5. The fundamental unit of life | connecting-the-dots

Summary of the Chapter

The chapter introduces the cell as the fundamental unit of life. Robert Hooke first observed and named "cells" in 1665 from a thin slice of cork. The cell theory, stating that all plants and animals are composed of cells and the cell is the basic unit of life, was presented by Schleiden and Schwann and later expanded by Virchow, who suggested all cells arise from pre-existing cells.

Components of a cell

The main structural components of a cell are the plasma membrane, nucleus, and cytoplasm.

Plasma membrane is the outermost covering that is selectively permeable, regulating the entry and exit of materials.
Nucleus plays a central role in cellular reproduction and contains chromosomes (DNA and protein), which hold information for inheritance.
Cytoplasm is the fluid content containing specialized cell organelles.

Plant cells also have a rigid cell wall outside the plasma membrane, mainly composed of cellulose, which provides structural strength.

Cell organelles

The chapter also discusses various cell organelles and their functions:

Endoplasmic Reticulum (ER): A network of tubes and sheets, serving as a transport channel and a manufacturing surface for proteins (Rough ER with ribosomes) and lipids (Smooth ER).
Golgi Apparatus: Involved in the storage, modification, and packaging of substances synthesized near the ER.
Lysosomes: The cell's waste disposal system, containing digestive enzymes, also known as 'suicide bags'.
Mitochondria: The 'powerhouses of the cell,' releasing energy in the form of ATP molecules.
Plastids (in plant cells): Include chloroplasts (for photosynthesis) and leucoplasts (for storage).
Vacuoles: Storage sacs, large in plant cells, providing turgidity and rigidity.

Finally, the chapter contrasts prokaryotic cells (primitive nucleus without a membrane, no membrane-bound organelles) and eukaryotic cells (well-defined nucleus with a nuclear membrane, and membrane-enclosed organelles).

Study Notes

1. Discovery and Basic Units of Life

Discovery of the Cell:
- Robert Hooke (1665) observed cells in a thin slice of cork using a self-designed microscope and called the little compartments "cells" (Latin for 'a little room').
- Leeuwenhoek (1674), using an improved microscope, first discovered free-living cells in pond water.
- Robert Brown (1831) discovered the nucleus in the cell.
- Purkinje (1839) coined the term 'protoplasm' for the fluid substance of the cell.
Cell Theory:
- Proposed by Schleiden (1838) and Schwann (1839).
- It states that all plants and animals are composed of cells, and the cell is the basic unit of life.
- Expanded by Virchow (1855) who suggested that all cells arise from pre-existing cells.
Types of Organisms:
- Unicellular Organisms (uni = single): A single cell constitutes a whole organism, e.g., Amoeba, Chlamydomonas, Paramoecium, and bacteria.
- Multicellular Organisms (multi = many): Many cells group together to form various body parts and assume different functions, e.g., some fungi, plants, and animals.

2. Structural Organization of a Cell

Three features found in almost every cell are: Plasma Membrane, Nucleus, and Cytoplasm.

Plasma Membrane (Cell Membrane):
- Structure: Outermost covering, flexible, made up of organic molecules called lipids and proteins.
- Function: Separates cell contents from the external environment, allows or permits the entry and exit of some materials.
- Selectively Permeable Membrane: It prevents the movement of some other materials, hence the name.
- Movement of Substances:
  - Diffusion: Movement of substances like CO₂ or O₂ from a region of high concentration to low concentration across the cell membrane.
  - Osmosis: Special case of diffusion; movement of water molecules through a selectively permeable membrane from high water concentration to low water concentration.
  - Endocytosis: Flexibility of the membrane enables the cell to engulf food and other material from its external environment (e.g., Amoeba).
Cell Wall (in Plant Cells, Fungi, and Bacteria):
- Location: Rigid outer covering, lying outside the plasma membrane.
- Composition: Mainly composed of cellulose, a complex substance.
- Function: Provides structural strength to plants.
- Plasmolysis: Phenomenon where a living plant cell loses water through osmosis, causing shrinkage or contraction of cell contents away from the cell wall.
- Withstanding Media: Cell walls allow these cells to withstand very dilute (hypotonic) external media without bursting, as the wall exerts equal pressure against the swollen cell.
Nucleus:
- Location: Darkly coloured, spherical or oval, dot-like structure near the center of most cells.
- Covering: Has a double-layered covering called the nuclear membrane with pores for material transfer to the cytoplasm.
- Contents: Contains chromosomes, which are composed of DNA (Deoxyribo Nucleic Acid) and protein. Functional segments of DNA are called genes.
- Function: Plays a central role in cellular reproduction (dividing to form new cells) and directs chemical activities for cell development and maturity.
Cytoplasm:
- Description: Large region of each cell enclosed by the cell membrane, taking up very little stain.
- Content: Fluid content inside the plasma membrane, containing many specialised cell organelles.
- Organelles: Perform specific functions (e.g., making new material, clearing waste) and are enclosed by membranes in eukaryotic cells.

