12.4 Studying Cells

Question 1

Define eukaryotic.

Answer 1

‘True nucleus’
DNA is enclosed by a nuclear membrane.
Have membrane bound organelles
e.g. animal cells

Question 2

Which two organelles of an animal cell are the only ones visible under a light microscope?

Answer 2

Nucleus
Cell membrane

Question 3

How are epithelial cells adapted for their role in absorption and secretion within the digestive system?

Answer 3

Cell membrane is folded into structures called microvilli which increase surface area for absorption.

Question 4

Give the structure and function of the nucleus.

Answer 4

Structure:
- Largest organelle in the cell.
- Nuclear envelope= double membrane.
- Nuclear pores= allow passage of molecules such as mRNA, in and out of the nucleus.
- Nucleoplasm= granular, jelly-like material.
- Chromatin= unwound, uncondensed DNA, wrapped around histone proteins.
- Nucleolus= which is where the rRNA is made to form ribosomes.

Function:
- Site of DNA replication and transcription (making mRNA).
- Contains DNA for each cell to synthesise proteins to control cell activity.

Question 5

Give the structure and function of the mitochondria.

Answer 5

Structure:
- Double membrane.
- Inner membrane is called the cristae.
- Fluid centre called the mitochondrial matrix.
- Prokaryotic, small, circular DNA and its own 70s ribosomes.

Function:
- Site of ATP production by aerobic respiration.
- DNA to code for the enzymes needed in respiration.

Question 6

Give the structure and function of the rough endoplasmic reticulum.

Answer 6

Structure:
- Made from highly folded membranes with 80s ribosomes embedded.
- Membrane is folded into flattened sacks called cisternae.
- Joined to the nucleus.

Function:
- Synthesises and transports proteins throughout the cell.

Question 7

Give the structure and function of the smooth endoplasmic reticulum.

Answer 7

Structure:
- Membrane is folded into flattened sacks called cisternae.

Function:
- Recombines glycerol and fatty acids to make triglycerides.
- Packages triglycerides into vesicles and transports them to the Golgi apparatus.

Question 8

Give the structure and function of the ribosomes.

Answer 8

Structure:
- Small
- Made up of 2 rRNA subunits.
- Eukaryotic cells contain 80s ribosomes.
- Prokaryotic cells, mitochondria and chloroplast contain 70s ribosomes.

Function:
- Site of protein synthesis.

Question 9

Give the structure and function of the Golgi apparatus and Golgi vesicles.

Answer 9

Structure:
- flattened sacs made of fluid-filled membranes pinch off smaller sacs called vesicles at their ends.

Function:
- Sorts, modifies and packages proteins and triglycerides into vesicles.
- Produce secretory enzymes.
- Form lysosomes.
- Add carbohydrates to proteins to form glycoproteins.

Question 10

Give the structure and function of lysosomes.

Answer 10

Structure:
- Bags of digestive/hydrolytic enzymes.
- Bound by a single membrane with no internal structure.

Function:
- Hydrolyse damaged, phagocytic and dead cells.
- Digest worn-out organelles for reuse of materials.
- Completely break down dead cells (autolysis).
- Exocytosis- release enzymes to outside of cell to destroy material.

Question 11

Give the structure and function of the cell surface membrane.

Answer 11

Structure:
- Phospholipid bilayer with molecules embedded within and attached on e.g. transport proteins, carbohydrates and cholesterol arranged in a fluid mosaic model.

Function:
- Controls passage of molecules in and out of the cell.

Question 12

Give the structure and function of centrioles. (ANIMAL ONLY)

Answer 12

Structure:
- Form a network of spindle fibres across the cell onto which the chromosomes attach.

Function:
- Pull the chromosomes/chromatids apart during mitosis.

Question 13

Give the structure of microvilli and how this speeds up absorption of digested food.

Answer 13

Structure:
- Finger-like projections of the cell membrane

Speeds up absorption by greatly increasing the surface area of the cell membrane.

Question 14

Give the structure and function of the vacuole. (PLANT ONLY)

Answer 14

Structure:
- Filled with fluid surrounded by a single membrane called a tonoplast.

Function:
- Make cells turgid and therefore provide support.
- Temporary store of sugars and amino acids.
- Pigments may colour petals to attract pollinators.

Question 15

Give the structure and function of chloroplasts. (PLANT ONLY)

Answer 15

Structure:
- Surrounded by a double membrane
- Contains thylakoids (folded membrane embedded with chlorophyll and ATP synthase enzyme to produce ATP), granum, stroma (fluid-filled part), and 70s ribosomes and prokaryotic DNA.

Function:
- Site of photosynthesis.

Question 16

Give the structure and function of the cell wall. (PLANTS, ALGAE AND FUNGI)

Answer 16

Structure:
- Plants- made of microfibrils of cellulose polymer. Very strong (with many weak H bonds between fibrils), permeable to most molecules, has plasmodesmata.
- Fungi- made of chitin, a nitrogen-containing polysaccharide.

Function:
- Provides structural strength to the cell.

Question 17

Define prokaryotic

Answer 17

‘Before nucleus’
Do not have nuclei or other membrane-bound organelles.
DNA is circular, does not form chromosomes, is found free in the cytoplasm, and is not associated with histones.
e.g. bacteria cells

Question 18

How do prokaryotic cells differ from eukaryotic cells?

