B3- Cell Structure

Question 1

Describe the function of the cell-surface membrane

Answer 1

• Selectively permeable→enables control ofpassage of substancesin / out of cell
• Molecules / receptors / antigenson surface→allow cellrecognition / signalling

Question 2

Describe the function of the nucleus

Answer 2

• Holds / storesgenetic informationwhich codes forpolypeptides(proteins)
• Site ofDNA replication
• Site oftranscription(part of protein synthesis), producingmRNA
• Nucleolusmakesribosomes/ rRNA

Question 3

Describe the structure of a ribosome

Answer 3

• Made ofribosomal RNAandprotein(two subunits)
• Nota membrane-bound organelle
• Site of protein synthesis (translation)

Question 4

Describe the structure of rough (rER) & smooth endoplasmic reticulum (sER)

Answer 4

Rough endoplasmic reticulum (RER)

• Surface covered in ribosomes
• Formed from continuous folds of membrane that is attached to the nuclear envelope
• Processes proteins made by the ribosomes

Smooth endoplasmic reticulum (SER)

• Does not have ribosomes on the surface, its function is distinct to the RER
• Involved in the production, processing and storage of lipids, carbohydrates and steroids

Question 5

Golgi apparatus and Golgi Vesicles

Answer 5

Golgi Apparatus

Modifies protein, eg. adds carbohydrates to produceglycoproteins
* Modifies lipids,eg. adds carbohydrates to makeglycolipids
* Packagesproteins / lipids intoGolgi vesicles
* Produceslysosomes(a type of Golgi vesicle)

Golgi Vesicle

• Transportsproteins / lipids to their required destination
• Eg. moves to andfuseswithcell-surface membrane

Question 6

Lysosomes

Answer 6

• Releasehydrolytic enzymes(lysozymes)
• To break down / hydrolysepathogensorworn-outcell components

Question 7

Mitochondria

Answer 7

• Site ofaerobic respiration
• To produceATPfor energy release
• Eg. for protein synthesis / vesicle movement / active transport

Question 8

Chloroplasts

Answer 8

• Absorbslightenergy forphotosynthesis
• To produceorganic substanceseg. carbohydrates / lipids

Question 9

Cell Wall

Answer 9

• Composed mainly ofcellulose(a polysaccharide) in plants / algae
• Composed ofchitin(a nitrogen-containing polysaccharide) in fungi
• Providesmechanical strengthto cell
• So prevents cell changing shape orburstingunder pressure due to osmosis

Question 10

Cell Vacuole

Answer 10

• Maintainsturgor pressurein cell (stopping plant wilting)
• Containscell sap→storessugars, amino acids,pigments and any waste chemicals

Question 11

What are the distinguishing features of prokaryotic cells?

Answer 11

• Cytoplasm lackingmembrane-bound organelles
• So genetic materialnotenclosed in a nucleus

Question 12

Explain why viruses are described as acellular and non-living

Answer 12

• Acellularnot made ofcells, no cell membrane / cytoplasm / organelles
• Non-livinghave no metabolism, cannot independently move / respire / replicate / excrete

Question 13

Describe the general structure of a virus particle

Answer 13

1. Nucleic acidssurrounded by acapsid(protein coat)
2. Attachment proteinsallow attachment to specific host cells
3. Nocytoplasm, ribosomes, cell wall, cell-surface membrane etc.
4. Some also surrounded by a lipid envelope eg. HIV

Question 14

Describe the difference between magnification and resolution

Answer 14

●Magnification= number of timesgreaterimage is than size of the real (actual) object

●Resolution =minimum distance apart 2 objects can be to bedistinguishedas separate objects

Question 15

Optical microscope

Answer 15

Lightfocused using
glasslenses
Lightpasses throughspecimen,
different structures absorb different amounts & wavelengths
Generates a2Dimage of across-section
Lowresolution due tolong wavelengthof light
Can’tseeinternal structureof organelles or ribosomes
Specimen =thin
Lowmagnification (x 1500)
Can viewlivingorganisms

Question 16

Transmission electron microscope (TEM)

Answer 16

Electronsfocused usingelectromagnets
Electronspass throughspecimen, denser partsabsorb moreand appeardarker
Generates a2Dimage of across-section
Very high resolution due toshort wavelengthof electrons
Canseeinternal structuresof organelles and ribosomes
Specimen =very thin
Highmagnification (x 1,000,000
Can only viewdead / dehydratedspecimens as uses avacuum
Complexpreparationsoartefactsoften present
Doesnotshow colour

Question 18

Scanning electron microscope (SEM)

Answer 18

Electronsfocused usingelectromagnets
Electronsdeflected/bounce offspecimen surface
Generates a3Dimage ofsurface
Highresolution due toshort wavelengthof electrons
Can’tsee internal structures
Specimen does not need to be thin
Highmagnification (x 1,000,000)
Can only viewdead / dehydrated
specimens as uses avacuum
Complexpreparationso
artefactsoften present
Doesnotshow colour

Question 19

Describe how the size of an object viewed with an optical microscope can be measured?

