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Cells and Tissues of Organisms


Chapter - 2: Cells and Tissues of Organisms

👉 All Chapter Contents 👈 

2.1  Living Cell

What is Living Cell?

A living cell is the basic unit of life that makes up all living organisms. It is a complex, self-contained structure that carries out all the necessary functions for an organism's survival and reproduction.

Living cells are composed of a variety of molecules, including proteins, carbohydrates, lipids, and nucleic acids, which work together to form specialized structures and perform specific functions. They have a cell membrane that separates the cell's interior from the external environment and controls the movement of molecules in and out of the cell.

Inside the cell, there are various organelles, including the nucleus, mitochondria, ribosomes, and others, each with a specific role in the cell's metabolism and function. Living cells are capable of carrying out metabolic processes such as respiration, photosynthesis, and protein synthesis, and they can respond to changes in their environment through signaling pathways.

Living cells come in a wide range of sizes and shapes, from tiny bacteria to complex multicellular organisms such as plants and animals. They are the fundamental building blocks of life and provide the basis for all biological processes.

Types of Cells

The two main types of cells: are prokaryotic and eukaryotic.

Prokaryotic cells are typically found in single-celled organisms such as bacteria and archaea. They are smaller and simpler in structure than eukaryotic cells. Prokaryotic cells lack a true nucleus and other membrane-bound organelles, and their genetic material is contained in a single circular chromosome. Instead of a nucleus, the DNA is found in a region called the nucleoid. Prokaryotic cells also have a cell wall that provides protection and support and may have pili or flagella for movement.

Eukaryotic cells, on the other hand, are more complex and are found in multicellular organisms such as plants and animals. Eukaryotic cells have a true nucleus that contains their genetic material, as well as other membrane-bound organelles such as mitochondria, chloroplasts (in plant cells), the endoplasmic reticulum, the Golgi apparatus, and lysosomes. The nucleus is enclosed by a nuclear membrane that separates it from the cytoplasm. Eukaryotic cells are typically larger than prokaryotic cells and have a cytoskeleton that provides support and helps with movement.

There are also differences between animal and plant cells. Animal cells lack a cell wall and have centrioles, while plant cells have a cell wall made of cellulose and contain chloroplasts for photosynthesis. Plant cells also have a large central vacuole that helps regulate the cell's water content and provide structural support.

Specialized cells also exist in multicellular organisms, such as muscle cells, nerve cells, and blood cells. Muscle cells are elongated and contain specialized proteins for contraction, while nerve cells have long extensions called axons that transmit signals. Blood cells come in several types, including red blood cells, which transport oxygen, and white blood cells, which play a role in the immune system.

In summary, prokaryotic and eukaryotic cells differ in size, structure, and complexity. While prokaryotic cells are simpler and lack a true nucleus, eukaryotic cells are more complex and contain a variety of organelles. Specialized cells also exist for specific functions within multicellular organisms.

What is a Somatic cell?

A somatic cell is any cell of an organism that is not a reproductive cell. In other words, it is any cell that is not involved in sexual reproduction.

Somatic cells make up the majority of an organism's body, and are responsible for carrying out its various functions. They can be further classified based on their function and location in the body. For example, muscle cells are specialized cells that allow movement, while nerve cells transmit signals throughout the body.

Somatic cells contain a full set of chromosomes, which are arranged in pairs. In humans, there are 23 pairs of chromosomes in each somatic cell, for a total of 46 chromosomes. These chromosomes carry the genetic information that determines an individual's traits and characteristics.

Somatic cells undergo a process called mitosis, where a single cell divides into two identical daughter cells. Mitosis is essential for the growth, development, and repair of tissues in the body. Unlike reproductive cells, somatic cells are not capable of undergoing meiosis, the process of cell division that produces gametes (sperm and egg cells) for sexual reproduction.

Abnormalities in somatic cells can lead to various diseases, including cancer. When somatic cells divide uncontrollably and form a mass of abnormal cells, it is called a tumor. Tumors can be benign, meaning they do not spread to other parts of the body, or malignant, meaning they can invade nearby tissues and spread to other parts of the body through a process called metastasis.

In summary, somatic cells are non-reproductive cells that make up the majority of an organism's body. They carry a full set of chromosomes and undergo mitosis for the growth, development, and repair of tissues. Abnormalities in somatic cells can lead to diseases such as cancer.

What is a gametic cell?

A gametic cell is a reproductive cell that is involved in sexual reproduction. Gametic cells are also known as germ cells, and they are specialized cells that carry genetic information from one generation to the next.

