Date post: | 21-May-2015 |
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Active Processes
• ATP is necessary to transport substances that are:– Too large– Non-soluble– Unable to move across its concentration gradient
• Active Transport– Primary & Secondary Active Transport
• Vesicular Transport– Endocytosis & Exocytosis
Primary Active Transport
•The energy to do work comes directly from the hydrolysis of ATP
•Example: Sodium-Potassium Pump
The Na+ - K+ Pump
• Na+-K+ ATPase maintains ↑ [K+] inside cells & ↑ [Na+] outside cells
• Binds 3 Na+ : 2 K+
• Creates cell membrane’s electrochemical gradient
• Crucial to cardiac & skeletal muscle, and nervous functions
• See video
Resting Membrane Potential
• Selective permeability allows for the generation of a membrane potential (voltage)
• At rest, the cell membrane has a (-) membrane potential
• Important to excitable tissue like nervous tissue
Secondary Active Transport
• Indirectly driven by primary active transport through the creation of ionic gradients
• Molecules or ions move from regions of lower concentration to regions of higher concentration
Vesicular Transport
• Large particles, macromolecules, and larger volumes of fluids do not fit through channels of protein pumps and must be transported in and out of the cell through vesicles
• Exocytosis: Out of the cell
• Endocytosis: Into the cell
Exocytosis
• Secretions within vesicles dock at the membrane and are released as the cell & vesicular membranes fuse
• Used during hormone secretion, neurotransmitter release, mucus secretion & waste elimination
Endocytosis: Clathrin-mediated
• The main process used for endocytosis
• Clathrin-mediated transport is used during phagocytosis, pinocytosis, and receptor-mediated endocytosis
• Clathrin is a lattice-like protein that cages in cargo for transport into the cell
Endocytosis: Phagocytosis
• Primarily used by defense cells like WBCs and macrophages
• Large, solid substances such as bacteria and dead cells are engulfed and subsequently destroyed
Endocytosis: Pinocytosis
• Droplets of extracellular fluid containing dissolved particles are folded into the plasma membrane
• Nutrients dissolved in extracellular fluid are taken into the cell
• Particularly important in the lining of the small intestine
Endocytosis: Receptor-mediated
• Selective mechanism for bringing specific macromolecules into the cell
• Receptors bind with their specific ligands (enzymes, insulin, hormones) and are endocytosed
The Cytoplasm
• Gel-like material, consistency of room temperature butter
• Contains the cytosol (viscous, clear liquid), organelles (“cell machinery”), the cytoskeleton (scaffolding & motor units), and inclusion bodies (stored nutrients, pigmentation)
• Most cellular activities take place in the cytoplasm
Mitochondria
• Organelle whose inner membrane is folded into shelf-like partitions called “cristae”
• Abundance depends on cell type
• Function: "Power plants of the cell”
Ribosomes
• Small granules dispersed throughout the cytoplasm and on the membranes of Rough ER, composed of ribosomal RNA and protein
• Function: protein synthesis
Endoplasmic Reticulum
• Network of interconnected parallel membranes that is continuous with the nuclear membrane
• Rough ER: Studded with ribosomes
• Smooth ER: No ribosomes
RER & Protein Synthesis
• Newly synthesized proteins are taken into the RER cisternae where they undergo modification before they are transported where needed
• RER is also a “membrane factory”. External face is site for phospholipid synthesis
Smooth Endoplasmic Reticulum• Contains enzymes that
catalyze the synthesis of lipids and steroids
• Plays a role in lipid metabolism and drug detoxification
• Catalyzes the decomposition of glycogen to release glucose
• In muscle, stores Ca+2
Golgi Apparatus
• Flattened membranous sacs ("cisternae") arranged in stacks associated with many vesicles
• Function: modification, packaging, and transport of proteins, the cell’s “traffic director”
Lysosomes
• Spherical membranous sacs containing digestive enzymes (acid hydrolase)
• Lysosome Function: “demolition crew of the cell“
Review: Endomembrane System
Peroxisomes
• Membrane sacs that look like lysosomes abundant in liver and kidney cells containing oxidases
• Oxidases detoxify harmful substances & neutralize free radicals by converting them into hydrogen peroxide and water