Unit 3: Cells & Subcellular Organelles — Long Questions

Cells are basic unit of life making every living organism. Some organisms are unicellular like amoebas and bacteria and some are multicellular such as plants and animals. All living organisms show the seven basic properties of life i.e., movement, respiration, homeostasis, growth, reproduction, excretion, and nutrition. These properties actually define the organisms. Cells perform all the fundamental activities that characterize living organisms.

• Movement: Cells can move. For example, sperm cells move with their flagella. White blood cells travel through the bloodstream to fight infections. Inside cells, organelles move to carry out vital functions.

• Nutrition: Cells obtain nutrients from their environment to produce energy, build cellular structures, and drive biochemical reactions.

• Respiration: Cells generate energy through respiration. This process breaks glucose to release ATP, the energy currency that powers cellular activities.

• Excretion: Cells remove waste products through diffusion and active transport, preventing toxic buildup.

• Homeostasis: Cells maintain a stable internal environment by regulating the movement of substances across their membranes.

• Growth: Cells grow by taking in nutrients and converting them into cellular components.

• Reproduction: Cells reproduce through mitosis and meiosis. Mitosis produces identical daughter cells for growth and repair, while meiosis creates gametes for sexual reproduction.

History Contribution of different scientists in formulation of cell theory is as follows:

• Robert Hooke: In 1665, English scientist Robert Hooke examined a thin slice of cork of oak tree under microscope. He observed that the cork was made of "many little boxes". Hooke also examined the pieces of stem and root of oak tree under microscope. He found that these were also made of similar little boxes. He concluded that the parts of plants were made of compartments. Hooke named these compartments as "cellulae"

• Anton van Leeuwenhoek: In 1673, a Dutch scientist Anton van Leeuwenhoek made a better microscope and observed living cells in pond water. He called these cells as animalcules.

• Jean Baptiste-de-Lamarck: In 1809, the French biologist Jean Baptiste de-Lamarck also observed cells when he examined the parts of animals and plants under microscope.

• Robert Brown: In 1831 the British botanist Robert Brown discovered nucleus in cell.

• Matthias Schleiden: In 1838, the German botanist Matthias Schleiden observed many parts of plants under microscope. He concluded that all plants were composed of cells.

• Theodor Schwann: The German zoologist Theodor Schwann concluded the same for animals.

• Rudolf Virchow: In 1885, the German physician Rudolf Virchow (1821-1902) observed that all cells come from other cells.

• Louis Pasteur In 1862, Louis Pasteur provided the experimental proof of this idea. These observations were combined to form a basic theory about cells. It is called cell theory.

Salient Features of cell theory
It has three essential points.

• All living organisms are composed of one or more cells.
• Cells are the basic units of structure and function in an organism.
• Cells come only from the division of pre-existing cells.

Validation of Cell Theory Cell theory can be validated to prove correct through several observations and experiments.

Example

• By using light microscopes and electron microscopes, scientists visualize (see) cell structures and find tangible (clear) evidence that cells are indeed the structural units of all living organisms.

• Through techniques like live-cell imaging and genetic studies, scientists can observe how cells replicate and give rise to new cells. These techniques validate the principle that all cells originate from pre-existing cells.

• Techniques like DNA sequencing reveal that cells share common genetic material and metabolic pathways, make the concept strong that the cell is the fundamental unit of life.

• Experiments, such as cell culture studies and tissue engineering, validate cell theory by demonstrating cellular growth, differentiation, and reproduction.

Exceptions to Cell Theory

While cell theory is widely accepted, there are important exceptions. For example:

• Viruses challenge cell theory because they are not made of cells and cannot carry out life processes independently. They require a host cell to replicate and are considered by many scientists to be at the border of living and non-living entities. Similarly, prions and viroids show properties of living organism but are not composed of cells. They are made of only DNA, RNA or proteins.

• Eukaryotic organelles mitochondria and chloroplasts have their own DNA and can replicate independently of the cell's nucleus. This suggests they may have originated from free-living prokaryotic cells.

• Some organisms, such as certain fungi and algae, have structures where multiple nuclei coexist within a syncytoplasmic mass. These structures create the confusion about the boundaries of individual cells as defined by traditional cell theory.

• Muscle cells (myocytes) in vertebrates can fuse to form multinucleated fibres, challenging the concept of a single cell as the basic unit in complex tissues.

Microscopy is the technique of using microscopes to observe and study objects that are too small to be seen with the naked eye. Microscopes use lenses and light or electron beams to magnify and make specimens bright and clear. Significance of microscopy is that further studies of internal structure of cells walls dependent upon microscopy.

A. Light Microscopy

(i) Working Principle

In light microscope, light is used to make the image of object. Light passes through object and then through two glass lenses. One lens produces an enlarged image of the object and the second lens magnifies the image more. After passing through object and lenses, the light forms enlarged clear image of object in viewer's eye.

(ii) Magnification and Resolution

Magnification The magnification of a light microscope is 1500X. It means it can magnify objects about 1500 times. Its resolving power is 0.2 micrometer (μm) and 1 μm = 1/1000 mm. In other words, the light microscope cannot distinguish objects smaller than 0.2 μm.

(iii) Advantage

Light microscopes are advantageous for viewing living organisms, but since individual cells are generally transparent, their components are not distinguishable unless they are coloured with special stains (coloured chemicals). Staining, however, usually kills the cells.

B. Electron Microscopy

(i) Working Principle

In electron microscope, a beam of electrons passes through the object. Magnetic lenses focus the electron beam on a screen or photographic film and make much enlarged image.

(ii) Resolving Power

Its resolving power is much greater than light microscope. It can clearly show objects as small as 0.2 nanometre (nm) and 1 nm = 1/1,000,000 mm. However, electron microscope cannot be used for living material because of the methods needed to prepare the specimens.

(iii) Types of Electron Microscope

Biologists use two types of electron microscopes.

(a) Transmission Electron Microscope (TEM) is used to view the internal structure of cell. TEM transmits a beam of electrons through a very thin specimen. It can magnify objects up to 250,000 times.

