Histology Wk 3 Flashcards
What are the functions of the plasma membrane
Establishes a flexible boundary, protects cellular contents and supports cell structure. Phospholipid bilateral separate substances inside and outside the cell
Selective permeability - regulates entry and exit of ions, nutrients and waste molecules throughout the membrane
Electrochemical gradients- establishes and maintains an electrical charge difference across the plasma membrane
Communication- contains receptors that recognise and respond to molecular signals
Thickness of membrane
7.5nm to 10nm
What is a phospholipid
Is amphipathic, with a phosphate group charge on the polar head and two long, non polar fatty acid chains, which can be straight (saturated) or kinked (at an unsaturated bond)
What is cholesterol and its function
A sterol lipid insert varying densities among the closely packed phospholipid fatty acids. Restricting their movements and modulating the fluidity of all membrane components
What are glycolipids
They are on the outer layer of the bilayer and contribute to a dedicate cell surface coating called the glycocalyx
How do phospholipids form a bilayer
Hydrophilic polar heads are in direct contact with water and hydrophobic non polar fatty acid chains are buried away from the membranes middle away from water
Why do some cytoplasmic membranes exhibit trilaminar appearance
After the fixation of osmium tetroxide, osmium binds the polar heads of the phospholipids and the oligosaccharide chains, producing the two dark outer lines that enclose the light band of osmium-free fatty acids
What are peripheral proteins
can be extracted from cell membranes with salt solu- tions, whereas integral proteins can be extracted only by using detergents to disrupt the lipids.
What are integral proteins
Integral proteins are firmly embedded in the lipid layers; those that completely span the bilayer are called transmem- brane proteins. Hydrophobic amino acids of these proteins inter- act with the hydrophobic fatty acid portions of the membrane lipids. Both the proteins and lipids may have externally exposed oligosaccharide chains.
What is a cryofracture
When cells are frozen and fractured (cryofracture), the lipid bilayer of membranes is often cleaved along the hydrophobic center. Splitting occurs along the line of weakness formed by
the fatty acid tails of phospholipids.
Most of the membrane seen are proteins or aggregates of proteins that remain attached to the half of the membrane adjacent to the cytoplasm
Each protein bulging on one surface has a corresponding depression (2) on the opposite surface.
Name the three types of transmembrane transport
Diffusion transports small, nonpolar molecules directly through the lipid bilayer. Lipophilic (fat-soluble) mol- ecules diffuse through membranes readily, water very slowly.
■ Channels are multipass proteins forming transmem- brane pores through which ions or small molecules pass selectively. Cells open and close specific channels for Na+, K+, Ca2+, and other ions in response to various physiological stimuli. Water molecules usually cross the plasma membrane through channel proteins called aquaporins.
■ Carriers are transmembrane proteins that bind small molecules and translocate them across the membrane via conformational changes.
Diffusion, channels, and carrier proteins operate pas- sively, allowing movement of substances across membranes down a concentration gradient due to its kinetic energy. In contrast, membrane pumps are enzymes engaged in active transport, utilizing energy from the hydrolysis of adenos- ine triphosphate (ATP) to move ions and other solutes across membranes, against often steep concentration gradients. Because they consume ATP pumps, they are often referred to as ATPases.
Describe the three types of endocytosis
Phagocytosis involves the extension from the cell of surface folds or pseudopodia which engulf particles such as bacteria, and then internalize this material into a cytoplasmic vacuole or phagosome.
(b) In pinocytosis the cell membrane forms similar folds or invaginates (dimples inward) to create a pit containing a drop of extracellular fluid. The pit pinches off inside the cell when the cell membrane fuses and forms a pinocytotic vesicle containing the fluid.
(c) Receptor-mediated endocytosis includes membrane proteins called receptors that bind specific molecules (ligands). When many such receptors are bound by their ligands, they aggregate in one membrane region, which then invaginates and pinches off to create a vesicle or endosome containing both the receptors and the bound ligands.
What are the major types of membrane receptors
Channel-linked receptors bind ligands such as neurotransmitters and open to allow influx of specific ions.
(b) Enzymatic receptors are usually protein kinases that are activated to phosphorylate (and usually activate) other proteins upon ligand binding
(C) G protein coupled receptors
A ligand binds to a receptor, causing a conformational change to activate receptor.
G protein binds to activated receptor.
GTP binds to G protein causing G-protein activation. Activated G protein leaves the
receptor. It attaches to and activates an effector protein. (an ion channel or an enzyme).
The activated effector protein makes secondary messenger available within the cell, which
leads to protein kinase enzyme activation.
What is exocytosis
,a cytoplasmic vesicle containing the molecules to be secreted fuses with the plasma membrane, resulting in the release of its contents into the extracellu- lar space without compromising the integrity of the plasma membrane
Exocytosis is triggered in many cells by a transient increase in cytosolic Ca2+. Membrane fusion during exocytosis is highly regulated, with selective interactions between several specific membrane proteins.
What are the two pathways of exocytosis
Constitutive secretion is used for products that are released from cells continuously, as soon as synthesis is complete, such as collagen subunits for the ECM.
■ Regulated secretion occurs in response to signals com- ing to the cells, such as the release of digestive enzymes from pancreatic cells in response to specific stimuli. Regulated exocytosis of stored products from epithelial cells usually occurs specifically at the apical domains of cells, constituting a major mechanism of glandular secre- tion
