1. Photosynthesis
IB topic 3.8 (page 83)

2. What is photosynthesis?
Photosynthesis is the production of organic compounds (carbohydrates, lipids, and proteins) using light energy and simple inorganic substances such as carbon dioxide and water
It is an example of energy conversion
Light energy is converted into chemical energy

3. First organisms
Prokaryotes were the first organisms to perform photosynthesis
About 3500 million years ago
They were joined billions of years later by algae and plants
One consequence of photosynthesis is the rise in oxygen concentration of the atmosphere, which began about million years ago

4. Why is this important?
Photosynthetic organisms produce foods that begin food chains
We count on the Sun as a constant energy source for warmth and food
Sunlight must be converted into a useful form of chemical energy for non- photosynthetic organisms
The most common chemical energy produced from photosynthesis is glucose
Most common molecule for fuel in cell respiration (remember?)

5. Pigments and light absorption
The vast majority of plants appear green to our eyes
Zoom into a leaf cell … the only structures in a leaf that are actually green are the chloroplasts
Plants contain a variety of pigments in chloroplasts
The photosynthetic pigment that dominates in most plant species is the molecule chlorophyll

6. Light absorption (cont.)
Light absorption in photosynthesis involves chemical substances called pigments
A white or transparent substance does not absorb visible light
Pigments are substances that do absorb light and therefore appear colored to us

7. Pigments
Pigments that absorb all colors appear black (b/c they emit no light)
Pigments that, for example, absorb all colors but blue appear blue to us (b/c this part of the sunlight is not absorbed)
Instead, the blue light is reflected and can pass into our eye

8. Pigments and light absorption
The 1st stage in photosynthesis is absorption of sunlight
Visible light is a form of electromagnetic radiation
Sunlight is a mixture of different wavelengths of visible light
Which we see at different colors
We call this the visible portion of the spectrum
Blue, green, and red

9. The electromagnetic spectrum

10. Absorption spectrum of several plant pigments

11. Visible light spectrum
Substances can do one of two things when they are struck by a particular wavelength (color) of light:
Absorb that wavelength (energy is being absorbed/used)
Reflect that wavelength (energy is not being absorbed/used; you will see that color)

12. Let’s Check In – Turn and Talk
You are walking outside with a friend who is wearing a red and white shirt. Explain why the shirt appears red and white
Sunlight is mixture of visible light
When sunlight hits the red pigments in the shirt, the blue and green wavelengths of light are absorbed
The red wavelengths are reflected, thus, our eyes see red
White  all wavelengths of light are reflected; eyes and brain interpret this mixture as white

13. How does this apply to photosynthesis?
Chlorophyll is a green pigment
Chlorophyll reflects green light and must absorb other wavelength of visible light
When a plant is hit by sunlight, the red and blue wavelengths of light are absorbed by chlorophyll and used for photosynthesis
Almost all of the energy of the green wavelengths is reflected, not absorbed
Do not try to grow plants in only green light

14. Fun Fact
Chlorophyll is not the only pigment type found in green plants
Most green plants also contain other pigments in smaller quantities that reflect colors like red and yellow
We see this in foliage – turning of the leaves in the fall
End of growing season,
Preparation for cold weather
These colors were always there in the leaf, but were hidden by the dominant color of chlorophyll until late in the season

15. Stages of photosynthesis
Photosynthesis produces sugar molecules as food source for the plant
Sugars (glucose) are held together by covalent bonds
Requires energy to create those covalent bonds
Sun is the source of that energy

16. Two stages of photosynthesis
1st stage:
Set of reactions that “trap” light energy and convert it to ATP (chemical energy)
2nd stage:
Set of reactions in which ATP is used to help bond carbon dioxide and water molecules together to create a sugar (glucose)

17. 1st stage – a deeper look Called light-dependent reactions
Chlorophyll (and other pigments) absorb light energy and convert to ATP
Light energy is also used to accomplish a reaction called photolysis of water
Water molecule is split into hydrogen and oxygen
The oxygen that is split away is released from the plant (waste product)
Good for us & millions of other organisms that need oxygen for aerobic respiration
Useful products for the plant formed during this stage are ATP and hydrogen

