Histology; The Cell

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Plasmalemma

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Plasma membrane, functions include maintaining structure, upholding selective permeability, facilitating receptor interactions and transducing signal into intracellular events

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Protoplasm

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Cytoplasm (betweem plasma membrane and nuclear envelope) and karyplasm (contents of the nucleus)

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Cytoplasm

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Aqueous solution with organic and inorganic compounds inside the cell

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Cytosol

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Cytoplasm + organelles

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Inclusions

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Metabolic by-products, storage forms of various nutrients and inert crystals and pigments

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Protein components of plasmalemma:
a) Integral proteins
b) Peripheral proteins

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a) Span the entire lipid bilayer and pass through membrane as transmembrane proteins.
b) Attached to cytoplasmic aspect or extracellular side of lipid bilayer

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Ion channels

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Transmembrane proteins and carrier proteins facilitate the passage of specific ions and molecules across the cell membrane

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Receptor sites

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Multipass proteins that make several passes through the membrane; contain receptor sites specific for particular signaling molecules

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Simple diffusion

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Non polar molecules (benzene, oxygen and nitrogen) and uncharged molecules (glycerol and water) can move across the cell by simple diffusion.

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Facilitated diffusion (passive transport)

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Hydrophilic molecules are transferred across the barrier by specialized transmembrane proteins; driven by concentration gradient

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Active transport

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Expending energy to transport ions (Na+, K+, Ca2+) and small molecules against the concentration gradient; require carrier proteins

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Channel proteins: gated or non-gated

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Most channel proteins are gated to prevent other molecules from entering. Incapable of transporting substances against a concentration gradient.

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Hydrophilic pores

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Ion channels formed so that hydrophobic amino acids interacts with phospholipids and hydrophilic amino acids face inwards, forming a polar lining for the channel

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Voltage-gated channels

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Gate goes from an open to an inactive position in which the passage is blocked (refractory period);
some are velocity-dependent

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Ligand-gated channels

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Require binding with a ligand (signaling molecule) to open the gate. The gate will remain open until the ligand dissociate from channel proteins.

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Ion channel-linked receptors, neurotransmitter-gated channels and nucleotide-gated channels

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Ligand-gated channels

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Mechanically-gated channels

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Physical manipulation is required to open the gate

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G-protein-gated ion channels

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Requires interaction between a receptor molecule and a G-protein coomplex; result in activation of G-protein and opening the channel

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Un-gated channels

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Potassium (K+) leak channels permit movement of K+ across it; direction reflect its concentration on the two sides of the membrane

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Aquaporins

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Multipass proteins designed for passage of water;
AqpZ is pure water transporter
- flip the water molecule halfway through the channel

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GlpF-multipass protein

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Transport glycerol
- restriction of pore size discriminate which molecules are able to pass

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Carrier proteins

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Utilize ATP-driven transport mechanism to ferry specific substances against a concentration gradient; when solute bind to the site the carrier protein undergo conformation changes

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Uniport

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Single type of molecule being transported

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Coupled transport; symport and antiport

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Two types of molecules being transported either in same direction (symport) or opposite direction (antiport)

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Na-K+ pump

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Active transport requiring ATP because of transport against concentration gradient; coupled antiport carrier protein with 2 binding sites for K+ on extracellular site and 3 binding sites for Na+ on cytoplasmic side

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ATPase

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Associated with active transport as it hydrolize ATP into ADP while conforming the carrier protein

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Ouabain

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A glycoside that binds to the same site as K+ and inhibits the Na-K+ pump

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Secondary active transport; coupled transport

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Facilitated by carrier proteins that enables symports and antiports to utilize the concentration differential to transport another molecule

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Glycocalyx

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Cell coat composed of oligosaccharides coating the cell surface. Carbohydrate chains are covalently attached to transmembrane proteins or phospholipids. Most important function is protection of the cell from interaction with inappropriate enteties.

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Cell communication

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Occur when signaling cells release signaling molecules that bind to the cell surface receptors of target cells

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Selective signaling process

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Synaptic signaling molecule; neurotransmitter released in synaptic cleft and only affect a single cell

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Paracrine signaling

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Signal molecule is released into the intercellular environment and affects the surrounding cells.

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Autocrine signaling

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Specialized type of paracrine signaling that affect the cell producing the signal

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Endocrine

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Signal molecule enters the bloodstream ferried by proteins to target cells situated at a distance from the signaling cell

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Signaling molecules

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Bind to extracellular and intracellular receptors to elicit a specific cellular response.

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Hydrophilic signaling molecules

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Acetylcholine is hydrophilic and cannot penetrate the cellmembrane, thus requiring a receptor on cell surface.

