem cells:

Xylem cells are tube-like and have thick walls for the conduction of water and support.

The function of Red blood cells:

Red blood cells are round to accommodate globular hemoglobin.

(b) Surface area to the volume ratio:

The function of root hair cells:

Root hair cells have a large surface area for the volume ratio maximum absorption of water and salts.

(c) Presence or absence of organelles:

1. Ceils involved in making secretions have more complex ER and Golgi apparatus
2. Cells involved in photosynthesis have chloroplasts.

(1) Function of nerve cells:

Nerve cells conduct nerve impulses and thus contribute to coordination in the body.

(2) Functions of Muscle cells:

Muscle cells undergo contraction and share their function in movements of the body.

(3) Functions of Red Blood cells:

Red blood cells carry oxygen and white blood cells carry foreign agents and so contribute to roles of transportation in blood and defense.

(4) Functions of skin cells:

Some skin cells act as physical barriers against foreign materials and some as receptors for temperature, touch, pain, etc.

(5) Functions of cells of bone:

The cells of bone deposit calcium in their extracellular spaces to make the bone tough and thus contribute to the supporting role of bones.

The cell works as an open system:

A cell works as an open system i.e. it takes in substances needed for its metabolic activities through cell membranes. Then it performs the metabolic processes assigned to it. Products and by-products are formed in metabolism. Cell either utilizes the products or transports them to other cells. The by-products are either stored or excreted out of the cell.

Cell size and surface area to volume Ratio:

Cells vary greatly in size.

Mycoplasmas:

The smallest cells are bacteria called mycoplasmas, with diameter between 0.1 µ to 1.0 µ m.

Bulkiest cells:

The bulkiest cells are bird eggs, and the longest cells are some muscle cells and nerve cells. Most cells lie between these extremes.

Note:

Most cells are small in size. Large cells have less surface area in relation to their volume while small cells of the same shape have more surface area.

The surface-to-volume relationship of cube-shaped cells:

The figure shows 1 large cell and 27 small cells. In both cases the total volume is the same:

Volume = 30 µ m 30 m x 30 µ m = 27,000 µ m³

Total surface areas:

In contrast to the total volume, the total surface areas are very different. Because the cubical shape has 6 sides, its surface area is 6 times the area of 1 side. The surface areas of the cubes are as follows:

Surface area of 1 large cube = 6 x (30 µ m x 30 µ m) = 5400 µ m²

Surface area of 1 small cube = 6 x (10 µ x 10 µ m) = 600 µ m² and

Surface area of 27 small cubes = 27 x 600 µ m = 16.200 µ m²

This relationship between cell size and surface area to volume ratio works to limit cell size.

As the size of a cell increases, cell volume increases more rapidly than its surface area.

Effect of cell size on surface area

Cell size and shape are related to cell function. Bird eggs are bulky because they contain a large number of nutrients for developing young ones. Long muscle cells are efficient in pulling body parts together. Lengthy nerve cells can transmit nerve signals between distant parts of an animal’s body. On the other hand, small cell size can also have many benefits. For example, human red blood cells are only 8µm in diameter and therefore can move through our tiniest blood vessels.

We know that cell membranes act as a barrier to most but not all molecules. That is why cell membranes are called differentially (or semi-) permeable membranes separating their inner cellular environment from the outer environment.

Functions of cell membranes:

While exchanging matter with the cell environment cell membrane maintains equilibrium inside the cell walls as outside. This control of the passage of molecules into and out of the cells is made possible through the phenomena of diffusion, osmosis, filtration, active transport, endocytosis, and exocytosis.

Diffusion:

Diffusion is the movement of a substance from an area of higher concentration to an area of lower concentration i.e. along a concentration gradient.

OR

Diffusion:

Diffusion is the net movement of molecules or ions from an area of higher concentration to an area of lower concentration i.e. along the concentration gradient.

Explanation:

Since the molecules of any substance (solid, liquid, or gas) are in motion then that substance is about 0 degrees Kelvin or -273 degrees C, energy is available for the movement of molecules from a higher potential state to a lower potential state. In a substance, the majority of the molecules move from higher to lower concentration, although there will be some that move from low to high. The overall movement is thus from high to low concentration. Eventually, if no input in the system the molecules will reach a state of equilibrium where they will be distributed equally throughout the system.

Passive Transport:

Because a cell doesn’t expend energy when molecules diffuse across its membrane, this diffusion is a type of passive transport.

Significance of diffusion:

Diffusion is one principle method of movement of substances within cells as well as the method for essential molecules to cross the cell membrane. Carbon dioxide and Oxygen are among the few simple molecules that can cross the molecules by diffusion. Gas exchange in gills and lungs operates by this process.

