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Flashcards in F214 - Photosynthesis Deck (32):

Define Autotrophs

Organisms that use light energy or chemical energy and inorganic molecules (carbon dioxide and water) to synthesise complex organic molecules


Define Heterotrophs

Organisms that ingest and digest complex organic molecules, releasing the chemical potential energy stored in them


Photosynthesis transforms light energy into...

...chemical potential energy that is then available to CONSUMERS and DECOMPOSERS


Autotrophs can synthesise complex molecules such as...

...carbohydrates, lipids, proteins, nucleic acids and vitamins.



-The first life on earth

-Were prokaryotes that synthesised complex organic molecules

-Used energy from EXERGONIC chemical reactions


Modern day example of chemoautotroph

Nitrifying bacteria which obtiain their energy by oxidising anmmonia to nitrite, or oxidising nitrite to nitrate Bacteria living near thermal oceanic vents, supporting very specific food chains



-Source of energy is sunlight -Raw materials are inorganic molecules (CO2 and H2O) -These are organisms whcih can undergo photosynthesis -These include plants and some bacteria called protoctists


Chemical equation for photosynthesis

6 CO2 + 6 H2O ---> C6H12O6 + 6 O2


Photosynthesis is a two stage process which happens in the...



Structure of Chloroplasts

-Most are disc shaped and between 2-10micrometres long

-Each is surrounded by a double membrane envelope

-Intermembrane space is aproximately 10-20 nm wide

-The outer membrane is permeable to small ions however the inner membrane is less permeable, with transport proteins embedded in it. The inner is folded into lamallae which are stacked into grana.

-Intergranal lamellae link each granum

-The two distinct regions in the chloroplasts are the stroma and the grana


Structure and fuctions of the Stroma

-A fluid filled matrix

-Reactions of the light-independant stage take place here

-Neccessary enzymes are already present for the reactions to occur

-Also contains starch grains, oil droplets, DNA and prokaryotic type ribosomes


Structure and function of the Grana


-Stacks of flattened membrane compartments called thylakoids

-The thylakoids are the site of light absorption and ATP synthesis (ie. the light dependent stage)


The thylakoids can only be viewed using...

...an electron microscope


Define Photosynthetic Pigment

Molecules that absorb light energy. Each pigment absorbs a range of wavelengths in the visible region and has its own distict peak of absorption. Other wavelengths are reflected.


Adaptations of the Chloroplast


-The inner membrane can control the movement of subtances between the cytoplasm and the stroma using it's transport proteins

-The many grana provide a large surface area for the light dependent reaction

-photosynthetic pigments are arranged into photosystems to allow maximum light absorption

-Proteins in the grana hold the photosystyems in place

-The stroma contains all neccessary enzymes to calayse the light independent stage

-Stroma surrounds grana so products from light dependent stage can readily be transported to the stroma for the light independent stage

-chloroplasts can make some of their own proteins needed for photosynthesis using the DNA and ribsomes present in the stroma


Structure of chlorophyll

-Chlorophyll is a mixture of pigments

-All have a long phytol (hydrocarbon) chain

-All have a porphyrin group, similar to the haem group in haemoglobin but with Mg in the structure rather than Fe



Accessory Pigments


-Otherwise known as Carotenoids

-They do not contain a porphoryn group

-They absorb blue light and reflet orange/yellow light

-Carotene (orange) and Xanthophyll (yellow) are the main carotenoid pigments


What is the equation for photolysis

2 H2O --> 4 H+ + 4 e- + O2


Define Photophosphorylation

The making of ATP from ADP and Pi in the presence of light


Define Electron Carriers

Molecules that carry electrons


Define Electron Acceptors

Chemicals that accept electrons from another compound. They are reduced while acting as oxidisng agents.


Non-Cyclic Photophosphorylation

1) A water molecule is split in the presence of light and an enzyme found in photosystem II producing 2 electrons and 2 protons (and half an O2 molecule as a biproduct)

2)The protons stay in the thylakoid space however the light excites the electrons so they are accepted by PSII, next by an electron acceptor, then to be transported down a chain of electron carriers (cytochromes)

3)The movement of these electrons releases energy with causes protons to move from the stroma into the thylakoid space

4)Due to the new concentration gradient, the H+ ions are trasported back into the stroma through channel proteins. These proteins are associated with ATP synthase. The proton movements draives the rotation, and so the activation, of this enzyme, causing ADP and Pi to form an ATP molecule

5) The protons which have no been transported into the stroma are now used to reduce NADP using NADP reductase


Cyclic Photophosphorylation

-Electrons are lost from PSI as they are excited by photons of light

-These are passed down electron carriers

-They end up at the chlorophyll of PSI where they started

-Produces a small amount of ATP

-Does not invloved the Photolysis of water and NADP


Difference between Cyclic and Non-Cyclic Phosphorylation

-Non Cyclic involves PSI and PSII, cyclic only involves PSI

-Non Cyclic involves photolysis of water, cylic does not

-Water is the electron donor in Non Cyclic, PSI is the electron donor in cyclic

-NADP is final electron acceptor in Non Cyclic, PSI is the final electron acceptor in cyclic

-Non cylic products are NADP, ATP and Oxygen. Cyclic product is ATP


How does CO2 reach the Calvin Cycle

-CO2 diffuses into the leaf through the open stomata

-Diffuses through the air spaces in the spongy mesophyll

-Reaches the palisade mesophyll

-DIffuses through the thin cellulose walls, cell surface membrane, the cytoplasm and the chloroplast envelope into the stroma


The Calvin Cycle

1) CO2 combines with Ribulose biphosphate (RuBP) (5C) catalysed by ribulose biphosphate carboxylate-oxygenase (rubisco)

2) This produces two molecules of glycerate 3-phosphate (GP) (3C) which can be used to make Amino Acids and Fatty Acids

3) 2GP is reduced by NADPH to form NADP and 2 molecules of triose phosphate (2TP). ATP is also used in this reaction to make ADP+Pi. Both NADPH and Atp are from the light dependent stage. TP can be used to make hexose sugars and glycerol.

4) 5/6 of the TP molecules are recylced back to RuBP using ATP from the light dependent stage


What is the optimum pH for RuBisCO?

pH 8 caused by the movement of protons into the thylakoid space from the stroma


Light intensity as a limiting factor

When light intensity is the limiting factor, the rate of photosynthesis is directly proportional to the light intensity, both moving up at constant rates alongside the other.

CO2 is in surplus and temperature is not a problem at this time


CO2 concentration as a limiting factor

Without a sufficient supply of CO2, the rate of photosynthesis can only be so high, depending on the temperature and light intensity.

When CO2 is the limiting factor, light intensity and temperature are no currently a problem and so are in surplus


Temperature as a limitng factor


The optimum temperature for photosynthesis in general is 25 degrees and so plants in hot and cold countries around the world have adapted to keep the inside of the leaf as close to this temperature as possible

Between 0-25 degrees, the photosynthetic rate aproximately doubles for every 10 degree rise however starts to fall again after 25 degrees as the enzymes stop working at full efficiency as they start to denature

High temperatures also cause the stomata to close to preserve water, meaning there is now a limited supply of CO2 for the plant as the gasous exchange has been slowed


3 ways of measuring the rate of photosynthesis

-Volume of oxygen produced over a given time

-Rate of CO2 uptake

-Rate of increase in dry mass of plants


What is a photosythometer and what is it's limitations

-Is a devicede used to measure the volume of oxygen produced by a plant and so measure the rate of photosynthesis

-Some of the oxygen is used by the plant for it's respiration and will not be released

-Nitrogen may be dissolved in the gas, skewing the results