Photosynthesis

1
Photosynthesis

• Chapter 7

2
Autotrophs are the Producers of the Biosphere

• I Autotrophs
• A. Plants
• 1.Make their own food
• 2.Sustain themselves without eating other
  organisms or even organic molecules
• 3.Producers of the biosphere
• a.Make organic molecules from very simple raw
  materials
• b.Produce biospheres food supply
• B. Land Autotrophs
• 1.Trees
• 2. Cacti
• C.Aquatic autotrophs
• 1.Algae
• 2.Photosynthetic bacteria
• II Light energy
• A.Used by plants, algae, and photosynthetic
  bacteria to drive the synthesis of organic
  molecules from CO2 and H2O
• B.Occurs in chloroplasts

7.1
3
Photosynthesis occurs in chloroplasts

• Chloroplasts carry out photosynthesis. They are
  found in all green part of the plant. Leaves have
  most chloroplasts and are the major sites of the
  process. Green color in plants is from
  chlorophyll pigments in the chloroplasts.
  Chlorophyll absorbs the light energy that
  chloroplasts put to work in making food
  molecules.
• Leaf structure is made of mesophyll. Mesophyll is
  green tissue in the interior of the leaf where
  chloroplasts are concentrated within the cells.
  Stomata are tiny pores where CO2 enters the leaf
  and O22 exits
• Chlorplast structure has a membrane which forms
  structural framework where many of the reactions
  of photosynthesis occur. The outer membrane,
  inner membrane, intermembrane space is between
  them . The stroma is found in the inner membrane
  it is a thick fluid where sugars are made from
  CO2. The thylakoids are an elaborate system of
  disklike membrane sacs. It also contains the
  third chloroplast compartment where it is
  concentrated in stacks called grana. The
  thylakoid membrane contains chlorophyll molecules
  that capture light energy. Also, it houses much
  of the molecular machinery that converts light
  energy to chemical energy.

7.2
4
Plants produce O2 gas by splitting water

• The leaves of plants in lakes have bubbles on
  them.
• These bubbles are really oxygen gas made during
  photosynthesis (oxygen gasO2).
• Jan Ingenhousz thought that O2 was made by being
  taken out of CO2.
• Scientists tested this by using an isotope of
  oxygen (18O) to see what happens to oxygen atoms
  during photosynthesis. They proved Ingenhousz
  wrong. The O2 that is produced during
  photosynthesis comes from H20 (water).

7.3
5
Photosynthesis is a redox process, as is cellular
respiration

• Photosynthesis is an oxidation-reduction (redox)
  process. It converts light energy to chemical
  energy.
• During photosynthesis, when the water molecules
  are split apart, making O2, they lose electrons
  and hydrogen ions and CO2 becomes sugar when the
  electrons and hydrogen ions are added. This is
  called oxidation.
• Cellular respiration is also a redox process. It
  harvests the energy thats held in a glucose
  molecule by oxidizing the sugar. It also makes O2
  into H2O.
• In cellular respiration, the electrons lose
  potential energy, but in photosynthesis, the
  electrons gain energy. The chlorophyll molecules
  provide this boost for the electrons.

7.4
6
Photosynthesis (overview)

• Photosynthesis occurs in two stages
• 1. Light reactions
• a. first stage of photosynthesis
• b. reactions that convert light energy to
  chemical energy and produce O2 as a waste
  product
• 2. Calvin cycle
• a. second step in photosynthesis
• b. series of reactions that assemble sugar
  molecules using CO2 as the energy-containing
  products of the light reactions
• Light reactions
• occur in thylakiod membranes of grana
• light energy used to make ATP from ADP and
  phosphate
• light energy also used to drive a transfer of
  electrons from H2O to NADP
• enzymes reduce MADP to NADPH by adding a pair of
  light-excited electrons and H
• when NADP is reduced to NADPH, water is
  oxidized, giving off O2

7.5
7
Photosynthesis (overview cont.)

• Calvin cycle
• occurs in stroma
• carbon fixation the incorporation of carbon from
  CO2 into organic compounds
• after carbon fixation, enzymes make sugars by
  breaking the fixed carbon down (add high-energy
  electrons and H)
• Calvin cycle does not directly require light, but
  most plants run the Calvin cycle during the day
  when it is powered by the light reactions

7.5
8
Photosystems capture solar power

• Theory of light wave states that most of lights
  properties are relative to photosynthesis
• light also behaves ass discrete packets of energy
  called photons
• photons are fixed quantities of light energy -
  the shorter the wavelengths greater energy
• pigment molecules absorb photons, one of the
  pigments electron gains energy - has been raised
  from ground state to excited state
• excited state is very unstable - electron loses
  excess energy and falls back to the ground state
• several things happen when energy is released in
  this process - heat may be released or light
  might be released
• chlorophyll in an intact chloroplast posses its
  excited electron to a neighboring molecules
• solar power transferred from chlorophyll to
  primary electron acception is the first step in
  the light reactions
• reaction center is what chlorophyll and the
  primary electron acception makes up (is a light
  gathering antenna molecules that absorbs photons
  and passes energy from molecules to molecules
  until it reaches the reaction center)
• photosystems is the combination of antenna
  molecules, reaction center and primary electron
  acceptor. This is the light-harvesting unit

