5. The Fundamental Unit of Life | Class 9 Science | PDF and Web notes

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  Chapter 5: THE FUNDAMENTAL UNIT OF LIFE

  WHAT ARE LIVING ORGANISMS MADE UP OF?

-    In 1665, Robert Hooke examined a thin slice of cork (a substance from the bark of a tree), under a self-designed microscope. He saw a honeycomb-like structure with many small compartments.

-    He called these boxes cells. (Latin, cell = ‘a little room’)

Activity

•    Take a small piece of an onion bulb and peel off the skin (epidermis) from the inner layer using forceps. Place it in a watch glass with water to prevent it from folding or drying out.

•    Put a drop of water on a glass slide. Transfer a piece of the peel from the watch glass to the slide using a brush. Put a drop of safranin. Place a cover slip, avoiding air bubbles with the help of a mounting needle.

•    Observe this temporary mount of onion peel under low power and high powers of a compound microscope.

-    These small similar structures are the basic building units of the onion bulb. These are called cells.

-    All organisms are made up of cells.

·    Robert Hooke: First discovered Cells.

·    Leeuwenhoek (1674): Discovered free-living cells in pond water.

·    Robert Brown (1831): Discovered cell nucleus.

·    Purkinje (1839): Coined the term protoplasm for the fluid substance of the cell.

·    Schleiden (1838) & Schwann (1839): Proposed the cell theory, that all the plants and animals are composed of cells and that the cell is the basic unit of life.

·    Virchow (1855): Expanded the cell theory by suggesting that all cells arise from pre-existing cells.

·    With the discovery of the electron microscope in 1940, it was possible to observe and understand the complex structure of the cell and its various organelles.

-    The invention of magnifying lenses led to the discovery of the microscopic world.

-    A single cell may constitute a whole organism, known as unicellular organisms. E.g., Amoeba, Chlamydomonas, Paramoecium and bacteria.

-    In multicellular organisms, many cells group together in a single body, assuming different functions to form various body parts. E.g., some fungi, plants and animals.

-    Every multicellular organism has come from a single cell. Cells divide to produce cells of their own kind. i.e., all cells come from pre-existing cells.

-    An organism may have different kinds of cells. E.g., cells in leaf peels and tip of roots are different.

-    The shape and size of cells are related to the specific function they perform.

-    Some cells like Amoeba have changing shapes. Some cells have more or less fixed shape. E.g., nerve cells.

-    There is a division of labour in multicellular organisms. i.e., different body parts perform different functions.

-    Division of labour is also seen within a single cell.  

  WHAT IS A CELL MADE UP OF?
WHAT IS THE STRUCTURAL ORGANISATION OF A CELL?

-    A cell can live and perform its functions through the organelles. They together make up the basic unit called the cell (fundamental structural unit of living organisms).

-    All cells have 3 features: plasma membrane, nucleus, and cytoplasm.

PLASMA MEMBRANE (CELL MEMBRANE)

-    It is the outermost covering of the cell that separates the contents of the cell from its external environment.

-    It allows or prevents the movement of materials in and out of the cell. So, cell membrane is called a selectively

permeable membrane.

-    Some substances like CO₂ or oxygen can move across the cell membrane by a process called diffusion.

-    Diffusion is the spontaneous movement of a substance from a region of high concentration to a region of low concentration. E.g.,

·  CO₂ concentration is higher inside the cells and lower outside the cell. So CO₂ moves out of the cell.

·  O₂ diffuses into the cell when the concentration of O₂ inside the cell decreases.

-    Water also obeys the law of diffusion. The movement of water molecules through a selectively permeable membrane is called osmosis.

-    The amount of substance dissolved in water affects the movement of water across plasma membrane. So, osmosis is the net diffusion of water across a selectively permeable membrane toward a higher solute concentration.

-    When an animal cell or a plant cell is put into a sugar or salt solution, one of the three things could happen:

1.   If the surrounding medium has a higher water concentration than the cell (outside solution is dilute), water enters the cell by osmosis, causing the cell to swell. Such a solution is called hypotonic solution.

2.   If the surrounding medium has the same water concentration as the cell, there will be no net movement of water across the cell membrane. Such a solution is called isotonic solution. Water crosses the cell membrane in both directions in equal amounts, resulting in no overall movement of water. The cell remains the same size.

3.   If the surrounding medium has a lower concentration of water than the cell (very concentrated solution), the cell loses water by osmosis. Such a solution is called hypertonic solution. Here, more water leaves the cell than enters it. So the cell shrinks.

