Topic 9 [HL]: Plants

Task One

• State the function of photosystem II, photosystem I and the calvin cycle in the production of glucose and sucrose.
• Photosynthesis is a two step process with light dependent and independent reactions. Light dependent reactions convert light energy to chemical energy and light independent reactions use the chemical energy to make organic molecules. 
• Light dependent reactions occur on thylakoid membrane and may occur via cyclic or non-cyclic processes; both processes include light exciting chlorophyll leading to release of electrons that pass through an electron transport chain, creating ATP (photophosphorylation)
• Non-Cyclic: Chlorophyll in photosystems 1 and 2 absorbs light, leading to photoactivation: a release of high energy electrons. Electrons from photosystem 2 go through ETC, producing ATP via chemiosmosis. Electrons from photosystem 1 reduce NADP+ to NADPH + H+. Electrons from photosystem 2 replace those lost in photosystem 1, electrons lost in photosystem 2 replaced by electrons generated from photolysis of water. Oxygen is by-product.
• Cyclic (only photosystem 1 involved): High energy electrons released by photoactivation pass through ETC (producing ATP) and then return to photosystem 1. Electrons in cyclic photophosphorylation do not produce NADPH + H+, which is required for light independent reactions. Thus, though cyclic phosphorylation constantly creates ATP, the chemical energy cannot be used to create organic molecules.
• Light independent reactions occur in the stroma and uses the ATP and NADPH + H+ from non-cyclic photophosphorylation. It is also known as the Calvin cycle and has three main steps:
1. Carbon Fixation: 

   Enzyme rubisco (RuBP carboxylase) catalyzes attachment of carbon dioxide to 5-carbon compound ribulose bisphosphate (RuBP). The new 6-carbon compound breaks down to two 3-carbon molecules: glycerinate-3-phosphate (G3P)

2. Reduction: 

   Each G3P molecule is phosphorylated by ATP and reduced by NADPH + H+, converting them to glyceraldehyde phosphate, a trios phosphate (TP)

3. Regeneration of RuBP: 

   For every 6 TP, only one can be used to create half a sugar molecule. The other 5 TP molecules are reorganized to regenerate RuBP (using ATP). The cycle repeats multiple times to construct chains of sugars (sucrose)

• Draw the molecular structures of glucose, fructose and sucrose. 

• Distinguish between the structure and function of the three polysaccharides of glucose. 
• glycogen- multiple branches, animal storage of glucose
• cellulose- branched and unbranched varieties, main structural component of plants
• starch- branched, plant storage glucose

• Compare the leaf and root structure and vascular tissue organization in stem and roots of monocotyledonous and dicotyledonous plants. [State which sugar cane is.]
• Leaves: Monocots have parallel veins from stem to tip; dicots have branching veins
• Root structure: Monocots do not have main roots but have smaller roots that branch out; dicots have main roots that grow downwards
• Vascular tissue organization: Monocots have vascular bundles spread across the stem, with bundles closer to the sides of the stem rather than the center; dicots have vascular bundles circular 

Task Two

• State the meaning of 'allocation of photo-assimilates'.
• The allocation/grouping of a certain amount of compounds formed through assimilation in light dependent reactions (monosaccharides) from a plants leaves to its sinks

• Explain what the author means by the phrase 'heterotrophic sinks'. 
• The areas of a plant in which sucrose is stored: fruits, roots, etc.

• Describe the structure and function of a plasmodesmata between the mesophyll and parenchyma cells. 
• Plasmodesmata are microscopic communication bridges between the cell walls of two plant cells; they facilitate the movement of sucrose from mesophyll cells to parenchyma cells

• Hypothesize the effect of a pH or temperature change on plasmodesmata structure and function. [Hypotheses must include a why!]
• A change in pH or temperature would denature proteins that make up the plasmodesmata, which would probably prevent it from functioning efficiently

• Distinguish the structure and function of xylem and phloem.
• Xylem vs. Phloem:
• Structure: Non-living vs living; thin walled sieve tubes w/ pores at ends vs tubes with no cross walls
• Function: Transports nutrients from leaves to sinks/growing parts vs water/minerals from roots to aerial parts (leaves)

Task Three

• Distinguish between, in your own words, the apoplastic route and symplastic route of ion movement in plants. 
• Apoplastic route: through the cell wall
• Symplastic route: through the cytoplasm (via plasmodesmata)

• Draw the pathway of sucrose from mesophyll cell [site of production], to the SE/CC. 

• Draw and annotate a diagram to describe the pressure flow model of water and sugar transport in xylem and phloem. [direction and substance are a must!] [you may add onto the previous drawing if you like].

• Annotate your drawing to distinguish between the different transport mechanisms [diffusion, osmosis, facilitative diffusion, primary active transport, secondary active transport ex. SUT1 ]  that occurs in the xylem/phloem pressure flow model of sucrose and water transport.