2018 Eukaryotic Cell Structure & Cell Membrane STQ MS

2018 Eukaryotic Cell Structure and Cell Membrane STQ MS2018 / H2 / ACJC PRELIM / P2 Q11Cholesterol is synthesised in the smooth endoplasmic reticulum (SER) in liver cells by a series of enzyme-catalysed reactions. Cholesterol is then transported to the Golgi apparatus where they are packaged into vesicles and subsequently released into a membrane-bound duct of the liver.Fig. 1.1 is an electron micrograph of a section of a liver tissue. Fig. 1.1(a)Name structure T in Fig. 1.1 and describe its role in liver cells.1.Structure T is the mitochondrion;2.Site of ATP synthesis for synthesis of cholesterol/glycogen (or other liver-specific functions);[2](b)Both prokaryotes and structure T have membrane proteins to help them perform the role described in (a). Suggest how prokaryotes perform this role.1.Presence of electron carriers / ATP synthase; membrane-bound duct ofliverTcholesterol

2.Embedded in cell surface membrane;3.Electron carriers use the energy from the transport of the electrons to pump H+ across the membrane, generating a proton gradient/pool;4.ATP synthase which uses the energy of the proton gradient/proton motive force/flow of protons for chemiosmotic synthesis of ATP;5.AVP;[3](c)Describe the role of cholesterol in the cell surface membrane.Maintain fluidity of the membrane by preventing close packing of phospholipids at low temperature / prevent phospholipids from moving too far apart when temperature is high;[1](d)Suggest how cholesterol is transported from the Golgi apparatus to the membrane-bound duct of the liver.1.Secretory vesicles containing cholesterol pinch off the GA and move along microtubules to the cell surface membrane;2.They fuse with the cell surface/duct membrane and discharge cholesterol outside the cell via exocytosis;[2] [Total: 8]2018 / H2 / IJC PRELIM / P2 Q12Fig. 1.1 shows an electron micrograph of a pancreatic cell that secretes large amounts of insulin that helps to regulate blood glucose level.

A Fig. 1.1(a)With reference to Fig. 1.1, (i)identify organelle A;Golgi apparatus/ body;[1](ii)describe two identifying features of organelle A that allows its identification in (a)(i).1.a stack of discrete, flattened, memb-bound sacs called cisternae;2.distinct memb with 2 faces cis/ convex & trans/ concave);3.vesicles pinching & joining from ends of each sac; (any 2)[2] Fig. 1.2 shows a diagram of the molecular structures of tristearin (a triglyceride) and

phosphatidylcholine (a phospholipid). Fig. 1.2(b)Table 1 shows a structural difference between the two molecules shown in Fig. 1.2.Complete Table 1 with two further structural differences other than in numbers of different types of atoms.Table 1structural featuretristearinphosphotidylcholinelength of fatty acid chainall same lengthdifferent lengthsdegree of saturationall FA tails are saturated1 FA tail is unsaturated / have C=Cfunctional grpglycerol & carboxylic acidglycerol, carboxylic acid & phosphatepresence of phosphate groupabsentpresent

no. of FA33[2](c)Triglyceride is used as energy storage while phospholipids are membrane components. Explain why phospholipids are suitable membrane components but not triglyceride.1.amphipathichas hydrophilic head and hydrophobic tail;2.able to form bilayerw hydrophilic heads interxt w aq medium in/s & o/s cell (while forming a hydrophobic core that prevents free movement of hydrophilic subst);[2]Cells in the pancreas secrete enzymes, such as amylase and trypsin, into a duct. The enzymes are packaged in vesicles so that they can be exported from these cells as shown in Fig. 1.3. (d) extracellular(d) cytoplasm Fig. 1.3(d)On Fig. 1.3, label the cytoplasm of the cell as 'cytoplasm' and extracellular fluid as 'extracellular'. [1]

(e)With reference to Fig. 1.3,(i)explain how enzymes that are secreted by cells in the pancreas are packaged into vesicles and exported, after their synthesis at the endoplasmic reticulum.1.enz packaged in tpt vesicles pinch off from surface of rERpinch off from surface of rER;2.tpt vesicles travel thru cytoplasm / or idea of transportfuse with A/ Golgi apparatus at cis face;3.protein modification occurs (adding phosphate grp/ adding, deleting, substituting sugar monomers) modification results in molecular identification tag on enz;4.vesicles containing enz pinch off from trans face of A/ Golgi apparatus;5.vesicles translocate towards CSMvia cytoskeleton (ATP req’d);6.memb vesicle fuses with CSMenz released out of cell via exocytosis;[6](ii)explain one property of the plasma membrane that allows vesicle formation.1.memb is fluidallows evagination of memb to form vesicles;2.phospholipid molecules are held by weak hydrophobic interxnsthus capable of lateral movement (within the monolayer);[2](f)Describe two advantages of having plasma membranes within the cell.1.allows compartmentalisation

to setup unique / optimum conditions for biochem rxns (e.g. acidic pH in lysosome);2.regulates movement of subst in and out of cell / organelle(by having prot trspters that only allows movement of specific subst);[2][Total: 18]2018 / H2 / MJC PRELIM / P2 Q1QUESTION 3Fig. 1.1 is an electron micrograph of part of a eukaryotic cell. Fig. 1.1(a)Identify the structures J and K. [2]J: Crista K: Mitochondrial matrix(b)Describe two structural features shown in Fig. 1.1 that identify G as the Golgi apparatus and not the rough endoplasmic reticulum. [2]G has no ribosomes while the rough ER is attached with ribosomes G has flattened membranous sacs that are stacked / no connection between membranes while the rER membranes are interconnected

G is not continuous with outer membrane of nuclear envelope while the rER is G has vesicles at ends of sacs / swellings at end of sacs (for vesicle formation) while there are no ends for vesicles formation in the rERG has a slight curvature compared to the rER

The Golgi apparatus is an organelle that is important for many functions. One example is the formation of lysosomes. Before lysosomal hydrolases can function in the lysosome, they must be sorted from other proteins. Fig. 1.2 shows how a glycosylated lysosomal hydrolase is sorted at the trans face of the Golgi apparatus. Fig. 1.2(c)Outline the pathway taken by the lysosomal hydrolase from its site of synthesis to the trans face of the Golgi apparatus. [3]After being synthesised by the ribosome on the rER, the lysosomal hydrolase enters the rER lumenER vesicle carrying lysosomal hydrolase buds off from rER and travels towards Golgi apparatusER vesicle fuses with cis face of Golgi apparatusRepeated budding and fusing of vesicles transports hydrolase to the trans face (d)With reference to Fig. 1.2, suggest how the Golgi apparatus is able to specifically sort the lysosomal hydrolase from other proteins. [2]Idea that Only the lysosomal hydrolase contains mannose-6-phosphate / other proteins are not glycosylated / not glycosylated with mannose-6-phosphate

Mannose-6-phosphate is complementary in shape to and binds to the mannose-6-phosphate receptor, allowing the lysosomal hydrolase to be packaged into the[Total: 9]2018 / H2 / MJC PRELIM / P2 Q2QUESTION 4(a)Contrast the processes of facilitated diffusion and active transport. [3]Facilitated diffusion involves transport of substances down a concentration gradient whereas active transport occurs against a concentration gradient.Facilitated diffusion does not require ATP whereas active transport requires ATP.Facilitated diffusion involves channel or carrier proteins whereas active transport only involves protein pump.(b)A group of students investigated the uptake of chloride ions in barley plants. They divided the plants into two groups and placed their roots in solutions containing radioactive chloride ions.Group A plants had a substance that inhibited respiration added to the solution.Group B did