Photosynthesis

Why is Photosynthesis Important?? Provides Oxygen to almost all organisms

need to break down food into energy(cell respiration-remember this for chapter 7)

Review of Plant Cell Structure

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Photosynthesis Converts radiant energy into chemical

energy stored in the bonds of organic compounds

Stored in plants as STARCH Done by Autotrophs Done in the Chloroplasts of the plant cell

I. The Chemistry of Photosynthesis The chemical reactions of

photosynthesis and respiration are opposites

Photosynthesis CO2 + H2O+Energy C6H12O6 + O2

Respiration C6H12O6 + O2 CO2 + H2O + Energy

A. Energy Intermediates

ATP-ADP

NADPH- NADP+ (+) H+

NADH- NAD+ (+) H+

FADH2- FAD+ (+) 2H+

ATP: Adenosine Triphosphate

Three Pieces:

Adenine

Ribose

Phosphate group

The adenine and ribose combine to make Adenosine

The energy obtained from ATP comes from the breaking apart of phosphate groups

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Phosphorylation

B. The ATP-ADP Cycle When the cell needs energy from a

specific activity…

1. The outermost phosphate of ATP is broken off by the enzyme ATPase

2. This phosphate group is transferred to another molecule in the process called PHOSPHORYLATION: when a phosphate group is added to a molecule.

3. The ADP then undergoes phosphorylation with the help of ATPsynthase to form a new ATP

Turning ADP into ADP Done with the aid of a proton pump

Movement of Hydrogen Ions across the membrane …. provides the energy needed to add a

phosphate group to the ADP, creating ATP

This is done by active transport

Characteristics of Light Pic

Humans can see light between the wavelengths 400nm and 700nm

Other organisms can see light at different wavelengths than we can!

Plants absorb light the best when the light is at a wavelength of around 450 or around 650nm

Plants do not absorb light between 500 and 600nm very well.

So, why are plants green??

Where does photosynthesis happen? Inside Chloroplasts

Specifically within the GRANA

Stacks of flattened sacs

Each stack is made of single sacs called Thylakoids.

Side Note: Single Stack is called Granum

STRUCTURES

Thylakoids Site of light absorption, where chlorophyll is

found Membrane of double lipid layers Embedded with proteins like ATPsynthaseIt is very important to keep track of where things happen in photosynthesis Some things happen between the two thylakoid

membranes, in a space calleds the thylakoid space

Some things happen on the membranes of the thylakoid, and these products are released into the Stroma: Protein rich solution around the grana

II. Stages of Photosynthesis Divided into 3 sets of reactions:

1. Light absorbtion by chlorophyll 2. Light dependent reactions

A. electron transfer B. Chemiosmosis

3. Light independent reaction

1. Light absorbtion by chlorophyll Radiant energy from the sun is absorbed

by pigments Chlorophylls

Chlorophyll a- most like absorbtion

Chlorophyll b- some light absorbtion

In the Chloroplasts Accessory Pigments:

Carotenoids- absorb some green and reflect yellow/orange They and chlorophyll b send absorbed light

energy to chlorophyll a

2. Light Dependent Reaction Light ReactionsConvert the light energy to chemical energy2 Main Stages:

A. Electron Transport- Converts the suns energy to electrical energy

B. Chemiosmosis- Converts the electrical energy to chemical bond energy

A. Electron TransferSunlight strikes the chlorophyll molecules and “excites” (e-)Photosystems: are units of several hundred chlorophyll molecules and carrier molecules.The photosystems are found in the: Thylakoid Membrane

The reaction starts in Photosystem II

Sunlight strikes chlorophyll in photosystem II, and excites the e-

e- move down chain of Electron Carriers(ETC)

2. e- move from high energy to a lower energy level found in photosystem I

** Giving off energy

3. At the same time… Light strikes photosystem I E- are excited and move along a transport

chain E- leaving photosystem I are replaced by

those from photosystem II

4. The energy is converted to energy intermediates:

A. NADPH is formed when the electron from photosystem I causes NADP+ to bond with H+

*E- lost in Photosystem II are replaced by electrons taken from water molecules

When H20 splits… hydrogen is used to make NADPH and ATP

Oxygen is given off as waste product: This is where you get all of the oxygen that you breathe!!

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B. Chemiosmosis The diffusion of chemicals through a

membrane resulting in the formation of ATP

1. During the breakdown of water, there is a build up of H+ ions in the thylakoid space

2. The movement of H+ ion across the thylakoid membrane triggers the phosphorylation of ADP to ATP

Done by ATP synthase

3. The ATP formed in the stroma is used in the 3rd reaction, during Carbon Fixation

What do we get out of the light reactions?? ATP, produced in the stroma

NADPH, produced in the stroma

Hydrogen ions, produced in the thylakoid space, pumped into the stroma to make NADPH and ATP

Oxygen, given off as by-product(waste_

3. Light Independent Reaction: Calvin Cycle(Dark Reactions)

Uses the energy formed in the light dependent reaction(ATP and NADPH) to form organic compounds.There are three possible pathways that can be followed- depends on the plants environment.

A. Calvin Cycle or C3 B. C4 Pathway C. CAM cycle

A. The Calvin Cycle(C3):The most common form of carbon fixation

1. CO2 diffuses into the stroma and an enzyme on the thylakoid membrane binds to CO2 to a five carbon – Ribulose Biphosphate molecule(RUBP)= 6 carbon molecule

2. The 6C molecule is unstable so it splits in to 2 PGA’s- Phosphoglyceric Acid(3C)

3. The 2 PGA’s get a phosphate from ATP and H from NADPH to form Phosphoglyceraldehyde(PGAL)(G3P), ADP and NADP

4. Most of the PGAL regenerates more RuBP. This is done by using ATP. The other PGAL is used to produce carbohydrates.

Every turn of the Calvin Cycle drops off 1C. So 3 turns per G3P. Two (G3P’s) produces a glucose molecule. 6 turns total.

Pic of cycle from other notes

B. C-4 Pathway Carbon Dioxide is bound to a compound

to form a four-carbon intermediate that enters the C-3 pathway

Fixes carbon 4 times faster than the C-3 pathway- allows plants to grow more quickly

Requires more energy than the C-3

Key Feature: The light dependent reactions and the Calvin Cycle occur in different areas of the leaf.

Many desert plants are C-4 Example Corn, Sugar Cane

C-4 occurs in sunny areas

C. CAM- Crassulacean acid metabolism Plants take in CO2 at night and store it Used in the daytime in The Calvin Cycle

Prevents excess water loss

Key Feature: The light dependent reactions and carbon fixation occur at different times.