1. Using Light to Make Food
CHAPTER PHOTOSYNTHESIS:
Using Light to Make Food

2. Photosynthesis – process by which plants use light to make food molecules from carbon dioxide and water

3. Autotrophs – plants that make their
own food (also known as producers)
Photosynthetic autotrophs—organisms
that use light as a source of energy to
make food

4. Chloroplasts
All green parts of a plant have chloroplasts (leaves are major sites of photosynthesis)
Green color of plants is due to green pigment w/in chloroplasts called chlorophyll
Chloroplasts mainly found in mesophyll cells – green tissue in interior of leaf

5. Consists of double membrane surrounding a thick fluid (stroma) – sugars are made from CO2 here
--CO2 enters leaf and O2 exits by way of tiny pores known as stomata

6. Thylakoids (green sacs) are suspended in stroma – light energy is captured here
Stack of these = Grana
Thylakoid

7. Redox Reactions 6CO2 + 12H2O C6H12O6 + 6H2O +6O2
-Photosynthesis STORES energy
ENDERGONIC
-Carbon dioxide is reduced to glucose
GAINING ELECTRONS
-Water is oxidized to oxygen
LOSING ELECTRONS
-Water is first consumed then produced

8. -Plants make sugar from carbon and oxygen in CO2 and from some hydrogen in H2O
-O2 is released, coming from the H2O, not from CO2

9. - Cellular respiration harvests energy by _______ the sugar & _______ O2 to H2O (respiration is energy-releasing, going from potential to kinetic by traveling down energy levels)
- Photosynthesis goes uphill (gaining potential and traveling up energy levels)
oxidizing
reducing

11. The Light Reactions
Convert light energy to chemical energy & O2 gas is waste product
Occur in thylakoid membranes
Stored in ATP & NADPH
No sugar produced

13. Visible Light
Sunlight is a type of energy called radiation (also known as electromagnetic energy)
Travels in rhythmic waves
Light reactions of photosynthesis only use certain wavelengths/colors of Visible Light

16. Light also behaves as discrete packets of energy called photons
A photon is a fixed quantity of light energy (shorter the wavelength, the greater the energy)

17. A photon of violet light packs almost twice as much energy as a photon of red light
Light may be reflected, transmitted, or absorbed (pigments are substances that absorb light)

18. Pigments involved in photosynthesis are chlorophyll a and b and carotenoids
The chlorophylls are green and carotenoids are shades of yellow and orange

19. Photosystems Pigments are clustered in thylakoid membranes
Chlorophyll a molecule & a primary electron acceptor make up the reaction center of the pigment assembly
Reaction center & other pigments function collectively as a light-gathering antenna that absorbs photons

22. Energy is passed from molecule to molecule until it reaches the reaction center
Combination of the antenna molecules, the reaction center, and the primary electron acceptor make up the photosystem
This is the light-harvesting unit of the chloroplast’s thylakoid membrane

23. Two Photosystems
Photosystem I is called P700 because the light it absorbs best is red light w/a wavelength of 700 nm
Photosystem II is called P680 because the light it absorbs best is orange shade of red light w/a wavelength of 680 nm

24. ATP, NADPH, and O2
First event in light reactions is the absorption of light energy
Second event is the excitation of electrons by light energy
Third event is formation of ATP & NADPH using energy made available by the cascade of energized electrons down electron transport chains

25. Transport chains are similar to the one that functions in cellular respiration
Consist of a series of electron-carrier molecules arranged in a membrane (the thylakoid of the chloroplast)

26. Chemiosmosis ATP is synthesized by chemiosmosis
Electron transport chains associated w/the chloroplast’s photosystems are arranged in thylakoid membranes
Electron transport chain in the chloroplast drives the transport of H+ through thylakoid membrane

27. Flow of H+ back through the membrane is harnessed by ATP synthase to make ATP
In photosynthesis this is called photophosphorylation
H+ ions, along w/electrons from the electron transport chain, join w/NADP+ to form NADPH

29. The Dark Reaction (Calvin Cycle)
Cyclic series of reactions that assemble sugar molecules using CO2 and energy-containing products of light reaction

31. Carbon Fixation (making sugar)
Calvin cycle is a sugar factory in the chloroplast (occurs in stroma)
CO2 from air & ATP & NADPH from light reactions are reactants of this process

32. Carbon Fixation (making sugar)
Using carbon from CO2, energy from ATP & high-energy electrons from NADPH, the Calvin cycle makes an energy rich sugar
Called glyceraldehyde-3 phosphate (G3P)
A plant uses this to make glucose or other organic molecules

34. Review
Sugar molecules that a plant produces are its food supply – they are expended during cellular respiration
Plants use sugars as building blocks for other organic compounds, including cellulose
Plants are the ultimate source of food for all other organisms

35. Adaptation for Saving Water
Most plants are C3 plants, which take carbon directly from CO2 in the air & use it in the Calvin cycle to build a 3-carbon molecule

36. Stomata in leaf surface usually close when the weather is hot & dry
Minimizes water loss
But CO2 and O2 are not exchanged as normal
Calvin cycle is diverted to an inefficient process called photorespiration

37. Some plants have special adaptations that enable them to save water & avoid photorespiration ex: corn, sugarcane
Special cells in C4 plants incorporate CO2 into a 4-carbon compound
It’s broken down to release CO2 (this initiates the Calvin cycle)

38. Rice, Soybean, many tree species Corn, sorghum, sugarcane
Characteristic
C3 Plants
C4 Plants
Origin
Temperate
Tropical
Examples
Rice, Soybean, many tree species
Corn, sorghum, sugarcane
Carbon dioxide fixation
3 carbon molecule
4 carbon molecule
Site of photosynthetic cycle
Mesophyll cells
Bundle sheath cells
Carbon dioxide concentration
Regulated by diffusion
Elevated high concentrations
Stomatal behavior
Open for longer periods
Open for shorter periods
Water use efficiency*
Not very efficient
Very efficient
Climatic adaptation
Mostly cooler, moderate climate
Mostly warmer, drier climate
Carbon dioxide saturation
High
Low
Light saturation
Photorespiration
*The ratio of carbon dioxide fixed to water used per unit area of the leaf.

39. CAM – Crassulacean Acid Metabolism
These plants are also adapted to hot, dry climates ex: cacti, pineapples, succulents (aloe)
Close stomata during day to prevent water loss & open at night
Carbon compounds stored in vacuoles at night
These compounds are broken down to release CO2 for photosynthesis during the day

40. Global Warming
In atmosphere, CO2 retains heat from sun that would otherwise radiate back into space
Burning of fossil fuels & wood releases excess CO2, which may be causing global warming
Greenhouse Effect

41. Forest Replacement Replace w/younger growth of trees
Increases photosynthesis, which reduces CO2, but burning at faster rate
Older trees also remove CO2, but at slower rate