12
Property of National Junior College H1 / H2 Biology Tutorial Ans March 2011 1 Sub-topic: Cell Structure and Functions Answers to Questions not discussed (For uploading on KMS) Qn 2) N00/P3/Q1 [Tutorial, Optional for St.] Fig.1.1 shows electron micrographs of three cell organelles A, B and C. Complete the table below describing the main identifying features and the main functions of the organelles shown in Fig. 1.1. (Note: naming the structure is not a substitute for describing main identifying features, thus no marks awarded even if structure is correctly named) organelle main identifying features main functions A o Bound by a double membrane o Inner membrane is folded extensively to form cristae (mitochondrion): no mark awarded o Involved in ATP synthesis to provide energy for the functioning of the cell B o Consists of a system of flattened membranous sacs called cisternae o Presence of ribosomes studded on the cytoplasmic surface of the membrane (RER): no mark awarded o Secretory proteins are synthesized on the RER o Proteins synthesized on the RER are transported within vesicles to the Golgi body C o Consists of interconnecting membrane-bound tubules o Absence of ribosomes (SER): no mark awarded o Site of lipid synthesis o Involved in drug detoxification Alternative Answer C o Membrane-bound sacs (Vesicle/Lysosome): no mark awarded o Contains hydrolytic enzymes, which hydrolyses the materials taken in by phagocytosis o For autolysis / self-destruction of dying cells by the release of the lysozyme into the cell o Transfer of lipids / proteins to Golgi body [6] CORE SYLLABUS CELLULAR FUNCTIONS Name: Date:

Cell Structure and Functions 2012 Tutorial - Answers Not Discussed for Uploading on KMS

Embed Size (px)

Citation preview

Page 1: Cell Structure and Functions 2012 Tutorial - Answers Not Discussed for Uploading on KMS

Property of National Junior College H1 / H2 Biology Tutorial Ans

March 2011 1

Sub-topic: Cell Structure and Functions

Answers to Questions not discussed (For uploading on KMS)

✔ Qn 2) N00/P3/Q1 [Tutorial, Optional for St.]

Fig.1.1 shows electron micrographs of three cell organelles A, B and C.

Complete the table below describing the main identifying features and the main functions of the organelles shown in Fig. 1.1. (Note: naming the structure is not a substitute for describing main identifying features, thus no marks awarded even if structure is correctly named)

organelle main identifying features main functions

A

o Bound by a double membrane

o Inner membrane is folded extensively to form cristae

(mitochondrion): no mark awarded

o Involved in ATP synthesis to

provide energy for the

functioning of the cell

B

o Consists of a system of flattened membranous sacs called cisternae

o Presence of ribosomes studded on the cytoplasmic surface of the membrane

(RER): no mark awarded

o Secretory proteins are

synthesized on the RER

o Proteins synthesized on the

RER are transported within

vesicles to the Golgi body

C

o Consists of interconnecting membrane-bound tubules

o Absence of ribosomes

(SER): no mark awarded

o Site of lipid synthesis

o Involved in drug detoxification

Alternative Answer

C

o Membrane-bound sacs

(Vesicle/Lysosome): no mark awarded

o Contains hydrolytic enzymes, which hydrolyses the materials taken in by phagocytosis

o For autolysis / self-destruction of dying cells by the release of the lysozyme into the cell

o Transfer of lipids / proteins to Golgi body

[6]

CORE SYLLABUS

CELLULAR FUNCTIONS

Name: Date:

Page 2: Cell Structure and Functions 2012 Tutorial - Answers Not Discussed for Uploading on KMS

Property of National Junior College H1 / H2 Biology

Jan 2012 2

✔ Qn 3) N02/P3/Q2

Centrioles replicate during interphase.

(a) State the main identifying features of centrioles.

Centrioles exists as a pair of rod-like structures positioned at right angles to each other

and are situated next to the nucleus.

Each centriole is a hollow cylinder composed of nine sets of triplet microtubules arranged in a ring.

[3]

Page 3: Cell Structure and Functions 2012 Tutorial - Answers Not Discussed for Uploading on KMS

Property of National Junior College H1 / H2 Biology

Jan 2012 3

Qn 5) N06/P2/Q6 Fig. 6.1 is an electron micrograph of a secretory cell from the hypothalamus of the brain. This cell synthesizes and releases ADH (vasopressin).

ADH is a peptide made of nine amino acids.

Page 4: Cell Structure and Functions 2012 Tutorial - Answers Not Discussed for Uploading on KMS

Property of National Junior College H1 / H2 Biology

Jan 2012 4

(a) Explain the role of the following structures in the synthesis of ADH.

mitochondria

Are the sites of aerobic respiration producing ATP.

