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Summary 13 Photosynthesis
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EVERYTHING you need to know about "Photosynthesis" for A-level Biology CIE
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PHOTOSYNTHESIS
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Biology AS Best summary notes
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Cell structure_ A level
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Photosynthesis13 November 2021 11:19
The Structure and Function of Chloroplasts
• Chloroplasts are biconvex discs about 3-10µm in diameter
• Numbers of chloroplasts within cells range from just a few to over 100 in a palisade
mesophyll cell, which is at the top of a leaf
• Each chloroplast is surrounded by an envelope of two phospholipid membranes
• In the middle of the chloroplast is a fluid medium known as the stroma
• In the stroma is an additional, substantial membrane system consisting of a series
of flattened, fluid-filled sacs called thylakoids
• These form stacks in some places, known as grana
The Grana:
• Each granum is joined to another by membrane links
• The grana provide a large surface area
• The grana hold groups of pigments, enzymes, and electron carriers needed for a
process known as the light dependent reaction
• The large surface area means that many of these molecules can be located here
• The pigment molecules are arranged in Light Harvesting Clusters for efficient light
absorption, and each cluster is known as a photosystem
• The light dependent reaction only occurs in the presence of light
The Stroma:
• The stroma, the fluid centre of the chloroplast, contains a loop of DNA, 70S
ribosomes, lipid droplets, and starch grains
• The chloroplasts are semi-autonomous in that it makes some of its own proteins,
although other chloroplast proteins are made by DNA in the nucleus
• The stroma is the site of the light independent reaction, which is often known as
the Calvin Cycle. Therefore it contains the base molecules and enzymes required for
this process to occur
Photosynthesis
• Photosynthesis is the trapping or fixation of carbon dioxide and its subsequent
reduction to carbohydrates, using hydrogen derived from water
• The whole process occurs within chloroplasts
• Two sets of reactions are involved, the light dependent reaction, for which light is
essential, and the light independent reaction, which does not need light
• However, the light independent reaction does require molecules made by the light
dependent reaction, so they occur in sequence
The Light Dependent Reactions:
• These only occur if there are suitable pigments able to absorb certain wavelengths
of light
• These pigments fall into two categories: primary pigments and accessory pigments
• The pigments are arranged in light harvesting clusters called photosystems
• There are two different types of photosystem, known as I and II
• In a photosystem, several hundred accessory pigments surround a primary
molecule, also known as the reaction centre (chlorophyll A)
• Different plants have evolved different pigments
• Some plants have plenty of access to light, while others live in darker areas, such as
below forest canopies
• This means they have evolved different pigments to absorb the different
wavelengths they can access
• So although chlorophyll is a pigment found in most plants, it may not be the most
common pigment in a plant, so not all plants are green
• Leaves come in a number of colours depending on available wavelength
• Once the light has been absorbed it is needed for two functions
• The first is to provide enough energy to split water (photolysis)
• This forms hydrogen and oxygen
• Oxygen is a waste product and exits the cell and leaf (if not used for respiration)
• Hydrogen is split into electrons and protons
• The second use of light is to provide energy, in the form of ATP, to carry forward to
the light independent reaction
• This is used to reduce carbon dioxide to carbohydrates
The Light Independent Reaction (Calvin Cycle):
• This set of reactions can happen with or without the presence of light
• It uses two molecules produced by the light dependent reaction, ATP and reduced
NADP (NADPH) to reduce carbon dioxide into carbohydrates
The Light Dependent Reactions
• The light harvesting clusters (photosystems) exist in pairs, embedded in the
thylakoids membranes within the chloroplast
• Photosystems I and II are positioned near each other, linked by a series of electron
carriers which behave in the same way as the ETC in the mitochondria
• Photosystem II (PS II) lies in front of photosystem I (PS I). The structure of PS I was
worked out before that of PS II
• As well as having an electron transport chain between the two photosystems, there
is a second, short ETC after PS I
• Both photosystems absorb light energy
• At the heart of each photosystem there is a molecule of chlorophyll A, which acts as
the primary pigment
• Surrounding this primary pigments are hundreds of other pigment molecules, the
accessory pigments
• Depending on the plant these vary, but common pigments include other chlorophyll
molecules (A and B), carotenoids, and xanthophylls
• Each of these pigments can absorb light energy of a slightly different wavelength
• The accessory pigments are able to absorb a fairly wide range of wavelengths
between them
• They funnel the light energy they each absorb from one molecule to the next all the
way to the primary pigment (the chlorophyll A molecule), also known as the
reaction centre
• Both photosystems are built in the same way and both absorb light energy
• The chlorophyll A in PS II absorbs light best at a wavelength of 680nm and the
chlorophyll A in PS I absorbs light best at a wavelength of 700nm
• For this reason, PS II may be referred to as P680, and PS I as P700
• The accessory pigments allow far more wavelengths of light to be absorbed to
maximise the efficient use of light
Absorption Spectrum:
• The absorption spectrum shows that the wavelengths of light best absorbed are in
the blue and red of the spectrum, whereas the wavelengths in the middle, the
greens and yellows, are not absorbed, but reflected
• The light absorbed at certain wavelengths provides enough energy to move the
electrons of certain biochemicals to a higher orbital
• This change puts the biochemical into an excited state which allows it to participate
in chemical reactions
• Wavelengths longer than visible red light, such as infrared, don't provide enough
energy to promote electrons to higher orbitals, infrared just makes substances
warmer
• Wavelengths shorter than visible blue or purple light (eg, ultraviolet) have so much
energy that they eject electrons out of the biochemical completely, causing
ionisation, which is dangerous
• It would be unlikely that a pigment could be made which could absorb all
wavelengths of light
• Also, it would be likely to cause the plants to overheat and enzymes would be
denatured
• Very few plants with black leaves do exist, but they tend to live in cool, shaded,
woodland regions, and in fact they possess pigment which are dark purple. Rather
than black
• A graph showing the relationship between the wavelength of light and rate of
photosynthesis is called the action spectrum
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