Photosynthesis

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Photosynthesis
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Definition

• Conversion of light energy into chemical energy.

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6CO2 6H2O ? C6H12O6 6O2
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Leaf Structure
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Cuticle

• Waxy covering that is secreted by the epidermal
  cells.

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Epidermis

• Single layer of cells that covers the upper and
  lower surface

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Stomate

• Minute pores in the epidermis through which gases
  diffuse.

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Guard Cells

• Surround the stomates.
• When filled with water, they swell and open the
  stomate.

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Mesophyll

• Middle section of leaf.
• Broken down into 2 layers- palisade and spongy.

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Palisade Mesophyll

• Filled with tall, skinny cells containing lots of
  chloroplasts.

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Spongy Mesophyll

• Cells are more spread out to create air pockets.

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Vein

• Transports water and food.

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Chloroplasts
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Stroma

• The liquid solution that fills the space.

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Grana

• Stacks of thylakoids.
• Maximize surface area.

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Thylakoid

• Flattened sacs in the chloroplast.

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Photosystems

• Embedded in the thylakoid membrane

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Photosystems

• Made of Light Harvesting Complexes (containing
  pigments) that surround a Reaction Center
  (containing 2 chlorophyll a molecules and a
  primary electron acceptor)

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Pigments

• Molecules that absorb some wavelengths of light.

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Pigments

• Each pigment absorbs light at a specific
  wavelength.
• Chlorophyll a- all green plants.

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Pigments

• Accessory pigments (chlorophyll b, beta carotene,
  etc.) collect light energy and transfer it to
  chlorophyll a OR dissipate extra energy

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Reaction Center

• P700 (Photosystem I) has a chlorophyll a whose
  absorption peaks at 700 nm
• P680 (Photosystem II) peaks at 680 nm

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Review

• Why are green plants green?????

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The electromagnetic spectrum
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Overview of Photo.

• Energy Capture
• Fixing Carbon

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Light Reactions

• Occurs across thylakoid membranes.

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Light Reactions- Summary

• Pigments absorb light energy.
• Electrons jump to higher states
• Electron Transport Chain forms ATP and NADPH

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Noncyclic Electron Transport

• 4 major steps (beginning from the end)
• Photosystem I loses an electron which helps
  reduce NADP to NADPH

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Noncyclic Electron Transport

• 2. Photosystem II loses an electron (via an
  E.T.C.) to Photosystem I.
• This also puts H into the thylakoid

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Noncyclic Electron Transport

• 3. Water is split into Oxygen, H and electrons.
• Electrons replace those lost by Photosystem II
• H are put into thylakoid

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Noncyclic Electron Transport

• 4. All the H in thylakoid diffuses through ATP
  synthase, which makes ATP.

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BUT.

• More ATP is needed for the Calvin Cycle so..

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Cyclic Electron Flow

• Electron in Photosystem I gets a boost.
• Energy is used to put more H into thylakoid.
• Electron goes back to Photosystem I

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Cyclic Electron Transport

• 4. All the H in thylakoid diffuses through ATP
  synthase, which makes ATP.

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Calvin Cycle

• Purpose
• To use chemical energy made in light reactions to
  make G3P

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Calvin Cycle

• 3 molecules of carbon dioxide combines with 3
  molecules of RuBP to form 3, 6-carbon sugars.
  The enzyme that combines them is called Rubisco.

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Calvin Cycle

• Each 6 carbon sugar splits into 2, 3-carbon
  sugars each called PGA

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Calvin Cycle

• ATPs and NADPHs are used to convert each PGA
  into G3P (now have 6 G3Ps)

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Calvin Cycle

• One G3P is released from the cycle
• More ATP is used to convert the remaining 5 G3Ps
  into 3 RuBPs

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Calvin Cycle

• G3P is used to make glucose, starch, amino acids
  (by adding NH3) lipids, etc.

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3- C (3 CO2)
18- C (6 G3P)
15- C (3 RuBP)

3- C (1 G3P)
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Photorespiration

• Instead of
• 6CO2 6 RuBP ? 2 G3P
• You Get
• 6O2 6 RuBP ? 1 G3P 1 glycolate

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Photorespiration

• Glycolate is a 2 carbon acid later converted to
  CO2

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Photorespiration

• When? Why?

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C4 Pathway

• An adaptation to reduce photorespiration and
  maximize photosynthesis

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C4 Pathway

• Examples- Sugar Cane, Corn, etc
• Conserves CO2 by fixing it with a middle man

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C4 Pathway

• Surrounding each vein is a layer of tightly
  packed cells called the bundle sheath

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C4
C3
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C4 Pathway

• Mesophyll cells surround bundle sheath (but do
  not contain rubisco, like in C3 plants)

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C4 Pathway

• These cells fix CO2 with a 3-carbon acid to form
  a 4-carbon acid (oxaloacetate)
• Oxaloacetate is rearranged to malate and
  transported to bundle sheath cells

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C4 Pathway

• There, the acid releases a CO2, which enters the
  Calvin Cycle

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C4 Pathway

• C4 plants grow very rapidly

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CAM Plants

• Examples- Jade, Cactus, etc
• Another way of conserving carbon dioxide by
  giving it to a middle man

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CAM Plants

• Open stomates at night and incorporate CO2 into
  organic acids (like C4 plants)

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CAM Plants

• Enzymes break down the organic acids releasing
  CO2, which then enters the Calvin Cycle
• During the day, stomates close to conserve water

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CAM Plants

• Not very efficient- CAM plants grow very slowly