Energy For Biological Processes

Pregunta	Respuesta
Why do organisms need energy?	Active transport, anabolic reactions (building molecules out of smaller units), movement
What is chemiosmosis?	The diffusion of protons from a region of high concentrations to a region of low concentration through a partially permeable membrane. The movement of the protons releases energy.
What does chemiosmosis depend on?	The creation of a proton concentration gradient - the energy to do this comes from excited electrons.
How are electrons excited?	When they absorb light energy or when they are released when chemical bonds are broken.
What is an electron transport chain?	A series of electron carriers, each with progressively lower energy levels, that create a proton concentration gradient.
How to electron transport chains work?	Excited electrons move from one electron carrier to another, which releases energy. This energy is used to pump protons across a membrane, creating a concentration difference across the membrane. The proton gradient is maintained as a result of the impermeability of the membrane to hydrogen ions.
How do protons move back through the membrane of an electron transport chain?	Through hydrophilic membrane channels linked to the enzyme ATP synthase. The flow of protons through these channels provides the energy used to synthesise ATP from ADP and and inorganic phosphate.
What is photosynthesis?	The process by which light energy is transformed into chemical energy trapped in bonds of complex organic molecules.
What are autotrophic organisms?	Organisms that can photosynthesise
What is the equation for photosynthesis?	Carbon dioxide + water --> glucose + oxygen 6CO2 + 6H2O --> C6H12O6 + 6O2
Describe the structure of chloroplasts	Double membrane containing a network of membranes that form flattened sacs called thylakoids, which are stacked to form grana. The grana are joined by membranous channels called lamellae. Complexes of pigments are embedded within the thylakoid membranes. The fluid enclosed in the chloroplast is the stroma.
What is an antennae complex?	A system of accessory pigments that absorb light energy of different wavelengths and transfer the energy to the reaction centre - where the primary pigment (commonly chlorophyll a) is present.
What is a photosystem?	The antennae and reaction centre together
Describe how chromatography is carried out to separate the different pigments in a plant extract	The mobile phase would be the solution containing a mixture of pigments and the stationary phase a thin layer of silica gel applied to glass. The different solubilities of the pigments in the mobile phase and their differing interactions with the stationary phase, lead to them moving at different rates. This results in the pigments being separated as they move through the silica gel. The retention value (Rf) for each pigment can be calculated using: distance travelled by pigment/distance travelled by solvent.
What are the two stages of photosynthesis?	Light dependent stage and the light independent stage
What occurs in the light dependent stage of photosynthesis?	Energy from the sun is absorbed and used to form ATP. Hydrogen ions from the water is used to reduce coenzyme NADP to NADPH. This occurs in three stages: non-cyclic photophosphorylation, photolysis and cyclic photophosphorylation.
What occurs in the light independent stage of photosynthesis?	Hydrogen from NADPH and carbon dioxide is used to build organic molecules, such as glucose, lipids and amino acids. ATP supplies the required energy. This stage involves the calvin cycle.
Describe the process of non-cyclic photophosphorylation	Two photosystems are involved: PSII followed by PSI. PSII absorbs light of wavelength 680nm and PSI 700nm. Light is absorbed by PSII which excites electrons. The excited electrons then pass through an electron transport chain. ATP is produced by chemiosmosis. The electrons lost from the reaction centre of PSII are replaced through the process of photolysis. At PSI electrons are again excited and travel along another electron transport chain. The electrons are replaced by the electrons that travelled along the first electron transport chain. ATP is again produced by chemiosmosis. The electrons leaving the electron transport chain following PSI are accepted, along with a H+, by coenzyme NADP to form reduced NADP.
Describe the process of photolysis	Water molecules are split into hydrogen ions, electrons and oxygen molecules using light energy. The electrons released replace the electrons lost from PSII. This is catalysed by the oxygen-evolving complex enzyme present in PSII. H2O --> 2H+ + 2e- + 1/2O2
How are the hydrogen ions produced in photolysis removed from the stroma?	The protons produced in photolysis are released into the lumen of the thylakoids, increasing proton concentration across the membrane. As they move back through the membrane down a concentration and electrochemical gradient, more ATP is produced through chemiosmosis. Once the hydrogen ions all return to the stroma they combine with NADP and an electron from PSI to form NADPH. This removes hydrogen ions and helps maintain the proton gradient across the thylakoid membranes.
Describe the process of cyclic photophosphorylation	Electrons leaving the electron transport chain after PSI are returned to PSI, producing ATP but no NADPH. This means PSI doesn't require electrons from PSII to replace its excited electrons.
Where does the light independent stage of photosynthesis take place?	The stroma of chloroplasts
How is the carbon in carbon dioxide fixed when it enters the stroma?	It combines with a 5C molecule called ribulose biphosphate (RuBP), forming an unstable 6C intermediate. This is catalysed by ribulose biphosphate carboxylase (RuBisCO) enzyme.
How is the unstable 6C compound formed when carbon is fixed to RuBP converted into triose phosphate?	It immediately breaks down to form two 3C glycerate 3-phosphate (GP) molecules. Each GP molecule is converted into 3C triose phosphate by reacting it with a hydrogen ion from NADPH using energy from ATP.
What are the three steps in the calvin cycle?	Fixation - carbon dioxide is incorporated into an organic molecule Reduction - GP is reduced to TP by the addition of a hydrogen ion Regeneration - RuBP is regenerated from the recycled TP
What happens to most of the TP produced in the calvin cycle ?	It is recycled to regenerate the RuBP so that the calvin cycle can continue.
What are the factors that affect the rate of photosynthesis?	Light intensity, carbon dioxide concentration and temperature
How does reducing light intensity affect the calvin cycle?	It will reduce the rate of the light dependent stage of photosynthesis, producing less NADPH and ATP. This will limit the conversion of GP to TP, so concentration of GP will increase and TP decrease. As there is less TP to regenerate RuBP, the concentration of RuBP will also decrease.
How does decreasing carbon dioxide concentration affect the calvin cycle?	Lower concentrations of CO2 means less is fixed onto RuBP to produce GP. This reduces the amount of TP. The concentration of RuBP will increase as it is still being formed from TP but not being used to fix carbon dioxide.
How does decreasing temperature affect the calvin cycle?	All reactions in the calvin cycle are catalysed by enzymes, which have less kinetic energy resulting in fewer successful collisions and a reduced rate of reaction at lower temperatures. Decreasing temperature results in lower concentrations of GP, TP and RuBP.

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