BIOL 101

Lecture Notes

 

Chapter 7 – Photosynthesis

 

Introduction

 

Van Helmont (about 1650’s), willow tree in a pot, added nothing to soil except water.  Five years later, found tree had gained 75 kg (150 lbs) while soil LOST only 1/000th this much mass.   He (incorrectly??) concluded that a plant gets most of its substance from water.

 

Priestley (1780), candle in closed container burns out, but if container is connected to container growing mint candle continues burning, can also sustain a mouse.  O2 not yet discovered but recognized plant “restores” the dead air.  Not reproducible always because he didn’t control for light

 

Jan Ingenhousz (1770’s, but after Priestly), showed plants can “restore” air only when exposed to LIGHT.

 

Later, Jean Senebier discovered that CO2 is taken up from air during photosynthesis.

 

Photosynthesis— CONVERSION of radiant energy from sunlight to chemical energy in the bonds of fuel molecules, consumes CO2, and generates oxygen, the “restoring principle” in Priestly’s experiments.

 

Photosynthesis provides the food supply for virtually ALL organisms.  Plants are autotrophs, they MAKE THEIR OWN FOOD.  Most other organisms get their food by consuming plants (except a few like bacteria that get their energy by breaking down chemical compounds such as sulfur-containing compounds in thermal vents in ocean floor)

 

                                            light energy

   Overall,  6 CO2  + 6 H2O         ®            C6H12O6  + 6 O2 

 

Unit 7.1 – Autotrophs are the Producers of the Biosphere

 

On land, the main food producers are plants

In aquatic environment, main food producers are algae (photosynthetic protists) and photosynthetic bacteria.  In plants and algae, photosynthesis  occurs in the chloroplasts.

 

Unit 7.2 – Photosynthesis Occurs in the Chloroplasts

 

Green color comes from pigment chlorophyll which is the molecular “antenna” that captures light energy.

 

            mesophyll—green tissue in leaf interior

            stomata—openings for gas exchange, intake CO2, release of O2

                chloroplasts—

 (inner and outer membranes and thylakoid membranes create compartments where different reactions occur).

 

Thylakoids—membranous sacks, stacked in grana

 

Intermembrane space

Stroma—enclosed by inner membrane—SUGARS MADE HERE!!

Thylakoid compartment

 

CHLOROPHYLL MOLECULES (to capture light energy) are embedded in thylakoid membrane, as are other important molecular components that convert light energy to chemical energy.

 

Unit 7.3 – Plants Produce O2 Gas by “Splitting Water”

 

In 1700’s Ingenhousz (the one who showed light required for plants to produce oxygen), thought plants produce O2 by extracting it from CO2—HE WAS WRONG!  It wasn’t until 1950’s that scientists showed O2 is extracted from water.   This was accomplished using a radioactive isotope of oxygen as a “tracer”.  Similar experiments have shown that the carbon in CO2 is incorporated into glucose.

 

Unit 7.4 – Photosynthesis is a Redox Process, as is Cellular Respiration

 

Like cellular respiration, photosynthesis involves a series of electron transfer or REDOX reactions.   Recall that:

 

oxidation involves LOSS of electrons

reduction involves GAIN of electrons

 

The two are always coupled together.

 

In contrast to respiration, in photosynthesis, the electrons are boosted UP instead of DOWN an energy hill!

 

Energy is transferred from sunlight to the electrons involved in atomic bonds in glucose.

 

In respiration, glucose was oxidized to CO2, and oxygen reduced to water

In photosynth, CO2 is reduced to glucose, and water oxidized to oxygen

 

Unit 7.5 – Overview – Photosynthesis Occurs in Two Stages, Linked by ATP and NADPH

Two stages:

 

1) “photo”  -- the “light reactions” (occur in the THYLAKOID MEMBRANES of grana), captures light energy, produces ATP (energy) and NADPH (high energy electrons) for...

                     

2) “synthesis” – the “dark reactions” (Calvin cycle—occur in the STROMA) building of glucose from CO2

SUMMARY OF LIGHT REACTIONS

 

In the “light reactions” (which occur in the thylakoid membrane), light energy is absorbed by chlorophyll:

ADP  +   P      ¾®   ATP (high energy currency)

 

 

                                                                        2e-

                     NADP+  +    2H                ¾¾¾®         NADPH         + H+

           (oxidized form)                                            (reduced form)

 

 

                                                               H2O      O2            

 

NADPH is an electron and hydrogen “shuttle”, structurally related to the NADH molecule seen in cellular respiration.  However, the function of NADPH is to help ASSIMILATE energy into glucose in photosynthesis, the function of NADH is to help EXTRACT energy FROM glucose in cellular respiration.

 

Note that in photosynthesis, the ultimate SOURCE of electrons for reducing NADP+ is WATER!!  The splitting of water occurs during the “light reactions”.

 

SUMMARY OF CALVIN CYCLE (DARK REACTIONS)

 

The process of capturing carbon dioxide from the atmosphere and incorporating it into biological molecules is called carbon fixation.  In the Calvin cycle, which occurs in the STROMA, carbon is fixed, then incorporated into GLUCOSE.  The source of high-energy electrons is NADPH, and the source of energy for this process is ATP.  

 

NOTE: the NADPH and ATP used in the Calvin cycle was generated earlier, in the light reactions!!

The Calvin cycle does not require light directly (which is why the Calvin cycle is said to consist of the “dark reactions”).   Think of the light reactions as the power plant that provides the energy, and the Calvin cycle as the “factory” where glucose is made.

