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CBSE NOTES CLASS 9 SCIENCE CHATER 5

THE FUNDAMENTAL UNIT OF LIFE

Cell

Components of Compound Microscope

How to Prepare a Slide for Mounting

Structure of Onion Peel

Cell Theory

Cell as structural and functional unit of life

Division of Labour

Features of a Cell

Plasma Membrane or Cell Membrane - Semipermeable membrane

Diffusion

Exchange of Gases in Humans

Osmosis

Hypotonic solution

Isotonic solution

Hypertonic solution

Endocytosis

Exocytosis

Cell Wall

Nucleus

Comparison of Prokaryotes & Eukaryotes

Cell organelles

Endoplasmic Reticulum (ER)

Golgi Apparatus

Lysosomes

Mitochondria

Plastids

Vacuoles

Differences between Plant and Animal Cells

CBSE NOTES CLASS 9 SCIENCE CHATER 5

THE FUNDAMENTAL UNIT OF LIFE

Cell

All the living organisms are made up of fundamental unit of life called cell.

While examining thin slice of cork (the bark of a tree), Robert Hooke saw that the cork was made of little room like structures resembling honeycomb

He called them cell meaning little room.

Components of Compound Microscope

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1. Base

It is the U shaped lower portion of the microscope on which the other parts of microscope are attached. Above the U shaped portion, there is a perpendicular portion, known as pillar.

2. Arm

On top of the pillar an arm is fixed. The point, at which the arm is fixed on the pillar, is known as inclination joint or swivel, which can be used to tilt the microscope at a desired angle.

Arm supports the body and is also used to carry the microscope.

A stage is fixed on the base and two knobs are fitted. These knobs are used for focusing. One is used for coarse adjustment and another one is used for fine adjustment.

Stage clips are used for fixing the sample on the stage.

3. Body Tube or Head

This is a tube attached to the knob of the arm. It houses the optical parts.

4. Optical Parts

There are two optical systems in a compound microscope: Eyepiece and Objective.

Eyepiece or Ocular is the lens through which we see at the top of the microscope. This can be changed depending upon the required magnification required.

Objective Lenses are the primary optical lenses on a microscope. They are housed in the nosepiece.

6. Stage

It is a platform having circular hole in the centre to allow passage of light from below. It is fixed to the stand. One mirror is fixed to the stand which is known as reflecting mirror. The specimen is placed on the stage

Stage Clips are used for fixing the specimen slide.

Aperture is the hole in the stage through which the base (transmitted) light reaches the stage.

Illuminator is the light source for a microscope, typically located in the base of the microscope.

Condenser is used to collect and focus the light from the illuminator on to the specimen. It is located under the stage.

How to Prepare a Slide for Mounting

Structure of Onion Peel

Image result for structure of onion cell

Cell Theory

The cell theory states that all the plants and animals are composed of cells. It was proposed by Schleiden (1838) and Schwann (1839).

The cell theory was further expanded by Virchow in 1855 by suggesting that “all cells arise from the pre-existing cells”.

Why is the cell called structural and functional unit of life?

Cells are basic building blocks of any organism. All organisms are made up of one or more cells. All activities related to life, like, nutrition, respiration, transportation, reproduction, excretion, control and coordination etc. are carried out within the cell.

Cells take different shape and size depending upon the functions they perform. All organs and systems are made up of cells, which have been specialized to do different functions.

Some of the different types of cells in humans

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Division of Labour

In multicellular organism there are different types of cells for performing different functions.

Cells take different shape and size depending upon the functions they perform. Each living cell has the capacity to perform certain basic functions that are characteristic of all living forms. Group of similar cells is called tissues. One or more type of tissues make body parts, i.e., organs and system.

Different parts of the body perform different functions. For example, the human body has a heart to pump blood, a stomach to digest food and so on.

Division of labour is also seen within a cell. Each such cell has got certain specific components within it known as cell organelles. Each kind of cell organelle performs a special function, such as making new material in the cell, clearing up the waste material from the cell and so on. A cell is able to live and perform all its functions because of these organelles. These organelles together constitute the basic unit called the cell.

Features of a Cell

The feature in almost every cell is same. They are Plasma membrane, Nucleus and Cytoplasm.

Plasma Membrane or Cell Membrane

Diffusion- Movement of substance from high concentration to low concentration, is called diffusion.

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Exchange of Gases in Humans

Exchange of carbon dioxide or oxygen with external environment is done through diffusion. When there is high concentration of CO2 in the blood, it moves out through the cell membrane by the process of diffusion.

Similarly, O2 enters the cell by the process of diffusion when the level or concentration of O2 inside the cell decreases.

Osmosis

The passage of water from a region of high water concentration through a semi-permeable membrane to a region of low water concentration, is called osmosis.

The osmosis depends on the concentration of solute in the water. Now there are 3 possibilities,

(i) If the medium surrounding the cell has a higher water concentration than the cell, meaning that the outside solution is very dilute, the cell will gain water by osmosis.

Water molecules move across the cell membrane in both directions, but more water will come into the cell than will leave. The net (overall) result is that water enters the cell. The cell is likely to swell up.

The process of osmosis towards the inside of a cell is called endosmosis.

A solution that has less concentration and more water than inside the cell is called hypotonic solution.

(ii) If the medium has exactly the same water concentration as the cell, there will be no net movement of water across the cell membrane.

Water crosses the cell membrane in both directions, but the amount going in is the same as the amount going out, so there is no overall movement of water. The cell will stay the same size.

A solution having same concentration across a semipermeable membrane, is called an isotonic solution.

