Photosynthesis

• What is photosynthesis?


  The process where light from the Sun is harvested and used to drive the production of chemicals, including ATP, and used to synthesise large organic molecules from inorganic molecules.
• What is the need for cellular respiration?


  Organisms have to grow, respond to changes in the environment. They have to find or make food and reproduce. All this activity depends on metabolic reactions and processes continually taking place in individual cells. Examples of these metabolic reactions include:

  -> Active transport

  -> Anabolic reactions

  -> Movement brought about by cilia/flagella
• What is the interrelationship between the processes of photosynthesis and respiration?

  
• What is the structure of chloroplasts?


  --> Surrounded by a double membrane (envelope)

  --> The intermembrane space (the space between the inner and outer membrane membrane - 10nm - 20nm wide)

  --> The outer membrane is permeable to small ions

  --> The inner membrane is selectively permeable but has transporter proteins embedded - and these are folded into lamellae.

  --> The membranes (lamellae) form flattened sacs called thylakoids. Thylakoids within a granum are connected to other thylakoids within another granum by intergranal lamellae

  --> Each stack of lamellae is called a granum (plural - grana)

  --> The fluid enclosed in the chloroplast is called the stroma

  The chloroplast also contains starch grains (a storage polysaccharide), ribosomes (site of protein synthesis), circular DNA (codes for enzymes, for example, rubisco)

  -> Chloroplasts have a biconvex shape which increases surface area

  
• What are the 2 sites of photosynthesis?


  Stroma -> Light-independent reaction (Calvin Cycle)

  Granum -> Light-dependent reaction

  - The grana is surrounded by the stroma, so the products of the light-dependent stage can easily pass into the stroma to be used in the light-independent stage

  
• What are the products of the light-dependent reaction?


  ATP

  Reduced NADP

  O2

  
• What are the products of the light-independent reaction?


  Glucose

  NADP

  ATP

  
• What are photosystems?


  Funnel-shaped structures that contain photosynthetic pigments

  --> The light harvesting system and the reaction centre make of photosystems

  
• What are photosynthetic pigments?


  Pigments that absorb specific wavelengths of light and traps the energy associated with light. Examples include chlorophyll a, b, carotene and xanthophyll --> Chlorophyll is a mixture of pigments

  
• What forms the light harvesting system?


  Photosynthetic pigments (e.g., chlorophyll b, xanthophyll) embedded in the thylakoid membrane
• What is the light harvesting system?


  Light energy of different wavelengths absorbed and this energy transferred to the reaction centre

  
• What is the reaction centre?


  Contains chlorophyll a and is where light-dependent reactions occur.

  
• What are the 2 forms of chlorophyll a (primary pigments)?


  P680- Photosystem II - peak of absorption is light of wavelength 680nm

  P700 (Photosystem I) - peak of absorption is light of wavelength 700nm
• What are accessory pigments?


  Other forms of chlorophyll a, chlorophyll b, carotenoids, xanthophylls

  --> Accessory pigments allow a broader range of wavelengths to be absorbed (more energy is captured from sunlight)

  
• PAG: Using thin layer chromatography (TLC) to separate photosynthetic pigments


  Photosynthetic pigments in chlorophyll can be easily separated by scraping/mashing a green leaf (using two microscope slides, for example).

  Spot the chlorophyll onto a slide coated with thin layer chromatography (TLC) material. When a solvent creeps up the slide, the pigments separate out because they have different Rf values

  
• How to calculate Rf value?

  
• Light-dependent reaction (overview)


  -> Takes place in thylakoid membranes (grana)

  -> Requires a continuous supply of light energy

  -> Chlorophylls absorb this light energy, which is converted into chemical energy through the formation of two compounds - ATP and reduced NADP

  -> Purpose: Harvest light energy to split water and to create ATP and reduced NADP

  
• What is photophosphorylation?


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

  
• What are the 4 key stages in light-dependent reaction?


  1) Non-cyclic photophosphorylation

  2) Cyclic photophosphorylation

  3) Photolysis

  4) Chemiosmosis

  
• Non-cyclic photophosphorylation


  1) A photon of light strikes PSII (P680) and its energy is channelled to the primary pigment reaction centre. The light energy excites a pair of electrons inside the chlorophyll molecule. The energised electrons escape from the pigment and are captured by an electron carrier (a protein with iron at its centre). The electron carriers are alternately reduced (as they gain an electron) and oxidised (as they lose an electron by passing it onto the next carrier), gradually release their energy as they pass through the electron transport chain. The electrons from PSII replace the lost electrons from PSI.

