Chapter 2 — The Cell

What is a cell?
Every living thing is made of tiny building blocks called cells. Some organisms are made of a single cell, while others have many cells joined together. Cells can be seen with a microscope, not with the naked eye. They vary in shape and size depending on their job. For example, blood cells are small and round, while plant cells are more box-like.

Why cells matter
Cells carry out all the activities of life: they take in food, release waste, grow, and reproduce. Each cell works like a small factory with parts that do special jobs. When many cells work together, they form tissues and organs. Thus, learning about cells helps us understand how plants, animals, and humans live and stay healthy.

Simple observation
You can see cells by preparing a thin slice of an onion or a cheek smear and looking under a microscope. Not every part of the cell is visible at low magnification, but the general shape and some structures can be seen. Scientists who study cells are called biologists; they use microscopes and experiments to learn more about cells and their functions.

Key ideas
Cells are the smallest living units; they have parts that perform different jobs; organisms can be single-celled or multicellular. This topic lays the foundation for studying parts of the cell and how they work.

Main cell parts
Cells have a few common parts that are found in many living things. The main parts you will learn are the cell membrane, cytoplasm, nucleus, and sometimes a cell wall and large vacuole in plants. Each part has a job that helps the cell survive and carry out activities.

Cell membrane
The cell membrane surrounds the cell. It holds the contents inside and controls what enters and leaves. Think of it like a gate or boundary that lets useful things in and wastes out.

Cytoplasm
The cytoplasm is a jelly-like substance that fills the cell. Many tiny parts float in it and carry out chemical reactions. It helps to move materials inside the cell and gives space for organelles to work.

Nucleus
The nucleus is often the most visible part under a microscope. It contains information needed for the cell to grow and reproduce. It acts like the control centre of the cell and holds genetic material that tells the cell what to do.

Plant extras
Plant cells usually have a rigid cell wall outside the membrane and one or more large vacuoles that store water and nutrients. Some plant cells also contain green structures called chloroplasts.

Why these parts matter
Each part must work well for the cell to be healthy. If the membrane breaks, the cell cannot control its contents; if the nucleus is damaged, the cell may not grow properly. Understanding parts helps explain how life functions at the smallest level.

What is the cell membrane?
The cell membrane is a thin, flexible layer that surrounds the cell. It separates the inside of the cell from the outside environment. Though the membrane is very thin, it plays a vital role in protecting the cell and maintaining its internal conditions.

How it works
The membrane controls movement into and out of the cell. Small molecules like water and oxygen pass through easily, while bigger molecules or unwanted substances are blocked or moved by special transport mechanisms. The membrane can be thought of as a selective gate that keeps helpful materials inside and lets waste out.

Protective and supportive role
Besides control of traffic, the membrane helps the cell keep its shape and stay connected to neighbouring cells. In animal tissues, membranes help cells stick to one another, forming tissues and layers. In plant cells, the membrane works with the cell wall outside it to give firmness.

Cell communication
The membrane also receives signals from the environment and other cells. Receptor molecules on the membrane detect hormones or nutrients and start responses inside the cell. This allows the cell to react to changes and coordinate activities.

Simple model
Imagine the membrane like a net with gates. Some gates are open, some are guarded. The behaviour of the membrane is key to how the cell lives and interacts in its surroundings.

What is cytoplasm?
The cytoplasm fills most of the cell and appears as a clear, jelly-like substance. It is made mostly of water but also contains salts, dissolved gases and many molecules needed for life. Organelles such as mitochondria, ribosomes and plastids are suspended in this jelly so they can work. The cytoplasm is not empty; it is an active place where many chemical reactions happen continuously to keep the cell alive.

Functions of the cytoplasm
One main role is to provide a medium for the movement of materials. Nutrients, waste products and signalling molecules move through the cytoplasm between organelles. The cytoplasm also helps in distributing materials made in one part of the cell to other parts where they are needed. For example, when proteins are made, they travel through the cytoplasm to reach the cell membrane or other organelles.

What is the cytoskeleton?
Within the cytoplasm there is a fine network of protein threads and tubes called the cytoskeleton. These are not visible under a simple microscope but can be seen by more powerful instruments. The cytoskeleton gives shape to the cell and holds organelles in their approximate places. It is like an internal skeleton for soft-bodied cells.