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Hydrophobic signaling molecules

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Steroid hormones and small non-polar molecules (NO) have the ability to diffuse through the cell membrane. These ligands require an intracellular receptor.

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Second messenger

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Binding of signal molecules activates intracellular second messenger system which iniatiates a cascade of reactions resulting in a response; cAMP, calcium, cGMP and diacylglycerol

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Steroid hormones

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Bind to intracellular hormone receptors which activates gene expression and transcription.

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G-protein linked receptors

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Multipass proteins that have two intracelular sites: one to bind the G-proteins and the other become phosphorylated during the receptor desensitization.

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GTPases

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Most cells posses two types: monomeric and trimeric, which can bind GTP and GDP.
GTPase is composed of a large a-subunit and two
b-subunit and g-subunit.

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Types of G-proteins

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Stimulatory (Gs), inhibitory (Gi), pertussis toxin-sensitive (Go), pertussin toxin-insensitive (Gbq) and transducin (Gt).
- Gi inhibits cAMP and thus prevents activation of adenylate cyclase

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Adenylate cyclase

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G-protein a-subunit bind to adenylate cyclase and activate it; formation of cAMP.
When ligand is released from G-protein linked receptor, a-subunit hydrolyze GTP to GDP and detaches from adenylate cyclase

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Cyclic adenosine monophosphate (cAMP)

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Second messenger as an intracellular signaling molecule that activate protein kinase A (A-kinase), creating a cascade of phosphorylation

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Go-protein

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Subunit that activates phospholipas C (enzyme) responsible for cleaving PIP2 to IP3 and diacylglycerol

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IP3

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Diffuses into E.R where it causes a release of Ca2+ into the cytosole that activate kinase C with diacylglycerol

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Kinase C

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Activates transcription of certain genes

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Calmodulin

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Binds to the excess Ca-ions and activates enzymes (CAM-kinases) that control smooth muscle contraction

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Ribosomes

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Small protein particles and ribosomal RNA (rRNA) consisting of two subunits (large and small) and is assembled in the nucleolus, then released into the cytosol as separate entities

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Small subunit of ribosomes

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Bind mRNA on the P-site for peptides tRNA, bind aminoacyl tRNA on the A-side and exits the tRNA on the E-site

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Large subunits of ribosomes

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Ribozymes catalyze the peptide bond formation

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Endoplasmic reticulum (ER)

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Largest membranous system of the cell with interconnected tubules and vesicles and a lumen refered to as cistern

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Smooth endoplasmic reticulum (SER)

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Sysytem of anastomosing tubules and occasional flattened membrane-bound vesicles.
Active in synthesis of steroids, cholesterol and triglycerides, and detoxification of toxic material

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Sarcoplasmic reticulum

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Function to storage and release calcium ions in muscle cells

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Rough endoplasmic reticulum (RER)

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Function to recognize and bind ribosomes on cytosolic surface using integral proteins in protein synthesis.
Additional functions is post-transitional modifications of the proteins including sulfation, folding and glycosylation.

- Proteins to be packed are synthesized in RER while proteins destined for cytosol are manufactured in cytosol.

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Integral proteins in RER

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Signal recognition particle receptor is a docking protein recognizing SRP, ribosome receptor protein (ribophorin I & II) and pore protein.

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tRNA

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Form covalent bonds with amino acids after recognizing the start codon on mRNA; forming aminoacyl tRNA.
Each tRNA reacts with a specific amino acid after recognition of anti-codon corresponding with mRNA

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Translation

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Protein synthesis on ribosomes; requires mRNA and tRNA

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Polysome

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Conglomeration of several ribosomes

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Synthesis of cytosolic proteins

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1. Initiator tRNA recognize AUG-codon on P-site
2. mRNA binds to small subunit or ribosome
3. Large subunit bind to small subunit
4. Acylated tRNA matches its anticodon with codon on mRNA on A-site
5. Amino acids on A-site and P-site form peptide bonds
6. The deaminated tRNA leaves the P-site and binds to E-site to be ejected

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Peptidyl transferas

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Catalyzes tRNA to transfer the amino acid from the P-site to the A-site

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Stop codon

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UAG, UAA or UGA; bind to the A-site and activate a releasing factor that cause the polypeptide chain to release from P-site, through E-site

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Synthesis of proteins in RER

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1. Identification lies in a small segment of mRNA after start codon (signal peptide)
2. Recognition of signal peptide by signal recognition particle (SRP) in cytosol
3. Attachment of SRP on P-site and transport to SRP-receptor on RER cytosolic surface
4. Docking of SRP receptor with SRP and attachment of ribosome to RER
5. Translocation of mRNA into cistern of RER by pore proteins
6. SRP is dislodged and frees the P-site