(b) Scanning Electron Microscope (SEM) is used to study the details of surfaces of cells or any other objects. The surfaces are coated with metal. When electron beam hits the metal, it is reflected and makes enlarged image. SEM's can magnify objects up to 100,000 times.

Light Microscope

1. Uses light (approx. 400-700 nm) as an illuminating source.
2. Lower magnification (usually 500X to 1500X) than an electron microscope.
3. Low resolution (may be 0.2μm) both live and dead specimens can be seen.
4. Specimen preparation takes about a few minutes or an hour.
5. The image is seen through the ocular lens. No screen needed.

Electron Microscope

1. Uses electron beams (approx. 1 nm) as an illuminating source.
2. Higher magnification (direct magnification is 16000X and photographic magnification is 1000000X).
3. High resolution (may be 0.2nm) only dead and the dried specimen can be seen.
4. Specimen preparation takes several days.
5. The image is received on a zinc sulphate fluorescent screen.

A. Structure Cell wall is a more or less solid layer surrounding a cell. It is found in bacteria, fungi, plants, and algae. When a cell wall is removed using cell wall degrading enzymes, the remaining components of the cell are called a protoplast. It is composed of three layers.

• Primary Wall: The primary wall is the actual cell wall of cell. It is composed of polysaccharides i.e. cellulose, hemicellulose and pectin. The cellulose microfibrils are aligned at all angles and are held together by hydrogen bonds. Many proteins are also present in primary walls.

• Middle Lamella: The middle lamella is a gelatinous layer that separates and holds the primary walls of the neighbouring cells. It is laid first, formed from the cell plate during cytokinesis, and the primary cell wall is then expanded inside the middle lamella. It contains magnesium and calcium pectates (salts of pectic acid).

• The secondary wall is found in some plant cells, after maturation, a secondary wall is made between protoplast and primary wall. Secondary cell walls contain lignin, cellulose and hemicellulose. Due to the presence of lignin, secondary wall is more rigid than the primary wall.

B. Functions

Cell walls have a number of functions:

• They provide rigidity to the cell for structural and mechanical support, maintain cell shape and the direction of cell growth and ultimately the architecture of the plant.

• The cell wall also prevents expansion when water enters the cell.

• Cell walls protect against pathogens and the environment and are a store of carbohydrates for the plant.

Plasmodesmata (singular, plasmodesma) These are small channels that directly connect the cytoplasm of neighbouring plant cells to each other. Plasmodesmata penetrate both the primary and secondary cell walls. They allow certain molecules to pass directly from one cell to another. So, they are important in cellular communication.

Plasmodesmata are formed during cell division when parts of the endoplasmic reticulum of the parent cell get trapped in the new cell wall.

The cell wall of algae contains cellulose and a variety of glycoproteins.

The cell wall of fungi is composed of chitin, the same carbohydrate that gives strength to the exoskeletons of insects.

The cell wall of prokaryotes (bacteria and cyanobacteria) is composed of peptidoglycan, that is a single large polymer of amino acids and sugar.

The cell wall of archaebacteria is composed of different polysaccharides and proteins, with no peptidoglycan.

Plasma Membrane

Plasma membrane is a basic component of prokaryotic as well as eukaryotic cells. It encloses the internal contents of cell and acts as semi-porous barrier. It controls the exchange of materials between cell and external environment. Its structure is explained as follows.

A. Structure of plasma membrane:

Fluid Mosaic Model The fluid mosaic model is a widely accepted concept that describes the dynamic nature of plasma membrane. It was proposed by two American biologists S.J. Singer and Garth Nicolson in 1972.

1. Lipid bilayer: According to the fluid mosaic model, the basic foundation of plasma membrane is a lipid bilayer. This bilayer is made of phospholipids. A collection of proteins float within the lipid bilayer.

2. Phospholipids: The phospholipids have a phosphate group at one end of each molecule. Phospholipids are characteristically hydrophilic ("water-loving") at their phosphate ends and hydrophobic ("water-fearing") along their tail regions containing C-H chains. In the lipid bilayer of plasma membrane, the hydrophobic lipid tails are oriented inwards and the hydrophilic phosphate groups are aligned outwards, either toward the cytoplasm of the cell or the extracellular environment.

3. Cholesterol: In eukaryotes, plasma membranes have cholesterol molecules, wedged into the phospholipid bilayer. They keep the fluidity of membrane at low temperatures.

4. Proteins: Many proteins float within the phospholipid bilayer of plasma membrane. Some other proteins simply adhere to the surfaces of the bilayer. The positioning of proteins is related to the organization of cytoskeleton.

Types and Functions of Proteins

Plasma membrane proteins function in several different ways.

• Selective transport: Many proteins play role in the selective transport of certain substances across the phospholipid bilayer, either acting as channels or active transport molecules.

• Attachment: Some proteins help in attachment of plasma membrane to cytoskeleton and external fibres.

• Identification Marks: Some proteins, on the exterior surface, attach with sugars and make identification marks.

• Receptors: Other proteins function as receptors, which bind messenger molecules (e.g. hormones) and transmit signals to the interior of cell.

• Some proteins also exhibit enzymatic activity, catalysing various reactions related to the plasma membrane.

5. Glycocalyx

The outside surface of plasma membrane has chains of sugars bonded to proteins and lipids. A protein with attached sugar is called a glycoprotein, whereas a lipid with attached sugar is called a glycolipid. The glycoproteins and glycolipids vary from species to species, from individual to individual in the same species, and even from one cell type to another in the same individual. The glycolipids and glycoproteins (collectively called glycocalyx) function as cell identification marks that are recognized by other cells.

Plasma membrane plays important role in prokaryotic and eukaryotic cells. Its major functions are:

(i) Plasma membranes serve as semi-porous barriers to the outside environment.

(ii) The membrane acts as a boundary, holding the cell constituents together.

(iii) The plasma membrane is permeable to specific molecules, however, and allows nutrients and other essential elements to enter the cell and waste materials to leave the cell.

(iv) Small molecules, such as oxygen, carbon dioxide, and water, are able to pass freely across the membrane, but the passage of larger molecules, such as amino acids and sugars, is carefully regulated.

