F214 module 3 Photosynthesis

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F214 Communication, Homeostasis and Energy
Module 3 Photosynthesis
4.3.1 Photosynthesis
a. Define the terms autotroph and heterotroph
Autotroph
o Organisms that can synthesise complex molecules like carbohydrates,
lipids, proteins, nucleic acid and vitamins from inorganic molecules as a
source of energy.
o They also use light energy and chemical energy.
Heterotroph
o Organisms which cannot make their own food.
o They digest complex organic molecules into simpler soluble ones which
they then use to synthesise complex molecules such as lipids, proteins and
nucleic acids.
o The chemical potential energy is released from the complex organic
molecules.
b. State that light energy is used during photosynthesis to produce complex
organic molecules
Summarised equation: 6CO2 + 6H2O (+ light energy) C6H12O6 + 6O2
c. Explain how respiration in plants and animals depends upon the products of
photosynthesis
Oxygen needed for aerobic respiration first appeared in the atmosphere due
to photosynthesis.
It allows phototrophs and heterotrophs to release the chemical potential
energy in complex organic molecules which were made during photosynthesis.
d. State than in plants photosynthesis is a two-stage process taking place in
chloroplasts
Photoautotrophs have organelles called chloroplasts where photosynthesis
takes place.
There are two stages to photosynthesis: the light dependant stage and the light
independent stage.
e. Explain how the structure of chloroplasts enables them to carry out their
functions

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Most chloroplasts are disk shaped and between 2-10m long.
Each chloroplast has an envelope which below is an intermembrane space about
10-20nm wide.
The outer membrane is permeable to many small ions.
The inner membrane is less permeable with transport proteins embedded. It is
folded into lamellae which are stacked up to form a granum.
The grana are attached by intergranal lamellae.
The stroma is a fluid filled matrix where the light dependant stage takes place
due to it containing the enzymes needed.…read more

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They are in the thylakoid membranes.
o They are arranged in funnel shaped structures called photosystems.
g. Explain the importance of photosynthetic pigments in photosynthesis
Chlorophylls
o Chlorophyll is a mixture of pigments.
o They have a long hydrocarbon chain and a porphyin group.
o When light hits them it causes a pair of electrons associated with
magnesium to become excited.
o There are two forms of chlorophyll a both of which appear green: P680
and P700.…read more

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The excited electrons pass to an electron acceptor and back to the
chlorophyll molecule it was from.
o There is no photolysis of water and no generation of reduced NADP.
o Small amounts of ATP which may be used in the light dependant reaction
or used in guard cells to bring potassium ions which lowers the water
potential which causes water to follow by osmosis causing the guard
cells to swell and open the stomata.…read more

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The carbon dioxide combines with the 5 carbon compound ribulose
bisphosphate (RuBP) which is catalysed by ribulose bisphosphate
carboxylase-oxygenase (rubisco). RuBP is carboxylated which is when it
is combined with carbon dioxide to have a carboxyl group.
o This makes two 3 carbon compounds glycerate 3 phosphate (GP). The
carbon dioxide is fixed.
o GP is reduced and phosphorylated to glycerate phosphate (TP) during
which ATP and reduced NADP from the light dependant reaction are
used.…read more

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So carbon dioxide uptake is reduced as is the rate of
photosynthesis.
o If carbon dioxide concentration is lower than 0.01% it allows RuBP to
build up so levels of GP and TP are reduced.
Light intensity
o It gives us a measure of how much energy is associated with the light.
o An increase in light intensity alters the role of the light dependant
reaction.
Because there is more light available, more electrons are excited.…read more

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Light intensity
o As light intensity increases, the rate of photosynthesis increases. It is
directly proportional.
o Light causes stomata to open to allow carbon dioxide into the leaves, it
is trapped by chlorophyll to excite electrons and it splits water
molecules to produce protons which are involved in
photophosphorylation.
o The rate of photosynthesis varies during the day as the light intensity
varies.
Temperature
o The enzyme catalysed reactions of the Calvin Cycle are affected by
changes in temperature.…read more

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