In humans, gametic cells are sperm cells in males and egg cells in females. These cells are produced through a process called meiosis, which is different from the process of mitosis which produces somatic cells. During meiosis, the chromosomes in the parent cell are divided into half, resulting in four daughter cells with only half the number of chromosomes as the parent cell. This ensures that when the sperm and egg cells fuse during fertilization, the resulting zygote will have the correct number of chromosomes.

Gametic cells are also unique because they undergo a process called genetic recombination, where the genetic material from the father and mother is shuffled and combined in new ways. This increases the genetic diversity of offspring and allows for natural selection to occur.

Unlike somatic cells, which divide through mitosis, gametic cells do not divide until they are ready for fertilization. In males, sperm cells are produced continuously from puberty onwards, while in females, egg cells are produced in a cyclical fashion and released from the ovaries during ovulation.

In summary, gametic cells are specialized reproductive cells that are involved in sexual reproduction. They are produced through meiosis, undergo genetic recombination, and have half the number of chromosomes as somatic cells. Gametic cells are unique in their role in passing genetic information from one generation to the next.

2.2  Main Organelles of Plant and Animal Cells and their Functions

Main Organelles of Plant and Animal Cells and their Functions

Both plant and animal cells have several organelles that perform specific functions. Here are the main organelles of plant and animal cells and their functions:

Nucleus: The nucleus is the control center of the cell and contains genetic material in the form of chromosomes. It regulates cell division, growth, and development.

Mitochondria: Mitochondria are responsible for generating energy in the cell through the process of cellular respiration. They convert glucose and oxygen into ATP (adenosine triphosphate), which is used as a source of energy for the cell.

Endoplasmic reticulum (ER): The ER is a network of tubules and sacs that transport materials throughout the cell. The rough ER is studded with ribosomes, which synthesize proteins and transport them to the Golgi apparatus. The smooth ER is involved in lipid synthesis and detoxification of drugs and toxins.

Golgi apparatus: The Golgi apparatus is responsible for packaging and sorting proteins and lipids into vesicles for transport to their final destination within the cell or for secretion outside the cell.

Lysosomes: Lysosomes are membrane-bound organelles that contain digestive enzymes. They break down waste materials and cellular debris, as well as foreign particles that are engulfed by the cell.

Chloroplasts (plant cells only): Chloroplasts are the site of photosynthesis in plant cells. They contain chlorophyll, which captures light energy and converts it into glucose and oxygen.

Cell wall (plant cells only): The cell wall is a rigid structure that surrounds plant cells and provides structural support and protection.

Vacuoles (plant cells): Vacuoles are large, membrane-bound organelles that store water, ions, and other molecules in plant cells. They also play a role in maintaining turgor pressure and regulating cell growth.

In summary, plant and animal cells share several organelles that perform similar functions, such as the nucleus and mitochondria. However, plant cells have unique organelles, such as chloroplasts and the cell wall, that allow them to carry out photosynthesis and maintain their structural integrity.

What is protoplasm?

Protoplasm is the living substance found within the cells of all living organisms. It is the material that makes up the cytoplasm, which includes the organelles, the cytoskeleton, and the various molecules within the cell. Protoplasm is composed of water, proteins, lipids, nucleic acids, and carbohydrates, and it is responsible for many of the vital functions that occur within the cell.

Protoplasm can be divided into two main components: the cytoplasm and the nucleoplasm. The cytoplasm is the gel-like substance that fills the cell and contains the organelles, while the nucleoplasm is the substance within the nucleus that contains the genetic material of the cell.

Protoplasm is important for many of the functions that occur within the cell, including metabolism, protein synthesis, cell division, and the maintenance of cell shape and structure. It is also responsible for the movement of materials within the cell and for communication between different parts of the cell.

In summary, protoplasm is a living substance found within the cells of all living organisms. It is composed of water, proteins, lipids, nucleic acids, and carbohydrates, and it is responsible for many of the vital functions that occur within the cell. Protoplasm is important for metabolism, protein synthesis, cell division, and the maintenance of cell shape and structure.

What is plasmalemma?

Plasmalemma, also known as the cell membrane, is a thin, semi-permeable membrane that surrounds the cytoplasm of a cell. It is composed of a lipid bilayer, which consists of two layers of phospholipid molecules with their hydrophobic tails facing each other and their hydrophilic heads facing outward.

The plasmalemma serves as a barrier between the internal and external environment of the cell, regulating the movement of substances into and out of the cell. Small, non-polar molecules such as oxygen and carbon dioxide can easily pass through the membrane through simple diffusion, while larger molecules such as proteins and carbohydrates require specialized transport proteins to cross the membrane.

In addition to its role as a selective barrier, the plasmalemma also plays a role in cell recognition and communication. It contains various proteins and receptors that allow cells to communicate with each other and respond to their environment.

