PHOTOSYNTHESIS

General Biology Lecture

Photosynthesis builds organic molecules for use by organisms throughout the food web. Photosynthesis takes place in chloroplasts and includes the light reactions when solar energy is captured, and the light independent reactions, during which carbohydrates are synthesized through the Calvin cycle. C₃ and C₄ photosynthesis and CAM photosynthesis are described.

  Photosynthesis, an energy conversion: light to chemical 

BIOLOGY AND SOCIETY: PLANT POWER FOR POWER PLANTS

     Historical Perspective 

1.   In the early 1600's it was a widely held notion that plants absorbed food from the soil (Aristotle wrote that leaves in fact were mere shading devices providing a comfort zone for roots; everyone new adding manure and humus to soil resulted in plants growing larger and growing faster.) 

2.   Mid 1600's van Helmont - a Belgian physician, devised simple experiment in which he grew a willow tree seedling for five years in a bucket containing a known quantity of soil.  The tree gained roughly 195 pounds while the soil lost less than one pound. 

        5 lb. willow                          200 lb. 

                      ----------->
                         5 yr
      200 lb soil                         199.8 lb soil 

Van Helmont concluded wrongly that plants obtain food from water.

If most of the dry material (dry biomass) didn’t come from the soil, then where did it come from?  It seems unlikely based on everyday experiences and normal perceptions we have based only on our bodily senses, but tremendous tonnage of biomass accumulates each day from thin air.  Photosynthesis is the process by which this happens—that seemingly weightless CO₂ is pulled from the air and accreted into an organic form, the form of carbohydrates.  Water from the soil plays an important role in supplying atoms of Hydrogen that are required in building carbohydrates.  Soil also provides inorganic nutrients that supply the other elements used in constructing other organic molecules such as amino acids and nucleotides (all but C, H, & O are from the soil, recall the mnemonic device for the chemical elements of life “C HOPKNS CaFe Mg B Mn CuZn Mo Cl Na”). 

3.  1700's Joseph Priestly (England) - put things under bell jars.  Mouse kept alive in bell jar with plant.
[draw jar with live, then dead mouse; draw w/plant & live mouse]
King of England presented Joseph Priestly with a medal and declared
"for these discoveries we are assured that no vegetable grows in vain... but cleanses and purifies our atmosphere"

4. 1882 German plant physiologist, T. W. Engelmann. 

               O₂ produced during photosynthesis
               some bacteria clustered around O₂ source
               light needed for Photosynthesis
               light consists of different wavelengths of energy
               pigments absorbed energy wavelengths differentially, that is the color of a 
               pigment indicates the wavelengths of light not absorbed but rather reflected

RADIO WAVES , INFRA RED,           VISIBLE LIGHT,        ULTRA-VIOLET, X-RAYS

Only visible portion of spectrum (wavelengths 400-750 nm) are important for photosynthesis.

T. W. Engelmann showed that violet and red light are used in photosynthesis and the yellow and green light less so.  He did this in an experiment with Spirogyra (filamentous algae) and 0₂ loving bacteria and the aid of a microscope fitted with a prism; the bacteria clustered around the portions of Spirogyra bathed by the blue and red portions of the spectrum.

4.     1940's  Calvin & Benson (U.S.)- used ¹⁴C radioactive isotope in C0₂ and traced path of
                                                           carbon fixation - conversion of inorganic CO₂ into organic carbohydrates.  

           Organic molecules are the key building blocks and energy stores for life.  Plants make (synthesize) their own organic molecules from
           inorganic atmospheric CO₂ .  To create organic molecules from CO₂ plants trap light energy to drive the metabolic pathways we call
           photosynthesis. 

            When eaten by animals or decay organisms, plants provide the organic molecules needed by those forms of life.  You also need plant-made organic molecules to supply the building blocks and energy needs required for growth and life itself.  You get these organic molecules by eating the plant that created them or by eating organic molecules originally found in plants but that now reside in some animal you may be about to eat.  Thus plants are the primary source and sole producer of original organic food.

