Summary of Photosynthesis

Summary of Photosynthesis

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Communication, homeostasis and energy
F214 Unit 3: Photosynthesis
A calorimeter can be used to
measure the light absorption chlorophyll b
of different photosynthetic
pigments. Findings are plotted
rate of photosynthesis
onto graphs called absorption
%age absorption
The process of photosynthesis is one which converts light energy into chemical energy to synthesis large organic spectra and the effectiveness
chlorophyll a
molecules from smaller, inorganic substrates. Plants and other photosynthetic organisms are therefore autotrophic of pigments is shown on the
(they make their own food). There are two main stages of photosynthesis, which both occur inside the chloroplast action spectrum which shows
intergranal lamella Inside chloroplasts are stacks rate of photosynthesis at
starch ribosomes different wavelengths of light.
of flattened membrane
. The table at the bottom of the
grain . . . . . single compartments called grana
. . (each single compartment page summarises the
. . . thylakoid
absorption spectra by stating
called a thylakoid). Thin
the colours/wavelengths each 400 500 600 700 400 500 600 700
membranal extensions connect
granum wavelength of light (nm) wavelength of light (nm)
. one granum to another, these pigment best absorbs
are called intergranal lamellae.
. .
. . . The fluid surrounding the LIGHT-DEPENDENT REACTIONS
. . . . grana is called the stroma. The
. . . . . chloroplast contains its own
The first of two stages in photosynthesis is the light-dependent stage, which takes place in the thylakoid membranes. Two photosystems are used in
this stage: photosystem II and photosystem I, as well as the many ATP synthase enzymes embedded in the membranes to manufacture ATP
DNA DNA and many ribosomes, so
stroma that is can make proteins Firstly, there is photolysis of water (splitting using light photons). Two molecules of water ­ 2H2O ­ are split into four electrons, four protons and one
pair of oxygen atoms. This occurs at photosystem II (which is the first photosystem to be used). The oxygen is actually a waste product and mostly
Adaptations of the organelle for its function during photosynthesis include:
leaves the leaf through the stomata, although some oxygen remains in the plant to be used for aerobic respiration
the granal membranes provide a large surface area for the attachment of the photosynthetic pigments The electrons however, are
(chlorophylls and carotenoids), electron carriers and enzymes for the light-dependent reactions +2H
accepted by the PSII, and the
a network of proteins in the grana hold the pigments in a very precise manner that forms the photosystems - light energy excites the electrons
allowing for maximum absorption of light causing them to move between
light light
the membranes have ATP synthase attached to them which via chemiosmosis help to manufacture ATP electron ADP + Pi the pigments until they are
the fluid of the stroma holds all of the enzymes needed to carry out the light-independent reactions carriers
accepted by an electron carrier
+ which passes the electrons
PHOTOSYNTHETIC PIGMENTS AND PHOTOSYSTEMS stroma through the membrane, carrier-
to-carrier, until one passes the
Embedded in the thylakoid membranes are coloured
electrons to photosystem I (PSI)
compounds called photosynthetic pigments which absorb Light Light 2e
light of certain wavelengths and reflect other light. Pigments The movement of electrons
chlorophyll b between electron carriers
are found in large numbers arranged into photosystems carotene thylakoid space
- ATP synthase (cytochromes) releases energy,
2e +
There are two types of pigment found in photosystems. The 2H
+ which is used to actively pump
chlorophylls are chlorophyll b and chlorophyll a. The 2H
H2O protons across the membrane
carotenoids are xanthophyll and carotene. Chlorophyll b, ½O2 against the gradient
xanthophyll and carotene are known as secondary pigments,
closest to the stroma side of the membrane, and absorb the xanthophyll The electrons are accepted by more carriers from PSI and are eventually taken to an enzyme called NADP reductase which reduces a coenzyme called
chlorophyll a
light, exciting a pair of electrons to be passed to chlorophyll a NADP (nicotinamide adenine dinucleotide phosphate). The hydrogen ions (protons) produced from the photolysis of water remain in the thylakoid
the primary pigment space, in order to build up a heavy concentration within the thylakoid. This maintains a constant concentration gradient down from the thylakoid to
the stroma (where there is a weaker concentration). This means that when protons are pumped into the thylakoid space between photosystems due
Peak absorption wavelength
Pigment Colour Function in photosynthesis to the release of energy from electron carriers (this release of energy and movement of protons is called chemiosmosis), protons always flow back
over the membrane down the gradient through specialised channel proteins associated with ATP synthase. As protons flow through these channels,
Chlorophyll a Yellow-green 430, 662 ATP synthase is activated and adds one phosphate group to a molecule of ADP to produce ATP (this is photophosphorylation)
Absorbs red and blue-purple light
Chlorophyll b Blue-green 453, 642 As hydrogen ions flow back across the membrane into the stroma, they meet up with the coenzyme NADP and the electrons received from the
Carotene Orange 450 electron carriers. The electrons and protons (two of each) combine to form two hydrogen atoms, and then the enzyme NADP reductase catalyses the
Absorb purple light, protect chlorophylls
reaction between NADP and the hydrogen to produce reduced NADP (NADPH or NADPH2). NADPH is one of the products of this stage and is used as
Xanthophyll Yellow 450-470 from damage from light and oxygen
the main substrate for the second stage of photosynthesis

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During the light-dependent reactions, photophosphorylation occurs ­ this is the addition of an inorganic phosphate group (Pi) to a molecule of ADP to The enzyme ribulose bisphosphate carboxylase oxygenase (RuBisCO) is used in the Calvin cycle to add carbon
produce ATP in the presence of light. There are two types of photophosphorylation found in this series of processes dioxide to ribulose bisphosphate, but as RuBisCO's name suggests, it is also capable of combining RuBP with oxygen.…read more




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