Answer 18

They also differ from eukaryotic cells in having:

• cytoplasm that lacks membrane-bound organelles
• smaller 70s ribosomes
• no nucleus; instead they have a single circular DNA molecule that is free in the cytoplasm and is not associated with proteins
• a cell wall that contains murein, a glycoprotein.

Question 19

Differentiate between prokaryotic and eukaryotic cells.

Answer 19

1. In prokaryotic cells, DNA is circular and not associated with histones (proteins) WHEREAS in eukaryotic cells, DNA is linear (a line) and is associated with histones (proteins).
2. In prokaryotic cells, there are no membrane-bond organelles WHEREAS in eukaryotic cells there are membrane-bound organelles (such as Golgi apparatus, RER, SER, lysosomes)
3. Prokaryotic cells have no true nucleus, DNA is free in the cytoplasm WHEREAS in eukaryotic cells, there is a nucleus and DNA is contained within the nuclear membrane.
4. Prokaryotic cells contain smaller 70s ribosomes WHEREAS eukaryotic cells contain larger 80s ribosomes.
5. Some prokaryotic cells have capsule, one or more flagella and one or more plasmids WHEREAS eukaryotic cells do not have a capsule.
6. Prokaryotic cells have cell walls made of murein/peptidoglycan WHEREAS eukaryotic (plant) cells have a cell wall made of cellulose.

Question 20

Are viruses living?

Answer 20

Viruses are acellular and non-living.
They are very small and require a living cell to replicate inside.

Question 21

Describe the structure of a virus.

Answer 21

The structure of virus particles includes genetic material, capsid and attachment protein.
- Contains DNA/RNA which can be single/double-stranded.
- Surrounded by a protein coat called a capsid.
- Has attachment proteins which enable it to bind to host cells.
- Has enzymes that it uses to replicate genetic information and insert into host cell DNA.

Question 22

Principles and limitations of a light microscope

Answer 22

• Specimens illuminated with light, organelles stained with coloured dye to make them visible.
• Living or dead specimens.
• Limited resolution, cant see cell organelles.

Question 23

Define resolution.

Answer 23

Ability to distinguish 2 objects close together.

Question 24

Relationship between wavelength and resolution.

Answer 24

The shorter the wavelength of the light is, the better the resolution.

Question 25

How could the student have used an eyepiece graticule to determine the mean diameter of the stomata?

Answer 25

• Measure each stomata using an eyepiece graticule
• Calibrate the eyepiece graticule against a stage micrometre
• Take at least 5 measurements and calculate a mean (more measurements are more accurate)

Question 26

Principles and limitations of TEM.

Answer 26

• Electrons pass through the specimen, less dense areas absorb fewer electrons and appear lighter, and denser areas absorb more electrons and appear darker.
• View organelles/internal structures.
• Specimens fixed in resin, sliced extremely thin, must be dead. (vacuum)
• Specimens stained using heavy metals- complex, can create artefacts.

Question 27

Principles and limitations of SEM.

Answer 27

-Specimens are not sliced
-Electrons are bounced off the surface of the specimen
-3D image

Question 28

Magnification equation

Answer 28

Magnification= Image size/ Actual Size

Question 29

Describe and explain how cell fractionation and centrifugation can be used to isolate mitochondria from a suspension of animal cells.

Answer 29

1. Cell homogenisation to break open cells and release organelles;
2. Filter to remove (large) debris/whole cells;
3. Use isotonic solution to prevent osmotic damage to mitochondria / organelles;
4. Keep cold to prevent/reduce damage to organelles by enzyme;
5. Use buffer to maintain pH and prevent protein/enzyme denaturation;
6. Use differential Centrifuge (at high speed/1000 g) to separate nuclei / cell fragments / heavy organelles;
7. Re-spin (supernatant / after nuclei/pellet removed) at higher speed to get mitochondria in pellet/at bottom;
8. Observe pellet with a microscope to identify mitochondria;

Question 30

Eukaryotic cells produce and release proteins. Outline the role of organelles in the production, transport and release of proteins from eukaryotic cells.

Answer 30

1. DNA in nucleus is code (for protein);
2. Ribosomes/rough endoplasmic reticulum produce (protein);
3. Mitochondria produce ATP (for protein synthesis);
4. Golgi apparatus package/modify; OR Carbohydrate added/glycoprotein produced by Golgi apparatus;
   5 Vesicles transport OR Rough endoplasmic reticulum transports;
5. (Vesicles) fuse with cell(-surface) membrane;

Question 31

Scientists use optical microscopes and transmission electron microscopes to investigate cell structure. Explain the advantages and limitations of using a TEM to investigate cell structure.

Answer 31

Advantages:
1 Small objects can be seen;
2 TEM has high resolution;
3 Electron wavelength is shorter;

Limitations:
4 Cannot look at living cells;
5 Must be in a vacuum;
6 Must cut section / thin specimen;
7 Preparation may create artefact;

Question 32

Scientists isolated mitochondria from liver cells. They broke the cells open in an ice-cold, buffered isotonic solution. Explain why the solution was:
a) Isotonic
b) Ice cold
c) buffered

Answer 32

a) Prevents osmosis / no (net) movement of water So organelle/named organelle does not burst/shrivel;
b) Reduce/prevent enzyme activity so organelles are not digested / damaged;
c) Maintain a constant pH so proteins do not denature;

Question 33

Give one advantage of using a TEM rather than a SEM.

Answer 33

1. Higher resolution;
2. higher (maximum) magnification / higher detail (of image);
   OR
3. Allows internal details / structures within (cells) to be seen / cross section to be taken;