Chapter 10: Photosynthesis

Modes of Nutrition
• Autotroph- self feeder
– The Producers!
– Almost all plants are photoautotrophs
• Light self feeders
• Heterotroph- other feeder
– Consumers
– Most heterotrophs are dependent on photoautotrophs as a
food (and oxygen) source

10.1 Photosynthesis converts light energy to the chemical energy of food
• Photosynthesis occurs in all green parts of plants but the leaf is the structure optimized for photosynthesis
• Plant Anatomy Vocabulary
– Mesophyll
• “middle leaf”
• The tissue in the interior of the leaf
– Stomata
• “mouth”
• Pores for the exchange of gasses (CO2 in, 02 out) , found mainly on the bottom of the
leaf
– Stroma
• The inner fluid of the chloroplast
– Thylakoid
• Membrane sacs in the stroma
• May be stacked and then called grana
• Inside these sacs is called the thylakoid space
– Chlorophyll
• Green pigment located in the thylakoid membranes of the chloroplast

6 CO2 + 12 H2O + Light energy --> C6H12O6 + 6 O2 + 6 H2O

General=CO2 + 2H2X --> [CH2O]+ 2 X

• Van Niel thought that plants split water to get the needed hydrogen atoms (and electrons!)

• Later this hypothesis was confirmed by using isotopes

• Plants form oxygen gas as a waste product like the sulfur bacteria form sulfur globules

Let’s review aerobic cellular respiration

• Sugar is oxidized when the hydrogen electrons are carried (slowly, gently) to oxygen and water is formed.

• The electrons lose potential energy as they go downhill to the more electronegative oxygen.

• This “lost” energy is harvested in the mitochondria to produce ATP. 

Photosynthesis reverses the direction electron flow.

• Water is split. Its hydrogen (a proton and an electron) is moved to carbon dioxide which is reduced to sugar.

• The electrons are being moved to oxygen (which is more electronegative) as it becomes sugar so they increase in potential energy. 

• This requires energy input. 

Part 1: The Light Reactions

• Covert solar energy to chemical energy

• Split water

2H2O -> 4 H+ + 4 e- + O2

– So now we need an electron acceptor, NADP+

– NADP+ Is just like NAD+ but with a phosphate 

group

• Solar energy reduces NADP+ to NADPH

• Makes ATP via photophosphorylation(photo energy)

Part 2: The Calvin Cycle 

The Dark Reactions (does not require light)

• Carbon is fixed – Carbon from CO2 is moved and attached to existing organic compounds

• Carbon is reduced

– gained ATP from NADPH which was made in the light reactions

• Sugar is made

– This synthesis reaction requires energy

– ATP was also made in the light reactions

10.2 The light reactions convert solar energy to the chemical energy of ATP and NADPH

• The light reactions occur in the thylakoid

• Light is electromagnetic energy that travels in rhythmic waves 

– Wavelength- distance from crest to crest

– Visible light 

• wavelengths we can see 

• part of the spectrum used in photosynthesis

• However, light also behaves as if it is a discrete particle

– Photons have a fixed amount of energy which is inversely related to the wavelength

Pigments

• Chlorophyll a- blue green

– Participates directly in light reactions

• Chlorophyll b- olive green

– Accessory pigment with slightly different structure (one functional group) 

– Gives a broader range of light absorption

• Carotenoids- yellow and orange

– Group of accessory pigments

– Also broaden the range but also provide photoprotection
Absorbed Light
-Absorbed photon's energy is given to another electron which moves to another (higher energy)orbital
-Excited state is unstable therefore when it falls back down heat is emitted, often times when its heat is in fluorescent.
Photosystem
 -Photo excites cholrophyll molecules which creates a domino action(transfer of energy) then goes to P680(PSII)  then e- then PEA.
ETC=Composed of Pq - Cytochrome complex - Pc
P700(PSI)

Chemiosmosis uses redox reactions in membranes to produce ATP.