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Chapter 7 Photosynthesis

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Presentation on theme: "Chapter 7 Photosynthesis"— Presentation transcript:

1 Chapter 7 Photosynthesis
Biology 2AP

2 Oxidation-Reduction Reactions
aka Redox reactions Electron transfer reactions Oxidation – lose electrons Reduction – gain electrons (LEOGR) Oxidation and reduction take place at the same time one molecule loses electrons while another molecule gains the electrons

3 Photosynthesis and Redox Reactions
H+ ions usually accompany electrons Oxidation in living things lose e-’s and H+ Reduction in living things gain e-’s and H+

4 Photosynthesis and Redox Reactions
6 CO2 + 6 H2O + Energy  C6H12O6+ 6 O2 Hydrogen atoms are transferred (lost) from water water is being oxidized Carbon dioxide gains the hydrogen atoms carbon dioxide is reduced Chloroplasts capture solar energy convert to chemical energy of ATP molecules

5 Photosynthesis and Redox Reactions
Coenzyme for redox NADP+ nicotinamide adenine dinucleotide phosphate active during photosynthesis has a positive charge accepts e-’s and H+ ions during photosynthesis NADP+ + 2 e- + H+  NADPH NADPH passes the e-’s and H+ ions to CO2 during photosynthesis

6 Cellular Respiration and Redox Reactions
C6H12O6+ 6 O2  6 CO2 + 6 H2O + Energy Glucose loses hydrogen atoms glucose is oxidized Oxygen gains hydrogen atoms oxygen is reduced Mitochondria use the energy released to make ATP

7 Cellular Respiration and Redox Reactions
Coenzyme for redox in cellular respiration NAD+ nicotinamide adenine nucleotide has a positive charge accepts electrons and H+ NAD+ + 2 e- + H+  NADH

8 Solar Energy Solar Energy + CO2 + H2O  Carbohydrate + O2
Solar energy is converted to chemical energy Energy is found in discrete packets photons

9 Light Light is part of the electromagnetic spectrum
Light behaves like waves Short wavelength – high energy Gamma ray, x-rays, uv Long wavelength – low energy Radio waves, microwaves, visible light Visible light – ROYGBIV Low energy, long wavelength  high energy, short wavelength

10

11 Light High energy photons Low energy photons Visible light
dangerous to cells can break down organic molecules Low energy photons do not damage cells increase vibrational or rotational energy do not break bonds Visible light only part of EMR spectrum used in photosynthesis just the right amount of energy to excite electrons

12 Energy Balance Sheet Only 42% of solar radiation reaches the earth’s surface most is w/in visible light range higher energy wavelengths are blocked by the ozone layer lower energy wavelengths are absorbed by water vapor and carbon dioxide Photosynthesis captures only 2% of the solar energy that reaches the earth’s surface plants incorporate only about 0.1 – 1.6% of this energy

13 Light and Pigments When light strikes an object, it can be
Absorbed Transmitted Reflected Pigment – a molecule that absorbs certain wavelengths more strongly than others The color observed are the colors not absorbed by the pigment

14 Absorption Spectrum Photosynthetic pigments can absorb various portions of visible light

15 Chloroplast pigments Found in the membrane of the thylakoids
Several different types of chlorophylls Chlorophyll a and chlorophyll b – most common Absorb different wavelength of light Chlorophyll a – absorbs more red and less blue Chlorophyll b – absorbs more blue and less red Since green light is not absorbed by the chlorophyll, it is reflected, which is why plants look green

16 Chloroplasts Pigments
Chlorophyll a involved in the light reactions Carotenoids Also in the thylakoid membrane Absorb green light Covered up by the chlorophyll Become visible when chlorophyll is lost in the fall Chlorophyll b and the carotenoids are accessory pigments they help plants to capture the energy in light

17 Action Spectrum Measure rate of photosynthesis (by measuring rate of O2 production) at each wavelength Action spectrum tells what portion of the EMR spectrum is used to perform photosynthesis sum of the action spectrum matches the absorption spectrum for chlorophyll a and b

18 Chloroplasts Where photosynthesis occurs in eukaryotes
O2 given off in photosynthesis comes from H2O (C.B. van Niel, 1930) Water is oxidized Carbon dioxide is reduced used CO2* (used heavy oxygen, 18O) O2 made did not contain 18O used heavy H2O (the oxygen is tagged) O2 made contained 18O

19 Chloroplasts Double membrane surrounds a fluid Stroma
enzyme rich solution where CO2 is attached to an organic molecule and reduced contains a membrane system flattened sacs = thylakoids stacked thylakoids = grana (plural; singular = granum) thylakoid space – space w/in a thylakoid connected to other thylakoid spaces chlorophyll and other pigments found w/in membranes of thylakoids

