Photosynthesis: light-dependent stage

what two stages does photosynthesis take place?

• The light-dependent reactions which rely on light directly

• the light-independent reactions, which do not use light directly though do rely on the products of the light-dependent reactions

where do light dependent reactions take place

• both light dependent an dlight independent reactions take place in the chloroplast

• The light-dependent reactions take place across the thylakoid membrane

where do light-independent reactions take place

stroma

main function of light dependent stage of photosynthesis

• During the light-dependent reactions light energy is converted into chemical energy in the form of ATP and reduced NADP. ATP supplies energy to build carbohydrates. They provide the source of reducing power and energy respectively in the light-independent reactions of photosynthesis to make glucose

• to break up water molecules in a photochemical reaction, providing hydrogen ions to reduce carbon dioxide and produce carbohydrates in the light-independent stage

why light energy needed in light dependent reactions

• enables the splitting of water molecules in a reaction known as photolysis

what does photolysis of one molecule of water produce

• 2 hydrogen ions (protons)

• 2 electrons (2e^-)

• One atom of oxygen (O)

• The hydrogen ions and electrons are used during the light-dependent reactions while the oxygen is given off as a waste product

how ATP and NADPH produced

• ATP and NADPH are produced during the light-dependent reactions as a result of a series of events that occur on the thylakoid membrane known as photophosphorylation

  • Photo = light

  • Phosphorylation = the addition of phosphate; in this case to ADP to form ATP

• when a photon of light hits a chlorophyll molecule, the energy is transferred to the electrons of the chlorophyll molecule

• the electrons are excited and are raised to higher energy levels

• if an electron is raised to a sufficiently high energy level it leaves the chlorophyll molecule compeltely

• the excited electron is collected by a carrier molecule called an electron acceptor and this results in the synthesis of ATP by either cyclic photophosphorylation snd non-cyclic photophosphorylation

• both these process occur at same time and in both cases ATP is formed as the excited electron is transferred along an electron transport chain

what are the 2 types of photophosphorylation that take place?

• Non-cyclic photophosphorylation

  • This produces both ATP and NADPH

• Cyclic photophosphorylation

  • This produces ATP only

what do both cyclic and non cyclic photophosphorylation invo

what happens in non cyclic photosphorphorylation

• light energy hits photo system 11 in thylakoid membrane (PS 1)

• 2 Electrons are excited to a higher energy level 

• The excited electrons from PS 11 is collected by am electron acceptor and pass along the electron transport chain, releasing energy as they do so. Now the chlorophyll molecule in PS2 os moissing 1 electron and so is unstable

• At the same time photolysis occurs to trstore electron lost from chlorophyll molecule. Ions produced due to dissociation of water molecules given to chlorophyll molecule in PS2 so its restored to its original state and ready to be excited again when hit by another photon of light.

• The energy released as the electrons pass down the electron transport chain through electron carriers provides energy to drive the process of chemiosmosis

  • H­­⁺ ions are pumped from a low concentration in the stroma to a high concentration in the thylakoid space, generating a concentration gradient across the thylakoid membrane 

  • H­­⁺ ions diffuse back across the thylakoid membrane into the stroma via ATP synthase enzymes embedded in the membrane

  • The movement of H­­⁺ ions cause the ATP synthase enzyme to catalyse the production of ATP

• At the end of the electron transport chain the electrons from photosystem II are passed to photosystem I

• Light energy also hits photosystem I, exciting another pair of electrons which leave the photosystem

• The excited electrons from photosystem I also pass along an electron transport chain

• These electrons combine with hydrogen ions from the photolysis of water and the coenzyme NADP to form reduced NADP

H⁺ + 2e^- + NADP⁺ → NADPH

• The reduced NADP and the ATP pass to the light-independent reactions

what is NADP converted to

• NADP is a type of molecule called a coenzyme; its role is to transfer hydrogen from one molecule to another

• during non cyclic photos phosphorylation water molecules are broken down porividng H+ to reduce NADP

• When NADP gains hydrogen it is reduced, and can be known as either reduced NADP or NADPH

• The useful products of the light-dependent reactions, ATP and NADPH, are transferred to the light-independent reactions within the chloroplast

how many electrons need to be gained for production of 1 molecule of oxygen

• many hydrogen ions are removed by NADP and many hydroxide ions remain

• the hydroxide ions react together to form oxygen and water

• electrons are freed as a result of the reaction and are absorbed by chlorophyll 4 chlorophyll molecules regain electron in the production of one molecule of oxygen

• 4OH- -4e- (lost ot chlorophyll)—> O2 + 2H2O

what happens in cyclic photophosphorylation

• Involves only photosystem 1 and drives production of ATP

• when light hits a chlorophyll molecules in PS1 electros excited to a higher energy level and leave the photosystem

• it is collected by an electron acceptor and transferred directly along an electron transport chain releasing energy as they do so

• The energy released as the electrons pass down the electron transport chain provides energy to drive the process of chemiosmosis

  • H­­⁺ ions are pumped from a low concentration in the stroma to a high concentration in the thylakoid space, generating a concentration gradient across the thylakoid membrane 

  • H­­⁺ ions diffuse back across the thylakoid membrane into the stroma via ATP synthase enzymes embedded in the membrane

  • The movement of H­­⁺ ions cause the ATP synthase enzyme to catalyse the production of ATP

• At the end of the electron transport chain the electrons rejoin photosystem I in a complete cycle; hence the term cyclic photophosphorylation

• The ATP produced enters the light-independent reaction

explain role of light in light dependent reactions in photosyntehsis

• because {it / light} is needed for photolysis (1)

• because {electrons released (from the water) are needed to replace those lost from the photosystems / hydrogen ions are used to reduce NADP} (1)

• because {it / light} is needed to {excite / release} the electrons of the {photosystems / chlorophyll / PS / photosynthetic pigments} (1) • electrons (from photosystems) used to produce ATP (and reduced NADP) (1)