This ATP could be used to activate amino acids / form peptide bonds / synthesise mRNA or tRNA or enzymes. (Any 1.)

nuclear envelope

The presence of nuclear pores on the nuclear envelope allows ADH mRNA synthesized within the nucleus to exit into the cytoplasm for protein synthesis.

nucleolus

Involved in the synthesis of rRNA, which is a component of ribosomes that are the sites of synthesis of ADH peptides.

[4]

(b) Describe how peptides, such as ADH, present in the cisternae of the rough endoplasmic reticulum are secreted out of the cell.

Peptides such as ADH in the rough endoplasmic reticulum (RER) are first packaged into transport vesicles that pinch / bud off from the RER.

These vesicles are transported / directed to cell surface membrane along microtubules.

The membrane of the vesicles fuses with the cell surface membrane,

emptying / releasing the contents within such as peptides by exocytosis. [4]

[Total: 8]

Page 5: Cell Structure and Functions 2012 Tutorial - Answers Not Discussed for Uploading on KMS

Property of National Junior College H1 / H2 Biology

Jan 2012 5

Qn 6) N10/P2/Q1

Fig. 1.1 is an electron micrograph of an animal cell.

(a) Identify the organelles labelled A and B.

In each case, state two visible features that enabled your identification.

A Mitochondrion

1. Has a double membrane

2. Presence of cristae which are infoldings of the inner membrane

B Rough Endoplasmic Reticulum

1. Made up of a network of tubules

2. Surface is studded with ribosomes [6]

Examiner’s comments: The features of mitochondria were well known, although some talked about rod-shapes or confused cristae with cisternae. Candidates must ensure that when asked for „visible features‟ that they only state features that they can see on the electron micrograph. Some listed sacs or tubes as features of the rough endoplasmic reticulum. These were not clearly visible on the diagram and reflect what the candidate had learnt rather than what they could see. Some weaker candidates,identified B as smooth ER or Golgi apparatus. They suggested that there were „no ribosomes‟, presumably interpreting the cisternae inside out. Candidates should be given the opportunity to interpret a variety of electron micrographs as part of their course to prepare them for questions such as this. Those few who misidentified the organelles often had difficulties in (b). It was not uncommon for misconceptions regarding respiration to surface here, with references to energy being „produced in the mitochondria‟, rather than being „released‟.

Page 6: Cell Structure and Functions 2012 Tutorial - Answers Not Discussed for Uploading on KMS

Property of National Junior College H1 / H2 Biology

Jan 2012 6

(b) (i) State the main function of each organelle.

A ATP Synthesis / Energy release (Reject: Energy production)

B Protein synthesis [2]

The majority of candidates correctly stated ATP synthesis for A and protein synthesis for B. Protein transport and vesicle formation were also allowed for B.

(ii) Explain how the functions of the two organelles A and B are linked.

Energy carriers synthesized by the mitochondria are required in the

process of translation that occurs at the ribosomes on the RER.

**GTP (Guanosine Triphosphate) is required for the initiation, elongation

termination stages of translation and for ribosome translocation.

(**students not expected to know, FYI) [2]

The majority of candidates explained how the products of A were useful in B, rather than vice versa. (comment doesn‟t seem to make sense bcos most mito proteins are synthesised by cytoplasmic ribo rather than RER)

(c) Suggest two advantages to eukaryotic cells of membrane-bound organelles.

Compartmentalisation in order to maintain optimum conditions for

specific enzyme reactions

Membranes of organelles can serve specific functions for example

enzymes for respiration reactions are embedded on the inner membrane of

mitochondria / pigments for light absorption embedded on thylakoid

membrane of chloroplasts. [2]

Many candidates correctly referred to „compartmentalisation‟. Candidates found it harder to qualify this with reference to the idea that different conditions are required to maintain optimum conditions for enzyme reactions or to increase surface area.

[Total: 12]

Page 7: Cell Structure and Functions 2012 Tutorial - Answers Not Discussed for Uploading on KMS

Property of National Junior College H1 / H2 Biology

Jan 2012 7

Non-compulsory Essay Ans

N99/P3/Q8

(a) Describe the structure and function of the endoplasmic reticulum. [6]

The endoplasmic reticulum (ER) is an organelle consisting of a network of

interconnecting membrane-bound sacs and tubules, namely the rough ER (RER) and

smooth ER (SER), which is continuous with the nuclear membrane.

The RER consists of membrane-bound flattened sacs / cisternae with ribosome’s

attached to their surface.

