1. Leaves and Photosynthesis

2. Leaf Structure and Function

3. Leaves Primarily light harvesting organs
Primary location of photosynthesis
Responsible for photon collection and carbon fixation
Usually thin, flat, which increases surface area
Increased surface area and stomatal openings lead to water transport through tree
Requires adaptations to prevent unnecessary water loss

4. Leaf Organization Epidermis Cuticle
Usually clear, nonphotosynthetic, outer cell wall thick compared to inner cell wall
Upper
Covers the upper surface
Lower
Covers lower surface
Cuticle
Made of waxy layer called cutin
Reduces water loss
Thicker in upper epidermis

5. The Mesophyll Major mass of photosynthetic tissue
Located between the upper and lower epidermis
Often two sublayers
Near upper epidermis, see palisade mesophyll
Longer parallel cells
Can be several layers, related to environment
Major site of photosynthesis
Lower cells called spongy mesophyll
Allow for gas exchange and gas movement within leaf
Also photosynthetic

7. Structure/Function in Leaves
Leaves are the site of photosynthesis – this is main function
Harvest light energy
Produce glucose and other sugars
Upper epidermis is clear to allow light to pass; is waxy to prevent excessive water loss
Mesophyll contains bulk of chloroplasts
Air space between mesophyll cells open to the open air through the stomata; CO2 diffuses into thin water layer on mesophyll cells

8. Water passes out of the leaf through stomata, drives the movement of water through the plant, bringing nutrients to the plant cells via the xylem; similarly water flowing back to the stem and roots carries sugar via the phloem

9. Mesophyll contains bulk of chloroplasts
Air space between mesophyll cells open to the open air through the stomata; CO2 diffuses into thin water layer on mesophyll cells
Water passes out of the leaf through stomata, drives the movement of water through the plant, bringing nutrients to the plant cells via the xylem; similarly water flowing back to the stem and roots carries sugar via the phloem

10. Photosynthesis: Capturing Energy

11. Autotrophs and Chemotrophs
Carbon fixation is the process of building complex carbon compounds from simple carbon compounds.
Organisms that fix carbon are autotrophs – they use carbon dioxide as a carbon source, and combine it with water
Photoautotrophs provide nearly all the energy used by living systems on Earth

12. Photoautotrophs: Organisms that gain energy from light in order to carry out carbon fixation
Photosynthetic plants, algae and bacteria
Use light energy to make ATP and carbohydrate
Chemoautotrophs: Organisms that use chemical energy only to cause carbon fixation and to build structure
Certain bacteria
Heterotrophs: Organisms that gain energy by eating other organisms, including autotrophs
Animals, nonphotosynthetic plants, nonphotosynthetic unicellular organisms (such as protists), bacteria, and fungi

13. The Chloroplast The site of light harvesting
The site of the start of carbohydrate synthesis

14. Chlorophyll, Part 1
Chlorophyll collects light energy (absorbs it) in a resonant porphyrin group that hangs out like a kite on the surface of the thylakoid
Chlorophyll a initiates the light-dependent reactions
Chlorophyll b is an accessory pigment
Carotenoids are yellow and orange pigments that capture light energy and pass electrons to chlorophyll

15. Partition of Function in the Chloroplast
The light-dependent reactions (the harvesting of light) occur on thylakoid membranes
The carbon fixation reactions (formation of carbohydrate) occur in the stroma

17. Overview of Photosynthesis
Photosynthesis is a redox reaction:
Carbon dioxide is reduced to sugar
Water is oxidized to molecular oxygen

18. How Photosynthesis Starts
hn absorbed by chlorophyll
Energy (as an electron) “falls” from one chlorophyll to the next.
Eventually falls into the reaction center; a special chlorophyll molecule
Biology, Sixth Edition Chapter 8, Photosynthesis: Capturing Energy

19. 2 major parts to photosynthesis
Light Dependent Reactions
Take place on grana on the thalakoid membranes.
A photon of light excites the chlorophyll.
This electron is passed through an electron transport system.
During this process water is broken apart.
Oxygen is produced
H+ ions build up and are picked up by NADP
ATP is produced

21. Protons Build up Inside Thylakoids
The activity of the ETC causes a gradient of protons across the thylakoid membrane

22. The Chloroplast ATP Synthase
Protons fall back through the chloroplast ATPsynthase
Makes ATP by combining ADP and phosphate in a process called chemiosmosis
Much like the mitochondrial IM ATPsynthase

23. The Light independentReactions
So-called because they do not directly need light
They occur in the stroma of the chloroplast
They fix carbon to make carbohydrate
They are the Calvin-Benson Cycle reactions
The power to run this cycle comes from the ATP generated in the first part of photosynthesis
The Hydrogen is transferred from NADPH to Carbon dioxide

