c1.3 photosynthesis Flashcards
what is photosynthesis?
photosynthesis is the process by which cells synthesise organic compounds from inorganic molecules in the presence of sunlight. this process requires a photosynthetic pigment and can only occur in certain organisms.
how does photosynthesis convert light energy to chemical energy?
photosynthetic organisms contain pigments that capture the light energy from the sun to create chemical energy. this chemical energy can then be used to synthesise organic compounds via anabolic reactions. the organic compounds can either contribute to cellular structure or be catabolically digested as an energy source.
what are the light independent reactions?
in plants, the light independent reactions occur within the fluid-filled interior of the chloroplast called the stroma. ATP and hydrogen (carried by reduced NADP) are transferred to the site of the light dependent reactions. the hydrogen is combined with carbon dioxide to form complex organic compounds. the carbon is fixed by the enzyme rubisco, with ATP providing the chemical energy required to join the molecules together. this process is also commonly known as the calvin cycle.
what are the light dependent reactions?
in plants, the light dependent reactions occur within the membranous discs called thylakoids (which are arranged into stacks called grana). light is absorbed by photosynthetic pigments, resulting in the production of ATP. light is also absorbed by water, which is split (photolysis) to produce oxygen and hydrogen (carried by reduced NADP). the hydrogen and ATP are used in the light independent reactions, the oxygen is released from the stomata as a waste product.
what are the pigments in photosynthetic organisms?
photosynthetic organisms do not rely on a single pigment to absorb light, but instead benefit from the combined action of many. in plants, the main photosynthetic pigment is chlorophyll a - but xanthophylls and carotenes are also used.
what are the common methods of separating photosynthetic pigments?
two of the most common techniques for separating photosynthetic pigments are paper chromatography (stationary - paper/cellulose) and TLC (stationary - silica gel). because photosynthetic pigments contain a non-polar tail, an organic solvent such as acetone mut be used to dissolve the pigments.
how do the pigment molecules absorb light energy?
in photosynthetic organisms, the absorption of light is mediated by specific pigment molecules. each pigment molecule contains electrons at discrete and specific energy levels. these electrons can absorb light at specific frequencies and become energised and delocalised. the energy from these excited electrons can be harnessed by the cell to make chemical energy.
what is the difference between absorption and action spectrum?
pigments absorb light as a source of energy for photosynthesis. the absorption spectrum indicates the wavelengths of light absorbed by each pigment. the level of absorbance can be quantitatively measured using a device called a spectrophotometer. the action spectrum indicates the overall rate of photosynthesis at each wavelength of light. the rate of photosynthesis can be measured by either the rate of carbon dioxide consumption or the level of oxygen consumption.
what does the correlation between absorption and action spectrum suggest?
there is a strong correlation between the cumulative absorption spectra of all pigments and the action spectrum. both display two main peaks - a larger peak at the blue region and a smaller peak at the red region. both also display a trough in the green/yellow portion of the visible spectra. collectively, this demonstrates that photosynthetic pigments absorb red and blue light effectively and reflect green light more than other colours.
what is the law of limiting factors?
when a chemical process depends on multiple conditions to occur, the reaction rate will be limited by the condition nearest to its minimum value. this is known as the law of limiting factors.
how does temperature affect the rate of photosynthesis?
temperature affects the rate of photosynthesis by impacting the frequency of successful enzyme-substrate collisions. at low temperatures, the respiration rate will be low as there is insufficient energy for frequent collisions. at high temperatures, the respiration rate will be low as the enzymes begin to denature and lose their functionality. rates will be highest at a temperature that reflects the optimum conditions for photosynthetic enzymes.
how does pH affect the rate of photosynthesis?
pH also affects the rate of photosynthesis by changing the charge and solubility of the enzymes involved. photosynthetic pigments will be highest at a pH that reflects optimum physiological conditions (typically ~ 7). any pH condition outside of an optimal range will cause the enzyme to denature, reducing the photosynthetic rate.
how does carbon dioxide concentration affect the rate of photosynthesis?
carbon dioxide is the main source of carbon used to synthesise organic compounds in the light independent reactions. increasing concentrations of carbon dioxide will result in higher rates of photosynthesis until all enzymes are saturated.
how does light intensity affect the rate of photosynthesis?
light is required for the photoactivation of pigments and the subsequent production of chemical energy. increasing the intensity of light will result in high rates of photosynthesis until all pigments are photoactivated.
how can carbon dioxide concentration be experimentally regulated?
the concentration of carbon dioxide can be experimentally regulated by using tablets of sodium bicarbonate, which dissolves in water to form carbonic acid (which readily dissociates to form carbon dioxide).
how can light intensity be experimentally regulated?
light intensity can be experimentally regulated by controlling the distance of a light source. in addition to light intensity, photosynthetic rates will also be impacted by the specific wavelength of light.
what are carbon dioxide enrichment experiments?
