PHOTOSYNTHESIS

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PHOTOSYNTHESIS

• Harnessing Energy

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Heterotrophs and Autotrophs

• All living organisms require organic compounds
  and energy for their cells.
• Depending on how organisms obtain these compounds
  and energy, we classify them as being
• heterotrophic (other feeding)
• or autotrophic (self-feeding).

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Heterotrophs and Autotrophs

• Heterotrophic organisms such as animals, fungi
  and many bacteria must consume food which
  provides organic compounds and energy for their
  cells.
• These heterotrophic organisms either ingest or
  absorb the organic matter of other living or dead
  organisms or their products.
• Autotrophic organisms such as plants, algae and
  several kinds of bacteria use do not ingest or
  absorb organic compounds.
• Autotrophic organisms use an external energy
  source to build organic compounds from simple
  inorganic compounds.
• Two different processes for trapping energy and
  creating organic matter from inorganic matter.
  These are chemosynthesis and photosynthesis.

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Chemosynthesis

• Chemosynthetic organisms use the chemical energy
  within inorganic molecules.
• This energy comes from oxidising reactions.
• These reactions involve the addition of oxygen to
  (or the removal of electrons from) a substance.
• Examples include bacteria who obtain energy by
  converting
• Ammonium ions (NH4) to nitrite ions (NO2-)
• Nitrite (NO2-) ions to nitrate (NO3-)
• Sulfide ions (S2-) to sulfate ions (SO42-)
• Whole communities of heterotrophic organisms live
  around volcanic vents on the deep ocean floors
  where light does not penetrate. They rely
  directly or indirectly on chemosynthetic bacteria
  for their food supply in much the same way as
  terrestrial communities depend on plants to trap
  energy.

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Photosynthesis

• Organisms such as plants, algae and some protists
  (such as phytoplankton) are able to trap light
  energy and make organic compounds, such as
  sugars, from simple compounds such as carbon
  dioxide and water.
• Photosynthesis is the process in which light
  energy is transformed into chemical energy stored
  in sugars.
• Organisms with this ability are termed producers.
  
• Other organisms, such as animals and fungi, that
  depend, directly or indirectly, on the organic
  compounds produced by producers, are called
  consumers.

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Photosynthesis

• In a typical producer, such as a terrestrial
  flowering plant, the complex series of reactions
  in photosynthesis can be summarised as follows
• carbon dioxide water ---------------------------
  gt glucose water oxygen
• 6CO2 12H2O ----------------------
  ----gt C6H12O 6H2O 6O2

light
chlorophyll
light
chlorophyll
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Where does photosynthesis occur?

• In a terrestrial flowering plant, only some cells
  are able to carry out photo synthesis and these
  are principally located in green leaves.
• The shape and structure of leaves equips them to
  carry out photosynthesis.

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Why are leaves so special?

• Their flat shape provides a large surface area
  exposed to sunlight.
• The presence of many stomata (pores) on one or
  both leaf surfaces provides access into the leaf
  for carbon dioxide.
• The thinness and the presence of internal air
  spaces in the leaves enables the ready diffusion
  of carbon dioxide to photosynthetic cells in the
  leaf tissue.
• The network of xylem vessels in the vascular
  tissue transports water to the photosynthetic
  cells.
• Each photosynthetic cell possesses many
  chloroplasts enabling it to trap the energy of
  sunlight.

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Chloroplasts

• Present in some cells of plants and algae.
• The boundary of each chloroplast is a double
  membrane (inner and outer).
• The inner membrane extends to form a system of
  membranous sacs called lamella or thylakoids.
• When several of these stack together they form
  grana.
• Chlorophyll is located in the grana.
• The semi-fluid substance between the grana is
  called the stroma.

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Chlorophyll

• Chlorophyll is pigment that absorbs or traps
  light.
• There are three types of chlorophyll a, b and
  c.
• Chlorophyll a is the major photosynthetic pigment
  and is found in all photosynthetic plants,
  protists, and cyanobacteria.
• Chlorophyll molecules are embedded in the
  membrane structure of grana.
• Chlorophylls absorb wavelengths of violet-to-blue
  and red light. They reflect green which is why
  leaves appear green.

