1. Photosynthesis & Plants
Stem and Leaf Anatomy

2. Photosynthesis Requires:
Carbon dioxide from the atmosphere
Plants must have air flow in the leaves to allow carbon dioxide into the leaf and oxygen to leave
Water from the soil
Roots absorb water, channel it into vascular tissue (veins), and send it through stems to the leaves
Sunlight
Comes from above, so plants must position their chloroplasts to best compete for sunlight

3. Conflicting needs pt. 1
More sunlight is good with regards to energy absorption, but causes more evaporation of water
Water must be in the liquid form for it to be used for photosynthesis
Water vapor can be easily lost from leaves (“transpiration”)

4. Broadleaf vs needle

5. Broadleaf vs grass

6. Leaves, external anatomy
Observe a variety of sample leaves
For each, consider its relative balance between light absorption and water conservation

7. Lab Activity – Stem Observation
Observe prepared slides of Zea stems and Tilia stems
Zea is corn. It is an annual plant (lives one growing season) with a herbaceous stem
Herbaceous stems are photosynthetic
Tilia is a woody perennial. It lives for several growing seasons.
Tilia stems are not photosynthetic
Note the growth rings
Compare and contrast the organization of vascular tissue
In the vascular bundles, identify the Xylem and Phloem cells
Xylem is used to transport water from roots to the stems and leaves
Phloem is used to transport dissolved sugars from the leaves to the stems and roots

8. Lab Activity – Leaf Observation
Observe prepared slides of Monocot and Dicot Leaf cross sections
For each type, note the concentration of photosynthetic cells
Consider what the anatomy of the leaf does with regard to
Absorbing sunlight
Allowing air flow
Providing a supply of water and preventing water loss by transpiration
Observe the arrangement of
Vascular bundles
Stomata
Palisade mesophyll
Spongy mesophyll

9. Leaf anatomy observations
Compare and Contrast with regard to :
Concentration of photosynthetic cells
Arrangement of vascular bundles
Location of stomata
Location and relative amount of air space

10. Stomata
Stomata are openings in the epidermis of the leaf to allow air to enter and leave
Stomata are controlled by a pair of guard cells that can open or close the stomate as needed

11. Guard cells and stomata
The guard cells have thick, rigid cell walls on the surfaces facing the stomate
When the guard cells swell up with water, the outer surfaces stretch, causing the cells to curve
The curved shape creates an opening that allows air flow

12. Conflicting needs pt. 2
More sunlight is good with regards to energy Air flow is necessary for gas exchange, but . . .
Space devoted to air flow is space that is not available for chloroplasts
More air flow allows more carbon dioxide into the leaf, and facilitates movement of oxygen out, but also allows water vapor to escape

13. photorespiration
Remember that the first step in the Calvin Cycle, the carboxylation of RuBP, is subject to competitive inhibition by O2.
The enzyme Rubisco can fix either CO2 or O2 to RuBP.
If photosynthesis were to progress with no air flow, eventually the concentration of CO2 would become so low, and O2 so high, that O2 would be fixed to RuBP at a high rate, bringing the Calvin Cycle to a grinding halt
Efficient photosynthesis depends upon maintaining a high concentration of CO2 in the leaf

14. Resolving Conflicts Ecologically
Plants that are good at conserving water are capable of thriving in dry climates, where they will not have any competition from plants that are not very good at conserving water
Plants that are good at maintaining air flow will thrive in climates that are moist. They will outcompete other plants that can’t maintain a constant supply of carbon dioxide. Unfortunately, they will dry out and die if they don’t also have a constant supply of water

15. The C3 Strategy
C3, as in 3 carbons. C3 refers to the 3 carbon compound PGA, which is the first stable step of the Calvin Cycle
C3 photosynthesis is the standard photosynthetic system as described in the textbook
C3 plants are generally very efficient at photosynthesis and exhibit a high level of productivity, but are more sensitive to water loss than C4 or CAM plants

16. C3 leaf anatomy

17. C3 leaf anatomy – labelled diagram

18. Palisade mesophyll
C3 leaves have a densely packed “palisade” layer at the top of the leaf to maximize light absorption and protect against water loss from the top of the leaf

19. Upper epidermis and cuticle
The upper epidermis has very few stomata, forming a sealed upper surface
The upper epidermis also secretes a waxy cuticle, forming a water-tight barrier

20. Spongy mesophyll
The lower half of the leaf consists largely of loosely aggregated cells with plenty of air space, associated with plentiful stomata in the lower epidermis, to allow the exchange of CO2 and O2

21. The trade off
The flow of air in the leaf minimizes photorespiration, but allows transpiration (water evaporation and loss )

22. The C4 Strategy C4 plants are better adapted to conserve water
The leaves are more compact, with less exposed surface, which helps to retain water.
The mesophyll is more tightly packed, with less overall air space, which minimizes evaporation of water
The tightly packed mesophyll forms a “bundle sheath” surrounding vascular bundles which provides a ready supply of water to photosynthetic cells

23. C4 leaf anatomy

24. The trade off
Less air flow means less water loss (transpiration), but also limits the ability of the leaf to exchange CO2 and O2, so the plant must provide CO2 from an alternative pathway

25. The c4 pathway (AKA – the malate shuttle)
C4 plants use an active transport system to concentrate CO2 into the cells of the bundle sheath
Mesophyll cells bind CO2 to phosphenol pyruvate, resulting in a high concentration of Malate, that diffuses into the bundle sheath

26. The c4 pathway (AKA – the malate shuttle)
Once malate enters the bundle sheath, it is decarboxylated, freeing CO2 which can then enter the calvin cycle
Regenerating PEP requires energy from ATP
Since 6 molecules of CO2 are required to make glucose, C4 plants require 6 more ATP’s to make a sugar than C3 plants do

27. C3 vs c4 compare/contrast
Each plant has variations in the density of cellular arrangement vs. air space that provides adaptive value for light absorption, air flow, and water conservation
Different types of plants will compete with each other for air, water, sunlight, soil and space.
C3 plants are better at competing for sunlight, but need more water.
C4 plants are better at conserving water, but require more sunlight energy

28. The CAM Strategy
CAM plants are succulents, with minimal exposed surface, a thick waxy cuticle, and maximum volume for storage of water.

29. The Cam strategy CAM stands for Crassulacean Acid Metabolism
CAM plants close stomata during the day, and open them at night, allowing for air flow with a minimum of water loss
CO2 entering the plant is immediately converted into larger molecules of water soluble organic acids
During the day, when sunlight is present, these acids can be decarboxylated to provide the CO2 needed for the Calvin Cycle
This allows the plant to maintain high CO2 levels, conserve water, and absorb sunlight all at the same time

30. CAM Photosynthesis