Cell - Class 8 Notes & Olympiad Questions

Cell

A cell is the basic structural and functional unit of all living organisms. It is the smallest independently functioning unit that can carry out the processes essential for life. From simple single-celled organisms like bacteria to complex multicellular organisms like plants and animals, all living things are made up of cells.

Discovery of the Cell

a) In 1665, an English scientist named Robert Hooke was examining thin slices of cork under a microscope.
b) Cork is a part of the bark of a tree, and Hooke wanted to see what it looked like under magnification.
c) To his surprise, he observed tiny box-like structures within the cork that looked like a honeycomb.
d) These structures were actually compartments in the cork, separated by walls or partitions.
e) Inspired by their resemblance to small rooms or cells in a monastery or convent, Hooke called these structures "cells."
f) However, it's essential to note that these were not living cells; they were the empty spaces left behind by dead plant cells, particularly the cell walls.
g) Despite this, Hooke's discovery was groundbreaking because it was the first time anyone had seen such structures under a microscope.
h) This discovery sparked a new field of study called cell biology and led to further investigations into the microscopic world.
i) Scientists continued to explore using more advanced microscopes and eventually observed living cells, the real building blocks of life.
j) Today, we know that all living things are made up of cells, and cells are like tiny building blocks that make up every living organism.
k) The discovery of the cell revolutionized biology and changed how we understand life at its smallest level.

Characteristics of Cells

Structure and Function: Cells have a specific structure that allows them to carry out various functions necessary for the survival and functioning of the organism. Different types of cells are specialized to perform specific tasks, such as muscle cells for movement, nerve cells for communication, and red blood cells for oxygen transport.

Basic Unit of Life: The cell is considered the fundamental unit of life because all living organisms are made up of one or more cells. Whether it's a single-celled organism like a bacterium or a complex multicellular organism like a human, all life starts and functions at the cellular level.

Reproduction and Growth: Cells have the ability to reproduce and grow. They can divide into two daughter cells through cell division, allowing the organism to develop and repair tissues.

Homeostasis: Cells maintain a stable internal environment, known as homeostasis, by regulating the balance of nutrients, water, and other substances within the cell.

Interactions: Cells communicate and interact with each other to coordinate the functions of different tissues and organs within an organism. These interactions are crucial for proper functioning and survival.

Organisms Show Variety in Cell Number, Shape and Size

1. Cell Number

a) Cells can vary significantly in number among different organisms. Some organisms, like unicellular bacteria, consist of a single cell that performs all essential functions for survival.
b) On the other hand, multicellular organisms, like humans and animals, are made up of numerous cells, ranging from billions to trillions. The number of cells in an organism depends on its complexity and specialised functions.

2. Cell Shape

a) Cells come in various shapes, and their shape is often related to their specific function. For example, nerve cells (neurons) are long and branched to facilitate the transmission of electrical signals over long distances.
b) Muscle cells are elongated and cylindrical to generate force and movement. Red blood cells are biconcave discs to increase their surface area for efficient oxygen exchange.

3. Cell Size

a) Cells can differ significantly in size, ranging from the tiniest bacterial cells, which may be only a few micrometres in length, to the much larger plant and animal cells, which can be tens to hundreds of micrometres in size.
b) The size of the cell is usually proportional to the overall size of the organism, but not always. Some single-celled organisms can be relatively large, while some multicellular organisms have small cells.
c) Some cells, like amoeba, have an irregular shape. These cells can constantly change their shape and form temporary extensions called pseudopodia. Pseudopodia are temporary projections that aid in cell movement and capturing food particles.

4. Cell Specialisation

a) In multicellular organisms, cells show specialisation based on their roles in the body. Specialised cells form tissues, which then combine to form organs.
b) Each specialised cell type performs specific functions crucial for the overall functioning of the organism. For example, muscle cells are specialised for contraction, and nerve cells are specialised for transmitting electrical signals.

5. Cell Complexity

Cells can vary in their complexity based on the type of organism they belong to. Prokaryotic cells, found in bacteria and archaea, are relatively simple and lack membrane-bound organelles. Eukaryotic cells, present in plants, animals, fungi, and protists, are more complex and contain membrane-bound organelles, such as the nucleus, mitochondria, and endoplasmic reticulum.

Prokaryotic and Eukaryotic Cells

Prokaryotic Cell

a) Prokaryotic cells are simple and smaller in size compared to eukaryotic cells.
b) They lack a distinct nucleus and other membrane-bound organelles.
c) The genetic material in prokaryotic cells is organized as a single circular DNA molecule located in the nucleoid region.
d) They do not have a true nucleus, and the DNA is not enclosed within a membrane.
e) Prokaryotic cells lack membrane-bound organelles like mitochondria, endoplasmic reticulum, and Golgi apparatus.
f) They have a cell wall made of peptidoglycan, which provides structural support and protection.
g) Prokaryotic cells are found in unicellular organisms like bacteria and archaea.

