The Fundamental Unit of Life Study Notes

Introduction to Cells

  • In 1665, Robert Hooke observed cork under a microscope and noted that it resembled a honeycomb structure containing compartments.

    • Hooke termed these compartments "cells" (from Latin "cella" meaning 'a little room').

    • This finding was pivotal as it was the first observation of living things being composed of separate units, leading to the term "cell" being used in biology to this day.

Living Organisms and Cellular Structure

5.1 What are Living Organisms Made Up of?

Activity 5.1: Observing Onion Cells

  • Peel the epidermis from the concave side of an onion bulb and immerse it in water to prevent folding or drying.

  • Prepare a slide with a drop of water and place the peel flat on it.

  • Add safranin solution and a cover slip, taking care to avoid air bubbles.

  • Observe the slide first under low power, then high power of a compound microscope.

Observation Questions

  • Can you identify any structures that resemble a figure of honeycomb or other predictable geometric layouts?

Unicellular vs. Multicellular Organisms

  • Unicellular organisms (e.g., Chlamydomonas, Paramecium, and bacteria) consist of a single cell.

  • Multicellular organisms include fungi, plants, and animals, made of multiple cells that function together.

    • Cell Division: Every multicellular organism originates from a single cell that divides to produce more cells. All cells arise from pre-existing cells.

Activity 5.2: Observing Different Cell Types

  • Perform temporary mounts of various plant parts to compare:

    1. Do all cells resemble each other in shape and size?

    2. Are there structural differences among cells from different parts of a plant?

    3. Can you observe cell structure similarities?

Historical Context of Cellular Discovery

Notable Discoveries:

  1. Robert Hooke (1665): Discovered cells in cork.

  2. Antonie van Leeuwenhoek (1674): Identified free-living cells in pond water using an improved microscope.

  3. Robert Brown (1831): Discovered the nucleus within the cell.

  4. Jan Evangelista Purkinje (1839): Coined "protoplasm" for the cell's fluid.

  5. Cell Theory by Schleiden and Schwann (1838-1839): Proposed that all living organisms are composed of cells and the cell is the unit of life.

  6. Virchow (1855): Expanded cell theory to state that all cells arise from pre-existing cells.

  7. Electron Microscope (1940): Enabled observation of complex cellular structures.

Role of Cells in Organism Structure

  • All organisms, multicellular and unicellular, are composed of cells which serve as the basic building units.

Cell Structure and Function

5.2 What is a Cell Made Up of?

5.2.1 Plasma Membrane

  • The plasma membrane is the cell's outermost covering, providing separation between the cell's interior and the external environment.

    • Characteristics:

    • Selectively permeable: allows certain materials to enter or exit.

    • Diffusion: Movement of substances from high to low concentration; used by gases like CO2 and O2 across membranes.

    • Osmosis: Movement of water through a selectively permeable membrane toward higher solute concentration.

    • Hypertonic Solution: Cell loses water (shrinkage).

    • Hypotonic Solution: Cell gains water (swelling).

    • Isotonic Solution: No net water movement across the membrane.

Activities on Osmosis

  1. Observing egg cells in hypotonic and hypertonic solutions for demonstration of osmotic effects.

  2. Observing dried raisins or apricots in water and sugar solutions.

5.2.2 Cell Wall

  • Present only in plant cells, the cell wall provides additional support and structure, mainly composed of cellulose.

    • Plasmolysis: When plant cells lose water, leading to shrinkage from the cell wall.

  • Plant cells can withstand osmotic pressure due to the structure of the cell wall.

Activity 5.6: Observing Plasmolysis

  • Mount leaf peels in water and then in concentrated sugar/salt solution to observe cell response and plasmolysis.

5.2.3 Nucleus

  • The nucleus is a membrane-bound structure containing chromosomes made of DNA, responsible for genetic information and inheritance.

    • It consists of:

    • Nuclear Membrane: Double-layer with pores allowing material transfer.

    • Chromosomes: Visible during cell division, composed of DNA and protein.

    • Genes: Functional DNA segments.

Nucleus in Prokaryotes vs. Eukaryotes

  • Prokaryotes: Lack a defined nuclear membrane, nucleus is termed a nucleoid; smaller and simpler.

  • Eukaryotes: Defined nucleus with nuclear membrane, contain extensive organelles.

5.2.4 Cytoplasm

  • Cytoplasm: Fluid inside the cell membrane containing organelles; involved in many cellular processes.

    • Eukaryotic cells have membrane-bound organelles, while prokaryotic cells do not, leading to differences in functionality between cell types.

Activities on Cytoplasm Observations

  • Observing onion and human cheek cells helps demonstrate the differences in structures, noting that the cytoplasm takes up little dye in the staining procedure.

5.2.5 Cell Organelles

Description of Key Organelles

  1. Endoplasmic Reticulum (ER)

    • Rough ER: Ribosomes present for protein synthesis.

    • Smooth ER: Involved in lipid synthesis and detoxification processes.

  2. Golgi Apparatus

    • Series of membrane-bound sacs that store, modify, and package substances synthesized in the ER.

  3. Lysosomes

    • Contain digestive enzymes for waste disposal and recycling of cellular components; involved in cell maintenance.

    • Known as