Unit 3: The Cell — Long Questions

The cell is the basic unit of life. Cells are the building blocks of living organisms, including plants, animals, and humans. Every living thing, from the smallest bacterium to the largest whale, is made of cells.
Size of Cells
i. Most of the cells are very small, and cannot be seen with the naked eye. Despite their size, cells are very complex and carry out many essential functions to keep living things alive and functioning, e.g. the size of RBCs is 8µm.
ii. Some cells are large enough to be seen with naked eye e.g., the egg cell of ostrich, a unicellular green algae Acetabularia, and a unicellular giant Amoeba.
Discovery of Cell and Nucleus
i. The basic structure of a cell was discovered by a curious scientist Robert Hooke. In 1665, using a simple microscope, Hooke examined a thin slice of cork and discovered tiny, box like structures that he called "cells." He could not study the details of the internal structure of cell.
ii. However, in the 19th century the quality of microscope improved. In 1831, while studying plant cells under microscope, Robert Brown observed the "nucleus". After that, many organelles were discovered in coming years.
Types of Cells
There are two basic types of cells:
1. Prokaryotic cell
2. Eukaryotic cell
Eukaryotic cell are more complex than the prokaryotic cells.

The difference between animal and plant cell is given below:
i. Animal Cell
An Animal cell has membrane bounded nucleus and organelles. Animal cells have no cell wall and chloroplast.
ii. Plant Cell
A plant cell is different from animal cell in having cell wall, single large vacuole and plastids.

Introduction
The cells of bacteria, fungi, plants and some protists have a rigid non-living wall around cell membrane. It is called cell wall.
Functions
It provides shape, strength, protection and support to the inner living matter (protoplasm) of the cell.
Structure
The plant cell wall is made of three layers i.e. middle lamella, primary wall, and secondary wall.
i. The primary wall is present just above the cell membrane. It is mainly composed of cellulose, hemicellulose, and pectin. Cellulose forms fibres that crisscross over one another to form strong primary wall.
ii. Middle lamella holds together the primary walls of adjacent cells. It contains magnesium, calcium and pectin.
iii. Some plant cells e.g., xylem cells make secondary wall on the inner side of primary wall. It is mainly made of cellulose, lignin and other chemicals.
Chemical Composition
i. The cell wall of algae is also composed of cellulose.
ii. The cell wall of prokaryotes is made of peptidoglycan (a single molecule made of amino acids and sugars).
iii. The cell wall of fungi is made of chitin.
iv. Plasmodemata (singular plasmodesma) are the channels in cell walls that allow the exchange of molecules between adjacent cells.

Introduction
All cells have a thin and elastic cell membrane around the cytoplasm of both plant and animal cells.
Function
It is selectively-permeable. It allows very few molecules to pass through it while blocks many other molecules.
Structure
i. Cell membrane is composed of proteins and lipids and small quantities of carbohydrates.
ii. The structure of cell membrane is described as fluid-mosaic model.
iii. According to this model the lipids make a fluid-like bilayer in which protein molecules are submerged.
iv. The lipids and proteins can move laterally. Due to these movements, the pattern or "mosaic." of lipids and proteins constantly changes.
v. Carbohydrates are joined with proteins (in the form of glycoproteins) or with lipids (in the form of glycolipids).
vi. Another lipid, cholesterol, is attached with the inner sides of the lipid bilayer. Cholesterol is absent in the membranes of most bacteria.
Examples
In eukaryotic cell many organelles e.g. mitochondria, chloroplasts Golgi apparatus and endoplasmic reticulum are also bounded by membrane.

Introduction
It is the jelly-like substance that fills the inside of a cell.
Chemical Composition
i. It's a complex mixture of water, proteins, enzymes, salts, and other substances.
ii. The liquid part of the cytoplasm that includes molecules and small particles, such as ribosomes, but not membrane-bound organelles is called cytosol.
Functions
i. Cytoplasm provides a medium for organelles to move and function.
ii. It also helps in the transport of materials throughout the cell.
iii. It acts as the site for various metabolic reactions e.g., Glycolysis (breakdown of glucose).
iv. It also stores food and wastes of the cell.

