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Sugar is oxidised to carbon dioxide
and water, making energy become available
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ATP is referred to as energy currency, because it is constantly recycled, ADP and a Pi bond
combine due to respiration or photosynthesis, and this ATP returns to its previous state once energy
is used for metabolic reactions or muscle movements
...
Much of the energy transferred is lost
as heat, but cells can still retain energy in the
form of ATP
...
Glycolysis, in which glucose is
converted to pyruvate
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The Link reaction, and Kreb’s cycle, in which pyruvate is converted to carbon dioxide
...
Oxidative phosphorylation (the electron transport chain), in which hydrogen removed in the
previous oxidation reactions, is converted to water, and most of the ATP is synthesised
...
The enzymes of glycolysis
are located in the cytoplasm outside the organelles,
rather than in the mitochondria, and it occurs in four
stages
...
At this stage, two
molecules of ATP are consumed per molecule
of glucose
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Oxidation- Of the three-carbon sugar molecules occurs by the removal of hydrogen
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This is a

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molecule that can accept hydrogen ions and electrons
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ATP formation- Occurs twice in the reactions, by which each triose phosphate molecule is
converted to pyruvate
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The Link Reaction:
Pyruvate diffuses into the matrix of the mitochondrion as it forms and is metabolised there
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The product of this oxidative decarboxylation
reaction is an acetyl group- a two carbon fragment
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The coenzyme involved is called coenzyme A, so acetyl coenzyme A is formed
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Pyruvate + NAD + Coenzyme A  NADH/H+ + Carbon Dioxide + Acetyl CoA
The Kreb’s Cycle:
Acetyl CoA reacts with four-carbon organic acid
(oxaloacetate, OAA)
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Then the citrate is converted back to the fourcarbon acid by the reactions of the Kreb’s cycle,
which involve the following changes –
•

•
•
•

Two molecules of carbon dioxide are
given off in separate decarboxylation
reactions
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Three molecules of reduced NAD are formed
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Because glucose is converted to two molecules of pyruvate in glycolysis, the Kreb’s cycle sequence of
reactions occurs twice for every molecule of glucose metabolised
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Reduced FAD
0
0
2
2

Oxidative Phosphorylation:
The hydrogen atoms or their electrons are transported along a series of carriers, from the reduced
NAD or FAD, to be combined with oxygen to form water
...
This final reaction to form water only occurs after the
energy level has been lowered by a series of transfers between carriers, bringing about the gradual
transfer of energy
...
Transfer of energy in this
manner is controlled and can be used by the cell
...
For every molecule of reduced NAD that is oxidised, approximately three molecules of
ATP are produced
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There are other pathways involved, making the value of 38
only approximate
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Electron carrier proteins are arranged in
the inner mitochondrial membrane in a
highly ordered way
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Here the H+ ions accumulate, causing the PH to drop
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Eventually, the protons do flow back into the matrix, via channels in ATP synthetase enzymes, also
found in the inner mitochondrial membrane
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Anaerobic respiration:
In the absence of oxygen, many organisms will continue to respire through a process known as
anaerobic respiration or fermentation
...
The Kreb’s cycle will also cease, and therefore glycolysis is
relied upon to produce the ATP
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The lack of oxygen causes a build up of pyruvic acid as there is no link reaction to use it up, and it is
then channelled into other biochemical pathways
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Both lactate and ethanol contain large quantities of chemical energy, indicating that the glucose has
only been partially broken down and explaining the low ATP yield
...
The muscles then need to
respire anaerobically, leading to a rising concentration of lactate, which gradually inhibits muscle
contractions
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Anaerobic fermentation in plants and yeast:
The ability of plant cells, and particularly yeast, to produce ethanol by fermentation is the basis of
the alcohol industry
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Definitions:
Aerobic respiration: The chemical breakdown of substrate molecules in cells to release energy in the
form of ATP when oxygen in present
...

Kreb’s cycle: An intermediate stage in aerobic respiration during which the products of glycolysis are
decarboxylated to form carbon dioxide and reduced coenzymes
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Coenzyme: An organic non-protein molecule that binds to an enzyme to allow it to carry out its
function
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ATP synthetase: An enzyme found in the inner mitochondrial membrane, which channels hydrogen
ions through the membrane during ATP synthesis by chemiosmosis
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2
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Plants
need to respire to produce ATP for cellular processes;
photosynthesis produces the respiratory substrates
required
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Higher plants have two types – chlorophylls and carotenoids
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Carotenoids absorb light energy,
which is passed on to chlorophyll, and so they are called accessory pigments
...
The chlorophyll molecules are attached to the membranes
of the chloroplast by their long tails, whilst their heads lie flat on the membrane surface in order to
maximise light absorption
...
The graph of
the amount of light absorbed is known as an absorption
spectrum
...
The action
spectrum for photosynthesis shows a very similar pattern to
the absorption spectrum, which provides good evidence to
support the idea of light being trapped in these pigments and
used in photosynthesis
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Almost all plants use chlorophylls as their primary pigments at the start of
photosynthesis, but the range of accessory pigments can vary depending upon the habitat
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The waste product is oxygen
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Light energy is trapped by chlorophyll, and becomes the chemical energy in glucose, and then ATP
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Sugars formed may temporarily be stored as starch, but most is used in metabolism
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Carbon Dioxide + Water + Light Energy –Chlorophyll in chloroplast-> Organic compounds + Oxygen
Chloroplasts:
Chloroplasts are the organelles in which the reactions of photosynthesis occur, and they are a type
of plastid similar to amyloplasts where starch is stored
...


