Fundamental Unit of Life Notes

Discovery of Cell

• Cells were first discovered by Robert Hooke in 1665.

• He observed a thin slice of cork under a self-made microscope.

• The cork resembled a honeycomb structure with small compartments.

• Hooke named these compartments "cells."

• Cork comes from the bark of a tree.

Microscopes

• The first microscope was built in 1595 by Hans and Zacharias Janssen in Holland.

• Later perfected in the 17th century by experts including Robert Hooke.

• Anton Von Leeuwenhoek made notable modifications in 1674.

• The electron microscope, discovered in 1940, helped in detailed observation of cells.

• The compound or light microscope uses light and two lenses to view objects.

History of Discoveries

• 1665 - Robert Hooke: Discovery of the cell.

• 1674 - Leeuwenhoek: Discovery of free-living cells in pond water.

• 1831 - Robert Brown: Discovery and naming of the nucleus.

• 1838-39 - M.J. Schleiden and Theodor Schwann: Formulation of the Cell Theory.

• 1855 - Rudolf Virchow: Expanded the cell theory.

The Cell Theory

• Presented by Matthias Schleiden and Theodor Schwann, and expanded by Rudolf Virchow.

• The cell theory states:

  • All living organisms are composed of cells.

  • The cell is the basic unit of life.

  • All cells arise from pre-existing cells (Rudolf Virchow).

Size and Shape of Cells

• Shape and size of cells are related to their specific function.

• Some cells (e.g., Amoeba) have changing shapes, while others (e.g., nerve cells) have fixed shapes.

• Smallest cell: Mycoplasma.

• Largest cell: Ostrich egg.

• Longest human cell: Nerve cell.

Unicellular and Multicellular Organisms

• Organisms are classified as unicellular or multicellular based on the number of cells.

• Unicellular organisms: Composed of only one cell (e.g., Amoeba, Chlamydomonas, Paramoecium).

• Multicellular organisms: Composed of more than one cell (e.g., Humans, Dogs, Plants, Fungi).

Unicellular vs Multicellular

• Unicellular Organisms:

  • Represented by a single cell.

  • All activities are performed by a single cell.

  • No division of labor.

  • Short lifespan.

  • Examples: Amoeba, Paramoecium.

• Multicellular Organisms:

  • Consist of a large number of cells.

  • Single cell performs one or few activities.

  • Division of labor exists with specialized cells.

  • Long lifespan.

  • Examples: Plants, Animals, Fungi.

Division of Labor in Cells

• Cell organelles perform different functions.

• Mitochondria: Produce energy.

• Lysosomes: Clean the cell.

• ER: Synthesizes lipids and proteins.

• Golgi: Packages synthesized materials.

Common Features of All Cells

• All cells have:

  1. Plasma membrane

  2. Cytoplasm

  3. Nucleus

Plasma Membrane/Cell Membrane

• A thin, flexible boundary between the cell and its environment, composed of lipid bilayer and proteins.

• Functions:

  • Provides structural support.

  • Plays a role in cell communication.

  • Controls the movement of substances in and out of the cell.

  • Selective permeability: Allows only selected substances to pass through.

Transport Across Plasma Membrane

• Two ways of transport:

  1. Diffusion

  2. Osmosis (a special type of diffusion)

Diffusion

• Movement of substances (solid, liquid, and gas) from a region of high concentration to a region of low concentration until evenly distributed.

Importance of Diffusion

• Helps in the spread of substances throughout the cytoplasm.

• Helps in the exchange of respiratory gases between cells and the environment.

• Important for plants and animals to get nutrition, air, and water.

• Transpiration is a diffusion process.

• Flowers spread aroma through diffusion to attract insects for pollination.

Diffusion of Oxygen and Carbon Dioxide

• $CO_2$ accumulates inside the cell due to cellular activities and moves out by diffusion from high to low concentration.

• Oxygen enters cells by diffusion when its concentration is higher outside than inside.

Movement of Water

• Water moves from an area of high water concentration to an area of low water concentration.

• Osmosis: Net diffusion of water across a selectively permeable membrane.

