Comprehensive Notes on Eukaryotic Cells (From Transcript)

Overview of Eukaryotic Cells

• Include algae, protozoa, fungi, higher plants, and animals
• Size range: 10-100\,\mu\text{m} in diameter
• Come in many shapes and arrangements; unlike bacteria, there are no standard names for eukaryotic cell morphology

Major structural features of eukaryotic cells

• Plant cell features (composite diagram shows half plant, half animal):
  • Peroxisome
  • Mitochondrion
  • Golgi complex
  • Microfilament
  • Vacuole
  • Microtubule
  • Chloroplast
  • Cytoplasm
  • Ribosome
  • Smooth endoplasmic reticulum (SER)
  • Rough endoplasmic reticulum (RER)
  • Plasma membrane
  • Cell wall
  • Nucleolus
  • Nucleus
• Animal cell features:
  • Nucleus
  • Nucleolus
  • Golgi complex
  • Cytoplasm
  • Basal body
  • Microfilament
  • Lysosome
  • Centrosome: centriole; pericentriolar material
  • Ribosome
  • Microtubule
  • Peroxisome
  • Rough endoplasmic reticulum
  • Mitochondrion
  • Smooth endoplasmic reticulum
  • Plasma membrane

Comparative context: prokaryotes vs. eukaryotes

• Chemically similar in core components:
  • Contain nucleic acids
  • Manufacture proteins, lipids, and carbohydrates
  • Carry out similar metabolic pathways (catabolism and anabolism)
• The major differences lie in cell structures and organization

The major differences between prokaryotic and eukaryotic cells (table-style summary)

• Size of cell:
  • Prokaryotic: typically 0.2-2.0\,\mu m in diameter
  • Eukaryotic: typically 10-100\,\mu m in diameter
• Nucleus:
  • Prokaryotic: typically no nuclear membrane or nucleoli (except Gemmata)
  • Eukaryotic: true nucleus with nuclear membrane and nucleoli
• Membrane-enclosed organelles:
  • Prokaryotic: relatively few
  • Eukaryotic: present (e.g., nuclei, lysosomes, Golgi, ER, mitochondria, chloroplasts)
• Flagella:
  • Prokaryotic: simple, primitive structure; chemically simple outer composition; varies
  • Eukaryotic: complex, multiple cytoskeletal components; flagella with 9+2 microtubule arrangement
• Glycocalyx:
  • Prokaryotic: usually present as capsule or slime layer
  • Eukaryotic: may be present; carbohydrate-rich layer involved in signaling/recognition
• Cell wall:
  • Prokaryotic: typically present and chemically complex (peptidoglycan in many bacteria)
  • Eukaryotic: when present, often simpler; algae/plants have cellulose; fungi have chitin; some eukaryotes lack a wall
• Plasma membrane composition:
  • Prokaryotic: lacks sterols; different carbohydrate content
  • Eukaryotic: contains sterols; carbohydrates involved in signaling and binding; receptor sites for ligands
• Cytoplasm/cytoskeleton:
  • Prokaryotic: cytoskeleton exists but less complex
  • Eukaryotic: complex cytoskeleton (microfilaments, intermediate filaments, microtubules) with cytoplasmic streaming
• Ribosomes:
  • Prokaryotic: smaller; typically 70S
  • Eukaryotic: larger; cytoplasmic/ER-associated ribosomes are 80S (composed of 40S + 60S subunits); mitochondria/chloroplasts contain 70S ribosomes
• Chromosome/DNA:
  • Prokaryotic: typically a single circular chromosome; histones generally absent
  • Eukaryotic: multiple linear chromosomes with histones
• Cell division:
  • Prokaryotic: binary fission
  • Eukaryotic: mitosis; sexual recombination via meiosis

Locomotion in eukaryotic cells

• Not all eukaryotic cells are motile; motility can be stage-specific in development
• Motility structures:
  • Flagella: long, few in number
  • Cilia: short, numerous
• Both anchored to the membrane by a basal body
• Both are built from microtubules in a 9+2 arrangement; microtubules are hollow tubes of tubulin; provide flexible cytoskeletal support

Flagellar and ciliary differences in function and structure

• Eukaryotic flagella/cilia rotate differently from prokaryotic flagella; prokaryotic flagella rotate like a boat propeller, while eukaryotic flagella beat in a wave pattern
• Diameter of eukaryotic flagella: around 200\,\text{nm}

Cell walls in eukaryotes

• Many eukaryotic cells have cell walls; functions include maintaining shape
• Algae and plants: cellulose in walls
• Fungi: chitin (polymer of N-acetylglucosamine, NAG)
• Yeast: walls composed primarily of glucan and mannan
• Protozoa: typically lack a conventional cell wall; have a pellicle (flexible protein covering)
• Glycocalyx: a sticky layer of carbohydrates (proteins and lipids anchored to the membrane) that aids attachment and cell–cell recognition

Peptidoglycan and antibiotics

• Eukaryotic cells do not contain peptidoglycan
• Antibiotics that target peptidoglycan (e.g., penicillins, cephalosporins) disrupt cross-linking in bacteria; this allows selective targeting with generally lower toxicity to humans

Plasma membrane: structure and transport

• Plasma membrane is structurally similar to bacterial membranes
• Eukaryotic membranes contain carbohydrates for signaling and binding; also serve as receptor sites and adhesion sites for bacteria/viruses
• Membranes contain sterols to reinforce against osmotic lysis
• Transport modes:
  • Simple diffusion
  • Facilitated diffusion
  • Active transport
  • Passive transport
  • Endocytosis