(v) Eukaryotic cells also have membranes around some of their interior organelles. Like the exterior plasma membrane, these membranes also regulate the flow of materials into and out of organelles.

Structure of cytoplasm

(i) The cytoplasm is a semi-viscous and semi transparent substance.

(ii) In eukaryotic cells, it is present between the plasma membrane and nuclear envelope.

(iii) In prokaryotic cells, it covers all the space beneath plasma membrane.

(iv) It consists of an aqueous ground substance, known as cytosol, which contains a variety of organelles and other inclusions.

(v) Cytosol contains water in which many organic (proteins, carbohydrates, lipids) and inorganic salts are completely or partially dissolved.

(vi) The cytoplasm contains discrete structures which are specific for various cellular functions and are called cell organelles. The organelles are generally enclosed by membrane except few such as ribosome.

Functions of cytoplasm

(i) The cytoplasm of the cell provides space for the proper functioning of the organelles.

(ii) It also acts as the site for various biochemical (metabolic) reactions for example Glycolysis (breakdown of glucose during aerobic respiration).

Techniques to study the structure of plasma membrane

• Transmission Electron Microscopy can reveal detailed structures of the lipid bilayer and associated electron proteins.

• Scanning Electron Microscopy is useful for examining the surface topology of cells and membranes.

• Confocal Microscopy uses laser scanning and fluorescence to create sharp, detailed images of the cell membrane.

• Total Internal Reflection Fluorescence Microscopy is used for high -resolution images of the membrane and its interactions with the cytoskeleton and other cellular components.

• Atomic Force Microscopy provides topographical images of cell membrane at high resolution.

• X-ray Crystallography is used to determine the atomic structure of membrane proteins.

• Lipidomics involves the comprehensive analysis of lipids in the cell membrane using techniques like mass spectrometry.

• Fluorescence Recovery After Photobleaching is used to study the mobility and dynamics of membrane proteins and lipids. It involves bleaching a fluorescently labelled region of the membrane with a laser and observing the recovery of fluorescence as unbleached molecules move into the area.

Nucleus

1. Location: A prominent nucleus is present in all eukaryotic cells (at centre in animal cells while pushed to side in plant cells). The spherical nucleus typically occupies about 10 percent of a eukaryotic cell's volume.

2. Functions of Nucleus

(i) It serves as information processing and administrative centre of the cell.

(ii) It performs two major functions: it stores the cell's hereditary material (DNA) and coordinates the cell's activities e.g., growth, protein synthesis and cell division.

3. Structure of Nucleus

a. Nucleoplasm: The semifluid matrix found inside the nucleus is called nucleoplasm. Within the nucleoplasm, most of the nuclear material consists of chromatin that organizes to form chromosomes during cell division. The nucleus also contains one or more nucleoli, which synthesize ribosomes.

b. The Nuclear Envelope: The nuclear envelope is a double-layered membrane that encloses the contents of the nucleus during most of the cell's lifecycle. The space between the double layers is called the perinuclear space and is connected with the rough endoplasmic reticulum. During cell division, the nuclear envelope disintegrates, but reforms in the daughter cells.

c. Nuclear Lamina: On the inner side of nuclear envelope, there is a protein lining, called nuclear lamina. It binds to chromatin to give it structural support.

d. Nuclear Pores

(i) The nuclear envelope has tiny holes known as nuclear pores. These pores regulate the passage of molecules between nucleus and cytoplasm.

(ii) Nuclear pores are permeable to small molecules. Some larger proteins, e.g., histones are also allowed to enter into nucleus.

(iii) A nuclear pore is made of an elaborate structure called the nuclear pore complex. It is composed of several subunits.

• Annular subunit (surrounding the inside of the pore)
• Column subunit (making the wall of the complex)
• Ring subunit (attached to the outer side of the column subunit)
• Lumenal subunit (anchoring the pore complex into the nuclear envelope)

e. Nucleolus

(i) The nucleolus is a prominent darkly stained structure in the nucleoplasm. There may be one or two nucleoli in a nucleus.

(ii) Nucleoli manufacture the subunits that combine to form ribosomes.

(iii) Nucleoli are formed at certain sites in chromosome, called Nucleolus Organizer Regions (NORs). The DNA found at NORs encodes the ribosomal RNA (rRNA).

(iv) The nucleolus consists of granular and fibrillar components and DNA. The granules consist of ribosomal subunits that have already been formed. The fibrils are composed of the raw materials of ribosome subunits i.e., rRNA molecules and associated proteins.

f. Chromatin and Chromosomes

(i) Nucleus contains string-like fibres, collectively called chromatin.

(ii) It is composed of DNA and proteins.

(iii) The structure of chromatin reveals that it is made of a series of bead-like structures, called nucleosomes.

(iv) In a nucleosome, DNA strand wraps around groups of small proteins called histones.

(v) During interphase (when the cell is carrying out its normal functions), the chromatin is dispersed throughout the nucleus in the form of a tangle of fibres.

(vi) When the cell begins to divide, all chromatin strands are compressed into specialized structures, the chromosomes.

(vii) A chromosome is made of arms, called chromatids and a central point called centromere.

(viii) The number of chromosomes within the nuclei of an organism's cells is species-specific. Human diploid cells (those that are not gametes) have 46 chromosomes. The chromosome number may be as low as 2, in some ants and roundworms, or more may be a thousand as in the Indian fern (Ophioglossum reticulatum), which has 1,260 chromosomes. It means that the number of chromosomes in a species does not correlate to the complexity of the organism.

A. Structure

1. Cisternae: It is a network of flattened sacs and branching tubules that extends throughout the cytoplasm in plant and animal cells. These sacs and tubules are called cisternae (singular cisterna).

2. Cisternal Space: All cisternae are interconnected so that the ER has only one large and highly convoluted lumen, called cisternal space. It takes up more than 10 percent of the total volume of a cell.

3. Double-Layered: The cisternae are also connected to the double-layered nuclear envelope. So, the ER provides a pipeline between nucleus and cytoplasm.