18. 2nd stage – a deeper look Called light independent reactions
ATP and hydrogen are used as forms of chemical energy to convert carbon dioxide and water into useful organic molecules for the plant
Remember? Carbon dioxide is one of the few inorganic molecules that contain carbon
Glucose (6C) is organic
Product of photosynthesis

19. 2nd stage – a deeper look (cont.)
Equation:
6CO2 + 6H2O  C6H12O6 + 6O2
Requires 6 molecules of carbon dioxide to form 1 glucose molecule
This conversion of an inorganic form of an element to an organic form is known as fixation

20. Fixation
Therefore, photosynthesis can be described as a series of reactions in which carbon dioxide and water are fixed into glucose, and oxygen is produced as a by- product
This fixation reaction requires energy
The energy to create the glucose comes directly from the ATP and hydrogen created in the 1st stage

21. Important to Note Energy traced back to sunlight
Glucose is one of many possible organic molecules that can form from photosynthesis

22. Measuring the rate of photosynthesis
Revisit equation:
6CO2 + 6H2O  C6H12O6 + 6O2
How does this equation compare to what you know about cellular respiration?
Products and reactants are reversed

23. Measuring the rate (cont.)
At any given time of year, a plant has a fairly consistent rate of cell respiration
Consistent night and day AND relatively low
Plants do not have muscle and other ATP demanding tissues like animals
They need ATP for various biochemical processes, but not as much

24. Measuring the rate (cont.)
The same consistency is not true for the rate of photosynthesis
Rate of photosynthesis depends on environmental factors
Intensity of light, temperature

25. Measuring the rate (cont.)
Daytime, especially on a warm, sunny day = high rate of photosynthesis
Rate of CO2 taken in and O2 released will both be very high
The plant is also doing cell respiration
Correction should be made for both CO2 and O2 levels
At night, the rate of photosynthesis may drop to zero
Giving off CO2 and taking in O2 just to maintain relatively low rate of cell respiration

26. Measuring the rate (cont.)
Measuring the rate of oxygen production or carbon dioxide intake is considered a direct measurement
As long as correction is made for cell respiration
Another common approach:
Keeping track of the change in biomass of experimental plants
Mass of plants is considered to be an indirect measurement
Increase or decrease in biomass may be traced to a whole variety of factors besides photosynthetic rate.
See Figure 3.26 in your text

27. Factors that affect rate of photosynthesis
Light intensity
Carbon dioxide concentration
Temperature
See page 87

28. Photosynthesis – the effect of light intensity
At low light intensities the rate of photosynthesis increases as light intensity is increased
But at high light intensities, further increases in light intensity have not effect
Several essential steps in photosynthesis are achieved using the light energy
ATP is produced (from ADP + phosphate)
Water molecules are split to release hydrogen
Needed later to make carbohydrate

29. Photosynthesis – the effect of CO2 concentration
Although CO2 concentration has been increasing over the past 100 years, it is still low enough to affect the rate of photosynthesis
At low CO2 levels, increases in concentration cause the rate of photosynthesis to rise
At high CO2 levels, further increases in concentration do not affect the rate of photosynthesis

30. Photosynthesis – the effects of temperature
At low temps, the rate of photosynthesis is very low or zero
As temp increases the rate increases until it reaches its maximum (optimum)
Above this temperature, the rate decreases and eventually becomes zero
Like enzymes, right?
Not coincidental; fixation of carbon dioxide it catalyzed by enzymes
In most plants the optimum temperature is between 25-35*C

31. Recap
Photosynthetic organisms produce organic molecules (glucose) to begin food chain
Photosynthetic pigments (chlorophyll) are used to absorb light energy to being the process
Most plants are green because chlorophyll reflect the green wavelengths of light and absorbs the red and blue wavelengths

32. Recap (cont.)
Photosynthesis occurs in 2 stages: light dependent and light independent
Light dependent
Produce ATP and hydrogen; gives off O2
Light independent
“fix” carbon dioxide and water into organic molecules (such as glucose) using ATP and hydrogen molecules produced during light dependent reactions

33. Recap (cont.)
The rate of photosynthesis can be measured directly by measuring either the production of oxygen or uptake of carbon dioxide
The rate of photosynthesis can be measured indirectly by measuring the biomass increase of a plant
Temperature, light intensity, and carbon dioxide can affect rate of photosynthesis