Roles of the cytoskeleton
The cytoskeleton helps cells to move and change shape. It supports the movement of organelles and vesicles along defined paths, using tiny motor proteins that carry cargo. During cell division, the cytoskeleton helps pull parts of the cell apart so two new cells form. In specialised cells, such as nerve or muscle cells, the cytoskeleton is arranged to help with long-distance transport or contraction.

Integration and importance
The cytoplasm and cytoskeleton work together: the cytoplasm provides the medium and chemicals for processes, while the cytoskeleton provides structure and movement pathways. Without them, organelles could not be organised, molecules would not reach their destinations, and cells could not maintain their proper shapes and functions.

The nucleus: control centre
The nucleus is a distinct, usually round part found in many cells. It is surrounded by its own membrane and contains the genetic material of the cell. The nucleus acts like a control centre, holding instructions that tell the cell how to grow, divide, and make useful proteins.

Genetic material
Genetic material carries codes for traits and cell activities. In simple words, it is the set of instructions for life. These instructions pass from parent cells to new cells, ensuring traits and functions are maintained. In many cells, the genetic material is arranged in structures that become visible when the cell divides.

Role in cell division
When a cell needs to make a copy of itself, the nucleus plays a key role. It ensures that the genetic instructions are copied and shared correctly with the new cell. If the nucleus did not do this accurately, the new cell might not work properly.

Nucleus and cell activities
The nucleus also helps control which parts of the genetic code are used at a given time. It sends messages to the cytoplasm to make particular proteins needed for growth, repair, or response to the environment.

Simple idea
Think of the nucleus as the principal in a school who keeps records and gives orders so that the school runs smoothly. The genetic material is like the record book that stores important information.

Comparing plant and animal cells
Plant and animal cells share many parts: both have a cell membrane, cytoplasm, and a nucleus. But plant cells have some special structures not found in most animal cells. These differences relate to the different life styles of plants and animals.

Cell wall
Plant cells have a rigid cell wall outside the cell membrane. The cell wall gives strength and keeps the plant upright. It also protects the cell and helps maintain shape when the cell takes in water.

Vacuole
Many plant cells have a large central vacuole that stores water, nutrients, and pigments. When full of water, the vacuole keeps the plant firm. Animal cells may have small vacuoles, but not a large central one.

Chloroplasts and photosynthesis
Plant cells may contain chloroplasts — green bodies that capture sunlight to make food by photosynthesis. This is why plants can make their own food while animals rely on eating others. Chloroplasts contain a green pigment called chlorophyll which absorbs light energy.

Animal cell features
Animal cells can have centrioles that help in cell division and many small vacuoles. Animal cells are often rounder and more flexible than plant cells.

Why the differences matter
These differences explain how plants and animals live. Plants need strong walls and chloroplasts to stand and make food; animals need flexibility to move and capture food. Understanding these contrasts helps explain the roles cells play in different organisms.

Mitochondria: power houses
Mitochondria are tiny, often rod-shaped organelles found in the cytoplasm of most cells. They are the sites where energy stored in food is released in a usable form. Because their job is to supply energy for the cell’s activities, mitochondria are commonly called the power houses of the cell. Each mitochondrion has an outer boundary and an inner folded membrane that increases its working area.

How mitochondria release energy
When food molecules such as sugars are broken down, chemical reactions inside mitochondria convert the energy of those molecules into a form the cell can use. This process uses oxygen in many organisms and produces carbon dioxide and water as waste products. The energy released is then used to build new molecules, power movement, and maintain cell activities such as active transport across the cell membrane.

Where mitochondria are most needed
Cells that do a lot of work contain more mitochondria. For example, muscle cells, which contract and require much energy, have many mitochondria. Similarly, root tips and growing tissues show active mitochondria because growth and division require energy. In single-celled organisms that move or feed actively, mitochondria are likewise plentiful.

Link with food and breathing
Mitochondria work with the processes that bring food and oxygen into the cell. In plants, chloroplasts make sugar by photosynthesis; mitochondria then use that sugar to release energy. In animals, food is digested and absorbed, and mitochondria use these nutrients with oxygen taken in by breathing to provide energy.

