Cell Structure and Function - MANS 433

Cell Structure and Function

• Course: MANS 433
• Instructor: Adam Gromley, Ph.D.
• Email: [email protected]
• Reading Assignments:
  • Essential Cell Biology, Chapter 1
  • Essential Cell Biology, Chapter 11
  • Marks' Basic Medical Biochemistry, Chapter 10

Lecture Objectives

• Identify the lipid, protein, and carbohydrate components of the plasma membrane
• Describe the basic functions of the plasma membrane, including selective permeability, cellular communication, and physical barrier functions
• Identify the major ions found inside the cell and in the extracellular environment, as well as explain the variations of ion concentrations across cell membranes
• Identify the six major membrane-bound organelles of the human cell
• Describe the structure and major functions of each organelle
• Identify genetic diseases associated with dysfunction of the nuclear lamina, mitochondria, lysosomes, peroxisomes, and Golgi apparatus
• Explain the molecular process affected in each
• Apply the knowledge to relevant clinical concepts

Cells

• Cells are small, membrane-enclosed units filled with a concentrated aqueous solution of chemicals
• They have the ability to create copies of themselves by growing and dividing in two
• A cell is the smallest unit of life

Classifications of Living Organisms

• All living organisms can be classified into two basic groups:
  • Prokaryotes
  • Eukaryotes

Components of Prokaryotic Cells (Bacteria)

• Typically possess a flagellum for movement
• Lack membrane-bound organelles
• DNA is in the cytoplasm, connected to the cell membrane
• Nucleoid
• Cell membrane is enclosed by a cell wall

Bacterial Cell Wall

• Composed of peptidoglycan (murein)
• Mesh-like layer with carbohydrate backbones that are cross-linked by peptide bridges
• Some classes of antibiotics inhibit bacterial cell wall formation, preventing peptidoglycan synthesis
• Examples:
  • Beta-lactams
  • Penicillin
  • Glycopeptide antibiotics
  • Vancomycin

Hierarchy of Multicellular Organisms

• Organs are comprised of tissues
• Tissues are made up of specialized cells
• Cells in organs perform specific functions
• Different cell types have similar components

Components of Eukaryotic Cells (Animal Cells)

• Plasma Membrane
• Membrane-enclosed organelles
• Nucleus
• Mitochondria
• Lysosomes
• Peroxisomes
• Endoplasmic Reticulum
• Golgi Apparatus
• Cytoskeleton

Prokaryotic vs. Eukaryotic Cells

• Prokaryotes are unicellular
• Eukaryotes are multicellular
• Prokaryotic cells have a cell wall
• Eukaryotic cells have a cell membrane
• Prokaryotic cells lack membrane-bound organelles, while eukaryotic cells have many
• Prokaryotic DNA is not enclosed in a nucleus. Eukaryotic DNA is enclosed in a nucleus
• Prokaryotic cells are typically smaller than eukaryotic cells

Plasma Membrane

• Sometimes referred to as plasmalemma
• Surrounds the cell and separates the cell's contents from the outside environment
• Composed of a phospholipid bilayer with: Two hydrophobic fatty acid tails and a hydrophilic head
• Selectively permeable to ions and organic molecules
• Associated proteins include:
  • Integral membrane proteins (permanently associated)
  • Peripheral membrane proteins (transiently associated)

Plasma Membrane: Major Components

• Lipids: About 50% of membrane volume, including
  • 75% phospholipids
  • 20% cholesterol
  • 5% glycolipids
  • Faces the outside (extracellular) environment
• Proteins:
  • ~ 50% of membrane volume

Lipids

• Three main families found in plasma membranes:
  • Phospholipids (most abundant)
  • Glycolipids (lipid attached to a sugar molecule)
  • Cholesterol (about 20% of the lipid composition of the plasma membrane)

Phospholipids

• Amphipathic (possess both hydrophobic and hydrophilic regions)
• Consist of five parts:
  • Hydrophilic head (polar)
  • Glycerol
  • Phosphate group
  • Two nonpolar hydrocarbon tails (originating from fatty acids)

