CBSE Class 9 Science Advanced Biology Fundamental Unit of Life Notes Set 01

Fundamental Unit of Life

A. Cell

A cell is the smallest unit of structure and function in a living organism. The body of every organism is put together from such units, and each unit performs the basic work of life. The name comes from the Latin word cellula, which means a small room. The branch of biology that deals with the structure and make-up of cells is called cytology.

Some milestones in the study of the cell:

B. Cell Theory

The cell theory was put forward by the two biologists Schleiden (a botanist) and Schwann (a zoologist). It was later enlarged by Rudolf Virchow, who stated that new cells come only from cells that already exist (omnis cellula e cellula). The theory says:

1. All plants and animals are made up of cells.

2. The cell is the basic unit of life.

3. All cells arise from pre-existing cells.

Exception: viruses do not have a cellular organisation, so they are the exception to the cell theory.

C. Types of Cell

(a) On the basis of organisation: prokaryotic and eukaryotic cells

Prokaryotic cells are primitive, simple cells. Their genetic material lies in the cytoplasm as a poorly defined nucleoid, because there is no nuclear membrane and no nucleolus. Bacteria are the usual example. Many bacteria also carry small extra circular pieces of DNA called plasmids, apart from the main DNA loop.

A bacterial (prokaryotic) cell

Eukaryotic cells are well developed cells. They have a true, advanced nucleus enclosed by a nuclear membrane, and they contain many membrane-bound organelles. Plant and animal cells are eukaryotic.

(b) On the basis of differentiation

1. Undifferentiated cells are unspecialised. They keep dividing by mitosis and so supply new cells for the growth and upkeep of tissues.

2. Differentiated cells are specialised cells. They are formed from undifferentiated cells when changes in structure and function take place during the development and growth of the organism.

3. Dedifferentiated cells are specialised cells that go back to a general, embryo-like, actively dividing state. This often happens during regeneration.

D. Cell Shape

Cells come in many shapes and sizes, and the shape of a cell is suited to the job it does. Most cells are roughly spherical, but a cell may also be elongated (nerve cell), branched (pigment cells), disc-like (red blood cell) or spindle-shaped (muscle cell).

Cells of various shapes

E. Cell Size

The size of a cell depends on the kind of organism. Some cells can be seen only under a microscope and some are big enough to see with the naked eye. Cell size ranges from about 0.2 µm to 18 cm.

1. A typical cell of a multicellular organism measures about 20 to 30 µm.

2. The largest cell is the egg of an ostrich: 18 cm across with its shell and 15 cm across without the shell.

3. The longest cell in animals is the nerve cell, which may be a metre or more in length. In plants the longest cell is the hemp fibre.

4. The smallest cells known are the PPLOs (pleuropneumonia-like organisms), for example Mycoplasma, which is only 0.1 µm across.

Brain cells (neurons) are the longest-lived cells of the body; they last for the whole life of a person.

Why smaller cells are more active: a small cell has a larger surface area for its volume. This allows faster diffusion of (i) nutrients into the cell, (ii) respiratory gases, that is oxygen into the cell and carbon dioxide out of it, and (iii) waste products out of the cell.

F. Components of a Cell

Inside a cell the work is divided among separate parts called cell organelles, each with its own function. Every cell has three basic components: (a) the cell coat, (b) the nucleus and (c) the cytoplasm. The chart below shows how the parts of a plant cell are arranged.

Components of a plant cell

(a) Cell coat

(i) Cell wall

The cell wall is the outermost covering of a plant cell. It is absent from animal cells. It is a rigid, strong, thick and porous non-living layer made mainly of cellulose and hemicellulose. The walls of two neighbouring cells are cemented together by a layer called the middle lamella, which is made of calcium and magnesium pectate. Cotton, jute and coconut fibres, which have great commercial value, are the cell walls of dead cells.

