Meditaliano IMAT Prep

Part 1: The Two Great Cellular Architectures

The primary division of life is between Prokaryotes (Bacteria and Archaea) and Eukaryotes (Eukarya). This division is based on fundamental differences in cellular organization, most notably the eukaryotic cell's compartmentalization through a true nucleus and membrane-bound organelles.

1.1 The Prokaryotic Cell

Prokaryotic cells are structurally simpler but metabolically diverse. They lack a nucleus and extensive internal membranes.

• Nucleoid: The region (not an organelle) where the single, circular chromosome is located, highly supercoiled and organized by nucleoid-associated proteins (NAPs).
• Cell Envelope: The structures enclosing the cytoplasm. In bacteria, this is key for classification:
  • Gram-positive: Thick peptidoglycan cell wall external to a single plasma membrane.
  • Gram-negative: A thin peptidoglycan wall located in the periplasmic space between the inner and outer plasma membranes.
• Cytoskeleton: Prokaryotes possess simple cytoskeletal homologs, such as FtsZ (a tubulin homolog for cell division) and MreB (an actin homolog for cell shape).

Image 1: The Two Great Cellular Architectures

1.2 The Eukaryotic Cell & Endosymbiotic Theory

Eukaryotic cells are defined by their complex compartmentalization. This complexity is believed to have arisen, in part, from endosymbiosis, a theory proposing that mitochondria and chloroplasts were once free-living prokaryotes engulfed by an ancestral host cell.

Diagram of an Animal Cell

Diagram of a Plant Cell

Part 2: The Information Core: Nucleus & Ribosomes

2.1 The Nucleus: Structure and Transport

The nucleus is the defining feature of the eukaryotic cell. It protects the genome and isolates the processes of transcription and RNA processing from translation.

• Nuclear Envelope: A double membrane (outer and inner) separated by a perinuclear space. The outer membrane is continuous with the endoplasmic reticulum.
• Nuclear Lamina: A meshwork of lamin proteins (a type of intermediate filament) on the inner surface of the nuclear envelope. It provides structural support and helps organize chromatin.
• Nuclear Pore Complex (NPC): An intricate, massive protein channel (made of nucleoporins) that regulates all traffic into and out of the nucleus. Small molecules diffuse freely, but large molecules (like proteins and RNA) require active, selective transport.

Image 2: The Nucleus and Intracellular Traffic

2.2 The Nucleolus and Ribosome Biogenesis

The nucleolus is a non-membranous structure within the nucleus, visible as a dense spot. It is the site of ribosome biogenesis.

• It consists of three regions: the Fibrillar Center (FC), Dense Fibrillar Component (DFC) where rRNA is transcribed, and the Granular Component (GC) where ribosomal subunits are assembled.
• Eukaryotic ribosomes are 80S, composed of a 60S large subunit and a 40S small subunit. They are assembled in the nucleolus and exported to the cytoplasm.
• Ribosomes translate mRNA into protein, operating at three sites: the A site (aminoacyl-tRNA), P site (peptidyl-tRNA), and E site (exit).

Structure of a Ribosome

Part 3: The Endomembrane System: A Cellular Factory

This system is a dynamic, interconnected network of membranes responsible for protein synthesis, modification, packaging, and transport, as well as lipid synthesis.

3.1 Endoplasmic Reticulum (ER)

The ER is a vast network of cisternae and tubules continuous with the outer nuclear membrane.

• Rough ER (RER): Studded with ribosomes. As proteins are synthesized (co-translational import), they enter the ER lumen, where they are folded by chaperone proteins (like BiP), undergo N-linked glycosylation, and are subject to quality control. Misfolded proteins trigger the Unfolded Protein Response (UPR).
• Smooth ER (SER): Lacks ribosomes. Its functions include: 1) Lipid synthesis (phospholipids, steroids), 2) Detoxification of drugs and poisons (e.g., via cytochrome P450 enzymes), and 3) Calcium sequestration (storage), which is critical for cell signaling (e.g., sarcoplasmic reticulum in muscle).

Image 6: Protein Sorting & ER Quality Control

3.2 The Golgi Apparatus: Sorting and Shipping

The Golgi consists of a stack of flattened, membrane-bound sacs called cisternae. It functions as a "post office," receiving cargo from the ER and modifying, sorting, and packaging it for its final destination. This occurs via the cisternae maturation model, where cisternae themselves move and mature from the *cis* (entry) face to the *trans* (exit) face.

