Energy Transfers in and between organisms

What are photosystems?

• The structure that photosynthetic pigments are arranged in.

• Can also be referred to as light harvesting systems or reaction centres.

• They are transmembrane protein complexes found in the thylakoid membrane (and grana and lamellae).

• They contain photosynthetic pigments that absorb light energy needed for photosynthesis.

• There is a chlorophyll in the centre of each photosystem.

• Chlorophyll absorbs wavelengths of light.

• PSI and PSII are the two main photosystems.

• PSI- chlorophyll absorbs light at a wavelength of 700nm at best

• PSII- 680nm at best

Why do plants have several photosynthetic pigments, other than chlorophyll?

• Because different pigments absorb different wavelengths of light so the plant increases the range of wavelengths that they can absorb, increasing their ability to photosynthesise.

• These accessory pigments aren’t directly involved but ensure that more light energy is captured for the chlorophyll to absorb.

• Some plants produce high levels of anthocyanins, dark red and purple pigments→ protect plants from UV radiation and to colour them

What are the two photosystems linked by?

• Electron carriers 

• These are proteins that transfer electrons between between them using a series of redox reactions.

• The chain of photosystems and proteins that electrons flow through in the thylakoid membranes, known as the electron transfer chain.

What 2 stages does photosynthesis consist of?

• LDR

• LIR

• in that order

Where do these reactions occur?

• LDR- Thylakoid

• LIR- Stroma

• both in chloroplast

What are light energy and water used for?

• To create ATP and reduce NADP

What occurs within LDR?

• Photoionisation of chlorophyll

• Photolysis (separate)

• Chemiosmosis

• Production of ATP and reduced NADP

Photoionisation of chlorophyll

• Light energy is absorbed by the chlorophyll and the energy results in electrons becoming excited and raising up an energy level to leave and go on to be passed through a series of electron carrier proteins embedded onto thylakoid membrane.

• Chlorophyll is left as a positively charged ion- has been oxidised.

Photolysis of water

• Light energy is absorbed by chlorophyll and splits water into oxygen, H+ and e-.

• 2H2O → O2 + 4e- + 4H+

• H+ go on to increase concentration of H+ in thylakoid space

• Electrons replace those lost from chlorophyll

• Oxygen is either used for respiration or diffuses out of the leaf through the stomata.

Electron transfer chain

• The electrons that gained energy and left the chlorophyll move along a series of proteins embedded within the thylakoid membrane.

• As these move, they release energy , some of which is used to actively pump the protons from the stroma into thylakoid (already lots of H+ inside thylakoid due to photolysis).

• An electrochemical gradient is created (wasn’t strong enough previously to allow it to diffuse through ATP synthase).

• The protons pass through the enzyme ATP synthase, in a process called chemiosmosis.

• Chemiosmosis stimulates the production of ATP (photophosphorylation) as well as NADPH by providing the energy required.

• Electron at end of chain combines with co-enzyme NADP and H+ to form NADPH.

What photosystem can split water?

• PSII as it has the correct enzyme complex

Products of the light dependent reaction

• ATP from photophosphorylation

• O2 from photolysis

• NADPH

What is photophosphorylation?

• ATP is soluble and easily diffuses around the cell to provide energy.

• ATP is synthesised from ADP and Pi directly after chemiosmosis (H+ moving through ATP synthase from thylakoid space to stroma).

What are the two types of photophosphorylation?

• Cyclic and Non-cyclic

• Cyclic only involves PSI whereas non-cyclic involves both.

Non-cyclic phosphorylation

• Chlorophyll in PSII absorbs light energy which excites electron.

• Electrons leave ( AND ARE REPLACED) and move along electron transport chain to PSI releasing energy.

• Energy released → active transport + chemiosmosis.

• PSII absorbs electrons that came from PSII and they are excited by light and move through some more electron carrier proteins, until they are finally taken up by NADP along with protons.

Cyclic photophosphorylation

• Electrons leave PSI and move down the ETC, releasing energy → active transport and chemiosmosis.

• Electrons at PSI can’t be replaced so they flow back to PSI  through ETC.

• No NADPH produced or O2 and only produces small amounts of ATP in comparison.

Describe the light independent reaction (Calvin cycle)

• CO2 reacts with ribulose biphosphate (RuBP)

• This has 5 carbons and one is gained from CO2 (carboxylate).

• Catalysed by enzyme rubsico

• This then breaks down (quickly) to form 2 lots of glycerate 3-phosphate (GP) which each have 3 carbons.

• This is reduced by NADPH and the energy for this is provided by ATP (as it converts to ADP and Pi, breaking a bond, releasing energy).

• 2 x triose phosphate are produced and 5/6 this goes on to form RuBP (using energy  released by ATP) and 1/6 times it forms useful organic products such as: glucose, amino acids and glycerol.

What is a limiting factor?

• Any factor that reduces the rate of photosynthesis

• CO2. light intensity and temperature.

Effect of temperature on rate of photosynthesis

• As temperature increases, rate increases

• As enzymes e.g. rubisco gain kinetic energy

• So more E-S complexes form

• Above an optimum temperature, rate decreases

• As enzymes denature as bonds in 3D tertiary structure break

• So fewer E-S complexes form.

