Cell: The Building Block of LifeClass 9 Science Notes

Cell: The Building Block of Life

All living organisms, from the smallest bacteria to the largest plants and animals, are made of cells. The cell is the basic structural and functional unit of life. Life itself is believed to have originated in water, possibly in environments like hot springs, similar to those found on early Earth about 3.5 billion years ago.

Organisms can be classified based on the number of cells they have:

• Unicellular organisms consist of a single cell, like bacteria and yeast.
• Multicellular organisms are made of millions of cells that work together, such as plants, fish, and humans.

In multicellular organisms, cells are organized into a hierarchy:

1. A group of similar cells performing a similar function forms a tissue.
2. Different tissues are organized to form an organ.
3. Several organs work together to form an organ system (e.g., the respiratory system includes the nasal cavity, trachea, and lungs).

Even in this complex organization, the cell remains the fundamental unit of structure and function.

How to Study Cells?

Most cells are too small to be seen with the naked eye. The ability of the eye to see two close objects as separate is called its limit of resolution. For the human eye, this limit is about $0.1$ mm. Objects smaller than this appear as a single point.

To study cells, scientists use microscopes, which use lenses to magnify objects.

• Magnification is the ability to make an object appear larger. The total magnification of a microscope is the product of the magnifying power of the eyepiece and the objective lens. For example, a 10X eyepiece and a 10X objective lens give a total magnification of 100X.
• Resolution is the measure of clarity, or the ability to distinguish between two close points.
• Contrast is the difference in brightness between various parts of an object.

Robert Hooke, in 1665, was the first person to observe cells. Using a self-designed microscope, he looked at a thin slice of cork and saw small, box-like compartments which he named "cells". Modern microscopes, like light microscopes and electron microscopes, have greatly improved magnification, resolution, and contrast, allowing for detailed study of cell structure.

Structure of a Cell

Cells interact with their surroundings through their boundary, exchanging substances with the external environment.

Cell membrane-The universal feature of a cell

The cell membrane, also called the plasma membrane, is a thin boundary that surrounds every cell, protecting its contents and defining its individuality.

A key property of the cell membrane is that it is selectively permeable. This means it allows some substances to pass through while blocking others. This control is crucial for the cell's survival.

Movement Across the Membrane:

• Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration. This happens naturally due to the random motion of particles and does not require a membrane.
• Osmosis is a special type of diffusion. It is the movement of water across a selectively permeable membrane from an area with more water (dilute solution) to an area with less water (concentrated solution).

Structure of the Cell Membrane: The structure of the cell membrane is explained by the fluid-mosaic model.

• It is composed of a lipid bilayer—two layers of fat molecules. These molecules have water-attracting "heads" facing outwards and water-repelling "tails" facing inwards.
• Proteins are embedded within this lipid bilayer, acting like gatekeepers to help certain substances pass through.
• The term fluid is used because the lipid and protein molecules can move sideways, rotate, and flip within the membrane.
• The term mosaic is used because the arrangement of proteins within the lipid layers resembles the tiles in a mosaic artwork.

Cell wall-The outer covering of cells

In addition to a cell membrane, the cells of plants, fungi, and bacteria have an additional rigid outer layer called the cell wall.

• Function: The cell wall provides structural support, helps the organism withstand environmental stresses (like wind and rain), and maintains the cell's shape. This is especially important for plants, which are stationary.
• Composition: The plant cell wall is primarily made of cellulose, a type of carbohydrate.
• Permeability: The cell wall is permeable, meaning water and dissolved minerals can pass through it freely. This, combined with the selectively permeable cell membrane, allows plant roots to absorb water and nutrients.

The Cell Interior-A Coordinated Working System

Most cells have three basic parts: a plasma membrane, cytoplasm, and a nucleus.

• Cytoplasm is the semi-fluid, jelly-like substance that fills the cell.
• Within the cytoplasm of more complex cells are many specialized, sub-cellular components called organelles.

Types of Cells: Cells are broadly categorized into two types based on their internal structure:

1. Prokaryotic cells (from pro, meaning 'primitive', and karyon, meaning 'nucleus'):
   • Lack a well-defined nucleus. Their genetic material is located in a region called the nucleoid.
   • Lack membrane-bound organelles.
   • They are typically smaller (1 to $10 \mu \mathrm{m}$) and are usually unicellular (e.g., bacteria).
2. Eukaryotic cells (from eu, meaning 'true', and karyon, meaning 'nucleus'):
   • Have a well-defined, membrane-bound nucleus that contains the genetic material.
   • Contain various membrane-bound organelles, each with a specific function.
   • They are larger (10 to $100 \mu \mathrm{m}$) and can be unicellular or form multicellular organisms (e.g., plants and animals).

Why do eukaryotic cells need these organelles?

Organelles allow eukaryotic cells to carry out many different life processes simultaneously in an organized way. They function like tiny organs within the cell, each performing a specific job like building materials, removing waste, or providing energy.

Nucleus-House of coded instructions

The nucleus is the control center of the eukaryotic cell.

• It is enclosed by a double-layered nuclear membrane with pores that allow materials to move between the nucleus and the cytoplasm.
• Inside the nucleus is a dense body called the nucleolus, where parts of ribosomes are made.
• The nucleus contains chromosomes, which carry genetic information. Chromosomes are composed of DNA (Deoxyribonucleic acid) and proteins.
• Genes are functional segments of DNA that contain instructions for building proteins and controlling inherited traits.
• In a non-dividing cell, the DNA exists as an entangled mass of thread-like structures called chromatin. When the cell is about to divide, the chromatin organizes into distinct, rod-shaped chromosomes.