3. Comparing Prokaryotic and Eukaryotic Cells

Feature	Prokaryotic Cell	Eukaryotic Cell
Size	Generally small (1-10 $\mu$ m)	Generally large (5-100 $\mu$ m)
Nuclear Region	Poorly defined (no nuclear membrane), called a nucleoid (contains only nucleic acids)	Well defined and surrounded by a nuclear membrane
Chromosome	Single	More than one chromosome
Organelles	Membrane-bound cell organelles absent	Membrane-enclosed organelles present

4. Cell Organelles and Their Functions

Endoplasmic Reticulum (ER):
- Structure: Large network of membrane-bound tubes and sheets (tubules or bags/vesicles).
- Types: Rough ER (RER) has ribosomes attached to its surface; Smooth ER (SER) does not.
- Function:
  - RER: Site of protein manufacture (by ribosomes).
  - SER: Helps in the manufacture of fat molecules (lipids).
  - Transport: Serves as channels for the transport of materials (especially proteins) between the cytoplasm and nucleus.
  - Detoxification: SER in vertebrate liver cells plays a crucial role in detoxifying poisons and drugs.
  - Membrane Biogenesis: Some manufactured proteins and lipids help in building the cell membrane.
Golgi Apparatus:
- Structure: System of membrane-bound vesicles arranged in stacks called cisterns, often connected to the ER membranes.
- Function:
  - Packaging and Dispatch: Packages and dispatches materials synthesized near the ER to various targets.
  - Storage and Modification: Functions include the storage, modification, and packaging of products in vesicles.
  - Lysosome Formation: Involved in the formation of lysosomes.
Lysosomes:
- Structure: Membrane-bound sacs filled with digestive enzymes (made by RER).
- Function:
  - Waste Disposal: Acts as the waste disposal system of the cell, digesting foreign material (like bacteria) and worn-out cell organelles.
  - Suicide Bags: Known as the 'suicide bags' of a cell because they may burst and the enzymes digest their own cell during a disturbance in cellular metabolism or when the cell is damaged.
Mitochondria:
- Nickname: Known as the powerhouses of the cell.
- Function: Releases the energy required for life in the form of ATP (Adenosine triphosphate) molecules; ATP is the energy currency of the cell.
- Structure: Has two membrane coverings: a very porous outer membrane and a deeply folded inner membrane, which creates a large surface area for ATP-generating chemical reactions.
- Special Feature: They have their own DNA and ribosomes and can make some of their own proteins.
Plastids (Only in Plant Cells):
- Types:
  - Chromoplasts: Coloured plastids.
    - Chloroplasts: Plastids containing the pigment chlorophyll, which are essential for photosynthesis.
  - Leucoplasts: White or colourless plastids.
    - Primary Function: Storage of materials such as starch, oils, and protein granules.
- Special Feature: Like mitochondria, plastids also have their own DNA and ribosomes.
Vacuoles:
- Structure: Storage sacs for solid or liquid contents.
- Size: Small in animal cells; very large in plant cells (central vacuole may occupy 50-90% of cell volume).
- Function:
  - Plant Cells: Full of cell sap, providing turgidity and rigidity to the cell, and storing important substances like amino acids, sugars, organic acids, and proteins.
  - Unicellular Organisms: Food vacuole contains consumed food items (e.g., Amoeba); specialized vacuoles expel excess water and waste.