Introduction
All eukaryotic cells have a prominent nucleus. In animal cells, it is present in the center. In mature plant cells, it is pushed to side due to a large central vacuole.
Structure
i. Nuclear Envelope
The nucleus is bounded by a double membrane known as nuclear envelope.
ii. Nuclear Pores
It is semi-permeable and has many small pores called nuclear pores.
iii. Nucleoplasm
The inner jelly-like material of nucleus is called nucleoplasm.
iv. Nucleoli
In nucleoplasm, there are one or more small bodies called nucleoli (singular; nucleolus). Here, ribosomes are assembled.
v. Chromatin
Nucleoplasm contains fine thread-like material known as chromatin. It is composed of deoxyribonucleic acid (DNA) and proteins.
vi. Chromosomes
When a cell starts dividing, its chromatin condenses and takes the shape of thick chromosomes. DNA contains genes which control all the activities of the cell. DNA is also responsible for the transmission of characteristics to the next generation. That is why it is called the hereditary material.
Functions
i. The nucleus serves as the cell's "control center".
ii. It oversees cellular activities by directing the production of proteins.
No Prominent Nucleus in Prokaryotic Cells
The prokaryotic cells do not contain a prominent nucleus. Their chromosome is made of DNA only and floats in cytoplasm.

Introduction
It is a network of thin tubes and filaments present throughout the cytoplasm.
Structure
It consists of three parts i.e. microtubules, microfilaments, and intermediate filaments.
i. Microtubules
Microtubules are hollow tubes made up of tubulin protein. This part holds organelles in place, maintains a cell's shape, and act as tracks for organelles. Microtubules also make mitotic spindle, cilia and flagella.
ii. Microfilaments
Microfilaments are finer than microtubules. These are made up of contractile proteins, mainly actin. They help in cell movement e.g., the crawling of white blood cells and the contraction of muscle cells.
iii. Intermediate Filaments
Intermediate filaments are rods made of variety of proteins, mainly keratin and vimentin. They anchor the nucleus and some other organelles in the cell. They also make cell-to-cell junctions.

Introduction
Ribosomes are tiny granular structures. They are the sites of protein synthesis.
Location
Ribosomes float freely in the cytoplasm and are also attached on the surface of rough endoplasmic reticulum.
Composition
They are composed of almost equal amounts of proteins and ribosomal RNA (rRNA).
Structure
Ribosomes are not bounded by membranes and so are also found in prokaryotes. Eukaryotic ribosomes are slightly larger than prokaryotic ones. Each ribosome consists of two subunits.
Functions
i. Ribosomes are the sites of protein synthesis.
ii. The two subunits of a ribosome unite during the process of protein synthesis.
iii. When a ribosome has finished its work, its subunits get separated again.

Introduction
It is a network of membrane-bounded channels present throughout the cytoplasm of eukaryotic cell.
Types
There are two types of endoplasmic reticulum.
(a) Rough Endoplasmic Reticulum (RER)
i. Numerous ribosomes are attached on its surface.
ii. RER serves the function in protein synthesis.
(b) Smooth Endoplasmic Reticulum (SER)
i. It lacks ribosomes.
ii. It is involved in lipid metabolism and in the transport of materials from one part of the cell to the other.
iii. It also detoxifies the harmful chemicals that have entered the cell.
iv. In muscle cells, the SER is also involved in contraction process.

Introduction
In 1898, an Italian physician Camillo Golgi discovered a set of flattened sacs in the cytoplasm.
Formation
These flattened sacs, called cisternae, are stacked over each other and make a structure known as Golgi apparatus.
Location
It is found in both plant and animal cells.
Function
i. It modifies molecules coming from rough ER and packs them into small membrane-bound sacs called Golgi vesicles.
ii. These sacs are kept in cell or are transported to exterior in the form of secretions.

Discovery
Lysosomes were discovered by Belgian scientist Christian Rene de Duye.
Definition
These are small membrane-bound vesicles that contain digestive enzymes.
Location
Lysosomes are predominantly found in animal cells.
Functions
i. Lysosomes bud off from Golgi apparatus.
ii. Cell engulfs the food material in the form of food vacuole.
iii. Lysosome fuses with food vacuole and its digestive enzymes break down the food present in vacuole.
iv. Lysosomes also have enzymes for breaking cellular wastes.
v. They also engulf the damaged organelles and break them.
vi. Lysosomes can store certain molecules for later use.

