CELL Introduction

👉 What is a Cell?

Cells are the smallest structural and functional units of every life.

Robert Hooke first discovered the cell in 1665 While examining a thin cork section under his self-made microscope, he observed cell structures like a honeycomb consisting of many compartments.

👉 What are the shapes of cells?

Cells have different shapes. Some of the common shapes are:

1. Oval
2. Spherical
3. Discoid
4. Columnar
5. Polygonl
6. Spindle

👉 Sizes of Cells

The size of the cells varies from tiny cells of bacteria (0.2 – 5.0 microns) to the very large egg of the ostrich (170mm). The nerve cells of the human body are as long as 90 cm in length.

👉Why is a cell called the structural and functional unit of life?

Every living being is made up of very tiny particles called cells. So, cells serve as basic structural units of all organisms.

All the metabolic activities of life take place at the cellular level, like respiration, reproduction, excretion, and digestion; a cell contains genetic material. Therefore, cells are called the structural and functional unit of life.

👉 A Brief History of the discovery of the cell.

• In 1665, Robert Hooke first discovered cells by using his primitive microscope.
• In 1674, Anton von Leeuwenhoek discovered the free living cells in pond water.
• In 1831, Robert Brown discovered the cell nucleus.
• In 1839, Purkiinje coined the term protoplasm.
• In 1838, Jacob Matthias, and 1839 Theodore Schwann independently asserted that all plants and animals are composed of cells and that the cells are the basic unit of life. It established the basis of cell theory.
• In 1855, Virchow refined cell theory.
• In 1932, after the invention of the electron microscope, Knoll and Ruska observed and understood the complex structure of the cell and its various organelles.

👉 Structure of onion cell as seen under a compound microscope.

👉 Classification of organisms Based on the type of cell/cells.

Organisms are classified into two types

1. Unicellular organisms: Those organisms that are made by a single cell are known as unicellular organisms.
   Example; Amoeb, Paramoecium, Bacteria, Chlamydomonas.

2. Multicellular organisms: Those organisms made by the constituent of many types of cells are known as multicellular organisms.
Examples: Plants, animals, some fungi etc.

The structural organisation of a cell

A cell is composed of many components known as organelles. Each organelle has a specific structure and function.

👉 List of components of a cell (Plants and Animals)/ Organelles

• Plasma Membrane
• Cell Wall (present only in plant cells)
• Nucleus
• Mitochondria
• Plastids (Present only in Plant cells)
• Endoplasmic Reticulum
• Golgi Body
• Lysosome
• Vacuoles
• Centrioles

We will discuss all of these cell organelles below.

1. Plasma Membrane

The plasma membrane is the outermost semi-permeable membrane of a cell. It is a flexible, thin, living and very delicate organelle of a cell.

📙 Important features of plasma membrane

• The plasma membrane is the outermost covering of the cell.
• It separates the contents of the cell from its external environment.
• It is made up of organic molecules called lipids and proteins.
• It can be seen with the help of an electron microscope only.
• It can engulf food and other materials from its external surroundings as seen in amoeba.

📙 The Functions of plasma membrane

• The plasma membrane gives shape to the cell.
• it protects the cell from injury.
• It maintains the individuality of the cell.
• Its outfolds increase the surface area for absorption.

📙 Position and structure of plasma membrane

A plasma membrane is a thin, elastic, microscopic, regenerative, transparent and semipermeable membrane outside the protoplasm.

Structure of a plasma membrane
It is a lipoproteinaceous and tri-laminar membrane in which the outer and inner layer is formed of globular protein molecules which move as icebergs in lipid layers. Carbohydrates are present only on its outer surface.

💡 Plasma membrane is called a selectively permeable membrane because, it permits the entry and exit of selected materials in and out of the cell.

2. Cell Wall

The cell wall is the rigid covering outside the plasma membrane of a plant cell.
It is composed of cellulose, a complex substance that provides structural strength to plants.

📙 The Functions of cell wall

• The cell wall gives shapes and rigidity of the cell.
• It protects the protoplast
• The growth of the cell wall determines the growth of the cell.
• It is involved in the movement of the materials in and out of the cell.
• It increases the size of the cell by continuous absorption.

Difference between Cell wall and cell membrane

3. Necleus

The centrally located spherical cellular component in the cytoplasm of a cell is called the nucleus.

The nucleus is enclosed by two layers of membrane called nuclear membrane. This layer separates the nucleus from the cytoplasm and has many pores which allow the transfer of materials between the nucleoplasm and the cytoplasm.

The nucleus contains chromosomes which are mainly composed of DNA(deoxyribonucleic acid) and can be seen during cell division.

There is another spherical structure called nucleolus which disappear and reappear during cell division stages.