III.       Importance of Photosynthesis:

            Fossil fuels - plants from 350 MYA (Carboniferous Period) died, were covered with sediments, and became coal.

            Food - 250,000 plant species, remarkable few used as human food.  Only 150 crop species.  The big four are:  rice, wheat, corn, &
                        beans  (the first 3 are grasses)

                        Removes CO₂ - reduces greenhouse gas.  – today only 0.033% CO₂ ;  earth billions of years ago had much higher CO₂ levels, much of this early CO₂ has been incorporated by plants into biomass (or into fossil fuels).  Our removal of forests and combustion of fossil fuels returns CO₂ to the atmosphere and results in: 1) theoretically with all other factors removed, faster plant growth & 2) global warming. 

            Created O₂ rich atmosphere (about 21 % O₂)

            Lead to the formation of the ozone (O₃) layer which filters harmful UV radiation). 

                                                           3O₂ ---> 2O₃ in stratosphere

            An O₂ rich atmosphere and ozone layer are prerequisites for life on land---a rather recent occurrence.  Only 10% of the time that life has been on earth has there been substantial terrestrial life.  For the first 3 billion years life on earth was all aquatic and only in the last 400 million years has terrestrial life appeared.  Photosynthesis performed by algae and seaweed changed remarkably the earth’s atmosphere.

IV.       Chemosynthesis - some bacteria use chemical energy to fix CO₂

            Thus not all organic molecules are created by plants alone, a minor amount of primary production occurs by chemoautotrophs, bacteria which use chemical bond energy of iron and sulfur compounds to fix carbon dioxide.

V.        Additional Vocabulary:

            Autotrophs – organisms that create their own food by fixing CO₂ into an organic form: requires outside energy source – sun in the case of photoautotrophs (ex. Algae, plants) & inorganic chemical energy in the case of  chemoautotrophs (some bacteria).

            Heterotrophs – organisms that feed on food already formed by autotrophs.  (ex. some non-green plants, all animals, some bacteria, all fungi)

Chapter Outline

THE BASICS OF PHOTOSYNTHESIS

Almost all plants are photosynthetic ______________________ as are some bacteria and protists.  They generate their own _______________ through photosynthesis.

I. Chloroplasts: Sites of Photosynthesis

Photosynthesis occurs in _______________________.

Chloroplasts:

1. ______________________________________

2. ______________________________________

3. ______________________________________

II. The Overall Equation for Photosynthesis

_______________________________________________________________________

In photosynthesis energized _____________________ are added to carbon dioxide to make ______________________.

__________________ provides the energy.

III. A Photosynthesis Road Map

Photosynthesis is composed of two processes:

1. __________________________________

2. __________________________________

THE LIGHT REACTIONS: CONVERTING SOLAR ENERGY TO CHEMICAL ENERGY

________________________ are chemical factories powered by the sun.  They convert _______________ to ____________________.

I. The Nature of Sunlight

Sunlight is a type of energy called ___________________ or ____________________________.  The full range of radiation is called the

_______________________________________.

A. What Colors of Light Drive Photosynthesis?

In 1883, German biologist ___________________________ performed an experiment using ________________ and _______________ and determined that certain types of _____________________ drive photosynthesis.

___________________ absorb select _____________________ of light that drive photosynthesis.

II. Chloroplast Pigments

Chloroplasts contain several pigments:

1. __________________________________

2. __________________________________

3. __________________________________

III. How Photosystems Harvest Light Energy

Light behaves as __________________, discrete packages of _________________.

Chlorophyll molecules absorb ____________________.  _________________ in the pigment gain energy.  The energy is released and used.

A photosystem:

1. _______________________________________

2. _______________________________________

IV. How the Light Reactions Generate ATP and NADPH

Light Dependent Reactions - require light  

             These reactions occur on the inner membranes of the chloroplast; these membranes hold the pigments and other molecules involved.  The photosynthetic pigments are arranged into discrete patches called photosystems.   It is these pigment molecules (largely chlorophyll) that absorb light energy.  Absorbed light energy is transferred to electrons, which in their energized state having absorbed light energy (blue and red light predominantly), leave the photosystems.  These removed electrons give up their energy absorbed from the sun to produce ATP during the Electron Transport System (more about ETS below).   To replace the electrons removed from photosystems, electrons are taken from two H₂O’s.  Removing electrons from two water molecules results in the water molecules splitting into four H⁺ and two O atoms.   The O atoms combine forming O₂ as a waste molecule or byproduct.  This process of splitting water molecules is called photolysis.