Osmosis with an egg

·    Remove the egg’s shell (calcium carbonate) by dissolving it in dilute hydrochloric acid. A thin outer skin now encloses the egg. Put the egg in pure water and observe after 5 minutes. The egg swells because water enters by osmosis.

·    Place a de-shelled egg in a concentrated salt solution and

observe after 5 minutes. The egg shrinks because water exits the egg to the salt solution.  

Activity

•    Put dried raisins or apricots in plain water and leave for some time. Then place them into a concentrated solution of sugar or salt.

o  Each gains water and swells when placed in water.

o  When placed in the concentrated solution it loses water, and consequently shrinks.

-    Unicellular freshwater organisms and most plant cells gain water through osmosis. Absorption of water by plant roots also occurs by osmosis.

-    Thus, diffusion is important in exchange of gases and water in the life of a cell.

-    Cells obtain nutrition from its environment. Different molecules move in and out of the cell through energy-dependent transport.

-    Structure of the plasma membrane can be observed only through an electron microscope.

-    Plasma membrane is made up of lipids and proteins.

-    The flexibility of the cell membrane enables the cell to engulf in food and other material from surroundings. It is called endocytosis. E.g., Amoeba.

CELL WALL

-    It is a rigid covering outer to the plasma membrane of plant cells.

-    Plant cell wall is mainly composed of cellulose. It provides structural strength to plants.

-    When a living plant cell loses water through osmosis, the contents of the cell shrink or contract away from the cell wall. This is called plasmolysis.

Activity

•    Mount a Rhoeo leaf peel in water on a slide and examine cells under the high power of a microscope. Note the green chloroplasts. Put a strong sugar or salt solution to the leaf on the slide. Wait for a minute and observe again. The cell contents shrink away from the cell wall (Plasmolysis).

•    Boil Rhoeo leaves for a few minutes to kill the cells. Mount one leaf on a slide and observe under a microscope. Put a strong sugar or salt solution on the mounted leaf. Wait for a minute and observe. It will not show plasmolysis because the dead cells are unable to undergo plasmolysis.

-    Only living cells can absorb water through osmosis.

-    Cell walls enable plant, fungi, and bacterial cells to resist bursting in very dilute (hypotonic) external media.

-    In such media the cells absorb water by osmosis. The cell swells, building up pressure against the cell wall. The wall exerts an equal pressure against the swollen cell.

-    Cells with cell walls can withstand greater changes in the surrounding medium than animal cells.

NUCLEUS

Iodine solution, Safranin or methylene blue can stain different regions of cells differently based on their chemical composition.

Activity

•    Gently scrape the inside surface of the cheek and spread the material obtained on a glass slide containing a drop of water.

•    Put a drop of methylene blue to colour the material. Put a cover-slip on it and observe under microscope.

•    Flat, irregular shaped cheek cells can be observed.

•    The darkly coloured, spherical or oval, dot-like structure near the centre of each cell is called nucleus.

-    Nucleus is covered by a double-layered nuclear membrane. It has pores for the transfer of material between the nucleus and cytoplasm.

-    Inside the nucleus are chromosomes. They are visible as rod-shaped structures when the cell is about to divide.

-    Chromosomes carry information for inheritance of characters in the form of DNA (Deoxyribo Nucleic Acid).

-    Chromosomes are composed of DNA and protein.

-    DNA contains the information necessary for constructing and organising cells.

-    Functional segments of DNA are called genes.

-    In non-dividing cells, DNA is present as part of chromatin material. It is an entangled mass of thread-like structures. It organizes into chromosomes when the cell is about to divide.

-    Functions of Nucleus:

·   For cellular reproduction. It is the process by which a cell divides into two new cells.

·   It determines the cell development and maturity by directing the chemical activities of the cell.

-    In organisms like bacteria, the nuclear region is poorly defined due to the absence of a nuclear membrane. It is called a nucleoid. It contains only nucleic acids.

-    Organisms without nuclear membrane are called prokaryotes. Organisms with cells having a nuclear membrane are called eukaryotes.

-    Prokaryotic cells lack many cell organelles present in eukaryotic cells. The functions of these organelles are performed by less organized parts of the cytoplasm. E.g., In photosynthetic prokaryotic bacteria, chlorophyll is associated with membranous vesicles, not with plastids as in eukaryotic cells.

CYTOPLASM

-    It is the fluid content inside the plasma membrane.

-    When stained, it takes up only very little stain.

-    It contains cell organelles enclosed by membranes.

-    Viruses lack membranes and show characteristics of life only when they enter a living cell and use its machinery to multiply. It indicates significance of membranes.

