Chapter 2: Cell: The Building Block of Life

Cells are usually too small to be seen with the naked eye because the human eye has a limited resolution. The chapter states that two points separated by about 0.1 mm can be seen as distinct by the unaided eye. Since most cells are smaller than this, microscopes are needed to study them.

Important points:

• A microscope magnifies tiny objects so that they appear larger.
• Robert Hooke was the first person to observe cells in 1665. He looked at a thin slice of cork through a self-designed microscope and named the box-like compartments cells.
• In school laboratories, light microscopes are used.
• Scientists also use electron microscopes, which show much finer details of cells at the nanometre scale.

Activity-based understanding:

The chapter explains how the size of a cell can be estimated using the field of view of a microscope. For example, if the visible field is 5000 µm and 25 cells fit across it, then the size of one cell is about 200 µm. This teaches students that microscopes are not only for seeing cells but also for estimating their size.

Key ideas:

• Magnification makes the object appear bigger.
• Resolution improves clarity.
• Contrast helps distinguish parts of the object.

A cell must interact with its surroundings and with other cells. These interactions take place through the cell boundary.

The cell membrane or plasma membrane is a thin boundary that surrounds the cell and protects its contents. It gives the cell its individuality.

Features of the cell membrane:

• It is selectively permeable.
• It allows some substances to pass through while blocking others.
• It controls exchange of materials between the cell and its environment.

Osmosis

The potato experiment in the chapter shows how water moves through the cell membrane:

• In plain water, the potato swells.
• In concentrated salt or sugar solution, the potato shrinks.

This happens due to osmosis, which is the movement of water through a selectively permeable membrane from a region of higher water concentration to lower water concentration.

Diffusion and osmosis

• Diffusion: movement of particles from higher concentration to lower concentration.
• Osmosis: diffusion of water across a selectively permeable membrane.

Solutions around a cell

• Isotonic solution: concentration outside = concentration inside
• Hypotonic solution: outside is more dilute
• Hypertonic solution: outside is more concentrated

Structure of the membrane

The chapter explains the fluid-mosaic model:

• The membrane is made of a lipid bilayer.
• Proteins are embedded in it.
• Lipids and proteins can move sideways.
• Proteins act like gatekeepers.

Plant cells, fungal cells, and bacterial cells have an additional outer layer called the cell wall.

Functions of the cell wall:

• Gives shape and rigidity
• Protects the cell
• Helps the plant remain upright
• Allows water and minerals to pass through because it is permeable

When plant cells are placed in concentrated sugar solution, the inner cell content shrinks but the outer boundary remains unchanged because the rigid cell wall maintains the shape. In animal cells, which do not have a cell wall, the cells shrink more easily.

Composition

The plant cell wall is mainly made of cellulose, a carbohydrate. The chapter also mentions that cellulose in our diet acts as roughage.

Plant cell vs animal cell

• Plant cells: fixed shape, rigid, have cell wall
• Animal cells: flexible, no cell wall, can change shape easily

Most cells have three main parts:

• Cell membrane
• Cytoplasm
• Nucleus

The cytoplasm is a jelly-like semi-fluid substance in which organelles are present.

Prokaryotic and eukaryotic cells

The chapter compares bacterial, plant, and animal cells.

Prokaryotic cells

• No well-defined nucleus
• No membrane-bound organelles
• Genetic material lies in a region called nucleoid
• Usually smaller and unicellular

Eukaryotic cells

• Have a well-defined nucleus
• Have membrane-bound organelles
• Larger and more complex
• Can be unicellular or multicellular

Comparison

• Prokaryotic cell size: about 1 to 10 µm
• Eukaryotic cell size: about 10 to 100 µm

The chapter also mentions cytoskeleton and cell inclusions in eukaryotic cells.

Eukaryotic cells perform many life processes at the same time. To manage this, they have different organelles that work like different departments of a factory. Some organelles make proteins, some store materials, some package products, and some produce energy.

The nucleus controls the activities of the cell.

Structure:

• Covered by a double-layered nuclear membrane
• Has nuclear pores for transfer of materials
• Contains nucleolus
• Contains chromatin and chromosomes

Functions:

• Stores genetic information
• Controls cell activities
• Helps in inheritance of characters

DNA, genes, chromatin, chromosomes

• DNA carries genetic information.
• Functional segments of DNA are called genes.
• In a non-dividing cell, DNA is present as chromatin.
• Before division, chromatin condenses into chromosomes.

The chapter also notes that mature human red blood cells do not have a nucleus, which allows more space for haemoglobin.

Ribosomes are tiny structures found:

• freely in the cytoplasm, or
• attached to the endoplasmic reticulum.

Function:

• They are the sites of protein synthesis.

