Download Chapter 8 – Photosynthesis

Chapter 8 – Photosynthesis
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Photosynthesis and Energy
o Plants make their own food
o They use the energy provided by the sun to take a simple gas, carbon
dioxide, join it to a carbohydrate (a sugar) and then energize that sugar
thus transforming it into food
o From Plants, a Great Bounty for Animals
 A by-product of photosynthesis is oxygen
 Plants break water molecules apart
 In doing so, they use electrons and protons from H2O but they
leave behind oxygen molecules (O2)
 Energy comes from the sun and then, in photosynthesis, is stored
in plants in the complex molecules carbohydrates
 Thus stored, these carbohydrates can be broken down and used
 The breakdown of food ends with the cellular respiration that
provides ATP
o Up and Down the Energy Hill Again
 Photosynthesis and cellular respiration are trips up and down the
energy hill
 Respiration – from more stored energy (in food) to less, as the food
was broken down to produce ATP
 Photosynthesis is a trip up the energy hill
 Electrons are removed from water, boosted to a more
energetic state by the power of sunlight and then brought
together with a sugar and carbon dioxide resulting in an
energy rich sugar
 Photosynthesis is the process by which certain groups of
organisms capture energy from sunlight and convert this solar
energy into chemical energy that is initially stored in a
carbohydrate
The Components of Photosynthesis
o Starts with the absorption of sunlight by leaves
o Absorption: light is taken in by the leaves, the leaves capture only a
portion of the light that falls on them
o Photosynthesis is driven by part of the visible light spectrum – mainly by
blue and red light of certain wavelengths
o This is why plants are green – they strongly scatter the green portion of the
visible light spectrum
o Where in the Plant Does Photosynthesis Occur?
 Leaf – one layer of outer (epidermal) cells at the top, another layer
of epidermal cells at the bottom and several layers of mesophyll
cells in between
 Chloroplasts are the sites of photosynthesis
 The stomata seen in the figure are the microscopic pores that let
carbon dioxide pass into leaves and water vapor pass out of them
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Chloroplasts – organelles within plant and algae cells that are the
sites of photosynthesis
 The chloroplast has outer and inner membranes at its periphery
 The interior of the chloroplast – there is a network of chloroplast
membranes, active in photosynthesis called thylakoids (thylakoids
often stack on top of one another creating structures called grana)
 Thylakoids are immersed in the liquid material of the chloroplast,
the stroma
 Thylakoid membranes and the stroma are the places where all the
steps of photosynthesis occur
 The thylakoid membranes are where photosynthesis start, with the
absorption of sunlight
 Any compound that strongly absorbs certain visible wavelengths of
sunlight is called a pigment
 Thylakoid membranes contain a pigment, called chlorophyll a that
can be defined as the primary pigment active in plant
photosynthesis
 Chlorophyll a is aided by several substances known as accessory
pigments, which do just what their name implies: aid chlorophyll a
in absorbing energetic rays from the sun after which they pass the
absorbed energy along
o There Are Two Essential Stages in Photosynthesis
 Two main stages of photosynthesis
 The first stage (the photo) – the power of sunlight will do two
things – string water of electrons and then boost these electrons to
a higher energy level
 The first stage ends when the original, energized electrons get
attached to a mobile electron carrier called NADP+
 The second stage (the synthesis) the electrons come together with
carbon dioxide and a sugar
 This sugar produces a high energy sugar – meaning food
 This second stage takes place in the stroma or the chloroplast
 The first stage of photosynthesis obviously depends directly on the
sunlight – these steps are sometimes referred to as light reactions
 The second set of steps is referred to as the Calvin Cycle
o The Working Units of the Light Reactions
 Light reactions take place within a set of working units
 Photosystem – an organized complex of molecules with a
thylakoid membrane that, in photosynthesis, collects solar energy
and transforms it into chemical energy
 First, there is a group of a few hundred pigment molecules that
serve to absorb the sunlight
 Majority of these molecules serve only as “antennae” that absorb
energy from the sun and pass it on
 The center of the antennae system – the reaction center – a pair of
special chlorophyll a molecules and associated compounds that
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first receive the solar energy from photosystem pigments and then
transform this solar energy into chemical energy
 Includes a molecule that could be thought of as the first
recipient of the absorbed solar energy the “primary electron