Ribosomes - The protein factories

Ribosomes are tiny structures responsible for protein synthesis. They can be found freely in the cytoplasm or attached to the Endoplasmic Reticulum.

Endoplasmic Reticulum (ER)-Manufacturing factory

The Endoplasmic Reticulum (ER) is a large network of membranes that extends from the nuclear membrane throughout the cytoplasm. It plays a key role in synthesizing and transporting proteins and lipids.

• Rough Endoplasmic Reticulum (RER): Its surface is studded with ribosomes, giving it a "rough" appearance. It is primarily involved in synthesizing and secreting proteins.
• Smooth Endoplasmic Reticulum (SER): It lacks ribosomes and has a "smooth" appearance. It is involved in synthesizing and storing fats (lipids) and some hormones.

Golgi apparatus-The packaging and shipping centres

The Golgi apparatus is a stack of flattened, sac-like structures. It functions like a post office for the cell.

• It receives proteins and lipids from the ER.
• It modifies, sorts, and packages these molecules into vesicles (small membrane-bound sacs) for transport to other parts of the cell, for secretion out of the cell, or for the formation of lysosomes.

Lysosomes-The clean-up system

Lysosomes are single-membrane sacs filled with digestive enzymes. They act as the cell's clean-up system.

• They break down unwanted materials like old proteins, damaged organelles, and foreign particles.
• The resulting simple molecules are released back into the cytoplasm to be reused.

Mitochondria-The powerhouse of the cell

Mitochondria are the "powerhouses of the cell" because they generate most of the cell's supply of energy.

• Each mitochondrion has two membranes: a smooth outer membrane and an inner membrane that is folded into finger-like projections called cristae. These folds increase the surface area for energy production.
• Inside the mitochondria, glucose and other molecules are broken down in a process called cellular respiration to release energy.
• This energy is stored in molecules of Adenosine Triphosphate (ATP), which serves as the energy currency for most cellular activities.

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

Plastids are organelles found in plant cells and some other eukaryotic organisms, involved in food synthesis and storage.

• Chloroplasts: These are the sites of photosynthesis. They contain the green pigment chlorophyll, which absorbs sunlight energy. Like mitochondria, they have a double membrane. Inside, disc-shaped structures contain the chlorophyll.
• Chromoplasts: These plastids contain pigments other than chlorophyll (yellow, orange, or red). They are responsible for the bright colors of flowers and fruits, which help attract pollinators and animals for seed dispersal.
• Leucoplasts: These are colorless plastids that store food, such as starch (in potatoes), oils, or proteins.

Vacuoles - The organelles for storage and support

Vacuoles are membrane-bound sacs that function in storage.

• Plant Cells: A mature plant cell typically has one large central vacuole. It is filled with a watery fluid called cell sap and stores water, minerals, sugars, and waste. By storing water, it helps maintain pressure inside the cell, keeping it firm. When a plant wilts, it's because the vacuoles have lost water.
• Animal Cells: If present, vacuoles in animal cells are much smaller and are used for temporary storage.

How do Normal Cells Grow and Divide?

Cells in our body grow and divide to replace old, dead, or damaged cells, and for overall growth of the organism. This process of forming new cells from pre-existing cells is called cell division. In eukaryotes, this is a controlled and orderly process called the cell cycle.

Cell division

There are two major types of cell division: mitosis and meiosis.

Mitosis

• Purpose: Growth, repair of damaged tissues, maintenance, and asexual reproduction.
• Process: A single parent cell divides once to produce two genetically identical daughter cells.
• Outcome: Each daughter cell has the same DNA and the same number of chromosomes as the parent cell. This is how most body cells (like skin cells) are replaced.

Meiosis

• Purpose: Production of gametes (sperm and egg cells) for sexual reproduction. This process creates genetic variation.
• Location: Occurs only in the cells of reproductive organs (testes in males, ovaries in females).
• Process: A single parent cell undergoes two successive divisions.
  1. The first division separates homologous chromosomes, reducing the chromosome number by half.
  2. The second division is similar to mitosis, where the two cells from the first division each divide again.
• Outcome: Four daughter cells are produced, each with half the number of chromosomes as the parent cell. When two gametes combine during fertilization, the original chromosome number is restored.

• Errors in mitosis can lead to uncontrolled cell division, which can form tumors.
• Errors in meiosis can result in gametes with an incorrect number of chromosomes, leading to genetic disorders or developmental problems.

Cell Theory-The Unifying Principle of Biology

The work of scientists Matthias Schleiden (1838), Theodor Schwann (1839), and Rudolf Virchow (1855) led to the formulation of the Cell Theory, a fundamental principle in biology.

The classical Cell Theory states:

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

Do cells grow and reproduce forever?

Cell growth and division are tightly controlled processes.

• Contact Inhibition: In many animal cells, division stops when cells come into contact with neighboring cells. Cancer cells lose this ability and divide uncontrollably, forming tumors.
• Life Span: Every cell has a definite life span. Old or damaged cells are programmed to die and are replaced by new ones. Programmed Cell Death (PCD) is an organized process of cell destruction essential for normal development (e.g., forming fingers by removing tissue between them) and maintaining health.
• Totipotency: Some cells, especially in plants, have the ability to develop into a complete new organism under the right conditions. This ability is called totipotency and is the basis for Plant Tissue Culture Technology.