Endoplasmic Reticulum (ER) — Manufacturing factory

The ER is a network-like organelle spread through the cytoplasm and connected to the outer nuclear membrane.

Types of ER:

Rough Endoplasmic Reticulum (RER)

• Has ribosomes attached
• Helps in protein synthesis and protein secretion

Smooth Endoplasmic Reticulum (SER)

• Has no ribosomes
• Helps in synthesis and storage of fats and hormones

The Golgi apparatus is made of stacks of flattened sacs.

Functions:

• Modifies proteins and lipids
• Sorts them
• Packages them into vesicles
• Sends them for transport or secretion
• Helps in lysosome formation

It acts like the post office of the cell.

Lysosomes are single membrane sacs filled with digestive enzymes.

Functions:

• Break down unwanted proteins, fats, and carbohydrates
• Digest damaged organelles
• Remove waste
• Keep the cell clean and healthy

The useful products released after digestion may be reused by the cell.

Mitochondria produce energy for cellular activities.

Structure:

• Double membrane
• Outer membrane is smooth
• Inner membrane is folded into cristae

Function:

• Site of cellular respiration
• Releases energy from food molecules
• Stores energy as ATP (Adenosine Triphosphate), the energy currency of the cell

Special feature:

Mitochondria have their own DNA and ribosomes, suggesting an evolutionary link with bacteria.

Plastids are found mainly in plant cells.

Chloroplasts

• Green plastids
• Contain chlorophyll
• Carry out photosynthesis
• Have double membrane
• Contain stroma
• Also have their own DNA and ribosomes

Chromoplasts

These plastids contain pigments other than chlorophyll.

Function:

• Give flowers, fruits, and some vegetables their yellow, orange, or red colours
• Help attract pollinators and seed-dispersing animals

Leucoplasts

• Colourless plastids
• Store food such as starch, oils, or proteins
• Found in storage organs like potato and taro

How do flowers, fruits, and vegetables acquire varied colours?

They get their bright colours mainly from chromoplasts, which contain pigments other than chlorophyll. These colours are useful because they attract insects and animals for pollination and seed dispersal.

A vacuole is a membrane-bound sac used for storage.

In plant cells:

• Usually one large central vacuole
• Filled with cell sap
• Stores water, minerals, sugars, and wastes
• Maintains internal pressure and keeps the cell firm

When plants lose water, the vacuole shrinks, cells become less firm, and the plant wilts.

In animal cells:

• Vacuoles may be present
• Usually smaller
• Help in temporary storage

Living organisms grow because their cells divide and form new cells. Cells do not grow endlessly in size. Instead, growth occurs mainly through cell division. This also helps in:

• repair of damaged tissues
• replacement of worn-out cells
• reproduction

The onion root tip activity shows different stages of cell division because cells there divide continuously.

Cell division is the process by which new cells are formed from pre-existing cells.

Importance:

• Growth
• Repair
• Maintenance
• Reproduction

There are two major types of cell division:

Mitosis

Mitosis is the most common type of cell division.

Features:

• One parent cell divides to form two daughter cells
• Daughter cells are genetically identical
• Same number of chromosomes as parent cell

Importance:

• Body growth
• Tissue repair
• Maintenance
• Asexual reproduction

Meiosis

Meiosis is a special kind of cell division that occurs in reproductive cells.

Features:

• One parent cell divides twice
• Produces four daughter cells
• Each daughter cell has half the number of chromosomes
• Produces gametes such as sperm and egg

Importance:

• Sexual reproduction
• Creates variation and diversity among organisms

Occurrence:

• In humans: testes and ovaries
• In plants: anthers and ovaries

Errors in division

The chapter explains that improper mitosis may lead to tumours and abnormal cell growth, while errors in meiosis can lead to genetic disorders, developmental problems, pregnancy loss, or reduced fertility.

The Cell Theory was developed through the work of:

• Matthias Schleiden — all plants are made of cells
• Theodor Schwann — all animals are made of cells
• Rudolf Virchow — all cells arise from pre-existing cells

Classical Cell Theory states:

1. All living organisms are made up of one or more cells.

2. The cell is the basic unit of structure and function in living beings.

3. All cells arise from pre-existing cells.

This theory unifies biology because it applies to all living beings, from bacteria to humans.

Do cells grow and reproduce forever?

No. Cells do not live forever.

Normal cell behaviour:

• grow in a controlled way
• perform specific functions
• die when no longer needed
• are replaced by new cells

Many animal cells stop dividing when they come in contact with neighbouring cells. This is called contact inhibition.

Cancer cells lose this control and continue dividing uncontrollably, forming tumours. Some tumours may spread to other body parts.

The chapter also mentions Programmed Cell Death (PCD), an important process that helps maintain balance in the body and is essential in development, such as forming fingers in an embryo.

