2. Cell the building block of life | Class 9 Science

CELL THE BUILDING BLOCK OF LIFE

Life was originated in water. This may have occurred in small water pools rather than in oceans. E.g., Hot springs.
The hot springs of Puga Valley (Ladakh, India) have near-boiling temperatures even in cold climate. This is similar to early Earth, about 3.5 billion years ago.
The unicellular, heat-loving bacteria that live in hot springs are called thermophiles.
Scientists in Birbal Sahni Institute of Palaeosciences, Lucknow, found that calcium carbonate formed rapidly around hot springs. This may have protected early organic molecules from radiation and extreme conditions, and helped form the first cell membrane.
All living organisms are made up of cells. It is the fundamental unit of structure and function in all living organisms.
Unicellular organisms: They have only one cell. E.g., bacteria, yeast etc.
Multicellular organisms: They have millions of cells that work together. E.g., plants, fish, birds, humans etc.
A group of similar cells performing similar functions forms tissues.
Different tissues are organised to form an organ.
Several organs work together to form organ systems. E.g., nasal pores, nasal cavity, trachea and lungs form respiratory system.

HOW TO STUDY CELLS?

Draw two dots on a paper. As they move closer, they cannot be seen separately beyond a limit. At about 25 cm (the near point of human eye), two points 0.1 mm apart can be seen as distinct. This is called the limit of resolution of the human eye (0.1 mm).
A convex lens or a combination of lenses (objective and eyepiece) is used for the magnification of an object.
Robert Hooke (1665) first observed a cell using a self-designed microscope (200-300X magnification). He observed small box-like compartments in a thin slice of cork and named them 'cells'.
In school laboratories, light microscopes (objective lenses of 10X, 40X etc.) are used for better magnification and resolution.

Structure of a light microscope

Electron microscopes use electron beams to produce highly magnified images at nanometre scale (10^-9 m).

Activity: Let us estimate the size of a cell

Take a transparent ruler with millimetre (mm) markings.
Place it on the stage of microscope, focus and measure the diameter of the circular field of view.
Convert the diameter from mm to micrometre (μm). E.g., 5 mm = 5000 μm.
[1 millimetre (mm) = 1000 μm]
Remove the ruler and place an onion peel slide.
Focus and count the number of cells present along the diameter in one straight line.
Estimate the real size of the cell using the formula:

Estimated size of the onion peel cell

Diameter of the visible field in micrometre

Number of cells along the diameter

If 25 cells are seen across the diameter, size of one onion cell = 5000 μm / 25 = 200 μm.
Total magnification depends on the eyepiece and objective lens. If both are 10X, total magnification is 100X. A 200 μm cell will appear 100 times larger.
Microscopes were improved by enhancing resolution (clarity), contrast (brightness difference), and magnification. E.g., lower surface of a Colocasia leaf can be seen using a scanning electron microscope.

STRUCTURE OF A CELL

Cells must interact with each other and their surroundings to function as a unit.
This occurs at the cell boundary, where substances move in and out.

Cell membrane - The universal feature of a cell

The cell membrane (plasma membrane) is a thin boundary that surrounds and protects the cell.
It is the barrier that defines the cell’s individuality.
It is selectively permeable, allowing some substances to pass while blocking others. E.g., In alveoli, cell membrane controls the movement of oxygen and CO₂.

Activity: Let us experiment

Cut a potato into two equal pieces.
Measure and record their initial weights.
Place one piece in Beaker A (plain water).
Place the other in Beaker B (20% salt/sugar solution).
Leave for about an hour until changes are seen.
Measure final weights.
Calculate the difference between initial & final weights.

Potato Experiment Setup — Experimental set-up, and initial and final states of potato pieces in
(a) plain water
(b) 20% salt solution

Observation:

Beaker A - The potato piece swells.
Beaker B - The potato piece shrinks.