  2) Light is used to split water into electrons, protons and oxygen: 2H20 --> 4H+ + 4e- + 02. The electrons replace those lost from PSII during photoactivation. The protons are used for ATP production and combine with electrons to reduce NADP and oxygen is released as a by-product

  3) The energy lost by electrons along the electron transport chain is used to pump protons across the thylakoid membrane into the thylakoid space. This produces a proton gradient (higher concentration of protons in the thylakoid space). The protons then diffuse through ATP synthase into the stroma and this movement powers ATP synthase to produce ATP from ADP and an inorganic phosphate (Pi) - this process is known as chemiosmosis. NADP takes up protons and electrons from PSI in the stroma and is reduced

  Reduced NADP is carried into the light-independent reaction

  
• What is chemiosmosis?*


  Involves the diffusion of hydrogen ions from a region of high concentration to a region of low concentration through a partially permeable membrane to produce ATP.

  --> This is done by facilitated diffusion.

  --> The energy that is released as protons flow down their gradient and is used in the attachment of ADP to an inorganic phosphate

  
• The role of water (Photolysis)**


  PSII contains an enzyme that, in the presence of light, can split water into H+ ions (protons), electrons and oxygen

  2H20 --> 4H+ + 4e- + O2.

  Water is the:

  -> source of protons (hydrogen ions) that will be used in photophosphorylation

  -> donates electrons to chlorophyll to replace those lost when light strikes chlorophyll
• Cyclic Photophosphorylation


  Light energy is absorbed by primary pigment reaction centre and excites electrons in the pigments in PSI only. Electrons are lost from the pigments of PSI and are transferred to an electron carrier molecule and passed along an electron transport chain, releasing energy. This energy powers active proton transport across the thylakoid membrane from the stroma into the thylakoid space.

  ATP is produced as protons flow back into the stroma through ATP synthase. As electrons are returned back to PSI, NADP is not reduced

  
• Light-independent stage (overview)


  -> Takes place in the stroma

  -> Does not directly require light

  -> Calvin cycle uses ATP and reduced NADP from the light-dependent reaction to fix carbon dioxide and eventually form glucose

  
• Fixation of carbon dioxide


  The fixation of carbon dioxide in the stroma maintains a concentration gradient that aids diffusion
• The Calvin Cycle (Light-independent stage)


  1) Carbon dioxide reaches the stroma via diffusion.

  2) Carbon dioxide combines with 5-carbon compound (RuBP) which is catalysed by the enzyme RuBisCo to form an unstable 6-carbon compound

  3) The unstable 6-carbon compound splits into two 3C GP molecules (glycerate-3-phosphate)

  4) GP is reduced and phosphorylated using reduced NADP and ATP from the light-dependent stage.

  5) This forms triose phosphate (TP)

  6) 5/6 molecules of TP are then recycled to produce more RuBP (process also uses ATP)

  7) TP can be used to produce more glucose (which can then be polymerised to produce starch/cellulose or isomerised to fructose/sucrose

  
• What are the uses of triose phosphate (TP)?


  TP can be used to synthesise organic compounds, for example:

  --> Some glucose is converted to sucrose, some to starch and some to cellulose

  --> Some TP is used to synthesise amino acids, fatty acids and glycerol

  
• What are the factors affecting photosynthesis?


  Light intensity

  Carbon dioxide concentration

  Temperature

  
• Light intensity


  As light intensity increases, ATP and reduced NADP are produced at a higher rate

  
• What happens when there is little/no light (low light intensity)?


  GP cannot be reduced to TP

  TP levels fall and GP accumulates

  TP levels falling means that RuBP cannot be regenerated
• Carbon dioxide concentration


  Increasing carbon dioxide concentration, increases the rate of carbon fixation in the Calvin cycle and therefore the rate of GP + TP production (increases rate of photosynthesis)

  
• What happens when carbon dioxide concentration is low?


  Less carbon dioxide is fixed and RuBP accumulates

  GP cannot be made and therefore TP cannot be made either
• Temperature


  An increase in temperature increases kinetic energy supplied to enzyme-controlled reactions in photosynthesis (such as carbon fixation) - this is around 25-30°
• What happens when the temperature is too high (30-45°)?


  Enzymes involved in photosynthesis denature. This would reduce the concentration of GP and TP and eventually RuBP as it would not regenerated due to the lack of TP.
• Water stress


  If a plant has sufficient water in the soil, the transpiration stream has a cooling effect on the plant. It also keeps plant cells turgid so they can function. Turgid guard cells keep the stomata open for gaseous exchange
• What if there isn't enough water available to the plant (water stress)?

  Roots are unable to take up enough water to replace that lost via transpiration

  Cells lose water and become plasmolysed

  Tissues become flaccid and wilt

  The rate of photosynthesis greatly reduces