Photosynthesis

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Photosynthesis

• Part 1 The light dependent reaction

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Obtaining Energy

• Autotroph
• Make food using the energy of the sun

• Heterotroph
• Must get energy from food instead of sunlight

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Light and Pigments

• Light from the sun appears white but it is
  actually composed of a variety of colors
  -Visible Light Spectrum ROY G BIV
• Light travels through space as waves of energy.
  Different colors have different wavelengths
• Pigments( like chlorophyll) absorb light

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Overview Photosynthesis

• Photosynthesis takes carbon dioxide, water and
  light and makes sugar and oxygen
• Cellular respiration takes oxygen and sugar and
  makes carbon dioxide and water
• 2 parts to photosynthesis
• 1) Light reactions
• 2) Calvin Cycle or Dark Reaction

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Photosynthesis Reaction
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Chloroplast Structure

• 2 membranes
• Outer
• Inner
• Other parts
• stroma
• thylakoid
• grana

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Thylakoids

• Located inside the inner membrane
• Flattened sacs
• Interconnected
• Surrounding stacks is a
• Fluid called STROMA

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Chlorophyll a and b

• Chlorophyll a
• absorbs less blue and more red
• Directly involved in light reactions of
  photosynthesis
• Neither absorb green light/instead they reflect.
  Thats why plants with lots of chlorophyll appear
  green

• Chlorophyll b
• assists a in capturing light energy

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More Pigments

• carotenoids(kuh-raht-uhn-oydz)
• red, yellow, brown accessory pigments
• absorb colors that a cant absorb and
  thereby help get more light, therefore more
  energy
• in nonphotosynthetic parts of plants, such as
  fruits, flowers, other colors are visible
• In fall , plants lose chlorophylls and the
  leaves take on the color of the cartenoids

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Photosystems I II Light Dependent Reaction

• Light systems begin when accessory pigment
  molecules in both photosystems absorb light. By
  absorbing light, those molecules acquire some of
  the energy that was carried by the light waves.
  In each photosystem, the acquired energy is
  passed quickly to another pigment until it
  reaches chlorophyll a molecules. From this point
  5 steps occur.

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What is a photosystem?

• Photosystem group of plant pigments that
  surround a chlorophyll a moleculethe reaction
  center. Found in the thylakoid

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Step 1

• Light energy forces e- to enter higher energy
  levels in the 2 chlorophyll a molecules of
  photosystem II. The e- are said to be excited

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Step II

• Excited e-, leave chlorophyll a
• Chlorophyll a, molecules undergo oxidation
  (loss of the e-)
• Primary Electron Acceptors accept e-
• That the chlorophyll a lost ( reduction)

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Step III

• Primary e- acceptor donates e- to electron
  transport chain (series of molecules in thylakoid
  that transfer e-)
• e- lose energy that the acquired when they were
  excited
• Energy that is lost is harnessed to move protons
  into the thylakoid

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Step IV

• Light is absorbed in photosystem I( bundle of
  pigments)
• E- move from a pair of chlorophyll a molecules
  in photosystem I to another primary e- acceptor.
  e- that are lost by chlorophyll a ---are
  replaced by e- that have passed through the e-
  transport chain from photosystem II

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Step IV Continued

• If the e- in Photosystem I were not replaced by
  electrons from Photosystem II, than both e-
  trains would stop.
• Replacement e- for photosystem II are replaced by
  the breakdown of water molecules by the thylakoid
• 2H20----- 4 (H) 4 (e-) 02
• For every 2 water split 4 e- become available
• The protons produced are left inside thylakoid
  and the Oxygen diffuses out of chloroplast

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Step V

• Primary e- acceptor of photosytem I donates e-
  to a different e- transport chain. E- combine
  with a proton and NADP.
• NADP becomes NADPH
• NADP ( organic molecules that accepts e- during
  re-dox reactions)
• NADP (Nicotinamide adenine dinucleotide
  phosphate)

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Making ATP In Light Reactions Chemiosmosis
Relies on a concentration gradient of protons
across the thylakoid membrane The concentration
of protons is higher in the thylakoid than The
stroma
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Chemiosmosis Continued

• The concentration gradient represents potential
  energy
• ATP SYNTHASE- located in thylakoid An enzyme
  that harnesses this energy and makes ATP by
  adding a phosphate group to ADP or adenosine
  diphosphate

• The energy driving this reaction is provided by
  the movement of protons from inside the thylakoid
  to the stroma through the ATP synthase
• ATP synthase converts potential energy into
  chemical energy stored in ATP
• Together ATP and NADPH provide energy for the
  next set of reactions