Cell Biology and Cytoskeleton

• Notes on topics 5, 6, 8, and 9

Topic 5: Cell Biology

• Eukaryotic endomembrane systems consist of interconnected organelles in the cytoplast
• Endomembrane system divides the cell into compartments for cellular functions
• Members routinely exchange materials including phospholipid bilayer
• Mitochondria and chloroplasts are "non-members" of the endomembrane system

Eukaryotic Cytoskeleton

• Microfilaments made of actin protein, facilitate movement like contraction, crawling, and pinching
• Vesicles are small phospholipid bilayer-bound sacs that are hydrophilic inside
• Exocytosis releases materials out of the cell
• Endocytosis brings materials into the cell through vesicle formations
• Actin prevents vesicles from binding to the plasma membrane
• Extended actin helps vesicles empty cargo via exocytosis or load cargo via endocytosis
• Microtubules, the largest cytoskeletal elements that are made of tubulin proteins help moves materials throughout the cell

Topic 6: Complex Multicellular Organisms

• Single-celled organisms perform all functions without specialized body parts
• Simple multicellular organisms consist of multiple grouped cells
• Cells can be separated and grow independently, lacking specialization
• Complex multicellular organisms consist of specialized cells that cannot exist independently
• Homeostasis maintains stable internal conditions
• Communication among cell groups is essential for homeostasis in multicellular organisms
• Stimulus causes changes in homeostatic variables
• Sensors detect changes in cells or tissues
• Integrators process information by comparing it to a set-point, deciding if adjustments are needed
• Effectors, such as target cells, bring homeostasis back to normal using messenger signals
• The effect opposes the original stimulus
• Ligands or hormones are signaling molecules from signaling cells
• Receptor proteins are molecules on a target cell that bind to ligands

Local Communication

• Paracrine communication occurs quickly and easily without long distances
• Neurons are not limited by distance or message type
• Skin uses endocrine signaling which takes time to deliver messages
• Receptors respond specifically to certain types of messages
• Ligands are signals received by target cells that trigger a response
• Hormones are ligands transported over long distances (endocrine)

Hormone Types

• Peptide, amino acid, and steroid hormone

Steroid Hormone

• Structure: Primarily modified from cholesterol
• Examples: testosterone, cortisol, and thyroxine
• Requires a transport protein, with albumin as the main transporter
• Hydrophobic
• Target cell receptors are typically internal and lipid-based, allowing direct passage through the plasma membrane

Amino Acid-Derived Hormones

• Structure: Produced from 1-2 amino acids, such as tyrosine and tryptophan
• Examples: epinephrine, adrenalin, acetylcholine, melatonin
• Freely dissolve in blood
• Hydrophilic

Peptide Hormones

• Structure: Shorter amino acid chains, less than 50
• Examples: insulin, oxytocin
• Freely dissolve in blood
• Hydrophilic
• Target cell receptors are located outside the cell

Topic 8: Direct Contact

• Molecules on one cell's surface are recognized by receptors on an adjacent cell
• Gap junctions in cardiac muscle are an example
• Direct contact allows neighboring cells to work as a unit

Paracrine Signaling

• Releases signals from a cell that affect neighboring cells, typically fewer than 20 cells away
• Coordinates responses among neighboring cells
• Examples: norepinephrine, epinephrine, acetylcholine

Endocrine Signaling

• Hormones released travel through the circulatory system to affect cells throughout the body
• Affects more than 20 cells away
• Coordinates responses among tissues far away
• Examples: insulin and testosterone

Synaptic Signaling

• Special form of paracrine signaling where nerve cells release neurotransmitters that bind to receptors
• Triggers a rapid, short-lived response in the target cell
• Examples: norepinephrine, epinephrine, acetylcholine
• Release occurs via diffusion to nearby cells
• Autocrine signaling releases a signal into extracellular space for the cell to act on itself

Cell Surface Receptors

• Extracellular part of the receptor with a ligand-binding domain
• Intracellular surface comprises proteins that relay messages into the cytoplasm
• Receptor Tyrosine Kinases: Ligand binds to the receptor, which transduces the message across the membrane

Receptor Tyrosine Kinases (RTK) Examples

• Insulin receptor
• Ligand = insulin
• Target = skeletal muscle cell
• Function = uses glucose energy (ATP production) supply
• Kinase is a receptor that transfers high-energy phosphates onto a molecule, typically from ATP
• Receptor activation happens when a ligand binds and the two receptor pieces dimerize
• Autophosphorylation occurs on the intracellular domain of the activated receptor, adding phosphates to amino acids, such as tyrosine, via kinase enzymes

Second Messenger Activation

• A different molecule carries the message inside the cytoplasm to another destination
• MAP Kinase Pathway: Amplifies the signal through a series of phosphorylating events
• MAP Kinase Pathways activate metabolic pathways to use incoming glucose for cell growth, proliferation, gene activity, and expression

Signal Transduction

• The process of signal transduction through G protein-coupled receptors is exemplified by the Adrenergic Receptor
• Ligand = epinephrine (amino-acid derived hormone)
• Target = liver cell
• Function- Signal transduction leads to mobilizing glucose from glycogen stores
• Receptor Activation: Ligand (epinephrine) binds, causing a shape change that attracts G-Protein along the intracellular surfaces towards it
• G-Protein Activation: Activated receptor causes the G Protein to swap out GDP for GTP which energizes/activates the protein
• G-Protein subunits: Alpha, Beta, and Gamma
• Activation of G Protein subunits triggers second messenger (cAMP and PKA) activation and amplification
• Kinase- Transfers high-energy phosphates onto substrates

Topic 9: Body Symmetry

• Bilateral Symmetry: Two distinct halves or sides
• Radial Symmetry: Distinct central point that repeating body sections develop from
• Asymmetrical: No clear development of body organization
• Cephalized/Centralized Nervous System: Contains different regions (central processing and peripheral sensory and motor)
• Nerve Net: Network of neurons with limited central processing units
• Sponges lack a functional nervous system, but contain genetic machinery
• Radial Symmetry: Body composed of similar sections centered around a central axis or point
• Nerve net: Loosely organized collection of neurons without a single concentrated processing center
• A nerve ring is a connected ring of neurons that may serve as a simplified processing region

Bilateral Symmetry Details

• Two halves, long ways, mirror images (central and peripheral regions)
• Centralized: Main processing tissues of the nervous system centralized between sensors and effectors
• Cephalized: Main cluster of processing neurons located in the head region Face off to prey and predators as well as contains sensory organs
• The brain is on site to process incoming information quicker

Neuron Anatomy

• Dendrites receive incoming signals and convert them to graded electrical signals
• Cell body or soma is the main location for cellular organelles and support structures
• Axon region relays electrical signals to the target cell via electrical or chemical means
• Myelinated Area (Axon Subregion): Conducts action potential towards the target cell
• Synaptic Terminal changes graded potentials to single action potentials
• Nodes of Ranvier are exposure points along the axon with no myelin covering
• Axon Hillock is the starting line for action potential
• Potential energy is involved with eletrical signals which uses energy stored across the membrane of the cell