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Carotenoids

• Carotenoids are accessory pigments found in all
  green plants.
• They absorb blue and green wavelengths and give a
  plant a yellow or orange color.
• In the autumn when chlorophyll breaks down, it is
  the accessory pigments which are responsible for
  the colour.
• Accessory pigments are better at absorbing light
  at different wavelengths to chlorophyll a. They
  do not retain energy, but transfer it to
  chlorophyll a to enhance its effectiveness.
• NB The red color of some autumn leaves is due to
  the anthocyanin pigments. These are not
  photosynthetic.

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Stages of Photosynthesis

• Photosynthesis from
• photo light
• synthesis put together
• The name reflects the two-stage nature of the
  process.
• Light-dependent stage involving trapping of light
  energy
• Light-independent stage in which energy trapped
  in the first stage is used to make organic
  compounds from carbon dioxide and water.

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Light-dependent Reaction

• Also known as the light reaction.
• Occur within the grana of the chloroplasts
• Requires the input of water as well as light
  energy.
• Can be summarised by the reaction below

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Steps in light-dependent reaction

• Sunlight is trapped by chlorophyll a (or other
  pigments) and light energy is converted to
  chemical energy.
• Absorbed energy is used to produce ATP and split
  water molecules to form H ions and oxygen (waste
  product). This involves the electron transport
  chain.
• H ions are gathered by a carrier molecule or
  acceptor molecule (NADP in this case).
• NADP becomes NADPH and transports H ions from
  the grana to the stroma.
• H ions and ATP produced in light-dependent
  reaction are utilised in light-independent
  reaction.

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Light dependent reaction
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Light-independent Reaction

• Also known as dark reaction or Calvin cycle.
• Occurs in the stroma and involves the reduction
  of carbon.
• Does not directly depend on light involvement but
  does dependent on previous stage occurring.
• Can be summarised by the reaction below

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Steps in light-independent reaction

• Carbon reduction (from CO2 to a sugar C(H2O)n)
  requires a supply of carbon dioxide and hydrogen
  ions, and an input of energy.
• Carbon dioxide can come from the air surrounding
  the leaf or from cellular respiration reactions.
• Energy required to drive these reactions comes
  from ATP and loaded carriers (NADPH molecules)
  produced during the light-dependent stage.
• H is the reducing agent and ATP is the source of
  energy for reducing carbon dioxide to organic
  compounds such as glucose and other sugars.
• Plants do not build sugars simply by joining CO2
  molecules together. Sugar formation involves a
  cyclic set of reactions in which intermediate
  substances are formed.

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Light independent reaction
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C3 plants

• In most plants the first step in carbon reduction
  reaction is the Calvin cycle.
• The first step of this reaction is
• Because the product of this reaction contains
  three carbon atoms, plants that carry out this
  reaction are known as C3 plants.

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The Calvin Cycle

• Each time the cycle proceeds, one carbon one
  carbon dioxide molecule enters the cycle and is
  fixed and reduced.
• To produce a 6-carbon compound that is released
  from the cycle, six turns of the cycle must take
  place.
• At the completion of each turn of the cycle, the
  starting compound is regenerated and so the cycle
  can proceed provided that CO2, ATP and NADPH are
  also available.
• The Calvin cycle in C3 plants occurs in nearly
  all trees and most shrubs and herbs.

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The Calvin Cycle
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C4 plants

• C4 plants occur mainly in hot, dry habitats and
  include important crop plants such as corn and
  sugar cane.
• The light independent reaction of these plants
  involves a series of reactions which precede the
  Calvin cycle. In C4 plants, the first step
  before the Calvin cycle is
• The 4-C compound undergoes further reactions and
  is transported to cells surrounding the vascular
  bundle.
• Once here, the 4-C compound releases a molecule
  of carbon dioxide which enters the normal Calvin
  cycle.

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Other variations on photosynthesis

• CAM (crassulacean acid metabolism) plants
• Plants such as pineapples and cacti close their
  stomata during the day and open them at night, at
  which time they take up carbon dioxide and
  convert it to four-carbon organic acids (e.g.
  crassulacean acid), which accumulate in the
  central vacuole.
• During the day, while the stomata are closed,
  carbon dioxide is released from these organic
  acids and used immediately for C3 photosynthesis.
  
• CAM plants are adapted to conditions of high
  daytime temperatures, intense sunlight and low
  soil moisture.
• Mistletoe
• Although mistletoe plants, Amyema species, can
  carry out photosynthesis, they are partly
  parasitic because they must obtain their mineral
  nutrients and water from a host.

