Cell Structure and Functions – Complete Study Guide for Class 8 Science

Introduction

Every living thing from a tiny bacterium to a towering tree or the human body is made of cells. The cell is the smallest unit of life capable of performing all essential life functions.

Understanding cell structure and functions is one of the most foundational topics in Class 8 Biology. It directly connects to exam questions, future biology chapters (like tissues, organs, and genetics), and helps explain how our own bodies work at a microscopic level.

Why Is This Topic Important?

• It forms the base of all biology studied in higher classes.
• Questions from this chapter regularly appear in CBSE board exams, Olympiads, and competitive tests.
• Understanding cells helps explain diseases, heredity, photosynthesis, and respiration all topics studied later.

Important Concepts and Definitions

What Is a Cell?

A cell is the structural and functional unit of all living organisms. It is the smallest unit of life that can carry out all basic life processes, including growth, reproduction, and energy production.

Example: Just as a brick is the basic unit of a building, a cell is the basic unit of a living organism.

What Is Cytology?

Cytology is the branch of biology that deals with the detailed study of cells their structure, composition, function, and behaviour.

What Is Protoplasm?

Protoplasm is the living content of a cell. It includes both the nucleoplasm (fluid inside the nucleus) and the cytoplasm (fluid outside the nucleus but inside the cell membrane).

Discovery of the Cell

Fun Fact: Robert Hooke named the small chambers he saw "cells" because they reminded him of the small rooms (cells) where monks lived in a monastery.

Cell Theory

Cell Theory was proposed by two German scientists — M.J. Schleiden (botanist) and T. Schwann (zoologist). It consists of three core principles:

1. All living things are composed of one or more cells.
2. Cells are the basic units of structure and function in all organisms.
3. All cells arise from pre-existing cells. (Added by Rudolf Virchow)

Note: Viruses are an exception to cell theory because they are not made of cells and cannot reproduce on their own.

Types of Organisms Based on Cell Number

Unicellular Organisms

Organisms made of a single cell that performs all life functions.

• Examples: Amoeba, Bacteria, Paramecium, Yeast

Multicellular Organisms

Organisms made of more than one cell, each specialised for specific functions.

• Examples: Humans, Plants, Animals
• Even a multicellular organism begins life as a single fertilised cell (zygote).

Cell Size and Shape

Cell Size

• Cell size varies greatly across organisms and even within the same organism.
• Most human body cells are 20–30 micrometres (μm) in diameter.
• The smallest cell is Mycoplasma (PPLO) about 0.1 μm.
• The largest animal cell is the ostrich egg 170 × 135 mm.
• The longest cell in the human body is the nerve cell (neuron) can be up to 3 feet long.
• Prokaryotic cells are generally smaller than eukaryotic cells.

Cell Shape

Cell shape is determined by its function. Different cells in the same body can have very different shapes.

Cell Structure: The Three Major Parts

Every living cell has three major functional structures:

1. Plasma Membrane
2. Nucleus
3. Cytoplasm

Plasma Membrane

What Is the Plasma Membrane?

The plasma membrane (also called the cell membrane) is the outermost boundary of an animal cell. It is a thin, living, elastic, and selectively permeable membrane composed of proteins and lipids.

It acts like a security gate controlling which molecules can enter or leave the cell.

Present in: Animal cells, Plant cells, and Bacterial cells

Functions of the Plasma Membrane

• Gives a definite shape to the cell.
• Acts as a barrier separating the cell's contents from its surrounding environment.
• Being selectively permeable, it regulates the movement of molecules (like water, oxygen, glucose) into and out of the cell.

Cell Wall

What Is the Cell Wall?

The cell wall is a rigid, non-living, freely permeable outer layer found only in plant cells (and fungi). It is located outside the plasma membrane.

• In plants, it is chemically made up of cellulose.
• In fungi (like mushrooms and yeast), it is made of chitin.
• The cell wall is freely permeable it does not control what passes through it.

Types of Plant Cell Wall

Middle Lamella: The first layer formed between two dividing cells; made of pectin.

Functions of the Cell Wall

• Provides structural strength and rigidity to plant cells.
• Gives cells a definite shape.
• Allows plant cells to withstand osmotic pressure changes without bursting.
• Protects the plant cell from external damage.

Nucleus

What Is the Nucleus?

The nucleus is the control centre of the cell. It was discovered by Robert Brown in 1831. It is usually an oval or round structure located at the centre of the cell.

The nucleus directs all cellular activities much like the brain controls the human body.