Importance for health and activity
If mitochondria do not function well, cells cannot make enough energy and the organism feels weak or tired. This explains why good food and breathing are important for healthy life. Mitochondria thus form a central part of a cell’s ability to do work and survive.

Vacuoles and their roles
Vacuoles are sac-like structures in the cell that store materials. In plant cells the central vacuole is large and filled mainly with water. This vacuole keeps the cell firm and supports the plant. It may also store sugars, salts and pigments. In animal cells vacuoles are usually small and store waste, food or other small substances. Vacuoles help maintain internal balance by holding useful or unwanted materials until the cell needs or disposes them.

Plastids and variety
Plastids are organelles found in plant cells only. The most familiar plastid is the chloroplast, which contains the green pigment chlorophyll and carries out photosynthesis. Other plastids store colours and food: for example, chromoplasts give colour to petals and fruits, while leucoplasts store starch in roots and seeds. Plastids help plants in storing materials, attracting pollinators, and preparing the next generation through seeds.

Ribosomes and protein making
Ribosomes are tiny organelles found in all cells. They are the sites where proteins are made. Proteins are needed for structure, enzymes and many cell functions. Ribosomes may float freely in the cytoplasm or attach to membranes, producing proteins that work inside the cell or are sent out.

Endoplasmic reticulum and Golgi body
The endoplasmic reticulum (ER) is a network of membranes that helps transport materials and makes some molecules. The Golgi body receives materials from the ER, modifies them and packages them in small vesicles for transport to other parts of the cell or outside. These organelles form a delivery system inside the cell, ensuring materials reach the correct destinations.

Lysosomes and cleaning
Lysosomes are small sacs containing enzymes that break down waste materials and worn-out organelles so useful parts can be recycled. This is important for cell health. All these organelles together make the cell a busy place where storage, building, transport and recycling happen continuously to keep the cell functioning.

Why movement matters
Every cell needs materials such as water, oxygen and food, and it must remove wastes. Movement of substances happens through the cell membrane by simple natural processes. Two important processes are diffusion and osmosis.

Diffusion
Diffusion is the movement of particles from a place of higher concentration to a place of lower concentration. For example, oxygen from the air moves into cells because there is more oxygen outside than inside. Diffusion does not need energy; it happens because particles move randomly and spread out.

Osmosis
Osmosis is a special type of diffusion for water. It is the movement of water from a region of more water (dilute) to a region of less water (concentrated) through a semipermeable membrane. In plant cells, osmosis helps the vacuole fill and keeps the cell firm. If a plant cell loses too much water, it wilts.

Examples and effects
Putting a plant in salty water causes water to leave its cells by osmosis, making the plant wilt. If a cell is placed in pure water, water moves in and the cell swells. In animal cells, too much water can cause bursting because animal cells lack a rigid cell wall.

Simple experiments
Students can test osmosis using a potato slice in salt water and fresh water to see changes in firmness. Understanding diffusion and osmosis explains how cells stay balanced and how fluids move in living organisms.

Nutrition in cells
Cells need nutrients for energy, growth and repair. Different organisms obtain food in different ways. Some cells make their own food using sunlight, while others obtain food by eating or absorbing it from the surroundings. Knowing how cells get food helps explain the basic life processes of plants, animals and single-celled organisms.

Photosynthesis in plant cells
Photosynthesis is the process by which green plant cells make sugar and oxygen using carbon dioxide from the air, water from the soil and energy from sunlight. This takes place in chloroplasts, which contain chlorophyll to capture light. Photosynthesis provides the primary source of food and energy for nearly all life on Earth. The sugar made may be used at once or stored as starch for later use.

How animals and other organisms get food
Animal cells do not have chloroplasts and cannot make their own food; animals must eat plants or other animals to obtain nutrients. Single-celled organisms show many ways of taking in food: some engulf food particles with their cell membrane, while others absorb dissolved nutrients directly. Once food is inside the cell, it is broken down by enzymes into smaller parts that can be used for energy and building new cell parts.

