1. Intro to Photosynthesis

2. Why photosynthesize?
Ultimately, all living organisms receive energy from the Sun.
Photosynthetic organisms (plants, algae, bacteria, and some protists) capture light energy and use it to convert inorganic CO2 into organic carbon, i.e. glucose (C6H12O6) or other simple sugars (CH2O).

4. The light energy from the Sun becomes stored energy in molecules of simple sugars.
In the process of cellular respiration, glucose molecules are broken down and chemical energy in the form of ATP is harvested.
Since heterotrophs cannot synthesize their own glucose, they obtain glucose from autotrophs/producers.

5. Equation for Photosynthesis:
From the atmosphere
From the Sun
6 CO2 + 6 H2O + Light Energy → [CH2O] + 6O2
A variety of simple sugars may be formed, though glucose (C6H12O6) is one of the most common
From the surrounding
environment

6. The Leaf
Leaves are specialized for photosynthesis. They regulate the flow of gases and capture light energy for photosynthesis.
The structure and arrangement of leaves maximizes the surface area exposed to sunlight and limits the distance gases need to travel.

7. Maple leaves are thin
and broad with a large
surface area
Pine leaves are thin and narrow. A single needle does not provide a sizable surface area, but a branch of needles do.

8. 3-D Cross-Section of a Leaf
A transparent colourless
layer that allows light to
pass through to the
mesophyll cells
Protects the leaf from excessive absorption
of light and evaporation of water
Where most of the photosynthesis takes place (abundant in chloroplasts).
A system of vessels that transport water, minerals, and carbohydrates within the plant.
Regulates the exchange of gases in the atmosphere
Photosynthetic epidermal cells that create microscopic openings called stomata.

9. Stomata
Guard cells control the size of a stoma by changing their shape in response to water movement by osmosis in the cells.
When a stoma is open, its guard cells are turgid (full of water).
When a stoma is closed, its guard cells are empty.

10. Transpiration
Stomata cover only 1% to 2% of a leaf’s surface but they are responsible for 85% of the water lost by the plant
Transpiration is important for photosynthesis because of two reasons

11. Transpiration assists photosynthesis by:
Creating a transpiration pull: this helps move water, minerals and other substances from the roots, where they are absorbed
It produces an evaporative cooling effect. This prevents leaves from heating to temperatures what could denature necessary enzymes

12. Opening and Closing Stomata
The direction of osmosis follows the diffusion of K+ (potassium ions) across the guard cell’s membrane.
The passive diffusion of K+ is coupled with the active transport of H + through proton-pumps.
When H+ is pumped out, K + flows in and water follows
In order for the proton pumps to function, ATP must be available

13. H+ are pumped out of guard cells actively by proton pumps
K+ diffuses into guard cells
H2O diffuse into cells by osmosis
Guard cells swell and open
 CO2 enters stoma

14. In general, stomata are open in the daytime and closed at night.
When the Sun comes out in the morning, it activates receptors in the guard cell membranes, stimulating proton pumps that drive protons out of the cells. This creates a gradient and as a result, K+ ions move into the cells and water follows
As the concentration of sucrose in guard cells decrease in the evening, water moves out of the cells and the stomata close.

15. Chloroplasts
Photosynthesis takes place within the chloroplasts of plant cells and algae

16. Unstacked thylakoids that connect grana
The protein-rich semi-liquid material in the interior space
Membrane-bound sacs that stack on top of one another to form columns called grana

18. Photosynthesis occurs partly within the stroma and partly with the thylakoid membrane.
The structure of the thylakoid system greatly increases the surface area of the thylakoid membrane and therefore significantly amplifies the efficiency of photosynthesis
A typical chloroplast contains approx. 60 grana, each consisting of 30 to 50 thylakoids

19. The thylakoid membrane contains light-gathering pigment molecules (i.e. chlorophyll) and electron transport chains which are essential for photosynthesis.
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