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Photosynthesis

Photosynthesis takes place in the chloroplasts, using light-harvesting pigments called chlorophyll

How glucose is used by the plant

The two main reactions of photosynthesis (pg 269)

The structure of the chloroplast (pg 269)

Thylakoid - stack of disc-like, double-membrane, structures, containing the light harvesting pigments embedded in the membrane Grana - stack of thylakoids Lamellae - membrane extensions that join up adjacent grana Stroma - the cytoplasm of the chloroplast, containing enzymes, DNA, 70S ribosomes, and starch and lipid granules (iodine test for starch)

Photosynthetic pigments

. Found in plants, algae and aquatic bacteria . Each pigment absorbs a fixed range of wavelengths . Having multiple pigments increases the wavelength of VISIBLE light absorbed . Pigments are arranged in photosystems

Photosystems

. a collection of pigments, called light-harvesting complexes (PSI and PSII) . All the light energy is funnelled to chlorophyll a, which absorbs the light and releases electrons . Each chlorophyll a contains Magnesium in the centre (link to magnesium-deficiency)

Light-Dependent Reaction (thylakoid membrane) pg 272

- Visible is absorbed by one of the many pigments in photosystem II, and light energy is passed inward from pigment to pigment until it reaches the reaction center. - Chlorophyll a in the reaction centre absorbs light energy and releases (two) electrons - photoionisation (Mg -> Mg2+ + 2e-) - The high-energy electron is passed to an electron carrier and replaced with an electron from water. - water is split in the process of photolysis, releasing electrons, H+ and O2

H2O -> 2H+ + ½ O2 + 2e-

- The high-energy electron travels down an electron transport chain, in a series of oxidation-reduction reactions, losing energy as it goes along the chain

- Some of the released energy drives pumping of H+ ions from the stroma into the thylakoid interior, building a gradient. H+ ions from the splitting of water also add to the gradient. - As H+ ions flow down their gradient and into the stroma, they pass through ATP synthase, driving ATP production in a process known as chemiosmosis. - Photosystem I also absorbs light energy and boosts an electron to a very high energy level, which is transferred to an electron carrier. The electrons are replaced by electrons from PSII (arriving via the electron transport chain). - The high-energy electron from PSI travels down a short second leg of the electron transport chain. - At the end of the chain, the electrons are passed to NADP+ to make NADPH (reduced NADP) (NADP+ reductase enzyme)

Calvin Cycle

- A CO2 molecule combines with a five-carbon acceptor molecule, ribulose-1,5-bisphosphate (RuBP). This step makes a six-carbon compound that splits into two molecules of a three-carbon compound, glycerate-3-Phosphate (GP). This reaction is catalyzed by the enzyme RuBP carboxylase/oxygenase, or rubisco. - Next, ATP and NADPH are used to reduce the GP into triose phosphate (TP), a three-carbon molecule. - Some TP molecules go to make glucose, while others must be recycled to regenerate the RuBP. Regeneration requires ATP. Six turns of the Calvin cycle are needed to make two (spare) TP molecule that can exit the cycle and go towards making glucose - uses 6CO2, 18 ATP, and 12 NADPH

Light compensation point (pg 276 -77, Fig 3)

The point at which the rates of photosynthesis and respiration are equal At light intensities below the light compensation point, the plant is starved because its rate of photosynthesis is less than its rate of respiration At light intensities above the light compensation point, the rate of photosynthesis is much higher than the rate of respiration. Thus, plants produce a great excess of glucose. Why do plants need to respire, if they produce ATP by photosynthesis?

How plants adapt to varying levels of light

Plants that are adapted to grow in the shade (i.e. forest floor) have a) lower maximum photosynthesis rate, b) lower light saturation range, and c) lower light compensation point

Rate of Light dependent vs Light Independent steps

At moderate light intensities, the "light" reaction limits the rate of PS With increasing light intensities, however, a point is eventually reached when the dark reaction is working at maximum capacity. Any further illumination is ineffective, and the process reaches a steady rate.

Effect of CO2 levels and temperature (pg 277, Fig 4)

In the atmosphere, the concentration of carbon dioxide ranges from 0.03 to 0.04 %. However, it is found that 0.1% of carbon dioxide in the atmosphere increases the rate of photosynthesis significantly. An optimum temperature ranging from 25oC to 35oC is required for a good rate of PS - between 0oC and 25oC, the rate of PS doubles for every 10oC rise in temperature

Measuring the Rate of Photosynthesis (pg 278)

Effect of Light on GP and TP levels (pg 280)- Draw graph

Calvin’s Lollipop experiment to determine the sequence

of reactions in the light-independent cycle (pg 279)