Facilitated diffusion:

Many molecules do not diffuse freely across cell membranes because of their size and charge. Such molecules are taken into or out of the cells with the help of transport proteins present in cell membranes.

When one of these transport proteins makes it possible for a substance to move down its concentration gradient (from higher to lower concentration) the process is called facilitated diffusion. The rate of facilitated diffusion is higher than simple diffusion.

Facilitated diffusion is a type of Passive transport:

Facilitated diffusion is also a type of Passive transport because there is no expenditure of energy in this process.

Osmosis:

Osmosis is the movement of water through a selectively permeable membrane from a solution of lesser solute concentration to a solution of higher solute concentration.

Rules of osmosis and concept of tonicity of solutions:

The rules of osmosis can be best understood through the concept of tonicity of solutions. The term tonicity refers to the relative concentration of solutes in the solutions being compared.

1. Hypertonic solutions are those in which more solute is present.
2. Hypotonic are those with less solute.
3. Isotonic solutions have equal concentrations of solute.

Explanation:

In a hypertonic solution, the solute molecules attract clusters of water molecules so that fewer water molecules are free to diffuse across the membrane. On the other hand, in a hypotonic solution, there are more free water molecules and there is a net movement of water from hypotonic solution to hypertonic solution.

Water balance problems in the animal cells:

OR

Animal cell in the hypotonic environment:

1. When an animal cell, such as the red blood cell, is placed in an isotonic solution, the volume of the cell remains constant because the rate at which water is entering the cell is equal to the rate at which it is moving out.
2. When a cell is placed in a hypotonic solution (which has a lower salt concentration than the cell) water enters and the cell swells and may rupture like an over-filled balloon.
3. An animal cell placed in a hypertonic solution (which has a higher salt concentration than the cell) will lose water and will shrink in size.
4. So in hypotonic environments (freshwater) animal cells must have ways to prevent the excessive entry of water and in hypertonic environments (seawater) they must have ways to prevent excessive loss of water.

Water balance problems in plant cells:

OR

Plant cell in the hypotonic environment:

1. Most plant cells live in a hypotonic environment because there is a low concentration of solutes in extracellular fluids than in their cells. As a result, water tends to move first inside the cell and then inside the vacuole.
2. When the vacuole increases in size the cytoplasm presses firmly against the interior of the cell wall, which expands a little.
3. Due to the strong cell wall, plant cell does not rupture but instead becomes rigid. The internal pressure of such a rigid cell is known as turgor pressure and this phenomenon is known as turgor.
4. In an isotonic environment the plant cells are flaccid (loose / not firm) because the net uptake of water is not enough to make it turgid.
5. In a hypertonic environment a plant cell loses water, causing the cytoplasm to shrink within the cell wall. The shrinking of cytoplasm is called plasmolysis.

Functions of stomata during Day time:

Stomata (openings) in the leaf epidermis are surrounded by guard cells. During daytime guard cells are making glucose and so are hypertonic (higher concentrations of glucose) than their nearby epidermis cells. Water enters them from other cells and they swell. In this form they assume a rigid bowed shape, creating a pore between them.

The function of Stomata during Night:

At night when there are low solute concentrations in guard cells water leaves them and they become flaccid. In this form both guard, and cells rest against one another closing the opening.

Most plant cells live in a hypotonic environment because there is a low concentration of solutes in extracellular fluids than in their cells. As a result, water tends to move first inside the cell and then inside the vacuole. When the vacuole increases in size the cytoplasm presses firmly against the interior of the cell wall, which expands a little. Due to the strong cell wall, plant cell does not rupture but instead becomes rigid. The internal pressure of such a rigid cell is known as turgor pressure and this phenomenon is known as turgor. 

The function of semi-permeable membranes:

The knowledge about semi-permeable membranes is applied for various purposes. We know that a semi-permeable membrane is capable of separating substances when a driving force is applied across it. Artificially synthesized semi-permeable membranes are used for the separation of bacteria from viruses because bacteria cannot cross a semi-permeable membrane. In advanced water treatment technologies, membrane-based filtration systems are used. Semi-permeable membranes efficiently separate salts from water under pressure.

Hypertonic and Hypotonic are related terms. Therefore, you must say what the solution is compared to.

Filtration:

Filtration is a process by which small molecules are forced to move across the semi-permeable membrane with the aid of hydrostatic (water) pressure or blood pressure.

The function of Filtration:

1. In the body of an animal blood pressure forces water and dissolved molecules to move through the semi-permeable membranes of the capillary wall cells.
2. In Filtration the pressure cannot force large molecules such as protein, to pass through the membrane pores.

Active transport:

Active Transport is the movement of molecules from an area of lower concentration to an area of higher concentration. This movement against the concentration gradient requires energy in the form of ATP.