Phospholipids in the Plasma Membrane

• Phosphatidylethanolamine
• Phosphatidylserine
• Phosphatidylcholine
• Sphingomyelin

Phospholipid Bilayer

• Phospholipids form a bilayer in aqueous solution
• Bilayer is asymmetrical
• Outer leaflet faces the extracellular environment
• Inner leaflet faces the cytoplasm
• Lipid bilayer is a two-dimensional fluid structure
• Individual phospholipids can exchange positions within the same monolayer (lateral diffusion) but rarely between layers (flip-flopping)

Glycolipids

• Backbone is a sphingosine molecule
• Contains a short carbohydrate molecule
• Found in the outer leaflet of the plasma membrane
• Have important roles in cell recognition
• Examples:
  • Cerebrosides
  • Gangliosides

Membrane Lipids and Signal Transduction

• Lipids found in the plasma membrane function in many different types of cellular signaling pathways

Cholesterol

• Constitutes roughly 20% of the lipid composition of the plasma membrane
• Fills the spaces between phospholipids
• Makes the plasma membrane more rigid and less permeable

Proteins in the Plasma Membrane

• Four ways proteins are associated with the plasma membrane
  • Transmembrane
  • Monolayer-associated
  • Lipid-linked
  • Protein-attached

Transmembrane Proteins

• Span both layers of the plasma membrane
• Four Classes:
  • Transporters
  • Anchors
  • Receptors
  • Enzymes

Fluidity of Membrane Proteins

• The plasma membrane is a fluid structure
• Membrane proteins can move laterally within the membrane

Functions of the Plasma Membrane

• Communication (has receptors for responding to molecular signals)
• Physical barrier (phospholipid bilayer separates interior and exterior)
• Intercellular connection (establishes a flexible boundary, protects cellular content, and supports cell structure)
• Selective permeability (regulates entry and exit of ions, nutrients, and waste molecules)

Lipid Bilayers and Their Impermeability to Solutes and Ions

• Small nonpolar molecules easily diffuse across the lipid bilayer
  • Examples:
    • Oxygen
    • Carbon dioxide
    • Nitrogen
  • Benzene
• Small uncharged polar molecules can cross the bilayer
• Water
• Ethanol
• Large uncharged polar molecules and ions require alternative methods to enter the cell
  • Examples:
    • Glucose
    • Amino acids
    • Na+
    • K+
    • H+
    • Cl-

Ion Concentrations Inside/Outside a Cell

• Ion concentration inside a cell differs from outside
• This difference creates an electrochemical gradient

Membrane-Enclosed Organelles

• A list of organelles located within eukaryotic cells.

Nucleus

• The most prominent organelle
• Stores DNA (information storage)
• Contains a nuclear envelope/membrane (two lipid bilayers; inner and outer nuclear membrane)
• Nuclear pore complex
• Chromatin
• Nucleolus

Nuclear Envelope Dysfunction (example: Progeria)

• Very rare, fatal childhood disease characterized by premature aging
• Associated with mutations in the nuclear lamina gene (lamin A)

Mitochondria

• Membrane-bound organelles (less than 1µm)
• Enclosed by two membranes (outer and inner)
• Inner membrane has cristae (in-foldings to increase surface area)
• Contains the matrix (space enclosed by inner membrane)
• Many copies per cell, the quantity depending on the cell type's metabolic needs

Functions of Mitochondria

• "Power plants of the cell"; produces ATP (energy)
• Oxidative phosphorylation
• Metabolic and biosynthetic processes, including
  • Beta oxidation of fatty acids
  • Heme synthesis
  • Steroid synthesis
• Apoptosis (programmed cell death)

Unique Characteristics of Mitochondria

• Self-replicate via fission
• All mitochondria in an organism originate from the mother
• "Non-Mendelian inheritance" (mDNA, not from both parents)
• Encodes some proteins required for function. The rest are encoded in the nuclear DNA.