• Cis Golgi Network (CGN): Receives vesicles from the ER.
• Medial & Trans Cisternae: Sites of further protein modification, such as O-linked glycosylation and modification of N-linked glycans.
• Trans Golgi Network (TGN): The final sorting station. Proteins are packaged into different vesicles for delivery to the plasma membrane (secretion), lysosomes, or other organelles.

The Endomembrane System (ER, Golgi, Vesicles)

3.3 Vesicular Transport and Lysosomes

Transport between these compartments is mediated by vesicles, which are "tagged" by specific coat proteins:

• COPII-coated vesicles: Anterograde transport (ER to Golgi).
• COPI-coated vesicles: Retrograde transport (Golgi back to ER, or within Golgi).
• Clathrin-coated vesicles: Transport from the TGN to endosomes/lysosomes, and in receptor-mediated endocytosis.

Vesicle fusion with the target membrane is mediated by SNARE proteins (v-SNAREs on the vesicle, t-SNAREs on the target membrane).

Lysosomes are the cell's digestive compartments, containing acid hydrolases. They are formed when transport vesicles from the TGN (tagged with a mannose-6-phosphate signal) fuse with late endosomes.

Part 4: Energy & Metabolism Hubs

4.1 Mitochondria

The mitochondrion is the primary site of cellular respiration and ATP synthesis. It is a dynamic organelle that undergoes constant fusion and fission.

• Outer Membrane: Contains porins, large channel proteins that allow free passage of small molecules.
• Intermembrane Space: The region between the membranes. Protons ($H^+$) are pumped here by the ETC. Contains cytochrome c, a key factor in initiating apoptosis (programmed cell death).
• Inner Membrane: Highly folded into cristae, dramatically increasing surface area. It is rich in the unique phospholipid cardiolipin and is impermeable to ions. It houses the Electron Transport Chain (ETC) and ATP Synthase.
• Matrix: The innermost compartment. Contains enzymes for the Krebs Cycle and fatty acid $\beta$-oxidation, as well as mitochondrial DNA (mtDNA) and 70S ribosomes.

Image 3: Energy & Metabolism Hubs

4.2 Chloroplasts

Found in plants and algae, chloroplasts are the site of photosynthesis. They are a type of plastid.

• Three-Membrane System: An outer membrane, inner membrane, and the internal thylakoid membrane system.
• Thylakoids: Flattened, interconnected sacs where the Light-Dependent Reactions occur. Stacks of thylakoids are called grana.
• Stroma: The fluid-filled space outside the thylakoids, analogous to the mitochondrial matrix. It contains enzymes for the Calvin Cycle, chloroplast DNA, and 70S ribosomes.

Structure of a Chloroplast

4.3 Peroxisomes

Peroxisomes are small, single-membrane metabolic compartments that are involved in diverse oxidative reactions. They acquire proteins from the cytosol and can arise from ER budding or by fission of existing peroxisomes.

• Beta-oxidation: They break down very-long-chain fatty acids.
• Detoxification: They neutralize toxic substances (like alcohol) by transferring hydrogen to oxygen, forming hydrogen peroxide ($H_2O_2$).
• Catalase: They contain the enzyme catalase, which immediately breaks down the toxic $H_2O_2$ into water and oxygen.

Structure of a Peroxisome

Part 5: The Cytoskeleton: Internal Scaffolding

The cytoskeleton is a dynamic network of protein fibers responsible for cell shape, internal organization, and movement.

Image 4: The Dynamic Cytoskeleton & Motors

Motor Proteins: The Engines of the Cell

Motor proteins use energy from ATP hydrolysis to move along cytoskeletal "tracks".

• Myosins: Move along actin filaments (e.g., in muscle contraction and cytokinesis).
• Kinesins: Most move toward the plus-end of microtubules (anterograde transport, e.g., moving vesicles to the cell periphery).
• Dyneins: Move toward the minus-end of microtubules (retrograde transport, e.g., moving vesicles to the cell center). Also responsible for the bending of cilia and flagella.

