2.1.1: Cell structure

Question 1

Outline how a student could prepare a temporary mount of tissue for a light microscope.

Answer 1

1. Obtain thin section of tissue e.g. using ultratome or by maceration.
2. Place plant tissue in a drop of water.
3. Stain tissue on a slide to make structures visible.
4. Add coverslip using mounted needle at 45° to avoid trapping air bubbles.

Question 2

Describe how light microscopes work.

Answer 2

1. Lenses focus rays of light and magnify the view of a thin slice of specimen.
2. Different structures absorb different amounts and wavelengths of light.
3. Reflected light is transmitted to the observer via the objective lens and eyepiece.

Question 3

Describe how a transmission electron microscope (TEM) works.

Answer 3

1. Pass a high energy beam of electrons through a thin slice of specimen.
2. More dense structures appear darker since they absorb more electrons.
3. Focus image onto fluorescent screen or photographic plate using magnetic lenses.

Question 4

Describe how a scanning electron microscope (SEM) works.

Answer 4

1. Focus a beam of electrons onto a specimen’s surface using electromagnetic lenses.
2. Reflected electrons hit a collecting device and are amplified to produce an image on a photographic plate.

Question 5

Describe how a laser scanning confocal microscope

works.

Answer 5

1. Focus a laser beam onto a small area on a sample’s surface using objective lenses.
2. Fluorophores in the sample emit photons.
3. Photomultiplier tube amplifies the signal onto a detector. An image is produced pixel by pixel in the correct order.

Question 6

How should the field of view in microscopy be

recorded?

Answer 6

Draw a diagram with a sharp pencil. Do not use sketchy lines or shading.

Include a scale bar.

Annotate visible structures.

Question 7

State an equation to calculate the actual size of a

structure from microscopy.

Answer 7

actual size = image size/magnification

Question 8

Define magnification and resolution.

Answer 8

Magnification: factor by which the image is larger than the actual specimen.

Resolution: smallest separation distance at which 2 separate structures can be distinguished from one another.

Question 9

Why do samples need to be stained for light

microscopes?

Answer 9

Coloured dye binds to the structures.
Facilitates absorption of wavelengths of light to produce image.
Differential staining: contrast between heavily & lightly stained areas distinguishes structures.

Question 10

State the magnification and resolution of a compound optical microscope.

Answer 10

magnification: x 2000
resolution: 200 nm

Question 11

State the magnification and resolution of a TEM.

Answer 11

magnification: x 500 000
resolution: 0.5 nm

Question 12

State the magnification and resolution of an SEM.

Answer 12

magnification: x 500 000
resolution: 3 - 10 nm

Question 13

Explain how to use an eyepiece graticule and stage micrometer to measure the size of a structure.

Answer 13

1. Place micrometer on stage to calibrate eyepiece graticule.
2. Line up scales on graticule and micrometer. Count how many graticule divisions are in 100μm on the micrometer.
3. Length of 1 eyepiece division = 100μm / number of divisions.
4. Use calibrated values to calculate actual length of structures.

Question 14

Describe the structure of the nucleus.

Answer 14

● Surrounded by a nuclear envelope, a semipermeable double membrane.
● Nuclear pores allow substances to enter/exit.
● Dense nucleolus made of RNA & proteins assembles ribosomes.

Question 15

Describe the function of the nucleus.

Answer 15

● Contains DNA coiled around chromatin into chromosomes.
● Controls cellular processes: gene expression determines specialisation & site of mRNA transcription, mitosis, semiconservative replication.

Question 16

Describe the structure and function of the endoplasmic reticulum (ER).

Answer 16

Cisternae: network of tubules & flattened sacs
extends from cell membrane & connects to nuclear envelope:
● Rough ER: many ribosomes attached for protein synthesis & transport.
● Smooth ER: lipid synthesis.

Question 17

Describe the structure and function of the Golgi

apparatus.

Answer 17

Planar stack of membrane-bound, flattened sacs, cis face aligns with rER. Molecules are processed in cisternae. Vesicles bud off trans face via exocytosis
● Modifies & packages proteins for export.
● Synthesises glycoproteins.

Question 18

Describe the structure and function of ribosomes.