Parts of the Nucleus

1. Nuclear Membrane (Nuclear Envelope)

• A double-layered membrane that separates nuclear contents from the cytoplasm.
• Perforated by nuclear pores that allow selective transport of molecules between the nucleus and cytoplasm.

2. Nucleoplasm

• A dense, granular fluid filling the nucleus.
• Contains chromatin material and the nucleolus.

3. Chromatin Material

• Made of DNA (deoxyribonucleic acid) and histone proteins.
• Exists as a highly coiled thread-like network during normal cell functioning.
• During cell division, chromatin condenses into rod-shaped structures called chromosomes.
• DNA stores and transmits hereditary information from one generation to the next.
• Segments of DNA = Genes (units of inheritance)

4. Nucleolus

• A round structure inside the nucleoplasm, not bounded by any membrane.
• Site where ribosomes are synthesised.

Functions of the Nucleus

• Controls all metabolic activities of the cell.
• Transmits hereditary traits (through DNA/genes) from parents to offspring.

Cytoplasm

The cytoplasm is the fluid portion of the cell located between the plasma membrane and the nuclear membrane. It houses all the cell organelles and provides a medium for chemical reactions.

Cell Organelles

Cell organelles are microscopic structures within the cytoplasm, each performing specific functions. They are analogous to the organs in our body.

1. Endoplasmic Reticulum (ER)

The ER is a network of membrane-bound tubules, vesicles, and sacs (cisternae) extending from the nuclear membrane throughout the cytoplasm.

Two types:

Primary Function: Acts as an internal transport system, moving molecules from one part of the cell to another.

2. Golgi Apparatus

• A membrane-bound organelle consisting of stacks of flattened sacs (cisternae) with small secretory vesicles on the sides.
• Often called the "post office" or "packaging centre" of the cell.
• Functions: Processing, packaging, and secreting proteins and other macromolecules.
• Much more common in glandular cells that secrete hormones or enzymes.

3. Lysosomes

• Membrane-bound vesicles containing lysosomal (hydrolytic) enzymes.
• Formed from pieces of the Golgi apparatus that break off.
• Function: Intracellular digestion break down worn-out organelles, bacteria, and waste materials.
• Also called "suicidal bags" because they can destroy the entire cell if ruptured.
• Common in animal, protist, and fungal cells; rare in plants.

4. Mitochondria

• Mostly rod-shaped structures with a double membrane.
• The inner membrane is folded into finger-like projections called cristae, which increase surface area.
• The fluid-filled space inside is called the matrix.

Functions:

• Site of aerobic respiration breakdown of glucose to release energy.
• Produce ATP (Adenosine Triphosphate) the cell's main energy currency.
• Nicknamed the "powerhouse of the cell".
• More mitochondria found in high-energy cells: muscle cells, liver cells, heart cells, sperm cells.
• Contain their own DNA and ribosomes (semi-autonomous organelle).

5. Ribosomes

• Tiny granular structures with no surrounding membrane.
• Found either free in the cytoplasm or attached to the Rough ER.
• Made of two subunits (large and small).
• Function: Protein synthesis (translation of mRNA into proteins).
• Present in both prokaryotic and eukaryotic cells.

6. Plastids (Only in Plant Cells)

Plastids are the largest cell organelles, found exclusively in plant cells. They are of three types based on their pigment:

Internal Structure of Chloroplast

• Bounded by two membranes.
• Inner space filled with colourless stroma (contains DNA and ribosomes).
• Inside stroma: stacks of flattened membrane sacs called thylakoids.
• One stack of thylakoids = granum (plural: grana).
• Different grana connected by stroma lamellae.
• Chlorophyll molecules in the grana capture light energy for photosynthesis.

7. Vacuoles

• Fluid-filled, membrane-bound sacs in the cytoplasm.
• In animal cells: many small, temporary vacuoles.
• In plant cells: one large, permanent central vacuole that occupies most of the cell's volume.
• The membrane surrounding the plant vacuole is called the tonoplast.
• Plant vacuoles store: sugars, organic acids, proteins, waste products, and water.
• Functions: maintain cell turgidity (rigidity of plant cells), store nutrients and waste.

8. Centrosome (Only in Animal Cells)

• Found only in animal cells, as a pair of granule-like structures near the nucleus.
• Contains two rod-like structures called centrioles arranged at right angles to each other.
• Function: Helps in cell division (forms the spindle fibre apparatus).