B. Function The ER manufactures, processes and transports a wide variety of biochemical compounds for use inside and outside of the cell.

C. Kinds of Endoplasmic Reticulum

a. Rough endoplasmic reticulum: The surface of rough endoplasmic reticulum (RER) is covered with ribosomes, giving it a bumpy appearance when viewed through the microscope.

Function This type endoplasmic reticulum is involved mainly in the production and processing of proteins. During processing of proteins, RER adds other chemicals (e.g. sugars) to the proteins. Then RER transports the processed proteins to areas of the cell where they are needed, or sends them to Golgi apparatus for further processing and modification.

b. Smooth endoplasmic reticulum (SER)

The surface of smooth endoplasmic reticulum (SER) lacks ribosomes. So, it appears more even under the microscope. In most cells, it is much less extensive than the rough endoplasmic reticulum.

Function

(i) Smooth endoplasmic reticulum is chiefly involved in the production of lipids, building blocks for carbohydrate metabolism.

(ii) Detoxification of drugs and poisons.

(iii) Smooth endoplasmic reticulum also plays a role in various cellular activities by storing calcium and doing calcium metabolism.

(iv) In muscle cells, smooth endoplasmic reticulum releases calcium to trigger muscle contractions.

Structure and role of ribosomes

• All living cells contain ribosomes that are tiny granular structures composed of approximately 60 percent ribosomal RNA (rRNA) and 40 percent protein.

• Ribosomes are not bound by a membrane and are much smaller than other organelles.

• In eukaryotic cells, ribosomes are mainly found attached to rough endoplasmic reticulum and some are scattered freely.

• In prokaryotic cells, all ribosomes are freely scattered in cytoplasm. Ribosomes serve as the protein production machinery for the cell.

• They are most abundant in cells that are active in protein synthesis, such as pancreas and brain cells.

• A typical cell contains several thousand ribosomes but some cell types may have a few million ribosomes.

• Eukaryote ribosomes are produced and assembled in the nucleolus. Ribosomal proteins enter the nucleus and combine with rRNA strands to create the two ribosomal subunits (one small and one large).

• The ribosome subunits leave the nucleus through the nuclear pores. In the cytoplasm, both subunits combine for the purpose of protein synthesis. When protein synthesis is not being done, the two subunits get separated.

• Protein synthesis requires the assistance of two other kinds of RNA molecules in addition to rRNA. Messenger RNA (mRNA) provides the instructions, which it has taken from the DNA. Transfer RNA (tRNA) brings amino acids to the ribosome. Once the chain of amino acids has been synthesized, the ribosome releases it.

• The subunits of a ribosome are described by their Svedberg (S) values, which are based upon their rate of sedimentation in a centrifuge.

• The complete ribosome in a eukaryotic cell has a Svedberg value of 80S. The smaller subunit has value of 40S while the larger subunit has 60S.

• Prokaryotic cells, on the other hand, contain 70S ribosomes, each of which consists of a 30S and a 50S subunit.

Structure of Mitochondria

• Mitochondria (sing., mitochondrion) are rod-shaped organelles that are considered the power generators of the cell.

• A mitochondrion is bounded by two membranes.

• There is a narrow intermembrane space between the two membranes.

• Beneath the inner membrane, there is a larger internal matrix.

• The outer membrane is smooth and acts like a sieve, filtering out molecules that are too big.

• The inner membrane is highly convoluted and forms many infoldings called cristae which increase the surface area.

• The inner surface of the cristae has knob-like extensions into the matrix, known as F-1 particles. These particles are actually the enzymes called ATP-synthase.

• Other complexes are also found in inner mitochondrial membrane, which serve as electron carriers in electron transport chain.

• Mitochondria are different from most other organelles.

• A mitochondrion has its own circular DNA (similar to the DNA of prokaryotes), all kinds of RNA and 70S ribosomes.

• A mitochondrion can replicate independently of the cell.

Functions of Mitochondria

• Mitochondria are the sites of cellular respiration.

• They generate adenosine triphosphate (ATP) from oxygen and nutrients. ATP is the chemical energy "currency" of the cell that powers the cell's metabolic activities.

• Enzymes in the mitochondrial matrix catalyse the steps of cellular respiration like Krebs cycle and ETC.

• The proteins that function as electron transport chain are found on the inner mitochondrial membrane.

Structure of Chloroplasts

• Importance: One of the most important characteristics of plants is their ability to conduct photosynthesis i.e., to make their own food by converting light energy into chemical energy. This process occurs in almost all plant species and is carried out in specialized organelles known as chloroplasts.

• Occurrence: All of the green structures in plants, including stems and un- ripened fruit, contain chloroplasts, but the majority of photosynthetic activity in most plants occurs in the leaves.

• Density: On the average, the chloroplast density on the surface of a leaf is about one-half million per square millimeter.

• Pigments: Chloroplasts contain the pigments chlorophyll "a" and chlorophyll "b", which are able to absorb the light energy needed for photosynthesis to occur.

• Intermembrane Space: The ellipsoid-shaped chloroplast is enclosed by two membranes and the area between the two membranes is called the intermembrane space.

• Stroma: A semi-fluid called stroma is present inside the inner membrane. It contains dissolved enzymes and comprises most of the chloroplast's volume.

• Membranes: The outer membrane is much more permeable than the inner layer. The inner membranes lie in close association with one another and fuse along their peripheries.

• Thylakoid: In this way, two adjacent membranes form a disk-shaped compartment called thylakoid.

• Grana: Many thylakoids form stacks called grana (singular granum). Each granum may contain a few to several thylakoids, and a chloroplast may contain a hundred or more grana.

• Lamellae: The lamellae are the non- green compartments that connect two grana.

• Own DNA: Like the mitochondrion, the chloroplast is different from most other organelles because it has its own DNA and reproduces independently of the cell in which it is found.

Functions of Chloroplast

• Light is absorbed by chlorophyll molecules embedded in the thylakoid disks.

• When these chlorophyll molecules absorb light, they emit electrons and thus ATPs are formed.

