Unit 3: Nucleus, Chromatin, and Membranes

Nucleus: Structure and Function

The nucleus is the "control center" of the eukaryotic cell. It houses the cell's genetic material (DNA) and coordinates all cellular activities like growth, metabolism, and reproduction.

Structure of the Nucleus

• Nuclear Envelope: A double membrane (two phospholipid bilayers) that encloses the nucleus, separating its contents from the cytoplasm. The outer membrane is continuous with the Endoplasmic Reticulum.
• Nuclear Pores: Large protein complexes that perforate the nuclear envelope. They regulate the passage of molecules (like mRNA, proteins, and ribosomes) between the nucleus and the cytoplasm.
• Nucleoplasm: The gel-like substance inside the nucleus, similar to cytoplasm.
• Nucleolus: A dense, non-membrane-bound structure within the nucleus. Its primary function is ribosome biogenesis (synthesizing rRNA and assembling ribosomal subunits).

Functions of the Nucleus

1. Genetic Storage: Contains the cell's chromosomes (DNA).
2. Control of Gene Expression: Regulates which genes are "turned on" (transcribed into mRNA).
3. DNA Replication: Site of DNA duplication before cell division.
4. Transcription: Site where mRNA is synthesized from a DNA template.
5. Ribosome Assembly: The nucleolus assembles ribosomal subunits.

Chromatin and Chromosomes

Inside the nucleus, DNA is not naked; it is highly organized and compacted by proteins.

Definitions:

• Chromatin: The complex of DNA and proteins (primarily histones) that forms chromosomes. This is the "unpacked" state of DNA found during interphase.
• Chromosomes: Highly condensed, thread-like structures of chromatin that become visible during cell division (mitosis and meiosis).

Levels of DNA Compaction:

1. Nucleosome: The basic unit of chromatin. It consists of DNA wrapped around a protein core of eight histone molecules (an octamer).
2. Solenoid: Nucleosomes are further coiled into a 30-nm fiber.
3. Loops & Scaffolds: These fibers are arranged in loops, which are anchored to a protein scaffold, leading to the fully condensed chromosome.

Types of Chromatin:

• Euchromatin: A loosely packed, "beads-on-a-string" form of chromatin. It is rich in genes and is transcriptionally active.
• Heterochromatin: A tightly packed, condensed form. It is transcriptionally inactive.

Components of Biological Membranes

Biological membranes (like the plasma membrane and organelle membranes) are essential for separating the cell from its environment and for compartmentalizing functions.

The main components are:

1. Phospholipids:
   • Form the fundamental lipid bilayer.
   • They are amphipathic:
     • Hydrophilic head: Phosphate group (polar, faces water).
     • Hydrophobic tail: Two fatty acid chains (nonpolar, face inwards).
2. Proteins:
   • Integral Proteins: Embedded within the bilayer, often spanning it entirely (transmembrane proteins). Function as channels, pumps, or receptors.
   • Peripheral Proteins: Loosely bound to the surface of the membrane (either inner or outer), often attached to integral proteins. Function as enzymes or anchors.
3. Cholesterol:
   • Found in animal cell membranes, nestled between phospholipids.
   • Functions as a fluidity buffer: at high temps, it restrains movement; at low temps, it prevents tight packing (freezing).
4. Carbohydrates:
   • Found only on the outer surface of the plasma membrane.
   • Covalently bonded to proteins (forming glycoproteins) or lipids (forming glycolipids).
   • This carbohydrate layer is called the glycocalyx.

Fluid Mosaic Model

Proposed by S.J. Singer and G.L. Nicolson in 1972, this model describes the structure of the cell membrane.

Key Tenets of the Fluid Mosaic Model:

1. Fluid: The membrane is a "fluid" structure. The phospholipids and many of the proteins can move laterally (sideways). It is not a rigid wall.
2. Mosaic: The membrane is a "mosaic" of various components—phospholipids, proteins, cholesterol, etc.—all fitted together.

Cell Recognition

Cell recognition is the ability of a cell to distinguish one type of cell from another. This is crucial for tissue formation, immune responses, and development.

• Mechanism: Mediated primarily by the glycocalyx (the glycoproteins and glycolipids on the cell surface).
• Function: The unique carbohydrate patterns act like "cellular name tags" or "ID badges."
• Example: ABO Blood Groups. The A, B, and O blood types are determined by different carbohydrate chains (antigens) on the surface of red blood cells. Your immune system recognizes its "self" antigens and attacks "foreign" antigens.

Membrane Transport

The cell membrane is selectively permeable—it controls what enters and leaves the cell.

Passive Transport (No Energy Required)

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

• 1. Simple Diffusion:
  • Small, nonpolar molecules (e.g., O₂, CO₂, steroid hormones) pass directly through the lipid bilayer.
• 2. Facilitated Diffusion:
  • Polar or charged molecules (e.g., glucose, ions like Na⁺, K⁺) cross the membrane with the help of transport proteins.
  • Channel Proteins: Form a hydrophilic pore (e.g., ion channels, aquaporins for water).
  • Carrier Proteins: Change shape to shuttle specific molecules across (e.g., glucose transporter).
• 3. Osmosis: A specific type of diffusion—the movement of water across a selectively permeable membrane from a region of high water concentration to a region of low water concentration.

Active Transport (Energy Required)

Movement of substances against their concentration gradient (from low to high concentration). This requires energy, usually in the form of ATP.

• 1. Primary Active Transport:
  • Energy from ATP hydrolysis is used directly to "pump" a substance.
  • Example: Sodium-Potassium (Na⁺/K⁺) Pump. Pumps 3 Na⁺ ions out of the cell and 2 K⁺ ions into the cell, maintaining the electrochemical gradient.
• 2. Secondary Active Transport (Cotransport):
  • Uses the concentration gradient established by a primary pump as an indirect energy source.
  • Example: A proton pump (primary) pumps H⁺ out. The H⁺ then diffuses back in (down its gradient) and "drags" a sucrose molecule with it (against its gradient).

Bulk Transport

Transport of large molecules or large amounts of material across the membrane using vesicles. This is an active process that requires energy.

• Endocytosis (Taking In):
  • Phagocytosis ("Cell Eating"): Engulfing large solid particles (e.g., bacterium).
  • Pinocytosis ("Cell Drinking"): Engulfing extracellular fluid and dissolved solutes.
  • Receptor-Mediated Endocytosis: Specific uptake of molecules (ligands) that bind to receptors on the cell surface.
• Exocytosis (Releasing Out):
  • Vesicles (often from the Golgi) fuse with the plasma membrane to release their contents outside the cell.
  • Example: Secretion of hormones, neurotransmitters, or enzymes.