📙 The Function Of a Nucleus

• Genetic information storage: The primary function of a nucleus is to store the genetic material of a cell in the form of DNA. The DNA contains the instructions for the synthesis of proteins and the overall functioning of a cell.
• Gene Expression and Transcription: The nucleus plays a key role in gene expression, which involves the process of using the information encoded in DNA to synthesize RNA (ribonucleic acid). Transcription, the first step of gene expression, occurs within the nucleus. RNA molecules, such as messenger RNA (mRNA), are transcribed from DNA and serve as templates for protein synthesis in the cytoplasm.
• Ribosomal RNA (rRNA) Synthesis: The nucleolus, a region within the nucleus, is responsible for the synthesis of ribosomal RNA (rRNA). Ribosomes, essential for protein synthesis, consist of both protein and rRNA. The nucleolus assembles the ribosomal subunits before they are transported to the cytoplasm, where they combine to form functional ribosomes.
• Cellular Metabolism and Regulation: The nucleus regulates various cellular activities by controlling gene expression. It responds to signals from the cell and the external environment, influencing processes such as cell growth, division, and differentiation. The nucleus helps maintain cellular homeostasis by coordinating the synthesis of proteins and other molecules necessary for cell function.
• Cell Reproduction and Mitosis: The nucleus is intimately involved in the process of cell division. During mitosis, the nucleus ensures the accurate replication and distribution of genetic material to daughter cells. The chromosomes condense, align, and are then separated into two identical nuclei, ensuring that each new cell receives the correct genetic information.

4. Mitochondria: The powerhouse of a cell

The word mitochondria comes from the Greek word, Mito = filament and chondrion = grain.

The mitochondria are tiny bodies of varying shapes viz cylindrical, oval, rod-shaped, spherical and size ranging from 0.2 – 2 micrometre distributed in the cytoplasm of a cell.

Mitochondria has two membranes (a) Inner membrane and (b) Outer membrane.
The outer membrane is a porous layer and the inner membrane is deeply folded (called cristae) to increase a large surface area for synthesis of ATP.

The inner surface which is attached to many tennis racket-like structures is called exosomes or F₁ particles.
The inner cavities are filled with proteinous semi-solid substances called matrix containing DNA, ribosomes and phosphate granules.

📙 The Functions of Mitochondria

• Mitochondria synthesise their own proteins with the help of their own DNA and ribosomes.
• They store energy as ATP. So, it is called the powerhouse of the cell.

5. Plastids (Present only in plant cells)

Plastids are the largest-sized eukaryotic structures, membrane-bound organelles of plant cells that produce or store food materials.

There are three types of plastids.

1. Chloroplasts
2. Chromoplasts and
3. Leucoplasts.

👉 Chloroplasts

Chloroplast is the green-coloured plastids. There is an intermediate space between the outer membrane and the inner membrane of a chloroplast. Inside the inner membrane, there is a semi-liquid, called stroma.

They are found in green parts of the plants and young stems.

📙 Function of Chloroplast: It manufactures plant food out of sunlight and water (photosynthesis).

👉 Chromoplasts

Chromoplasts are specialized plastids found in plant cells. Chromoplast does not play a direct role in photosynthesis. Instead, they accumulate and store pigments such as carotenoids, which gives fruits and vegetables their red, orange and yellow colour.

📙 Function of Chromoplast: Synthesis and storage of pigments that contribute to the colours of fruits, flowers and roots.

👉 Leucoplasts

Leucoplasts are colourless or white plastids. They are found in many storage parts of the plants.

📙 Functions of Leucoplasts: They store starch, oils and protein granules.

6. Endoplasmic reticulum

The system of interconnected membrane-bound channels present in the cytoplasm and connected to the nuclear membrane is called the endoplasmic reticulum.

There are two types of endoplasmic reticulum.

1. Smooth Endoplasmic reticulum and
2. Rough endoplasmic reticulum

Both SER and RER are interconnected.

The endoplasmic reticulum serves as a network of traffic pathways and transports materials from one cell region to another.

👉Smooth Endoplasmic reticulum (SER)

When the surface of the membrane does not bear ribosomes, it is called smooth endoplasmic reticulum.

📙 Functions of Smooth endoplasmic reticulum (SER)

1. It synthesises hormones and steroids.
2. It forms visual pigment from Vitamin A in retinal cells.
3. Detoxification of drugs and poison.

👉 Rough Endoplasmic reticulum

When the outer membrane is covered with ribosomes giving them a rough appearance, it is called Rough endoplasmic reticulum.

📙 Functions of Rough endoplasmic reticulum

1. It provides a channel for quick transport of materials.
2. It provides a surface for the attachment of ribosomes.
3. It provides the ultrastructural skeletal framework.

Difference between Smooth endoplasmic reticulum and Rough endoplasmic reticulum.