The Fundamental Unit of Life

What are Living Organisms Made Up of?

Robert Hooke’s Discovery

• In 1665, Robert Hooke examined a thin slice of cork using a self-designed microscope.
• He saw that the cork looked like a honeycomb with many tiny compartments.
• Cork comes from the bark of a tree.
• Hooke called these compartments “cells,” which means “a little room” in Latin.
• This was the first time anyone observed that living things are made of separate units called cells.
• The term “cell” is still used in biology today.

Conclusion

• Cells are the fundamental units of life.
• All living things are made of cells, whether they are simple or complex organisms.

More to Know About Cells

Discovery of Cells

• Robert Hooke (1665): First observed cells in a cork slice using a primitive microscope.
• Leeuwenhoek (1674): Discovered free-living cells in pond water with an improved microscope.
• Robert Brown (1831): Discovered the nucleus in cells.
• Purkinje (1839): Coined the term ‘protoplasm’ for the fluid substance in cells.
• Schleiden (1838) and Schwann (1839): Presented the cell theory, stating that all plants and animals are made of cells, the basic unit of life.
• Virchow (1855): Expanded the cell theory by stating that all cells arise from pre-existing cells.
• Electron Microscope (1940): Allowed detailed observation of cell structure and organelles.

Unicellular and Multicellular Organisms

• Unicellular Organisms: Single cell constitutes the entire organism (e.g., Amoeba, Chlamydomonas, Paramoecium, bacteria).
• Multicellular Organisms: Many cells group together to form a single organism with different functions (e.g., fungi, plants, animals).
  • Cell Division
    • All multicellular organisms come from a single cell.
    • Cells divide to produce more cells of their own kind.

Diversity of Cells

• Different Shapes and Sizes: Cells can vary in shape and size based on their function (e.g., nerve cells have a typical shape, Amoeba changes shape).
• Specialized Functions: Each cell performs basic functions necessary for life.

Division of Labour in Cells

• Multicellular Organisms: Different body parts perform different functions (e.g., heart pumps blood, stomach digests food).
• Within a Single Cell: Cell organelles (specific components within a cell) perform specific functions, such as:
  • Making new material
  • Clearing waste
• Common Organelles: All cells have the same types of organelles regardless of their function or the organism they are in.

What is a Cell Made Up of?

Structural Organization of a Cell

• Main Features of a Cell:
  • Plasma Membrane (Cell Membrane)
  • Nucleus
  • Cytoplasm

1. Plasma Membrane or Cell Membrane

• Function:
  • Outermost covering separating cell contents from the environment.
  • Selectively permeable: controls entry and exit of materials.
• Movement of Substances:
  • Diffusion: Movement from high to low concentration (e.g., CO2, O2).
  • Osmosis: Movement of water from high to low concentration through a selectively permeable membrane.

Examples of Osmosis:

1. Hypotonic Solution:
   • Higher water concentration outside the cell.
   • Water enters the cell, cell swells.
2. Isotonic Solution:
   • Equal water concentration inside and outside the cell.
   • No net movement, cell remains the same size.
3. Hypertonic Solution:
   • Lower water concentration outside the cell.
   • Water leaves the cell, cell shrinks.

Activities:

Key Points:

• Unicellular Freshwater Organisms and Plant Cells:
  • Gain water through osmosis.
  • Plant roots absorb water by osmosis.
• Plasma Membrane Structure:
  • Made of lipids and proteins.
  • Flexible and observed via an electron microscope.
• Endocytosis:
  • Cell membrane engulfs food and material from the environment (e.g., Amoeba).

2. Cell Wall

• Plant Cells:
  • In addition to the plasma membrane, plant cells have a rigid outer covering called the cell wall.
  • Composed mainly of cellulose, providing structural strength.
• Plasmolysis:
  • Occurs when a plant cell loses water through osmosis, causing the cell contents to shrink away from the cell wall.
  • Can be observed using a Rhoeo leaf in different solutions (Activity 5.6).
• Function:
  • Cell walls allow plant, fungi, and bacterial cells to withstand dilute external media without bursting.
  • Helps cells take up water by osmosis and withstand pressure.

3. Nucleus

• Observation:
  • Staining cells (e.g., with iodine or methylene blue) helps observe the nucleus.
  • Onion peel cells and cheek cells show a darkly coloured nucleus near the centre.
• Structure:
  • Double-layered nuclear membrane with pores for material transfer between nucleus and cytoplasm.
  • Contains chromosomes (visible when the cell divides) made of DNA and proteins.
  • DNA contains genes that carry hereditary information.
• Function:
  • Central role in cellular reproduction (cell division).
  • Directs chemical activities, determining cell development and form.
• Prokaryotes vs. Eukaryotes:
  • Prokaryotes: No nuclear membrane, nuclear region called nucleoid (e.g., bacteria).
  • Eukaryotes: Cells with a nuclear membrane and organized organelles.