Prokaryotic Cell	Eukaryotic Cell
Size: Small (1-10 μm)	Large (5-100 μm)
Nuclear region: Poorly defined (nucleoid). No nuclear membrane.	Well defined and surrounded by a nuclear membrane.
Single chromosome	More than one chromosome
Membrane-bound cell organelles are absent	Present

CELL ORGANELLES

-    These are membrane-bound structures in a cell that perform various chemical activities to support structure & function of cell. E.g., nucleus, endoplasmic reticulum, Golgi apparatus, lysosomes, mitochondria and plastids.

-    Some organelles are seen only under electron microscope.

(i) ENDOPLASMIC RETICULUM (ER)

-    It is a network of membrane-bound tubes and sheets.

-    It looks like long tubules or vesicles.

-    ER membrane is structurally similar to plasma membrane.

-    ER is 2 types:

       a.    Rough endoplasmic reticulum (RER): Appears rough due to ribosomes on its surface.

Ribosomes are present in all active cells and are the sites of protein synthesis.

       b.    Smooth endoplasmic reticulum (SER):  Helps in the manufacture of fat molecules or lipids.

-    Some of these proteins and lipids help in building the cell membrane. This is called membrane biogenesis.

-    Some other proteins and lipids function as enzymes and hormones.

Functions of ER:

·   Act as channels to transport materials (mainly proteins) within cytoplasm or between the cytoplasm & nucleus.

·   Functions as a cytoplasmic framework providing a surface for some biochemical activities of the cell.

·   In liver cells, SER detoxify poisons and drugs.

(ii) GOLGI APPARATUS

-    First described by Camillo Golgi.

-    Consists of membrane-bound vesicles (flattened sacs) arranged in stacks called cisterns.

-    These membranes often connected with ER membranes forming part of a complex cellular membrane system.

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

Functions of Golgi apparatus:

·   Storage, modification & packaging of products in vesicles.

·   Synthesis of complex sugars from simple sugars.

·   Formation of lysosomes.

(iii) LYSOSOMES

-    Membrane-bound sacs filled with digestive enzymes made by RER.

-    These are cell’s waste disposal system. They keep the cell clean by digesting foreign material (e.g., bacteria, food) and worn-out cell organelles.

-    They break down complex organic substances into simpler substances using digestive enzymes.

-    If cellular metabolism is disturbed (e.g., cell gets damaged), lysosomes may burst and the enzymes digest their own cell. Hence, they are also called ‘suicide bags’.

(iv) MITOCHONDRIA

-    These are known as the powerhouses of the cell.

-    Mitochondria have two membranes: Porous outer membrane and deeply folded inner membrane. The folds increase surface area for ATP production.

-    Mitochondria release energy in the form of ATP (Adenosine triphosphate - energy currency of the cell).

-    The energy in ATP is used for making new chemical compounds and for mechanical work.

-    Mitochondria have their own DNA and ribosomes. Hence, they can make some of their own proteins.

(V) PLASTIDS

-    These are present only in plant cells.

-    They have their own DNA and ribosomes.

-    They are 2 types – chromoplasts (coloured plastids) and leucoplasts (white or colourless plastids).

-    Chromoplasts with the pigment chlorophyll are called chloroplasts. These are essential for photosynthesis.

-    Chloroplasts also contain yellow or orange pigments.

-    Chloroplast has many membrane layers embedded in a material called stroma. They are similar to mitochondria in external structure.

-    Leucoplasts store materials like starch, oils and protein.

(vi) VACUOLES

-    These are storage sacs for solid or liquid contents.

-    Animal cells have small sized vacuoles.

-    Plant cells have very large vacuoles. Central vacuole of some plant cells occupies 50-90% of the cell volume.

-    Plant vacuoles are full of cell sap. It provides turgidity and rigidity to the cell. They contain substances such as amino acids, sugars, organic acids and proteins.

-    In single-celled organisms like Amoeba, the food vacuole contains ingested food. Specialized vacuoles in some unicellular organisms help expel excess water & wastes.

Cell Division

-    It is the process by which new cells are made.

-    It is needed for body growth, to replace old, dead and injured cells, and for gamete formation.

-    2 types: mitosis and meiosis.

Mitosis:

-    In this, each cell (mother cell) divides to form two identical daughter cells.

-    Daughter cells have the same number of chromosomes as mother cell.

-    It helps in growth and repair of tissues in organisms.

Mitosis

Meiosis:

-    It occurs in reproductive organs or tissues to form gametes (sperm and eggs).

-    It involves two consecutive divisions forming 4 new cells.

-    Each new cell has only half the number of chromosomes than that of the mother cells. This helps to maintain the chromosome number during fertilization.

Meiosis