1. Where It Starts - Photosynthesis
Chapter 7
Where It Starts - Photosynthesis

2. Photosynthesis - Intro
Photosynthesis – makes food (sugar and other compounds) by
using sunlight as an energy source
carbon dioxide as the carbon source
Releasing water and oxygen

3. 7.0 Sunlight and Survival
Plants are autotrophs, or self-nourishing organisms; they get Carbon and energy from the environment and make their own food.
The first autotrophs filled Earth’s atmosphere with oxygen, creating an ozone (O3) layer
The ozone layer became a shield against deadly UV rays from the sun, allowing life to move out of the ocean and diversify.

4. Other types of organisms
Heterotrophs – Cannot obtain energy and C from the environment. They feed on autotrophs, one another, and organic wastes.
Plants are photoautotrophs. They make sugars and other compounds using sunlight as the energy source and CO2 and release great amounts of oxygen.
Plants around the world produce 220 billions tons of sugar each year

5. Prokaryotes
The first prokaryotes were chemoautotrophs, they did not have the enzymes needed. (So they extracted energy and carbon from methane and hydrogen sulfide, there was little free oxygen.)
Some prokaryotes evolved to neutralize toxic oxygen radicals and flourished. Those that did not perished.

6. 7.1 Sunlight as an Energy Source
Plants (Photoautotrophs) utilize the energy from the sun, in the form of photons , particles of wavelengths of visible light.
Organisms use only a small range of wavelengths for photosynthesis
(380 – 750nm)
Light energy is packed as photons.

7. Electromagnetic Spectrum
Shortest Gamma rays*
wavelength X-rays*
UV radiation*
Visible light
Infrared radiation
Microwaves
Longest Radio waves
wavelength * alters or breaks bonds in DNA and Proteins. Threat to all organisms.

8. Photons Packets of light energy
Each type of photon has fixed amount of energy
Photons having most energy travel as shortest wavelength (blue-violet light)
Photons having least energy travel in longer wavelengths. (red Light, pg. 108)

9. Visible Light Wavelengths humans perceive as different colors
Violet (380 nm) to red (750 nm)
Longer wavelengths, lower energy
Figure 7-2 Page 108

10. Pigments
Pigments are a class of molecules that absorb photons in certain wavelengths only.
The color you see is the wavelength not absorbed
Light-catching part of molecule often has alternating single and double bonds
These bonds contain electrons that are capable of being moved to higher energy levels by absorbing light

11. Pigment Molecules
Pigment molecules on the thylakoid membranes absorb photons.
Chlorophyll a (major) pigments absorb violet and red, but reflect green & yellow (leaves)
Chlorophyll b (accessory) pigments reflects green & blue.
Carotenoid pigments absorb blue-violet and blue-green but reflect yellow, orange, and red
The light-catching portion is the flattened ring structure (see page 109)

12. Pigment Molecules
Xanthophylls reflect yellow, brown, purple, or blue light
Anthocyanins – reflect red and purple light in fruit and flowers
Phycobilins – reflect red or blue-green light and are accessory pigments found in red algae and cyanobacteria

13. Pigments in Photosynthesis
Bacteria
Pigments in plasma membranes
Plants
Pigments and proteins organized into photosystems that are embedded in thylakoid membrane system

14. 7.2 Harvesting the Rainbow
Wilhelm Theodor Engelmann, a botanist, knew that plants use sunlight, water and something in the air.
What he wanted to know what which parts of sunlight do plants favor?
He designed an experiment using photosynthetic alga, Cladophora and aerobic bacterial cells.

15. Englemann’s Experiment
He knew that certain bacterial cells will move toward places where oxygen concentration is high
He also understood that Photosynthesis produces oxygen
He identified violet and red light are best at driving photosynthesis, as most of the bacterial cells gathered at this point.

16. 7.3 Overview of Photosynthesis Reactions
Photosynthesis proceeds in two reaction stages. See page 111.
Light-dependent reactions -thylakoid membrane
Sunlight energy is converted to chemical bond energy of ATP
Water molecules are split, and typically the coenzyme NADP+ accepts the released hydrogen and electrons, thus becoming NADPH, oxygen is released

17. Photosynthesis – Second Stage
Light Independent reactions - stroma
Runs on energy delivered by ATP.
This energy drives the synthesis of glucose and other carbohydrates.
The building blocks are the hydrogen atoms and electron from NADPH, as well as carbon and oxygen atoms stripped from carbon dioxide and water.

18. light-dependant reactions light-independant reactions
Two stages of Photosynthesis
SUNLIGHT
H2O O2
CO2
NADPH, ATP
light-dependant reactions
light-independant reactions
NADP+, ADP
sugars
CHLOROPLAST
Fig. 7-6c, p.111

19. 7.4 Light-Dependent Reactions
Occurs in the thylakoid membrane of the chloroplast
Hundreds of Pigments absorb light energy, give up e-, which is transferred to a photosystem.
Two molecules of chlorophyll a are at the center of a photosystem.
Chloroplasts have two kinds of photosystems, type I and type II. (Focus on type II)

20. Light Dependent continued
The freed e- enter an electron transfer chain, an orderly array of enzymes and co- enzymes.
This is the first step in the conversion of light to chemical energy.
Water molecules split, ATP and NADH form, and oxygen is released.

21. Light Dependent continued
Pigments (PS II)that gave up electrons get replacement electrons (from water molecules –through the process of photolysis)
H+ moves from the stroma into the thylakoid compartment.
The e- now enter photosystem 1
This is also known as the noncyclic pathway of ATP formation.

