Cell The Building Block of Life Class 9 PPT Notes PDF (Ch 2)

• Scientists believe life originated in water, in small warm water pools or hot springs, about 3.5 billion years ago
• Heat-loving bacteria called thermophiles (single-celled) live in such hot springs, for example in the Puga Valley in Ladakh
• Calcium carbonate around hot springs may have helped form the first protective membrane, the barrier that defines a cell

All living organisms are made up of cells, the basic level at which life exists. A unicellular organism is a single cell. A multicellular organism is made of millions of cells working together.

Cell → Tissue → Organ → Organ system → Organism. A cell is the basic unit; a tissue is a group of similar cells with the same function; an organ is made of different tissues working together; an organ system is made of organs working together. Even inside tissues and organs, the cell stays the fundamental unit of structure and function.

• Resolution is the ability of the eye to see two close objects as separate and distinct
• Two points closer than 0.1 mm look like one point, so the limit of resolution of the human eye is 0.1 mm
• The near point is 25 cm
• A cell is usually too small to be seen by the unaided eye

• Robert Hooke in 1665 was the first to observe a cell
• He used a self-designed microscope (about 200 to 300X)
• He examined a thin slice of cork and saw tiny box-like compartments
• He named them “cells”, from cellula, meaning little rooms

• Parts: eyepiece, body tube, objective lens, coarse and fine adjustment knobs, handle (arm), stage, mirror and base
• It uses visible light and objective lenses (10X, 40X) for magnification and resolution
• A convex lens (or a combination: objective + eyepiece) makes objects appear larger
• Total magnification = magnification of the eyepiece × magnification of the objective. Example: 10X × 10X = 100X, and 10X × 40X = 400X

1. Place a mm ruler on the microscope stage and measure the diameter of the field of view in mm
2. Convert mm to µm (1 mm = 1000 µm), for example 5 mm = 5000 µm
3. Place an onion-peel slide and count the cells along the diameter

Worked example: 5000 µm ÷ 25 cells = 200 µm per onion cell.

• It uses a beam of electrons instead of light
• It reveals fine cell details at the nanometre scale (1 nm is one-billionth of a metre)
• Microscopes improved in three ways: resolution (clarity), contrast (brightness difference) and magnification (apparent size)
• Example: the lower surface of a Colocasia leaf under a scanning electron microscope shows stomata

• A thin boundary that surrounds and protects the cell, and defines its individuality
• It is selectively permeable: it allows some substances through and blocks others
• It is present in all cells, even single-celled organisms

Cut two equal potato pieces, weigh them, and place them in Beaker A (plain water) and Beaker B (20% salt or sugar solution). Wait 1 hour and reweigh. The potato in Beaker A swells (gains weight) and the potato in Beaker B shrinks (loses weight). The cell membrane lets water move in and out, but not salt or sugar molecules.

In plants, water enters the root cells by osmosis.

• The membrane is about 7 to 10 nm thick and made of lipids and proteins
• Molecules can move sideways, flip and rotate, so the membrane is fluid
• The proteins (“gatekeepers”) are arranged like tiles in a mosaic

• An extra rigid covering outside the membrane in plant, fungal and bacterial cells
• Made mainly of cellulose (many glucose units)
• It gives shape, rigidity and support, and helps plants stay upright
• It is permeable: water and dissolved minerals pass through
• Cellulose in our diet is roughage, which helps digestion

Onion or Rhoeo peel cells are box-shaped and regularly arranged, whereas human cheek cells are irregular with no fixed shape. In a 20% sugar solution the outer wall of the plant cell stays the same while the inner content shrinks and pulls away from the wall. The cheek cells shrink considerably, because they have no wall. The rigid cell wall keeps plant cells in shape; animal cells have no wall.

• Plasma membrane: the selectively permeable boundary
• Cytoplasm: a semi-fluid, jelly-like substance that also holds tiny organelles (sub-cellular components), most visible only under an electron microscope
• Nucleus: the prominent control centre

Pro = primitive, eu = true, karyon = nucleus. Bacterial cells are prokaryotic; plant and animal cells are eukaryotic.

Acellular means not made of cells, and too small to be seen under a light microscope.