Activities:

4. Cytoplasm

• Definition:
  • Fluid content inside the plasma membrane.
  • Contains many specialized cell organelles.
  • Takes up very little stain when observed under a microscope.
• Cell Organelles:
  • Perform specific functions for the cell.
  • Enclosed by membranes in eukaryotes.
  • Absent in prokaryotes, which lack a defined nuclear region and membrane-bound organelles.
• Viruses:
  • Lack membranes and do not show characteristics of life until they enter a living body.

5. Cell Organelles

Every cell has a membrane that separates its contents from the external environment. Complex cells, like those in multicellular organisms, need membrane-bound structures called organelles to perform various functions. Eukaryotic cells have these organelles, unlike prokaryotic cells. Let’s look at some important cell organelles.

a. Endoplasmic Reticulum (ER)

• Structure: Network of membrane-bound tubes and sheets.
• Types:
  • Rough ER (RER): Has ribosomes, makes proteins.
  • Smooth ER (SER): Makes lipids (fats), helps detoxify poisons and drugs.
• Functions:
  • Transports materials (especially proteins) within the cell.
  • Provides a surface for biochemical activities.

b. Golgi Apparatus

• Structure: Stacks of flattened sacs called cisterns.
• Functions:
  • Modifies, sorts, and packages proteins and lipids from the ER.
  • Forms lysosomes.
  • Makes complex sugars from simple sugars.

Do You Know?

• Camillo Golgi:
  • Born in 1843, studied medicine, investigated the nervous system.
  • Developed the ‘black reaction’ staining method using silver nitrate.
  • Shared the Nobel Prize in 1906 for work on the structure of the nervous system.

c. Lysosomes

• Structure: Membrane-bound sacs filled with digestive enzymes.
• Functions:
  • Digests foreign materials and worn-out cell parts.
  • Breaks down complex substances into simpler ones.
• Nickname: “Suicide bags” of the cell because they can digest the cell itself if they burst.

d. Mitochondria

• Structure: Double membrane, inner membrane is folded.
• Functions:
  • Produces energy in the form of ATP (Adenosine triphosphate).
  • Known as the “powerhouses” of the cell.
• Unique Feature: Have their own DNA and ribosomes, can make some proteins.

e. Plastids

• Types:
  • Chromoplasts: Coloured plastids, includes chloroplasts for photosynthesis.
  • Leucoplasts: White or colourless plastids, store starch, oils, and proteins.
• Chloroplasts: Contain chlorophyll for photosynthesis, have their own DNA and ribosomes.

f. Vacuoles

• Structure: Storage sacs for solids or liquids.
• Size: Large in plant cells, small in animal cells.
• Functions:
  • Provide turgidity and rigidity to plant cells.
  • Store important substances like amino acids, sugars, and proteins.
  • In single-celled organisms, help in digestion and expelling excess water and wastes.

Key Points

• Cell Structure: Cells have a specific organization of membranes and organelles.
• Functions: Perform tasks like respiration, obtaining nutrition, waste removal, and protein formation.
• Basic Unit: Cells are the fundamental structural and functional units of life.

Cell Division

New cells are formed in organisms to:

• Grow
• Replace old, dead, and injured cells
• Form gametes for reproduction

Types of Cell Division

1. Mitosis
   • Most cells divide this way for growth.
   • One mother cell divides into two identical daughter cells.
   • Daughter cells have the same number of chromosomes as the mother cell.
   • Helps in growth and tissue repair.
2. Meiosis
   • Occurs in reproductive organs to form gametes.
   • Involves two consecutive divisions.
   • Produces four new cells with half the number of chromosomes compared to the mother cell.

Why Chromosome Number Reduces in Meiosis

• To ensure the offspring have the correct number of chromosomes after fertilization.

Chapter Summary:

• The fundamental organizational unit of life is the cell.
• Cells are enclosed by a plasma membrane composed of lipids and proteins.
• The cell membrane is an active part of the cell and regulates the movement of materials between the cell’s interior and the outer environment.
• In plant cells, a cell wall composed mainly of cellulose is located outside the cell membrane.
• The cell wall enables the cells of plants, fungi, and bacteria to exist in hypotonic media without bursting.
• The nucleus in eukaryotes is separated from the cytoplasm by a double-layered membrane and directs the life processes of the cell.
• The ER functions as both a passageway for intracellular transport and a manufacturing surface.
• The Golgi apparatus consists of stacks of membrane-bound vesicles that function in the storage, modification, and packaging of substances manufactured in the cell.
• Most plant cells have large membranous organelles called plastids, which are of two types: chromoplasts and leucoplasts.
• Chromoplasts that contain chlorophyll are called chloroplasts, and they perform photosynthesis.
• The primary function of leucoplasts is storage.
• Most mature plant cells have a large central vacuole that helps to maintain the turgidity of the cell and stores important substances, including wastes.
• Prokaryotic cells have no membrane-bound organelles, their chromosomes are composed only of nucleic acid, and they have only very small ribosomes as organelles.
• Cells in organisms divide for body growth, replacing dead cells, and forming gametes for reproduction.