exam qns + ans Flashcards
Suggest two advantages to eukaryotic cells of having membrane-bound organelles.
Compartmentalisation of cell contents allows for:
MP1: different conditions in different compartments. to maintain optimum conditions for each enzymatic reaction
MP2: to increase surface area for the embedding of enzymes and proteins into the membrane
MP3: isolates harmful substances from the rest of the cell
Describe two functions of Golgi body.
MP1: Chemical modification of proteins / lipids occurs in GA such as glycosylation / addition of
short carbohydrate chains to form glycoproteins / glycolipids;
MP2: Proteins are transported to the Golgi apparatus where they are temporarily stored and
concentrated;
MP3: The vesicles containing protein to be secreted and lysosomal enzymes bud off / pinch off the Golgi apparatus as secretory vesicles and lysosomes respectively;
Outline the structure and function of the Golgi body in cells. [6]
- Golgi apparatus (GA) consists of one or several stacks of flattened, curved single membrane-bound sacs called cisternae;
- New cisternae are formed at cis / receiving / forming face by fusing with transport vesicles from the rER and sER
- Golgi vesicles bud off from shipping / trans / secreting face whereby its movement to their target
sites is guided by microtubules / cytoskeleton;
+ any 3 functions (i.e. maximum 3 marks):
- Chemical modification of proteins / lipids occurs in GA such as glycosylation / addition of short carbohydrate chains to form glycoproteins / glycolipids;
- Proteins are transported to the Golgi apparatus where they are temporarily stored and concentrated;
- The vesicles containing protein to be secreted and lysosomal enzymes bud off / pinch off the Golgi apparatus as secretory vesicles and lysosomes respectively;
- Synthesis and secretion of polysaccharides such as pectin to form new plant cell wall;
- Transport of lipids via Golgi vesicles to cell surface membrane to enter the lymphatic system;
Describe the properties of the phospholipid bilayer and the aquaporin channels in relation to the movement of water across the cell surface membrane.
- Water is a polar molecule, therefore it is unable to pass through hydrophobic core of phospholipid bilayer;
- As phospholipid bilayer is fluid and water is a small molecule, some water molecules will pass through small temporary gaps in the membrane via osmosis;
- Aquaporins have hydrophilic pores which allows water to pass through the phospholipid bilayer;
- Aquaporins have hydrophobic regions / amino acids which form hydrophobic interactions with hydrophobic fatty acid tails of the phospholipid bilayer, allowing it to be embedded in the membrane;
Explain how proteins are held in the membrane.
- Proteins have hydrophobic regions which form hydrophobic interactions with hydrophobic fatty acid tails of the phospholipid bilayer
- The hydrophilic regions form hydrogen bonds with the aqueous medium and hydrophilic phosphate heads of the phospholipid molecules.
- They are attached to cytoskeleton on the cytoplasmic side;
Explain how endocytosis allows foreign material such as aluminium silicate to be taken up by cells.
- Phagocytosis involves the extension of pseudopodia to engulf the foreign material;
- As the cell surface membrane pinches off, a phagocytic vesicle is formed enclosing the foreign material
Describe the role of proteins, glycolipids and cholesterol in the structure of the cell surface membrane. [8]
- Channel proteins aid in facilitated diffusion as they have hydrophilic pores which allow hydrophilic molecules such as charged ions and polar molecules to pass through the hydrophobic fatty acid tails of the phospholipid bilayer;
- Carrier proteins aid in facilitated diffusion / active transport as they have specific binding sites and undergo change in conformation when a solute binds to it;
- Some proteins function as enzymes such as adenylate/adenylyl cyclase; Receptor proteins have sites of attachments which allow for cell signalling between cells;
- Glycoproteins are involved in cell-cell recognition where cells are able to recognize and bind to one another;
- Glycoproteins function in intercellular junctions for cell-cell adhesion which allows cells to bind with each other to form a tissue;
- Formed by addition of short carbohydrate chains to phospholipids/lipids on outer surface of cell membrane;
- For cell-cell recognition
- For cell-cell adhesion
- They act as receptor sites for signal molecules to bind which allows for cell signalling;
- For electrical insulation (e.g. myelin sheath which insulates axons);
- For protection (e.g. glycocalyx which protects the cell surface membrane);
- Regulate fluidity of cell membrane;
- Maintains mechanical stability of cell membrane;
- Prevents leakage of small polar molecules;
Explain how compounds are taken up by cells. [8]
- Passive transport is the movement of substances down a concentration gradient without the use of ATP and types of such transport are diffusion, facilitated diffusion and osmosis.
- Small hydrophobic non-polar molecules such as O2 move across the phospholipid bilayer by diffusion;
- Charged ions like Na+ and K+ move across the phospholipid bilayer by facilitated diffusion, through channel proteins, which form hydrophilic pores in the membrane;
- (Large) polar molecules like glucose also move across the phospholipid bilayer via facilitated diffusion, through carrier proteins, which have specific binding sites and undergo change in conformation when a solute binds to it;
- Water molecules move across the phospholipid bilayer via osmosis, through channel proteins known as aquaporins, or through temporary gaps between phospholipids in the membrane;
- Osmosis is the net movement of water molecules down a water potential gradient from a region of less negative water potential to a region of more negative water potential across a partially/selectively permeable membrane until a dynamic equilibrium is reached;
- Some compounds may be taken up by cells through active transport, which is the movement of substances against a concentration gradient using energy in the form of ATP and pumps;
- Some compounds are taken up by cells through endocytosis, where large molecules are taken up using energy in the form of ATP;
- Cell surface membrane pinches/buds off to form a vesicle containing the substance
- Pinocytosis is an unspecific process which involves the uptake of droplets of extracellular fluid via tiny vesicles via the invagination of the cell surface membrane; (against a concentration gradient)
- Phagocytosis involves the uptake of large, solid materials via the extension of pseudopodia, which are then enclosed in phagocytic vesicles (phagosomes) and digested by hydrolytic enzymes in lysosomes;
explain how the binding of insulin to the insulin tyrosine kinase receptor triggers a response in the target cell that reduces the blood glucose concentration
- insulin binds to the ligand binding site of the receptor-tyrosine kinase (RTK) that has a complementary shape to insulin
- the binding triggers dimerisation of RTK proteins and a three-dimensional conformational change that activates the tyrosine kinase activity in the intracellular receptor tails
- the receptor tail carries out cross-phosphorylation that adds phosphate from ATP to the tyrosine amino acid residue on the tail of the other RTK protein
- this fully activated RTK phosphorylates other relay proteins that bind to the phosphorylated tyrosine on the RTK
- Signal amplification occurs when these relay proteins activate other downstream proteins and kinases in a phosphorylation cascade
- Various cellular responses take place eg. increase in GLUT4 on the plasma membrane for glucose uptake from the blood/ increase glycogenesis by glycogen synthesis/ increased rate of glycolysis
