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Photosynthesis: Purpose

Photosynthesis: Basics

12H2O + 6CO2 6O2 + C6H12O6 + 6H2O

Photosynthesis is the process by which plants capture light energy to manufacture sugars.

You will be familiar with one of these two equations

6H2O + 6CO2 6O2 + C6H12O6

Light

Light

Chlorophyll

Chlorophyll

You can see that these equations represent the same reaction; one has just canceled some water.

You need to memorise these if you haven’t already.

Wednesday, April 21, 2010

Photosynthesis: Location


The chloroplast is the location of photosynthesis.

* The thylakoids have a disc shape and are bound by a membrane. They contain chlorophyll.

* These thylakoid discs are arranged in stacks called grana (sing. granum).

* The grana are surrounded by a fluid called the stroma.

* The structures are easily identifiable in an electron micrograph.


Photosynthesis: Two Stages


There are two stages in photosynthesis:

The light reactions begin on the thylakoid membrane and are powered by light energy. Its purpose is to produce ATP and NADPH to drive the next stage.

The dark reactions occur in the stroma and is powered by ATP and NADPH. Its purpose is to produce sugar.


1. Light Reactions

Photosynthesis: Light Reactions

The light reactions begin in the thylakoid membranes where chlorophyll

pigments absorb light energy.

Electrons in chlorophyll are energised by this absorption and passed through

an Electron Transport Chain.

The energy yielded by this process is used to convert NADP+ to NADPH

It is also used to split water to drive the conversion of ADP to ATP.

The light reaction can involve a multitude of reactions involving the movement of protons and electrons among different molecules. Focus on the major features rather than trying to understand everything.


1. Light Reactions: Complex


The light reactions of photosynthesis (often called the light-dependent reactions) can be enormously complicated- even more so than the video shown here. While you needn’t learn all of it, an appreciation of the underlying biochemical complexity is necessary.

Watch and note:

* How light energy affects chlorophyll.

* How light absorption is turned into chemical energy.

* Why water is split

* How oxygen is formed as a byproduct.

Video Link


http://www.youtube.com/watch?v=eY1ReqiYwYs

http://www.youtube.com/watch?v=eY1ReqiYwYs

1. Light Reactions: Simplified



1. Light Reactions: Inputs and Outputs


IN OUT

You do need to remember:* Where the light reactions occur* What goes in, what goes out

Light

Water Oxygen


1. Light Reactions: Coenzymes


Coenzymes are relatively small organic non-protein molecules.

They assist enzyme catalysed reactions by acting as carriers of electrons and protons. They may also carry specific atoms or groups of atoms, such as phosphate, that are required for, or produced by, chemical reactions.

The coenzymes we study in VCE Biology cycle between “loaded” and “unloaded” states.

CoenzymeAbbreviationAbbreviation

FunctionCoenzymeUnloaded Loaded

Function

Adenosine Triphosphate ADP ATP Energy Transfer in many processes

Nicotinamide adenine dinucleotide phosphate NADP NADPH

Transfer of electrons and protons in photosynthesis

Nicotinamide adenine dinucleotide NAD+ NADHTransfer of electrons and protons

in respiration

Flavin adenine dinucleotide FAD FADH2Transfer of electrons and protons

in respiration


2. Dark Reactions: Complex

Photosynthesis: Dark Reactions

Web Video

The next stage of photosynthesis takes place in the stroma. Once again, you needn’t learn all of it but having an impression of how the reaction takes place is important.

You will probably encounter different names for this same process such as;* Dark Reactions* Calvin-Benson Cycle* Light Independent Reactions

Watch the video and note:

* How many carbons are added to the calvin cycle.

* How many carbons leave the calvin cycle.

* The roles of RuBP and G3P

* How glucose is finally obtained.


http://www.youtube.com/watch?v=mHU27qYJNU0

http://www.youtube.com/watch?v=mHU27qYJNU0

2. Dark Reactions: Simplified



2. Dark Reactions: The Basics


IN OUT

You do need to remember:* Where the dark reactions occur* What goes in, what goes out

Carbon Dioxide 3C Sugar


2. Dark Reactions: Rubisco


RuBisCo (ribulose 1,5-bisphosphate carboxylase oxygenase) is a very large enzyme that catalyses the fixing of CO2 to the 5 carbon RuBP compound during the calvin cycle.

It is the most abundant enzyme on the planet and is usually the limiting factor to glucose production.

RuBisCo requires Mg2+ ions to bind to the active sites before it is activated to fix CO2.

Metal ions that bind to an enzyme to activate or increase catalytic activity are called cofactors.


An Overview of the C3 Pathway

Photosynthesis: Overview


Alternative Photosynthetic Pathways- C4

Photosynthesis: C4

There are three major photosynthetic pathways in plants. The pathway just described is known as the C3 pathway for the manner in which it cuts 3 carbon molecules.

The C4 pathway allows * plants to fix CO2 while stomata remain closed * before moving it to another location to drive the Calvin cycle

C4 plants are more competitive under high temperatures.

They constitute about 3% of the world’s plants and include corn and sugar cane.


Alternative Photosynthetic Pathways- CAM

Photosynthesis: CAM

The third pathway is CAM (Crassulacean Acid Metabolism). CAM is an adaptation for plants in hot, arid environments- typically succulents like cacti.

These plants open their stomata at night and fix CO2

to form malate. The CO2 is stored in this form until daylight when it is used to drive photosynthesis.

This pathway is used by about 10%

of the world’s plants.


Wavelength Absorption

Photosynthesis: Wavelength Absorption

The colours of visible light are characterised by different wavelengths. Objects appear coloured only because particular wavelengths of light are reflected. This reflected light is detected by our retina and then interpreted as a colour by our brain. Wavelengths that are not reflected are absorbed.

Photosynthetic pigments are responsible for absorbing light to drive the photosynthesis. Thus...

Plants appear green

Chlorophyll reflects green light

Green light is poor for photosynthesis

Photosynthetic pigments of different colours exist including.....

* Chlorophyll a (blue-green) * Chlorophyll b (yellow green)

.....and some other pigment families such as carotenoids (orange) and xanthophylls (yellow)


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