1. Photosynthesis
Topic 3.8 and 8.2

2. Autotrophs
Plants are autotrophs (meaning “self-feeders” in Greek) in that they make their own food and thus sustain themselves without eating other organisms or even organic molecules.
Chloroplasts of plant cells capture light energy that has traveled 150 million kilometers from the sun and convert it to chemical energy that is stored in sugar and other organic molecules.

3. Producers
Plants, algae, some prokaryotes make their own organic molecules and are the ultimate source of organic molecules for almost all other organisms.
Often referred to as the producers of the biosphere because they produce its food supply
All organisms that produce organic molecules from inorganic molecules using the energy of light are called photoautotrophs.

4. Chloroplasts
All green parts of a plant have chloroplasts in their cells and can carry out photosynthesis.
In most plants, leaves have the most chloroplasts and are the major sites of photosynthesis.
Chloroplasts are concentrated in the cells of the mesophyll, the green tissue in the interior of the leaf.
Each mesophyll has numerous chloroplasts
Carbon dioxide enters the leaf and oxygen exits via tiny pores called stomata.
Water absorbed by the roots is delivered to the leaves in veins.

5. Chloroplasts
Membranes in the chloroplast form the framework where many of the reactions of photosynthesis occur, just as mitochondrial membranes do for cell respiration.
Similar to mitochondria, chloroplast has an outer membrane and an inner membrane, with an intermembrane space between them.
Inner membrane is filled with a thick fluid called stroma
Stroma is where sugars are made from carbon dioxide and water
Within stroma is a system of interconnected membranous sacs called thylakoids
Enclose a third compartment called the thylakoid space
Built into thylakoid membranes are the chlorophyll molecules that capture light energy.
Membranes also house much of the machinery that converts light energy to chemical energy.
In some places, thylakoids are concentrated in stacks called grana.

6. Chloroplast

7. Photosynthesis is a redox
6CO2 + 6H2O  C6H12O6 + 6O2
When water molecules are split apart, yielding O2, they are actually oxidized; that is, they lose electrons along with hydrogen ions
Meanwhile, CO2 is reduced to sugar as electrons and hydrogen ions are added to it.
Overall, cell respiration harvest energy stored in a glucose molecule by:
oxidizing the sugar and reducing O2 to H2O, involving a number of energy-releasing redox reactions,
with electrons losing potential energy as they travel down an energy hill from sugar to O2.
Along the way, the mitochondria uses some of the energy to synthesize ATP.
In contrast, photosynthesis redox reactions involve an uphill climb.
As water is oxidized and CO2 is reduced, electrons gain energy by being boosted up an energy hill.
Light energy captured by chlorophyll molecules in the chloroplast provides the boost for the electrons.
Photosynthesis converts light energy to chemical energy and stores it in sugar molecules.

8. Photosynthesis: Overview
Photo, from the Greek word for light, refers to the first stage.
Synthesis, meaning “putting together” refers to the sugar construction in the second stage

9. Photosynthesis Overview
Occurs in two stages:
Light reactions
Include the steps that convert light energy to chemical energy stored in ATP and NADPH and produce O2 gas as a waste product.
Occur in thylakoid membranes
Light energy absorbed by chlorophyll is used to make ATP from ADP and phophate.
Also used to drive a transfer of electrons from water to NADP+, an electron carrier similar to NAD+ that carries electrons in cellular respiration.
NADP+ gets reduced to NADPH via enzymes by adding a pair of light-excited electrons along with an H+
Reaction temporarily stores energized electrons which originally came form water that is split and O2 is released.

10. Photosynthesis Overview
Dark reactions, or Calvin Cycle
Occurs in the stroma
Does not require light directly
Cyclic series of reactions that assembles sugar molecules using CO2 and the energy-containing products (NADPH and ATP) of the light reactions.
Incorporation of carbon from CO2 into organic compounds is called carbon fixation.
After carbon fixation, enzymes of the cycle make sugars by further reducing the carbon compounds.

11. Light Reactions: Converting Solar Energy to Chemical Energy
Electromagnetic energy
type of energy that is sunlight
Travels in space as rhythmic waves analagous to those made by a pebble dropped in a puddle of water
Distance between the crests of two adjacent waves is called a wavelength.
In the electromagnetic spectrum, shorter wavelengths have more energy than longer ones.
Visible light- the radiation your eyes can see as different colors, consists of wavelengths from about 380 nm to 750 nm

12. Light Reactions: Converting Solar Energy to Chemical Energy
Figure 7.6B shows what happens to visible light in the chloroplast.
Light absorbing molecules called pigments, built into the thylakoid membranes, absorb some wavelengths of light and reflect or transmit other wavelengths.
We do not see the absorbed wavelengths; their energy has been absorbed by pigment molecules
We see green wavelengths when we look at plants that the pigments transmit and reflect.