The chloroplast is contained by a double membrane, with the outer membrane being a continuous
boundary, but the inner membrane ‘infolds’ to form branching membranes called
lamellae/thylakoids
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These hold the chlorophyll
...

Photosynthesis is made up of two stages: The light
dependent reactions, and the light independent reactions
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Alongside this, ATP is generated from ADP and phosphate, also using energy from light
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Oxygen is given off as a waste product of the light-dependent reactions,
and this stage occurs in the grana
...
Rather than
a random spread of chlorophyll molecules being present, they are grouped together in structures
called photosystems, held in the thylakoid membranes of the grana
...
They all
‘funnel’ the energy into a single chlorophyll molecule within the photosystem, known as the reaction
centre
...

There are two types of photosystem present in the thylakoid membranes, identified by the
wavelength of light that the chlorophyll of the reaction centre absorbs
...
This reaction
centre is referred to as P700
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The two photosystems occur grouped together in the thylakoid membranes of the grana, along with
certain proteins that function specifically as either,
•
•
•
•

Enzymes catalysing the splitting or water into hydrogen ions, electrons, and oxygen atoms
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Enzymes catalysing the conversion of oxidised H-carrier (NADP+) to reduced carrier
(NADPH/H+)
Electron carrier molecules (large proteins)
...
High energy electrons are
released from this chlorophyll molecule as a result, and these electrons bring about the biochemical
changes o the light-dependent reactions
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The excited electrons from photosystem II are picked up, and passed along, a chain of electroncarriers
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Here, they accumulate, causing the PH to drop
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This is an example of chemiosmosis
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Therefore, the electrons
have been transferred from photosystem II to photosystem I
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The positively charged ‘vacancies’ in photosystem II are powerful enough to
cause photolysis in the presence of a specific enzyme
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The oxygen atoms combine to form molecular oxygen, which is the waste product
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In the grana of the chloroplasts, the synthesis of ATP is coupled to electron transport via the
movement of protons by chemiosmosis
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At the same time, ATP is
synthesised from ADP and Pi, in a process called phosphorylation
...
The photophosphorylation reaction in which they are involved is described
as non-cyclic photophosphorylation, because the pathway of electrons is linear
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They are passed
to NADP+ two at a time, which, with the addition of hydrogen ions from photolysis, is reduced to
form NADPH/H+
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ATP and NADP do not normally accumulate, as they are immediately used in the fixation of carbon
dioxide in the surrounding stroma in the light independent reactions
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The Light-Independent Reactions:
In these reactions, carbon dioxide is converted to carbohydrate
...
Carbon dioxide readily diffuses into the chloroplast where it is
built up into sugars in a cyclic process called the Calvin cycle
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The stroma is packed with rubisco, which easily makes up the bulk of all protein in the
green plant
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The product is two molecules of a three-carbon
compound, glycerate-3-phosphate (GP)
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Some of the GALP is converted into
products of photosynthesis, such as glucose,
or amino acids and fatty acids
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But the bulk of GALP is
converted to more acceptor molecules,
enabling fixation of carbon dioxide to
continue
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GP combines with two protons from
NADPH/H+ using energy from the conversion of ATP to ADP, and forms water, released as a byproduct, and GALP
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Photosynthesis and Plant Metabolism:
The first sugar produced in photosynthesis is a three-carbon compound, GP
...
The remainder is
converted into the carbohydrate products of photosynthesis, mainly glucose and starch, or serves as
intermediates that are starting points for all other metabolites the plant requires
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The intermediates of respiration are also the
starting points for the synthesis of other metabolites
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Lipids, including membrane lipids
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Nucleic acids, growth factors, vitamins, hormones and pigments
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In general, temperature has no

effect on the rate at low light intensities but shows a familiar response to increased temperature
at high light intensities
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Definitions:
Accessory pigments: Light-absorbing molecules that pass on the energy they absorb to chlorophyll
molecules at the start of photosynthesis
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Hydrophobic: Substances that repel water
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Absorption spectrum: A graph of the amount of light absorbed at different wavelengths by a
pigment
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Autotrophic nutrition: The synthesis of larger organic molecules from simpler inorganic compounds
as carried out by plants during photosynthesis
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Stroma: The aqueous matrix found inside chloroplasts
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Photophosphorylation: The production of ATP from ADP using energy from light during
photosynthesis
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Non-cyclic photophosphorylation: The production of ATP from ADP using light energy in a linear
series of reactions
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Rubisco: The abbreviation for the enzyme ribulose bisphosphate carboxylase, which is important in
fixing atmospheric carbon dioxide during photosynthesis
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Limiting factor: In any process controlled by more than one factor, then the rate of the overall
process will be determined by the availability of the least favourable factor

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