Inference:

In beaker A, the weight of the potato increases. In Beaker B, the weight of the potato decreases.
This is because the cell membrane allows water to move in and out but not the sugar or salt molecules.
Water moves from a dilute solution (more water, less solute) to a concentrated solution (less water, more solute) through a selectively permeable membrane until concentrations become equal. It is called osmosis.
Particles of matter intermix due to a difference in their concentrations (concentration gradient).
Diffusion is the net movement of particles from higher to lower concentration even without a membrane.
Osmosis is the diffusion of water across a selectively permeable membrane. In plants, water from the soil enters root cells by osmosis.

Effect of a Concentrated Solution on Mung Bean Seeds after soaking in water for 12 hours

Water moves out of the seed cells and into the concentrated (hypertonic) solution.
The inner cell contents and membrane shrink away from the rigid cell wall.
The seeds become rubbery, but the cell wall maintains their basic external shape.
Loss of water damages the seeds’ ability to germinate.

Effect of solutions of different concentrations on a cell:

	Solute concentration of extracellular medium = Solute concentration of intracellular medium
	Solute concentration of extracellular medium < Solute concentration of intracellular medium
	Solute concentration of extracellular medium > Solute concentration of intracellular medium

All cells communicate with surroundings and neighbouring cells through the cell membrane.
Cell membrane is extremely thin, 7 to 10 nm thick (1 nm = 0.000001 mm).
It is made up of lipids (fats) and proteins.

Structure of cell membrane (fluid-mosaic model):

The membrane has a lipid bilayer (two layers of fat molecules with water attracting heads outwards and water repelling tails inwards) with proteins embedded in them.

Structure of a cell membrane

The molecules can move sideways, flip and rotate within the membrane. Hence, it is fluid.
Membrane proteins act as gatekeepers, helping substances pass through.
Since the molecules are arranged like tiles in a mosaic, it is called the 'mosaic' model.

Activity: Let us investigate

Prepare temporary slides of a thin peel of onion leaf or Rhoeo (Cradle lily) leaf and mount it with safranin using cover slip. Observe under a microscope.
Prepare a temporary slide of cheek cells by gently scraping the inner side of cheek with a cotton swab or a toothpick. Spread the cells on a glass slide. Add a drop of water, stain with methylene blue and place a coverslip. Observe under a microscope.

Observation:

Onion peel cells or Rhoeo leaf peel cells are box-shaped and regularly arranged because they possess a cell wall. Cheek cells are irregularly arranged due to the absence of cell wall.

Activity:

Prepare two slides of a Rhoeo leaf peel and human cheek cells, and put 20% sugar solution on them. Observe under a microscope after half an hour.

Observation:

Plant cells remain the same but their inner content shrinks, and the space between the inner and outer boundaries increases. Cheek cells shrink considerably.

Cell wall - The outer covering of cells

Cell wall is an additional rigid covering outside the cell membrane of plant cells.
Functions:
- Plants cannot move from place to place, so cell wall acts as a rigid structure to withstand environmental stresses like wind and rain.
- Cell wall helps leaves and flowers remain firm, and maintain their shapes and help plants stay upright.
- Cell wall is permeable, i.e., water and some dissolved minerals can pass through it. This helps plant roots absorb water and nutrients from the soil.
In concentrated sugar solution, plant cells lose water by osmosis. But they do not shrink because the cell wall maintains shape. The inner content shrinks as the membrane pulls away from the cell wall. This shows that cell wall keeps plant cells firm in their original shape.
Animal cells lack a cell wall. So, they lose water and shrink in concentrated sugar solutions.
Due to the lack of cell wall, animal cells can change shape easily. This flexibility supports the movement and functioning of tissues.
Plant cell wall is mainly made of cellulose (a carbohydrate formed by glucose units linked together).
Cellulose acts as roughage in diet and helps digestion.
Fungi and bacteria also have cell wall. It provides protection, structural support and maintains cell shape.