• Using these ATPs, in the stroma, low-energy carbon dioxide is transformed into a high-energy compound like glucose.

Ans. Structure of Golgi Apparatus

★ Discovery: Golgi apparatus was discovered by Camillo Golgi.

• Cisternae: Golgi apparatus consists of five to eight cup-shaped, membrane-covered sacs called cisternae that are stacked over each other.
• It is found in the cells of plant, animal and unicellular eukaryotes.
• In some unicellular flagellates, the Golgi apparatus may consist of 60 cisternae.
• The number of Golgi apparatuses in a cell varies according to its function.
• Animal cells generally contain between 10 – 20 Golgi stacks in their Golgi apparatus.
• This complex is usually located close to the nucleus.
• Each Golgi stack has two distinct faces. The 'cis' face is found near the endoplasmic reticulum. The 'trans' face is positioned near the plasma membrane.

Functions of Golgi Apparatus

• The Golgi apparatus is the distribution and shipping department for the cell's chemical products.
• It modifies proteins and lipids that have been built in the endoplasmic reticulum and prepares them for export outside the cell or for transport to other locations in the cell.
• Small vesicles that contain proteins, carbohydrates, phospholipids and other molecules, bud off from the ER. These vesicles move through the cytoplasm until they reach the 'cis' face of Golgi apparatus. The vesicles fuse with Golgi apparatus and release their molecules into it.
• Here, the compounds are further processed. Enzymes present in the Golgi lumen convert them into glycoproteins and glycolipids.
• The product is extruded from the 'trans' face of the Golgi apparatus in a vesicle and directed to its final destination inside or outside the cell. The exported products are known as secretions.
• Other products are returned to the endoplasmic reticulum or may undergo maturation to become lysosomes.
• In addition, the Golgi apparatus in plant cells produces pectin and other polysaccharides specifically needed for plant structure and metabolism.

Ans. Structure of Lysosomes

★ Discovery: Lysosomes were discovered by a Belgian scientist Christian Rene de Duve. They contain strong digestive enzymes.

• Lysosomes are spherical organelles bounded by a single membrane.
• They serve as digestive compartments of the cell.
• Lysosomes are found in most eukaryotic cells.
• In animals, they are most numerous in disease-fighting cells, such as white blood cells.
• This is because white blood cells must digest materials like bacteria, viruses, and other foreign intruders.
• Lysosomes have about 40 different hydrolytic enzymes, all of which are manufactured in the endoplasmic reticulum and modified in the Golgi apparatus.

Function of lysosomes

• The membrane covering of the lysosome protects the rest of the cell from the harsh digestive enzymes contained in the lysosomes, which would otherwise cause significant damage.
• They are also involved in breaking the cellular materials that have exceeded their lifetime or are no longer useful. In this regard, the lysosomes perform autophagy (self-eating).
• They break down cellular waste products, fats, carbohydrates, proteins, and other macromolecules into simple compounds, which are then transferred back into the cytoplasm for making new materials (recycling).
• Lysosomes also function in the elimination of whole cell. Selective cell death is a mechanism used by multicellular organisms in their development. For example, when a tadpole develops into a frog, the cells of the tail are destroyed by the enzymes of lysosomes (apoptosis).

Lysosomal Storage Diseases

In lysosomal storage diseases the patient lacks one of the hydrolytic enzymes of lysosome. The abnormal lysosome fills with indigestible substances, which interfere with cellular functions. Following are the examples of storage diseases

• In Pompe's disease the lysosome lacks a glycogen-digesting enzyme. So, harmful amounts of glycogen accumulate in liver cells.
• In Tay-Sachs disease an essential lipid- digesting enzyme is missing. Accumulation of these lipids in the nerve cells of brain damages the nervous system, causes mental retardation, and death in early childhood.

Ans. Structure

Peroxisomes are single membrane bounded organelles in all eukaryotic cells. These were discovered by Christian de Duve, who also discovered lysosomes. Peroxisomes contain a variety of enzymes. Many of these enzymes are oxidative that carry out oxidation i.e. the removal of electrons and hydrogens.

Functions

• These enzymes primarily function to rid the cell of toxic substances.
• Some peroxisomes, such as those in liver cells, detoxify alcohol and other harmful compounds by carrying out their oxidation.
• Some peroxisomes contain catalase enzymes, which break down hydrogen peroxide (a common by-product of cellular metabolism) into water and oxygen.

2H₂O₂ → Catalase → 2H₂O + O₂
(Hydrogen peroxide) (Water) (Oxygen)

• Defects in peroxisomes cause a number of metabolic disorders. The most serious of these disorders is Zellweger syndrome, which is characterized by absence or reduced number of peroxisomes in the cells. It is congenital disorder (present at birth) and has no cure or effective treatment and usually causes death within the first year of life.

Glyoxisomes

Location Glyoxysomes are similar to peroxisomes but are found only in plant cells. They are most abundant in the cells of lipid-rich seeds (e.g. castor beans and soyabeans).

Functions These organelles contain enzymes that convert lipids into carbohydrates. During germination, these organelles convert stored lipids into carbohydrates that provide energy for seed germination.

Ans. (i) Definition

Vacuoles are membrane bound sacs sound both in animal and plant cells. Vacuoles have different functions.

(ii) Position and formation is plant cells

Many plants cells have a large, single central vacuole. This large vacuole develops by fusion of smaller vacuoles in plants sells. It occupies 80% or more space in plant cells.

(iii) Structure

(a) Tonoplast: Vacuole is plant cells is surrounded by membrane called tonoplast.
(b) Cell sap: The material inside the vacuole is called cell sap. The cell sap differs markedly from the surrounding cytoplasm.

(iv) Functions

(a) In mature plant cells: A single large vacuole provides structural support, as well as serves functions in storage, waste disposal, protection, and growth.

(b) In animal cells: Vacuoles are much smaller, and are more commonly used to temporarily store materials or to transport substances.