Overall, the plasmalemma is a critical component of the cell, regulating the movement of substances into and out of the cell, while also playing a role in cell recognition and communication.


Mitochondria are organelles found in most eukaryotic cells, including animal and plant cells. They are often referred to as the "powerhouses" of the cell, as they are responsible for generating energy in the form of ATP (adenosine triphosphate) through the process of cellular respiration.

Mitochondria have a distinctive shape, consisting of an outer membrane and an inner membrane that is folded into structures called cristae. The inner membrane is where most of the ATP synthesis occurs, while the outer membrane acts as a barrier and controls the entry and exit of molecules.

The process of cellular respiration occurs within the mitochondria and involves a series of complex biochemical reactions. In the presence of oxygen, glucose is broken down in a series of steps, with each step releasing energy that is used to produce ATP. Carbon dioxide and water are also produced as byproducts of this process.

In addition to their role in energy production, mitochondria also play a role in regulating cell metabolism, calcium signaling, and programmed cell death (apoptosis). Defects in mitochondrial function have been implicated in a wide range of diseases, including cancer, neurodegenerative disorders, and metabolic disorders.

Overall, mitochondria are essential organelles that are critical for generating energy in eukaryotic cells. They play a key role in cellular metabolism, signaling, and regulation, and defects in mitochondrial function can lead to a variety of diseases and disorders.

What is plastid?

Plastids are organelles found in plant cells and some algae. They are responsible for a variety of functions, including photosynthesis, storage of starches and lipids, and the synthesis of pigments and other essential molecules.

There are several types of plastids, each with a different function and structure. Chloroplasts are the most well-known type of plastid and are responsible for carrying out photosynthesis. They contain chlorophyll, the pigment that gives plants their green color and is responsible for converting sunlight into energy in the form of sugars.

Other types of plastids include amyloplasts, which are responsible for storing starches, and elaioplasts, which store lipids. Chromoplasts are another type of plastid that contain pigments such as carotenoids, which give fruits and flowers their distinctive colors.

Plastids have a double membrane structure, similar to that of mitochondria, and contain their own DNA and ribosomes. They are able to divide and replicate independently of the cell, allowing them to increase in number as needed.

Overall, plastids are essential organelles found in plant cells that are responsible for a wide range of functions. They are involved in photosynthesis, the storage of essential molecules, and the synthesis of pigments and other compounds. Plastids are able to divide and replicate independently of the cell and are an important component of plant cell biology.

2.3  Roles of different cells in the proper functioning of plants and animals

The tissue cell is the structural and functional unit of organisms. The functions of unicellular and multicellular organisms are performed in different ways.

Plant Tissue

An ensemble of similar or different types of cells with the same origin and performing a specific function is called tissue.

Types of Plant Tissue

Plant tissues are of two types.

a) Meristematic tissue

b) Permanent tissue

Simple Tissue

The permanent tissue which contains cells of the same shape, size, and structure, is called simple tissue.

Types of simple tissue

On the basis of the nature of cells, simple tissues are divided into three types: 

1) Parenchyma

2) Collenchyma and

3) Sclerenchyma.

1) Parenchyma

Parenchyma cells are seen in almost every part of a plant. Cells of this type are living, isodiametric, thin walled and turgid with protoplasm. Intercellular space is found in parenchyma cells. The cells are thin walled, and the walls are composed of cellulose. When chloroplasts are present in this type of cells, they are called chlorenchyma. The parenchyma cells with air filled spaces are called aerenchyma, and are usually big in size and found in aquatic plants. The main functions of parenchyma cells are to organise the body or body parts of plants to produce, transport and store food.

2) Collenchyma

2.4  Organ and System


A part of the animal body formed by the combination of one or more than one types of tissue performing a particular function is called an organ.


The branch of biology where organs are discussed is called Morphology.

2.5  Microscope

Light Microscope

The compound microscope is able to observe very small objects with the help of light is called a light microscope.

Types of light microscope

Light microscopes are of two types: simple microscopes and compound microscopes.

Electron Microscope

The microscope in which electrons are used instead of light is called the electron microscope. 

Simple microscope

A simple microscope is a magnifying glass that has a double convex lens with a short focal length.

Uses of Simple Microscope

  • It is used in pedology (a study of soil particles).
  • It is used by a dermatologist to find out various skin diseases.
  • It is used in microbiology to study samples of algae, fungi, etc.
  • It is used by jewelers to get a magnified view of the fine parts of the jewelry.

Compound Microscope

A compound microscope is a microscope that uses multiple lenses to enlarge the image of a simple. 

Secondary Biology


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