Class 8 Science Chapter 3 Living Beings & Their Structure

Class 8 Science Chapter 3 – Living Beings and Their Structure covers one of the most fundamental topics in biology. In this chapter, you will explore what living beings are, how they are organized at the cellular level, how scientists classify organisms, the role of microorganisms in our world, and how food is preserved using various methods. Whether you are preparing for your exams or building your science foundation, this comprehensive guide to Class 8 Science Chapter 3 Living Beings and Their Structure will help you understand every concept clearly and confidently.

 Introduction to Living Beings

• Living beings → Organisms that show growth, reproduction, metabolism, response to stimuli, adaptation, and cellular organization.
• All living beings are made up of cells (basic unit of life).
• Two types:
  • Unicellular organisms (e.g., Amoeba, Bacteria) → Single cell performs all life functions.
  • Multicellular organisms (e.g., Humans, Plants) → Specialized cells form tissues, organs, and systems.

Exam Tip: Always state “cell is the structural and functional unit of life” in definitions. Cell: Fundamental Unit of Life

• Cell → Smallest structural and functional unit of life.
• Discovered by Robert Hooke (1665) in cork.
• Cell Theory (Schleiden & Schwann, 1839):
  1. All living beings are made of cells.
  2. Cell is the basic unit of life.
  3. New cells arise from pre-existing cells (Virchow, 1855).
  4. Robert Hooke’s Discovery

 Shape and Size of Cells

• Shape: Cells show different shapes based on function.
  • Spherical (e.g., egg cell), spindle-shaped (e.g., muscle cell), elongated (nerve cell), disc-shaped (RBC).
• Size: Ranges from 0.1 µm (bacteria) to 170 mm (ostrich egg).

Pitfall: Do not confuse cell size with organism size — larger animals do not always have bigger cells, just more cells.

 Structure of Cell

General Structure:

• Plant Cell: Cell wall, cell membrane, cytoplasm, nucleus, plastids, vacuoles.
• Animal Cell: Cell membrane, cytoplasm, nucleus, centrioles, lysosomes.

a. Cell Wall (only in plant cells)

• Functions:
  1. Provides rigidity and support.
  2. Protects against mechanical injury.
  3. Allows exchange of materials via plasmodesmata.
  4. Maintains cell shape.

b. Cell Membrane (Plasma Membrane)

• Functions:
  1. Selectively permeable → controls entry/exit of substances.
  2. Protects internal cell contents.
  3. Helps in communication and signaling.
  4. Maintains homeostasis.

c. Cytoplasm

• Functions:
  1. Site for biochemical reactions.
  2. Holds organelles in place.
  3. Provides medium for transport.
  4. Stores vital chemicals.

d. Cell Organelles and Inclusions

Specialized structures performing specific functions.

e. Mitochondria (Powerhouse of cell)

• Functions:
  1. Produces ATP (energy currency).
  2. Site of respiration.
  3. Regulates metabolism.
  4. Stores genetic material (mtDNA).

f. Plastids (in plant cells only)

• Chloroplast: Photosynthesis, stores chlorophyll.
• Chromoplast: Provides color to fruits/flowers.
• Leucoplast: Storage of starch, oil, proteins.

g. Ribosomes

• Functions:
  1. Protein synthesis (“protein factory”).
  2. Found free or attached to ER.
  3. Supports enzymatic activity.
  4. Maintains cell metabolism.

h. Endoplasmic Reticulum (ER)

• Rough ER (RER):
  1. Protein synthesis.
  2. Membrane formation.
  3. Intracellular transport.
• Smooth ER (SER):
  1. Lipid synthesis.
  2. Detoxification of harmful substances.
  3. Storage of calcium ions.
  4. Intracellular transport.

i. Golgi Bodies

• Functions:
  1. Packaging and secretion.
  2. Formation of lysosomes.
  3. Transport of proteins and lipids.
  4. Formation of cell plate during division.

j. Lysosomes (“Suicidal bags”)

• Functions:
  1. Intracellular digestion.
  2. Destroys worn-out organelles.
  3. Protects against pathogens.
  4. Helps in autolysis (cell death).

k. Vacuoles

• Functions:
  1. Storage of water, salts, proteins.
  2. Maintains turgor pressure.
  3. Provides rigidity.
  4. Removes waste.

l. Nucleus

• Components:
  1. Nuclear envelope – Double membrane.
  2. Nuclear membrane – Controls exchange with cytoplasm.
  3. Nucleoplasm – Fluid containing DNA & proteins.
  4. Chromatin – Contains hereditary material.
  5. Nucleolus – Synthesizes rRNA & ribosomes.
• Functions:
  1. Controls all cell activities.
  2. Stores genetic material.
  3. Directs protein synthesis.
  4. Controls cell division.

