Questionthank you so much for your notes on photosynthesis, you're a life saver!! xx Answer

Oh you’re welcome!! I’m glad someone got some use from it

Tropism is directional growth in response to stimuli. Most commonly used by plants in order to maintain a position most favourable for survival.

Like Taxis, Tropisms can be positive (towards a stimulus) or negative (away from a stimulus)


The most common forms of tropism are

  • Phototropism - growth in response to light intensity (for example, plants leaves show positive phototropism as they grow their leaves to face the highest light intensities)
  • Geotropism - growth in response to gravity (for example, the roots of a plant show positive geotrpism as they grow downwards)
  • Hydrotropism - growth in response to amount of water available (for example, plant roots show positive hydrotropism as they grow in the direction of water)

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This will probably be updated at some point

PPQ, MS & Spec

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Kinesis is a non-directional movement in a response to stimuli. This means that unlike taxis, kinesis cannot be positive (towards stimuli) or negative (away from stimuli) but instead is random in order to increase the organisms chances of survival.

Like Taxis, Kinesis also occurs in different forms such as photokineses, geokinesis and chemokinesis; for example…

Photokinesis light sensitive kinesis (for example, cockroachs show photokinesis when exposed to light as they scatter in random directions)

The more intense the stimulus is the more rapid the movement and the frequency of changes in direction increases.

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(^^^that diagram is not mine or anything, I just found it on the internet)

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PPQ, MS & Spec

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Taxis is directional movement in response to stimuli. Although more common in invertebrates, even some bacteria and protoctists show taxes.

Positive taxis is directional movement towards the stimuli
Negative taxis is directional movement away from the stimuli

Some different types of Taxis are…

  • Chemotaxis -chemically generated taxis (for example, phagocitic white blood cells show positive chemotaxis for a pathogen as they move towards it)
  • Phototaxis light sensitive taxis (for example, moths show positive chemotaxis as they move towards light)
  • Geotaxis - taxis in repsonse to gravity (for example, earthworms show positive geotaxis as they burrow down into the soil)
  • Rheotaxis - taxes in response to movement (for example, salmon use positive rheotaxis to swim upstream as they are swimming with the water)

Taxis is a protective/survival response to stimuli & changes in the environment that increases the chances of the organisms survival

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PPQs, MS & Spec

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Light Dependent reactions of photosynthesis

Light Dependent Reactions of Photosynthesis animation

Photosystem II

(This video goes into more detail than necessarily needed for AQA Biology Unit 4 but it’s still good to see)

So, first of all a photon of light hits a chlorophyll that is embedded within Photosystem II in the Thylakoid Membrane of a Chloroplast

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(Please excuse my awful drawings, I try)

This excites electrons in a chlorophyll to a higher energy level. The photosystem is oxidized (Remember OILRIG: Oxidation is loss, reduction is gain…of electrons).The charge separation between the electrons and PSII also drives the photolysis of water over at the Oxygen Evolving Complex, but as water is a very stable molecule it takes 4 photons to split it! The word equation for this reaction looks like this…

                                              (photolysis)
                                   2(H2O)          ——>       O2 + 4H+ + 4e-

What happens to the products of photolysis?
Well, Oxygen at this stage is a biproduct as we don’t need it and so it simply out of the chloroplast, out of the cell (if not used for respiration) and eventually out of the plant completely and into the air. The protons (H+) however, build up in the Thylakoid Lumen and contributes to a proton gradient. Whereas the electrons are used to reduce the chlorophyll and replace the excited electrons so that the photosystem is ready for another photon to oxidize it again and so the cycles continues.

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An electron acceptor is reduced as it accepts the excited electron from Photosystem II

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The Electron Acceptor is re-oxidized as it enters The Electron Transport Chain (a chain of proteins in the Thylakoid Membrane …shock horror) and a series of REDOX reactions occurs. As the electrons move down the ETC energy is released which is used to pump protons from the stroma and into the lumen (protons from the photolysis of water remain in the lumen) creating a high concentration of protons in the Thylakoid Lumen.

Photophosphorylation
The high concentration of protons in the Thylakoid Lumen causes the H+ to move down a concentration gradient through the Thylakoid Membrane via ATPsynthase. This provides the energy for the photophosphorylation of ADP into ATP.
The yield of ATP is not as high as Oxidative Phosphorylation during Respiration.

Note: Pi = Inorganic Phosphate.

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Anyway! Back to the electrons

Once the electrons have completed the series of REDOX reactions they replace an electron (that has been excited to a higher energy level by a photon) in Photosystem I.
The rest is alot similar to what went on in Photosystem II

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The excited electron then reduces another Electron Acceptor

The electron acceptor is then oxidized as the electrons are passed onto another Electron Transport Chain

This time, instead of being passed onto another Photosystem, the electrons recombine with protons in the stroma to reduce NADP to form NADPH - the reducing power for the Light Independent Reactions

The whole point of the Light Dependent Reactions is to produce NADPH which are the reducing agents for The Calvin Cycle

Before we kick off this whole Photosynthesis thing we just need to clarify what exactly this mess is

Photosystem II laaa

This bad boy isn’t as complicated as he looks, he’s a Photosystem (Photsystem II to be exact) and basically it’s just a collection or pigments and proteins arranged in a certain way. The green stringy things you see in this diagram are chlorophyll.

In the Light Dependent reactions of Photosynthesis there are two of these photosystemy things involved - Photosystem II and Photosystem I (imaginative names), both of which are located in the Thylakoid Membrane of a chloroplast

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Photosystem II also contains what is known as the Oxygen Evolving Complex which does just what it says on the tin - produces Oxygen, Protons & and Electrons from the photolysis of water molecules.

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As we know Photosynthesis is basically just backwards respiration so the word equation would look like this

                                            (light energy)
Water + Carbon Dioxide           ——>        Glucose + Oxygen



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And this happens in two stages; the Light Dependent and Light Independent reactions.

The purpose of the Light Dependent reactions is to produce Reduced NADP (NADPH) and ATP with Oxygen as the biproduct and all this takes place in the thylakoid membrane of a chloroplast.

Although The Light Independent reactions (The Calvin Cycle) don’t need light themselves they cannot continue in the absence of it because they need ATP and NADPH which are the products of the Light Dependent reactions. This stage takes place solely within the stroma of the chloroplast and involves a series of redox reactions ultimately forming glucose.