Endocytosis: engulfing large particles

• Three main types:
  • Phagocytosis: membrane extensions (pseudopods) surround target and internalize it
  • Pinocytosis (cellular drinking): membrane folds inward to create a vesicle
  • Receptor-mediated endocytosis: ligand binds receptor, triggering uptake; a common route for viral entry

Internal structure: cytoplasm and cytoskeleton

• Cytoplasm: everything inside the plasma membrane and outside the nucleus
• Cytoskeleton:
  • Network of microfilaments, intermediate filaments, and microtubules
  • Provides shape, organization, and tracks for movement of cellular products

Organelles: membrane-bound structures with specialized functions

• Note: not every cell has every organelle described

The nucleus (eukaryotic hallmark)

• Found only in eukaryotic cells
• Shape: spherical or oval; often the largest organelle
• Nucleus contains most genetic material and is surrounded by a double membrane called the nuclear envelope
• DNA associates with histones to form chromatin when not replicating; histones regulate transcription by keeping DNA less accessible when needed
• Nucleolus(es): sites of ribosomal RNA (rRNA) synthesis; rRNA is a key ribosome component
• Nuclear pores: permit regulated access to DNA; transcription in nucleus and translation in cytoplasm are spatially separated

The nucleus: visual features

• Nucleolus, Nuclear envelope, Nuclear pores, Chromatin

Ribosomes

• Function: synthesize proteins
• Eukaryotic cytoplasmic/ER-associated ribosomes form the 80S ribosome, consisting of subunits 40S + 60S
• Free ribosomes synthesize proteins for the cytoplasm
• Rough endoplasmic reticulum (RER) ribosomes synthesize proteins that are often exported or inserted into membranes
• Mitochondrial ribosomes are 70S, suggesting a prokaryotic origin

Endoplasmic reticulum (ER)

• Network of membranous sacs (cisternae) attached to the nuclear envelope
• Rough ER: studded with ribosomes; proteins synthesized into the ER lumen and modified (folding, lipid and carbohydrate attachment, etc.)
• Smooth ER: no ribosomes; synthesizes phospholipids, fats, and sterols

Golgi complex

• Stacks of cisternae (like pita bread); acts as the cell’s post office
• Proteins transported between cisternae via transport vesicles; vesicles bud from one cisterna and fuse with the next
• Within cisternae, proteins undergo further modifications
• Final products are packaged into secretory vesicles for delivery to their destinations

Mitochondria

• Found throughout the cytoplasm; number varies by cell type
• Structure: double membrane with highly folded inner membrane; cristae increase surface area; matrix fills the inner compartment
• Function: generation of ATP (energy) via oxidative phosphorylation
• Contain some DNA and ribosomes; support endosymbiotic origin theory

Lysosomes

• Formed by budding off the Golgi complex
• Single membrane; lack internal membrane-bound structures
• Contain over 40 digestive enzymes for breaking down organic molecules
• Important in immune defenses (to be discussed later)

Peroxisomes

• Similar to lysosomes but smaller; arise by division of existing peroxisomes
• Detoxify cells by oxidizing toxic compounds into less-toxic forms (e.g., alcohol → H2O2 → H2O + O2)

Vacuoles

• Membrane-bound cavities in the cytoplasm; size variable (roughly 5-90% of total cell size)
• Derived from the Golgi complex
• Functions vary by cell type: storage of sugars, organic acids, proteins, energy sources, waste products, toxins, and water

Chloroplasts (in algae and green plants)

• Contain chlorophyll and the enzymes needed for photosynthesis
• Structure includes thylakoids organized into grana; chlorophyll is stored here for light capture
• Like mitochondria, chloroplasts have 70S ribosomes and their own DNA; they replicate independently

Endosymbiotic theory: why eukaryotes are complex

• Proposes that eukaryotic cells arose from simpler prokaryotic ancestors via endosymbiosis
• Key idea: larger prokaryotes engulfed smaller bacteria; rather than digesting them, they formed a symbiotic relationship
• Over time, engulfed bacteria lost much of their DNA and became organelles (mitochondria, chloroplasts); some also contributed to organelles like flagella in some lineages
• Multiple such events could explain the origin of several organelles

Evidence supporting the endosymbiotic theory

• Mitochondria and chloroplasts resemble bacteria in size and shape
• They contain circular DNA, similar to bacterial genomes
• They reproduce independently of the host cell (binary fission-like replication)
• Mitochondrial and chloroplast ribosomes resemble prokaryotic ribosomes and are inhibited by antibiotics that target prokaryotic ribosomes

Summary: key takeaways

• Eukaryotic cells are complex, larger, and organize multiple membrane-bound organelles to compartmentalize functions
• The nucleus safeguards genetic material and coordinates transcription; the cytoskeleton provides structure and transport pathways
• Energy production centers (mitochondria) and, in plants/algae, photosynthetic centers (chloroplasts) exemplify endosymbiotic origins
• Endocytosis allows uptake of large particles and receptor-mediated pathways enable specific uptake (including viral entry)
• Differences from prokaryotes include the presence of a nucleus, membrane-bound organelles, a more complex cytoskeleton, and linear chromosomes with histones