LG 5 Outline Photosynthesis Photosynthesis: An Overview Role of Electrons – CO2 Fixation – Water – Chloroplast Structure – Light-Dependent Reactions Pigment Molecules – Photosystems – Photosystem II – Photosystem I – Light-Independent Reactions Phase I – Phase II – Phase III – The C4 Cycle Stomata – The C4 Cycle –

Photosynthesis: An Overview Summary Reaction: 6CO2 + 6H2O + light energy  C6H12O6 + 6O2 (carbon dioxide + water  glucose + oxygen) Role of Electrons Light energy pushes electrons to high energy levels. These electrons are used to convert inorganic CO2 to an organic form. This process is called CO2 fixation.

CO2 Fixation Electrons are added to CO2 (a reduction). Protons are also added. This converts CO2 to a carbohydrate.

Water Water is the source of the electrons and protons needed for carbon fixation. Oxygen generated from the splitting of water is released to the environment: 2 H2O 4 H+ + 4e- + O2

Chloroplast Structure Surrounded by a outer membrane with an inner membrane lying just inside. Between them is an inter-membrane compartment.

Fluid within the inner membrane is called stroma. These form flattened closed sacs called thylakoids. Thylakoids are arranged into stacks of grana which are joined together by tubular membranes called stromal lamellae.

Light-Dependent Reactions Pigment Molecules Light is absorbed by molecules of green pigment called chlorophyll and yellow-orange pigments called carotenoids. They are found embedded in the thylakoid membranes. Chlorophyll absorbs light in the blue and red spectra and reflects green light.

When photons from sunlight hit these pigment molecules, electrons jump to higher energy levels. Excited electrons are transferred to nearby electron-accepting molecules. Chlorophyll molecules become oxidized and the acceptor molecules are reduced.

Chlorophylls and Carotenoids Chlorophyll comes in two varieties, a and b, which differ very slightly in their chemical structure. Carotenoids Absorb light at different wavelengths than chlorophyll and pass this energy on to chlorophyll molecules thus expanding the range of wavelengths used in photosynthesis.

Photosystems Pigment molecules along with proteins form complexes called photosystems in the thylakoid membranes and stromal lamellae. There are two types, photosystem I and II. Photosystem I is also known as P700 because it absorbs light at a wavelength of 700 nm. Photosystem II is known as P680 for the same reason.

Photosystem II Electrons from the excited chlorophyll molecules split water into electrons, protons, and oxygen. Electrons from water first flow through photosystem II when light excites P680. They flow “downhill” in energy level through an electron transfer system connecting photosystems II and I. Electrons release free energy at each transfer and some is used to create a gradient of H+ across the membrane. ATP is synthesized as a result.

Photosystem I Electrons from photosystem II pass to photosystem I and become excited again in P700 through energy absorbed from light. They pass through an electron transfer system which leads to the final electron acceptor NADP+ which becomes reduced to NADPH using two electrons and two protons from the surrounding water solution. One proton is released.

This is called noncyclic electron flow because there is a one-way flow of electrons from H2O to NADP+ . The overall yield of this pathway is one molecule of NADPH and one molecule of ATP for each pair of electrons produced from the splitting of water. ATP synthesis in this pathway is known as photophosphorylation.

Light Independent Reactions The Calvin Cycle The NADPH and ATP from the light dependent reaction are used to fix CO2 in a circuit known as the Calvin Cycle. Phase I – A carbon atom from CO2 is added to ribulose 1,5-bisphosphate (RuBP) to produce two three-carbon molecules of 3-phosphoglycerate (3PGA).

Phase II – Reduction reactions using NADPH and ATP from the light-dependent reactions convert 3PGA into G3P, another three-carbon molecule. These are used to build glucose and other organic compounds.

Phase III – Some of the G3P molecules are used to produce more RuBP so the cycle can begin again.

The C4 Cycle Stomata Plants take in and release oxygen and carbon dioxide through small openings in their leaves called stomata. Water is also released through the stomata.

The C4 Cycle In warm climates, stomata close during the day to conserve water. This results in a build up of oxygen and a decrease of carbon dioxide in the leaves of the plant. This results in a process called photorespiration which produces a toxic molecule that reduces the efficiency of photosynthesis.

Plants in these climates have evolved an alternate pathway called the C4 Cycle to deal with this problem. It uses enzymes that are not affected by the high oxygen concentrations.

LG 5 Terms CO2 Fixation – Chloroplast – Stroma – Thylakoids – Grana – Chlorophyll – Photosystem II – Photosystem I – Calvin Cycle – C4 Cycle –