Answer 18

Formed of protein & rRNA.

Have large subunit which joins amino acids & small subunit with mRNA binding site.

Question 19

Describe the relationship between the organelles

involved in the production and secretion of proteins.

Answer 19

The ribosomes that synthesise proteins are attached to the rER. The Golgi apparatus, which modifies proteins for secretion, aligns with the rER.

Question 20

Describe the structure of a mitochondrion.

Answer 20

● Surrounded by double membrane.
● Folded inner membrane forms cristae: site of electron transport chain.
● Fluid matrix: contains mitochondrial DNA, respiratory enzymes, lipids, proteins.

Question 21

Describe the structure of a chloroplast.

Answer 21

● Vesicular plastid with double membrane.
● Thylakoids: flattened discs stack to form grana; contain photosystems with chlorophyll.
● Intergranal lamellae: tubes attach thylakoids in adjacent grana.
● Stroma: fluid-filled matrix.

Question 22

State the function of mitochondria and chloroplasts.

Answer 22

● Mitochondria: site of aerobic respiration to produce ATP.

● Chloroplasts: site of photosynthesis to convert solar energy to chemical energy.

Question 23

Describe the structure and function of a lysosome.

Answer 23

Sac surrounded by single membrane embedded H+ pump maintains acidic conditions contains digestive hydrolase enzymes.
Glycoprotein coat protects cell interior:
● digests contents of phagosome
● exocytosis of digestive enzymes

Question 24

Describe the structure and function of a plant cell wall.

Answer 24

● Made of cellulose microfibrils for mechanical support.
● Plasmodesmata form part of apoplast pathway to allow molecules to pass between cells.
● Middle lamella separates adjacent cell walls.

Question 25

What are bacterial and fungal cell walls made of?

Answer 25

bacteria: peptidoglycan (murein)
fungi: chitin

Question 26

Describe the structure and function of centrioles.

Answer 26

● Spherical group of 9 microtubules arranged in triples.
● Located in centrosomes.
● Migrate to opposite poles of cell during prophase & spindle fibres form between them.

Question 27

Describe the structure and function of the

cell-surface plasma membrane.

Answer 27

‘Fluid mosaic’ phospholipid bilayer with extrinsic & intrinsic proteins embedded.
● Isolates cytoplasm from extracellular environment.
● Selectively permeable to regulate transport of substances.
● Involved in cell signalling / cell recognition.

Question 28

Explain the role of cholesterol, glycoproteins & glycolipids in the cell- surface membrane.

Answer 28

● Cholesterol: steroid molecule connects phospholipids & reduces fluidity.
● Glycoproteins: cell signalling, cell recognition (antigens) & binding cells together.
● Glycolipids: cell signalling & cell recognition.

Question 29

Describe the structure and function of flagella.

Answer 29

● Hollow helical tube made of the protein flagellin.

● Rotates to propel (usually unicellular) organism.

Question 30

Describe the structure and function of cilia.

Answer 30

● Hairlike protrusions on eukaryotic cells.

● Move back and forth rhythmically to sweep foreign substances e.g. dust or pathogens away / to enable the cell to move.

Question 31

Why is the cytoskeleton important?

Answer 31

● Provides mechanical strength.
● Aids transport within cells.
● Enables cell movement.

Question 32

Compare eukaryotic and prokaryotic cells.

Answer 32

Both have:
● cell membrane
● cytoplasm
● ribosomes

Question 33

Contrast eukaryotic and prokaryotic cells.

Answer 33

Prokaryotic:

• small cells & always unicellular
• no membrane-bound organelles & no nucleus
• circular DNA not associated with proteins
• small ribosomes (70S)
• binary fission - always asexual reproduction
• cellulose cell wall (plants)/ chitin (fungi)
• capsule, sometimes plasmids & cytoskeleton

Eukaryotic:

• larger cells & often multicellular
• always have organelles & nucleus
• linear chromosomes associated with histones
• larger ribosomes (80S)
• mitosis & meiosis - sexual and/or asexual
• absent in animal cells, in plants they are made of murein (also known as peptidoglycan)
• no capsule, no plasmids, always cytoskeleton