lecture 17 - carbohydrates part 2 and lipids and membranes part 1 Flashcards
Describe the structure of cellulose
structural molecule of plant cell walls, giving strength and stability
unbranched polymer of glucose with beta 1-4 linkages
stabilising H-bonds between adjacent glucose units give cellulose its tensile strength - not found in alpha linkage form
What are glycoproteins?
glycosidic bonds between
proteins and carbohydrates
N-linked glycosidic linkage to asparagine
O-linked glycosidic linkage to serine or threonine
What are glycoconjugates?
oligosaccharides can be attached to protein and lipids by glycosidic bonds
glycoprotein normally on extracellular domain:
N-linked glycosidic linkage to asparagine
O-linked glycosidic linkage to serine or threonine
e.g. cell adhesion molecules are glycoproteins
sugars provide essential “address label” on many proteins, e.g. lysosomal enzymes carry characteristic mannose 6-phosphate tag
sugar groups may also be important in the mechanics of protein trafficking within the cell, e.g. nuclear pore complex proteins are glycosylated, and nuclear import is blocked by lectins that bind to the sugars
glycolipids are found predominantly in plasma membranes of cells in the vertebrate nervous system- may be important both in cell recognition and in electrical insulation
How does glycosylation take place?
Different glycosylation enzymes are present in each compartment (cisternae) of the Golgi
N-linked glycosylationbegins in the endoplasmic reticulum (ER) and continues in the Golgi
O-linked glycosylation occurs in the Golgi
What are biological membranes?
Boundaries of cells are formed by biological membranes:
• define inside and outside of the cell
• selectively permeable to small molecules
• dynamic and fluid structures
• composed of lipids (self-assembling) and proteins (function)
What are the membrane-bound organelles of eukaryotes?
mitochondria / chloroplasts nuclear envelope endoplasmic reticulum Golgi complex peroxisomes lysosomes vesicles
What are the common features of biological membranes?
Membranes are sheet-like structures
• Consist of lipids, proteins and carbohydrates
• Membranes are formed from lipids with hydrophobic tails and hydrophilic head
groups (membrane lipids are amphipathic)
• Specific proteins give distinct functions
• Membranes are non-covalent structures held together by many interactions
• Membranes are asymmetric (outer leaflet / inner leaflet)
• Membranes are fluid structures (rapid 2D diffusion, fluid mosaic model)
• Most cell membranes are electrically polarised
Describe how fatty acids are key constituents of
membrane lipids
Ability of membrane lipids to form bilayer is due to their hydrophobic properties
Hydrophobic properties are due to fatty acid hydrocarbon tails
Fatty acids spontaneously form a micelle in water, not a bilayer
What are the key features of lipids?
most common lipids comprise fatty acids covalently bound to alcohol, e.g. glycerol or sphingosine; many variations in structure
two major types of lipids:
hydrophobic (non-polar), e.g. triglycerides
amphipathic (polar head and non-polar tail) e.g. phospholipids
Phospholipid polar head oriented toward the solvent and non-polar tails away from the solvent in aqueous solution - important in biological membranes
non-polar lipids are insoluble in aqueous solvents
amphipathic lipids form micelles (unstable) or self-healing bilayers (stable)
complex lipids form the basis of biological membranes and include:
phospholipids (lipid +phosphate)
sphingolipids (lipid + sphingosine)
glycosphingolipids (sphingolipid + carbohydrate)
Triglycerides (fats) are major molecules of fuel storage:
fats are stored predominantly in adipocytes in mammals
stored fats metabolised to acetyl coA by beta oxidation in mitochondria, or formed into ketone bodies for utilisation as preferred fuel of mammalian heart muscle and of brain on starvation
