Chroloplasts By Tibor Cemicky

What are chroloplasts? Chloroplasts are organelles found in plant cells and other eukaryotic organisms that conduct photosynthesis. Chloroplasts capture light energy to conserve free energy in the form of ATP and reduce NADP to NADPH through a set of processes called photosynthesis.

Structure of Chroloplasts Outer Membrane: holds the structure together, freely permeable to molecules Stroma: complex mix of enzymes and water, important for the Calvin cycle, contains naked DNA and ribosomes Inner membrane: contains many transporters: (integral membrane proteins) that regulate the passage in an out of the chloroplast of small molecules like sugars and proteins Intermembrane space: oxidative phosphorylation Granum (pl, grana): Stack of thylakoids Thylakoid: They are the site of the light-dependent reactions of photosynthesis Lamella: acts like the skeleton of the chloroplast, keeps all of the grana a safe distance from each other Lumen: produces ATP using the energy from sunlight during process called called photophosphorylation

Limiting Factors of Photosynthesis Limiting factor is one which is nearest to the minimum and therefore limits the rate of the reaction. Even if the other 2 factors increase 10x the rate of photosynthesis will stay the same because of the limiting factor. There are 3 main limiting factors in photosynthesis: –Light Intensity –CO2 concentration –Temparature

Limiting Factor 1: Light Intensity At low light intensity there is a shortage of the products of the light-dependent reactions: ATP and NADPH. The rate-limiting step in the Calvin cycle is the point where glycerate 3-phosphate is reduced. Light intensity is not ussually the limiting factor unless the plant is heavily shaded or the sun is rising or setting.

Limiting Factor 2: Co2 Concentration When concentration of co2 is low the rate- limiting step in the Calvin cycle is the point where co2 is fixed produce to produce glycerate 3-phosphate. RuBP and NADPH start to accumulate in the plant. Low concentration of co2 is often the limiting factor because the level of co2 in the atmosphere is never very high.

Limiting Factor 3: Temperature When temperature is low all the enzymes which catalyze the reactions of the Calvin cycle work slowly causing the rate of photosynthesis to decrease. When temperature is high, RuBP carboxylase does not work effectively, so the rate-limiting step in the Calvin cycle is the point where co2 is fixed. NADPH accumulates in both cases

Cyclic Photophosphorylation When light is not the limiting factor NADPH tends to accumulate in the stroma and there is a shortage of NADP+. The normal flow of electrons in the thylakoid membrane is inhibited because NADP+ is needed as final acceptor of electrons. An alternative route can be used that allows ATP production even when NADP+ is not availible. This pathway is called cyclic photophosphorylation

Process of Cyclic Photophosphorylation 1.Photosystem I absorbs light and is photoactivated 2.Excited electrons are passed from photosystem I to a carrier in the chain between photosystem II and photosystem I 3.The electrons pass along the chain of carriers back to photosystem I 4.The electron flow causes pumping of protons across the thylakoid membrane 5.A proton gradient is formed and this allows production of ATP through ATP synthase