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Which type of photosynthesis is best?
C3 plants C4 plants CAM plants
Make stable molecules with 3 carbons Use more ATP to produce stable 4 carbon sugar, so in cooler climates C3 is more efficient Carbon dioxide released by the metabolism of cellular acids is used in C3 photosynthesis
Up to 50 of carbon dioxide absorbed through leaves is released before it can be used to make glucose by photosynthesis. Close their stomata during day and open them at night to prevent water loss in hot climates
In warm conditions the photosynthetic enzyme binds with oxygen instead of carbon dioxide Have photosynthetic enzymes that never bind with oxygen and are more efficient during hot weather than C3 plants
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The importance of PGAL

• PGAL (phosphoglyceraldehyde) is an important 3
  carbon compound formed during the Calvin cycle.
• PGAL is the starting point for the production of
  sugars in the cytosol outside the chloroplast
• Two PGAL molecules can join to form fructose (6
  carbon monosaccharide).
• Fructose can be converted to glucose.
• Fructose and glucose can combine to form sucrose
  disaccharide form in which carbohydrates are
  transported from the leaf to other parts of the
  plant via the phloem
• Many glucose units can combine to form starch
  (storage molecule in plants).

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The importance of sugars

• All cells can use sugars as a starting point for
  the manufacture of other carbohydrates and
  lipids.
• They can react sugars with with nitrogen to form
  non-essential amino acids and nitrogenous bases
  that are found in nucleic acids.
• The chemical energy is starch is used directly or
  indirectly by consumers in cellular respiration
  to produce ATP for their energy requirements.

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Factors that influence photosynthesis

• Light intensity
• Carbon dioxide availability
• Temperature
• Indirect factors

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Light intensity

• The rate of photosynthesis usually increases with
  light intensity until there is another limiting
  factor, such as the saturation of chloroplasts.
• About 20 of light that hits the leaf is
  reflected.
• Only about 1 of light absorbed by the leaf is
  converted to chemical energy.

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Carbon dioxide

• For most plants, carbon dioxide from air
  dissolves in extracellular fluid before entering
  photosynthetic cells.
• There are local variations in carbon dioxide
  levels in air, in different habitats and at
  different times of the day.
• Aquatic plants can also use hydrogen carbonate
  (carbonic acid), which forms when carbon dioxide
  dissolves in water.
• CO2 released as a product of cellular respiration
  can also be used for photosynthesis, but usually
  only provides a small amount of the total carbon
  dioxide requirements.
• The degree to which the level of carbon dioxide
  affects the rate of photosynthesis is different
  for C3, C4 and CAM plants.
• C4 and CAM plants are more efficient than C3
  plants at trapping carbon dioxide when it is
  warm.

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Compensation point

• At low levels of light intensity, the rate of
  photosynthesis is less than the rate of cellular
  respiration, so there is net output of carbon
  dioxide by plants.
• The light intensity at which the rate of carbon
  dioxide produced by cellular respiration equals
  the rate of carbon dioxide used in photosynthesis
  is known as the light compensation point.

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Temperature

• Photosynthesis increases with increasing
  temperature until around 20-40oC, depending on
  plant species, then it declines again.
• Plants that live in hotter climates are at higher
  end of the range.
• In C3 plants, oxygen displaces trapped carbon
  dioxide more rapidly as temperature increases
  (enzyme binds oxygen instead of carbon dioxide).

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Indirect factors

• Water
• Required in photosynthesis
• Only 1 of water passing up the xylem is used in
  photosynthesis. The rest is used in other
  chemical reactions, to hydrate cells or is lost
  in transpiration.
• If there is not enough water to hydrate the cells
  and keep them turgid, the stomata close. This
  prevents carbon dioxide entering the leaves,
  therefore photosynthesis decreases.
• Level of chlorophyll
• Limits photosynthesis
• Yellow leaves will have a lower rate of
  photosynthesis.
• Nitrogen and Magnesium
• Chlorophyll contains the elements nitrogen and
  magnesium.
• If the soil is deficient in one or both these
  elements, the plants cannot make sufficient
  chlorophyll.

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Rate of photosynthesis

• Any of the factors that influence photosynthesis
  may limit the rate of photosynthesis.

• Photosynthesis will be limited by only one factor
  at a time, but if conditions in an individual
  chloroplast change, the particular factor that is
  limiting may also change.
• For example, carbon dioxide levels that are
  adequate (not limiting) in conditions of low
  light may become limiting if light intensity
  increases.