Table: Difference between Plant Cell and Animal Cell

Feature	Plant Cell	Animal Cell
Cell wall	Present	Absent
Plastids	Present (Chloroplast, etc.)	Absent
Vacuoles	Large, central	Small, temporary
Centrioles	Absent	Present
Shape	Regular, rectangular	Irregular, round

 Interrelationship among Cells, Tissues and Organs

• Cells → Basic unit of life.
• Tissues → Group of similar cells performing same function (e.g., muscle tissue, xylem).
• Organs → Different tissues combine to form organs (e.g., heart, leaf).
• Organ Systems → Group of organs performing common functions (e.g., digestive system, respiratory system).

Key Point: Cells → Tissues → Organs → Organ Systems → Organism

Interesting Facts

• Largest human cell → Ovum.
• Longest cell → Nerve cell (over 1 m).
• Mitochondria and chloroplast have their own DNA → considered semi-autonomous.

Mnemonics / Memory Aids

• Cell Organelles: “MR. G. L. V. N.” → Mitochondria, Ribosomes, Golgi bodies, Lysosomes, Vacuoles, Nucleus.
• Plastids Types: “C-C-L” → Chloroplast, Chromoplast, Leucoplast.

Summary / Quick Revision

• Living beings are made of cells → structural and functional unit of life.
• Cell size varies from microscopic bacteria to large ostrich egg.
• Plant cells have cell wall, plastids, large vacuole; animal cells lack these but have centrioles.
• Major organelles: Mitochondria (ATP), Ribosomes (Protein), ER (Transport), Golgi (Secretion), Lysosomes (Digestion), Nucleus (Control center).
• Cells → Tissues → Organs → Organ Systems → Organism.

Classification of Living Beings – Class 8 Science Chapter 3

Classification → Systematic arrangement of living beings into groups based on similarities and differences.

Importance / Why Classification?

• Makes study of organisms easier and systematic.
• Helps to identify and understand relationships among organisms.
• Explains evolutionary trends.
• Provides uniformity in naming (taxonomy).

 Two-Kingdom System of Classification

Proposed by → Carl Linnaeus.
Division → Plant Kingdom & Animal Kingdom.

Basis of classification

1. Mobility → Plants are immobile, animals are mobile.
2. Nutrition → Plants → autotrophic, Animals → heterotrophic.
3. Simplicity → Plants considered simple, animals more complex.

Salient Features

• Simple and easy to understand.
• All organisms divided into two broad groups.

Drawbacks (4 points)

1. Could not classify microorganisms (bacteria, protozoa, fungi).
2. Misplaced unicellular organisms (e.g., Euglena has both plant and animal features).
3. Did not consider mode of reproduction.
4. Ignored evolutionary relationships.

Five-Kingdom System of Classification

Proposed by → R.H. Whittaker.
Kingdoms → Monera, Protista, Fungi, Plantae, Animalia.

Bases of Classification

1. Cellular structure → Prokaryotic / Eukaryotic.
2. Nutrition → Autotrophic / Heterotrophic / Saprophytic.
3. Body structure → Unicellular / Multicellular.
4. Reproduction → Asexual / Sexual.
5. Mobility → Mobile / Immobile.

Salient Features (4 points)

1. Scientific and evolutionary basis.
2. Clear distinction of unicellular and multicellular organisms.
3. Proper place for fungi, protists, and bacteria.
4. More accurate than two-kingdom system.

 Micro-organisms

Definition → Microscopic living organisms not visible to the naked eye, observed under a microscope.

Main Features (4 points)

1. Mostly unicellular (some multicellular fungi).
2. Found everywhere (air, water, soil, inside living beings).
3. Very small size, require microscope.
4. Can be beneficial or harmful.

a. Bacteria

Characteristics (4 points):

1. Prokaryotic, unicellular.
2. Reproduce by binary fission.
3. Some autotrophic, others heterotrophic.
4. Found in soil, water, human body.

b. Viruses

Characteristics (4 points):

1. Borderline of living and non-living.
2. Non-cellular, made of protein coat + nucleic acid.
3. Can only reproduce inside host cell.
4. Cause diseases (e.g., influenza, HIV).

c. Amoeba

Characteristics (4 points):

1. Unicellular protozoan.
2. Moves by pseudopodia (false feet).
3. Engulfs food by phagocytosis.
4. Found in freshwater.

d. Fungi

Characteristics (4 points):

1. Eukaryotic, mostly multicellular (yeast is unicellular).
2. Cell wall made of chitin.
3. Saprophytic (feed on dead matter).
4. Examples: Mushroom, yeast, molds.