(c) Turgor pressure: The central vacuole in plant cells plays an important structural role for the plant. This role of the vacuole is related to its ability to control turgor pressure. Turgor pressure makes the rigidity of the cell. Under optimum conditions, a plant receives adequate amounts of water and the central vacuoles of its cells swell as the liquid collects within them. It creates a high turgor pressure, to maintain the structural integrity of the plant, along with the support from the cell wall.

(d) Store the pigments: Vacuoles also often store the pigments that give certain flowers and other pollinators.

(e) Protection: Vacuoles also release molecules that are poisonous to various insects and animals, thus discouraging them from consuming the plant.

Ans. Centrioles

In the cells of animals and most protists, centrioles are organelles associated with the assembly and organization of the fibres of cytoskeleton i.e., microtubules (including spindle fibres).

In eukaryotic cells centrioles occur in pairs. The two centrioles are located at right angles to one another near the nuclear envelope. In ciliated or flagellated cells centrioles are involved in the formation of cilia and flagella. Each cilium and flagellum are anchored by a centriole, known as basal body.

The cells of plants and fungi lack centrioles and basal bodies, and their microtubules and spindle fibres are organized from the structures of cytoplasm.

Ans. It is a network of protein fibres present in cytoplasm. It includes the following types of fibres:

1. Microfilaments

a. These are present in all eukaryotic cells.
b. Microfilaments are solid rods made of a globular protein, called actin.
c. Microfilaments disassemble and re-assemble and help the cells to change shape and move.
d. Microfilaments also enable a dividing cell to pinch off into two cells.
e. In association with myosin, microfilaments help in cellular contraction.

2. Microtubules

a. These straight, hollow cylinders are composed of subunits. Each subunit is made of two different tubulin proteins known as alpha-tubulin and beta-tubulin.
b. Microtubules give structure and shape to a cell.
c. They also serve as highways for the transport of organelles.
d. Microtubules are the major components of cilia and flagella, and participate in the formation of spindle fibres during cell division.

3. Intermediate filaments

a. These are found only in some higher animal groups.
b. They are made of different proteins but the most common type of protein subunit is vimentin.
c. Some cells may have intermediate filaments made of other proteins. For example, skin cells contain a protein keratin.

Ans. Cilia and Flagella

• Cilium (plural cilia) and flagellum (plural flagella) are the locomotor appendages that protrude from certain cells.
• They are thin, tail-like projections extending from the cell body.
• Cilia are short in length and are usually numerous in number; while flagella are longer but less numerous.
• Cilia are rare in plants. Many protozoan (ciliates) possess cilia e.g., paramecium.
• Larger eukaryotes such as mammals have cilia on some cells' surfaces. For example, in humans, cilia are found in the lining of the trachea where they sweep mucus and dirt out of breathing tubes.

Structure

• The core of eukaryotic cilia and flagella is called axoneme.
• It contains two central microtubules that are surrounded by an outer ring of nine doublet microtubules.
• Dynein molecules are located around the circumference of the axoneme.
• These dynein molecules bridge the gaps between adjacent microtubule doublets.
• Biologists refer to this organization as a "9 + 2" structure.
• A plasma membrane surrounds the entire axoneme.
• At the base of the cilium its organizing centre, called basal body, is present. Basal body has the same basic structure of the outer ring of axoneme, but each of the nine sets of outer filaments is composed of three microtubules, rather than a doublet of microtubules.
• The basal body is actually the modified centriole.
• Prokaryotic flagella have a completely different structure built from the protein flagellin.

Ans. Bacteria and archaea are made up of prokaryotic cells. Following are the characteristics of prokaryotic cells.

• Prokaryotic cells are much simpler than eukaryotic ones.
• Most prokaryotic cells range from 2 to 8 µm in length i.e., about one-tenth of the size of a typical eukaryotic cell.
• A prokaryotic cell lacks a nucleus.
• The much less extensive DNA of prokaryotic cell is present in the more-or-less central region known as nucleoid (nucleus-like).
• A prokaryotic cell also lacks other membrane-bounded organelles like endoplasmic reticulum, mitochondria, chloroplasts, Golgi apparatus, lysosomes, peroxisomes etc.
• The entire cytoplasm of a prokaryotic cell is one unit with no internal support structures.
• Ribosomes are present in prokaryotic cell but these are smaller in size than those of eukaryotic cells.
• The Svedberg values (sedimentation rates) of the smaller and larger subunits of ribosomes of prokaryotic cells are 30S and 50S respectively. The sedimentation rate of a complete ribosome is 70S.
• Surrounding the plasma membrane of most prokaryotic cells is a cell wall but it does not contain cellulose. It is composed of peptidoglycan that is a single large polymer of amino acids and sugar.
• In bacteria the cell wall may also be surrounded by a capsule and may also have extensions for attachment known as pili (singular pillus).
• Prokaryotic flagella are made of repeating units of the protein flagellin and they do not contain microtubule triplets.
• Mitosis and meiosis are missing in prokaryotic cell and it divides by direct division (binary fission).

Ans.

Sr. No. | Characteristics | Eukaryotic Cell | Prokaryotic Cell
1. | Distinct Nucleus | Present | Absent
2. | Number of chromosomes | More than one | One—but not true chromosome: plasmids
3. | Cell Type | Usually multicellular | Usually unicellular (some cyanobacteria may be multicellular)
| Examples | Protozoan, Algae, Fungi, Animals, Plants | Bacteria and Archaea
4. | Lysosomes and peroxisomes | Present | Absent
5. | Microtubules | Present | Absent or rare
6. | Endoplasmic reticulum | Present | Absent
7. | Mitochondria | Present | Absent
8. | Cytoskeleton | Present | May be absent
9. | Vacuoles | Present | Present
10. | Ribosomes | Larger | Smaller
11. | Gogi apparatus | Present | Absent
12. | Chloroplasts | Present (in plants) | Absent; chlorophyll scattered in the cytoplasm
13. | Cell Division | Mitosis or meiosis | Mitosis and meiosis are missing; cell divides by direct division (binary fission)
14. | Flagella | Membrane bounded; contains two central microtubules surrounded by an outer ring of nine doublet microtubules. | Not membrane bounded; made of repeating units of flagellin; do not contain microtubule triplets
15. | Cell Wall | Only in plant cells and fungi (chemically simpler) | Composed of peptidoglycan (a single large polymer of amino acids and sugar)
16. | Cell size | 10-100 um | 1-10 um

Ans. Definition Cell signalling is the ability of cells to respond to stimuli from their environment producing cellular responses. It involves the transmission of signals between cells through a series of molecular events, often leading to a cellular response.