Pause and Ponder

What argument would you give for the necessity of a cell wall in plants usually fixed in one place versus in animals usually moving from one place to the other?
Answer: Plants are fixed in one place, so the cell wall gives them support and protection. Animals move, so they need flexible cells without a rigid wall.
What consequences would you predict for a plant cell if its cell wall were to become as flexible as a cell membrane?
Answer: The plant cell could lose its shape, become weak, and may burst or shrink easily.
Why is it important to cut the two potato pieces in roughly equal size and measure their initial weight before placing them in different liquids?
Answer: This ensures a fair test. Measuring the initial weight helps to calculate the exact mass change caused by osmosis after the potato is removed from the liquid.

THE CELL INTERIOR - A COORDINATED WORKING SYSTEM

Most cells have 3 basic parts: Plasma membrane, Cytoplasm (semi-fluid, jelly-like substance) & Nucleus.
Besides the nucleus, the cytoplasm contains sub-cellular components called organelles. Most of them are visible only under an electron microscope.
A bacterial cell lacks a well-defined nucleus (genetic material without membrane) and membrane-bound organelles. Such cells are called prokaryotic cells (pro = primitive, karyon = nucleus).
In prokaryotic cells, most cellular activities occur in the cytoplasm.
Plant and animal cells have a well-defined nucleus (genetic material enclosed by a membrane) and membrane-bound organelles. They are called eukaryotic cells (eu = true, karyon = nucleus).

Comparison of different kinds of cells based on their structure

No.	Cell structures	Bacterial cell	Plant cell	Animal cell
1.	Cell membrane	Present	Present	Present
2.	Cell wall	Present	Present	Absent
3.	Cytoplasm	Present	Present	Present
4.	Well-defined nucleus	Absent	Present	Present
5.	Primitive nucleus (nucleoid)	Present	Absent	Absent
6.	Membrane-bound organelles	Absent	Present	Present

Comparison between prokaryotic and eukaryotic cells

Characteristics	Prokaryotic cell	Eukaryotic cell
Primitive nucleus	Present	Absent
Diameter of a typical cell	1 to 10 μm	10 to 100 μm
Number of cells in an organism	Usually unicellular	Unicellular or multicellular
Membrane-bound organelles	Absent	Present
Membrane-bound nucleus	Absent	Present

Ready to Go Beyond

Viruses, viroids, & prions are acellular infectious agents that are too small to be seen under a light microscope.

Viruses: Genetic material with a protein coat.
Viroids: Only genetic material, No protein coat.
Prions: Misfolded proteins, No genetic material.

In eukaryotic cells, a network of fine fibres forms the cytoskeleton. It provides support, maintains cell shape, and enables cell movement and internal transport.

In plant cells, the cytoplasm may also store starch or crystals of calcium oxalate or silica. These are called cell inclusions.

Why do eukaryotic cells need these organelles?

Eukaryotic cells perform various life processes in different cell organelles simultaneously. E.g., building new materials, removing waste, and providing energy.
Thus, a cell is like a tiny living factory, with each part doing a specific job.

Nucleus - House of coded instructions

The nucleus has a double-layered covering called the nuclear membrane. It has pores that allow the transfer of material between the nucleus and the cytoplasm.
The nucleolus is a dense round body in the nucleus where ribosomal subunits are formed. These subunits move to the cytoplasm where one large and one small subunits join to form ribosome.

Structure of a nucleus

The nucleus contains chromosomes. They are visible as rod-shaped structures during cell division.
Chromosomes carry information for inheritance of characters in the form of DNA (Deoxyribonucleic acid).
Chromosomes are made of DNA and proteins.
DNA contains genetic information.
Functional units of DNA are called genes.
In a non-dividing cell, DNA exists as part of chromatin (entangled thread-like structures).
Before cell division, chromatin is organised into chromosomes.

Threads of Curiosity

Some cells are specialised for specific functions. E.g., Mature human red blood cells (RBCs) lack a nucleus (enucleate). This provides more space for haemoglobin to carry oxygen.
Without a nucleus, RBCs cannot repair or divide. Hence, their lifespan is short (120 days).
Platelets, lens cells etc. also lack nucleus.
In prokaryotic cells, DNA is a single circular molecule associated with proteins. This region is called nucleoid.