• Virus: genetic material plus a protein coat
• Viroid: only genetic material, no protein coat
• Prion: a misfolded protein, no genetic material

• A double-layered nuclear membrane with pores controls the transfer between the nucleus and the cytoplasm
• The nucleolus is a dense, round body that makes ribosomal subunits
• The nucleus controls all the activities of the cell
• Chromosomes are visible as rods only when the cell is about to divide; in a non-dividing cell the DNA stays as chromatin (entangled threads)
• Chromosomes = DNA + proteins, and they carry the information of inheritance; functional segments of DNA are genes
• Mature red blood cells have no nucleus, which leaves more space for haemoglobin; their lifespan is about 120 days

• An outer membrane (smooth, porous) and an inner membrane folded into cristae, which increase the surface area for reactions
• Glucose is broken down to release energy by cellular respiration, and the energy is stored as ATP, the cell’s energy currency
• Mitochondria have their own DNA and ribosomes

• A chloroplast (a plastid) contains the green pigment chlorophyll, which absorbs sunlight for photosynthesis
• Plastids are double-membrane-bound like mitochondria, with a semi-fluid stroma and disc-shaped membranes with chlorophyll
• Sugars made are stored as starch in the stroma
• Mitochondria and plastids have their own DNA and ribosomes, which suggests a shared evolutionary history with bacteria

• A plant cell usually has one large central vacuole with a selectively permeable membrane; it stores water, minerals, sugars and waste, and keeps the cell firm (turgor). With low water the plant wilts
• Animal cells have small vacuoles for temporary storage
• The cytoskeleton is a network of fine fibres in eukaryotic cells that gives structural support, maintains cell shape, and enables movement and internal transport
• Cell inclusions are stored substances in the cytoplasm, for example starch in plant cells and crystals of calcium oxalate or silica

1. Scientists studied the full DNA of a simple bacterium using a computer
2. They chemically synthesised an exact copy of that DNA in the lab
3. They removed the DNA from another related bacterium, keeping its cytoplasm and membrane
4. They inserted the synthetic DNA, and the cell grew and divided following the new instructions

DNA controls the structure and activities of a cell. Only the DNA was synthetic, not a whole new cell made from scratch.

• To replace old, dead or damaged cells, for example healing a skin cut or regrowing hair
• Growth happens because cells divide, not just because they get bigger (cells grow only up to a certain size)
• About 1% of body cells (hundreds of billions) are replaced every day

Activity 2.5: onion root tip. Grow roots, fix them in aceto-alcohol, stain with aceto-carmine, squash on a slide, and observe the different stages of cell division under the microscope.

Eukaryotic cells divide in an orderly way called the cell cycle. The stages of mitosis are DNA replication, prophase, metaphase, anaphase and telophase with cytokinesis.

• Faulty mitosis leads to uncontrolled division, tumours and abnormal chromosome numbers
• Faulty meiosis leads to genetic disorders, developmental problems, early pregnancy loss and reduced fertility
• Cell culture is growing plant or animal cells outside the body in a nutrient-rich medium under sterile conditions (with the right temperature, pH and moisture). It is used to make biochemicals, medicines and vaccines and to study cells

• Arun Kumar Sharma (India) is famous for work on chromosomes, plant taxonomy, evolution and development, and invented lab methods to study plant chromosomes. He won the Shanti Swarup Bhatnagar award and the Padma Bhushan
• Gottlieb Haberlandt (Austria, 1902) proposed that any living plant cell can grow into a whole plant under the right conditions, called totipotency, and laid the foundation of plant tissue culture technology

The three postulates:

1. All living organisms are made of one or more cells
2. The cell is the basic unit of structure and function
3. All cells arise from pre-existing cells

The cell theory unifies all biology, from bacteria to humans.

• Every cell has a definite life span: old cells die and are replaced
• Cancer cells lose this control and divide uncontrollably, forming tumours (benign or malignant; malignant tumours can spread)
• Programmed Cell Death (PCD) is a controlled, genetic self-destruction of cells, for example the removal of the cells between the fingers in an embryo
• Contact inhibition: animal cells stop dividing when they touch their neighbours. Plant cells, with their rigid wall, do not show contact inhibition

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