Prokaryotic vs Eukaryotic Cells

Cells are classified into two types based on the complexity of their structure:

Plant Cell vs Animal Cell

Organisation of Life: Cell to Organism

Living organisms are organised in a hierarchy from the simplest to the most complex level:

Cell → Tissues → Organs → Organ Systems → Organism

• Cell: Smallest unit of life; specialised cells perform specific functions.
• Tissue: A group of similar cells working together (e.g., muscle tissue, photosynthetic tissue).
• Organ: A collection of different tissues working together (e.g., heart, stomach, leaf).
• Organ System: A group of interconnected organs performing a major function (e.g., digestive system, root system).
• Organism: A complete living being made of multiple organ systems working in harmony.

Plant organ systems: Root system + Shoot system (stem, leaves, flowers, fruits).

Important Terms for the Exam

Enhanced Study Notes & Quick Revision

Memory Tricks

To remember organelle functions:

"Mighty Gorillas Rarely Like Performing Circus Routines"

• Mitochondria → Energy (powerhouse)
• Golgi apparatus → Packaging/secretion
• Ribosomes → Protein synthesis
• Lysosomes → Digestion (suicidal bag)
• Plastids → Photosynthesis/colour/storage
• Chloroplast → Captures light (photosynthesis)
• Rough ER → Protein transport

To remember plastid types:

"Charlie Loves Chocolates"

• Chloroplast → Green, photosynthesis
• Leucoplast → Colourless, storage
• Chromoplast → Coloured (not green), flowers/fruits

Prokaryote vs Eukaryote:

Pro = Before/Primitive (No true nucleus) Eu = True (Has a true nucleus)

Summary Table: Quick Revision

Solved Examples (20 Questions)

Q1. What is a cell and why is it called the basic unit of life?

Answer: A cell is the smallest unit of life that can perform all basic life functions growth, nutrition, respiration, excretion, and reproduction. Every living organism is composed of cells, making it the fundamental structural and functional unit of life. Even the simplest organism like Amoeba is a single cell that carries out all life processes independently.

Q2. Who discovered the cell and how?

Answer: Robert Hooke discovered the cell in 1665. He used a crude microscope to observe a thin slice of dried cork (bark of a tree). He saw small, box-like compartments and named them "cells" because they resembled the small rooms where monks lived. However, these were dead cell walls. Anton van Leeuwenhoek later (1675) was the first to observe living cells such as bacteria and protozoa.

Q3. State the three principles of Cell Theory.

Answer:

1. All living things are composed of one or more cells.
2. Cells are the basic units of structure and function in an organism.
3. All cells arise from pre-existing cells. (Added by Rudolf Virchow)

Q4. Why is the mitochondrion called the "powerhouse" of the cell?

Answer: Mitochondria carry out aerobic respiration, during which glucose and oxygen are used to produce ATP (Adenosine Triphosphate). ATP is the primary energy molecule used by cells for all activities. Since mitochondria generate most of the cell's energy supply, they are called the powerhouse of the cell.

Q5. What is the difference between Rough ER and Smooth ER?

Answer:

Q6. Why are lysosomes called "suicidal bags"?

Answer: Lysosomes contain powerful hydrolytic (digestive) enzymes. Under normal conditions, they digest worn-out organelles and foreign materials. However, if the lysosome membrane ruptures, these enzymes spill into the cytoplasm and digest the entire cell from within, leading to cell death. This is why they are called suicidal bags.

Q7. What are the three types of plastids and their functions?

Answer:

• Chloroplasts: Contain green pigment chlorophyll; carry out photosynthesis to produce food.
• Chromoplasts: Contain coloured pigments (yellow, red, orange); give colour to flowers and fruits, attracting pollinators and dispersers.
• Leucoplasts: Colourless; store starch, proteins, and oils.

Q8. How does a plant cell differ from an animal cell?

Answer: Key differences:

• Plant cells have a cell wall (cellulose); animal cells do not.
• Plant cells have plastids (chloroplasts, etc.); animal cells generally do not.
• Plant cells have a large central vacuole; animal cell vacuoles are small and temporary.
• Animal cells have centrioles; plant cells do not.

Q9. What is the role of the Golgi apparatus in the cell?

Answer: The Golgi apparatus serves as the "post office" of the cell. It:

• Processes and modifies proteins and lipids received from the ER.
• Packages them into vesicles.
• Ships them to destinations inside or outside the cell (secretion). It is especially abundant in glandular cells that produce secretions like hormones and enzymes.

Q10. What is the function of the nucleus in a cell?

