Photosynthesis Edexcel A Level Biology (A) SNAB
Photosynthesis: An Overview
Photosynthesis is a series of chemical reactions occurring in producers like plants and algae.
It converts light energy into chemical energy, stored in the biomass of producers.
Light energy splits water molecules (H_2O), releasing hydrogen and oxygen.
Oxygen is released into the atmosphere as a waste product.
Hydrogen combines with carbon dioxide to produce glucose.
Chemical energy is stored in glucose molecules, functioning as fuel for respiration. Hydrogen is essentially stored in glucose.
ATP as a Supply of Energy
All organisms need a constant energy supply to maintain cells and stay alive.
Energy is required for:
Building new molecules from digestion products (anabolic reactions).
Moving substances across cell membranes (active transport) or within cells.
Muscle contraction.
Conduction of nerve impulses.
Adenosine triphosphate (ATP) is used to transfer and supply energy within cells; it's the universal energy currency.
ATP diffuses within cells to where it's needed.
ATP is a phosphorylated nucleotide, similar in structure to DNA and RNA nucleotides.
A nucleotide consists of a nitrogenous base, a sugar, and a single phosphate group.
ATP contains adenine, a ribose sugar, and three phosphate groups (hence, triphosphate).
Removal of one phosphate creates ADP (adenosine diphosphate); removal of two creates AMP (adenosine monophosphate).
ATP is produced during respiration by adding inorganic phosphate (P_i) to ADP:
ADP + P_i \rightarrow ATP
Glucose breakdown in respiration releases the energy to phosphorylate ADP.
Hydrolysis (breakdown) of ATP releases an inorganic phosphate and a small amount of energy:
ATP \rightarrow ADP + P_i
Dephosphorylation is the removal of a phosphate group.
ATP hydrolysis is catalyzed by the enzyme ATPase.
ADP and inorganic phosphate from ATP hydrolysis can be recycled to make more ATP.
Chloroplasts: Structure & Function
Chloroplasts are organelles in plant cells where photosynthesis occurs.
Each chloroplast is surrounded by a double membrane called the chloroplast envelope; each membrane is a phospholipid bilayer.
The stroma is the cytoplasm-like fluid within the chloroplast, containing enzymes, sugars, ribosomes, and chloroplast DNA.
Chloroplasts have a separate system of membranes in the stroma, consisting of flattened fluid-filled sacs called thylakoids, each surrounded by a thylakoid membrane.
Thylakoids stack up to form grana (singular: granum), which are connected by membranous channels called lamellae (singular: lamella).
Essential components for photosynthesis are in the thylakoid membranes:
ATP synthase enzymes.
Photosystems (containing photosynthetic pigments like chlorophyll a, chlorophyll b, and carotene).
The chloroplast envelope encloses the chloroplast, keeping components close.
Transport proteins in the inner membrane control molecule flow between the stroma and cytoplasm.
The stroma contains enzymes catalyzing photosynthesis reactions.
Chloroplast DNA contains genes coding for some proteins used in photosynthesis.
Ribosomes enable translation of proteins coded by chloroplast DNA.
The thylakoid membrane has a space between its two membranes (thylakoid space), where conditions can differ from the stroma (e.g., a proton gradient).
Grana create a large surface area, maximizing photosystem number for light absorption.
Grana provide more membrane area for proteins like electron carriers and ATP synthase.
Photosystems exist in two types: photosystem I and photosystem II, containing different pigment combinations.
Each photosystem absorbs light at a different wavelength (e.g., photosystem I absorbs at 700 nm, photosystem II at 680 nm).
Light-dependent Reactions
Photosynthesis occurs in two stages:
Light-dependent reactions (rely directly on light).
Light-independent reactions (don't use light directly but rely on products of light-dependent reactions).
Light-dependent reactions take place across the thylakoid membrane.
Light-independent reactions take place in the stroma.
Light energy enables splitting of water molecules (photolysis):
H_2O \rightarrow 2H^+ + 2e^- + O
Photolysis of one water molecule produces 2 hydrogen ions (protons), 2 electrons, and one oxygen atom.
Hydrogen ions and electrons are used during the light-dependent reactions, while oxygen is released as waste.
Light energy is converted into chemical energy in the form of ATP and reduced NADP.
NADP is a coenzyme that transfers hydrogen from one molecule to another; when it gains hydrogen, it is reduced (NADPH).
Reduction is gain of electrons, gain of hydrogen, or loss of oxygen; oxidation is loss of electrons, loss of hydrogen, or gain of oxygen.
Reduced NADP can reduce other molecules by giving away hydrogen; NADP can oxidize other molecules by receiving hydrogen.
ATP and NADPH are transferred to the light-independent reactions.
ATP and NADPH are produced during light-dependent reactions via photophosphorylation (addition of phosphate to ADP to form ATP).
Two types of photophosphorylation:
Non-cyclic: produces both ATP and NADPH.
Cyclic: produces ATP only.
Both involve a series of membrane proteins making up the electron transport chain, where electrons pass from one protein to another, releasing energy.
Energy released as electrons pass down the electron transport chain is used to produce ATP via chemiosmosis.
