The Cell Chapter Notes | Biology for SSS 1 PDF Download

Introduction

• The chapter also covers the cell theory.
• Differences between plant and animal cells.
• The roles of various cell organelles.

Through activities like observing onion peel and cheek cells, students learn to identify cell components and understand their importance in living organisms.

Warm Up Activity

• Identify and arrange the levels of organization of organisms in successive order from given diagrams.
• Cells are the building blocks of living organisms, similar to bricks in a house.
• Cells are the structural and functional units of life, performing all life functions.

Discovery of the Cell

• Cells were discovered after the invention of the microscope.
• Antonie van Leeuwenhoek made a simple microscope magnifying up to 300 times.
• Leeuwenhoek observed blood cells in a frog’s foot-web and single-celled organisms in water.
• Robert Hooke developed a compound microscope with two lenses.
• Hooke observed cork slices and named the box-like structures "cells" in 1665.
• The term "cell" comes from the Latin word "cellula," meaning a small room.
• Cork cells observed by Hooke were dead, showing only cell walls.
• A living cell contains cytoplasm and nucleus, together called protoplasm.
• Cytology is the branch of biology that studies cells.

Cell Theory

• Improved microscopes in the 19th century helped scientists study cells better.
• Matthias Schleiden (1838) found that all plant parts are made of cells.
• Theodor Schwann (1839) discovered that animal parts are also made of cells.
• Rudolph Virchow (1855) stated that new cells come from pre-existing cells.
• Cell theory states:
  • All living beings are made of cells.
  • Cells are the basic structural and functional units of life.
  • New cells are formed by the division of existing cells.

Variation in Cell Number, Shape, and Size

Number of Cells

• Organisms are classified based on the number of cells.
• Unicellular organisms have one cell that performs all life functions (e.g., Amoeba, Paramecium, Euglena, bacteria).
• Multicellular organisms have many cells, with specialized cells for different functions (e.g., neem, peepal, rose, fish, frog, lion).

Shape of Cells

• Cell shape depends on the function they perform.
• Oval: Chlamydomonas (a unicellular green alga).
• Irregular: Amoeba and human white blood cells, aiding in engulfing harmful substances.
• Oblong: Paramecium (slipper-shaped).
• Elongated: Muscle cells, which contract and relax for bone movement.
• Thread-like: Nerve cells, for sending messages across the body.
• Cubical or rectangular: Leaf cells in plants.
• Plant cells can be round, oval, cubical, rectangular, or polygonal.

Size of Cells

• Cell size is measured in micrometres (1 micron = 1/1000th of a millimetre).
• Most cells are microscopic, visible only under a microscope.
• Bacterial cells: 0.2 to 0.5 microns in diameter.
• Human red blood cells: 6-8 microns.
• Ostrich egg: Largest cell, about 23 cm.
• Nerve cells in elephants: Up to 3 meters long.

Cell: The Functional Unit of Life

Structure of the Cell

• All cells have three main parts: cell membrane, cytoplasm, and nucleus.
• Cytoplasm and nucleus together form protoplasm, the living substance of the cell.

Cell Membrane
Thin, flexible, semipermeable covering of the cell.
Functions:

• Separates cell from surroundings.
• Protects internal components.
• Gives shape to the cell.
• Controls movement of materials in and out.

Plant cells have an additional non-living cell wall made of cellulose.
Cell wall functions:

• Provides rigidity and strength.
• Gives definite shape.
• Limits entry of large molecules and pathogens.

Cytoplasm

• Jelly-like, colourless, translucent fluid between cell membrane and nucleus.
• Contains ions, amino acids, carbohydrates, enzymes, salts, and cell organelles.
• All life functions occur in the cytoplasm.

Nucleus

• Small spherical body in the cytoplasm, surrounded by a double nuclear membrane.
• Contains nucleoplasm with nucleoli and chromatin network.
• Chromatin forms chromosomes during cell division, carrying genes for inheritance.
• Chromosome number is fixed for each organism.
• Functions:
  • Controls all cell activities (control centre).
  • Transfers characters from parents to offspring.
  • Plays a key role in cell division.

Cell Organelles

Plastids

Types:

• Chloroplasts: Green plastids with chlorophyll, trap solar energy for photosynthesis (kitchen of the cell).
• Chromoplasts: Coloured plastids in fruits and flowers (yellow, orange, red).
• Leucoplasts: Colourless, in roots and stems, store starch, proteins, and fats.

Vacuoles

• Sac-like structures with cell sap, bound by a single membrane (tonoplast).
• Plant cells have large vacuoles; animal cells have small or no vacuoles.
• Functions:
  • Maintain cell turgidity (shape).
  • Store food, water, and waste.

Functions of Different Cell Organelles

• Vacuole: Stores water, useful materials, pigments, and waste.
• Plastids:
  • Chloroplasts: Contain chlorophyll for photosynthesis.
  • Leucoplasts: Store food (starch).
  • Chromoplasts: Contain non-chlorophyll pigments.
• Nucleus: Controls cell activities, transfers hereditary characters, aids cell division.

Differences between Plant and Animal Cells

• Have a cell wall.
• Contain plastids.
• Have large vacuoles.
• Thin layer of cytoplasm.

Animal cells:

• No cell wall.
• No plastids.
• Small or no vacuoles.
• Denser, granular cytoplasm filling the cell.

Keywords

• Cell: Structural and functional unit of living organisms.
• Cytology: Study of cells.
• Cell membrane: Thin, semipermeable membrane around the cell.
• Cell wall: Protective layer outside plant cell membrane.
• Cytoplasm: Jelly-like fluid between cell membrane and nucleus.
• Nucleus: Contains genetic material, controls cell activities.
• Chromosomes: Thread-like structures with genes in the nucleus.
• Plastids: Plant cell organelles with pigments or food storage.