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Chapter 5 — The Fundamental Unit of Life (Notes without Activities and Questions)

Introduction: Discovery of the Cell

• In 1665, Robert Hooke examined a thin slice of cork (from tree bark) using a self-designed microscope.

• He saw a honeycomb-like structure made of many small compartments.

• He named these compartments cells.

• Cell comes from a Latin word meaning “a little room.”

• This observation was important because it was the first time someone noticed that living things appear to be made of separate basic units, and the term cell is still used today.

5.1 What are Living Organisms Made Up of?

Unicellular vs Multicellular

• Some organisms are made of one cell and live independently:

• Examples: Amoeba, Chlamydomonas, Paramoecium, bacteria

• These are unicellular organisms (uni = single).

• Many organisms have many cells working together:

• Examples: some fungi, plants, animals

• These are multicellular organisms (multi = many).

Key idea: All multicellular organisms start from one cell

• Every multicellular organism begins from a single cell.

• Cells divide to produce cells of their own kind.

• Therefore: all cells come from pre-existing cells.

Cells show variety

• Cells can differ in shape and size depending on their function.

• Amoeba has changing shape.

• Some cells have fixed shapes, e.g. nerve cells.

• Human body has different cell types (examples): blood cells, bone cells, muscle cells, nerve cells, fat cells, sperm, ovum.

Division of labour (in organisms and within cells)

• In multicellular organisms: different organs do different functions (heart pumps blood, stomach digests food, etc.).

• Within a cell: different cell organelles perform different jobs (manufacturing, transport, waste removal, etc.).

• Organelles together make the cell able to live and function.

Cell discovery and cell theory (timeline)

• Robert Hooke (1665): observed cork cells with a primitive microscope.

• Leeuwenhoek (1674): observed free-living cells in pond water for the first time.

• Robert Brown (1831): discovered the nucleus.

• Purkinje (1839): coined the term protoplasm for the fluid substance of the cell.

• Schleiden (1838) & Schwann (1839): proposed cell theory (plants and animals are made of cells; cell is basic unit of life).

• Virchow (1855): added that all cells arise from pre-existing cells.

• Electron microscope (1940): made it possible to observe and understand cell organelles in detail.

5.2 What is a Cell Made Up of? (Structural organisation)

Basic parts seen in most cells

1. Plasma membrane

2. Nucleus

3. Cytoplasm

5.2.1 Plasma Membrane (Cell Membrane)

• Outermost covering of the cell.

• Separates cell contents from the outside environment.

• Selectively permeable: allows some substances to enter/exit and restricts others.

Diffusion

• Movement of substances from high concentration to low concentration.

• Helps in gas exchange:

• CO₂ (waste) diffuses out when higher inside the cell.

• O₂ diffuses in when lower inside the cell.

Osmosis

• Movement of water through a selectively permeable membrane from higher water concentration to lower water concentration until equilibrium.

Effect of solutions on cells

• Hypotonic solution (dilute outside): water enters → cell swells.

• Isotonic solution: no net water movement → cell remains same size.

• Hypertonic solution (concentrated outside): water leaves → cell shrinks.

Other important points

• Cell membrane is flexible and made of lipids and proteins (detailed structure seen with electron microscope).

• Cells can engulf food/material due to flexibility → endocytosis (example: Amoeba).

5.2.2 Cell Wall (Plant cells)

• Plant cells have a rigid outer covering outside plasma membrane called cell wall.

• Mainly made of cellulose (gives structural strength).

Plasmolysis

• When a living plant cell loses water through osmosis, the cell contents shrink away from the cell wall → plasmolysis.

Importance of cell wall

• Helps plant/fungal/bacterial cells survive in hypotonic (very dilute) medium without bursting.

• Cell wall provides resistance against swelling pressure.

5.2.3 Nucleus

• A dark, usually spherical/oval structure inside the cell.

• Covered by a double-layered nuclear membrane with pores.

Chromosomes, DNA, genes

• Nucleus contains chromosomes (visible as rod-shaped structures when the cell is about to divide).

• Chromosomes carry hereditary information in the form of DNA.

• Chromosomes = DNA + protein.

• Functional segments of DNA are called genes.

• In a non-dividing cell, DNA is present as chromatin (thread-like material).

• Before division, chromatin condenses into chromosomes.

Functions of nucleus

• Controls cell activities and plays a key role in cell reproduction and development.

Prokaryotes vs Eukaryotes (based on nucleus)

• Prokaryotes: no nuclear membrane; nuclear region is poorly defined → nucleoid (e.g., bacteria).

• Eukaryotes: well-defined nucleus with nuclear membrane.

5.2.4 Cytoplasm

• Fluid content inside the plasma membrane.

• Contains cell organelles.

• Many life processes occur here.

• Membrane-bound organelles are present in eukaryotes but absent in prokaryotes.

Viruses (membrane significance)

• Viruses lack membranes and don’t show life characteristics until they enter a living cell and use its machinery to multiply.

5.2.5 Cell Organelles (main ones)

Endoplasmic Reticulum (ER)

• Network of membrane-bound tubes/sheets.

• RER: has ribosomes; helps in protein synthesis and transport.

• SER: helps in lipid (fat) synthesis; important in membrane biogenesis; helps detoxification in liver cells.

Golgi Apparatus

• Stacks of membrane-bound flattened sacs (cisterns).

• Functions: storage, modification, packaging, and dispatch of materials.

• Helps in formation of lysosomes.

• Can form complex sugars from simple sugars.

Lysosomes

• Membrane-bound sacs filled with digestive enzymes (enzymes made by RER).

• Digest foreign material and worn-out organelles.

• Can burst and digest the cell → called “suicide bags.”

Mitochondria

• Powerhouses of the cell.

• Double membrane; inner membrane folded to increase surface area.

• Produce ATP (energy currency of the cell).

• Have their own DNA and ribosomes.

Plastids (only plant cells)

• Chromoplasts (coloured); chloroplasts contain chlorophyll and do photosynthesis.

• Leucoplasts (colourless): storage of starch, oils, proteins.

• Have their own DNA and ribosomes.

Vacuoles

• Storage sacs.

• Large central vacuole in plant cells (cell sap) provides turgidity/rigidity and stores substances including wastes.

• Small vacuoles in animal cells.

• In Amoeba, food vacuole stores ingested food.

Cell as the fundamental unit

• Cell structure and function depend on organisation of membrane and organelles.

• Therefore, the cell is the fundamental structural and functional unit of life.

Cell Division

• New cells form for growth, replacement of damaged/dead cells, and reproduction.

Mitosis

• One mother cell → two identical daughter cells.

• Same chromosome number as mother cell.

• For growth and repair.

Meiosis

• Two consecutive divisions.

• One cell → four daughter cells.

• Daughter cells have half the chromosomes (for gamete formation).

What you have learnt (key points)

• Cell is the fundamental organisational unit of life.

• Plasma membrane is made of lipids and proteins and regulates movement of materials.

• Plant cells have a cellulose cell wall outside the membrane.

• Cell wall prevents bursting in hypotonic conditions.

• Eukaryotic nucleus is enclosed by double membrane and directs life processes.

• ER: transport + manufacturing surface.

• Golgi: modification, packaging, dispatch; forms lysosomes.

• Plastids in plants: chromoplasts/chloroplasts (photosynthesis) and leucoplasts (storage).

• Vacuole maintains turgidity and stores substances.

• Prokaryotes lack membrane-bound organelles and nuclear membrane.

• Cells divide for growth, repair, and forming gametes.