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 ultra tome 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 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 SEM works

Answer 4

1. Focus a beam of electrons onto a specimens 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 samples surface
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 two 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 and lightly stained areas distinguishes structures

Question 10

State the magnification and resolution of a compound light microscope

Answer 10

Magnification - x2000
Resolution - 200 nm

Question 11

State the magnification and resolution of a TEM

Answer 11

Magnification - x500000
Resolution - 0.5nm

Question 12

State the magnification and resolution of a SEM

Answer 12

Magnification - x500,000
Resolution - 3-10nm

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 graticules and micrometer. Count how many graticule divisions are in 100um on the micrometer
3. Length of 1 eyepiece division = 100um / 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 and proteins assemble 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, semi conservative

Question 16

Describe the structure and function of the ER

Answer 16

Cisternae: network of tubules and flattened sacs extends from cell membranes and 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 and rRNA
Have large subunit which joins amino acids and 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 synthesis 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 stacks to form grana which contain photosystems with chlorophyll
  Intergranal lamallae: 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 phagsome
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 from part of apoplast pathway to allow molecules to pass between cells
• Middle lamella separates adjacent cell wells

Question 25

What are bacterial and fungal walls made of?

Answer 25

Bacteria : peptidoglycan Fungi : Chitin

Question 26

Describe the structure and function of the centrioles

Answer 26

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

Question 27

Describe the structure and function of the cell-surface plasma membrane

Answer 27

'Fluid mosaic' phospholipid bilayer with extrinsic and 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 cholestrol, glycoproteins and 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 and cell recognition

Question 29

Describe the structure and function of the 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 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 and always unicellular No membrane bound organelles and no nucleus Circular DNA not associated with proteins Small ribosomes (70S) Binary Fission - always asexual reproduction Made of murein (also known as peptidoglycan Capsule, sometimes plasmids 7 cytoskeleton Eukaryotic : Larger cells and often multicellular Always have organelles and nucleus Linear chromosomes associated with histones Larger ribosomes (80S) Mitosis and meiosis - sexual and/or asexual Absent in animal cells, cellulose cell wall (plants) / chitin (Fungus) No capsule, No plasmids, always cytoskeleton