Functions of Active Transport:

In this process carrier proteins in the cell membrane use energy to move the molecules against the concentration gradient. For example, the membranes of nerve cells have carrier proteins in the form of a “Sodium-potassium pump”. In resting (not conducting nerve impulse) nerve cells, this pump spends energy (ATP) to maintain higher concentrations of K+ and lower concentrations of Na+ inside the cell. For this purpose, the pump actively moves Na+ to the outside of the cell where they are already in higher concentration. Similarly, this pump moves K+ from the outside to the inside of the cell where they are in higher concentration.

Filtration.

Endocytosis:

It is a special type of movement of material across the cell membrane. In endocytosis, bulky materials, rather than individual molecules, are moved across the cell membrane.

Steps involved in Endocytosis:

Endocytosis occurs in the following steps:

1. A portion of the cell membrane invaginates (depressed inward).
2. The material from the outside is taken inside the invagination.
3. The open ends of the invagination seal form a small vesicle.
4. The vesicle detaches from the cell membrane and moves into the cytoplasm.

Types of Endocytosis:

The two forms of Endocytosis are phagocytosis (cellular eating) and pinocytosis (cellular drinking). In phagocytosis, the cell takes in solid material while in pinocytosis cell takes in liquids in the form of droplets.

Exocytosis:

It is the process through which bulky materials are exported.

Steps involved in Exocytosis:

Exocytosis occurs in the following steps:

1. The bulky material is packed inside a membrane and a vesicle is formed.
2. The vesicle moves out to the cell membrane
3. The vesicle fuses with the membrane and releases its contents into the extracellular environment.

Functions of Exocytosis:

This process adds a new membrane, which replaces the part of the cell membrane lost during endocytosis.

In a colony of cells, there are many cells and each cell performs all general functions on its own. Such a group does not get tissue level of organization because cells are not specific and there is no coordination among them.

Tissues.

Animal Tissues:

In the bodies of animals, there are four major categories of tissues: epithelial tissue, connective tissue, muscle tissue, and nervous tissue.

1. Epithelial tissue:

Epithelial tissue covers the outside of the body and lines organs and cavities. The cells in this type of tissue are very closely packed together and joined with little space between them.

The function of Epithelial tissue:

Epithelial tissue helps to protect organisms from microorganisms, injury, and fluid loss. These tissues are commonly classified on the base of the shape of the cells as well as the number of cell layers.

Types of Epithelial tissue and their function:

Some types include:

2. Connective Tissue

As the name implies, connective tissue serves a "connecting" function. It supports and binds other tissues. Unlike epithelial tissue, connective tissue typically has cells scattered throughout an extracellular matrix.

Types of Connective Tissue:

There are many types of this tissue.

3. Muscle Tissue:

Muscle tissue consists of bundles of long cells called muscle fibers. It is the most abundant tissue in a typical animal. The cells of this tissue can the contract

Voluntary Action Muscle Tissue:

The skeletal muscles are voluntary in action i.e. their contraction is under the control of our will.

 Involuntary Action Muscle Tissue:

The smooth and cardiac muscles are involuntary in action i.e. their contraction is not under the control of our will.

1. Nervous Tissue:

Animal's survival depends on its ability to respond appropriately to stimuli from its environment. This ability requires the transmission of information from one part of the body to another. Nervous tissue forms a communication system and performs this task.

Specification of nervous tissue:

This tissue is mainly composed of nerve cells, or neurons, which are specialized to conduct messages in the form of nerve impulses.

Location of nervous tissue:

Nervous tissue is found in the brain, spinal cord, and nerves.

Barrier and protective function

Skeletal.

Exercise does not increase the number of our skeletal muscle cells; it is simply enlarging those already present.

Major categories of tissues in plants: There are two major categories of tissues in plants i.e. simple tissues and compound(complex) tissues.

Simple Tissues:

The tissues which are made up of a single type of cells are called simple tissues.

1. Meristematic tissues
2. Permanent tissues

Meristematic Tissues:

These tissues are composed of cells, which can divide.

Properties of Meristematic tissues:

The cells of meristematic cells have the following properties:

1. They have thick cytoplasm, with a large nucleus in the center and with small or no vacuoles.
2. They are thin-walled.
3. They are alike and there are no intercellular spaces among them.

In-plant cell division occurs solely in meristematic regions.

Types of Meristematic Tissues:

Two main types of meristematic tissues are recognized in plants:

1. Apical Meristems:

Apical meristems are located at the apices or tips of roots and shoots. When they divide, they cause an increase in the length of the plant. Such growth is called primary growth.

1. Lateral Meristems:

Lateral meristems are located on the lateral sides of roots and shoots. By dividing they are responsible for the horizontal expansion of the parts of the plant. Such growth is called secondary growth.