The ribosomes are involved in the synthesis of proteins for export from the cell or for

incorporation into membranes.

The SER is a system of interconnected tubules with no ribosomes attached.

The SER is mainly involved in the synthesis of lipids and steroids / cholesterol /

reproductive hormones.

Detoxifying enzymes are present on the SER, making it an important site for drug

detoxification.

(b) Explain the roles of the Golgi apparatus and lysosomes in the cell. [10]

Proteins synthesized at the RER are transported in transport vesicles to the Golgi

apparatus, where the vesicles fuse with the cis face of the Golgi apparatus

and the proteins enter the Golgi for chemical modifications.

One example of chemical modification is glycosylation, where carbohydrates are

added to proteins to form glycoproteins.

As the chemically modified proteins move towards the trans face of the Golgi, they

undergo further sorting and packaging into secretory / Golgi vesicles.

The modified proteins are then released via secretory / Golgi vesicles that bud off

from the trans face of the Golgi apparatus.

If the proteins are destined for secretion from the cell, the secretory / Golgi vesicles

will move towards the cell surface and release the proteins via exocytosis.

Page 8: Cell Structure and Functions 2012 Tutorial - Answers Not Discussed for Uploading on KMS

Property of National Junior College H1 / H2 Biology

Jan 2012 8

Lysosomes are vesicles containing hydrolytic / digestive enzymes produced by the

Golgi apparatus.

In cells that ingest insoluble particles / foreign bacteria by endocytosis /

phagocytosis, lysosomes fuse with endocytic / phagocytic vesicles

to hydrolyse / digest the ingested material or to destroy bacteria, releasing its

contents into the cell.

Lysosomes are involved in autophagy in which unwanted / worn-out organelles are

destroyed by the fusion of lysosomes with vesicles containing the unwanted

organelles.

In autolysis (the self-destruction of dying cells), lysosomes rupture to release

hydrolytic enzymes into the cytoplasm, resulting in the destruction of the cell and all

its organelles.

Examiners’ Comments: Too many candidates referred to the lysosomes attaching to the ingested material / organelle and releasing its contents. Note: Lysosomes do not attach to the ingested material directly. Fusion of lysosome membrane and vesicle containing ingested material occurs.

N03/P3/Q7

(a) Describe the main structural features of rough endoplasmic reticulum, mitochondria and

ribosomes. [8]

The RER consists of a system of flattened membranous sacs called cisternae

Ribosomes are studded on the cytoplasmic surface of the membrane

The RER membrane is continuous with the nuclear membrane

A mitochondrion is an organelle enclosed by a double membrane.

The outer membrane is smooth while the inner membrane is folded to form

cristae.

Embedded within the inner membrane are proteins of the Electron Transport

Chain and ATP synthases / stalked particles.

The inner membrane surrounds the fluid mitochondrial matrix,

Page 9: Cell Structure and Functions 2012 Tutorial - Answers Not Discussed for Uploading on KMS

Property of National Junior College H1 / H2 Biology

Jan 2012 9

which contains Krebs cycle enzymes, mitochondrial DNA and 70S ribosomes.

A ribosome is a non-membranous organelle.

The 80S ribosomes in eukaryotic cells comprise two subunits, a large 60S subunit

and a small 40 S subunit.

The constituents of ribosomes are ribosomal RNA and ribosomal proteins.

(b) Outlines the functions of rough endoplasmic reticulum, mitochondria and ribosomes. [6]

The RER is the site of synthesis of two types of proteins (secretory and membrane proteins).

Secretory proteins destined for export from the cell will be packaged into vesicles that bud off the RER.

Membrane proteins that are synthesized will be incorporated into plasma membranes.

Mitochondria are involved in cellular respiration, which is a series of biochemical

reactions that result in the synthesis of ATP.

In aerobic respiration, the highly convoluted cristae greatly increase the surface

area for enzymes and electron carrier proteins, which facilitate oxidative

phosphorylation, which generates most of the cells’ ATP.

Within the fluid matrix are enzymes that catalyse reactions in the Krebs cycle,

breaking down pyruvate to generate high-energy electrons for the ETC in the inner

mitochondrial membrane.

The ribosome is the site of protein synthesis.

There are binding sites for tRNA molecules on the large ribosomal subunit, and an mRNA binding site on the small ribosomal subunit, which facilitates the complementary base-pairing of tRNA anti-codons with mRNA codons during the process of translation.

[Part (c) on the use of electron microscope is not in the syllabus.]