Positive Effects of Microorganisms (4 points)

1. Used in medicine (antibiotics like penicillin).
2. Decompose organic matter, recycle nutrients.
3. Used in food production (curd, bread, alcohol).
4. Nitrogen fixation by bacteria (Rhizobium).

 Harmful Effects of Microorganisms (4 points)

1. Cause diseases in humans (TB, cholera, malaria, COVID-19).
2. Spoil food items.
3. Destroy crops (fungal diseases).
4. Cause decay of materials (wood, leather).

Interesting Facts

• Bacteria are the oldest known living organisms.
• Viruses can crystallize outside the host like chemicals.
• Penicillin, the first antibiotic, was discovered from the fungus Penicillium notatum.

Mnemonics / Memory Aids

• 5 Kingdoms: M-P-F-P-A → Monera, Protista, Fungi, Plantae, Animalia.
• Microorganisms studied: BVAF → Bacteria, Viruses, Amoeba, Fungi.

Summary / Quick Revision

• Classification organizes organisms systematically for easy study.
• Two-Kingdom system → Simple but inaccurate, only plants and animals.
• Five-Kingdom system → More scientific, includes Monera, Protista, Fungi, Plantae, Animalia.
• Microorganisms → Bacteria, Viruses, Amoeba, Fungi.
• Positive effects → Medicine, food, decomposition, nitrogen fixation.
• Harmful effects → Diseases, food spoilage, crop destruction, material decay.

 Methods of Food Preservation

Food Preservation → Process of treating and handling food to prevent spoilage, extend shelf life, and ensure safety.

Why are foods preserved?

a. Prevent spoilage → Stops growth of microorganisms and enzymes.
b. Extend shelf life → Food lasts longer for storage and transport.
c. Maintain nutrient content → Reduces nutrient loss.
d. Reduce food waste → Prevents unnecessary loss of food.
e. Enable seasonal availability → Seasonal foods available year-round.
f. Enhance food safety → Destroys harmful microbes and toxins.

 Dry Preservation

Principle → Removal of moisture → microorganisms cannot grow.

i. Dehydration

• Drying food by sun, hot air, or machines.
• Example: dried fruits, grains, fish.

ii. Salting

• Salt draws out water (osmosis) and prevents microbial growth.
• Example: salted fish, pickles, meat.

iii. Smoking

• Exposing food to smoke (contains antimicrobial chemicals).
• Example: smoked meat, fish.

 Wet Preservation

Principle → Use of liquid medium or chemicals to prevent spoilage.

i. Canning

• Food sealed in airtight cans/jars, then sterilized by heat.
• Prevents entry of microbes.
• Example: canned fruits, vegetables, soups.

ii. Pickling

• Food preserved in salt, vinegar, or oil.
• Acidic medium prevents microbial growth.
• Example: pickled mango, cucumber.

iii. Fermentation

• Use of beneficial microbes to produce acids/alcohol.
• Example: curd, sauerkraut, kimchi, wine.

 Cold Preservation

Principle → Low temperature slows microbial growth and enzyme action.

i. Refrigeration

• Storage at 0–5°C.
• Example: milk, fruits, vegetables.

ii. Freezing

• Storage below –18°C.
• Example: frozen meat, fish, ice cream.

iii. Flash Freezing

• Very rapid freezing at extremely low temperatures.
• Prevents formation of large ice crystals → maintains texture and nutrients.
• Example: frozen peas, seafood.

Interesting Facts

• Honey is a natural preservative — low water content prevents bacterial growth.
• Freeze-drying is used by astronauts to carry food in space.
• Old civilizations used salting and smoking long before modern refrigeration.

Mnemonics / Memory Aids

• Dry Preservation: D-S-S → Dehydration, Salting, Smoking.
• Wet Preservation: C-P-F → Canning, Pickling, Fermentation.
• Cold Preservation: R-F-F → Refrigeration, Freezing, Flash Freezing.

This completes the full summary of Class 8 Science Chapter 3 Living Beings and Their Structure. Let’s do
a quick revision of all key concepts covered.