Cell Theory and Eukaryotic Cells

• Eukaryotic cells perform different functions in different spaces or compartments, separated by membranes, allowing for a division of labor.
• Organelles are compartments within cells that carry out different functions.
• A typical animal cell contains a membrane, nucleus, and organelles, but cells in reality vary greatly
• The first recognition of cells as building blocks occurred with the invention of the microscope in the 17th century.
• Robert Hooke (1635-1703) made significant contributions to microscopy.
• Matthias Jakob Schleiden (1804-1881), Theodor Schwann (1810-82), and Rudolf Virchow (1821-1902) used microscopes to study microstructures in plant and animal tissues, leading to the understanding that tissues are subdivided into small compartments.

Cell Theory

• All living organisms consist of one or more cells.
• The cell represents the fundamental unit of life.
• New cells originate from pre-existing cells through division.
• All cells are enclosed by a plasma membrane.
• Some cells expel genetic material.
• Not all cells house DNA within a nucleus.
• All cells perform metabolism; without metabolism, an organism is not alive.
• Not all cells are capable of movement.

Cell Types

• Eukaryotic cells possess a nucleus and can be single-celled or multi-cellular.
• Plants, including some algae, which are single-celled, are eukaryotes.
• Animals are eukaryotes.
• Fungi (molds, yeasts, mushrooms) are multicellular eukaryotes.
• Protozoans are single-celled eukaryotes.
• Prokaryotic cells lack a nucleus and are always single-celled.
• Bacteria (formerly eubacteria), including photosynthetic cyanobacteria (formerly known as blue-green algae), are prokaryotes.
• Archaea (formerly archaebacteria) thrive in extreme environments, such as high salt concentrations (halophiles) and hot sulfur springs (thermoacidophiles); they are prokaryotes.
• Ancestral prokaryotes, the first cells, arose approximately 1 billion years after the Earth's formation, leading to diverse branches of life over time.

Cellular Organization

• Cellular organization includes organelles and subcellular structures.
• The cell membrane consists of a bilayer of phospholipids, cholesterol, and embedded transmembrane proteins.
• Phospholipids are amphipathic, having a hydrophilic phosphate group and hydrophobic lipid tails.
• Dynamic lipid rafts concentrate specific transmembrane proteins.

Main Roles of Cellular Organization

• Barrier: separating the inside from the outside, maintaining crucial differences, such as potential differences in neurons.
• Communication with the external environment.
• Import and export of substances.
• Acting as an electrical capacitor.

Cytoskeleton

• The cytoskeleton fills the cell like a gel.
• It comprises a network of protein fibers with diverse functions.
• Actin filaments, intermediate filaments, and microtubules are part of the cytoskeleton.

Cytoskeleton Roles

• Maintenance of cell shape and stability.
• Adaptation of cell shape.
• Facilitating cell division.
• Enabling motility
• Movement of particles within cells, such as mitochondria to the synaptic terminal.
• Cytoplasm (cytosol) requires a precise salt balance to maintain cell functions.
• The cytoplasm is an aqueous solution with a slightly basic pH (~7.2) and specific ion composition.
• The cytoplasm contains a high concentration of proteins (20-30%), including metabolic enzymes and intracellular messengers.
• Transfer RNAs (tRNAs) are present in the cytoplasm and are involved in protein synthesis.
• Free ribosomes, which produce proteins, are located in the cytoplasm.
• Inclusion bodies, which store glycogen granules, are found in the cytoplasm.

Nucleus

• The nucleus is a defining feature of eukaryotes and the most prominent organelle.
• It is surrounded by two layers of membrane, creating a nuclear envelope.
• The nucleus contains nucleoplasm, a water-based soluble solvent.
• The nucleus contains chromosomal DNA, packaging proteins (histones), and gene regulatory proteins.
• The nucleus is the site of RNA synthesis and processing (RNA splicing).
• The nucleolus is the site of ribosome synthesis (and signal recognition particle), and can capture gene regulatory proteins, regulating gene expression by silencing genes.

Endoplasmic Reticulum (ER)

• The endoplasmic reticulum (ER) is a network of interconnected membrane vesicles (cisternae) continuous with the outer nuclear membrane.
• Rough ER (rER) is studded with ribosomes and synthesizes secreted and transmembrane proteins.
• Smooth ER (sER) synthesizes lipids and steroid hormones, facilitates detoxification (in the liver), and releases glucose from the liver.

Golgi Apparatus/Complex

• The Golgi apparatus/complex consists of stacked flattened membrane vesicles.
• The Golgi apparatus modifies proteins destined for secretion and transmembrane proteins.
• rER => cis Golgi => trans Golgi => secretory vesicles.

Mitochondria

• Mitochondria are often oval or sausage-shaped, 0.5-1 x 1-2 mm.
• Mitochondria can make up to 25% of the cytoplasm.
• The double membrane of mitochondria has an inner membrane folded into cristae (high transmembrane protein content).
• Mitochondria contain circular DNA (mtDNA) and ribosomes in the matrix.
• Mitochondria have alternative genetic code in their matrix and hold their own genetic information; mutations in mitochondrial DNA can lead to disorders.

Mitochondria Functions

• Respiration/oxidative phosphorylation (electron transport chain) produces ATP (chemical energy).
• The citric acid (Krebs) cycle occurs in mitochondria.
• Heat is a byproduct of energy production in mitochondria.
• Mitochondria store Ca2+.
• Programmed cell death (apoptosis) occurs to remove specific cell types and prevent attacks on the body's structures; failure can result in autoimmune disorders.
• Mitochondrial disorders can arise from mutations in mtDNA (e.g., Kearns-Sayre syndrome) or defects in nuclear genes encoding mitochondrial proteins (e.g., hereditary spastic paraplegia).

Lysosomes

• Lysosomes are membrane-bound organelles in animal cells.
• They degrade unwanted proteins and particles taken up by the cell, as well as no longer needed membranes and organelles.
• Lysosomes use an acidic pH of 4.5-5 for optimal activity of degradative enzymes.
• They are separated into compartments with low pH.
• Macrophages use lysosomes to degrade pathogens (bacteria, viruses, and other cells).

Peroxisomes

• Peroxisomes degrade fatty acids and toxic compounds.
• Fatty acid oxidation in peroxisomes produces precursors for biosynthetic pathways.
• Oxidation in peroxisomes produces H2O2 (corrosive).
• The enzyme catalase neutralizes H2O2: 2 H2O2 => 2 H2O + O2.
• Peroxisomes are involved in the detoxification of ethanol in the liver.
• Individuals with fewer of these enzymes may remain intoxicated for longer but experience less of a hangover.
• C2H5OH + H2O2 => CH3CHO + 2 H2O.