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

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H1 / H2 Biology Tutorial Ans

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CORE SYLLABUS CELLULAR FUNCTIONSSub-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 o o A o main identifying features Bound by a double membrane Inner membrane is folded extensively to form cristae (mitochondrion): no mark awarded Consists of a system of flattened membranous sacs called cisternae Presence of ribosomes studded on the cytoplasmic surface of the membrane (RER): no mark awarded o C o Consists of interconnecting membrane-bound tubules Absence of ribosomes (SER): no mark awarded o Alternative Answer C o Membrane-bound sacs o Contains hydrolytic enzymes, which hydrolyses the materials taken in by phagocytosis For autolysis / self-destruction of dying cells by the release of the lysozyme into the cell Transfer of lipids / proteins to Golgi body [6]March 2011 1

main functions o Involved in ATP synthesis to provide energy for the functioning of the cell o Secretory proteins are synthesized on the RER o Proteins synthesized on the RER are transported within vesicles to the Golgi body o o Site of lipid synthesis Involved in drug detoxification

o B

(Vesicle/Lysosome): no mark awarded o

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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]

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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.

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(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]

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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] Examiners 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.

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(b) (i) State the main function of each organelle. A ATP Synthesis / Energy release (Reject: Energy production) B Protein synthesis 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 [2]

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 doesnt 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]

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Non-compulsory Essay AnsN99/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 ribosomes 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.

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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 riboso

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