Introduction
All living organisms are made up of cells. A cell is the basic structural and functional unit of life. Some organisms like bacteria and yeast have only one cell and are called unicellular, while plants, animals, and humans have many cells and are called multicellular. In multicellular organisms, similar cells form tissues, tissues form organs, and organs work together in organ systems. Yet, the cell remains the basic unit of structure and function in every living organism.
The chapter also connects the idea of cells to the origin of life. It explains that early protective membranes may have helped in the formation of the first cells. This shows why the cell boundary is so important in living systems.
2.1 How to Study Cells?
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.
2.2 Structure of a Cell
A cell must interact with its surroundings and with other cells. These interactions take place through the cell boundary.
2.2.1 Cell membrane — The universal feature of a cell
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.
2.2.2 Cell wall — The outer covering of cells
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
2.3 The Cell Interior — A Coordinated Working System
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.
2.3.1 Why do eukaryotic cells need these organelles?
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.
Nucleus — House of coded instructions
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 — The protein factories
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
Golgi apparatus — The packaging and shipping centres
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 — The clean-up system
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 — The powerhouse of 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 — Centre for food synthesis in plant cells and beyond
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.
Vacuoles — The organelles for storage and support
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
2.4 How do Normal Cells Grow and Divide?
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.
2.4.1 Cell division
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.
2.5 Cell Theory — The Unifying Principle of Biology
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:
- All living organisms are made up of one or more cells.
- The cell is the basic unit of structure and function in living beings.
- 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
Contact inhibition
Many animal cells stop dividing when they come in contact with neighbouring cells. This is called contact inhibition.
Cancer cells
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.
Summary of the Chapter
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.