Lec 1 - Cell Membranes Flashcards
What is the physiological roles of lipids in the cell membrane and in tears?
Meibomian glands in the eyes produce lipids that help in the prevention of evaporation of tears. If these glands aren’t producing enough lipids then the eye can become dry (keratitis sicca or xerophthalmia) and can even cause corneal erosion.
How does polarity of molecules affect their diffusion through the cell membrane?
Lipid soluble molecules permeate or diffuse through the lipid bilayer easily where as water soluble molecules do not. The reason for this is because the bilayer has a polar head and non polar tails embedded in the middle. The nonpolar tails don’t allow polar molecules to pass. Instead these polar molecules must come into the cell in another fashion (channels, endocytosis, etc.).
What is the difference between micelles, liposomes, and endosomes?
Micells have polar heads in a sphere shape with all their tails coming together to a point in the middle (monolayer). These molecules are involved in the process of lipid absorption in the gut. Liposomes have a bilayer of phospholipids, where there are polar heads in the middle surrounding an aqueous center and the outer layer with nonpolar tails between the two. Liposomes are used for delivery of drugs, especially hydrophilic drugs. An endosome is a vesicle formed from the lipid bilayer of the cell membrane when an outside particle is brought into the cell by a form of phagocytosis.
What is the fluid-mosaic model of the cell membrane?
This is the idea that lipids and proteins move or float within the membrane due to the thermal energy. This fluidity decreases with high cholesterol and saturation of tail fatty acids. This fluid-mosaic model also shows that proteins are not uniformly distributed throughout the membrane due to the constant movement of the molecules. Also, the inner and outer layer of lipids are not symmetrical. Some components of the membrane are not always free to float (ex: sequestration - segregation of the nicotinic receptors at the motor end plate)
What is the role of cholesterol in regulation of cell membrane fluidity and permeability?
Cholesterol is an important constituent of animal cell membranes and is synthesized and excreted by liver cells. It increases the “tightness” of membranes (decreases permeability to water and urea) and causes a decrease in membrane fluidity. Cholesterol also serves as a precursor for bile acids and steroid hormones.
How is cholesterol transported through the cell membrane and in the blood?
Cholesterol travels from the liver to the tissues and then back in the blood wrapped by specific proteins known as lipoproteins. LDL cover cholesterol when it circulates in the blood. The LDLs seve as ligands which bind to the specific LDL receptors imbedded in the cell membrane, triggering its invagination at certain regions/pits. HDL helps to shuffle cholesterol back to the liver, so high levels of HDL indicates low risk of atherosclerosis. In contrast, high levels of LDL in the blood indicates high risk of atherosclerosis. This shows that LDL is building up in the blood because it either isn’t being taken up into the cell properly, or there is just too much to take up.
What functions are performed by integral proteins and integrins?
Integral proteins function as channels to regulate the flux of ions and water in or out of the cell. They also function as transporters or carrier proteins, transporting glucose, amino acids, and other substances which can’t diffuse across the phospholipid bilayer. They can also act as receptors, causing a signal due to the binding of a ligand to this molecule (neurotransmitter, drugs, etc.). They can also act as enzymes (catalyzing reactions), structural proteins (cell to cell membrane, extracellular matrix, or cytoskeleton), and cell identity markers. Integrins provide lines of communication between the extracellular matrix proteins and the cytoskeleton inside
What are the functional roles of various cell junctions?
Tight junctions can prevent passage of molecules between cells (brain blood barrier, renal epithelium). Adherins junctions (endothelial cells) and desmosomes (some layers of the epidermis) contain cadherins which mark tissues with low proliferative activity.
Which adhesion molecules indicate tissues with low and high proliferation?
Cadherins mark tissue with low proliferative activity and Integrins mark tissue with high proliferative activity.
Which proteins, junctions, and cells are affected by autoimmune blistering disease?
Pemphigoid has several forms and all affect hemidesmosomes (connection between the epidermis and dermis - integrin junctions). Pemphigus affects the connections between desmosomal connections (connection between keratinocytes - epithelial cells that produce keratin - cadherin junctions)
What is the role of cell adhesion receptors and cytoskeleton in phagocytosis?
AKA clatherin dependent endocytosis. Ligand ginds with the receptors which causes a change in the cytoskeletal framwork and causes membrane invagination and formation of a vesicle that is brought into the cell and fuses with a lysozome and causes the breakdown of the material inside the phagolysozome. This is used for cholesterol uptake into cells from the bloodstream
Which substances are taken into cells by receptor mediated endocytosis?
Large particulate matter - bacteria, cholesterol crystal, collagen, cells, etc. Iron is taken up by clathrin mediated endocytosis via transferrin in the blood. Also botulinum toxin and other toxins are also taken up.
Why is cholesterol clinically important and what are the manifestations of its accumulation in tissues?
Cholesterol is covered by LDL when it circulates in the blood. LDLs serve as ligands which bind to the specific LDL receptors imbedded in the cell membrane, triggering its invagination at the certain region/pits. The cytoplasmic side of the pits is coated with a specialized protein known as clathrin. Mutation of LDL receptors causes familial hypercholesterolemia due to a decrease in binding of LDLs and cholesterol uptake. This disease causes atherosclerotic plaques on coronary and other blood vessels; this can cause myocardial infarction and stroke.
What are cadherins?
Cadherins are located in cell junctions and mark tissues with low proliferative activity. They are the “glue” that keeps cells together. They are present in desmosomes connecting epithelial cells to eachother.
What are integrins involved in?
Tissue growth, development, remodeling, wound healing, cell motility, diapedesis (leukocyte extravasation), and phagocytosis. They are responsible for adhesion of the first, actively proliferating basal layer of epidermal cells to the basal lamina via hemidesmosomes. During wound healing, hemidesmosomes can be disassembled to allow keratinocyte movement over the affected sites.