Eukaryotic Cell

a) Eukaryotic cells are more complex and larger in size compared to prokaryotic cells.
b) They have a true nucleus, where the genetic material is enclosed within a double membrane.
c) The nucleus houses the cell's DNA in the form of multiple linear chromosomes.
d) Eukaryotic cells have various membrane-bound organelles, such as mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes, and more.
e) These organelles compartmentalize the cell's functions and allow for specialised processes to occur.
f) Eukaryotic cells may have a cell wall (e.g., in plant cells) or lack one (e.g., in animal cells).
g) They are found in multicellular organisms like plants, animals, fungi, and protists.

6. Cell Arrangement

In multicellular organisms, cells are organized into tissues, organs, and organ systems. The arrangement and coordination of cells in these structures allow for the efficient functioning of the entire organism. Different organs and tissues are composed of various types of cells working together to perform specific functions.

Parts of the Cell

A cell is a complex and organised structure with various components called organelles, each having specific functions that contribute to the cell's overall functioning. Here are the main parts of the cell:

Cell Membrane (Plasma Membrane)

a) The cell membrane is a thin, flexible barrier that surrounds the cell and separates its internal contents from the external environment.
b) It is composed of a lipid bilayer with embedded proteins.
c) The cell membrane regulates the movement of substances in and out of the cell, maintaining its internal environment and allowing essential materials to enter while removing waste products.

Cytoplasm

a) The cytoplasm is a gel-like substance that fills the cell's interior, surrounding the organelles.
b) It contains various organelles suspended in a fluid called cytosol.
c) Many cellular processes, including protein synthesis and metabolism, occur in the cytoplasm.
d) The cell membrane separates the cell's internal contents (cytoplasm and nucleus) from the external environment.
e) It forms a boundary that defines the cell's shape and structure.
f) The cell membrane protects the cell from harmful substances and controls the entry and exit of molecules and ions.
g) It is selectively permeable, allowing only specific substances to pass through while restricting others.
h) The cell membrane facilitates the movement of essential molecules in and out of the cell through specialized transport proteins and channels.
i) It plays a crucial role in maintaining the internal balance of the cell's environment.
j) The cell membrane contains receptors and signalling molecules for communication with the surrounding environment and neighbouring cells.

Nucleus

a) The nucleus is the control centre of the cell and houses the cell's genetic material in the form of chromosomes made of DNA.
b) It is surrounded by a double-layered membrane called the nuclear envelope, which has pores that allow the movement of molecules between the nucleus and cytoplasm.
c) The nucleus regulates gene expression and controls cellular activities, including growth, division, and reproduction.

Endoplasmic Reticulum (ER)

a) The endoplasmic reticulum is a complex network of membranes throughout the cell.
b) There are two types of ER: rough ER, studded with ribosomes on its surface, and smooth ER, lacking ribosomes.
c) Rough ER is involved in protein synthesis and modification, while smooth ER plays a role in lipid metabolism and detoxification.

Golgi Apparatus (Golgi Body)

a) The Golgi apparatus consists of stacks of flattened membranous sacs.
b) It receives proteins and lipids from the ER, modifies them, and sorts them for transport to specific destinations within or outside the cell.
c) The Golgi apparatus is responsible for packaging and distributing cellular products.

Mitochondria

a) Mitochondria are double-membraned organelles known as the powerhouse of the cell.
b) They generate adenosine triphosphate (ATP), the cell's primary energy currency, through cellular respiration.
c) Mitochondria contain their own DNA and replicate independently within the cell.

Lysosomes

a) Lysosomes are small sac-like organelles containing digestive enzymes.
b) They break down cellular waste, damaged organelles, and foreign materials, aiding in cell recycling and maintaining cellular health.

Vacuoles

a) Vacuoles are membrane-bound sacs that store water, nutrients, waste products, and other cellular materials.
b) In plant cells, a large central vacuole helps maintain turgor pressure and stores nutrients.

Ribosomes

a) Ribosomes are tiny structures responsible for protein synthesis.
b) They can be found free in the cytoplasm or attached to the rough ER.
c) Ribosomes read the genetic information from the nucleus and assemble amino acids into proteins.
d) Each part of the cell has its specific function, and together, they work harmoniously to ensure the cell's survival and proper functioning within the organism. The coordinated activities of these organelles allow cells to carry out essential processes and maintain the life of the organism.

Comparison of Plant and Animal Cells

Similarities Between Plant and Animal Cells

Cell Membrane: Present in both plant and animal cells, surrounds the cell and controls the movement of substances.
Nucleus: Both plant and animal cells have a nucleus, housing the genetic material.
Cytoplasm: Both cells have cytoplasm, where various cell organelles are located.
Mitochondria: Both cells have mitochondria, which are responsible for producing energy through cellular respiration.
Endoplasmic Reticulum: Both cells have endoplasmic reticulum, involved in protein synthesis and lipid metabolism.
Golgi Apparatus: Present in both cells, involved in the processing and packaging of proteins.