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Signal peptidase

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Enzyme that cleaves the signal protein into amino acid components

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Golgi apparatus

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Synthesize carbohydrates and modify and sort proteins manufactured in RER

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Golgi stack's; 3 levels of cisternea

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1. Cis-face (cis-Golgi-network); entry face of RER proteins
2. Medial face (intermediate face); ERGIC
3. Trans-face; exit face when the protein has been modified and ready to be packaged

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Golgi vesicle transport

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Vesicles transporting proteins have a proteinaceous coat by coatamers (COP I, II and clathrin)
- Vesicles to ERGIC are alwas COP II coated and later take on COP I coat until trans-Golgi where they change into clathrin coat

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Golgi surface markers and receptors

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Receptors on surface of coats interact with microtubules and motor protein complexes;
- dynein drives toward microtubule organizing center MTOC
- kinesin drives vesicle leaving Golgi

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Cargo leaving TGN (trans-Golgi-network)

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Vesicles who either fuse with cell membrane as proteins or lipids or is released into extracellular space
- vesicles can also fuse with late endosomes and become a lysosome

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Exocytosis

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Vesicles follow a default pathway to be immediately releas their contents into intracellular space

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Constitutive secretory pathway

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All vesicles that participate in non-selective transport have a coat of 7-unit protein complex; coatamer that remains with vesicle until target is reached
- pathway of actin filaments driven by myosin II or via microtubels

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Regulated secretory pathway

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Pathway to lysosomes and secretory vesicles.
In polarized cells they remain localized until a signal cause them to release their contents

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Lysosome synthesis

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1. Phosphorylation of mannose of lysosomal proteins (lysosomal hydrolases) in cis-face
2. In trans-face mannose-6-phosphate (M6P) that acts as a signal
3. Binding to receptors (transmembrane proteins) in TGN; creating a small pit (clathrin triskelions)
4. Protein complex is pinched off and form clathrin-coated vesicles
5. Clathrin coat is shed when vesicle fuse with late endosome

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Adaptin

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Intermediary protein that differentiate between the different clathrin coats between the receptor molecule and clathrin

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Endocytosis

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When the cell ingest macromolecules from the extracellular space;
- larger molecules ingested are phagosomes
- smaller molecules ingested are pinocytic vesicles

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Phagocytosis

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Engulfing larger particular matter usually performed by phagocytes (neutrophils and monocytes); Fc-receptors bind Fc-regions of antibody which leads to pseudopods surrounding the microorganism and internalize it
- monocytes leaving the bloodstream and entering connective tissues become macrophages

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Pinocytosis

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Most active transport process (membrane trafficking); depends on cargo receptors (transmembrane proteins) associated with a ligand extracellularly and clathrin coat intracellularly

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Clathrin triskelions

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Form clathrin-coated pits which become pinocytic vesicles enclosing the ligand

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Dynamin (GTPase)

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Pinches off the pinocytic vesicles and release it into the cytoplasm

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Endosomes

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Early and late endosomes; endosomal compartment
Membrane contain ATP-linked H+pumps to acidify the interior (pH 6-5,5)

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Recycling endosome

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Pinocytic vesicle fuse with endosome to release the ligand into the lumen and receptor molecules are returned to plasma membrane (transcytis)

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Lysosomes

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Vesicle containing 40 different types of acid hydrolases (proteases, nucleases, lipases and glycolases)
Also contain H+pumps to maintain pH 5
Aid in digestion of macromolecules and senescent organelles
- Lysosomes recieve their hydrolysis enzymes from TGN in different vesicles through late endosomes

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Autophagosomes

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Organelles that capture cellular components (including proteins, membrane fragments and whole organelles) and deliver them to lysosomes

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Peroxisomes

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Self-replicating organelles containing oxidative enzymes (urate oxidase, catalase, D-amino acid oxidase) that catabolise long-chained fatty acids (beta oxidation) forming CoA (acetyl coenzyme A) and hydrogen peroxidase

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Hydrogen peroxidase (H2O2)

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Detoxifies various noxious agents (ethanol) and kill microorganisms

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Proteasomes

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Protein complexes responsible for proteolysis of malformed and ubiquitin-tagged proteins. Enzymes helping in degredation are; ubiquitin-activating enzyme, ubiquitin-conjugating enzyme and a number of ubiquitin ligases

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Ubiquination

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Several ubiquitin molecules attaches to a lysine residue of the candidate protein forming a polyubiquinated protein (tagged)

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Mitochondria

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Organell possesses its own DNA and perform oxidative phosphorylation and lipid synthesis; producing ATP for energy-requiring activites