7.6
9

• Two types of photosystems
• photosystem 1- chlorophyll of the molecule is the
  reaction center is called P700 because it
  absorbed red light the best
• Photosystem 11 - is called P680 had a more orange
  shade
• are actually identical chlorophyll molecules but
  their associations with different proteins in the
  thylakoid membrane accounts for the slight
  difference

7.7
10
In the light reactions, electron transport
chains generate ATP, NADPH and O2

• Key events in lights interaction of
  photosynthesis
• 1.absorbing light energy
• 2.excitation of electron by that energy
• 3.forming of ATP and NADPH using energy made from
  the cascade of energized electrons down transport
  chains
• consist of a series of electron carrier molecules
  arranged in a membrane
• transport chains are similar to the functions of
  the cellular respiration chain
• the membrane is the thylakoid membrane of
  chloroplast

7.8
11
Chemiosmosis powers ATP synthesis in the light
reactions

• The thylakoid membrane of the chloroplast
  contains 2 photosystems an electron transport
  chain.
• In photosystem I, the hydrogen is drawn back from
  the thylakoid compartment through the ATP
  synthase help the ADP combine with the
  phosphorylate to make ATP, called
  photophosphorylation. The final electron
  acceptor is NADP.
• In photosystem II, lightH2O oxidizes the
  hydrogen, creating hydrogen ions releasing the
  O2. The electrons that were removed travel along
  the membrane part of their energy is used to
  transport the hydrogen ion from the stroma to the
  thylakoid compartment.

7.9
12
ATP NADPH power sugar synthesis in the Calvin
Cycle

• 3 molecules of CO23RuBP makes 6(3-PGA). 2
  chemical reactions occur, then they take in 6
  ATP. They use the energy oxidize 6 molecules
  of NADPH.
• This reduces the 6 molecules of 3-PGA produces
  6 molecules of G3P.
• 1 of them leaves the cycle, is the net product
  of photosynthesis.
• The remaining 5G3Ps go back reacting with 3ATP
  to form 3RuBP. The cycle is repeated 6 times to
  make 2G3Ps, which makes 1 glucose molecule.

7.10
13
Photosynthesis uses light energy to make food
molecules
PSphotosynthesis

• Light reactions take place in the thylakoid
  membranes
• 1. Two photosystems in the membranes capture
  solar energy, using it to energize electrons
• 2. Water is split and O2 is released
• 3. The photosystems transfer energized electrons
  to electron transport chains where energy is
  harvested and used to make NADPH and ATP
• sugar factory of chloroplastsCalvin cycle2nd
  stage of PS
• in the stroma- enzymes of cycle combine CO2 and
  RuBP (CO2 RuBP G3P)
• sugar molecules from G3P are plants own food
  supply
• stockpile extra sugars as starch- major source of
  food for animals
• Photosynthesis depended on by everything

7.11
14
C3,C4, and CAM PlantsC4 and CAM Plants Have
Special Adaptations That Save H2O
7.12
15
C3 Plants

• Plants in which the Calvin cycle used CO2
  directly from the air C3
• ex. wheat and rice
• -on a hot day C3 plants close their stomata
• closing its stomata reduces H2O loss and prevents
  CO2 from entering the leaf
• CO2 levels can become low in the leaf while O2
  from the light reactions increase
• first enzyme of the Calvin cycle(rubisco)
  incorporates O2 instead of CO2 2 carbon
  compounds created instead of regular 3 carbon
• plant cell breaks the 2-carbon compound down to
  CO2 and H2O
• process called photorespiration

16
C4 Plants

• C4 plants keep stomata closed most of the time
  during warm weather, and continues making sugars
  by photosynthesis
• enzyme fixes carbon into 4-C compound instead of
  into 3-PGA
• C4enzyme cannot switch over to fixing O2
• C4 enzyme can continue to fix carbon even when
  CO2 concentration in the leaf is much lower than
  the O2 concentration
• C4 compound donates the CO2 to the Calvin cycle
  so sugar continues to be created
• ex. corn and sugar cane

17
CAM Plants

• CAM plants conserve water by opening its stomata
  and admitting CO2 only at night
• when CO2 enters the leaves it is fixed into 4-C
  compound
• 4-Carbon compound banks CO2 at night and releases
  it to the Calvin cycle during the day- keeps
  photosynthesis operating during the day
• CAMcrassulacean acid metabolism
• ex. pineapples and cact.

18
Old Growth Trees
Timber Industry
Conservationists

• Contain a lot of marketable lumber
• replace old trees with seedlings so it would
  increase photosynthesis

• Home to many species that cant survive anywhere
  else
• large photosynthesizers remove a lot of
  potentially harmful CO2 from the atmosphere
• when old trees are harvested much less than half
  their bulk becomes lumber. The remains are
  either left behind to decompose or burned, which
  releases more carbon dioxide than the new trees
  they planted can take up.

7.13
19
Mario Molina

• Mario is one of the many scientists studying
  the ozone layer. In 1995 he shared a Nobel Prize
  for his research on how certain pollutants are
  damaging that layer. In your text book he
  explains why the ozone layer is important. So,
  for further information see page 122 in your
  textbook.

7.14