The material of the wall differs from group to group:

Functions of the cell wall:

1. It gives the cell its shape and keeps that shape.

2. It gives strength, so the plant can bear stress and strain.

3. It is permeable, so molecules of different sizes can pass through it.

Cell wall: T.S. of a plant cell and L.S. of the walls of two adjacent cells (S₁, S₂, S₃ are the layers of the secondary wall)

(ii) Cell membrane

The cell membrane is also called the plasma membrane or plasmalemma; the name plasma membrane was given by Nageli. It is present in plant and animal cells alike. In an animal cell it is the outermost boundary, while in a plant cell it lies just inside the cell wall.

It is made of lipids and proteins. A virus has no membrane of its own. It shows the characters of life only after getting inside a living host, whose cell machinery it then uses to make copies of itself.

Singer and Nicolson proposed the fluid mosaic model in which protein molecules lie scattered in, and across, a double layer of lipid molecules, like tiles of a mosaic floating in a fluid. The membrane is in a quasi-fluid state and is about 75 Å thick. It is flexible: it can fold, break and join up again. It is selectively permeable, which means it lets only some substances enter or leave.

Fluid mosaic model of the cell membrane

Functions of the plasma membrane:

1. It gives the cell its shape.

2. It protects the inner parts of the cell mechanically.

3. Only useful substances are allowed in.

4. It makes the cell specific, that is, different from other cells.

5. It is the boundary that separates the cytoplasm from the surroundings.

Movement of substances across the plasma membrane

1. Diffusion. Molecules or ions move from a region where they are more concentrated to a region where they are less concentrated. Oxygen and carbon dioxide cross the membrane in this way.

2. Osmosis. It is the movement of water (the solvent) through a semipermeable membrane from a solution of lower concentration to a solution of higher concentration. It can be described as the diffusion of the solvent. Movement of solvent into the cell is endosmosis and movement out of the cell is exosmosis. Diffusion and osmosis need no energy from the cell, so they are called passive transport.

3. Mediated transport. Certain molecules need the help of carrier proteins, known as transport proteins or permeases, to get across the membrane. A permease forms a tiny passage through the phospholipid bilayer so that the solute can cross. Permeases are quite specific: each recognises and carries only a small group of chemicals, or even a single substance.

4. Facilitated transport (facilitated diffusion). Here a permease helps a molecule to diffuse across a membrane that it could not otherwise pass.

Types of solution on the basis of concentration

1. Isotonic solution: the concentration of the solution outside the cell equals that of the cytoplasm. There is no net movement of water.

2. Hypertonic solution: the solution outside is more concentrated than the cell contents. The cell loses water and shrinks (it is plasmolysed).

3. Hypotonic solution: the solution outside is more dilute than the cytoplasm. Water enters, the cell swells up and may burst.

Shrinking of the protoplasm is called plasmolysis in a plant cell and crenation in an animal cell. The taking in of bulk material from outside into the cell is endocytosis. It is of two kinds: phagocytosis (cell eating) and pinocytosis (cell drinking).

Movement of water in animal and plant cells placed in hypotonic, isotonic and hypertonic solutions

(b) Nucleus

The nucleus is called the headquarters of the cell. It was discovered by Robert Brown in 1831. It is a dense, double-membrane-bound mass of protoplasm that controls all the chemical activities of the cell and holds its genetic information, so it acts as the controller or director of the cell.

Nucleus (with the rough endoplasmic reticulum around it)

The nucleus has four parts:

1. Nuclear envelope (nuclear membrane, karyotheca). Two membranes surround the nucleus and keep the nucleoplasm apart from the cytoplasm. The envelope has tiny openings called nuclear pores.

2. Nucleoplasm (nuclear sap, karyolymph). It is the protoplasm enclosed inside the nuclear envelope and contains the chromatin threads and the nucleolus.

3. Nucleolus. Discovered by Fontana. A nucleus usually has one nucleolus, though sometimes there are more. It is where RNA is stored.