Page 10: Cell Structure and Functions 2012 Tutorial - Answers Not Discussed for Uploading on KMS

Property of National Junior College H1 / H2 Biology

Jan 2012 10

N05/P2/Q7

(a) Describe the main differences in structure between plant and animal cells. [6]

Plant Cell Animal Cell

Cellulose cell wall present Cellulose cell wall absent

Large central vacuole present Smaller vesicles / vacuoles present

Chloroplasts present Chloroplasts absent

Tonoplast present Tonoplast absent

Centrioles absent A pair of centrioles present

Lysosomes absent Lysosomes present

Presence of plasmodesmata Plasmodesmata absent

(b) Explain how lysosomes differ from ribosomes. [8]

Answers under essays for discussion.

(c) Outline the structure and function of the Golgi body in cells. [6]

Structure: Made of flattened membrane sacs called cisternae Each of the flattened sacs has an internal space / lumen Each Golgi body has a cis and a trans face The cis face is located nearest the nucleus and receives materials from vesicles from

the RER The trans face is located closest to the plasma membrane and transports Golgi vesicles

for secretion from the cell Function: Involved in the chemical modification of proteins. E.g. glycosylation to form glycoproteins Involved in the formation of secretory vesicles or lysosomes Golgi bodies of plant cell produce extracellular polysaccharides that are used for the

formation of the cell wall

Page 11: Cell Structure and Functions 2012 Tutorial - Answers Not Discussed for Uploading on KMS

Property of National Junior College H1 / H2 Biology

Jan 2012 11

STRETCH-A-CISE

i) Identify the organelles P, Q and R. [3]

P: Rough Endoplasmic Reticulum; Q: Golgi Apparatus; R: Secretory vesicle / Vesicle

P: Rough Endoplasmic Reticulum; Q: Transport Vesicle / vesicle; R: Golgi Apparatus

P: Vesicle ; Q: Rough Endoplasmic Reticulum; R: Golgi Apparatus [3]

NJC Promos 2011 / Q1b) An experiment was carried out to determine what happens to amino acids after they are absorbed by animal cells. The cells were incubated for 5 minutes in a medium containing radioactively labelled amino acids. The radioactive amino acids were then washed off the cells’ surface and the cells were incubated in a medium containing only non-radioactive amino acids.

Samples of the cells were removed from the medium every five minutes for 40 minutes. For each sample, the levels of radioactivity in three different membranous organelles, P, Q and R, were determined. The results of the experiment are shown in Fig. 1.2 below.

Radioactivity

/arbitrary unit

Time/ minutes

P Q

R

Page 12: Cell Structure and Functions 2012 Tutorial - Answers Not Discussed for Uploading on KMS

Property of National Junior College H1 / H2 Biology

Jan 2012 12

Examiner’s Comments:

A straightforward question requiring students to recall the endomembrane system. A variety of answers were accepted, with the main focus on the relative order that students put the relevant organelles in. (Thus single answers for only one organelle without any appropriate relative order to other organelles were not awarded the mark) Some students wrote “ribosomes” as one of their answers forgetting that the question specified that P, Q & R were membraneous organelles. “Plasma membrane” was also a common wrong answer for R (it is not considered an organelle).

ii) With reference to Fig. 1.2, explain the changes in the level of radioactivity of the three organelles with time.

Radioactivity in P increases 1st after 0 mins, to 80 arbitary units, followed by Q and then R ; (data) (award for at least two organelles’ data described)

Ribosomes on the RER; add amino acids to the polypeptide chain / use amino acids to synthesise protein.

Vesicles containing radioactive proteins fuse with the Golgi Apparatus where modifications of the proteins take place / eg. of modification

(Transport) Vesicles containing radioactive proteins bud off the RER and move toward the Golgi Apparatus OR (Secretory) Vesicles bud off the GA and move toward the plasma membrane.

[3]

iii) Suggest what will happen to the level of radioactivity in the cell after 40 minutes. Explain your answer.

It will drop to a low level but not disappear / reach zero because some radioactive proteins may be retained within the cell OR some radioactive proteins are embedded in the cell’s membranes (OWTTE)

[1]

Examiner’s Comments: A few students rightly pointed out that radioactive proteins that were in the cell could possibly be degraded. However do remember that the (radioactive) amino acids will probably be recycled to synthesise other proteins thus leading to a low level of radioactivity in the cell even after some time.

Examiner’s Comments 1. A number of students did not realise that the molecules being transported from one

organelle to the next were not amino acids on their own, but rather after synthesis into polypeptides at the RER, it would be proteins that were transported.

2. Another common problem with phrasing: students wrote “protein buds off RER / GA” which is inaccurate. It is the VESICLE that buds off, the proteins are contained inside it.