13. Light Reactions: Converting Solar Energy to Chemical Energy
Pigments of chloroplast:
Chlorophyll a
Absorbs mainly blue-violet and red light
Participates directly in the light reactions
Looks grass-green because it reflects mainly green light
Chlorophyll b
Absorbs mainly blue and orange light and reflects (appears) yellow-green.
Broadens the range of light that a plant can use by conveying absorbed energy to chlorophyll a, which then puts the energy to work in the light reactions
Carotenoids
Absorb mainly blue-green light and reflects yellow-orange
Some may pass energy to chlorophyll a, as chlorophyll b does
Have a protective function: absorb and dissipate excessive light energy that would other-wise damange chlorophyll

14. Light Reactions: Converting Solar Energy to Chemical Energy
The theory of light as waves explains most of light’s properties.
However, light also behaves as discrete packets of energy called photons:
A fixed quantity of light energy, and the shorter the wavelength, the greater the energy.
Each type of pigment absorbs certain wavelengths of light because it is able to absorb the specific amounts of energy in those photons.

15. Light Reactions: Converting Solar Energy to Chemical Energy
Photosystems
Clusters of chlorophyll molecules along with other pigments and proteins in the thylakoid membrane
Consists of a number of light-harvesting complexes surrounding a reaction center.
Have chlorophyll a, chlorophyll b, and carotenoid pigments that function collectively as a light-gathering antenna.
Pigments absorb photons and pass the energy from molecule to molecule until it reaches the reaction center.

16. Light Reactions: Converting Solar Energy to Chemical Energy
Photosystems:
Reaction center
A protein complex that contains a chlorophyll a molecule and a molecule called the primary electron acceptor:
Captures a light-excited electron from the reaction-center chlorophyll molecule and passes it to an electron transport chain

17. Light Reactions: Converting Solar Energy to Chemical Energy
Photosystems:
Two types: Photosystem I and Photosytem II:
Photosystem I:
Occurs second in light reactions
Reaction center is called P700 because the wavelength of light it absorbs best is 700 nm
Photosystem II:
Occurs first in light reactions
Chlorophyll a molecule in reaction center is called P680 because the light it absorbs best is red light with a wavelength of 680nm

18. Light Reactions: Converting Solar Energy to Chemical Energy

19. Light Reactions: Converting Solar Energy to Chemical Energy
Light energy is transformed into the chemical energy of ATP and NADPH
In this process, electrons removed from water molecules pass from photosystem II to photosystem I to NADP+
Between the two photosystems, the electrons move down an electron transport chain and provide energy for ATP production.

20. Light Reactions: Converting Solar Energy to Chemical Energy
Flow of electrons in light reactions (Figure 7.8A):
A pigment molecule in a light-harvesting complex absorbs a photon of light. The energy is passed to other pigment molecules and finally to the reaction center of Photosystem II, where it excites an electron of chlorophyll P680 to a higher energy level.
The electron is captured by the primary electron acceptor.
Water is split, and its electrons are supplied one by one to P680, replacign those lost to the primary electron acceptor. The oxygen atom compbines with an oxygen from another split water molecule to form a molecule of O2.

21. Light Reactions: Converting Solar Energy to Chemical Energy
Flow of electrons in light reactions (Figure 7.8A):
4. each photoexcited electron passes from photosystem II to photosystem I via an electron transport chain. The exergonic “fall” of electrons provides energy for the synthesis of ATP.
5. Meanwhile, light energy excites an electron of chlorophyll P700 in the reaction center of photosystem I. The primary electron acceptor captures the excited electron and an electron from the bottom of the electron transport chain replaces the lost electron in P700.
6. The excited electrons of photosystem I is passed through a short electron transport chain to NADP+, reducing it to NADPH

22. Light Reactions: Converting Solar Energy to Chemical Energy
Chemiosmosis
Drives ATP synthesis using the potential energy of a concentration gradient of hydrogen ions across a membrane
Gradient is created when an electron transport chain pumps hydrogen ions across a membrane as it passes electrons down the chain.