• 25 is optimum and 10-15 is too low.

Effect of light intensity on photosynthesis

• As light intensity increases, rate increases

• Because LDR increases → more Photoionisation of chlorophyll so more ATP and NADP produced.

• So light-independent reaction increases as more GP reduced to triosephosphate (within a given time) and more triosephosphate regenerates RuBP.

• Stops after a certain light intensity → another factor is limiting.

• Too high UV levels can damage chlorophyll.

Effect of CO2 on photosynthesis

• As CO2 increases, rate increases

• LIR increases as more CO2 combines with RuBP to form GP

• So more GP reduced to TP

• So more TP converted to organic substances and more RuBP regenerated.

• Above a certain CO2 concentration, rate stops increasing → another factor is limiting

• Plants work best at 0.4% (0.04 is atmosphere) and any higher can cause stomata to close.

Water Availability- less relevant

• Too much water can cause roots to become waterlogged → can’t respire → no ATP for active transport of mineral → chlorosis as not enough Mg is being absorbed to make chlorophyll → reduce photosynthesis

Agricultural practices

• For maximum photosynthesis and therefore plant growth, common agricultural practices incorporate techniques to remove limiting factors.

• Artificial lighting, heating a greenhouse and burning fuel (to release CO2).

• Extent of each technique needs to be considered in terms of profit, meaning profit from extra yield should be greater than costs in order of being cost-effective.

What is the aim of the chromatography practical?

• To investigate different pigments isolated from different leaves through chromatography.

What 5 pigments does chlorophyll typically consist of?

• Chlorophyll a- blue/green

• Chlorophyll b- yellow/green

• Carotene- orange

• Xanthophyll- yellow

• Phaeophytin - grey

• Each pigment absorbs a different wavelength of light and plants are adapted to their environment by having different proportions of each pigments to maximise light energy absorption.

Hypothesis for chromatography practical

• The combination of pigments and proportion of each pigment will differ in leaves from plants from different environments.

Why draw line in pencil and why place it 5mm higher than where the solvent will sit?

• So line doesn’t dissolve as it would with ink?

• So pigment isn’t washed off by solvent.

Why do we dry and add more pigment (3-5 mm)?

• So that when pigments dissolve and separate, they can still be seen.

Why do we use forceps to remove the paper, rather than fingers?

• To prevent irritation to skin

• So sebum doesn’t contaminate chromatogram

Why does the line representing the solvent front need to be drawn immediately?

• Because it will evaporate and no longer be visible → can’t calculate Rf value.

• Can also draw circle around pigment marks because colour can fade.

What is used to identify the pigments?

• Rf values

• Rf values are constant for each pigment dissolved in a particular solvent as it depends on the solubility of the pigments in that particular solvent.

Formula for Rf value

• Distant travelled by pigment/ Distance travelled by solvent

• Measure pigment from middle to standardise and allow fair comparison as pigments are usually quite spread out.

Conclusion of Chromatography practical

• Compare two chromatograms and the different Rf values to see if the different plant species contain the same pigments.

Why should you make sure the chromatography paper is vertical and straight?

• So the pigments move straight up the paper, to avoid them running off the side of the paper and being washed off.

Why was carbon-14 isotope used in the Calvin experiment?

• Because it is radioactive so the equipment used could trace the movement of carbon through the Calvin cycle, and visualised how it was distributed in the plant material.

• This is how the cycle was discovered and understood.

Why are certain pieces of equipment used?

• Funnel- to add algae

• Syringe- To inject radioactive carbon isotope (carbon dioxide)

• Rapid action tap- For taking samples at precise times and rapidly in quick succession (can open and close really quick)

• Hot methanol- To denature enzymes and stop reaction. Can analyse quantity of radioactive carbon isotope in all the different molecules.

• Flat lollipop flask- To increase surface area for light and to continuously supply CO2 to plant material while simultaneously removing the carbohydrate molecules that were produced.

Calvin Experiment: Why?

1. Isolation of chloroplast- link to T2.

2. Incorporation of CO2- CO2 injected into apparatus and left for set period of time under experimental conditions to allow CO2 to be fully incorporated into all the carbon containing compounds.

3. Perfusion of the chloroplast- Simultaneously inject carbon-14 while taking samples through rapid action tap.

4. Measurement of radioactivity- Autoradiography- Allows for quantitative measurement of radioactive substances in carbon containing compounds.

5. Analysis on next flashcard

Why is there a high amount of radioactive substances in GP in the light?

• Because it contains 6 carbons (more than RuBP)

What caused the amount of radioactively labelled glucose to decrease in the dark?

• Because the amount of TP is decreasing.

• This is because LDR has stopped so ATP and NADPH has stopped being produced, so LIR can’t continue and form glucose.

Why did GP levels rise in the dark?

• LDR stopped so less NADPH and ATP produced to reduce GP into TP.

• Build up of GP.

Why did RuBP levels decrease in the dark?

• Because GP stops being reduced as ATP and NADPH are no longer being produced.

• Therefore TP isn’t forming and RuBP can’t be regenerated.

• Initial RuBP does convert to 2 GP molecules so that increases (see previous flashcards).

• GP would stop increasing soon though when RuBP runs out.