Photosynthesis

Objectives

1. PS converts light energy into chemical energy --- food

2. PS occurs in two stages

1. Light Reactions convert Light to ATP and NADPH

2. Calvin Cycle uses ATP and NADPH to convert Carbon dioxide into sugar

3. There are alternative mechanisms that have evolved in dry climates

Heterotrophs rely on other organisms for energy

Autotrophs create their own food

Photoautotrophs use sunlight to supply energy for the reaction to occur

Chloroplasts structure

-Most chloroplasts are found in the mesophyll cells

*Note Structure and Function

Atomic tracking -Notice water is a reactant and product

-Oxygen comes from water, it was once thought to have come from Carbon Dioxide

p. 188

Photosynthesis Overview

Stage One: Light RXNS

Stage Two: Dark RXNS

NADP+ is the electron transporter

Photophosphorylation is the formation of ATP using the energy from the sun

Carbon fixation is the creation of a sugar in the Dark RXNS

Sunlight does what????

-Chlorophyll is a pigment

-Chlorophyll a absorbs a certain wavelength

-Chlorophyll b absorbs a certain wavelength

-Location of the pigments is in the thylakoids

-Sunlight excites the electrons in the head of the pigment to an excited state.

-Molecule wants to be at its ground state so gives off energy either as heat, light or to another molecule

Light Harvesting

-Phytosystems I and II

-Light harvesting complexes consist of various pigments – act like antenna for the photosystem – increase surface area for light capture

- Chlorophyll a have ability to transfer electron to primary electron acceptor

Linear Electron Flow

Steps 1-3

-Sun excites an electron which travels from one pigment to another until it reaches P680 (wavelength).

-When an electron from P680 travels to the primary electron acceptor it is left as P680+

-P680+ is the strongest biological oxidizer known

-Electrons must fill the void – come from water – O2 is immediately formed.

Step 4-5

-Electrons travel down an electron transport chain to photosystem I

-Increases proton motive force while traveling

-Chemiosmosis makes ATP

Steps 6-8

-Sun excites an electron at PS I leaving an electron hole, P700+, that is filled with the electron from PSII

-Electron travels down another electron transport chain finally reducing NADP+ to NADPH

**Cyclic Electron Flow only uses Photosystem I

Chemiosmosis Comparison

-Proton Motive Force Driven

-ATP Synthase makes ATP

-High [H] in the inner space

*ATP produced in the light reactions goes to the dark reactions

Phase One: Carbon Fixation

-Carbon Dioxide attaches to make an unstable 6 carbon sugar

-Rubisco is the most abundant enzyme in plant cells and possibly in the world

Phase 2: Reduction

-Phosphorylation of the three carbon sugar takes place

-NADPH reduces to G3P, the same product as the first stage of Glycolysis

Accounting:

-3 carbon Dioxide molecules Yield 6 G3P molecules

-Only three G3P molecules are tallied as the rest move on to stage three

Phase 3: Regeneration

-Use of three more ATP

- 3 carbon molecule is changed to a 5 carbon molecule, which can be used again in the Calvin Cycle

-PEP Carboxylase has a higher affinity for Carbon Dioxide then Rubisco -- Better in lower concentrations when stomata is partially closed

-Carbon Dioxide Pump

Plant Adaptations

-Gas and Water Exchange occurs in Stomata

--Dry hot areas water leaves the plant faster during the day.

--Photorespiration occurs when Carbon Dioxide is not available – Necessary but depletes Organic material needed in the Calvin Cycle

CAM plants

-Open stomata during the night and close during the day

-Carbon Dioxide is converted to an organic acid until needed during the day