acceptor”
 The absorbed energy is then passed on to the pair of chlorophull a
molecules in a reaction center
 Electrons from this pair are “moved” in a couple of ways
 Physically these elements are transferred to the primary
electron receptor
 Also movement up the energy hill
 The energy from sunlight is pumping the electrons up the hill in
photosynthesis
o Energy Transfer in Photosynthesis Works Through Redox Reactions
 Any time one substance loses (or “donates”) electrons to another it
is said to have been oxidized
 The substance that gained electrons is said to have been reduced
 This is a redox reaction
 Many of the photosynthetic steps are redox reactions
 The movement of electrons with the reaction center – from the
chlorphyll a molecules to the primary electron acceptor – is a
redox reaction
 The chlorophyll molecules are oxidized by the primary electron
acceptor
 Electrons are passed on by means of one electron carrier oxidizing
another
 The electrons are moving both up and down the energy hill
 With the energy supplied by the sun, electrons are pumped up a
couple of formidable energy gradients only to come partway back
down them as they “seek” their lowest energy state
 They are releasing energy as they fall
Stage 1: The Steps of the Light Reactions
o First step is that solar energy, collected by photosystem II’s antennae
molecules arrives at the reaction center
o This energy then gives a boost to an electron in the reaction center in the
two ways noted: the electron physicall moves to another part of the
reaction center complex, the primary electron acceptor, and it is also
pumped up the energy hill
o The reaction center chlorophyll has lost an electron
o That loss leaves an energy hole in this chlorophyll making it an oxidizing
agent
o A special enzyme in the reaction center splits water molecules that lie
within the thylakoid compartment
o These water molecules are now being oxidized which means they are
losing electrons
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o These electrons travel to the reaction center, where they will be the next
electrons in line for an energy boost
o A Chain of Redox Reactions and Another Boost from the Sun
 After arriving at the primary electron acceptor they fall back down
the energy hill as they are transferred through a series of electron
transport molecules each oxidizing its predecessor
 They arrive at photosystem I which includes a slightly different
kind fo reaction center
 This center is also receiving solar energy and uses it to boost
electrons to a higher energy state
 Electrons are transferred down the second energy hill – until they
are received by the electron carrier NADP+
 In accepting electrons NADP+ becomes reduced to NADPH an
electron carrier that ferries the electrons into the next stage of
photosynthesis
 This second stage is the Calvin cycle which will yield the high
energy sugar that is the essential product of photosynthesis
o The Physical Movement of Electrons in the Light Reactions
 Electrons have physically moved through the chloroplast
 Started out in the water of the thylakoid, through the thylakoid
membrane and ended up in the stroma attached to NADPH
What Makes the Light Reactions so Important?
o Two momentous things that have taken place within these steps: the
splitting of water and the transformation of solar energy to chemical
energy
o The Splitting of Water: Electrons and Oxygen
 The splitting of water provides the traveling electrons
 Oxygen accounts for 21 percent of the Earth’s atmosphere
 In the water splitting hydrogen atoms are removed from H2O
while oxygen is left behind
 When two liberated oxygen atoms come together in the thylakoid
compartment the result is O2 – the form of oxygen that exists in
the atmosphere
o The Transformation of Solar Energy to Chemical Energy
 Transformation of solar energy to chemical energy is the second
major feat that takes place
 Energy of the sunlight moves to a chlorophyll molecule in a
reaction center, it boosts an electron there from what is known as a
ground state to an excited state
 This electron is moving farther out from the nucleus of an atom
 One fate is for the electrons to drop back down to the original state
in the process releasing as heat then energy they have absorbed
 Another possible fate is for falling electrons to release part of their
energy as light in the process known as fluorescence
 In photosynthesis the energized electrons are transferred to a
different molecule and they don’t fall back to their ground state
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They are passed on a redox reaction
Electrons are taken from water, boosted to a higher energy state by
the sun and transferred to other molecules
o Production of ATP
 A primary function of the fall of electrons between photosystems II
and I is a release of energy that is used for the production of ATP
 ATP will be used to power the reactions that are coming up in the
second stage of photosynthesis
