topic 6.2 photosynthesis reactions -...

Topic 6.2 Photosynthesis ReactionsHow is energy from light converted to chemical energy?

Metabolism relies on High-energy Molecules!

Glucose is produced to store energy for later use by the organism.

The energy-rich molecules ATP and NADPH are a form of energy readily usable by the organism to drive metabolic and other cellular reactions.

ATP: Adenosine Triphosphate

Provides energy required by living organisms for carrying out chemical reactions and driving transport mechanisms across membranes

The bond holding a 3rd phosphate molecule is high-energy.

Energy released

Energy required

NADP+: Nicotinamide Dinucleotide Phosphate

H+, 2e-

Red

uctio

n

NADPH is an electron-carrier (the high-energy form), similar to ATP

NADP+ is an electron-acceptor molecule (low-energy form), similar to ADP

Oxidation

“OIL RIG”

Oxidation Is Loss of e-Reduction Is Gain of e-

Energy released

Energy required

PhotosynthesisDivided into 2 main sets of reactions:

1. Light-dependant reactions (light reactions)2. Light-independent reactions (dark reactions)

Light-Dependent Reactions

Now relax!

We will go through it step by step!

Step 1: Light is converted into chemical energy

Photosystem II and Photosystem I are chlorophyll molecules in the thylakoid membrane that absorb light energy (photons)

The absorbed photon excites an electron to a higher level

(light energy is converted to chemical energy!)


Water 2 Hydrogen + 2 electrons + ½ Oxygen

H2O 2H+ + 2e- + ½ O2

The excited electron is then passed onto the Electron Transport Chain (ETC)

Electrons are replaced by photolysis (water is broken down by light)

Light

Light


Step 2: Electron Transport Chain

Electron Transport Chain (ETC): a series of progressively stronger electron acceptors where each time the electron is passed on energy is released

In photosynthesis, the ETC generates ATP and NADPH which are used in the light-independent reactions


ATP Generation by Chemiosmosis

1. ETC transports H+ ions from stroma into thylakoid lumen 2. H+ ions accumulate in lumen creating a positive (electrical)

AND chemical concentration gradient3. The gradient facilitates passive transport of H+ ions through

ATP synthase to the stroma4. ATP synthase uses the energy produced by flow of H+ ions

to generate ATP from ADP and P molecules. 5. The ATP molecule is then released in the stroma and used for

the light-independent reactions


H+ ions transported into lumen

H+ concentration gradient formed

H+ ions flow into stroma, powering ATP synthase

ATP is formed from ADP and P


NADPH Generation

1. Photosystem I passes 2 excited electrons to ETC2. NADP+ accepts the 2 electrons and a Hydrogen ion to form

NADPH3. The NADPH molecule is released in the stroma and used for

the light-independent reactions

Step 2: Electron Transport Chain Photosystem I passes excited electrons to ETC

NADP+ accepts 2 electrons and H+, forming NADPH

Note: Light-Independent reactions do not REQUIRE light to proceed.

Although sometimes called ‘dark reactions’ they occur in the light/day as well as in the dark/night!

PhotosynthesisDivided into 2 main sets of reactions:

1. Light-dependant reactions (light reactions)2. Light-independent reactions (dark reactions)

FYI: Named for Melvin Calvin who won Nobel Prize in 1961 for his work on discovering steps of the pathway.

Light-Independent Reactions:The Calvin Cycle

You do NOT need to know all these steps!!! FYI only

The calvin cycle is the LAST stage of photosynthesis.

It performs a process called ‘Carbon Fixation”, turning carbon dioxide into G3P

G3P is a carbon-based molecule used to form glucose (and other sugars) useable by the organism

Light-Independent Reactions: Calvin Cycle

Glucose

other Carbohydrates(sucrose, cellulose, starch…)

Storage

Cellular Respiration

other organic compounds

G3PCO2Calvin Cycle

(in stroma)

Light-Independent Reactions: Calvin Cycle

Each cycle requires 3 ATP, 2 NADPH and 1 CO2 molecule

- ATP and NADPH are supplied by light-dependent reactions- CO2 diffuses into the cell from the surrounding air

Each cycle produces 3 ADP, 3P and 3NADP+ and 1 G3P molecule

- ADP, P and NADP+ are returned the light-dependent reactions

It takes 6 ‘rounds’ of the calvin cycle to produce enough G3P for 1 glucose

So, 18 ATP, 12 NADPH and 6 CO2 are required by Calvin cycle to produce 1 glucose molecule

Coming up next...

Monday: we will finish chapter 6 - rate of photosynthesis and applications of photosynthesis

Tuesday: Review class

- if you have any suggestions/requests let me know!

Wednesday: Chapter 6 Quiz - photosynthesis

We will start cellular respiration (Chapter 7) after quiz

Have a great weekend!

http://www.youtube.com/watch?v=iLDbW_XvxHQ

Photosynthesis

Think about it…What reaction uses/produces each reactant and product? Where does this reaction occur? What is each step of the reaction? What is the purpose of the reaction?

Looking back to leaf structure...

“Designed” to allow flow of gas molecules

Stomata and air spaces allow gas exchange.

The veins allow nutrient/chemical exchange.

Flow is facilitated by diffusion.

Why do you think the

stomata are mainly

located on the lower surface?

Rate of Photosynthesis - Limiting Factors

1. Light - rate increases as intensity of light increases

Eventually the rate is limited “plateaus”

http://www.rsc.org/learn-chemistry/content/filerepository/CMP/00/001/068/Rate%20of%20photosynthesis%20limiting%20factors.pdf



Rate of Photosynthesis - Limiting Factors

2. CO2 - as concentration of CO2 increases there is a rapid increase in rate of photosynthesis as plant incorporates the available CO2

Eventually the rate is limited “plateaus”




Rate of Photosynthesis - Limiting Factors3. Temperature - The Calvin cycle relies on many enzymes to be carried out. As the optimal Temp required by the enzymes is reached the overall rate of photosynthesis increases

But, as Temp continues to increase the enzymes are damaged and photosynthesis stops




Applications of Photosynthesis - Agriculture

Higher levels of fertilizer linked to higher rates of photosynthesis and bigger plants

Applications of Photosynthesis - Environment

Toxic algal blooms and eutrophication in lakes is linked to increased rate of photosynthesis. Why?

Applications of Photosynthesis - Technology

Current research into using blue-green algae (cyanobacteria) as a source of biofuel (sustainable energy-source).

Coming up next...

Tuesday: Review class

- if you have any suggestions/requests let me know!

Wednesday: Chapter 6 Quiz on photosynthesis

We will start cellular respiration (Chapter 7) after quiz so bring your binders/notes

topic 6.2 photosynthesis reactions -...

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