Cellular Structure and Organelles

Cellular Structure

• Eukaryotic cells contain various organelles, each with a specific function.
• The cell's structure is organized by the cytoskeleton, including microtubules, microfilaments, and intermediate filaments.
• Organelles are not simply suspended in the cytosol; they are anchored and moved by the cytoskeleton.

Lysosomes

• Lysosomes are membrane-bound organelles found in eukaryotic cells.
• Lysosomes contain hydrolytic enzymes to break down biomolecules like proteins, lipids, carbohydrates, and nucleic acids.
• They are crucial for waste material degradation, autophagy (recycling cellular components), and defense against pathogens.
• The low internal pH aids the hydrolytic enzymes in degrading molecules.

Vacuoles

• Vacuoles are membrane-bound sacs in plant and fungal cells, acting similarly to lysosomes.
• In plants, the central vacuole stores water and ions (K+, Cl-).

Peroxisomes

• Peroxisomes are roughly spherical organelles often with a granular or crystalline core.
• They contain enzymes like catalase, used to remove hydrogen atoms and produce hydrogen peroxide (H₂O₂).
• Peroxisomes use oxygen to break down fatty acids.
• Liver peroxisomes break down alcohol.
• Peroxisomes have a role in metabolic cooperation with other organelles such as chloroplasts and mitochondria.

Mitochondria

• Mitochondria are organelles where cellular respiration occurs.
• They have an outer and inner membrane with infoldings (cristae).
• The mitochondrial matrix contains free ribosomes.

Chloroplasts

• Chloroplasts are organelles that contain chlorophyll, enabling photosynthesis.
• Chloroplasts have thylakoid membranes where light reactions of photosynthesis take place.
• Chloroplasts are found in plants and algae.
• They possess their own genome, transcription machinery, as well as ribosomes.

Endosymbiont Theory

• Endosymbiosis proposes that mitochondria and chloroplasts originated from free-living prokaryotes that were engulfed by ancestral eukaryotic cells.
• Mitochondria and chloroplasts have their own genomes, transcription machinery, and ribosomes, supporting this theory.
• Eukaryotic cells developed from engulfing an oxygen-using non-photosynthetic prokaryote that became mitochondria, and engulfing a photosynthetic prokaryote forming chloroplasts.

Lysosomal Storage Disorders

• Lysosomal storage disorders affect ~50 genes associated with malfunctioning lysosomes. 

• Tay-Sachs disease is a lysosomal storage disorder caused by an enzyme defect that digests gangliosides(lipid), resulting in brain lipid buildup and neurological dysfunction.

• Gaucher disease causes a deficiency of the enzyme glucocerebrosidase leading to glucocerebroside build-up in the spleen, liver and bone marrow, resulting in enlarged liver and spleen, fatigue, anemia, bone pain and fractures, and easy bruising and bleeding.

The Cytoskeleton

• Organelles are not free-floating within eukaryotic cells.
• Three types of cytoskeletal fibers are: microtubules, microfilaments, and intermediate filaments. 
• Microtubules provide structural support, maintain cell shape, and play roles in long-distance intracellular transport, cell division (mitotic spindle), and movement of organelles and vesicles.
• Microfilaments are the smallest, providing support, cell shape maintenance, short-distance intracellular transport, and roles in cell movement and division.
• Intermediate filaments are rope-like, providing mechanical strength, maintaining cell shape, and anchoring organelles.

Membrane Structure

• Biological membranes are composed of a fluid bilayer of phospholipids with embedded amphipathic proteins.
• Phospholipids move laterally within the membrane plane; infrequent flip-flops occur.
• Hydrophilic heads face the exterior; hydrophobic tails face the interior.
• Integral membrane proteins pass through the bilayer.
• Peripheral membrane proteins are associated with the membrane's surface.
• Cholesterol influences membrane fluidity.
• Unsaturated fatty acids increase fluidity at lower temperatures by introducing kinks in the tails, preventing tight packing.

General Concepts

• Eukaryotic cells have internal membranes that compartmentalize their functions.
• The genetic instructions reside in the nucleus and are carried out by ribosomes.
• The endomembrane system regulates protein traffic.
• Mitochondria and chloroplasts change energy forms and carry out metabolic functions.
• The cytoskeleton organizes cell structures and activities.
• Extracellular components and connections between cells coordinate cellular activities. 
• A cell's function results from the collective action of its parts.