QUESTION 1
Chloroplasts are the site of photosynthesis. Reactants are water and carbon dioxide; sunlight is source of
(solar) energy. Products are glucose and oxygen.
sunlight
(water + carbon dioxide →glucose + oxygen).
Chloroplasts are lens-shaped / elongated to increase surface area (SA) to Volume (Vol) ratio. This increases
uptake of CO2 and H2O into chloroplast and the output of O2 and glucose from chloroplast, so increasing
photosynthetic capacity.
Grana are stacks of thylakoid membranes within the chloroplast. The chlorophyll molecules are embedded
in these membranes; the large number / stacks of membranes provides a large SA for large amounts of
chlorophyll. Chlorophyll captures solar energy and convert it to chemical energy (ATP) which is used to
bond carbon dioxide and hydrogen (from water) to form glucose (in stroma). Oxygen is produced /
released as a waste product. The more chlorophyll, the greater the amount of solar energy captured hence
the greater the photosynthetic capacity.
Chloroplasts are located just below the plasma membrane / outer layer of cytoplasm. Reasons are :
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Receives higher light intensity than if located deeper in cytoplasm, so increasing rate of
photosynthesis.
Shortens the diffusion distance for carbon dioxide and water which diffuse through the plasma
membrane to and into the chloroplasts. Chloroplasts obtain the reactants faster than if embedded
deeper in cytoplasm so increasing photosynthetic capacity
Shortens the diffusion distance for oxygen which diffuses out of the chloroplasts and to and
through plasma membrane to then exit the leaf. This ensures more rapid removal of a waste
product than if chloroplasts embedded deeper in cytoplasm.
QUESTION 2
a) A typical cell moves (cycles) between periods of cell division / mitosis and periods of growth and
normal activity (interphase), then back to division. After mitosis, the cell grows and carries out
essential processes such as synthesis of lipids, proteins, nucleotides. DNA replication occurs
followed by preparation for mitosis then mitosis itself. The cycle then repeats.
b) DNA contains the genetic code / code for synthesis of proteins / enzymes in the sequence of its
bases.
New / all cells need the code in order to synthesise the essential proteins needed for the cell to
grow and function / carry out its role. Therefore, DNA must replicate to ensure that all new cells
have a complete copy of the code.
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c) Key step in replication are :
DNA helix unwinds
two strands separate (‘unzip’) between bases
new base pairs join / bond to each old / separated bases
two DNA molecules are produced when pairing is complete
Base pairing is Adenine (A) – thymine (T) and Cytosine (C) - guanine (G)
Base pairing is essential as it provides the mechanism / allows for DNA replication.
When the two DNA strands separate between the bases, a new base can bond to an exposed base
according to the base pairing rule. As a result two identical DNA molecules are formed.
Replication is semi conservative as in each of the two DNA molecules, one strand is old and the other
strand is new. The two old strands have been the template for the formation of the two new strands.
DNA contains the genetic code in the order / sequence of its bases. Each DNA molecule formed in
replication needs a complete copy of the code; it is semi-conservative replication that allows the DNA
molecules formed to have the complete genetic code.
QUESTION 3
(Rate of) Chemical reactions increase as temperature increases as there are more (successful) collisions
between the reacting particles.
Enzyme activity increases with increasing temperature as more (successful) collisions between reactant(s)
and enzyme.
At optimum temperature, maximum catalysis is occurring and reaction is at its highest rate.
Enzymes are (biological) catalysts and speed up / control biochemical reactions such as DNA replication.
Enzymes are specific in the reactions they catalyse as each enzyme has an active site that only fits with that
of the reactant(s) eg one enzyme unwinds DNA helix, another enzyme separates the bases in DNA
replication.
Note : names of particular enzymes not required.
The active site is dependent on weak hydrogen bonds holding the enzyme in shape.
As temperatures increase above the optimum, the hydrogen bonds break and the active site is lost as the
enzyme denatures. Therefore, the enzyme can no longer catalyse its particular reaction and the overall
reaction eg DNA replication ceases.