 The suns power has provided energy that is stored in two forms –
ATP and the energetic electrons in NADPH
Stage 2: The Calvin Cycle
o The energy captured in the light reactions will be used
o It will power a process by which carbon dioxide taken from the
atmosphere – will be joined to a sugar, with the resulting product
energized, thus creating a carbohydrate
o Energized Sugar Comes from a Cycle of Reactions
 Calvin Cycle – or the C3 cycle – is the set of steps in
photosynthesis in which energetic electrons are brought together
with carbon dioxide and a sugar to produce an energetic
carbohydrate
 Synthesis of photosynthesis – a bring together of elements
 The first stems can be thought of as a process of fixation – of a gas
being incorporated into an organic molecule
 Carbon dioxide, which comes in through the leaves of the plant, is
being fixed into the starting sugar, which is called RuBP
 It is the way life builds itself up with materials that lie outside of
life
 The next reactions in the cycle are the energizing steps of the
process
 Low energy sugar receives the energetic products of the light
reactions
 The RuBP derivatives first interact with ATP and then receive
energetic electrons from NADPH
 A relatively low energy sugar has been energized from which
position it can now serve as food
 The energized sugar – G3P – is the essential product of
photosynthesis
 One molecule of it is netted with each turn of the Calvin cycle
 The remainder of the cycle can be thought of as a preparation of
molecules for another trip around the cycle
o The Ultimate Product of Photosynthesis
 G3P can be turned into many things
 Put two G3P molecules together and you get the more familiar six
carbon sugar glucose
 The ultimate product of photosynthesis is the whole plant
Photorespiration and the C4 Pathway
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o A glitch in this process is called photorespiration and it takes place
within the Calvin cycle
o Cycle begins with carbon dioxide being joined to a low energy sugar
o What brings these two compounds together is an enzyme called rubisco
o It is extremely powerful but slow moving and prone to making errors
o Rubisco is the bridge between the living world and the nonliving world
o It is the molecule that allows plants to capture the carbon atoms that are
the building blocks of all living things
o One of the problems is its speed – rubsico can only manage 3 reactions per
second as opposed to the average of 1000
o Because rubisco works so slowly, every plant must have a lot of it as a
result rubisco is the most abundant protein on Earth
o Rubsico has a problem in even binding with the right substance - it can
bind to oxygen as well as to carbon dioxide
o Rubisco may fix up to one molecule of O2 for every three of the carbon
dioxide
o Photorespiration is especially likely to take place when temperatures rise
because heat prompts the stomata on leaves to close to preserve water
o The effect of closed stomata is not only that water is kept in but that CO2
is kept out
o With a deficit of CO2 rubisco tends to bind more frequently with oxygen
o The result is plants that don’t grow as much
o Photorespiration: a process in which the enzyme rubisco undercuts
carbon fixation in photosynthesis by binding with oxygen instead of with
carbon dioxide
o Rubisco: can be defined as an enzyme that allows organisms to
incorporate atmospheric carbon dioxide into their own sugars during the
process of photosynthesis
o C4 Photosynthesis is a form of photosynthesis in which carbon dioxide is
first fixed to a four carbon molecule and then transferred to special cells in
which the Calvin cycle is undertaken, bundle sheath cells
o The C4 Pathway Is Not Always Advantageous
 C4 Pathway is known to be employed most notably in some
grasses, and in corn, sugar can and sorghum – vastly more C3 than
C4 plants
 C4 fixation does not confer an across the board advantage
 It has a cost, which is the expenditure of ATP in shuttling carbon
dioxide to the bundle sheath cells
 C4 fixation is advantageous only where the weather is warm
enough to bring about a significant increase in photorespiration
Another Photosynthetic Variation: CAM Plants
o When plants live in climates that are not just warm but dry, a large part of
their survival comes down to retaining water
o Photosynthesis, however, works against water retention
o When CO2 can pass in, water vapor can pass out
o Plans we call succulents – like cacti – have a solution for this problem –
close their stomata during the day and open them at night
o The plants then carry out C4 metabolism at night but only up to a point
o They fix carbon dioxide into an initial four carbon molecule and then
stand pat
o The CO2 stays banked in them awaiting the energy of the next days sun
o CAM Photosynthesis is a form of photosynthesis, undertaken by plants in
hot, dry climates, in which carbon fixation takes place at night and the
Calvin cycle occurs during the day
o CAM is an acronym: crassulacean acid metabolism