Photosynthesis
Cuticle
the waxy coating protection from excess absorption of light and evaporation of water in a leaf
Epedermis layer
On the leaf, allows light to get to the mesophyll layer (spongy and palisade) where the majority of photosynthesis happens
Guard cells
On the leaf, create opening around stomata, o2 and co2 exchange and transpiration of water
vascular bundles
On the leaf, xylem (moves water and minerals) and phloem (moves carbohydrates)
Chloroplast structure
Photoosynthesis takes place inside chloroplasts inside leaf cells (in eukaryotes). Chloroplasts have 3 membranes, outer, inner, and thylakoid membrane which is folded to form thylakoids (which are arranged in stacks called grana). Chlorophyll and other pigments involved in photosynthesis are embedded in the thylakoid membrane
Photolysis
the use of energy consumed in photosynthesis, the splitting of water molecules
Light dependent reactions
Uses light energy to split water, releasing H+ ions that can be used by ATP synthase to produce ATP. NADP+ is reduced to NADPH and H+. They occur on the thylakoid membranes. Consists of photolysis of water and photophosphorylation. ATP and NADPH are used in the light independent reactions, creating oxygen as a waste product.
Light independent reactions
Use ATP and H+ ions to "fix" CO2, making glucose. Glucose phosphate (6C) is produced and either stored as starch, used for cellulose (growth), or used in respiration. CO2, NADPH, and ATP are used (Co2 from atmosphere, and the other two produced from light dependent reactions). Occurs in the stroma of the chloroplast, consists of carbon fixation, the Calvin Cycle, and the synthesis of carbohydrates.
Photoexcitation
The light reactions of photosynthesis involve the use of photosystems (a cluster of pigment molecules bound to proteins, along with a primary electron acceptor, in thylakoid membrane). There are 2 photosystems involved, Photosystem II (P680) which absorbs light best at a wavelength of 680nm, and Photosystem I (P700) which absorbs light best at a wavelength of 700nm.
Antenna complex
A series of accessory pigments that absorb photons and transfers energy from pigment to pigment until it reaches the reaction centre. They work towards the goal of the highest rate of photosynthesis possible as they each aosrb different wavelengths of light. The greater the nummber of accessory pigments, the greater the rate. Without them, photosynthesis would not go past light dependent reactions.
Reaction centre
Made of chlorophyll a and associated proteins. An electron from chlorophyll a molecule absorbs the energyt and one of its electrons becomes "excited" (jumps to a higher energy level). Reaction centre is P680 in photosystem II, and P700 in photosystem I. A REDOX reaction occurs, the electron is transferred to a primary electron acceptor.
Steps of Light dependent reaction - PSII
Chlorphyll in photosystem II absorbs light. Photons of light are absorbed by a pigment in PSII and are transferred to other pigment molecules until it reaches one of the chlorophyll a molecules in the reaction centre. The photon energy causes one of the electrons to jump to a higher state. It is captured by the electron acceptor plastoquinone. Plastoquinone takes two electrons and two protons and gets converted to plastoquinol.
Light dependent reactions (photoactivated electrons)
photoactivated electrons are passed along the membrane by electron carriers. Plasiquinol transfers electrons to the b6-f complex (cytochrome complex) where it then gets converted back to plastoquinone to be used again in PSII. Energy from photoactivated electrons are used to pump protons across the thylakoid membrane. Hydrogen ions accumulate in the thylakoid space, and oxygen is a waste product. Electrons are replaced through photolysis of water.
Light dependent reactions (non-cyclic photophosphorylation)
As hydrogen ions accumulate, it generates a high H+ concentration gradient. These protons move through ATP synthase (chemiosmosis). The flow of protons through ATP synthase couples ADP and Pi to make ATP in a process called non-cyclic photophosphorylation. NADP+ is reduced to NADPH. Electrons from the b6-f complex move to PSI by a carrier protein (plastocyanin), and then activated by light. These are then received by ferredoxin (carrier protein) and transferred to NADP+ reductase to reduce NADP+ wth a H+ ion. NADPH is carried to the light independent reactions, the concentration gradient is maintained.
Non cyclic photophosphorylation
once an e- is lost by a reaction centre chlorophyll molecule within a photosystem, it does not return to that system, but instead it ends up in NADPH. Non-cyclic electron flow will eventually generate 1 NADPH and ~1 ATP. However the light-independent reactions require 3 ATP to 2 NADPH.
cyclic photophosphorylation
Cyclic electron flow uses PS1 only. Photon ejects electron from PSI. Electron is passed to Fd, goes through b6-f complex and back to PS1. Therefore generation of proton gradient for ATP synthesis, but not NADPH. No oxygen is made by the process. This is done when there is an abundance of NADPH.
Thylakoids used for light dependent reactions
Cyanobacteria have thylakoids that are variable in shape and are attached to the plasma membrane. Eukaryotic algae and plants have two types of thylakoid inside the chloroplast: disc shaped ones arranged in stacks called grana, and unstaked ones, known as stroma lamellae, they form connections betweem thylakoids in grana. There is evidence that components are not evenly distributed between grana and stroma lamellae - psII and b6-f complexes are mostly in grana, ATP synthase and psI are mostly in stroma lamellae
Carbon fixation
Phase 1 of the Calvin Cycle. RuBP is carboxylated with the addition of CO2. This reaction is catalysed by the enzyme rubisco. The ^C product immediately splits into 2 molecules of glycerate-3-phosphate (G3P).
Reduction
Phase 2 of the calvin cycle. Reduction of G3P. It is removed from the Calvin cycle to be used to make glucose and other organic compounds. NADPH from the light dependent reactions are used to produce NADP+ to go back to the light dependent reactions.