22. cross-section through a disk-shaped fold in the thylakoid membrane
LIGHT-
HARVESTING
COMPLEX
PHOTOSYSTEM II
P 680
sunlight
PHOTOSYSTEM I
P 700 H+
NADPH e- e- e- e- e- e-
NADP + + H+ e-
H2O H+ H+ H+ H+
thylakoid
compartment H+ H+ H+ H+ H+ H+ O2 H+
thylakoid
membrane
stroma
ADP + Pi
ATP
cross-section through a disk-shaped fold in the thylakoid membrane H+
Fig. 7-8, p.113

23. Noncyclic pathway of ATP
This pathway of photosynthesis photon energy forces electrons out of photosystem II to an electron transfer chain, which sets up H+ gradient that drive ATP formation, and ultimately end up in NADPH.
The electrons are not cycled back into photosystem II.

24. Cyclic Pathway of ATP
Photosystem I may run independently so that cells can continue to make ATP.
It is a cyclic because the electrons that leave photosystem I get cycled back to it.
These electrons pass through an ETC that moves H+ into the thylakoid compartments. This drives ATP formation, but no NADPH forms.

25. 7.5 Energy Flow in Photosynthesis
Most pigments in photosystem are harvester pigments
When excited by light energy, these pigments transfer energy to adjacent pigment molecules
Each transfer involves energy loss

26. Photosystem Function: Reaction Center
Energy is reduced to a level that can be captured by molecule of chlorophyll a
This molecule P700 (I) or P680 (II) is the reaction center of a photosystem
Reaction center accepts energy and donates electron to acceptor molecule

27. Cyclic Electron Flow Electrons
are donated by P700 in photosystem I to acceptor molecule
flow through electron transfer chain and back to P700 to be reused
Electron flow drives ATP formation
This is the process of the first anaerobic photoautotrophs. See page 114

28. Noncyclic Electron Flow
Two-step pathway for light absorption and electron excitation
Uses two photosystems: type I and type II
e_ that leave PS II are not returned to it.
Produces ATP and NADPH
Involves photolysis - splitting of water
See page 114

29. Chemiosmotic Model of ATP Formation
Electrical and H+ concentration gradients are created between thylakoid compartment and stroma
H+ flows down gradients into stroma through ATP synthesis ( ATP synthase)
Flow of ions drives formation of ATP

30. 7.6 Light-Independent Reactions
Synthesis part of photosynthesis – make sugar
Takes place in the stroma and can proceed in the dark
Steps – carbon fixation, Rubisco mediated
Calvin-Benson cycle – a series of enzyme mediated reactions

31. Carbon Fixation
A Carbon atom from CO2 becomes attached to an organic compound.
Plants get the carbon dioxide from the air, and algae get it from carbon dioxide dissolved in water.
Rubisco (ribulose bisphosphate carboylase/oxygenase meditates this step in most plants.

32. Calvin-Benson Cycle This is known as the sugar factory
It is a series of enzyme mediated reactions that take place in the stroma.
Reactants – carbon dioxide attaches to rubisco, and splits into phosphoglycerate. ATP, and NADPH
Products – Glucose, ADP, and NADP+
This is cyclic and RuBP is regenerated

33. phosphorylated glucose
Calvin- Benson Cycle
Six turns to make one glucose molecule
6CO2
ATP
6 RuBP
12 PGA 12
6 ADP
12 ADP +
Calvin-Benson
cycle
12 Pi
ATP 12
NADPH
4 Pi
12 NADP+
10 PGAL
12 PGAL
1 Pi 1
phosphorylated glucose
Fig. 7-10b, p.115

34. 7.7 The C3 Pathway Gases diffuse in/out of a plant through stomata.
In Calvin-Benson cycle, the first stable intermediate is a three-carbon PGA
Because the first intermediate has three carbons, the pathway is called the C3 pathway

35. Photorespiration in C3 Plants
On hot, dry days stomata close
Inside leaf
Oxygen levels rise
Carbon dioxide levels drop
Rubisco attaches RuBP to oxygen instead of carbon dioxide (photorespiration)
Only one PGAL forms instead of two
See page 117

36. C4 Plants Carbon dioxide is fixed twice
In mesophyll cells, carbon dioxide is fixed to form four-carbon oxaloacetate
Oxaloacetate is transferred to bundle-sheath cells
Carbon dioxide is released and fixed again in Calvin-Benson cycle. See page 117.

37. CAM Plants Crassulacean Acid Metabolism
Carbon is fixed twice (in same cells)
Never uses oxygen, no matter the concentration
Night – stomata open
Carbon dioxide is fixed to form organic acids
Day
Carbon dioxide is released and fixed in Calvin-Benson cycle. See page 117.

38. 7.8 Autotrophs and the Biosphere
Photoautotrophs
Carbon source is carbon dioxide
Energy source is sunlight
Heterotrophs
Get carbon and energy by eating autotrophs or one another

39. Linked Processes Photosynthesis Aerobic Respiration
Energy-storing pathway
Releases oxygen
Requires carbon dioxide
Aerobic Respiration
Energy-releasing pathway
Requires oxygen
Releases carbon dioxide

40. Photoautotrophs
Capture sunlight energy and use it to carry out photosynthesis
Plants
Some bacteria
Many protistans
See Figure 7-14, on page 120 for an overview