Week 12 - Exam 3

Describe "light" reactions (the reactions that need light)?

protons move from the stroma to the thylakoid lumen

Summarize the reactions of the Calvin cycle (the three phases)? What changes occur in the carbon skeletons of the intermediates?

1. carbon fixation 3 CO2 and 3 RuBP react with the help of Rubisco (an enzyme) to make 2 3-carbon intermediates 2. reduction the 3-carbon intermediates get phosphorylated and reduced using the products of the energy transduction phase/light reactions in order to make triose phosphate products (G3P and DHAP) 3. regeneration of RuBP 1 of the triose phosphates leaves the cycle, the other 5 stay behind to regenerate RuBP using ATP

What are the two major events in photosynthesis?

1. energy transduction reactions aka light reactions -take light and use it to move electrons that come from H2O in order to produce ATP and NADPH, and release O2 2. carbon assimilation reactions aka dark reactions -fix inorganic CO2 into organic carbohydrates (food) using the products of the light reactions

What is the role of ATP and NADPH in the Calvin cycle?

ATP is used to phosphorylate the 3 carbon intermediates that are made from the 3 RuBP and 3 CO2 molecules and NADPH gets oxidized so the intermediates can get reduced

Describe the process of photophosphorylation? What drives the process?

As electrons spontaneously flow through the Z-scheme, release free energy, which is coupled to the transport of H+ against their gradient across the membrane through Cytb6/f from the stroma to the thylakoid lumen. Then the H+ gradient can be used to drive ATP synthesis. The process is driven by the free energy released from electron transport which creates an electrochemical H+ gradient across the thylakoid membrane between the lumen and the stroma

What is chemiosmosis?

Chemiosmosis is the movement of ions across a selectively permeable membrane, down their electrochemical gradient

Compare and contrast chemiosmosis during oxidative phosphorylation in mitochondria and photophosphorylation in chloroplasts?

For oxidative phosphorylation in mitochondria chemiosmosis involves the movement of H+ ions down their gradient from the intermembrane space through the ATP synthase complex into the mitochondrial matrix For photophosphorylation in chloroplasts chemiosmosis involves the movement of H+ ions down their gradient from the thylakoid lumen into the stroma through the ATP synthase complex

Compare and contrast electron transport during aerobic respiration and photosynthesis?

In the mitochondria the flow of electrons is driven by electronegativity differences from less electronegative to more electronegative proteins in the ETC. Everything flows spontaneously. In photosynthesis, energy needs to be added in the middle of the electron transport process. (1) electrons begin at P680 (2) sunlight photoexcites the electron at P680 (3) the PSII electron spontaneously flows to the acceptor molecule which is now comparatively more electronegative (4) the electron keeps flowing to more electronegative proteins until it gets to P700 (5) the electron at P700 gets excited again and then spontaneously flows to the acceptor molecule which is now comparatively more electronegative (6) the electron keeps flowing to more electronegative proteins until it gets to electron acceptor NADP+, which becomes NADPH The electron void left by the electrons moving through the chain are replaced by electrons from H2O, which gets oxidized to release H+ and O2 as a byproduct.

Why isn't it accurate to refer to the Calvin cycle as the "dark" reactions?

It's not accurate to refer to the calvin cycle as dark reactions because it doesn't directly require light, but it needs the products of light reactions to happen. Both processes need to happen simultaneously

Describe the structure and function of a leaf? What are the stomata and guard cells? What is their structure and function?

Leaves have an upper and lower epidermis with pores called stomata, which are surrounded by guard cells. The guard cells open and close the pores/stomata to exchange O2 and CO2 with the environment. This lets CO2 enter the calvin cycle.

What is more electronegative: oxygen or P680?

P680 is more electronegative otherwise it wouldn't be able to replace its protons by taking H+ from the oxygen

How are photoreduction, photophosphorylation, and carbon fixation involved in photosynthesis? Where in the cell do each of these events occur?

Photophosphorylation = using H+ gradient to synthesize ATP, where H+ gradient is created by coupling H+ movement with exergonic movement of electrons that are helped by photons Carbon fixation = reacting CO2 and RuBP to form 3-carbon intermediates Photoreduction = reducing the 3 carbon intermediates and using the ATP from photophosphorylation to phosphorylate the 3-carbon intermediates of carbon fixation to make triose phosphates G3P/DHAP

What four different factors influence/regulate the Calvin cycle? Why are they important?

The presence of light and changes in pH regulate the Calvin cycle. So as the energy transduction/light reaction phase occurs, electrons coupled to energonic movement of H+ against gradient from stroma to thylakoid lumen, which causes the pH of the stroma to increase. The calvin cycle happens in the stroma, and the required enzymes are activated by a high pH (low H+ concentration). So the Calvin cycle will only take place when there is light. So light -> electron movement -> H+ gradient -> creates ATP and NADPH + higher pH -> activate calvin cycle enzymes and provide energy for Calvin cycle Also, redox status of the calvin cycle enzymes. When there's light, electrons can be transported to the enzymes to reduce them and activate them. If no light, the enzymes remain oxidized and inactive. Also, genetic regulation

Where in the cell does carbon assimilation take place? What is carbon assimilation a part of?

aIn the stroma carbon assimilation is a part of the calvin cycle