Types of Lateral Meristems:

Lateral meristems are of further two types i.e. vascular cambium and cork cambium.

Vascular Cambium:

Vascular cambium is present between the xylem and phloem tissues. Its cell divides and forms new xylem tissue toward the center and a new phloem tissue toward the outside.

Cork Cambium:

Cork cambium is present on the outer lateral sides and its cells are responsible for making the characteristic cork layer.    

Intercalary meristem:

Intercalary meristem occurs in form of small patches of meristem present among the mature tissues. These are common in grasses and help in the regeneration of parts removes by herbivores etc.

Permanent tissues:

Permanent tissues originate from the meristematic tissues. These tissues are composed of cells, which cannot divide.

Types of Permanent Tissues:

They are further classified into the following types:

1. Epidermal Tissues:

Epidermal tissues are composed of a single layer of cells and they cover the plant body. They act as a barrier between the environment and the internal plant tissues.

Functions of Epidermal tissues:

They are also responsible for the absorption of water and minerals primarily in the root region. On stem and leaves, they secrete Cutin (the coating of cutin is called the cuticle) which prevents evaporation.

Epidermal tissues also have some specialized cells that perform specific functions.

For Example:

Root hairs:

Root hairs absorb water and minerals.

Leaf hairs:

Leaf hairs (1-2 cells) reflect light to protect against overheating and excessive water loss. The layer of leaf hairs acts to hold in a layer of humidity “trapped”. This layer also prevents air from moving directly against the stomata which would encourage water loss.

Stomata:

Stomata are made by guard cells and are most abundant on the underside of leaves. They regulate the diffusion of CO₂ into the leaf for photosynthesis as well as regulate the loss of water from the leaf.

Salt Glands:

Salt glands are the waste bins for the excess salts absorbed from the soil. They form a crust of salt in leaves which reflects light to prevent overheating.

1. Ground Tissues:

Ground tissues are simple tissues made up of parenchyma cells. Parenchyma cells are the most abundant cells in plants.

The function of ground tissues:

Overall they are spherical but flat at the point of contact. They have thin primary cell walls and large vacuoles for the storage of food. In leaves, are cold mesophyll and are the sites of photosynthesis. In other parts, they are the sites of respiration and protein synthesis.

1. Support tissues:

These tissues provide strength and flexibility to the plants.

Types of support tissues:

They are further of two types:

1. Collenchyma tissue:

They are found just beneath the epidermis in the cortex of young herbaceous stems and the midribs of leaves and petals of flowers, they are made of elongated cells with evenly thickened primary cell walls. They are flexible and function to support the organs in which they are found.

1. Sclerenchyma tissue:

They are composed of cells with rigid secondary cell walls. The cell walls are hardened with lines in lignin, which is the main chemical component of wood. Mature sclerenchyma cells cannot elongate and most of them are dead.

Types of Sclerenchyma tissue:

There are two types of cells in Sclerenchyma tissues:

• Fiber cells:

Fiber cells are associated with phloem and xylem tissues.

• Sclerite cells:

Sclerite cells are found in seed coats.

Compound (complex) tissues:

A plant tissue composed of more than one type of cell is called a compound or complex tissue. Xylem and phloem tissues found only in vascular plants are examples of compound tissues.

Types of compound (complex) tissues:

1) Xylem tissue:

Xylem tissue is responsible for the transport of water and dissolved substances from roots to the aerial parts. Due to the presence of lignin, the secondary walls of its cells are thick and rigid. That is why xylem tissue provides support to the plant body.

Types of cells found in xylem tissue:

The following types of cells are found in xylem tissues:

1. Vessel elements of cells:

   These are short and wide cells with thick secondary cell walls. The vessel cells are dead and hollow. They lack end walls and join together to form long tubes.

2. Tracheid:

   These are long cylinder cells with overlapping ends. Water moves upward from tracheid to tracheid.

2) Phloem Tissue:

Phloem tissue is responsible for the conduction of dissolved organic matter (food) between different parts of the plant body.

Types of cells found in phloem tissue:

The following types of cells are found in phloem tissue:

1. Sieve tube cells:

   These are long cells. Their end walls have small pores and are called sieve plates. Many sieve tube cells join to form long pipelines called sieve tubes. During development, they lose their nuclei and ribosomes et cans possess little protoplasm.

2. Companion cells:

   Each sieve tube cell is accompanied by a companion cell. The companion cells contain functional DNA and ribosomes and they make proteins for the sieve tube cells.

Leaf hairs:

Leaf hairs reflect light and are important for plants in dry regions. Here excess light may lead to photobleaching of pigments and excess absorption may overheat the tissues.

Most parenchyma cells can develop the ability to divide and differentiate into other types of cells and they do so during the process of repairing and injury.