Link between photosynthesis and respiration
The sugar produced by photosynthesis is the raw material for respiration in both plants and animals. In respiration, mitochondria release energy from sugar using oxygen. Thus, photosynthesis and respiration form a cycle: plants make food and oxygen, and animals (and plants) use them to obtain energy and release carbon dioxide and water.

Simple takeaway
Plants make food from sunlight; animals obtain food by eating. Both types of cells use this food to release energy needed for life, showing how all living things are linked.

Why cell division is needed
Growth, repair and reproduction in living things require new cells. Cells multiply by dividing into two. This simple process ensures that organisms can grow from young to adult, heal wounds, and replace old cells that wear out. Without cell division, bodies could not grow or repair damaged tissues.

Basic steps in cell division
At its simplest, cell division involves copying the cell’s contents and then splitting into two. First, the cell makes a copy of its genetic material so that each new cell will have the same instructions. Next, the cell divides its cytoplasm and organelles so both parts are equipped. Finally, the cell splits to form two daughter cells that are usually like the parent cell. For class 6, it is enough to understand these basic steps rather than the detailed stages.

Where division happens most
Some parts of plants and animals show rapid cell division. In plants, the root tip and shoot tip have regions where cells divide quickly to help growth. In animals, growing tissues such as skin and bone marrow produce many new cells. When you cut your skin, cells at the edge divide more rapidly to repair the wound.

Growth and differentiation
After cells divide, they may take on special shapes and jobs in a process called differentiation. For example, some new cells become muscle cells, others become nerve cells, depending on the body’s needs. This specialisation allows multicellular organisms to have different tissues and organs that work together.

Simple safety and observation
Teachers may show prepared slides of dividing cells from plant root tips to illustrate the concept. Observing these helps students see that division is a normal, controlled process that produces two similar new cells needed for life processes.

Single-celled organisms
Some living things are made of only one cell. These are called unicellular organisms. Each single cell performs all life activities: it eats, breathes, moves, and reproduces. Examples include many bacteria and some protozoa. Despite being very small, these organisms are complete and independent because their single cell contains structures needed for life.

How single cells work
A unicellular organism must perform every task itself. It takes in food particles or dissolved nutrients, digests them, removes wastes and often moves to find better conditions. Special structures within the cell act like tools for these jobs. For example, some single-celled organisms have tiny hair-like structures to swim, or contractile vacuoles to pump out excess water.

Many-celled organisms
Most plants and animals are multicellular, made of many cells that work together. In multicellular organisms, cells become specialised to do particular jobs; for example, root hair cells absorb water, leaf cells make food, and nerve cells carry messages. Specialised cells group to form tissues, which together form organs and organ systems.

Advantages of multicellularity
Being multicellular allows organisms to grow larger, be more complex, and survive damage better. If some cells are injured, others can continue to function or help repair the damage. Specialisation means work can be divided: some cells focus on movement, others on digestion or protection. This cooperation makes complex life possible.

Simple comparison
Unicellular life shows how a single cell can be self-sufficient; multicellular life shows how many cells cooperating and specialising can build complex bodies. Both ways of life are successful in different environments and together they show the diversity of living things.

Using a simple microscope
A microscope lets us see small objects like cells. Learn to place the slide on the stage, use low power first, and focus slowly. Always start with the lowest magnification to locate the specimen, then switch to higher power if needed. Use the coarse knob for low power and the fine knob for higher focus.

Preparing a wet mount slide
To view onion cells or cheek cells, place a thin specimen on a glass slide, add a drop of water or stain, and cover with a cover slip. Handle the cover slip carefully to avoid air bubbles. Label your slide and keep it clean after use.

Making labelled drawings
Observe the specimen and draw what you see, not what you think it should look like. Use a pencil, draw with clear lines, and write labels with lines pointing to parts. Include the magnification used. Accurate drawing and labelling are important skills in science assessments.

Safety and care
Always carry a microscope with two hands, clean lenses with lens paper only, and store equipment safely. Do not touch stains or strong chemicals with bare hands. Be careful while handling glass slides to avoid cuts.

Learning by doing
Practical work helps students understand cell structure much better than theory alone. Repeated practice with slides and drawings builds confidence and observation skills.