Functions
i. Mitochondria (Singular: mitochondrion) are the "powerhouse" of the cell because they produce energy.
ii. They perform the reactions of aerobic respiration in which oxygen is used to break food (glucose) to release energy (ATP - adenosine triphosphate).

Structure
i. Mitochondria are double membrane bounded organelles present only in eukaryotes.
ii. The outer membrane of mitochondria is smooth but the inner membrane forms many folds. These folds are called cristae (singular: crista). They increase the surface area for respiration.
iii. The inner fluid-like material is called matrix.
iv. Mitochondria contain their own DNA and ribosomes. They can multiply within the cell on their own.

Ribosomes of Mitochondria
The ribosomes of mitochondria are more similar to prokaryotic ribosomes than to eukaryotic ribosomes

Introduction
Plastids are present in the cells of plants and photosynthetic protists (algae).

Types.
There are three main types of plastids:
1. Chloroplasts
2. Chromoplasts
3. Leucoplasts.

1. Chloroplasts

Introduction
Chloroplasts are green plastids present in the cells of green parts of plants and in algae.

Function
They contain photosynthetic pigments e.g., the green pigment chlorophyll. They carry out photosynthesis. With the help of their photosynthetic pigments, they capture light energy and convert it into chemical energy in the form of glucose.

Structure
i. Membranes
Like mitochondria, chloroplast is enclosed within two membranes.
ii. Grana
On the internal side of inner membrane, there are many sets of stacked membranes. These stacks are called grana (singular, granum)
iii. Thylakoid
The sac-like structures which make a granum are called thylakoids. Photosynthetic pigments are present on the surface of thylakoids.
iv. Stroma
A fluid called stroma surrounds the thylakoids.
v. DNA and Ribosomes
Like mitochondria chloroplasts also contain DNA and ribosomes.

2. Chromoplasts

Introduction
Chromoplasts are the plastids that contain pigments such as carotenoids.

Location
These pigments are associated with bright colours and are present in the cells of flower petals and fruits.

Functions
Chromoplasts give colours to flower petals and fruits, thus helping in pollination and dispersal of fruit and seeds.

3. Leucoplasts

Introduction
Leuocplasts are plastids that have no pigments.

Function
They are involved in the storage of starches, lipids, and proteins.

Location
They are present in the cells of those parts where food is stored e.g., underground stems, seeds, roots etc.

Introduction
These are single membrane-bound sacs filled with fluid.

Vacuoles in an Animal Cell
i. Animal cell may have many small temporary vacuoles.
ii. They contain water and food substances.
iii. Some freshwater organisms like amoeba and sponges have contractile vacuoles which collect and pump out extra water and other wastes.
iv. Some cells ingest food by forming food vacuoles. Food vacuoles also store food.

Vacuole in a Plant Cell / How does turgor pressure develops in plants cells?
i. Most mature plant cells have a single, large, central vacuole.
ii. It is formed by the fusion of many small vacuoles.
iii. The membrane of plant vacuole is called tonoplast and the sap inside plant vacuole is called cell sap (It is a watery solution of salts).
iv. Due to this large central vacuole, the nucleus is pushed to a side. This outward pressure of the vacuole on the cytoplasm and cell wall makes plant cells turgid. This pressure is called turgor pressure and the process is called turgor.

Introduction
Centrioles are barrel-shaped organelles found in the cells of animals and most protists. They are absent in prokaryotes, higher plants and fungi. In animal cell there is a pair of centrioles, both centrioles are at right angle to each other. This pair is called a centrosome and it is located near the nuclear envelope.

Structure
Each centriole is formed of 9 triplets of microtubule (made up of tubulin protein).

Functions
i. At the start of cell division, the pair of centrioles duplicates.
ii. The new pairs move to the opposite pole of the cell. There, they form spindle fibres.
iii. The cells which have cilia or flagella contain centriole near cell membranes.
iv. These centrioles are called basal bodies.
v. Basal bodies are responsible for the formation of cilia and flagella.