4. Chromatin threads. A network of long, fine threads, tangled together, which takes up dark stain.

Nuclear pores take part in the exchange of substances between the nucleoplasm and the cytoplasm. Chromosomes are made of DNA and histone proteins. The DNA holds all the genetic information passed on to the next generation, and a functional piece of DNA is a gene. When the cell is about to divide, the chromatin threads coil up and shorten into rod-like chromosomes; each chromosome then consists of two identical halves called chromatids, joined at the centromere.

Functions of the nucleus:

1. It directs all the chemical (metabolic) work of the cell.

2. It brings about growth by directing the making of structural proteins.

3. Ribosomes are assembled with its help.

4. It regulates the cell cycle.

5. It carries the genes and hands the hereditary characters down to the next generation.

Comparison of plant cell and animal cell

Plant cell

Animal cell

Differences between prokaryotic and eukaryotic cells

(c) Cytoplasm

Kolliker discovered the cytoplasm in 1862. It is the region where both building-up (biosynthetic) and breaking-down (catabolic) pathways take place. It has two parts:

1. Cytosol: the watery, dissolved part. It contains fibrous proteins that form the cytoskeleton.

2. Cytoplasmic inclusions: the many living and non-living structures lying in the cytoplasm. The living ones are the cell organelles (also called protoplasmic inclusions or organoids). The non-living ones are called deutoplasmic or ergastic bodies.

Role of the cytoplasm:

1. It helps in distributing nutrients, metabolites and enzymes inside the cell.

2. It helps in the exchange of materials between the cell organelles.

3. It is the site of chemical reactions such as glycolysis (a step of respiration) and the synthesis of fatty acids.

(d) Cell organelles

Cell organelles are the living parts, smaller than the cell, that lie in the cytoplasm. According to their membranes they fall into three groups:

1. Double-membrane organelles: plastids and mitochondria.

2. Single-membrane organelles: endoplasmic reticulum, Golgi apparatus, lysosomes, peroxisomes, glyoxysomes and others.

3. Non-membranous organelles: ribosomes and similar structures.

Double-membrane organelles

1. Plastids

Plastids are double-membrane organelles found in plants and some protists. They trap solar energy, make nutrient molecules and store materials. The word plastid was coined by E. Haeckel. Plastids usually contain pigments and may make and store various substances. Depending on the pigment they hold, they are of three types:

While a fruit ripens its colour changes from green to yellow or red, because the chlorophyll breaks down and the carotene pigments that were hidden by it show up.

Chloroplast

The chloroplast is a double-membrane, usually disc-shaped organelle that occurs only in plant cells. The book credits A. V. Leeuwenhoek with discovering it, and it was named by Schimper. Besides the disc or rhombic form found in plants, chloroplasts of algae can be U-shaped, spiral, coiled or ribbon-shaped. Each chloroplast has two parts:

1. Grana (the lamellar system). Grana are stacks of flat sacs lying on top of one another. Each granum is built from piled-up closed compartments called thylakoids. Each thylakoid carries quantasomes, which are the photosynthetic units, and each quantasome holds about 230 chlorophyll molecules. Grana are the site of the light reaction of photosynthesis because they hold chlorophyll.

2. Stroma (matrix). It is the granular, transparent ground substance in which the grana are embedded. It also holds lipid droplets, starch grains, ribosomes and a loop of DNA. The stroma is the site of the dark reaction of photosynthesis.

Internal structure of a chloroplast

2. Mitochondria

Kolliker first observed mitochondria in insect cells, and Benda gave them their name. A mitochondrion is rod-shaped and is present in the cytoplasm of all eukaryotic cells except the mammalian red blood cells. It is absent from prokaryotes. The cells that are metabolically most active contain the largest number of mitochondria. It is called the "power house of the cell" or the "storage battery".

It has two membranes. The outer membrane has specific proteins, and the inner membrane is thrown into folds called cristae, which divide the inside into chambers. The cristae carry the enzymes for respiratory cycles such as the Krebs cycle, and the units that make ATP are called oxysomes (F₀–F₁ particles). The gap between the two membranes is the perimitochondrial space, and the fluid filling the inside is the matrix.