Ribosomes - The protein factories

Ribosomes are tiny structures present freely in the cytoplasm or attached to the endoplasmic reticulum.
These are the sites of protein synthesis.

Endoplasmic Reticulum (ER) - Manufacturing factory

A large organelle forming a network in the cytoplasm.
It is continuous with the outer membrane of nuclear envelop.
Function: Synthesis and transport of proteins, fats (lipids), and some hormones in specialised cells.
There are 2 types of ER:
- Rough Endoplasmic Reticulum (RER): Has ribosomes on its surface, giving a rough appearance.
  Function: Protein synthesis and protein secretion (e.g., in gland cells like pancreatic cells).
- Smooth Endoplasmic Reticulum (SER): Lacks ribosomes, so it appears smooth.
  Function: Synthesis and storage of fats & hormones.

Golgi apparatus - Packaging & shipping centre

It is made of stacks of flattened, sac-like structures.
It is functionally linked to the ER, cell membrane and other organelles.
Functions: It acts like the cell's post office. It modifies, sorts, and packages proteins/lipids into vesicles for transport, secretion, or lysosome formation.

Meet a Scientist

Golgi apparatus was first observed by Camillo Golgi (Italy, 1898) in barn owl nerve cells.
Using special staining techniques, he observed a thread-like network. Early microscopes could not resolve it clearly, so its existence was doubted. It was later confirmed using electron microscopes.

Lysosomes - The clean-up system

Cells produce wastes and worn-out organelles.
Lysosomes are single membrane-bound sacs filled with enzymes. They break down unwanted proteins, carbohydrates, fats and damaged cell parts.
The breakdown products are released into cytoplasm, where they may be reused in other cellular processes.

Threads of Curiosity

Human sperm cells contain lysosomal enzymes.
When a sperm meets an egg, these enzymes help break down the outer layer of the egg for fertilisation.

Mitochondria - The powerhouse of the cell

Mitochondria are called the 'powerhouses of the cell' because they supply energy for cellular activities.
Each mitochondrion is surrounded by 2 membranes:
- Outer membrane: Smooth and porous.
- Inner membrane: Folded into finger-like projections called cristae, which increase the surface area for chemical reactions and facilitate energy production.
In mitochondria, glucose and other molecules are broken down to release energy during cellular respiration. The energy is stored as a molecule called Adenosine Triphosphate (ATP). It acts as the energy currency used for cellular activities.

Structure of a mitochondrion

Plastids - Centre for food synthesis in the plant cells and beyond

Plastids are organelles in plants for food synthesis and storage. E.g., Chloroplasts, chromoplasts, leucoplasts.
Chloroplasts: They are double-membrane-bound organelles, like mitochondria. They contain a semi-fluid substance called the stroma.
Stroma contains disc-shaped membrane structures with a green pigment chlorophyll.
Chlorophyll absorbs light energy during photosynthesis (synthesise food in presence of sunlight). The sugars formed are stored in stroma along with starch granules.

Structure of a chloroplast

Mitochondria and plastids have some features that are similar to certain bacteria. E.g., they have their own DNA and ribosomes. So they can make some of their own proteins. This suggests that mitochondria and plastids share an evolutionary history with bacteria.
Chromoplasts (Greek, chroma = colour): These are plastids which contain pigments other than chlorophyll. Found in flower petals and fruits to give bright colours. This helps to attract pollinators for pollination and fruit-eating animals for seed dispersal.
Leucoplasts (Greek, leukos = white): These are colourless plastids that lack pigments. They store food material (starch, oils or proteins). They are classified based on the type of food they store. E.g., leucoplasts in potato and taro (Colocasia) cells store starch.

Vacuoles - The organelles for storage & support

A mature plant cell has one large central vacuole with a selectively permeable membrane. It is filled with a watery fluid called cell sap.
The vacuole stores water, minerals, sugars and waste material. By storing water, it maintains internal pressure and keeps the cell firm.
When a plant lacks enough water, the vacuole loses water, the cells become less firm, and the plant wilts.
In animal cells, vacuoles are sometimes present. They are smaller and help in temporary storage of materials.