20

21 Reactions of Photosynthesis
F.F. Blackman, 1905 suggested two sets of reactions involved in photosynthesis when light is maximally absorbed, the rate of photosynthesis can still be increased by raising the temperature raising the temperature affects enzymes 1st set of reactions: light dependent reactions 2nd set: light independent reactions

22 Light Dependent Reactions
Occur in the thylakoid membranes where chlorophyll a and b and carotenoids are located chlorophyll a appears blue-green reflects blue-green, absorbs all other wavelengths chlorophyll b appears yellow-green reflects yellow-green, absorbs all other wavelengths Energy capturing reactions pigments capture sun’s energy use sun’s energy to excite electrons remove electrons from water

23 Light Dependent Reactions
e-’s move from chlorophyll a to the electron transport system ATP ADP + P NADP+ + 2e- + H+  NADPH NADPH “holds” energy in the form of energized electrons used to reduced CO2 later

24 Light Independent Reactions
Occurs in the stroma of the chloroplast Can take place in either the light or the dark The synthesis reactions uses ATP and NADPH formed in the thylakoids reduce CO2

25 Photosystems I and II Light gathering units in the thylakoid membrane
Named for the order in which they were discovered Contain closely packed molecules of chlorophyll a and b and accessory pigments pigment molecules are an antenna complex solar energy is passed from one pigment to another concentrated in the reaction center chlorophyll a molecule energy is used to excite electrons from the chlorophyll a molecule electrons escape to a nearby electron acceptor molecule

26

27 Electron Pathways Cyclic electron pathway Noncyclic electron pathway
generates only ATP cyclic photophosphorylation Noncyclic electron pathway generates ATP and NADPH ATP production during this pathway is known as noncyclic photophosphorylation Cell regulates proportion of ATP to NADPH by the relative activity of these two pathways

28 Cyclic Electron Pathway
High energy e-’s leave the PS I reaction center chlorophyll a molecule these e-’s will eventually return to the PSI chlorophyll a molecule e- enter the electron transport system a series of carriers (molecules) pass electrons from one molecule to the other energy is released and “stored” H+ ions are pumped from the stroma into the thylakoid space high concentration of H+ inside thylakoid space electrochemical gradient

29 Cyclic Electron Pathway
Some photosynthetic bacteria use only this pathway In plants, use cyclic pathway to generate more ATP for other reactions taking place in the stroma Use cyclic pathway when carbon dioxide is in limited supply when carbohydrates are not being produced so NADPH would not be necessary

30

31 Noncyclic Electron Pathway
Photosystem II antenna complex absorbs solar energy high energy electrons leave reaction center chlorophyll a molecule H2O  2H+ + 2 e- + ½ O2 electrons from water replace the electrons that left PS II O2 is released from the chloroplast and the plant H+ ions stay in the thylakoid space to contribute to the electrochemical gradient

32 Noncyclic Electron Pathway
High energy electrons that leave PS II captured by an electron acceptor sent to an electron transport system passed from one carrier to the next energy is released and “stored” H+ ions are pumped from the stroma into the thylakoid space high concentration of H+ inside thylakoid space electrochemical gradient enter PS I

33 Noncyclic Electron Pathway
PS I antenna complex absorbs solar energy electron is excited leaves reaction center chlorophyll a captured by an electron acceptor passed to NADP+ NADP+ + 2 e- + H+  NADPH The NADPH and ATP produced in the thylakoid membrane used by enzymes in the stroma in the light independent reactions

34 ATP Production H+ “stored” in the thylakoid space
from water being oxidized H2O  2 e- + 2 H+ + ½ O2 from e- moving down the electron transport system H+ pumped into thylakoid space using energy given off

35 ATP Production - Chemiosmosis
Flow of H+ ions from high concentration to low concentration across the thylakoid membrane from the thylakoid space to the stroma provides energy for ATP synthase catalyzes ADP + P  ATP

36

37 Complexes found in the Thylakoid Membrane
PS II protein complex light gathering antenna complex oxidizes water produces O2 gives off high energy electrons

38 Complexes found in the Thylakoid Membrane
Cytochrome Complex transporter of electrons between PS II and PS I H+ pumped into thylakoid space PS I protein complex light gathering antenna complex associated w/ enzymes to reduce NADP+ to NADPH

39 Complexes found in the Thylakoid Membrane
ATP synthase complex Has an H+ channel protruding ATP synthase H+ flows down its concentration gradient from thylakoid space to stroma ADP + P  ATP

