Robert Hooke's Discovery of Cells

• Observation of Cork (1665)
  • Robert Hooke, while examining a thin cork slice with a self-designed microscope, noticed that the structure resembled a honeycomb, consisting of small compartments.
  • He named these compartments "cells," derived from the Latin word meaning "a little room."
• Significance of the Discovery
  • This marked the first observation that living organisms are made up of individual units or cells.
  • The term "cell" is still widely used in biology today.

Understanding Cells

5.1 What are Living Organisms Made Up Of?

Activity 5.1

• Experiment with Onion Peel
  1. Take a small piece from an onion bulb.
  2. Using forceps, peel off the epidermis from the concave side of the onion and immediately place it in a watch glass with water to prevent folding or drying out.
  3. Transfer the peel to a glass slide with a drop of water, ensuring the peel is flat. (A thin camel hair brush may be needed.)
  4. Add a drop of safranin solution to the peel and apply a cover slip, avoiding air bubbles with a mounting needle.
  5. Observe the slide under both low and high power of a compound microscope.
• Observation Prompt
  • What structures are visible? Can they be drawn and do they resemble figure 5.2?

Unicellular and Multicellular Organisms

• Types of Organisms
  • Unicellular Organisms: Organisms like Chlamydomonas, Paramecium, and bacteria that consist of a single cell (uni = single).
  • Multicellular Organisms: Organisms consisting of multiple cells that perform different functions and make up different body parts, such as fungi, plants, and animals.
• Cell Division
  • Every multicellular organism starts from a single cell. Cells divide to create more cells through the process of cell division, originating from pre-existing cells.

Activity 5.2

• Temporary Mount Preparation
  • Practice making temporary mounts of leaf peels and the tips of onion roots.
  • Investigate the following questions:
  • (a) Do all cells appear similar in shape and size?
  • (b) Do all cells have the same structure?
  • (c) Are there differences among cells from different parts of a plant?
  • (d) What similarities exist among the cells?
• Human Cell Types
  • Explore pictures illustrating various human cells: Sperm, Bone cell, Smooth muscle cell, Blood cells, Ovum.

Cell Structure and Theory

• Commonality Across Onion Cells
  • Similar cell structures found regardless of the size of onion bulbs, indicating that all organisms are made up of cells.
• Historical Context
  • Robert Hooke (1665) was the first to discover cells in cork.
  • Antonie van Leeuwenhoek (1674) discovered free-living cells in pond water.
  • Robert Brown (1831) discovered the nucleus in cells.
  • Purkinje (1839) introduced the term "protoplasm" for the fluid in cells.
  • Cell theory (1838-1855): Proposed by Schleiden and Schwann, it states all organisms are composed of cells, and Virchow added that all cells arise from pre-existing cells.
  • The electron microscope's invention in 1940 revealed complex cellular structures and organelles.

Cell Functions

• Cell Shape and Function
  • The shape and size of cells relate to their specific functions.
  • For example, Amoeba can change shapes, while nerve cells have a fixed structure.
• Division of Labour
  • In multicellular organisms, different body parts perform distinct functions—this division of labour is also evident within individual cells.

Parts of a Cell

5.2 What is a Cell Made Up Of?

Structural Organization of a Cell

• Common Features in Cells
  • When studying a cell, three main features are typically found: plasma membrane, nucleus, and cytoplasm.

5.2.1 Plasma Membrane or Cell Membrane

• Function
  • The plasma membrane is the outer covering that separates the cell's contents from the external environment.
  • It allows selective movement of substances in and out of the cell, hence termed selectively permeable.
• Diffusion Processes
  • Diffusion: The process where substances like O2 and CO2 move across the membrane from high to low concentration areas.
  • Example: CO2 moves out when higher concentration is inside than outside.
  • Osmosis: The movement of water across the plasma membrane is a specific type of diffusion, defined by:
  • Hypotonic Solution: Surrounding medium with higher water concentration than cell; cell swells as it gains water.
  • Isotonic Solution: Equal water concentrations; no net movement.
  • Hypertonic Solution: More concentrated externally; cell shrinks as it loses water.

Activity 5.3

• Osmosis Experiment with Eggs
  • Remove an egg shell using dilute hydrochloric acid, leaving the skin intact. Place it in pure water and observe swelling after 5 minutes.
  • Place another de-shelled egg in a concentrated salt solution to observe shrinking after 5 minutes.

5.2.2 Cell Wall

• Plant Cells
  • Plant cells possess a rigid outer cell wall, mainly composed of cellulose, providing structural strength.
  • Plasmolysis: When a living plant cell loses water and shrinks away from the cell wall — this can be observed with specific activities (e.g., Rhoeo leaves experiment).

5.2.3 Nucleus

• Nuclear Structure
  • The nucleus, often visible as a darkly colored dot in cells, is surrounded by a double-layered nuclear membrane with pores allowing material transfer.
• Genetic Material
  • Contains chromosomes made of DNA, responsible for hereditary traits, organized into chromatin in non-dividing cells.

5.2.4 Cytoplasm

• Description
  • The fluid part enclosed by the plasma membrane containing organelles.
  • Notably absent in prokaryotes where it retains a simpler structure compared to eukaryotes.

5.2.5 Cell Organelles

• Overview
  • Eukaryotic cells contain membrane-bound organelles, each performing specific functions:
  • Endoplasmic Reticulum (ER):
  • Rough ER: Studded with ribosomes, aids in protein synthesis and transport.
  • Smooth ER: Functions in lipid synthesis and detoxification.
  • Golgi Apparatus: Processes, modifies, and packages proteins and lipids for distribution.
  • Lysosomes: Contain enzymes for digesting waste material and foreign substances; known as the "suicide bags" of the cell due to their potential to digest cell components during metabolism disturbances.
  • Mitochondria: Known as the "powerhouses" of the cell; generate ATP (energy currency) and possess their own DNA.
  • Plastids: Found only in plants; includes chloroplasts (for photosynthesis) and leucoplasts (for storage).
  • Vacuoles: Storage sacs, large in plant cells, playing a critical role in turgidity and storage of substances.

Cell Division

• Cell Division Types
  • Mitosis: The process by which a mother cell divides to produce two identical daughter cells for growth and tissue repair.
  • Meiosis: Involved in the production of gametes, resulting in four new cells with half the chromosome number of the mother cell.

Summary of Key Concepts

• The cell is the fundamental organizational unit of life.
• Cells are enclosed by a plasma membrane made of lipids and proteins, responsible for regulating material movement in and out.
• Plant cells have a rigid cell wall that permits resistance in hypotonic environments.
• The nucleus in eukaryotic cells guides life processes, coordinating activities within the cell.
• Organelles, such as the ER and Golgi apparatus, support diverse cellular functions essential for life and growth.
• Prokaryotes lack membrane-bound organelles, while eukaryotes contain complex structures necessary for higher-order functions.
• Cell division is critical for growth, repair, and reproduction in organisms.

Exercises

1. Compare the differences between plant and animal cells.
2. Explain prokaryotic vs. eukaryotic cells.
3. Discuss the implications of a ruptured plasma membrane.
4. Assess the role of the Golgi apparatus; what would happen without it?
5. Identify the "powerhouse" of the cell.
6. Detail how lipids and proteins for the cell membrane are synthesized.
7. Investigate how an Amoeba acquires food.
8. Define osmosis clearly.
9. Conduct a specified osmosis experiment with potato cups to observe water movement and effects.
10. Distinguish between mitosis and meiosis regarding their functions in cell division.