Mitochondrion

Functions:

1. Its main work is to produce and store energy as ATP.

2. It is the site of the Krebs cycle of respiration because it holds the enzymes for it.

3. The oxysomes contain the enzymes for ATP production.

Plastids and mitochondria are semi-autonomous organelles, because each has its own ribosomes, DNA and enzymes that help it to make some of its own proteins.

Single-membrane organelles

1. Endoplasmic reticulum (ER)

The ER is a network of membranes spread through the cytoplasm. It was discovered by Porter, Claude and Fullam. It occurs in every cell except prokaryotic cells and mammalian red blood cells. It has three components:

Cisternae are long, flat, parallel and unbranched tubules that lie in layers around the nucleus. They are found in cells that are active in making proteins.

Vesicles are rounded or spherical and are found in cells that are active in synthesis.

Tubules are small tubes with smooth walls and a tubular space inside. They occur in cells that do not secrete and in cells that make steroids.

Endoplasmic reticulum

The ER of striated muscle is called the sarcoplasmic reticulum. The ER is of two kinds, smooth (SER) and rough (RER):

Functions of the ER:

1. Its folded membranes give a large surface on which many of the cell's chemical reactions can take place.

2. It acts as an inner framework that supports the jelly-like cytoplasm.

3. It works as a transport system inside the cell for many substances.

4. Substances made in the cell can be stored in different parts of the ER.

5. Proteins are made on the surface of the rough ER by its ribosomes. These proteins are either used inside the cell or sent out of it.

6. Lipids are made on the smooth ER together with the Golgi complex. The smooth ER also helps to make sex hormones such as testosterone in the testis and oestrogen in the ovary.

2. Golgi apparatus

The Golgi apparatus was discovered by Camillo Golgi in 1898 in the nerve cells of an owl. It lies near the nucleus. It is made of a pile of flattened sacs called cisternae along with small vesicles and tubules. The Golgi bodies are pleomorphic, meaning that their parts differ in structure and shape in different cells.

Golgi apparatus

Functions:

1. It takes part in secretion; it stores, modifies and condenses (packages) materials in membranes.

2. It forms the acrosome of the sperm.

3. It forms lysosomes.

4. It makes material for the cell wall.

5. It helps in the formation of the plasma membrane.

6. It forms the cell plate (phragmoplast) when a plant cell divides.

In plant cells the Golgi body is called dictyosome. It is absent from prokaryotes, mammalian red blood cells and sieve cells. The acrosome is a cap-like sac at the head of a sperm; its enzymes dissolve the covering of the egg at the time of fertilisation.

3. Lysosomes

Lysosomes were discovered by Christian de Duve. The name comes from lyso (digestive) and soma (body). A lysosome is a small vesicle or sac scattered in the cytoplasm with no definite shape or size. It contains about 40 kinds of digestive enzymes called hydrolases. Lysosomes occur in animal cells and in a few plant cells, and they are polymorphic, which means they occur in different forms.

Lysosomes in pinocytosis, phagocytosis and autophagy

Functions:

1. Their main work is digestion inside the cell (intracellular digestion; phagy means eating).

2. They are the waste-disposal system of the cell.

3. They digest foreign material and worn-out cells.

4. When the metabolism of the cell is disturbed or the cell is damaged, lysosomes burst, the enzymes spill into the cytoplasm and digest the cell itself. This is why they are called "suicidal bags".

4. Peroxisomes

The term peroxisome was first used by de Duve; they are also called uricosomes. They are ovoid or granular bodies with a single unit membrane and a diameter of 0.5 to 1.0 µm.

Non-membranous organelles

1. Ribosomes

A ribosome is made of protein and RNA. Ribosomes were first reported by Claude and were named by G. Palade. They are tiny granules that can be seen only under an electron microscope. They are either attached to the outer face of the endoplasmic reticulum or lie free in the cytoplasm. Ribosomes are the one kind of organelle found in every type of cell, and because they make proteins they are known as the "protein factories".