Regeneration of RuBP
Phase 3 of the Calvin cycle. Most of the triose phosphate produced is used to regenerate RuBP. Some of the triose phosphate molecules are linked to form glucose phosphate, which can be converted to starch via condensation. 3 Molecules of CO2 are required to regenerate RuBP however only hald a glucose molecule is made.
Glucose phosphate used to create:
Starch for energy storage, cellulose for cell wall formation, sucrose, fatty acids and glycerol are produced from intermediate g3p, and amino acids are created when nitrogen is added to the hydrogen backbone of g3p.
Calvin's experiment (Lolipop vessel)
Calvin used chlorella algae and placed it in a thin glass vessel (lolipop vessel). IThe algae was given plenty of light, carbon dioxide, and hydrogen carbonate containing normal carbon. At the start of the experiment the carbon compounds were replaced with compounds with radioactive carbon. Samples of algae were taken at different time intervals and carbon was seperated by chromatography and autoradiography. It showed that RuBP was phosphorylated. After only 5 seconds, there was more g3p than any other compound (indicates it is the first product of carbon fixation). The next compound to be detected was triose phosphate. After 30 seconds a range of different compounds occur showing the intermediate and final products of light independent reactions.
Palisade cells adaptations
they are found close to the top surface of leaves. They contain a high density of chloroplasts to enable efficient absorption of light.
Thylakoid membranes and grana adaptations
They provide a large surface area for light absorption and light dependent reactions. Chlorophyll (and other pigments) molecules are grouped together to form the photosystems which are embedded in the membrane along with the electron carriers. Folds in thylakoid allow photosystems and electron carriers to be close together.
Thylakoid spaces adaptations
the spaces collect H+ for chemiosmosis, the low volume enables the H+ gradient to be generated rapidly. H+ flows back to the stroma, down the H+ gradient, through the ATP synthase channels embedded in the thylakoids membrane to produce ATP
Stroma adaptations
contains rubisco for carboxylation of RuBP, a suitable ph and all the other enzymes required for the Calvin cycle
Chloroplasts and mitochondria
Chloroplast envelop and outer mitochondrial membrane (membranes which compartmentalise the organelles in the cell's cytoplasm)
Thylakoid membrane and inner mitochondrial membrane (carry out electron transport chain, have ATP synthase, generate ATP, and make us eof chemiosmosis of H+ ions)
Stacked membranes and invaginated membranes (maximise surface area for reactions)
low volume intermembrane spaces (rapid generation of H+ concentration gradient)
Stroma and matrix (Fluid medium for diffusion of moolecules and contains enzymes for cyclic reactions)
The great oxidation event
Early earth's atmosphere contained little oxygen. It wasn't until 2.2 BYA that oxyen levels rose to 2% resulting in a drop of methane - first glaciation. Oxidation dissolved iron and allowed it to precipitate to form bands across the seafloor. Oxygen led to the production of oxidised compounds. Formation of ozone layer, allowing for evolution of a wider range of organisms.
Photosynthesis overall equation
6C(carbon from carbon dioxide used to produce glucose)O2 + 6H2(water is split, hydrogen used for glucose)O + light energy = C6H12(used in respiration or starch or cellulose)O6 + 6O2
Light absorption by chlorophyll
Chlorophyll is the photosynthetic pigment that dominates over other pigments in most plant species. Chlorophyll absorbs all wavelengths of visible light except for green, which is reflects.
Absorption spectrum
Varies depending on the type of photosynthetic pigment present. Represents the amount of light energy being absorbed by the photosynthetic pigment. For the plant, this spectrum represents the light aborbed by all pigments present. Chlorophylls a and b have high absorption of light energy in the violet-blue and red light wavelengths. Carotenoids absorb light energy at different wavelengths than chlorophyll a and b. Other pigments are not as efficient at aborbing light as chlorophyll a and b. Not all plants have the same absorption spectrum.
Action spectrum
Varies depending on the type of photosynthetic pigment present. Represents the rate of photosynthetic process being carried out by the pigment. For the plant, this spectrum represents the rate of photosynthesis as a result of all the pigments present. Chlorophylls a and b have a relatively high efficieny rate of photosynthesis, carotenoids allow for photosynthesis at different wavelengths. Other pigments are not as effective as chlorophyll a and b.
Why do leaves turn brown in autumn?
some leaves have accessory pigments which cannot be normally seen when chlorophyll is active. When temperatures cool down, chlorophyll breaks down before the accessory pigments, leaving them to show through.
Chromatography
Used to seperate mixtures. A mixture is dissolved in a fluid (mobile phase) and passed through a static material (stationary phase) like alcohol or ethanol. Different components of the mixture travel at different speeds, causing them to seperate. The further a pigment travels, the more soluble it is in the solvent. A retardation factor can be calculated (distance component travels/distance solvent travels, rf = a/x). Paper chromatography uses paper (cellulose) as the stationary bed. Thin layer chromatography uses a thin layer of adsorbent which runs faster and has better separation.
Light intensity on photosynthesis
At very low light levels, the plant will be respiring, not only photozynthesizing. As light intensity increases then the rate of photosynthesis increases. At high light intensities the rate becomes constant, even with further increases in light intensity there are no increases in rate as the chlorophyll system can be damaged by intense light levels.
CO2 concentration on photosynthesis
As the concentration of CO2 increase the rate of reaction increases.
Temperature on photosynthesis
same as temperature on enzymes
free air carbon dioxide enrichment experiments (FACE)
Scientists use greenhouses to control the environment in order to perform experiments on photosynthesis. FACE allows scientists to examine the effects of increasing carbon dioxide levels on plants in natural and agricultural ecosystems.