Introduction
Some cells have thin, hair-like projections called cilia (singular: cilium) and flagella (singular: flagellum).

Structure
i. Cilia are short in length and are usually numerous in number, while flagella are longer but less in number.
ii. Eukaryotic cilia and flagella consist of nine pairs of microtubules which surround a single central pair of microtubules.
iii. Cilia and flagella are connected to the basal body.
iv. Prokaryotic cells also have flagella but their structure is completely different.
v. Prokaryotic flagella are made of a protein called flagellum.

Function
The function of cilia and flagella is movement.

[Table structure preserved]

Component | Description | Location | Function

Cytoplasm | Jelly-like, with organelles in it | Between plasma membrane and nuclear envelope | Provides the site to cell organelles, site of metabolic reactions

Cell membrane | A partially permeable membrane that forms a boundary around the cytoplasm | Around cytoplasm | Prevents cell contents; controls what substances enter and leave the cell

Nucleus | A spherical or oval organelle containing DNA | In the centre in animal cells, on a side in plant cells | Controls cell division; controls cell activities

Cell wall | A tough, non-living outer layer made of cellulose | Around the outside of plasma membrane | Provides mechanical support; allows water and salts to pass

Large Vacuole | A fluid-filled space surrounded by a membrane | Inside the cytoplasm of planit cells | Contains salts and water, helps to keep plant cells turgid

Chloroplast | An organelle containing chlorophyll | Inside the cytoplasm of some plant cells | Traps light energy for photosynthesis

Plant and animal cells have distinct structural differences that reflect their specialized functions and adaptations. Here are some structural advantages of both plant and animal cells.

Advantages of Plant Cell Structures
i. Cell wall
Plant cells have a rigid cell wall made of cellulose. It provides structural support and protection.
ii. Chloroplasts
They contain chloroplasts, which are responsible for photosynthesis. Chloroplasts convert light energy into chemical energy, allowing plants to produce food.
iii. Large Central Vacuole
The large central vacuole stores water, nutrients, and waste products. It provides turgor pressure that maintains cell shape.
iv. Plasmodesmata
Plant cells are interconnected by plasmodesmata, channels that allow direct communication and transport of substances between cells.

Advantages of Animal Cell Structures
i. Centrioles
Animal cells have centrioles which make spindle fibres. This ensures the accurate distribution of chromosomes during cell division.
ii. Lysosomes
They contain lysosomes, filled with digestive enzymes that break down waste materials. Lysosomes contribute to cellular cleanup and recycling.
iii. Flagella and Cilia
Some animal cells have structures called flagella and cilia, which are involved in movement. For example, sperm cells have a flagellum that propels them toward the egg for fertilization.
iv. Lack of Cell Wall
They lack a rigid cell wall, allowing them to change shape easily. This flexibility is crucial for cell movements, such as white blood cells moving to sites of infection or injury.

Introduction
In multicellular organisms, all cells are not exactly alike. Rather, there are different types of cells. Each type has a special structure and performs special function. When cells are formed by cell division, they are all similar. After their formation, cells undergo the process of specialization or differentiation. During this process, they get special sizes, structures, and metabolic features. As a result, they become, specialized.
Some examples of the specialized cells of plants and animals are given below:

Cell Specialization in Plants

1. Mesophyll Cells

Introduction
These are green cells present in leaves.

Function
i. They are specialized for photosynthesis.
ii. They contain large number of chloroplasts, which contain the green pigment chlorophyll necessary for capturing light energy.
iii. Their shape and arrangement in leaves is suitable for maximum absorption of light.

2. Epidermal Cells

They are flat and tightly packed cells that make the outer layer (epidermis) of plant organs.

Functions
i. Epidermis protects the internal tissues.
ii. Modified cells of epidermis also perform other functions.

Examples
(a) Epidermis of root contains root hair cells. These cells make extensions called root hairs to absorb water and minerals from soil.
(b) Similarly, lower epidermis of leaves contains guard cells which regulate opening, and closing of stomata.

Cell specialization in Animals

3. Muscle Cells

Introduction
Muscle cells are specialized animal cells that can contract. They are elongated cells filled with actin and other contractile proteins.