Steps of Cell Signalling

1. Signal Reception

Cell signalling begins when a signal molecule (ligand) binds to a receptor on the membrane of a target cell. These receptors are typically proteins embedded in the cell membrane but can also be located inside the cell. Each receptor is specific to a particular ligand.

2. Signal Transduction

Once the receptor binds to the ligand, it undergoes a conformational change that activates an intracellular signalling pathway. This often involves a series of interactions and modifications, creating a signalling cascade that amplifies the signal. Small molecules like cAMP (cyclic AMP), calcium ions, and inositol triphosphate (IP3) can act as second messengers, transmitting the signal from the receptor to target molecules inside the cell.

3. Cellular Response

The signal transduction pathway often leads to changes in gene expression, turning specific genes on or off. This can result in various cellular responses, such as cell growth, division, differentiation, or apoptosis (programmed cell death). Signalling can also lead to changes in cellular metabolism, enzyme activity, or the opening and closing of ion channels.

Ans. Definition Cell signalling pathways involve the transmission of signals from the cell's exterior to its interior, resulting in a specific cellular response.

Types There are two main types of signalling pathways.

(i) Protein/Peptide Signalling
(ii) Steroid Signalling

(i) Protein/Peptide Signalling

• Protein or peptide signalling molecules are water-soluble. So, they cannot pass through plasma membrane.
• When such ligand approaches the cell surface, it binds to its specific receptor on plasma membrane.
• This binding causes a conformational change in the receptor protein and activates it. The activated receptor triggers a series of reactions within the cell.
• These reactions generate second messenger like cyclic AMP (cAMP) which starts changes e.g., changes in gene expression. The pathway can lead to changes in metabolism, cell growth, division, or apoptosis.

(ii) Steroid Signalling

• Steroid hormones, being lipophilic, can diffuse through the plasma membrane of the target cell.
• Once inside, they bind to specific intracellular receptors located in the cytoplasm or nucleus. This binding results in the formation of active receptor- hormone complex which moves into the nucleus if it was not already there.
• Inside nucleus, the receptor-hormone complex binds to specific DNA sequences in target genes.
• This binding regulates the transcription of these genes, leading to increased or decreased production of specific proteins.

Ans. Definition The movement of molecules across plasma membrane without any expenditure of energy is called passive transport.

Types The following are the types of passive transport.

a. Diffusion

Diffusion is the net movement of a substance (liquid or gas) from an area of higher concentration to one of lower concentration i.e., along concentration gradient. Because a cell does not expend energy when molecules diffuse across its membrane, the diffusion of molecules is a type of passive transport.

b. Facilitated Diffusion

Some molecules are taken into or out of the cells with the help of transport-proteins present in plasma membranes. When a transport protein helps a substance to move it down its concentration gradient (from higher to lower concentration), the process is called facilitated diffusion. It is also a type of passive transport because no energy is used in facilitated diffusion. The rate of facilitated diffusion depends on how many transport-protein molecules are available in the membrane. The main types of transport proteins involved in facilitated diffusion are:

1. Channel Proteins: These proteins form hydrophilic channels across the membrane that allow specific molecules or ions to pass through. They can be gated or non-gated. Gated channels open or close in response to specific stimuli (such as voltage changes, ligand binding, or mechanical stress). Examples include ion channels (allow the passage of specific ions e.g., Na⁺, K⁺, Ca²⁺) and aquaporin (facilitate the rapid transport of water molecules) while non-gated channels remain open constantly.

2. Carrier Proteins (Transporters): These proteins bind to the specific molecule they transport, undergo a conformational change, and move the molecule across the membrane. They are highly specific for the molecule they transport. They can become saturated, meaning there is a maximum rate of transport when all carrier proteins are occupied. Examples include glucose transporters (facilitate the transport of glucose) and amino acid transporters (transport specific amino acids into or out of the cell).

Ans.

| | Simple Diffusion | Facilitated Diffusion |
| Mechanism | Substances move from higher concentration to lower concentration directly through the lipid bilayer of plasma membrane. | Substances move from higher concentration to lower concentration through specific transport proteins embedded in plasma membrane. |
| Energy Requirement | It is a passive transport mechanism, requiring no energy input from cell. | It is also a passive transport mechanism and does not require energy. |
| Types of Molecules | Typically involves small, nonpolar molecules such as oxygen, carbon dioxide and lipid-soluble substances. | Primarily involves polar or charged molecules, such as glucose, amino acids, and ions, which cannot easily pass through the hydrophobic core of the lipid bilayer. |
| Rate of Movement | The rate depends on the concentration gradient temperature and the permeability of the membrane. | The rate can be affected by the number and availability of transport proteins and can reach a maximum rate when all transport proteins are saturated. |

Ans. Osmosis

Definition The process by which water molecules diffuse across a cell membrane from an area of higher concentration to an area of lower concentration is called osmosis.

Passive transport Because water is moving from a higher to lower concentration, osmosis does not require cells to expend energy. Therefore, osmosis is the passive transport of water.

Direction The direction of osmosis depends on the concentration of solutes on the two sides of membrane. Water always moves from hypotonic solution (with lower solute concentration) to hypertonic solution (with higher solute concentration).

Selectively permeable membrane Osmosis occurs through selectively permeable membrane, which allows water to pass while restricting many solutes.

Cell turgor Osmosis is crucial for maintaining cell turgor, which is vital for plant cells, and for balancing the internal water content in cells.

The direction and rate of osmosis are influenced by the osmotic gradient and the permeability of the membrane to water

Aquaporins Specialized proteins called aquaporins facilitate the rapid transport of water molecules across the cell membrane, ensuring efficient regulation of cellular hydration and volume.