(iii) If the medium has a lower concentration of water than the cell, meaning that it is a very concentrated solution, the cell will lose water by osmosis.

Water crosses the cell membrane in both directions, but more water leaves the cell than enters it. Therefore the cell will shrink.

The process of osmosis towards the outside of a cell is called exosmosis.

A solution that has a greater concentration of solutes on the outside of a cell compared with the inside of a cell is called hypertonic solution.

Examples of Osmosis/Diffusion

Endocytosis

The movement of food macromolecules such as proteins or polysaccharides into the cell is called endocytosis.

Exocytosis

The movement of waste out of the cell is called exocytosis.

Cell Wall

Nucleus

Criteria

Prokaryotes

Eukaryotes

Size

generally small (1-10 μm)

generally large. (5-500 μm)

Nuclear region

Not well defined and not surrounded by a nuclear membrane and known as nucleoid.

Well defined and surrounded by a nuclear membrane

Chromosomes

Single chromosome

More than one chromosome

Cell Organelles

Membrane-bound cell organelles absent

Membrane-bound cell organelles present

Examples

bacteria, blue green algae

fungi, plant cell and animal cell

Cell organelles

Every cell has fluid matrix (other than nucleus), which is called cytoplasm.

The nucleus and cytoplasm together are called as protoplasm.

Large and complex cells, including cells from multicellular organisms, need a lot of chemical activities to support their complicated structure and function. To keep these activities of different kinds separate from each other, these cells use membrane-bound little structures (or organelles).

Endoplasmic Reticulum (ER)

The endoplasmic reticulum (ER) is a large network of membrane-bound tubes and sheets. It looks like long tubules or round or oblong bags (vesicles).

The ER membrane is similar in structure to the plasma membrane.

Types of ER

Rough endoplasmic reticulum (RER)

RER looks rough under a microscope because it has particles called ribosomes attached to its surface. The ribosomes, which are present in all active eukaryotic cells, are the sites of protein manufacture.

Smooth endoplasmic reticulum (SER)

The SER looks smooth because Ribosomes are not attached to its surface. It helps in the manufacture of fat molecules or lipids.

The manufactured proteins and lipids are sent to various places in the cell depending on need, using the ER.

This process of building the cell membrane by using proteins and lipids is known as membrane biogenesis.

Functions of ER

Golgi Apparatus

It was discovered by Camillo Golgi in 1897.

It consists of membrane-bound vesicles arranged approximately parallel to each other in stacks called cisterns.

These membranes often have connections with the membranes of ER.

The material synthesized near the ER is packaged and dispatched to various targets inside and outside the cell through the Golgi apparatus.

Functions of Golgi bodies

Lysosomes

Structurally, lysosomes are membrane-bound sacs filled with digestive enzymes. These enzymes are made by RER.

Lysosomes are the waste disposal system of the cell. Lysosomes help to keep the cell clean by digesting any foreign material as well as worn-out cell organelles. Foreign materials entering the cell, such as bacteria or food, as well as old organelles end up in the lysosomes, which break them up into small pieces, with the help of powerful digestive enzymes capable of breaking down all organic material.

During the disturbance in cellular metabolism, for example, when the cell gets damaged, lysosomes may burst and the enzymes digest their own cell. Therefore, lysosomes are also known as the ‘suicide bags’ of a cell.

Mitochondria

Mitochondria are known as the powerhouses of the cell. The energy required for various chemical activities needed for life is released by mitochondria in the form of ATP (Adenosine triphopshate) molecules.

ATP is known as the energy currency of the cell.

The body uses energy stored in ATP for making new chemical compounds and for mechanical work.

Mitochondria have two membrane coverings instead of just one. The outer membrane is very porous while the inner membrane is deeply folded. These folds create a large surface area for ATP-generating chemical reactions.

Mitochondria have their own DNA and ribosomes. Therefore, mitochondria are able to make some of their own proteins.

Plastids

Plastids are present only in plant cells. There are two types of plastids, The internal organisation of the plastids consists of numerous membrane layers embedded in a material called the stroma.

Plastids also have their own DNA and ribosomes.

Chromoplasts (coloured plastids)

Plastids containing the pigment chlorophyll are known as chloroplasts. Chloroplasts are important for photosynthesis in plants. Chloroplasts also contain various yellow or orange pigments in addition to chlorophyll.

Leucoplasts (white or colourless plastids)

Leucoplasts are primarily organelles in which materials such as starch, oils and protein granules are stored.

Vacuoles

Vacuoles are storage sacs for solid or liquid contents. Vacuoles are small sized in animal cells while plant cells have very large vacuoles.

The central vacuole of some plant cells may occupy 50-90% of the cell volume.

In plant cells vacuoles are full of cell sap and provide turgidity and rigidity to the cell.

Many substances like amino acids, sugars, organic acids and proteins are stored in vacuoles.

In single-celled organisms like Amoeba, the food vacuole contains the food items that it has consumed. In some unicellular organisms, specialised vacuoles also play important roles in expelling excess water and wastes from the cell.

Differences between Plant and Animal Cells

Criteria

Animal Cell

Plant Cell

Cell wall

Absent

Present

Vacoules

Small size

Large size

Plastids

Absent

Present

Nucleus

Lies in the centre

Lies on one side

Cell the Fundamental Unit of Life

Each cell acquires its structure and ability to function because of the organization of its membrane and organelles in specific ways.

The cell has a basic structural organisation. This helps the cells to perform functions like respiration, obtaining nutrition, and clearing of waste material, or forming new proteins.

Thus, the cell is the fundamental structural unit of living organisms. It is also the basic functional unit of life.