23. Light Reactions: Converting Solar Energy to Chemical Energy
Chemiosmosis (ctd)
Relationship between chloroplast structure and function in light reactions:
The two photosystems and e.t.c. are all located in the thylakoid membrane of a chloroplast.
As photoexcited electrons are passed down the e.t.c. connecting the two photosystems, H+ are pumped across the membrane from the stroma into the thylakoid space. This generates a concentration gradient across the membrane.

24. Light Reactions: Converting Solar Energy to Chemical Energy
Chemiosmosis (ctd)
Similar ATP synthase complex in mitochondria
Energy of concentration gradient drives H+ back across the membrane through ATP synthase
ATP synthase couples the flow of H+ to the phosphorylation of ADP: called photophosphorylation

25. Light Reactions: Converting Solar Energy to Chemical Energy
Chemiosmosis (ctd)
In photosynthesis, light energy is used to drive electrons to the top of the transport chain (whereas, cell respiration, high-energy electrons pass down the e.t.c. coming from oxidation of food molecules)
Chloroplasts transform light energy into the chemical energy of ATP (whereas, mitochondria transfer chemical energy from food to ATP)
In photosynthesis, the final electron acceptor is NADP+ (whereas, in cell respiration, O2 is)
In photosynthesis, electrons are stored in at a high state of potential energy in NADPH (whereas, in cell respiration, they are at a low energy level in H20)

26. Light Reactions: Converting Solar Energy to Chemical Energy

27. Dark Reaction (Calvin Cycle) Converting CO2 to sugars
Figure 7.10A: Overview of Calvin Cycle
CO2 (from air), energy from ATP and high energy electrons from NADPH (both generated by light reactions) , the Calvin Cycle constructs an energy-rich, three-carbon sugar, glyceraldehyde-3-phosphate (G3P).
A plant cell uses G3P to make glucose and other organic molecules as needed.

28. Dark Reaction (Calvin Cycle) Converting CO2 to sugars
Figure 7.10B: Details of the Calvin Cycle
Carbon fixation: the enzyme rubisco attaches CO2 to RuBP (5-C). The unstable 6-C product splits into two molecules called 3-PGA.
For three CO2, six 3-PGA result
Reduction: NADPH reduces the organic acid six 3-PGA to six molecules G3P with the assistance of ATP

29. Dark Reaction (Calvin Cycle) Converting CO2 to sugars
3. Release of one molecule of G3P:
Five G3Ps remain in the cycle, and one G3P will leave. Plant cells use two G3P molecules to make one molecule of glucose.
4. Regeneration of RuBP
energy from ATP drives a series of chemical reactions to rearrange the atoms in the five G3P molecules to form three RuBP molecules. These can start another turn of the cycle.

30. Dark Reaction (Calvin Cycle) Converting CO2 to sugars

31. Absorption spectrum
As light meets matter, it may be reflected, transmitted, or absorbed.
Pigments
Substances that absorb visible light
Different pigments absorb light of different wavelengths
Chlorophylls absorb red and blue-violet light and appear green
Carotenoids absorb blue-violet and appear orange, yellow, or red
Measured with a spectrophotometer.

32. Absorption spectrum Absorption spectrum
Light absorption vs. the wavelength
Absorption spectrum of different photosynthetic pigments provides clues to their role in photosynthesis, since light can only perform work if it is absorbed.
Accessory pigments (chlorophyll b and carotenoids) absorb wavelengths of light that chlorophyll a cannot, pass the energy to chlorophyll a, broadening the spectrum that can effectively drive photosynthesis.

33. Absorption Spectrum Action spectrum
Profiles the effectiveness of different wavelength light in fueling photosynthesis.
It is obtained by plotting wavelength against some measure of photosynthetic rate.

34. Action Spectrum

35. Abiotic factors impact on photosynthetic rate
Photosynthetic rate is depended on environmental factors:
Amount of light available
Level of carbon dioxide
temperature

36. Light intensity
Up to a certain intensity, photosynthesis increases as more light is available to the chlorophyll.
When all the chlorophyll molecules are activated (saturated) by the light, more light has no further effect.

37. Light Intensity

38. Temperature
Increased temperature increases photosynthetic rate until an optimal temperature is reached.
Above the optimal temperature, enzymes cannot function properly and photosynthesis will decrease.

39. Temperature

40. Carbon Dioxide levels
Increased carbon dioxide levels increases photosynthesis, unless limited by another factor, then levels off.

41. Carbon Dioxide

42. Measuring photosynthesis
Production of oxygen or uptake of carbon dioxide
Increase in biomass