Science Exploration Notes Class 9 English Medium 2026-27

Science Exploration Chapter 2. Cell: The Building Block of Life

CBSE Notes New Syllabus: Class 9

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Chapter 2. Cell: The Building Block of Life

This chapter introduces the cell as the basic structural and functional unit of life. It explains how cells were discovered, how they are studied using microscopes, the structure of cells, different types of cells, cell organelles, and the importance of cell division in the continuity of life. 

Chapter Review

Important Terms & Definitions

Cell – The basic structural and functional unit of all living organisms. 

Unicellular Organism – An organism made up of a single cell, such as bacteria and yeast. 

Multicellular Organism – An organism made up of many cells that work together, such as plants and animals. 

Tissue – A group of similar cells performing the same function.

Organ – A structure made of different tissues working together to perform a specific function.

Organ System – A group of organs working together to carry out major life processes. 

Microscope – An instrument used to observe objects that are too small to be seen with the naked eye. 

Cell Membrane (Plasma Membrane) – A selectively permeable membrane that surrounds the cell and controls the movement of substances. 

Cell Wall – A rigid outer covering present in plant cells, fungi and bacteria that provides protection and support. 

Cytoplasm – A jelly-like substance inside the cell where most cell organelles are present. 

Nucleus – The control centre of the cell containing genetic material (DNA). 

Prokaryotic Cell – A cell without a true nucleus and membrane-bound organelles. 

Eukaryotic Cell – A cell with a well-defined nucleus and membrane-bound organelles. 

Diffusion – Movement of particles from a region of higher concentration to lower concentration. 

Osmosis – Movement of water through a selectively permeable membrane from a dilute solution to a concentrated solution. 

Chromosome – Rod-shaped structure containing DNA and genes responsible for inheritance. 

Gene – Functional segment of DNA that carries hereditary information. 

DNA (Deoxyribonucleic Acid) – The genetic material responsible for inheritance.

Important Scientists

• Robert Hooke (1665) – First observed cork cells using a microscope and coined the term "cell". 
• Camillo Golgi (1898) – Discovered the Golgi apparatus. 

Important Concepts

• Cell is the basic unit of life – Every living organism is made up of one or more cells.
• Cells form tissues – Similar cells combine to form tissues.
• Tissues form organs – Different tissues work together to perform specific functions.
• Organs form organ systems – Organ systems carry out major life activities.
• Microscopes made cell study possible – Light and electron microscopes reveal structures invisible to the naked eye.
• Cell membrane controls transport – It regulates movement of substances into and out of the cell.
• Cell wall provides rigidity – Plant cells remain firm because of the cell wall.
• Osmosis maintains water balance – Water moves across the plasma membrane according to concentration.
• Cell organelles perform specialised functions – Each organelle has a specific role.
• Nucleus controls cellular activities – It stores genetic information and regulates cell functions.
• Mitochondria produce energy – They generate ATP during cellular respiration.
• Chloroplast performs photosynthesis – Green plants prepare food in chloroplasts.
• Ribosomes synthesise proteins – They are called the protein factories of the cell.
• Golgi apparatus packages materials – It modifies, packages and transports proteins and lipids.
• Lysosomes digest wastes – They clean damaged cell parts.

Important Comparisons

• Plant Cell vs Animal Cell – Plant cells possess a cell wall and chloroplasts, whereas animal cells do not.
• Prokaryotic vs Eukaryotic Cells – Prokaryotes lack a true nucleus, while eukaryotes possess a membrane-bound nucleus. 
• Diffusion vs Osmosis – Diffusion involves movement of particles, whereas osmosis involves movement of water only.

Important Activities

• Estimating the size of a cell – Using a light microscope and field of view. 
• Potato Osmosis Experiment – Demonstrates osmosis using plain water and salt solution. 
• Observation of Plant and Animal Cells – Comparing onion peel and cheek cells under a microscope. 

Important Facts

• Limit of Resolution of Human Eye – Approximately 0.1 mm. 
• Cell Membrane Thickness – About 7–10 nanometres. 
• Typical Size of Prokaryotic Cells – 1–10 µm. 
• Typical Size of Eukaryotic Cells – 10–100 µm. 
• Mature Human RBCs – Lack a nucleus and survive for about 120 days. 
• ATP – Known as the energy currency of the cell. 

Exam Booster

• Remember – Robert Hooke discovered cells in cork in 1665.
• Remember – Cell membrane is selectively permeable, whereas the cell wall is freely permeable.
• Remember – Mitochondria are called the powerhouse of the cell.
• Remember – Ribosomes are the sites of protein synthesis.
• Remember – Chloroplasts are found only in green plant cells.
• Remember – Genes are present on chromosomes.
• Remember – Osmosis is frequently asked in competency-based and case-study questions.
• Remember – Differences between plant & animal cells and prokaryotic & eukaryotic cells are highly important for CBSE examinations.