40 Thylakoid Membrane

41 Light Independent Reactions
Light not needed for these reactions CO2 enters leaf Use ATP and NADP produced in light dependent reactions reduce CO2 provide electrons and energy Occurs in the stroma Series of reactions – the Calvin cycle

42 PGAL/G3P Glyceraldehyde-3-phosphate Product of the Calvin cycle
Converted to other organic molecules fructose phosphate fatty acid synthesis amino acid synthesis glucose phosphate sucrose starch cellulose

43 Calvin Cycle Light Independent Reactions Synthesize carbohydrates
Include carbon dioxide fixation carbon dioxide reduction regeneration of RuBP (ribulose bisphosphate)

44 Carbon Dioxide Fixation
Attachment of CO2 to an organic compound 1st step of Calvin cycle CO2 combines w/ RuBP RuBP carboxylase enzyme for reaaction makes up 20 – 50% of the protein content in chloroplasts relatively slow

45 Reducing Carbon Dioxide
6-Carbon molecule breaks down to two 3-Carbon molecules PGA (3-phosphoglycerate) PGA  PGAP  PGAL 2 steps Reduction requires NADPH + H+  NADP+ supplies electrons for reduction requires ATP ADP + P supplies energy

46 Regeneration of RuBP 3 turns of the Calvin cycle needed
net gain of 1 PGAL molecule 2 PGAL molecules needed to form 1 glucose molecule so 6 turns of the Calvin cycle needed to form 1 glucose molecule 5 molecules of PGAL (5 x 3 C molecule) needed to reform 3 RuBP (3 x 5 C molecule)

47 Calvin Cycle aka C3 cycle
1st molecule in cycle identified by Melvin Calvin was PGA Different plant species fix CO2 in different ways C3 plants (Calvin Cycle used to fix CO2) C4 plants CAM plants

48 C4 Photosynthesis Structure of leaf of C3 plant differs from that of a C4 plant C3 plant mesophyll cells contain well formed chloroplasts chloroplasts arranged in parallel layers C4 plant bundle sheath cells and mesophyll cells contain chloroplasts mesophyll cells are arranged concentrically around bundle sheath cells

49 C4 Photosynthesis C3 Plants C4 Plants
RuBP carboxylase used to fix CO2 to RuBP RuBP breaks down to give two 3-Carbon molecules, PGA C4 Plants PEP carboxylase used to fix CO2 to PEP (phosphoenolpyruvate, a 3-Carbon molecule) make oxoaloacetate, a 4-Carbon molecule

50 C4 Plants Oxaloacetate  malate Include
malate is pumped into bundle sheath cells CO2 then enters Calvin cycle in the bundle sheath cells Include sugarcane corn Bermuda grass Net photosynthetic rate is 2 – 3 times that of C3 plants avoids photorespiration

51 Photorespiration Occurs in C3 plants Leaves have little openings
stomates water leaves through the stomates CO2 enters Hot and dry weather stomates close to conserve water CO2 concentration decreases in leaves O2 concentration increases

52 Photorespiration O2, not CO2, combines w/ RuBP carboxylase
make 1 molecule of PGA release 1 molecule of CO2 Photorespiration occurs in the presence of light (photo-) O2 is taken up CO2 is given off (respiration)

53 Photorespiration Does not occur in C4 leaves When stomates are closed
CO2 is still delivered to the bundle sheath cells CO2 fixation occurs in bundle sheath cells Fix CO2 by forming a C4 molecule prior to the Calvin Cycle

54 Advantages to... C3 plants have the advantage in moderate weather
C4 plants have the advantage in hot and ry weather Early summer C3 plants predominate wheat, rice, oats Kentucky bluegrass, creeping bent grass Late summer C4 plants predominate crabgrass

55 CAM Photosynthesis Crassulacean acid metabolism Crassulaceae
family of flowering succulent plants live in warm, arid regions CAM first discovered in these plants prevalent among most succulent plants in desert environments minimal photosynthesis

56 CAM Photosynthesis Stomates close in the day Stomates open at night
no CO2 enters light dependent reactions still take place make ATP and NADPH Stomates open at night take in CO2 CO2 combines w/ PEP forms 4-Carbon molecules stored in vacuoles in mesophyll cells In the day, these C4 molecules release CO2 to the Calvin cycle when ATP and NADPH are available

57 C4 vs. CAM C4 plants CAM Plants
CO2 uptake is physically separated from Calvin cycle CO2 uptake in mesophyll cells to make a 4 Carbon molecule 4 Carbon molecule pumped into bundle cells releases CO2 for the Calvin cycle CAM Plants CO2 uptake is separated from Calvin cycle by time CO2 uptake is at night make a 4 Carbon molecule 4 Carbon molecule releases CO2 in the daytime for the Calvin cycle

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