Two types of photosystems cooperate in the light reactions:

1. _______________________________

2. _______________________________

The light reactions occur in the __________________________.

An electron transport chain:

1. _______________________________

2. _______________________________

Electron transport system – a difficult concept, but go with the following analogy:  The electrons that begin the electron transport system have potential energy stored from the sun just as water behind a dam stores potential energy from the sun (Recall the sun is needed to elevate water, via evaporation and rainfall, from sea level to the level of the dam.  Similarly, sunlight elevates the potential energy of electrons).  The electrons are transported between various molecules in the membranes of the chloroplast such that they give up their potential energy in a form that is used to add a phosphate to ADP, thus making ATP.  This is very much like the water behind a dam giving up its potential energy to generate electricity as it (the water) is transferred via the pull of gravity through the turbines in the dam.    

 In summary of the Light Dependent Reactions, O₂ has been produced as a byproduct and light energy has been converted into the chemical bond energy of ATP.  But ATP is not food, it is incapable of being stored, and thus photosynthesis is not yet complete (see next page for final processes in photosynthesis)

THE CALVIN CYCLE: MAKING SUGAR FROM CARBON DIOXIDE

The Calvin cycle:

1. _________________________________

2. _________________________________

Light Independent Reactions- do not require light.

    =Carbon fixation – the conversion of inorganic CO₂ into carbohydrates.

    =Calvin cycle, Calvin-Benson cycle, or C₃ cycle.

                        1940's - Calvin & Benson - used ¹⁴C radioactive isotope and traced the path of carbon fixation - conversion of inorganic CO₂ into organic carbohydrates.  Today this reaction series bears their name as the Calvin-Benson Cycle or Calvin Cycle.

                       The actual synthesis reactions that produce carbohydrates during photosynthesis do not directly require light.  These so-called light independent reactions do however require the ATP made during the light reactions.  Without ATP, no energy would be available to drive the carbon fixation process.  Carbon and Oxygen atoms from atmospheric CO₂ and the H⁺ from soil drawn H₂O made available by photolysis are combined to make carbohydrates during the Calvin Cycle.  All the chemicals involved are dissolved in the solution that fills the chloroplast, thus the internal membranes of the chloroplast are not directly involved in the Light Independent Reactions.  As carbohydrates accumulate they are transported to other plant organs or stored as starch granules inside the chloroplast.

In summary of the Light Independent reactions:

·        consumes CO₂  to produce C₆H₁₂O₆

·        consumes ATP made during light reactions (ATP is the energy source that directly drives the carbon fixation reactions)

I. Water-Saving Adaptations to C₄ and CAM Plants

C₃ plants:

1. ______________________________________

2. ______________________________________

C₄ plants:

1. ______________________________________

2. ______________________________________

CAM plants:

1. ______________________________________

THE ENVIRONMENTAL IMPACT OF PHOTOSYNTHESIS

Photosynthesis has an enormous impact on the atmosphere:

1. _________________________________________

I. How Photosynthesis Moderates Global Warning

Greenhouses used to grow plants indoors:

1. ________________________________________

A similar process, the greenhouse effect:

1. ________________________________________

2. ________________________________________

Greenhouse gases are the most likely cause of ______________________________, a slow but steady rise in the Earth's surface temperature.  Destruction of _____________________ may be increasing this effect.

EVOLUTION CONNECTION: THE OXYGEN REVOLUTION

The atmospheric oxygen we breathe is a by-product of ___________________________.  ______________________ were the first organisms to carry out photosynthesis.  The production of _____________________ changed the Earth forever.  The _________________________ was a major episode in the history of life on Earth.

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