1 Exam Questions

Question 1

Outline the use of human embryonic stem cells to treat Stargardt’s disease 2

Answer 1

• inherited form of blindness / retina degeneration
• inject differentiated stem cells from embryonic stem cells into eye
• clinical tests done on human and animals

Question 2

compare the genetic material of pro and eukaryotes

[6]

Answer 2

prokaryotes

• plamids
• circular
• no histone proteins, naked DNA
• one chromosome
• no introns
• nucleoid region
• one replication point

eukaryotes

• no plasmids
• linear
• histone proteins
• many chromosomes
• introns and exons
• nucleus
• many replication point

both

• use DNA
• mitochondria and chloroplasts similar to pro

Question 3

what are pili used for

Answer 3

adhesion to other membranes or conjugation

Question 4

discuss the endosymbiotic theory for the origin of eukaryotes

[6]

Answer 4

eukaryotes evolved from prokaryotes;

mitochondria/chloroplasts evolved from (independent) prokaryotic cells;

taken in by larger (heterotrophic) cell by endocytosis;

theory supported by characteristics of chloroplasts/mitochondria;

[2 max] for mitochondria/chloroplast characteristics:

mitochondria/chloroplasts have naked DNA;

mitochondria/chloroplasts divide/carry out fission;

mitochondria/chloroplasts have 70S ribosomes / synthezise own proteins;

mitochondria/chloroplasts have double membranes;

cristae similar to mesosomes / thylakoid have similar structures in prokaryotes;

but theory cannot be falsified as it predicts something occurring in the past;

theory does not explain the origins of cilia/flagella/linear chromosomes/meiosis;

weaker evidence that cilia/flagella evolved from attached bacteria/spirochetes;

Question 5

describe briefly the endosymbiotic theory [3]

Answer 5

eukaryotes evolved from prokaryotes ✔

b. prokaryotes engulfed other prokaryotes without digesting them ✔
c. engulfed aerobic cell/prokaryote became mitochondria ✔
d. engulfed photosynthetic cell/ prokaryotes became chloroplasts ✔
e. these organelles have a double membrane «due to the engulfing process» ✔
f. mitochondria/chloroplasts contain DNA/small ribosomes/70S ribosomes ✔

Question 6

Discuss the evidence for the theory that mitochondria may have evolved
from free-living prokaryotes by endosymbiosis. [3]

Answer 6

a. «double» membrane may have formed when engulfed ✔

b. replicate by binary fission like free-living prokaryotes
OR
reproduce separate from «host» cell replication ✔

c. they have their own «circular» DNA AND reproduce on their own ✔
d. they have «70s» ribosomes AND can manufacture «their own» proteins ✔
e. have organelle«s» similar to free-living prokaryotes ✔
f. similar in size to free-living prokaryotes ✔
g. mitochondrial inner membranes manufacture ATP like bacterial membranes ✔
h. currently there is no free-living prokaryote like a mitochondrion ✔

Question 7

Discuss the cell theory and its limitations. [6]

Answer 7

cell theory:

a. cell theory is the accepted explanation of life ✔
b. organisms are composed of «one or more» cells ✔
c. cells are the basic/fundamental/smallest units of life ✔
d. cells can only come from pre-existing cells ✔
e. spontaneous generation of life has been disproven ✔

limitations:
f. striated muscle cells contain many nuclei «while most eukaryotic cells have
one nucleus»
OR
red blood cells have no nucleus «while most eukaryotic cells have one nucleus» ✔

g. giant algae have complex single cell structure
OR
organisms as large as giant algae would be expected to be multicellular, but they have only one cell with one nucleus ✔

h. aseptate fungal hyphae are tube-like structures that contain no cell membranes between the many nuclei
OR
slime molds contain many nuclei ✔

i. viruses have some characteristics of living organisms but are not cells ✔
j. if all cells come from pre-existing cells, where did the first one come from? ✔

Question 8

every cell is surrounded by cell surface membranes which regulate the movement of materials into and out of the cell.

Discuss alternative models of membrane structures including evidence for and against each model [8]

Answer 8

1. early evidence showed membranes are partially permeable and organic solvents penetrate faster than water
2. suggests they have non polar regions
3. chemical analysis showed membranes consist mainly of proteins and lipids
4. layer of phospholipids spread over water, orientate tehmselves into monolayer with nonpolar tails out of water and polar head in water surface
5. when shaken with water form micelles/particles with tails inwards away form water
6. davson -danielli model proposed phospolipid bilayer ocated with protein molecules on both surfaces
7. evidence from electron microscope supported this
8. three layered structure / railway tracks / dark bands with light between
9. model did not account for hydrophobic proteins due to low resolution
10. fluorescent labelling / freeze fracturing used to investigate membrane structure
11. led by singer-nihcolson / fluid mosaci model with proteins floating in fluid lipid bilayer
12. shwos particles /proteins project partially and sometimes right thourhg lipid bilayer
13. indicated peripheral and intergral proteins present

Question 9

explain the importance of the surface area to volume ration as a limiting factor of cells size (3)

Answer 9

the rate of material / heat exchange / diffusion is proportional to surface area;

the rate of metabolism is proportional to mass / volume;

as a cell grows, the volume increases faster than the surface area / as a cell grows, surface area to volume ratio decreases;

(without cell division) material / heat cannot be exchanged fast enough to meet the needs of the cell;

small cells have larger ratio (than larger cells) / ratio decreases as size increases; surface area / membrane must be large enough to absorb nutrients / oxygen / substances needed;

surface area / membrane must be large enough to excrete / pass out waste products;

need for materials is determined by (cell) volume;

cell size is limited (by SA / vol ratio) / cells divide when they reach a certain size; reference to diffusion across / through membrane / surface area;

rate of transport of materials across membrane;

energy / heat is a function of surface area;

rate of metabolism is a function of mass to volume ratio;

as size increases the ratio between SA and volume decreases;

causing problems in transport / rate of exchange;