iesc105

Introduction to Cells

  • Historical Context

    • Robert Hooke observed cork, noting it resembled honeycomb compartments in 1665 through his self-designed microscope.

    • Hooke coined the term "cells" from the Latin word meaning 'a little room'.

    • This observation marked a significant milestone in biology, highlighting that living organisms consist of discrete units known as cells.

What are Living Organisms Made Up of?

  • Activity 5.1: Observing Onions

    • Peel the epidermis from the inner layer of an onion bulb using forceps.

    • Place the peel in a watch glass of water to prevent drying.

    • Transfer a flat peel piece to a glass slide with water, apply a drop of safranin, and cover with a slip.

    • Observe under a microscope to identify structures like in Fig. 5.2.

Types of Organisms

  • Unicellular vs. Multicellular Organisms

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

    • Multicellular organisms, such as fungi and plants, are composed of many cells that perform distinct functions.

    • All multicellular organisms originate from a single cell due to cell division (cells arise from pre-existing cells).

Activity 5.2

  • Temporary mounts of various plant parts can be prepared to observe differences in size, shape, and structure among cells.

    • Questions for reflection:

    • (a) Do all cells appear similar in shape and size?

    • (b) Do all cells have the same structure?

    • (c) What differences can be identified among cells from varying plant tissues?

    • (d) What similarities exist among these cells?

Historical Discoveries Relating to Cells

  • Significant Figures:

    • 1665 - Robert Hooke discovered cells in cork.

    • 1674 - Leeuwenhoek discovered free-living cells in pond water utilizing an improved microscope.

    • 1831 - Robert Brown identified the nucleus within cells.

    • 1839 - Purkinje termed 'protoplasm' for the cell's fluid substance.

    • 1838-1839 - Schleiden and Schwann proposed the cell theory: all organisms are composed of cells, which are the basic units of life.

    • 1855 - Virchow added that all cells come from pre-existing cells.

    • 1940 - Electron microscope invention allowed complex cell structures to be observed.

Characteristics of Cells

  • Cells can exist independently as in the case of Amoeba, while others form complex multicellular structures.

  • The structure and function of cells are linked, and variations in shape reflect their specific roles.

Cell Functions and Organization

  • Basic functions are divided among specialized organelles within cells, each carrying out particular tasks that contribute to overall cellular function.

Structural Organization of a Cell

  • Key features visible in nearly all cells under a microscope:

    • Plasma Membrane

    • Nucleus

    • Cytoplasm

5.2.1 Plasma Membrane

  • This is the outer layer separating the cell's interior from its environment, characterized by being selectively permeable.

  • It regulates the entry and exit of substances through diffusion and osmosis.

    • Diffusion: Movement of substances from high to low concentration (e.g., CO2 exit and O2 entry).

    • Osmosis: Specific for water movement across the membrane.

Effects of Solutions on Cells

  1. Hypotonic Solution: Higher water concentration than the cell; water enters, swelling the cell.

  2. Isotonic Solution: Equal water concentration; no net movement, cell size remains the same.

  3. Hypertonic Solution: Lower water concentration than the cell; water exits, causing cell shrinkage.

Activity 5.3

  • Explore the effects of different solutions on an egg and dried fruits to observe osmotic phenomena.

5.2.2 Cell Wall

  • Plant cells possess a rigid cell wall made primarily of cellulose, which provides structural support and allows for specific osmotic responses (plasmolysis).

Activity 5.6

  • Observe plasmolysis in living vs. dead cells.

5.2.3 Nucleus

  • The nucleus, containing genetic material and surrounded by a nuclear membrane, is critical for cellular function and reproduction.

  • Chromosomes within the nucleus carry DNA, forming the basis of hereditary information.

    • Prokaryotes: Cells lacking a defined nucleus (nucleoid).

    • Eukaryotes: Cells with a defined nuclear membrane.

5.2.4 Cytoplasm

  • The cytoplasm is the region within the plasma membrane that houses organelles.

  • Differences in cellular organization exist between prokaryotic and eukaryotic cells; the former lacks membrane-bound organelles.

5.2.5 Cell Organelles

  • Organelles, membrane-bound structures within eukaryotic cells, facilitate various biochemical processes:

    • Endoplasmic Reticulum (ER): Network of membranes with two types:

    • Rough ER (RER): Contains ribosomes, synthesizing