The chapter explains that the cell is the building block of life and the basic structural and functional unit of all living organisms. Cells may be unicellular or multicellular, but in every case they perform the essential functions of life. Because cells are too small to be seen with the naked eye, microscopes are used to study them. Robert Hooke first observed cells, and later improvements in light and electron microscopes helped scientists understand cell structure better.

Every cell is surrounded by a cell membrane, which is selectively permeable and controls the movement of substances. Plant, fungal, and bacterial cells also have a cell wall, which gives them shape, support, and protection. Inside the cell, the cytoplasm contains many organelles that work together like a coordinated system. The nucleus controls cell activities and contains DNA, genes, chromatin, and chromosomes. Ribosomes make proteins, ER helps in manufacturing substances, Golgi apparatus packages and transports materials, lysosomes digest wastes, mitochondria release energy, plastids help in food synthesis and storage, and vacuoles store materials and maintain firmness in plant cells.

The chapter also explains the difference between prokaryotic and eukaryotic cells. Prokaryotic cells are simpler and lack a true nucleus and membrane-bound organelles, while eukaryotic cells are more complex and highly organised. Cell division is another major idea in the chapter. Mitosis produces two identical daughter cells for growth and repair, while meiosis produces four gametes with half the chromosome number for sexual reproduction and variation. Finally, the chapter presents the Cell Theory, which states that all living organisms are made of cells, the cell is the basic unit of life, and all cells arise from pre-existing cells.

Cell — The basic structural and functional unit of life. All living organisms are made of cells.

Unicellular organism — An organism made up of only one cell, such as bacteria or yeast.

Multicellular organism — An organism made up of many cells, such as plants, animals, and humans.

Tissue — A group of similar cells that perform a similar function.

Organ — A structure formed by different tissues working together to perform a specific function.

Organ system — A group of organs working together to perform a major life function.

Resolution — The ability to distinguish two very close points as separate and distinct.

Limit of resolution — The minimum distance at which two points can still be seen separately by the eye or an instrument.

Magnification — The process of making an object appear larger than its actual size.

Microscope — An instrument used to see tiny objects that cannot be seen clearly with the naked eye.

Light microscope — A microscope that uses visible light and lenses to magnify small objects.

Electron microscope — A very powerful microscope that uses a beam of electrons to see very tiny structures in great detail.

Cell membrane / Plasma membrane — A thin outer boundary of the cell that surrounds and protects the cell contents and controls movement of substances in and out.

Selectively permeable membrane — A membrane that allows only certain substances to pass through it while blocking others.

Diffusion — The movement of particles from a region of higher concentration to a region of lower concentration.

Osmosis — The movement of water through a selectively permeable membrane from a region of higher water concentration to lower water concentration.

Concentration gradient — The difference in concentration of particles between two regions.

Isotonic solution — A solution in which the concentration outside the cell is equal to the concentration inside the cell.

Hypotonic solution — A solution in which the concentration outside the cell is lower than inside the cell.

Hypertonic solution — A solution in which the concentration outside the cell is higher than inside the cell.

Fluid-mosaic model — The model that explains the structure of the cell membrane as a lipid bilayer with proteins embedded in it.

Lipid bilayer — The double layer of lipid molecules that forms the basic structure of the cell membrane.

Protein — A complex biological molecule that helps in structure, transport, movement, and many functions inside living cells. In membranes, proteins help substances pass through.

Cell wall — A rigid outer covering outside the cell membrane in plants, fungi, and bacteria that gives shape, support, and protection.

Permeable — A condition in which substances such as water and some dissolved materials can pass through.

Cellulose — A carbohydrate that forms the main part of the plant cell wall.

Cytoplasm — The jelly-like semi-fluid substance inside the cell in which cell organelles are present.

Organelle — A specialised structure inside the cell that performs a particular function.

Prokaryotic cell — A cell that does not have a well-defined nucleus and lacks membrane-bound organelles, such as a bacterial cell.

Eukaryotic cell — A cell that has a well-defined nucleus and membrane-bound organelles, such as plant and animal cells.

Membrane-bound organelles — Organelles that are surrounded by their own membranes.

Nucleoid — The region in a prokaryotic cell where genetic material is present without a surrounding membrane.

Cytoskeleton — A network of fine fibres in eukaryotic cells that gives support, maintains shape, and helps movement and transport.

Cell inclusions — Stored materials in the cytoplasm such as starch or crystals.

Nucleus — The control centre of the cell that contains genetic material and directs cell activities.

Nuclear membrane — The double-layered covering around the nucleus.

Nuclear pore — Small openings in the nuclear membrane through which materials move between the nucleus and cytoplasm.

Nucleolus — The dense round body inside the nucleus where ribosomal subunits are formed.

