2.1-Cell Structure

Question 1

Define Prokaryotic Cells.

Answer 1

DNA is ‘free’ in cytoplasm, no organelles e.g. bacteria & archaea.

Question 1

Define Eukaryotic Cells

Answer 1

DNA is contained in a nucleus, contains membrane-bound specialised organelles.

Question 2

State the relationship between a system and specialised cells.

Answer 2

Specialised cells —> tissues that perform specific function —> organs made of several tissue types —> organ systems.

Question 3

Describe the structure and function of the Cell-Surface Membrane.

Answer 3

‘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 4

Explain the role of cholesterol in the cell-surface membrane.

Answer 4

A steroid molecule that connects phospholipids and reduces fluidity.

Question 5

Explain the role of glycoprotiens in the cell-surface membrane.

Answer 5

Cell signalling, cell recognition (antigens) and binding cells together.

Question 6

Explain the role of glycolipids in the cell-surface membrane.

Answer 6

Cell signalling and cell recognition.

Question 7

Describe structure of the nucleus.

Answer 7

• Surrounded by nuclear envelope, a semi-permeable double membrane.
• Nuclear pores allow substances to enter/ exit.
• Dense nucleolus made of RNA and proteins assembles ribosomes.

Question 8

Describe the function of the nucleus.

Answer 8

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

Question 9

Describe the structure of mitochondrion.

Answer 9

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

Question 10

Describe the structure of a chloroplast.

Answer 10

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

Question 11

State the function of mitochondria.

Answer 11

Site of aerobic respiration to produce ATP.

Question 12

State the function of chloroplasts.

Answer 12

Site of photosynthesis to convert solar energy to chemical energy.

Question 13

Describe the structure and function of the Golgi Apparatus.

Answer 13

• Planar stack of membrane-bound, flattened sacs cis face aligns with rER.
• Molecules are processed cisternae vesicles bud off trans face via exocytosis:
  
  • Modifies & packages proteins for
    export
  • Synthesis glycoproteins

Question 14

Describe the structure and function of a lysosome.

Answer 14

Sac surrrounded by single membrane embedded H+ pump maintains acidic conditions conditions contains digestive hydrolase enzymes glycoprotein coat protects cell interior:
- Digests contents of phagosome
- Exocytosis of digestive enzymes

Question 15

Describe the structure and function of a ribosome.

Answer 15

Formed of protein & rRNA. Free in cytoplasm or attached to ER.
- Site of protein syntheis via translation:
large subunit: joins amino acids
small subunit: contains mRNA binding site.

Question 16

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

Answer 16

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

Question 17

Describe the structure of the Cell Wall.

Answer 17

Bacteria:
- Made of the polysaccharide murein.

Plants:
- Made of cellulose microfibrils plasmodesmata allow molecules to pass between cells, middle lamella acts as boundary between adjacent cell walls.

Question 18

Functions of the Cell Wall.

Answer 18

• Mechanical strength and support.
• Physical barrier against pathogens.
• Part of apoplast pathway (plants) to enable easy
  diffusion of water.

Question 19

Structure + Function of the Cell Vacuole in plants.

Answer 19

Surrounded by single membrane: tonoplast contains cell sap: mineral ions, water, enzymes, soluble pigments.

• Controls turgor pressure.
• Absorbs and hydrolyses potentially harmful substances
  to detoxify cytoplasm.

Question 20

Explain some common cell adaptations.

Answer 20

• Folded membrane or microvilli increase surface area
  e.g. for diffusion.
• Many mitochondria = large amounts of ATP for active
  transport.
• Walls one cell thick to reduce distance of diffusion
  pathway.

Question 21

State the role of plasmids in prokaryotes.

Answer 21

• Small ring of DNA that carries non-essential genes.
• Can be exchanged between bacterial cells via
  conjugation.

Question 22

State the role of flagella in prokaryotes.

Answer 22

• Rotating tail propels (usually unicellular) organism.

Question 23

State the role of the capsule in prokaryotes.

Answer 23

Polysaccharide layer:
- Prevents desiccation.
- Acts as food reserve.
- Provides mechanical protection against phagocytosis & external chemicals.
- Sticks cells together.

Question 24

Compare eukaryotic and prokaryotic cells.

Answer 24

Both have:
- Cell membarane.
- Cytoplasm.
- Ribosomes (don’t count as an organelle since not
membrane-bound.

Question 25

Contrast eukaryotic and prokaryotic cells.

Answer 25

Prokaryotic:
- Small cells and always unicellular.
- No membrane-bound organelles & no nucleus.
- Circular DNA not associated with proteins.
- Small ribosomes (70s).
- Binary fusion - always asexual reproduction.
- Murein cell walls.
- Capsule, sometimes plasmids & cytoskeleton.

Eukaryotic:
- Larger cells and often multicellular.
- Always have organelles and nucleus.
- Linear chromosomes associated with histones.
- Larger ribosomes (80s).
- Mitosis & meiosis - sexual and/or asexual.
- Cellulose cell wall (plants)/ chitin (fungi).
- No capsule, no plasmids, always cytoskeleton.

Question 26

Why are viruses referred to as ‘particles’ and not cells?

Answer 26

Acellular & non-living: no cytoplasm, cannot self-reproduce, no metabolism.

Question 27

Describe the structure of a viral particle.

Answer 27

• Linear genetic material (DNA or RNA) & viral
  enzymes e.g. reverse transcriptase.
• Surrounded by capsid (protein coat made of
  capsomeres).
• No cytoplasm.

Question 28

Describe the structure of an enveloped virus.

Answer 28

• Simple virus surrounded by matrix protein.
• Matrix protein surrounded by envelope derived
  from cell membrane of host cell.
• Attachment proteins on surface.

Question 29

State the role of the capsid on viral particles.

Answer 29

• Protect nucleic acid from degradation by restriction
  endonucleases.
• Surface sites enable viral particle to bind to & enter
  host cells or inject their genetic material.

Question 30

State the role of attachment proteins on viral particles.

Answer 30

Enable viral particle to bind to complementary sites on host cell: entry via endosymbiosis.

Question 31

Describe how optical microscopes work.

Answer 31

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 32

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

Answer 32

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. Avoid coverslip using mounted needle at 45* to
   avoid trapping air bubbles.

Question 33

Suggest the advantages and limitations of using an optical microscope.

Answer 33

+ Colour image
+ Can show living structures.
+ Affordable apparatus.
- 2D image
- Lower resolution that electron microscopes = c
cannot see ultrastructure.

Question 34

Describe how a Transmission Electron Microscope (TEM) works.

Answer 34

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

Question 35

Advantages and limitations of using a TEM.

Answer 35

+ Electrons have shorter wavelength than light = high resolution, so ultrastructure visible.
+ High magnification (x500000).
- 2D image.
- Requires a vacuum = cannot show living structures.
- Extensive preparation may introduce artefacts.
- No colour image.

Question 36

Describe how a Scanning Electron Microscope (SEM) works.

Answer 36

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 37

Advantages and limitations of using a SEM.

Answer 37

+ 3D image.
+ Electrons have shorter wavelength than light = high resolution.
- Requires a vacuum = cannot show living structures.
- No colour image.
- Only shows outer surface.

Question 38

Define magnification.

Answer 38

Factor by which the image is larger than the actual specimen.