Answer 19

1. Line up(scale of) eyepiece graticule with (scale of)stage micrometre
2. Calibrateeyepiece graticule - usestage micrometreto calculatesize of divisionsoneyepiece graticule
3. Take micrometre away and use graticule to measurehow many divisionsmake up the object
4. Calculate size of object bymultiplyingnumber of divisions by size of division
5. Recalibrateeyepiece graticule at different magnifications

Question 20

Describe and explain the principles of cell fractionation and ultracentrifugation as used to separate cell components?

Answer 20

Homogenisetissue / use a blender

• Disruptscell membrane,breaking opencells and releasing contents / organelles

Place in acold, isotonic, bufferedsolution

• Cold toreduce enzyme activity→so organelles not broken down / damaged
• Isotonic so water doesn’t move in or out of organelles byosmosis→so they don’tburst
• Buffered to keeppH constant→soenzymesdon’tdenature

Filter homogenate

• Remove large, unwanteddebriseg. whole cells, connective tissue

Ultracentrifugation- separates organelles in order ofdensity / mass

• Centrifuge homogenate in a tube at ahigh speed
• Removepelletofheaviestorganelle andrespin supernatantat ahigher speed
• Repeat atincreasing speedsuntil separated out, each time pellet made oflighterorganelles (nuclei→chloroplasts / mitochondria→lysosomes→ER→ribosom s)

Question 21

Interphase

Answer 21

●(S phase)DNA replicatessemi-conservatively○Leading to2 chromatids(identical copies) joined at acentromere
●(G1/G2) number oforganelles& volume ofcytoplasmincreases, protein synthesis

Question 22

Mitosis

Answer 22

● Nucleus divides
● To produce 2 nuclei withidenticalcopies of DNA produced by parent cell

Question 23

Cytokinesis

Answer 23

● Cytoplasmandcell membrane(normally) divide
● To form 2 new genetically identical daughter cells

Question 24

Stage 1 Prophase

Answer 24

●Chromosomescondense, becomingshorter / thicker(so visible)○Appear as2 sister chromatidsjoined by acentromere
* Nuclear envelope breaks down
* Centriolesmove to opposite poles formingspindle network

Question 25

Stage 2 Metaphase

Answer 25

Spindle fibres attach to chromosomes by their centromeres * Chromosomes align along equator

Question 26

Stage 3 Anaphase

Answer 26

* Spindle fibres shorten / contract * Centromere divides * Pulling chromatids (from each pair) to opposite poles of cell

Question 27

Stage 4 Telophase

Answer 27

* Chromosomes uncoil, becoming longer / thinner * Nuclear envelopes reform = 2 nuclei * Spindle fibres / centrioles break down

Question 28

Why do some eukaryotic cells not undergo the cell cycle?

Answer 28

- Within **multicellular organisms, not all cells** retain the ability to divide (eg. neurons) - Only cells that do retain this ability go through a **cell cycle**

Question 29

Explain the importance of mitosis in the life of an organism?

Answer 29

- Growth of multicellular organisms by increasing cell number - Replacing cells to repair damaged tissues - Asexual reproduction

Question 30

Describe how tumours and cancers form?

Answer 30

- Mutations in DNA / genes controlling mitosis can lead to uncontrolled cell division - Tumour formed if this results in mass of abnormal cells - Malignant tumour = cancerous, can spread (metastasis) - Benign tumour = non-cancerous

Question 31

Suggest how cancer treatments control rate of cell division

Answer 31

Some disrupt spindle fibre activity / formation - So chromosomes can’t attach to spindle by their centromere - So chromatids can’t be separated to opposite poles (no anaphase) - So prevents / slows mitosis ● Some prevent DNA replication during interphase - So can’t make 2 copies of each chromosome (chromatids) - So prevents / slows mitosis

Question 32

Describe how prokaryotic cells replicate

Answer 32

Binary fission: 1. Replication of circular DNA 2. Replication of plasmids 3. Division of cytoplasm to produce 2 daughter cells ◦ Single copy of circular DNA ◦ Variable number of copies of plasmids

Question 33

Describe how viruses replicate

Answer 33

Being non-living, viruses do not undergo cell division. 1. Attachment proteins attach to complementary receptors on host cell 2. Inject viral nucleic acid (DNA/RNA) into host cell 3. Infected host cell replicates virus particles: 1. Nucleic acid replicated 2. Cell produces viral protein / capsid / enzymes 3. Virus assembled then released