Unit 1: Introduction to Cell Biology & Cell Membrane

1. Introduction to Cell Biology

Cell Biology is the study of cells—their structure, function, organization, and the processes they carry out. It is foundational to all other fields of biology.

The Cell Theory

The classical cell theory, proposed by Schleiden, Schwann, and Virchow, has three main tenets:

1. All living organisms are composed of one or more cells.
2. The cell is the basic structural and functional unit of life.
3. All cells arise from pre-existing cells.

Modern cell theory adds to this, stating that cells contain hereditary information (DNA) passed on during division and that all cells have a similar basic chemical composition.

2. Ultrastructure of Prokaryotic Cells

Prokaryotic cells (from Greek: pro- "before", karyon- "nucleus") are structurally simpler than eukaryotic cells. They include Bacteria and Archaea.

[Image of the ultrastructure of a prokaryotic bacterial cell]

Key Structures:

• No Nucleus: Genetic material (a single, circular DNA molecule) is located in a region called the nucleoid. It is not enclosed by a membrane.
• Cell Wall: A rigid outer layer (outside the plasma membrane) that provides shape and protection. In bacteria, it is made of peptidoglycan.
• Plasma Membrane: A phospholipid bilayer that controls transport.
• Cytoplasm: The entire content within the cell membrane, including:
  • Cytosol: The jelly-like fluid.
  • Ribosomes: Small (70S) complexes responsible for protein synthesis.
  • Inclusions: Storage granules (e.g., for nutrients).
• No Membrane-Bound Organelles: Lacks mitochondria, ER, Golgi, etc.
• Other Features (optional):
  • Flagella: Long, whip-like tails for motility.
  • Pili/Fimbriae: Short, hair-like appendages for attachment.
  • Capsule: A sticky outer layer for protection.

3. Ultrastructure of Eukaryotic Cells

Eukaryotic cells (from Greek: eu- "true", karyon- "nucleus") are larger and more complex, with extensive internal compartmentalization. They include animals, plants, fungi, and protists.

Key Structures:

• True Nucleus: Contains the cell's genetic material (multiple linear chromosomes) enclosed within a nuclear envelope (a double membrane).
• Plasma Membrane: The outer boundary (in animal cells) or found just inside the cell wall (in plant cells).
• Cytoplasm: The region between the nucleus and the plasma membrane, consisting of:
  • Cytosol: The fluid portion.
  • Membrane-Bound Organelles: (See Unit 2 for details) e.g., Mitochondria, Endoplasmic Reticulum (ER), Golgi apparatus, Lysosomes, Peroxisomes, Vacuoles.
  • Ribosomes: Larger (80S) than in prokaryotes.
  • Cytoskeleton: A network of protein fibers (microtubules, microfilaments, intermediate filaments) for structure, movement, and transport.
• Features specific to Plants/Fungi:
  • Cell Wall: Made of cellulose (in plants) or chitin (in fungi).
  • Chloroplasts: (In plants) The site of photosynthesis.
  • Large Central Vacuole: (In plants) Maintains turgor pressure.

4. Prokaryotic vs. Eukaryotic Cells

5. Cell Membrane: Components of Biological Membranes

The cell membrane (or plasma membrane) is a dynamic, fluid barrier that separates the cell's interior from the outside environment. Its main components are:

1. Lipids (Phospholipids and Cholesterol):
   • Phospholipids: The fundamental component. They are amphipathic (hydrophilic head, hydrophobic tail) and spontaneously form a lipid bilayer.
   • Cholesterol: (In animal cells) A steroid lipid that embeds within the bilayer. It acts as a "fluidity buffer," preventing the membrane from becoming too fluid (at high temps) or too rigid (at low temps).
2. Proteins (Integral and Peripheral):
   • Integral Proteins: Penetrate the hydrophobic core of the bilayer (e.g., transmembrane proteins that span the entire membrane).
   • Peripheral Proteins: Loosely bound to the surface of the membrane (either on the cytosolic or extracellular side).
3. Carbohydrates (Glycolipids and Glycoproteins):
   • These are short carbohydrate chains found only on the outer (extracellular) surface of the membrane.
   • Glycolipid: Carbohydrate attached to a lipid.
   • Glycoprotein: Carbohydrate attached to a protein.
   • Together, they form the glycocalyx, which is crucial for cell recognition.

6. Fluid Mosaic Model

[Image of the Fluid Mosaic Model of the cell membrane]

Proposed by Singer and Nicolson in 1972, this is the accepted model for membrane structure.

The Fluid Mosaic Model describes the cell membrane as a mosaic of proteins (and other components) that are "floating" or embedded within a fluid bilayer of phospholipids.

• Fluid: The phospholipids and most of the proteins are not static; they can move laterally (sideways) within the membrane. This fluidity is essential for membrane function.
• Mosaic: The membrane is a "patchwork" of different components (lipids, proteins, carbohydrates) arranged together.

7. Cell Recognition

Cell recognition is the ability of a cell to distinguish one type of cell from another. This is vital for forming tissues and for the immune system to identify "self" vs. "non-self" (foreign) cells.

• Role of Glycocalyx: This process is mediated primarily by the diverse and complex carbohydrate chains of the glycocalyx (glycolipids and glycoproteins).
• Example (ABO Blood Groups): The A, B, and O blood types are determined by different carbohydrate antigens (glycolipids) on the surface of red blood cells.

8. Membrane Transport

The cell membrane is selectively permeable—it controls what enters and leaves the cell. Transport mechanisms are divided into two main categories:

a) Passive Transport (No Energy Required)

Moves substances down their concentration gradient (from high to low concentration).

• Simple Diffusion: Small, nonpolar molecules (like O₂, CO₂) pass directly through the lipid bilayer.
• Facilitated Diffusion: Polar molecules (like glucose) or ions (like Na⁺) cross the membrane with the help of transport proteins (either channel or carrier proteins).
• Osmosis: The diffusion of water across a selectively permeable membrane from a region of low solute concentration to high solute concentration.

b) Active Transport (Requires Energy)

Moves substances against their concentration gradient (from low to high concentration). This requires energy, usually from ATP.

• Protein Pumps: Carrier proteins that use ATP to "pump" specific ions or molecules across the membrane.
• Example (Na⁺/K⁺ Pump): A vital pump in animal cells that actively pumps 3 Na⁺ ions *out* of the cell and 2 K⁺ ions *into* the cell for every ATP molecule hydrolyzed. This maintains the cell's electrochemical gradient.

[Image of types of membrane transport: simple diffusion, facilitated diffusion, and active transport]