In non-cyclic photophosphorylation:
Light hits photosystem II, exciting two electrons to a higher energy level.
Excited electrons leave photosystem II and pass to the first protein in the electron transport chain.
Electrons from photolysis of water replace those leaving photosystem II.
As electrons pass down the electron transport chain, energy is released, enabling chemiosmosis.
H^+ ions are pumped from low concentration in the stroma to high concentration in the thylakoid space, generating a concentration gradient.
H^+ ions diffuse back into the stroma via ATP synthase, catalyzing ATP production.
Electrons from photosystem II are passed to photosystem I.
Light hits photosystem I, exciting another pair of electrons, which pass along another electron transport chain.
These electrons combine with hydrogen ions and NADP to form reduced NADP:
H^+ + 2e^- + NADP \rightarrow NADPH
Reduced NADP and ATP pass to light-independent reactions.
Cyclic photophosphorylation:
Light hits photosystem I, exciting electrons.
Excited electrons pass along the electron transport chain, releasing energy.
Energy drives chemiosmosis (pumping H^+ ions).
H^+ ions diffuse back, driving ATP synthase.
Electrons rejoin photosystem I, completing the cycle.
ATP produced enters the light-independent reaction.
Light-independent Reactions
Also known as the Calvin cycle.
Produces complex organic molecules like starch, sucrose, and cellulose.
Requires ATP and reduced NADP from light-dependent reactions.
Three main steps:
Fixation: Carbon dioxide combines with ribulose bisphosphate (RuBP, a 5-carbon compound), yielding two molecules of glycerate 3-phosphate (GP, a 3-carbon compound).
Carbon dioxide combines with RuBP, catalyzed by rubisco.
The resulting 6-carbon compound is unstable and splits into two GP molecules.
Reduction: GP is reduced to glyceraldehyde 3-phosphate (GALP, another 3-carbon compound) using reduced NADP and ATP.
Energy from ATP and hydrogen from reduced NADP reduce GP to GALP.
Regeneration: RuBP is regenerated from GALP using ATP.
For every turn, only one-sixth of a glucose molecule is produced (two GALP molecules contain six carbon atoms, five of which are needed to regenerate RuBP).
Six turns of the Calvin cycle are required to produce one molecule of glucose (6-carbon).
Five-sixths of GALP are used to regenerate RuBP.
Products of Photosynthesis
Intermediate molecules of the Calvin cycle (GP and GALP) are used to produce other biological molecules.
GP is used to produce:
Amino acids (for protein synthesis).
Fatty acids (for lipid molecules like triglycerides and phospholipids).
GALP is used to produce:
Hexose sugars (e.g., glucose).
Glycerol (for building lipid molecules).
Nucleic acids (for DNA and RNA).
Glucose can be converted to other hexose sugars (e.g., sucrose for transport in phloem).
Hexose sugars can be joined to make polysaccharides (starch and cellulose).
Glucose can be used in respiration while other biological molecules are used to build new plant biomass. These molecules are passed to consumers when plant tissue is eaten.
Practical: The Hill Reaction
Light-dependent reactions in the thylakoid membrane release high-energy electrons from chlorophyll molecules in photosystems.
These electrons are picked up by NADP along with hydrogen ions from photolysis, forming NADPH (reduced NADP).
NADP is the electron acceptor in this process.
The formation of NADPH this way is the Hill reaction.
The rate of the Hill reaction can be studied using indicators like DCPIP and methylene blue.
These indicators accept electrons instead of NADP, causing them to change color:
Oxidized DCPIP (blue) \rightarrow reduced DCPIP (colorless).
Oxidized methylene blue (blue) \rightarrow reduced methylene blue (colorless).
The rate at which the indicator changes color (blue to colorless) measures the rate of the Hill reaction.
A faster color change means a faster reaction rate.
Apparatus:
Leaves (e.g., spinach).
Pestle and mortar or food blender.
Isolation solution (sucrose, potassium chloride, pH 7 buffer).
Funnel and filter paper/cloth.
Beaker, centrifuge, centrifuge tubes, glass rods.
Ice-cold water bath, colorimeter, cuvettes.
Test tubes and rack, lamp, DCPIP indicator, dropping pipette.
Method:
Grind leaves with isolation solution to break apart tissues.
Filter the liquid to remove large pieces of leaf tissue.
Centrifuge to form a chloroplast pellet.
Discard the liquid and resuspend the pellet in fresh isolation medium.
Transfer to an ice-cold water bath to slow down activity.
Place chloroplast extract in a test tube rack set at a distance from a lamp.
Add DCPIP solution and mix.
Remove a sample and place in a cuvette.
Take an absorbance reading with a colorimeter.
Repeat every minute for 10 minutes.
Repeat the experiment at least twice more.
Results:
Absorbance decreases over time as DCPIP is reduced, changing the solution from blue to colorless.
Plot a graph of absorbance against time to show the rate of the Hill reaction.
Changing variables like light intensity, light wavelength, or temperature would allow the effect of different variables on the rate of the Hill reaction to be studied. The presence of chloroplasts in the solution causes it to appear green rather than colorless.