24. The Calvin Cycle:Phases 1 & 2
1. Carbon uptake
Adds carbon dioxide to 5C ribulose bisphosphate (RuBP)
Catalyzed by RUBISCO; ribulose bisphosphate carboxylase
2. Carbon reduction phase
Citrate is made and broken to form phosphoglycerate (PGA)
PGA is rearranged and phosphorylated by ATP
NADPH reduces the backbone further to form glyceraldehyde-3-phosphate (PGAL or G3P)

26. The Calvin Cycle: Phase 3
3. Reformation of RuBP:
PGAL (G3P) is rearranged,
& phosphorylated
With further investment of ATP…
To make RuBP, a bisphosphorylated compound
Alternatively,
PGAL is shuttled out of the cycle to produce glucose and other carbohydrates elsewhere

28. When RUBISCO Doesn’t Work
In high light and temperature:
Photosynthesis is very active
Water is easily lost
Leaf stomata close (small pores on the underside of the leaf) to protect against water loss
Oxygen builds up

29. On hot, dry days, plants close their stomata
Conserving water but limiting access to CO2
Causing oxygen to build up

30. RUBISCO has mixed affinity for oxygen and CO2
It binds O2 when O2 is abundant
PGAL is NOT produced

31. Photorespiration And as a result, photorespiration occurs:
No carbohydrate is produced
Instead, CO2 and H2O are produced
NO ATP is produced, however
Photosynthetic efficiency is degraded

32. Different enzyme is present as an adaptation in C4 and CAM plants
Alternative mechanisms of carbon fixation have evolved in hot, arid climates
Different enzyme is present as an adaptation in C4 and CAM plants
PEP carboxylase

33. Evolutionary “Work-Arounds” Avoid Photorespiration
1. The C4 pathway
Physically sequesters the carbon dioxide-requiring RUBISCO (carbon fixation) away from high oxygen
Uses compartmentation with biological membranes (in different cells)
In crabgrass, corn, and sugar cane
2. Crassulacean acid metabolism (CAM)
Carries out C fixation separated in time from high temperature and high oxygen
In desert plants: succulents and cacti (also lilies and orchids)
Biology, Sixth Edition Chapter 8, Photosynthesis: Capturing Energy

34. C3 versus C4 Anatomy
Bundle sheath cells are arranged differently in C3 and C4 plants
Biology, Sixth Edition Chapter 8, Photosynthesis: Capturing Energy

35. C4 Plants C4 plants minimize the cost of photorespiration
By incorporating CO2 into four carbon compounds in mesophyll cells

36. These four carbon compounds
Are exported to bundle sheath cells, where they release CO2 used in the Calvin cycle

37. C4 leaf anatomy and the C4 pathway
CO2
Mesophyll cell
Bundle-
sheath
cell
Vein
(vascular tissue)
Photosynthetic
cells of C4 plant
leaf
Stoma
Mesophyll
C4 leaf anatomy
PEP carboxylase
Oxaloacetate (4 C)
PEP (3 C)
Malate (4 C)
ADP
ATP
Sheath
Pyruate (3 C)
CALVIN
CYCLE
Sugar
Vascular
tissue
Figure 10.19

38. The CAM pathway is similar to the C4 pathway
Spatial separation of steps. In C4 plants, carbon fixation and the Calvin cycle occur in different
types of cells.
(a)
Temporal separation of steps. In CAM plants, carbon fixation and the Calvin cycle occur in the same cells at different times.
(b)
Pineapple
Sugarcane
Bundle-
sheath cell
Mesophyll Cell
Organic acid
CALVIN CYCLE
Sugar
CO2 C4
CAM
CO2 incorporated
into four-carbon
organic acids
(carbon fixation)
Night
Day 1 2
Organic acids
release CO2 to
Calvin cycle
Figure 10.20

39. CAM Plants
CAM plants
Open their stomata at night, incorporating CO2 into organic acids

40. During the day, the stomata close
And the CO2 is released from the organic acids for use in the Calvin cycle

41. The CAM pathway Seen in xeric plants (very dry conditions; desert)
Separation of function by TIME instead of location / compartmentation
PEP carboxylase works at night when the stomata are open (they close during the day in desert plants)
OA is made and malate derived from it is stored in the vacuole
During the day, CO2 is derived from the malate and made available to the Calvin cycle.
Not as efficient at supporting rapid growth as is seen in C4 plants like crabgrass and corn, is good adaptation for desert plants
Biology, Sixth Edition Chapter 8, Photosynthesis: Capturing Energy

42. Calvin cycle is all-important!
In C3, C4 and CAM plants, the Calvin Cycle always receives the carbon dioxide that is made into carbohydrate...
. . . under all conditions!
Scientists are very interested in getting C4 chemistry genetically introduced into plants to increase crop efficiency.
Biology, Sixth Edition Chapter 8, Photosynthesis: Capturing Energy