Answer: The nucleus:

• Acts as the control centre of the cell, directing all metabolic activities.
• Contains DNA (in the form of chromatin/chromosomes), which carries genetic information.
• Transmits hereditary traits from parents to offspring through genes.
• Houses the nucleolus, which produces ribosomes.

Short Answer Questions

Q11. Name the energy currency of the cell. Which organelle produces it?

Answer: The energy currency of the cell is ATP (Adenosine Triphosphate). It is produced by the mitochondria during cellular respiration.

Q12. What is the nucleoid in prokaryotic cells?

Answer: In prokaryotic cells, there is no true nucleus. The DNA is not enclosed by a nuclear membrane. Instead, it exists as a single, highly coiled circular DNA strand in direct contact with the cytoplasm. This undefined nuclear region is called the nucleoid.

Q13. Name two cell organelles that contain their own DNA.

Answer: Mitochondria and Chloroplasts both contain their own DNA and ribosomes, making them semi-autonomous organelles. This supports the endosymbiotic theory of their origin.

Q14. What is the tonoplast?

Answer: The tonoplast is the membrane that surrounds the central vacuole in plant cells. It regulates the movement of substances in and out of the vacuole and helps maintain the osmotic pressure of the plant cell.

Q15. Give the hierarchy of biological organisation from cell to organism.

Answer:

Cell → Tissue → Organ → Organ System → Organism

• Cell: Basic unit of life (e.g., muscle cell)
• Tissue: Similar cells working together (e.g., muscle tissue)
• Organ: Different tissues working together (e.g., heart)
• Organ system: Organs working together (e.g., circulatory system)
• Organism: Complete living being (e.g., human)

Assertion-Reason Questions

Q16.Assertion (A): Mitochondria are more numerous in muscle cells than in skin cells. Reason (R): Mitochondria produce ATP, and muscle cells require more energy for contraction.

Answer: Both A and R are correct, and R is the correct explanation of A. Muscle cells are highly active and require a constant large supply of energy (ATP). Mitochondria are the sites of ATP production, so more mitochondria are present in muscle cells.

Q17.Assertion (A): Ribosomes are found in both prokaryotic and eukaryotic cells. Reason (R): Ribosomes are essential for protein synthesis in all living cells.

Answer: Both A and R are correct, and R is the correct explanation of A. Since all cells need proteins to function and survive, ribosomes the sites of protein synthesis are present universally in all cell types.

Case-Based Questions

Q18. A student observed two types of cells under a microscope. Cell X had a well-defined nuclear membrane, membrane-bound organelles, and a large central vacuole. Cell Y had no nuclear membrane; its DNA was in direct contact with the cytoplasm, and it had no membrane-bound organelles.

(a) Identify Cell X and Cell Y.(b) Give one example each of Cell X and Cell Y.(c) Name the undefined nuclear region in Cell Y.

Answer:

• (a) Cell X = Eukaryotic cell (plant cell); Cell Y = Prokaryotic cell
• (b) Cell X example: Onion peel cell / Human cheek cell; Cell Y example: Bacteria / Blue-green algae
• (c) The undefined nuclear region in Cell Y is called the nucleoid.

Q19. A biology teacher said: "One organelle in the cell acts like a post office, another like a power station, and yet another like a recycling centre."

(a) Which organelle acts like a post office?

(b) Which organelle acts like a power station?

(c) Which organelle acts like a recycling centre?

Answer:

• (a) Golgi apparatus — packages and ships proteins (like a post office)
• (b) Mitochondria — generates ATP/energy (like a power station)
• (c) Lysosomes — breaks down waste and worn-out materials (like a recycling centre)

Long Answer Question

Q20. Describe the internal structure of a chloroplast and explain how it is adapted for photosynthesis.

Answer:

Structure of Chloroplast:

• Bounded by two membranes (outer and inner).
• Interior filled with colourless fluid called stroma contains DNA, ribosomes, and enzymes needed for photosynthesis.
• Within the stroma are stacks of flattened, membrane-bound sacs called thylakoids.
• One stack of thylakoids = granum (plural: grana).
• Different grana are connected by tubular structures called stroma lamellae.
• Chlorophyll (green pigment) is embedded in the thylakoid membranes within the grana.

Adaptation for Photosynthesis:

• The large stroma area provides space for the enzymes that convert CO₂ into glucose (Calvin cycle/dark reactions).
• The thylakoid membranes house chlorophyll to absorb sunlight efficiently (light reactions).
• The stacked granum structure maximises the surface area for light absorption.
• Presence of own DNA and ribosomes allows the chloroplast to partially self-replicate and produce its own proteins.