1. Introduction to Photosynthesis Chapter 8
OCC BIO-161
Dave Werner

2. Essential Questions Why study Photosynthesis?
What will you gain from understanding Photosynthesis?
How does Photosynthesis relate to Cellular Respiration?

3. Animations Light & Dark Reactions Podcast – Overview of Photosynthesis
Photosynthesis Rap

5. 1. ALL LIFE REQUIRES ENERGY 2
1.ALL LIFE REQUIRES ENERGY 2.Animals, fungi, and most protists obtain their energy by consuming, directly or indirectly, organic food from their environment (heterotrophs) 3.Some organisms (autotrophs) have the ability to capture the energy of the sun to synthesize their own organic food (green plants, algae) 4.THE ULTIMATE SOURCE OF ALL ENERGY ON EARTH IS THE SUN 5.PHOTOSYNTHESIS is the link between life on earth and the sun 6.It is a set of reactions which convert light energy from the sun into chemical bond energy of glucose and ATP

7. Light and Pigments
The Nature of Sunlight

8. The Nature of Sunlight
light is a form of energy known as electromagnetic radiation
light travels in rhythmic waves which are disturbances of electrical and magnetic fields

10. The Nature of Sunlight
the distance between crests of electromagnetic waves is called the wavelength
the entire range of radiation is known as the electromagnetic spectrum

12. Light Energy (Fig.8.4)
the narrow range from about 380 to 750nm in wavelength is detectable by the human eye and is called visible light
the model of light as waves explains many of its properties, but in certain respects it behaves as though it consists of discrete particles
these particles called photons act like objects in that each of them has a fixed quantity of energy
the amount of energy is inversely related to the wavelength of light (shorter wavelengths have more energy)
Why are plants green?

13. Photosynthetic Pigments
as light meets matter, it may be reflected, transmitted or absorbed
substances that absorb light are called pigments
if a pigment is illuminated in white light, the color we see is the color most reflected or transmitted by the pigment

15. Light perception
the major pigment in leaves, chlorophyll, appears green because it absorbs red and blue light while transmitted and reflecting green
chlorophyll is actually a family of pigments with similar chemical structures

16. Photoexcitation of Chlorophyll
when energy is absorbed by a molecule of pigment, one of the molecules electrons is elevated to from its ground state to a higher orbital around the nucleus (excited state)

18. Photoexcitation of Chlorophyll
the only photons absorbed are those whose energy is exactly equal to the energy difference between the ground state and an excited state
the energy of the photon is converted to the potential energy of an electron, making the electron less stable
generally, when pigments absorb light, their excited electrons drop back down to the ground state very quickly releasing their energy as heat and/or light (fluorescence)

20. Overview of Photosynthesis Reactions
Photosynthesis can be summarized with this chemical equation:
6CO2 + 12H2O + LIGHT ENERGY/ENZYMES --> C6H12O6 + 6O2+ 6H2O

21. 6CO2 + 12H2O + LIGHT ENERGY -> C6H12O6 + 6O2 + 6H2O
The chemical change is the reverse of cellular respiration
The low energy inorganic compounds (CO2 and water) are converted into the high potential organic molecule (glucose)

22. The Chloroplasts: Sites of Photosynthesis
The primary function of this specialized organelle is to convert light energy into ATP and NADPH (nicotinamide adenine dinucleotide phosphate)
Chloroplasts are found mainly in the cells of the mesophyll (about 50/cell), the green tissue on the interior of the leaf

24. Leaf (Fig.8.1)
Carbon dioxide enters the leaf, and oxygen exits, by way of microscopic pores called stomata
The double membrane of the chloroplast regulates transport of materials in and out
Chloroplasts are filled with an aqueous solution called the stroma which contains all the necessary enzymes for photosynthesis

27. Chloroplast (Fig. 8.1)
The conversion from light energy to ATP and NADPH occurs in the thylakoid membranes within the stroma
The thylakoid membranes contain all of the pigments involved in the process including chlorophyll (green pigment) and other carotenoids
The thylakoids are organized into closely packed stacks called grana

29. Choloroplast
Within these thylakoids and grana, light energy is converted into ATP and NADPH – these are said to be LIGHT-DEPENDENT REACTIONS
The reactions that actually convert CO2 to carbohydrate are LIGHT-INDEPENDENT REACTIONS or DARK REACTIONS

30. The Light Reactions - Video
Must take place in the presence of light
Steps that convert solar energy to chemical energy
Light absorbed by chlorophyll drives a transfer of electrons from water to an acceptor named NADP+ which temporarily stores the energized electrons

32. Light Reactions
Water is split in the process and thus it is the light reactions of photosynthesis that give off O2 as a by-product
The light reactions also generate ATP by powering the addition of a phosphate group to ADP, a process called photophosphorylation. This is done w/ the help of ATP Synthase b/c H+ cannot diffuse through the membrane.
THE LIGHT REACTIONS PRODUCE NO SUGAR

33. Electron Transport (Fig.8.2)
chloroplasts and mitochondria generate ATP by the same basic mechanism of chemiosmosis
an electron transport chain (ETC) embedded in the thylakoid membrane pumps protons across the membrane as electrons are passed through a series of carriers producing a proton-motive force (potential energy stored in the proton gradient)

