Chapter 6 – Photosynthesis Can we directly utilize the energy of the sun? What is the energy rich molecule that our cells use? ATP What do we use as energy molecules prior to the production of ATP? Carbohydrates, Lipids, Proteins Where do we get these molecules from? Plant & animal sources How do plants acquire their energy? SUN Photosynthesis is nothing more than the conversion of one form of energy (light) to another (chemical in the form of carbohydrates)
Chapter 6 – Photosynthesis How is light energy quantified? Light energy is a form of electromagnetic radiation Electromagnetic radiation can be described in terms of a stream of mass-less particles, each traveling in a wave-like pattern and moving at the speed of light. Each mass-less particle contains a certain amount (or bundle) of energy. Each bundle of energy is called a photon, and all electromagnetic radiation consists of these photons. The only difference between the various types of electromagnetic radiation is the amount of energy found in the photons. Radio waves have photons with low energies, microwaves have a little more energy than radio waves, infrared has still more, then visible, ultraviolet, X-rays, and... the most energetic of all... the gamma-rays WHY?
Chapter 6 – Photosynthesis
Chapter 6 – Photosynthesis
Chapter 6 – Photosynthesis Why do the different forms of electromagnetic radiation have different amounts of energy associated with them? Wavelength is the key. Wavelength is the distance between adjacent peaks in a series of periodic waves Think about the waves at the beach. They are hitting you at waist level, but their frequency or wavelength is long. You absorb the energy, rock back on your heels, & come back to the standing position.
Wavelength
Chapter 6 – Photosynthesis Same scenario. Think about the waves at the beach. They are hitting you at waist level, but their frequency or wavelength is much shorter. You absorb the energy, rock back on your heels, & then you are hit by another wave. You rock back again, & then you are hit by another wave, until eventually you are knocked over Shorter the wavelength, the greater the energy of the electromagnetic radiation! It’s a matter of how many particles are hitting the target per unit time.
Chapter 6 – Photosynthesis Visible light is the electromagnetic radiation that falls between the wavelengths of 390nm –760nm. nm = nanometers = 10
-9 m
Each wavelength of light is associated with a particular color & these colors can be separated & observed when light is passed through a prism 390 nm = violets
550 nm = greens
750 nm = reds
Which color light has the most energy? Which the least? How do we perceive color & how is it related to the wavelength of light? Why is a plant leaf green? Perceived light is reflected light. When we perceive a plant leaf as green, it just means that green light is being reflected by the leaf & not absorbed KEY POINT = Different molecules absorb different wavelengths of light Think about the spectrophotometer & the methylene blue experiment! What color light was minimally & maximally absorbed?
Chapter 6 – Photosynthesis What happens when a photon is absorbed by a molecule? The energy of the photon is used to cause a change in the molecule that has absorbed it. Typically, electrons get pushed to a higher energy orbital & then fall back, releasing energy as heat or as another photon (fluoresence) Think of a rock getting thrust up a hill after being hit & then falling back down. What are the light absorbing molecules found in plants that drive photosynthesis? Chlorophylls & carotenoids Based on the absorption spectrum seen on the next page, what color is chlorophyll? If I told you that the carotenoids are yellow/orange in color, what color light do you think they absorb?
Chapter 6 – Photosynthesis
Chapter 6 – Photosynthesis
Chapter 6 – Photosynthesis
Chapter 6 – Photosynthesis Where are these pigments found? Structure & function of chloroplasts 1.
Double membrane organelle
2.
Inner membrane surrounds a central space called the stroma A. Site for the fixation of carbon dioxide into carbohydrates
3.
Thylakoids are flattened membrane bound discs found within the stroma A. Organized into stacks called grana B. The thylakoid membrane contains the light absorbing pigments & surrounds the thylakoid space C. The thylakoid membrane contains the necessary machinery to convert the light energy into chemical energy (ATP, NADPH)
Chapter 6 – Photosynthesis
Chapter 6 – Photosynthesis What or how do these molecules convert the energy in the photons to chemical energy? PHOTOSYNTHESIS
6 CO2 + 6 H2O + light energy
C6H12O6 + 6 O2
Photosynthesis can be broken down into 2 steps: 1.
Light dependent reactions – conversion of light energy to chemical energy
2.
Light independent reactions – fixation of carbon dioxide into carbohydrates, utilizing the energy created in the light dependent reactions
Chapter 6 – Photosynthesis
Chapter 6 – Photosynthesis Light dependent reactions 1.
Require the participation of 2 light gathering units called photosystem I & photosystem II
2.
Each photosystem is composed of tightly packed pigments which serve as antenna to gather light energy
3.
Once absorbed the energy is passed from one pigment to the next until it is concentrated into one chlorophyll a molecule, the reaction center chlorophyll a
4.
The energy that was absorbed by this chlorophyll is sufficient to excite electrons off it
5.
The moving electrons (kinetic energy) are used to create ATP
How are these electrons used to create energy?