Each ribosome has two unequal subunits: a larger, dome-shaped one and a smaller, oval one. The size of a ribosome is expressed by its sedimentation coefficient in the centrifuge (S units). The cytoplasmic ribosomes of eukaryotes are of the 80S type (subunits 60S and 40S). Prokaryotes, and the organelles mitochondria and chloroplasts, have 70S ribosomes (subunits 50S and 30S).

Prokaryotic (70S) and eukaryotic (80S) ribosomes

2. Centrosome

The centrosome is found in animal cells and is absent from plant cells apart from a few exceptions. It lies near the nucleus and is made of a clear, uniform mass of cytoplasm called the centrosphere with one or a pair of microtubular centrioles in it. It is concerned with cell division and with the formation of the basal bodies of cilia and flagella.

3. Microtubules

Microtubules are extremely thin, hollow, tube-like fibres. They may lie free or form parts of other structures such as centrioles, basal bodies, cilia and flagella. They strengthen the structures they are part of and bring about different movements inside the cell.

Ergastic bodies

Ergastic bodies are the non-living inclusions of the cytoplasm.

1. Vacuole. A vacuole is a membrane-bound region of the cytoplasm filled with water and other substances. Its single boundary membrane is the tonoplast. Animal cells have several small vacuoles, whereas a plant cell has one large vacuole that takes up about 90% of the cell volume. Vacuoles help to maintain the osmotic pressure of the cell, store toxic waste products of the plant and contain coloured pigments such as anthocyanins.

2. Granules. These are tiny particles, crystals or oil drops. Starch grains and fat droplets store food in the cell.

G. Microscope

A microscope is an instrument for studying objects too small to be seen with the naked eye or with a hand lens. It has more than one lens. The first compound microscope was built by the Janssens (Zacharias Janssen and his father) in 1590. The electron microscope was designed in 1932 by Knoll and Ruska of Germany.

(a) Structure of a compound microscope

The microscope used in schools is the compound microscope. Its parts are:

1. Base: the metallic, horse-shoe shaped foot which carries the whole weight of the instrument.

2. Handle (arm): the curved part used to hold and carry the microscope.

3. Stage: a strong, rectangular, horizontal metal plate fixed to the arm, on which the slide is kept.

4. Stage clips: two clips on the stage that hold the slide in place.

5. Condenser: lies below the stage and concentrates the light rays on the object.

6. Body tube: a wide, hollow tube joined to the upper part of the arm; the lenses are fixed to it.

7. Adjustment screws: the bigger coarse adjustment screw moves the body tube up and down; the smaller fine adjustment screw is used for final, fine focusing.

8. Reflecting mirror: reflects light so that it passes through the object being viewed.

Compound microscope

(b) Preparation of a microscopic slide

The object to be viewed is the specimen. It is held on a microscopic slide, a thin sheet of glass, and covered by a much thinner glass sheet called the coverslip. The steps are:

1. Take a clean glass slide.

2. Using a dropper, place one drop of water at the centre of the slide.

3. Gently place the specimen in the drop of water with the help of a brush.

4. A transparent specimen is first stained with a suitable chemical.

5. Hold the coverslip so that its edge touches the edge of the drop, and lower it gently on to the water.

6. Dry any extra water that comes out from under the coverslip with blotting paper. The finished slide must be clean and dry.

Preparation of a slide

Activity: observing an onion peel

Aim: to observe plant cells (onion peel or Rhoeo leaf) under a microscope.

Materials: an onion, a glass slide, a coverslip, a stain (safranin) and a microscope.

Procedure: Cut a slice of the onion and peel off its outer skin. Take out the thin transparent membrane lying between two fleshy layers and place it on a clean slide. Add a drop of water, put on a coverslip and look at it under the microscope. Prepare a second slide in the same way, but stain the peel with safranin, and observe it as well. Staining is done to make the nucleus stand out.

Making a slide of an onion peel

Observations:

1. A number of cells lie side by side.

2. Each cell has a clear boundary, the cell wall.

3. A nucleus is seen near the centre of each cell.

4. Some large empty-looking spaces are present in the cell. These are vacuoles.