Types
i. Skeletal Muscle Cells
Skeletal muscle cells are long, striated. They are attached to bones. They are voluntary in action and their contractions move the skeleton for body movements and locomotion.
ii. Cardiac Muscle Cells
Cardiac muscle cells are branched and striated. They are found in the heart walls. They are involuntary in action and their contractions result in the pumping action of heart.
iii. Smooth Muscle Cells
Smooth muscle cells are spindle shaped and non-striated. They are involuntary in action and are present in the walls of many internal organs. For example, smooth muscles in the alimentary canal contract to move food forward, while those in blood vessels regulate blood flow.

4. Neurons

Functions
These are the specialized cells of the nervous system. They are responsible for transmitting messages (nerve impulses) throughout the body. To, perform this function, they have a unique structure.

Structure
i. A neuron consists of a cell body and two types of cytoplasmic extensions.
ii. Dendrites, the shorter extensions, receive nerve impulses and transmit them to the cell body.
iii. Axons, the longer extensions, carry nerve impulses away from the cell body.

5. Red Blood Cell (Erythrocyte)

Functions
These blood cells are specialized to carry oxygen from the lungs to the body's tissues.

Shape
i. They are biconcave disk-shaped cells.
ii. This shape provides more surface area to absorb and release oxygen.
iii. They are filled with haemoglobin that actually carries oxygen.
iv. In mammals, the mature red blood cells do not contain nucleus, mitochondria, and endoplasmic reticulum etc.
v. It helps to accommodate more haemoglobin.

6. Liver Cell

Introduction
They are also called hepatocytes.

Functions
i. They are specialized for a lot of important functions like storage of glycogen, iron and some vitamins; detoxification of toxic substances; production of clotting proteins of blood, recycling of old red blood cells etc.
ii. They have prominent nuclei for maximum activities required for making enzymes and other proteins.
iii. Large number of mitochondria provide the necessary ATP for energy-intensive processes.
iv. Expansive network of SER helps for extensive detoxification and lipid synthesis.
v. There are large number of peroxisomes which contain enzymes to neutralize toxic substances.
vi. Small ducts are present between liver cells which collect and transport their secretion (bile) to the Gall bladder.

Introduction
Division of labour refers to the specialization of different parts of a system to perform specific tasks more efficiently. It is a fundamental principle that enhances efficiency and functionality in biological systems (both within and across cells).

Division of Labour within Cells

Introduction

Within Cells Within a cell, this concept is exemplified by the various organelles that each carry out distinct functions necessary for the cell's survival.

Examples
Mitochondria generate energy, endoplasmic reticulum synthesizes proteins and lipids, and lysosomes break down waste materials. In this way, the function of each organelle contributes to the cell's overall survival, growth, and functioning.

Division of Labour Across Cells

Introduction
In multicellular organisms, the division of labour extends across cells. Each type of cell performs a specific role and contributes to the overall functions of the organism. Example. muscle cells are specialized for contraction and movement, nerve cells for transmitting messages, and red blood cells for carrying oxygen. This intercellular specialization allows complex organisms to perform a wide range of functions.

Zygote An unspecialized cell
i. In sexually reproducing organisms, all different types of cells arise from a single cell (zygote).
ii. The zygote is an unspecialized cell but it has the ability to make new cells which can differentiate into specialized cells.

Stem Cells - Specialized Cells
iii. Such unspecialized cell that has the ability to differentiate into a variety of specialized cell types is called stem cell.

Division of Stem Cells
iv. During development, when the earliest stem cell (zygote) divides, it makes different cell lines.
v. The cells of each line differentiate into skin cells, muscle cells, nerve cells, blood cells etc.

Functions of Stem Cells
Stem cells also remain in different parts of the body throughout life. These stem cells can divide and differentiate into specific cells as the body needs them. They can also regenerate damaged tissue under the right conditions.

Examples
(a) Stem cells present in skin help in wound healing.
(b) Stem cells present in liver also help it to repair after damage.
(c) Stem cells present in the bone marrow differentiate to make different types of blood cells and immune cells.
(d) In some parts of the body, such as the gut and bone marrow, adult stem cells regularly divide to produce new tissues for maintenance and repair.