Osmosis

• Defined as the diffusion of water or solvent across a semipermeable membrane from a region of high concentration to a region of lower concentration.

• Plasma membrane functions as the semipermeable membrane.

• Affected by the amount of substances dissolved in water (solutes).

Types of Osmosis

• Two types:

  1. Endosmosis: Osmotic entry of water into the cell when placed in a solution of low concentration.

  2. Exosmosis: Osmotic withdrawal of water from the cell when placed in a solution of high concentration.

Endocytosis

• The flexible cell membrane enables the cell to engulf food and other materials from the external environment.

• Amoeba acquires food through endocytosis.

Types of Solutions

• Based on solute concentration, there are three types of solutions:

  1. Hypertonic solution

  2. Hypotonic solution

  3. Isotonic solution

Animal Cell in Sugar/Salt Solution

• Hypotonic Solution:

  • Higher water concentration outside the cell causes water to enter by osmosis.

  • Cells swell.

• Isotonic Solution:

  • Similar water concentration inside and outside the cell.

  • No net movement of water; cell size remains constant.

• Hypertonic Solution:

  • Lower water concentration outside the cell causes water to move out by osmosis.

  • Cells shrink.

Importance of Osmosis

• Unicellular freshwater organisms and most plant cells gain water through osmosis.

• Absorption of water by plant roots is an example of osmosis.

• Active transport: Different molecules move in and out of the cell through a type of transport requiring energy.

Diffusion vs. Osmosis

• Diffusion:

  1. Spontaneous movement of substances from high to low concentration.

  2. Applicable to solids, liquids, and gases.

  3. Semipermeable membrane is not essential.

  4. Faster than osmosis.

• Osmosis:

  1. Movement of water from its high concentration to its low concentration across a semipermeable membrane.

  2. Applicable only to liquids.

  3. Semipermeable membrane is essential.

  4. Slower than diffusion.

Activity - Osmosis with an Egg

• (a) Egg in pure water:

  • Egg swells due to water entering by osmosis after removing the shell (calcium carbonate).

• (b) Egg in concentrated salt solution:

  • Egg shrinks due to water passing out of the egg into the salt solution.

Activity - Dried Raisins

• (a) Raisins in plain water:

  • Gain water and swell.

• (b) Raisins in concentrated solution:

  • Lose water and shrink.

Homework Questions

• What will happen if excess fertilizer is added to green grass lawn?

• What will happen if salt is added to cut pieces of raw mango?

• If cells of onion peel and RBC are kept in hypotonic solution:

  • RBC will burst easily, while cells of onion peel will resist bursting to some extent.

Cell Wall

• A rigid, semi-transparent, and protective covering present outside plant cells, prokaryotes, and fungi.

• Made up of cellulose in plants.

Functions of Cell Wall

• Provides structural strength to plant cells.

• Permits cells to withstand dilute external media (hypotonic solution) without bursting.

• Gives a definite shape to cells.

• Protects the cell against mechanical injury.

• Cell wall is formed of cellulose.

Plasmolysis

• Cytoplasm and cell membrane shrink away from the cell wall when plant cells are placed in a hypertonic solution.

Turgor Pressure and Wall Pressure

• Turgor pressure: Pressure exerted by the cell content against the cell wall due to endosmosis in hypotonic solution.

• Wall pressure: Pressure exerted by the cell wall against the cell content.

• When turgor pressure exceeds wall pressure, the cell wall may burst.

Activity - Plasmolysis

• Living cells, not dead cells, undergo osmosis.

The effect of Osmosis in Animal cell & Plant cell

• Plasmolysis in plant cells shown

Cell Wall vs. Cell Membrane

• Cell Wall:

  • Occurs in plant cells.

  • Lies on the outside of the cell.

  • Rigid and thick.

  • Permeable.

  • Formed of cellulose.

  • Provides protection and strength.

• Cell Membrane:

  • Occurs in plant and animal cells.

  • Lies on the outside of the cell in animals and inside the cell in plants.

  • Flexible and thin.

  • Selectively permeable.

  • Formed of lipids and proteins.

  • Holds cellular content and controls transport of substances.

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