Mitochondrial Dysfunction (examples, symptoms, etc)

• Neuromuscular disease symptoms
• Mitochondrial defects, especially if high-energy metabolism is involved
• Caused by mutations in mitochondrial or nuclear DNA
• Vaguely defined symptoms

Lysosomes

• Small (less than 0.5µm), membrane-bound organelles
• Contain hydrolytic enzymes (digestive enzymes)
• "Stomach of the cell"
• Breaks down various substances and organelles
• Important for innate and adaptive immunity (pathogen digestion by phagocytosis)

Lysosomal Dysfunction

• lysosomal storage diseases cause abnormal accumulation of substances inside the cell. This may increase the affected tissue’s mass (Brain in neurodegeneration for instance)
• Affects children often and causes them to die after a few years.

Tay-Sachs Disease

• Deficiency in the enzyme hexosaminidase A (Hex A)
• Enzyme normally breaks down ganglioside in the brain
• Accumulation of ganglioside results in gangliosides neuronal cell death
• Infantile Tay-Sachs: Neurological damage starts around 3-6 months and progresses until the child dies between 2-4 years old

Hurler Syndrome

• Defect in the enzyme a-L-iduronidase
• Enzyme that breaks down glycosaminoglycans (GAGs)
• Accumulation of undigested GAGs within the lysosomes
• Affected individuals exhibit skeletal abnormalities and intellectual disabilities

Peroxisomes

• Small, membrane-bound vesicles similar to lysosomes
• Contain enzymes for oxidative reactions—including catalase (to convert H2O2 to H2O + O2)
• Involved in the breakdown of very long-chain fatty acids, involved in various metabolic and biosynthetic processes
• Involved in degradation of substances like ethanol.

Peroxisomal Dysfunction (example: Zellweger Syndrome).

• Autosomal recessive disorders from mutations in peroxins
• Results in the accumulation of very long-chain fatty acids.
• Zellweger syndrome occurs with impaired brain development, liver and kidney diseases
• Diagnosed (often) through fatty acid levels in the blood
• Usually fatal within the first year of life.

Endoplasmic Reticulum (ER)

• Membrane-bound system of interconnected sacs and tubes
• Continuous with nuclear membrane
• Two types:
  • Smooth ER (major site for new membrane synthesis)
  • Rough ER (large areas associated with ribosomes; make secretory proteins)
• Sarcoplasmic Reticulum (smooth ER in muscle cells) -important for calcium storage
• Unfolded protein response (UPR)—quality control of newly synthesized proteins; commits “cellular suicide” when proteins do not fold properly.

Protein Translation in the ER

• Proteins destined for the plasma membrane or incorporation into vesicles are synthesized
• A signal sequence directs the ribosomes to the ER to ensure correct processing

Golgi Apparatus

• Located near the nucleus; ER-derived
• Stacks of flattened membrane-enclosed sacs (cisternae)
• Has polarity: cis face (receives) and trans face (releases)
• Modifications, packaging, and sorting of proteins and enzymes destined for lysosomes or plasma membranes

Protein Trafficking and Sorting

• Different pathways for transporting proteins in the cell, including packing into vesicles, transport to different organelles (like lysosomes), and secretion.

Golgi Apparatus Dysfunction (example: I-cell disease)

• Rare inherited condition clinically characterized by defective physical growth and intellectual disabilities
• Deficiency of a phosphorylating enzyme (normally found in the Golgi apparatus)
• Consequently, lysosomal enzymes coming from the rough ER are not phosphorylated
• Unphosphorylated enzymes aren't directed to lysosomes; they are released outside the cell
• Lysosomes lack necessary enzymes; accumulation of undigrated proteins (secreted)

This quiz covers key concepts from Essential Cell Biology and Marks' Basic Medical Biochemistry, focusing on the structure and functions of the plasma membrane and organelles. Students will explore topics such as membrane composition, ion concentrations, and genetic diseases related to organelle dysfunction.