34. ETC: ATP Synthesis in Chloroplasts
ATP synthase in the membrane couples the diffusion of hydrogen ions down their gradient to the phosphorylation of ADP
in contrast to oxidative phosphorylation in mitochondria, chloroplasts use light energy (not chemical energy in food) to drive electrons to the top of the transport chain
the proton pump of the thylakoid membrane moves hydrogen ions from the stroma to the thylakoid space which functions as the H+ reservoir
the membrane makes ATP in the stroma as hydrogen ions diffuse back down their gradient through ATP synthase

36. Restoring PSII (Fig. 8.13) 2H2O → 4H + 4e- + O2
Can you explain how PSII is restored?

37. Chemiosmosis Happens during Light Rxns.
Concentration gradient of protons across thylakoid membrane.
Where do the protons come from?
ATP Synthase – protein that harnesses energy.
Some Protons are used to make NADPH from NADP+.
NADPH & ATP drive next set of rxns.

38. Energy Flow in Photosynthesis
in its native environment of the thylakoid membrane, chlorophyll is organized along with proteins, pigments,  and other kinds of smaller organic molecules into photosystems
a photosystem has a light gathering "antenna complex" consisting of a few hundred chlorophyll a, chlorophyll b, and carotenoid molecules
all of the antenna molecules absorb photons of light and the energy is transmitted from pigment molecule to pigment molecule until it reaches the reaction center

39. Photosystems II and I (Fig.8.13)
there are two photosystems in the thylakoid membranes, photosystem II and photosystem I
the chlorophyll a in both photosystems is identical, it is their association with different proteins that affects their light absorbing properties

41. The Calvin Cycle (Dark Reactions)
The "Dark Reactions" include the biochemical, enzyme-catalyzed reactions involved in the synthesis of carbohydrate from carbon dioxide; these are collectively know as the Calvin-Benson cycle

42. The Dark Reactions (Calvin Cycle) Video
Light is not required directly for these reactions to occur
These reactions incorporate CO2 from the air into organic material through a process known as carbon fixation
The fixed carbon is then reduced to carbohydrate by the addition of electrons
The reducing power is provided by NADPH and ATP provided by the light reactions
Dark reactions in most plants occur during daylight so that the light reactions can regenerate NADPH and ATP
These reactions occur in the stroma

43. The Reactions In the Stroma (Fig.8.16)
1. CO2 combines with RuBP to form 2 molecules of PGA.
2. Each molecule of PGA is converted into a molecule of PGAL.
3. Most of the PGAL is converted back into RuBP, but some PGAL can be used to make a variety of organic compounds.
For every three CO2 that enter the Calvin-Benson cycle via rubisco, a total of six molecules of 3-phosphoglyerate (PGA) are made

46. The Balance Sheet for Photosynthesis
Each turn of the Calvin Cycle fixes one CO2 molecule.
It take 3 turns to produce each PGAL molecule.
3 turns uses nine ATP molecules and 6 NADPH molecules. Why nine?
A variety of organic compounds are created: amino acids, lipids, and carbohydrates.

47. The Metabolic Fates of Glucose:
About 50% of the glucose formed is used immediately to meet the plants energy needs
Excess glucose can be converted to starch within the stroma of the chloroplast or in specialized storage cells of roots, tubers, seeds, and fruits
REMEMBER, plants actively metabolize glucose (cellular respiration) and grow in the dark and in the light
The glucose may be converted to sucrose (glucose + fructose) for transport (via the phloem cells) to the non-photosynthetic leaves, roots, and stems
The glucose may be converted to CELLULOSE, to build cell walls, especially in plant cells that are still growing and maturing

48. Alternative Pathways (Fig.8.20)
Calvin Cycle=most common pathway for carbon fixation.
Plants in harsh conditions fix carbon through alternate pathways and then release it to enter the Calvin cycle.

49. C3 Plants Most plants are C3 plants
C3 plants exclusively fix carbon through the Calvin cycle b/c of 3-carbon PGA.
Build up of O2, which slows down sugar production.

50. C4 Pathway - Video Corn, sugar cane, crabgrass.
Fix CO2 into 4-Carbon compounds = C4 pathway.
Partially close stomata during hottest part of the day.
Lose about ½ as much water as C3 plants when producing same amount of carbs.

52. CAM Pathway
CAM plants have adapted to dry conditions by opening their stomata during the night and closing them during the day, opposite to how other plants behave
when the stomata are open CO2 is incorporated into a variety of organic acids in a method of carbon fixation call crassulacean acid metabolism (CAM)
the mesophyll cells of CAM plants store the organic acids they make during the night in their vacuoles until morning when the stomata close
CO2 is released from the acids during the day for incorporation into the Calvin cycle
Lose less water than C3 & C4 plants.
Desert plants - Cactus

53. Rate of Photosynthesis

54. Now let's revisit the summary equation for photosynthesis & note how each of the raw materials end up in the chloroplasts so that the whole photosynthesis deal can go down.

55. Now let's do the same, except pay attention to what happens to the products of photosynthesis.