Chapter 6 – Photosynthesis Arnold’s diagram of the electron transport system & the production of ATP
Chapter 6 – Photosynthesis
Chapter 6 – Photosynthesis Light dependent reactions Cyclic electron pathway 1.
Once the electrons are excited, they pass through the electron carriers
2.
As they pass through the carriers, hydrogen ions are pumped from the stroma into the thylakoid space
3.
The electrons eventually return to the reaction center - cyclic
4.
The hydrogen ions, which are now concentrated in the thylakoid space, will diffuse down their concentration gradient back into the stroma through a molecule called ATP synthase (a combination of a membrane channel & an enzyme bounded to a membrane)
5.
ATP synthase harnesses the energy of these flowing hydrogens to produce ATP from ADP & Pi
Just like water flowing through a hydroelectric dam!
Chapter 6 – Photosynthesis
Chapter 6 – Photosynthesis Light dependent reactions Non cyclic electron pathway 1.
Starts with photosystem II absorbing light & “kicking off” electrons
2.
These electrons flow through the electron transport system ultimately creating ATP
3.
These electrons flow to photosystem I replacing electrons that have been “excited off” by the absorption of light
4.
These electrons are used to reduce NADP+ (nicotinamide adenine dinucleotide phosphate) to NADPH – NADPH is necessary to the production of carbohydrates in the light-independent reactions
How does photosystem II replace the electrons it lost? Water is broken down, liberating hydrogen ions, oxygen, & electrons.
6 CO2 + 6 H2O + light energy
C6H12O6 + 6 O2
Chapter 6 – Photosynthesis
Chapter 6 – Photosynthesis What happens following the conversion of light energy to ATP & NADPH? Light independent reactions 1.
Occurs in the stroma
2. CO2 is fixed into PGAL (glyceraldehyde-3-phosphate), a 3-carbon sugar via the Calvin Cycle 3.
PGAL is a precursor to your common 6 carbon sugars (glucose)
Calvin Cycle 1.
Begins with the fixation of CO2 to ribulose bisphosphate (5 carbon sugar) with the aid of the enzyme rubisco (ribulose bisphosphate carboxylase)
2.
The resulting 6 carbon sugar is split into 2 3-phosphoglycerate molecules (3 carbon sugars)
3.
They are reduced & converted to PGAL via the utilization of ATP & NADPH
4.
Some of the PGAL is used to make glucose, the rest to regenerate ribulose bisphosphate so that the cycle can fix more CO2 again
Chapter 6 – Photosynthesis
Chapter 6 – Photosynthesis
Chapter 6 – Photosynthesis Different forms of photosynthesis Are all photosynthetic pathways the same?
C3 vs C4 vs CAM C3 photosynthesis 1.
We’ve already covered it.
Does it have any limitations? Where does the CO2 come from to ultimately make the glucose? From the atmosphere How does the CO2 enter the leaf? Via stomata During hot weather would a plant want to have its stomata open continuously? NO! It would dry out & thus it conserves water by keeping the stomata closed But is this a problem?
Chapter 6 – Photosynthesis
C3 photosynthesis (continued) As CO2 is depleted, oxygen levels are increasing Rubisco not only interacts with CO2, but also O2 When O2 binds to rubisco, photorespiration is initiated where ribulose bisphosphate is broken down ultimately into CO2 The only problem is NO ATP or any other useful molecule is being produced Thus the energy used to create sugars & regenerate ribulose bisphosphate is wasted to create CO2 again
Chapter 6 – Photosynthesis Adaptations to prevent photorespiration
C4 photosynthesis Utilizes an enzyme (phosphoenolpyruvate (PEP) carboxylase) that can fix CO2 even when CO2 is low, & not fix O2 at all (useful when the stomata are closed) This enzyme fixes CO2 into the molecule oxaloacetate, which then transported to cells containing the Calvin cycle enzymes Oxaloacetate is broken down, liberating CO2, thus fueling the Calvin cycle even when the stomata are closed Thus high levels if CO2 are maintained for the Calvin cycle to function properly
Chapter 6 – Photosynthesis Adaptations to prevent photorespiration CAM photosynthesis Utilizes the same 4 carbon oxaloacetate CO2 donating molecule The difference is that CAM plants maximize the production of oxaloacetate at night when their stomata are open They store the oxaloacetate in vacuoles until the next day
Chapter 6 – Photosynthesis
Chapter 6 – Photosynthesis
Chapter 6 – Photosynthesis
Chapter 6 – Photosynthesis PRACTICE QUESTIONS 1.
Define photon, wavelength.
2.
What is the relationship between wavelength & energy?
3.
What happens to a molecule when it absorbs the energy of a photon?
4.
What are the light absorbing pigments in plants? Where are they specifically located?
5.
What are the functions of the light dependent & light independent reactions?
6.
How are ATP & NADPH synthesized in the light dependent reactions?
7.
How is oxygen produced during photosynthesis?
8.
How is carbon dioxide fixed during photosynthesis?
9.
What enzyme initiates the fixation of carbon dioxide?
10. What are the differences between C3, C4, & CAM plants with regard to fixing carbon dioxide?
