Cell Energetics Flashcards
photosynthesis
Requires water (H2O), carbon dioxide (CO2) and light.
Roots of plants absorb water and transport it to the leaves through xylem, CO2 enters the plant (while O2 exits) through microscopic pores in leaves called stomata, and individual photons are absorbed from the sun in the plant by the pigment chlorophyll.
**It is important that O2 levels remain low (explained later) for the Calvin Cycle (Stage 2)
Inside the cell of plants are plastids called chloroplasts, which contain thylakoids (which store chlorophyll), which contain lumen inside and Stroma outside; stacks of thylakoids are called grana.
Light Dependent Reactions (Stage 1)
PS II (wavelength p680)
-Photons strike the chlorophyll in Photosystem 2 (PS II) and an electron absorbs the energy and becomes excited (photoexcitation), but has nothing to do with this energy.
-The electron travels from PS II (1st protein complex of 4) along the Electron Transport Chain (via mobile electron carriers) to harness the energy; across a series of reactions the electrons lose their energy.
-The energised electrons from PS II that were picked up by electron carriers are transported to the 2nd protein complex, the Cytochrome Complex.
-The cytochrome complex stands in between PS II and PS I (which are all embedded in the thylakoid membrane), and uses the lost energy from the electrons to pump hydrogen ions into the thylakoid.-Now missing an electron, the chlorophyll molecule will split a H20 molecule in cooperation with PS II to steal its electron and replenish the one it lost, the by-products of this process are hydrogen ions (single protons) and oxygen.
**By-product of oxygen is the reason behind the survival of living organisms.
-Protons accumulated from the splitting of H2O and via the cytochrome complex now in the thylakoid then move towards the enzyme ATP Synthase, which uses that energy to pack an inorganic phosphate (Pi) onto ADP to create ATP.
PS I (wavelength p700)
The ‘tired’ electrons becomes reenergised by some photons as it reaches Photosystem 1 (PS I) and travels on another electron carrier, where all of the energy is used to help make NADPH (similar to ATP) by an enzyme, which combines 2 electrons and 1 Hydrogen ion with NADP+. (NADP+ + 2 e- + 1 H+ = NADPH).
**Products of light dependent reactions: Chemical energy in the form of ATP and NADPH, as well as the by-product O2.
Light Independent Reactions/Calvin Cycle (Stage 2)
**Doesn’t require light energy from photons, uses energy derived from ATP and NADPH to fuel processes
Carbon Fixation (1st phase of Calvin Cycle)
-Begins in the Stroma (empty space of chloroplast). Involves fixing a CO2 molecule onto Ribulose-Biphosphate or RuBP (the starting and end point of the ‘cycle’), with the help of an enzyme called RuBisCo (which converts inorganic carbon into organic carbon), which causes it to become unstable.
-So this new 6-Carbon chain has to break apart, creating 2 molecules of 3-Phopshoglycerate, which occurs to 3 molecules of RuBP (explained later).
**If RuBisCo reacts with O2 instead of CO2, it creates a toxic by-product known as phosphogycolate (process of photorespiration) believed to alter enzymatic functions.
Reduction (2nd phase)
- ATP combines a phosphate group onto the 3-Phosphoglycerate molecules and then NADPH packs some electrons, creating 2 molecules of Glyceraldehyde 3-Phosphate (G3P).
- G3P is a high-energy, 3-carbon compound that plants can convert into many carbohydrates such as glucose, or cellulose or starch.
Regeneration (3rd and last phase)
- 9 ATP molecules and 6 NADPHs are required to convert the 3 RuBPs started with into 6 G3Ps, however only 1 G3P can leave the cycle - with the other 5 G3Ps needed to regenerate the original 3 RuBPs, completing 1 round of the cycle.
- *G3P is considered the ultimate product of photosynthesis as the glucose gained from it is the energy used by all cells.
