PART 4: Photosynthesis

PART 4: Photosynthesis

1. Leaf: A flattened structure of a higher plant, typically green and bladelike, that is attached to a stem directly or via a stalk; photosynthesis occurs in the leaf.

• Cuticle: waxy covering on leaves produced by the upper epidermis to protect the leaf’s water from evaporation.
• Palisade parenchyma: a layer of cells just below the upper epidermis that contains lots of chloroplasts.
• Chloroplasts: organelles found only in plants that are the site of photosynthesis; chloroplasts absorb all light except green light; chloroplasts are made of two main parts: stroma and grana (which is made of thylakoids).
• Stroma: fluid-filled region of a chloroplast.
• Grana: stacked disc structures in chloroplasts.
• Thylakoids: the discs that make up the grana. It is within the membranes of thylakoids that the chemical process of photo respiration takes place.

• Spongy parenchyma: irregular shaped cells found below the palisade parenchyma that allows diffusion of gases within the leaf, especially CO2.
• Vein (Vascular Bundle): a string of tissue found amongst the spongy parenchyma that transports substances throughout the cell. It is made of xylem and phloem (both further discussed in Part 9).
• Lower epidermis: layer of cells, similar to the upper epidermis, at the bottom of the leaf organization. The difference is that there are holes that allow gas exchange and transpiration called stomates; guard cells surround the stomates and control their opening and closing.

2. Photosynthesis: as briefly explained in Part 3, it is the process by which light energy is converted to chemical energy. There are two stages of photosynthesis: light reactions (Vocab #3) and dark reactions (Vocab #4).

• Photons: the energy units of solar energy.
• Photophosphorylation: The production of ATP using the energy of sunlight. Cyclic photophosphorylation is when the energy cycles through using one photosystem (see Light Reactions), whereas noncyclic is when the energy travels through two photosystems to produce energy.

• Pigments: substances found in animal tissue, plant tissue and fluids that give it a distinct color.

3. Light reactions: the first of two major steps of photosynthesis. Light is absorbed by chlorophyll pigments, exciting their electrons; these electrons are then used to form ATPs from ADPs and phosphates, and to form NADPH from NADP+ (nearly identical electron carrier to NAD+(H) except that these are in chloroplasts). O2 is produced as a by-product because water is split during the previous reactions.

• Photosystems: light-capturing units that contain and organize the antenna pigments and reaction-centre complexes. There are two (note that the one labelled “II” is first because it’s first in the process but was discovered second):
• Photosystem II: the first photosystem of the light reactions; best absorbs light at 680 nm. It is here that water is split to form oxygen (noncyclic). This splitting is called photolysis. From here, electrons are sent down an electron transport chain (where some ATP is produced) towards the next photosystem.
• Photosystem I: the second photosystem of the light reactions; best absorbs light at 700 nm. It is here that NADPH is produced in noncyclic photophosphorylation. In cyclic, this photosystem works by itself, behaving like photosystem II.

• Antenna (accessory) pigments: a collection of pigments (clustered into a unit of the thylakoid called an antenna complex) that gather light for photosynthesis.
• Cartenoid: yellow or orange antenna pigments (the fact that they’re not green broadens the spectrum of colors that can power photosynthesis).
• Chlorophyll α [alpha]: a green antenna pigment that converts light energy into chemical energy.
• Chlorophyll β [beta]: a green antenna pigment that supplies energy to chlorophyll a.
• Reaction-center complex: complex of proteins associated with chlorophyll α molecules and a primary electron acceptor; this is where reactions take place (using the gathered light of the antenna pigments) that excite electrons, donating them to an electron carrier.

4. Dark reactions: reactions that are the products of the light reactions (ATP and NADPH) to make sugars. In the stroma of the chloroplast, CO2 from the air undergoes carbon fixation (conversion of carbon molecules into carbohydrates). There are three ways this can happen: C3, C4, and CAM.

• C3 Pathway (The Calvin Cycle): the system of chemical reactions used in the most common plants. It uses carbon dioxide and ribulose biphosphate (RuBP: 5-carbon molecule) to form an unstable 6-carbon molecule using the enzyme rubisco (RuBP carboxylase). This unstable molecule is split into PGA molecules, and then into G3P (base molecule of sugar) using ATP and NADPH from the light reactions. Only 1 for every 5 G3Ps is used for sugars – the others are recycled through the Calvin Cycle. The ATP and NADPH that was used in the reactions are now recycled back to the light reactions as ADP and NADP+. When C3 plants are exposed to too much bright light, the Calvin Cycle is altered by photorespiration.
• Photorespiration: when the Calvin Cycle is altered by excess bright light to fixate oxygen instead of CO2, which permanently makes the CO2 fixation less efficient.

• C4 Pathway: a more efficient series of reactions for carbon fixation that plants such as corn and sugar cane have adapted to in order to live in dry, hot climates. Carbon dioxide combines with PEP (phoenolpyruvate) using PEP carboxylase (enzyme). This forms oxaloacetate which is converted to maltate which later breaks down to pyruvate (which moves to mitochondria to produce more ATP) and CO2 (which continues on to the Calvin Cycle). Considered more efficient because it produces more products per CO2 molecule.
• CAM (crassulacean acid metabolism) photosynthesis: how plants in climates with extremely low water levels (i.e., deserts) photosynthesize. The stomates are closed during the day to prevent water loss from evaporation. Similarly to C4, PEP carboxylase fixes CO2 to oxaloacetate but then to malic acid to be stored in the central vacuole. This stores as much CO2 as possible while the stomates are open at night which, for the rest of the next day, will be used for photosynthesis through the Calvin Cycle after breaking the malic acid into CO2 again.

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