Motor Proteins (Kinesin and Dynein) on a Microtubule

Part 6: Cell Junctions, Walls, and ECM (Advanced)

6.1 Animal Cell Junctions

In tissues, cells are connected to each other and to the extracellular matrix (ECM) by specialized junctions.

• Tight Junctions (Zonula Occludens): Formed by claudins and occludins. They seal the space between epithelial cells, preventing paracellular transport (leakage between cells).
• Adherens Junctions (Zonula Adherens): Formed by cadherins. They link the actin cytoskeletons of adjacent cells, forming an "adhesion belt".
• Desmosomes (Macula Adherens): Formed by cadherins (desmoglein, desmocollin). They link the intermediate filaments of adjacent cells, providing strong, spot-like adhesion.
• Gap Junctions: Formed by connexin proteins. They create channels that allow for the direct passage of small molecules and ions between adjacent cells, coupling them metabolically and electrically.

Types of Animal Cell Junctions

6.2 Plant Cell Structures

Plant cells have unique structures reflecting their sessile, autotrophic lifestyle.

• Cell Wall: A rigid outer layer composed of cellulose microfibrils embedded in a matrix of hemicellulose and pectin. Provides structural support and protection.
• Central Vacuole: A very large organelle (up to 90% of cell volume) enclosed by a membrane called the tonoplast. It stores water, maintains turgor pressure, and often contains digestive enzymes (acting as a lysosome).
• Plasmodesmata: Channels that pass through the cell walls of adjacent plant cells, connecting their cytoplasm. They are the functional equivalent of gap junctions.

Part 7: The Plasma Membrane and Transport

7.1 The Fluid Mosaic Model Revisited

The fluid mosaic model describes the plasma membrane as a fluid structure with a "mosaic" of various proteins embedded in or attached to a bilayer of phospholipids. The membrane is selectively permeable. Cholesterol molecules help maintain membrane fluidity at different temperatures.

Image 5: Advanced Cell Structures & Transport

7.2 Passive Transport: No Energy Required

Passive transport is the diffusion of a substance across a membrane with no energy investment. Substances move down their concentration gradient.

• Simple Diffusion: The net movement of small, nonpolar molecules like $O_2$, $CO_2$, and lipids directly through the lipid bilayer.
• Facilitated Diffusion: The passage of polar molecules (like glucose) and ions (like $Na^+$) with the help of specific transmembrane transport proteins (channels or carriers).
• Osmosis: The diffusion of free water across a selectively permeable membrane.

Osmosis in Animal Cells (RBCs)

In hypotonic solution, animal cells lyse (burst). In hypertonic, they shrivel.

Osmosis in Plant Cells

Plants prefer hypotonic (turgid). In hypertonic, they undergo plasmolysis.

7.3 Active Transport: Energy Required

Active transport uses energy (usually ATP) to move solutes against their concentration gradient.
• Primary Active Transport: Directly uses ATP. The sodium-potassium pump is a key example (3 $Na^+$ out, 2 $K^+$ in).
• Secondary Active Transport (Cotransport): Uses the concentration gradient created by a primary active transporter.

Diagram: Sodium-Potassium Pump (Primary Active Transport)

7.4 Bulk Transport: Vesicles for Large Molecules

Large molecules cross the membrane in bulk via vesicles, which requires energy.

• Exocytosis: The cell secretes molecules by the fusion of vesicles with the plasma membrane.
• Endocytosis: The cell takes in molecules by forming new vesicles. Includes phagocytosis ("cellular eating"), pinocytosis ("cellular drinking"), and receptor-mediated endocytosis (highly specific).

Bulk Transport: Endocytosis and Exocytosis

Part 8: Acellular Structures - Viruses (Advanced)

Viruses are infectious particles that lie on the border of life. They are obligate intracellular parasites, meaning they lack the machinery for self-replication and must hijack a host cell's resources to reproduce. They are not considered cells and thus fall outside the cell theory.

Viral Structure

• Genetic Material: The viral genome can be DNA or RNA, single- or double-stranded.
• Capsid: A protein shell that encloses and protects the genome.
• Envelope (Optional): Many viruses have a lipid envelope derived from the host cell membrane, which contains viral glycoproteins used for attachment.

Basic Structure of a Virus

Interactive Practice Quiz (Lesson 6)

Test your understanding of all topics (Advanced Structure, Junctions, Transport, Viruses). Choose the best answer for each question and then submit to see your results.