★ Reverse osmosis

• It is a widely used technology for purifying water by removing contaminants and impurities.
• Unlike natural osmosis, which moves water a lower to a higher solute concentration, Reverse osmosis applies external pressure to push water through a semi-permeable membrane from a higher to a lower solute concentration.
• This process effectively filters out dissolved salts and other impurities, providing clean and safe drinking water
• Reverse osmosis is commonly used in water treatment plants desalination facilities and even in household water purification systems.

Ans. Definition The movement of substances across plasma membrane from lower concentration to higher concentration with the expenditure of energy is known as active transport.

Types

Following types of active transport occur through plasma membrane.

1. Active transport through carrier proteins

In this process, carrier (transport) proteins in the plasma membrane use energy to move the molecules against the concentration gradient. For example, the membranes of nerve cells have carrier proteins in the form of "sodium-potassium pump". In a resting (not conducting nerve impulse) nerve cell, this pump spends energy (ATP) to maintain higher concentrations of K⁺ and lower concentrations of Na⁺ inside the cell. For this purpose, the pump actively moves Na⁺ to the outside of the cell and K⁺ to the inside of the cell.

2. Endocytosis

In endocytosis, bulky materials are moved into the cell across plasma membrane. During endocytosis a portion of plasma membrane invaginates (depressed inward). The material from outside is taken inside the invagination, and its ends seal. Thus, a small vesicle is formed. It detaches from the plasma membrane and moves into cytoplasm. The two common forms of endocytosis are phagocytosis and pinocytosis.

a. In phagocytosis, cell takes in solid material while in
b. pinocytosis, cell takes in liquids in the form of droplets.

★ Receptor-mediated endocytosis

Specific receptor proteins of plasma membrane pick up material from outside and pinch inside to form a vesicle. For example, the cells of liver have receptor proteins for cholesterol. Cholesterol circulates in our blood in the form of low-density-lipoproteins (LDLs). The receptor proteins of plasma membrane of liver cells, recognize and take up LDLs from the blood by receptor-mediated endocytosis.

3. Exocytosis

It is the process through which bulky material is exported out of cell. In exocytosis, the bulky material is packed inside a membrane and a vesicle is formed. The vesicle moves to the plasma membrane and fuses with it to release its contents into the extracellular environment.

Ans. Stem cells are unique cells with the remarkable ability to develop into many different cell types in the body. When a stem cell divides, each new cell has the potential either to remain a stem cell or to become specialized cell, such as a muscle cell, a red blood cell, or a brain cell. The following are the major categories of stem cells on the basis of the number of types of cells which they can make.

1. Totipotent: These stem cells can differentiate into all possible cell types. For example, zygotic and the cells produced by the first few divisions in zygote.

2. Pluripotent: These cells can turn into almost any cell. For example, cells from the early embryo.

3. Multipotent: These cells can differentiate into a closely related family of cells. For example, the hematopoietic stem cells can become red and white blood cells or platelets.

4. Oligopotent: These can differentiate into a few different cell types. For example, adult lymphoid or myeloid stem cells.

5. Unipotent: These can only produce cells of one kind, which is their own type. However, they are still stem cells because they can renew themselves. For example, adult muscle stem cells.

Q34. Write uses of stem cells.

Ans. Following are the uses of stem cells

(i) Regenerative Medicine: Stem cells have the potential to repair or replace damaged tissues and organs. Therefore, they are used for treating conditions such as spinal cord injuries, type 1 diabetes, Parkinson's disease, and heart disease.

(ii) Drug Testing and Development: By differentiating stem cells into specific cell types, researchers can create models of human diseases, allowing for more accurate testing of drug effects and reducing the reliance on animal models.

(iii) Personalized Medicine: Stem cells can be derived from a patient's own cells, reducing the risk of immune rejection when used in treatments. This personalized approach can lead to more effective and safer therapies.

Ans. Following are the uses of stem cells

(i) Regenerative Medicine: Stem cells have the potential to repair or replace damaged tissues and organs. Therefore, they are used for treating conditions such as spinal cord injuries, type 1 diabetes, Parkinson's disease, and heart disease.

(ii) Drug Testing and Development: By differentiating stem cells into specific cell types, researchers can create models of human diseases, allowing for more accurate testing of drug effects and reducing the reliance on animal models.

(iii) Personalized Medicine: Stem cells can be derived from a patient's own cells, reducing the risk of immune rejection when used in treatments. This personalized approach can lead to more effective and safer therapies.

Ans. Categories of stem cells

The following are the major categories of stem cells on the basis of their origin.

1. Embryonic Stem Cells (ESCs)

ESCs are derived from the inner cell mass of blastocysts (early-stage embryos). These stems cells are pluripotent, meaning they can differentiate into nearly all cell types in the body. They have high differentiation potential, making them extremely versatile for research and therapy. Ethical concerns of using ESCs include the use of human embryos, risk of teratoma formation, and potential immune rejection.

2. Adult Stem Cells (ASCs)

These stem cells are found in various tissues throughout the body, such as bone marrow, fat, and blood. They are multipotent, meaning they can differentiate into a limited range of cell types related to their tissue of origin. Using them involves less ethical controversy, lower risk of immune rejection when derived from the patient's own tissues. However, they have limited differentiation potential and are harder to isolate and culture.

3. Induced Pluripotent Stem Cells (iPSCs)

They are generated in the lab by reprogramming adult somatic cells to a pluripotent state using specific transcription factors. They are pluripotent, similar to embryonic stem cells. Advantages of using iPSCs: They do not have the ethical controversies linked to embryonic stem cells, as they do not require the destruction of embryos. They can be generated from a patient's own cells, minimizing the risk of immune rejection. They offer potential for regenerating damaged tissues and organs (e.g., new heart cells for patients with heart disease or new neurons for patients with neurodegenerative conditions).

Disadvantages of using iPSCs The reprogramming process can introduce genetic changes that may affect iPSCs. They could form tumours (teratomas) when transplanted into patients. Directing iPSCs to differentiate into specific, fully functional cell types remains a complex task. It is still difficult to ensure that these cells function properly.