The Fundamental Unit of Life Class 9 Notes Science Chapter 5

Introduction

• In 1665, Robert Hooke observed cork under a microscope and noted its honeycomb-like structure composed of small compartments.
• Cork is derived from tree bark, and Hooke referred to these compartments as "cells," with the term "cell" originating from the Latin word for "a little room."
• This observation marked the first instance of recognizing that living organisms are made up of distinct units.
• The term "cell" continues to be used in biology to describe these fundamental units.
• Robert Hooke's discovery of cells in cork was a significant milestone in the history of science.

Cells can have different Shaped and Sizes 

What are Living Organisms Made Up of?

• Living organisms are composed of structures that resemble each other and together form larger structures, akin to an onion bulb.
• Cells are the basic building units of living organisms, visible as small structures under a microscope.
• All organisms, including onions, are made up of cells, which can exist singly or in groups.
• Unicellular organisms like Amoeba and bacteria consist of a single cell forming the entire organism.
• Multicellular organisms, like some fungi, plants, and animals, are made up of many cells that perform different functions in various body parts.
• Cells divide to produce cells of their own kind, indicating that all cells come from pre-existing cells.
• Cells can vary in shape and size based on the specific functions they perform.
• Each living cell has the ability to carry out basic functions characteristic of all living forms.
• Within multicellular organisms, as with human beings, there is a division of labor among different body parts.
• Cells contain specialized components called cell organelles that perform specific functions necessary for the cell's survival and operation.
• All cells have the same organelles, regardless of their function or the organism they belong to.

Discovery of Cell & Cell Theory

• Cells were first identified by Robert Hooke in 1665 when he observed them in a cork slice using a basic microscope.
• Leeuwenhoek, in 1674, using an improved microscope, discovered free-living cells in pond water.
• In 1831, Robert Brown identified the nucleus within cells.
• In 1839, Purkinje introduced the term 'protoplasm' for the cell's fluid substance.
• The cell theory, proposed by Schleiden (1838) and Schwann (1839), states that all plants and animals are made up of cells, with the cell being the fundamental unit of life.
• Virchow (1855) expanded the cell theory by suggesting that all cells come from pre-existing cells.
• The electron microscope, invented in 1940, allowed for the observation and comprehension of the intricate structure of cells and their organelles.

What is a Cell Made Up Of?

Cells consist of organelles like plasma membrane, nucleus, and cytoplasm which allow cell activities and interactions.

The Structure of Cells in Plants and Animals 

Plasma Membrane or Cell Membrane

• The plasma membrane is the outermost layer of a cell, controlling material entry and exit.
• Substances like CO2 and O2 move in and out of cells through diffusion.
• Osmosis is the movement of water across a selectively permeable membrane based on solute concentration.
• Cells in hypotonic solutions gain water, isotonic solutions have no net water movement, and hypertonic solutions cause water loss.

• Diffusion and osmosis are vital for gas and water exchange in cells.
• Cells also obtain nutrients through energy-requiring transport processes.
• The plasma membrane, made of lipids and proteins, is observed with an electron microscope and enables processes like endocytosis.

The Plasma Membrane 

Cell Wall

• Plant cells have a rigid outer covering called the cell wall, composed mainly of cellulose.
• Plasmolysis is the shrinkage of living plant cell contents away from the cell wall due to water loss.
• Cell walls allow cells to withstand hypotonic conditions by preventing bursting and maintaining structural integrity.

Nucleus

• The nucleus, with a double-layered covering, contains chromosomes carrying genetic information.

Nucleus of a Cell

• Chromosomes are composed of DNA and protein, with genes as functional segments of DNA.
• The nucleus plays a central role in cellular reproduction and directs the cell's chemical activities.
• Prokaryotes lack a nuclear membrane and well-defined organelles, unlike eukaryotes.

Eukaryotic and Prokaryotic Cells 

Cytoplasm

• Cytoplasm is the fluid content inside the plasma membrane, containing specialised cell organelles.
• Cell organelles have specific functions and are membrane-enclosed in eukaryotic cells.
• Viruses lack membranes and exhibit life characteristics only when inside a living host cell.

Cell Organelles

• Cells use membrane-bound organelles to separate different chemical activities.
• Organelles like endoplasmic reticulum, Golgi apparatus, lysosomes, mitochondria, and plastids are crucial for cell functions.

Different Cell Organelles

Endoplasmic Reticulum (ER)

• Large network of membrane-bound tubes and sheets.
• Consists of rough ER (RER) with ribosomes for protein synthesis and smooth ER (SER) for lipid production.
• Functions include material transport within the cell and detoxification in liver cells.

Endoplasmic Reticulum

Golgi Apparatus

• System of membrane-bound vesicles involved in packaging and modifying products.
• Participates in sugar synthesis and formation of lysosomes.

Golgi Apparatus

Lysosomes

• Membrane-bound sacs with digestive enzymes for waste disposal.
• Break down foreign material and worn-out organelles, known as the "suicide bags" of cells.

Lysosome 

Mitochondria

• Considered the powerhouses of the cell, producing ATP for energy.
• Contain their own DNA and ribosomes, aiding in protein synthesis.

Mitochondria

Plastids

• Present in plant cells, including chloroplasts for photosynthesis and leucoplasts for storage.
• Similar to mitochondria in structure, have their own DNA and ribosomes.

Types of Plastids

Vacuoles

• Storage sacs in cells, crucial for maintaining cell rigidity and storing various substances.
• Plant cells have large vacuoles, while animal cells have smaller ones.

Vacuoles in plant cells

Cell Division

• Process of cell division includes mitosis for growth and repair, and meiosis for gamete formation.
• Mitosis results in two identical daughter cells with the same chromosome number as the mother cell.
• Meiosis produces four cells with half the number of chromosomes, important for sexual reproduction.