 

Unit 7.6 – Visible Radiation Drives the Light Reactions

 

The distance between the crests of two adjacent waves is called the wavelength.   The smaller the wavelength, the higher the energy of the light!

DIFFERENT COLORS of light represent DIFFERENT WAVELENGTHS in the spectrum of visible light. 

 

Pigments are molecules that absorb light.  Chloroplasts contain several important pigments, each absorbs light of different wavelengths:

 

            chlorophyll a  -- participates directly in the light reactions

 

            chlorophyll b  

                                        broaden range of useful wavelengths, conveying light they absorb to

            carotenoids       chlorophyll a

 

Wavelengths that are not absorbed by these pigments are transmitted THROUGH the leaf, or REFLECTED by the leaf, giving the characteristic GREEN color.

 

Unit 7.7 – Photosystems Capture Solar Power

 

Although light can be described as “wave-like”, it also behaves like “particles”.  The “particles” are called photons.  The shorter the wavelength, the MORE ENERGY a photon of light contains.

 

When a pigment molecule absorbs a photon of light (of the correct wavelength), an electron in the pigment is “boosted” or “exited” to a higher energy level (an excited state).  The exited electron may be passed on or transferred to a neighboring molecule.  Recall that electron-transfer reactions like this are called “redox” reactions.

 

The pigment molecules (chlorophylls a and b, and the carotenoids) in a leaf are CLUSTERED TOGETHER in assemblies of 200-300 molecules.   Together, these pigments act as an antenna to capture light energy.  When one of these pigments absorbs a photon of light, an excited electron is generated that is passed from pigment molecule to pigment molecule until it reaches  a special chlorophyll a molecule (the reaction center) in the antenna assembly.  The reaction center is associated with another molecule called the primary electron acceptor that will act as the FIRST electron-carrier in an electron-transport chain. 

 

The COMBINATION of the antenna molecules, reaction center, and primary electron acceptor is called a photosystem.   There are actually TWO kinds of photosystems in the thylakoid membrane:

 

Photosystem II-- has a reaction center called P680 (a chlorophyll a molecule that preferentially absorbs light of 680 nm wavelength).

 

Photosystem I-- has a reaction center called P700 (a chlorophyll a molecule that preferentially absorbs light of 700 nm wavelength).

 

These two photosystems are LINKED TOGETHER in the light reactions.

 

Unit 7.8 – In the Light Reactions, Electron Transport Chains Generate ATP, NADPH, and O2

 

THE FUNCTION OF THE LIGHT REACTIONS:

 

To produce ATP and NADPH that will be used in the Calvin Cycle (“dark reactions”) to drive synthesis of glucose.  

 

Water is split to yield both electrons and hydrogen atoms (H) that will be carried by the “electron-shuttle” NADPH.  Oxygen (O2) is produced as water is split.

PRODUCTS of the light reactions: ATP, NADPH, O2

 

Steps in the light reactions:

 

1)        Light is absorbed at photosystem II and funneled to P680 reaction center. 

2)        Water is split, and electrons are boosted (“exited”) by absorbed energy

3)        Excited electrons pass from P680 to primary electron acceptor

4)        Electrons pass through a FIRST electron transport chain—ATP is generated by chemiosmotic phosphorylation

5)        Electrons enter photosystem I, where they receive ANOTHER BOOST from light absorbed at  700 reaction center.

6)        Electrons pass down a SECOND electron transport chain, to NADP+, reducing it to NADPH.

 

Thus, TWO electron-transport chains are involved in the light reactions.

 

Unit 7.9 – Chemiosmosis Powers ATP Synthesis in the Light Reactions

 

As we saw in Chapter 6, in cellular respiration, some ATP synthesis occurs by “substrate-level phosphorylation”, and but most occurs by “chemiosmotic phosphorylation” . 

 

Recall that chemiosmotic phosphorylation differs from substrate-level phosphorylation in that chemiosmotic phosphorylation involves the hydrogen ion concentration gradient generated by the electron-transport chain.  In photosynthesis, the ATP is generated ENTIRELY by chemiosmotic phosphorylation.  Because the energy to drive chemiosmotic phosphorylation in photosynthesis is provided by LIGHT, the process is called photophosphorylation. 

 

The processes of chemiosmotic ATP synthesis in cellular respiration and in the light reactions of photosynthesis are very similar and share these common features:

1)        electrons flow from a high-energy state to a low-energy state as they are passed from one electron-carrier in the chain to the next.

2)        electron-carriers in the chain pump hydrogen ions (H+) across membrane to generate a concentration gradient.

3)        H+ ions seeking to return back across the membrane are channeled through a special enzyme “port” (ATP synthase), that adds a phosphate group to ADP to generate ATP.

 

However, the processes of chemiosmotic ATP synthesis in cellular respiration and photosynthesis differ in two important respects:

1)        In respiration, the final electron acceptor in the electron-transport chain is OXYGEN.  In photosynthesis, the final electron acceptor in the electron-transport chain is NADP+.

2)        In respiration, H+ ions are pumped across the inner mitochondrial membrane.  In photosynthesis, H+ ions are pumped across the thylakoid membrane, into the thylakoid compartment.

 

Unit 7.10 – ATP and NADPH Power Sugar Synthesis in the Calvin Cycle

 

Inputs = CO2, + ATP and NADPH (from light reactions!)

Products = GLUCOSE (C6H12O6)!!  + ADP + + NADP+

 

An important intermediate in the Calvin Cycle is a 3-carbon molecule called glcyeraldehyde-3-phosphate or G3P, that is used not only for the production of glucose, but can be used by plants as a BUILDING BLOCK for other organic molecules as well.