Chromatin — The thread-like form of DNA present in a non-dividing cell.

Chromosome — A condensed rod-shaped structure formed from chromatin during cell division; it carries genetic information.

DNA (Deoxyribonucleic acid) — The molecule that carries genetic information in living organisms.

Gene — A functional segment of DNA that carries information for inherited traits.

Inheritance — The passing of characters from parents to offspring.

Ribosome — A tiny cell structure that is the site of protein synthesis.

Protein synthesis — The process by which proteins are made in the cell.

Endoplasmic Reticulum (ER) — A network-like organelle that helps in the synthesis and transport of proteins, lipids, and some hormones.

Rough Endoplasmic Reticulum (RER) — The type of ER that has ribosomes on its surface and mainly makes proteins.

Smooth Endoplasmic Reticulum (SER) — The type of ER that lacks ribosomes and mainly makes and stores fats and some hormones.

Golgi apparatus — A stack of flattened sacs that modifies, sorts, packages, and transports proteins and lipids.

Vesicle — A small membrane-bound sac used for transport or storage of materials inside the cell.

Lysosome — A membrane-bound sac containing digestive enzymes that breaks down wastes and worn-out cell parts.

Enzyme — A biological substance that speeds up chemical reactions in living organisms.

Mitochondrion / Mitochondria — The organelle that releases energy from food and is called the powerhouse of the cell.

Cristae — Finger-like folds of the inner membrane of mitochondria that increase surface area for energy-producing reactions.

Cellular respiration — The process by which food is broken down to release energy in the cell.

ATP (Adenosine Triphosphate) — The energy currency of the cell that stores and supplies usable energy.

Plastid — A type of organelle found in plant cells that helps in food synthesis, storage, or colour formation.

Chloroplast — A green plastid containing chlorophyll that performs photosynthesis.

Chlorophyll — The green pigment that absorbs sunlight for photosynthesis.

Photosynthesis — The process by which green plants prepare food in the presence of sunlight.

Stroma — The semi-fluid substance inside a chloroplast.

Chromoplast — A plastid containing coloured pigments such as yellow, orange, or red.

Leucoplast — A colourless plastid that stores food such as starch, oils, or proteins.

Pigment — A coloured substance present in cells or plastids.

Vacuole — A membrane-bound storage sac in the cell that stores water, minerals, sugars, and wastes.

Cell sap — The watery fluid present inside the vacuole of a plant cell.

Wilted — The drooping condition of a plant when its cells lose water and firmness.

Cell division — The process by which new cells are formed from pre-existing cells.

Mitosis — A type of cell division that produces two genetically identical daughter cells with the same number of chromosomes as the parent cell.

Meiosis — A type of cell division that produces four daughter cells with half the number of chromosomes; it forms gametes.

Daughter cells — The new cells formed after cell division.

Gamete — A reproductive cell such as sperm or egg having half the number of chromosomes.

Fertilisation — The fusion of male and female gametes to form a new individual.

Genetic diversity — Differences in genetic makeup among individuals of the same species.

Cell cycle — The controlled and orderly sequence of events through which a eukaryotic cell grows and divides.

Cell culture — The process of growing plant or animal cells outside the body under special conditions.

Synthetic biology — A field of science in which scientists design or build biological systems or parts artificially.

Cell Theory — The theory stating that all living organisms are made of cells, the cell is the basic unit of life, and all cells arise from pre-existing cells.

Contact inhibition — The process in which many animal cells stop dividing when they come in contact with neighbouring cells.

Tumour — A mass of abnormal cells formed due to uncontrolled cell division.

Cancer cell — An abnormal cell that divides uncontrollably and may form tumours.

Programmed Cell Death (PCD) — A genetically controlled process in which cells die in an orderly way for proper growth and balance in the body.

Totipotency — The ability of a living plant cell to develop into a complete plant under suitable conditions.

Thermophiles — Heat-loving bacteria that can survive in very hot environments like hot springs.

Acellular — Not made up of cells.

Virus — A tiny acellular infectious agent made of genetic material and a protein coat.

Viroid — A tiny infectious agent made only of genetic material and lacking a protein coat.

Prion — An infectious misfolded protein that lacks genetic material.

For quick revision, these are the most important terms from the chapter:

Cell, Cell membrane, Osmosis, Diffusion, Cell wall, Cytoplasm, Nucleus, Chromosome, DNA, Gene, Prokaryotic cell, Eukaryotic cell, Ribosome, ER, Golgi apparatus, Lysosome, Mitochondria, ATP, Plastid, Chloroplast, Chromoplast, Leucoplast, Vacuole, Cell division, Mitosis, Meiosis, Gamete, Cell Theory, Contact inhibition, Tumour.