5.6. Photosynthesis

• ATP is the form in which energy is useful to living organisms. Respiration produces this when needed
• Bond energy- energy is used to break bonds and is released when bonds are made. The same quantity of energy is involved whether bonds are formed or broken. All bonds formed and broken have to be taken into account in an overall reaction.
• ATP releases energy when it is hydrolysed to form ADP and Pi
• Photosynthesis equation- 6CO2 + 6H20 —> C6H1206 + 602
• What is glucose?

  a complex, organic molecule
• Chloroplasts- contain thylakoids, granum, lamellae, inter-membrane space, starch grains, lipid droplet, stroma
• The fluid filled matrix inside the chloroplasts is called the stroma, and the grana are stacks of thylakoid membranes
• The masses of folded membranes within the chloroplasts provide a large surface area
• Pigments are substances that can absorb some wavelengths of light. The wavelengths not absorbed are reflected/transmitted through the substance. Unabsorbed wavelengths reach our eyes, so we see these colours
• Photosynthetic pigments: chlorophyll an and b absorb slightly different wavelengths, but mainly in the red/orange and blue/purple regions. Carotenoids absorb mainly the shorter wavelength area (blue/purple/green). Xantophylls are yellow in colour, so mainly absorb blue
• Chlorophyll a is the primary pigment, found in the primary reaction centre. All the other pigments are referred to as accessory pigments as their role is to absorb the light and pass the energy from one pigment to another. This energy ultimately enters the primary reaction centre
• Pigment molecules are embedded in the thylakoid membranes
• A range of pigments are needed so that a large amount of energy is absorbed for photosynthesis
• Within the thylakoid membranes are pigments, arranged in complexes with other proteins called photosystems.
• There are two types of photosystem (I) and (II). Photosystem II is found predominantly in the thylakoid stacks and between the stacks and inter-granal lamellae which contain photosystem I.
• The light dependant reaction in photosynthesis occurs on the thylakoid membranes. The photosystems contain pigments to absorb light energy so that it can be converted to chemical energy in the form of ATP. Photosystems are thought of as funnel-shaped structures held in place by proteins.
• The light-dependent stage of photosynthesis occurs in the grana (thylakoids) of chloroplasts and involves the photosystems. it involves the direct use of light energy.
• The light-dependent stage consists of: 1. Light harvesting at the photosystems 2. Photolysis of water 3. Photophosphorylation- the production of ATP in the presence of light 4. The formation of reduced NADP (Oxygen, the by-product of photosynthesis, is also produced in the light-dependent stage)
• What is grana?

  Stacks of thylakoids that contain photosynthetic pigments
• Describe what happens to protons that have moved through ATP synthase?

  They are accepted along with electrons from the electron transport chain, by NADP. This is facilitated by NADP reductase (NADP has been reduced to NADPH)
• What is photolysis and where exactly does it take place?

  The enzyme-catalysed splitting of water molecules, in the presence of light. It takes place in PSII on thylakoid membranes of chloroplasts
• Explain why chloroplasts in guard cells only contain PSI?

  Produces ATP which actively pumps potassium ions into the cell, lowering the water potential so water follows by osmosis. This causes the guard cell to become turgid and the stomata to open
• Describe the structure of a photosystem

  A light-harvesting complex that channels light towards a reaction centre (containing a primary pigment called chlorophyll a); accessory pigments channel photons towards the reaction centre
• Outline the role of water in photosynthesis.

  Electron donor; source of protons/hydrogen ions; source of by-product, oxygen; keeps cells turgid so they can function (all metabolic reactions need to be in solution)
• Name the primary and accessory pigments?


  Primary: Chlorophyll a Accessory: Carotenoids, xanthophylls, chlorophyll b
• Describe the differences between non-cyclic and cyclic photophosphorylation.

  NC- involves PSI and PSII; produces ATP, oxygen and reduced NADP (NADP is the final electron acceptor). C- involves PSI only; produces ATP only as electrons are recycled back to PSI and NADP is not reduced
• Describe the differences between photosystem I and photosystem II?

  PSI contains chlorophyll a that absorbs light at a peak of 700nm (P700). PSII contains chlorophyll a that absorbs light at a peak of 680nm. Electrons are excited to a higher energy level in PSI than PSII
• What is the structure that connects grana?

  Intergranal lamella
• Chemiosmosis is the movement (diffusion) of protons across ATP synthase down the electrochemical gradient to generate ATP
• Ferrodoxin is also known as NADP reductase.  
• The electron carrier proteins (electron transport chain) are proteins containing iron (Fe) which are reduced and oxidised as the electrons move through them (we call these redox reactions).
• What pumps the hydrogen ions into the thylakoid space?

  A cytochrome complex (electron carrier)
• What does the Benedict's test test for?

  Reducing sugar
• During photosynthesis, what is pumped across the membrane and into the thylakoid space of the chloroplast?

  Hydrogen ions/ protons
• Rf values differ for different pigments according to the solvent used. Why is this the case?

  The molecules of pigments will have different solubilities in different solvents
• In chemiosmosis, where does the energy to pump protons come from?

  An electron passing down the first electron transport chain in the light-dependent reaction gives energy.
• How can we test for starch?

  Iodine solution
• Describe how a proton gradient develops between the thylakoid lumen and the stroma.

  Energy from electrons moving through a chain of electron carriers is used to actively pump protons across the thylakoid membrane into the thylakoid lumen.
• Suggest why a lack of iron in soil may reduce growth in plants.

  Iron is needed for electron carriers in the chain (on thylakoid membranes) and for ferredoxin. A reduction of electron carriers could reduce the rate of photosynthesis.
• Is photosynthesis exothermic or endothermic?

  Endothermic, as the overall reaction takes in energy in the form of light