Many physiological effects are mediated by secreted proteins (example, hormones, neurotransmittors, etc.), such as intercellular communication
In what organelles are secreted proteins formed?
Ribosomes, rough endoplasmic reticulum, and Golgi complex
What is the process by which secreted proteins are released from cells?
What is the basic process of secretion in the cytoplasm?
1. mRNA finds ribosomes on rER, forms proteins
2. Transport vesicles from rER transport proteins to Golgi complex, where proteins have post-translation modifications and are secreted within secretory vesicles which fuse with plasma membrane
How does translocation in the rER of secretory proteins occur?
Using ribosomes, mRNA builds polypeptide chain through pore into rER using a signal peptide which recognizes pore, eventually forming a protein which is released into the cell via a transport vesicle
How do secretory proteins recognize where to thread through the rER?
Signal peptide recognizes pore, and is eventually cleaved off once polypeptide is formed
What happens to the signal peptide in a secretory protein once the polypeptide is fully threaded through?
Signal peptidase cuts signal peptide, releasing protein, and signal peptide is broken down
Do all proteins threaded through rER pore continue through the process of becoming secretory proteins? Why or why not?
No - some proteins have signal peptide and go through pore, but a stop transfer sequence of amino acids jams polypeptide in membrane - when signal peptide is cleaved, protein remains in membrane
Why are post-translational modifications of secretory proteins necessary and where do they occur?
Post-translation modifications give proteins their function - they occur mostly in the Golgi body, modifying as they pass through the membranes
What are the two different phases that secretory proteins go through as they pass through the Golgi body?
Cis (forming) phase and trans (mature) phase
What occurs once secretory vesicles are fully formed?
Can either remain in cytoplasm until signal or can be released immediately - they have proteins in their membrane recognized by plasma membrane, allowing secretory vesicles to fuse with plasma membrane and spew contents out of cell via exocytosis
Uptake of particles - pockets in cell membrane take up particles, forming phagocytic vesicle which fuses with lysosome
Uptake of fluids - pockets in cell membrane take up dissolved molecules, forming pinocytic vesicle which fuses with lysosome
Surface of cell has receptors which are specific to certain molecules - when cluster of receptors infolds, it contains concentrated molecules, formed a clathrin-coated vesicle which enter a system of vesicular compartments in cytoplasm for processing
What is the function of the Golgi body in endocytosis?
Has an endocytic sorting station at one end (as well as the secretory sorting station at the other end)
What is the function of lysosomes in the cell?
Contain hydrolytic enzymes capable of breaking down a wide variety of macromolecules
Where are lysosomal proteins formed and through what process?
Formed in the rER in a process similar to secretory proteins
What is autophagy and where is it commonly found?
Digesting oneself through lysosomal proteins - occurs during development of complex organs, etc. and is referred to as programmed cell death
What is an example of a lysosomal storage disease?
Tay-Sachs - missing a certain lipase, causing lipid to accumulate in the brain cells, resulting in impaired neuronal function and death
What three fibrous proteins compose the cytoskeleton of a cell?
Microfilamets, microtubules, and intermediate filaments
What are characteristics of a microfilament?
2 chains of molecules intertwined
Made up of actin (used in contraction)
Diameter of ~ 7 nm
What are characteristics of microtubules?
~ 25 nm n diameter
Made up of a spiral arrangement of tubulin
What are the four functions of microtubules?
1. Maintaining cell shape
2. Intracellular transport vesicles (axonal transport)
4. Mitotic spindle
How do microtubules function in intracellular (axonal) transport?
Vesicles attach to outside of microtubules, which acts as a "highway" for the vesicles - secretory vesicles attach via kinesin molecules, which move vesicles down microtubule
How do microtubules act in cilia function?
Cilia is composed fo 9 microtubule doublets and 2 microtubules in the centre which slide up and down via dynein (a molecular motor) allowing cilia to move
How do microtubules function in the mitotic spindle?
Chromosomes attach to microtubules and are pulled to opposite ends of the cell
What are cell membranes composed of?
Lipids (phospholipids and cholesterol), proteins (integral and peripheral) and carbohydrates (glycoproteins and glycolipids)
Fluid mosaic of phospholipids, proteins, and carbohydrates
What is a phospholipid composed of?
Head (polar) - amino acid, phosphate, and glycerol
Tail (non polar) - fatty acids
Membrane proteins and functions
Loop through membrane several times, with an amino group at one end and a carboxyl group at the other, to act in:
1. Transport and channel proteins
5. Attach to cytoskeleton
What is the function of the loops of membrane proteins in extracellular fluid?
How do membrane proteins form channels to act in transport?
Several transmembrane proteins cluster together, forming a channel in the middle for ions - can control movement of ions by changing its shape
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