CO2 enrichment is the process of increasing the amount of carbon dioxide to a level higher than what is normally in fresh air. CO2 enrichment experiments are commonly undertaken as a means of predicting future rates of photosynthesis and plant growth in response to human activity.
what are enclosed greenhouse experiments?
carbon dioxide levels can be artificially increased in indoor greenhouses by adding CO2from compressed gas tanks or by adding fermentation buckets that continuously produce CO2. enclosed greenhouses act as a closed system, which allow for the control of a range of extraneous variables (such as temperature and light). however the conditions do not reflect those that occur in the natural environment and only plants that grow in small spaces can be measured.
what are free air carbon dioxide enrichment experiments?
free air carbon dioxide enrichment experiments involve the placement of pipes which emit CO2around an experimental area. the concentration of carbon dioxide is monitored by sensors which then adjust the flow of CO2from the pipes.
FACE experiments represent open systems which incorporate natural conditions like rainfall and temperature fluctuations. FACE experiments can also measure the effects of CO2enrichment on larger trees and consider the impact of competition between plant species. the disadvantage of experimenting on open systems is that certain conditions (such as sunlight) cannot be controlled.
what are photosystems?
photosynthetic organisms do not rely on a single pigment to absorb light, but instead benefit from the combined action of many. these photosynthetic pigments are grouped into molecular arrays called photosystems that are located within a membrane. by grouping pigments that have individualised absorption spectra together, the cell maximises its light absorption.
how does energy get to the reaction centre in photosystems?
when a pigment is energised by light, it releases high energy electrons. chlorophyll and additional accessory pigments transfer their energised electrons to a central reaction centre containing a specialised chlorophyll. there are two main photosystems involved in photosynthesis and they differ in their type of reaction centre.
photosystem I tends to absorb longer wavelengths of light (700nm) while photosystem II tends to absorb a slightly shorter wavelength of light (680nm)
what reactions happen at the photosystems?
photosystem II accepts de-energised electrons from water and donates excited electrons to an ETC.
photosystem I accepts the de-energised from the ETC and donates excited electrons to a hydrogen carrier (NADP). photosystem I can also donate an excited electron back into the ETC (cyclic photophosphorylation). this can allow for further ATP production but does not allow for the synthesis of organic compounds.
what are the products made at photosystem II?
the photolysis of water generates protons and electrons that are used in the light dependent reactions, but oxygen is a waste product in this process. before the evolution of photosynthetic organisms, any free oxygen produced on earth was chemically captured and stored.
how has oxygen impacted the environment?
earth’s oceans initially had high levels of dissolved iron. when iron reacts with oxygen gas it undergoes a chemical reaction to form an insoluble precipitate (iron oxide). the build up of iron precipitates created oceanic deposits called banded iron formations. when iron in the ocean was completely consumed, oxygen gas started accumulating in the atmosphere.
free oxygen is toxic to obligate anaerobes and an increase in atmospheric oxygen levels may have wiped out many of these species. the rise in atmospheric oxygen levels was a critical determinant to the evolution of aerobically respiring organisms. the oxygenation of the atmosphere also led to the development of an ozone layer which limited exposure to harmful radiation.
what is photophosphorylation?
ETCs consist of several electron-shuttling carrier proteins and the transmembrane enzyme ATP synthase. as energised electrons from the photosystems are passed through the chain they lose energy, which is used to translocate protons from the stroma into the thylakoid. this build up of protons creates an electrochemical gradient. the protons return to the stroma via ATP synthase (chemiosmosis) - the passage of protons is used to catalyse the synthesis of ATP. this process is called photophosphorylation.
what is cyclic photophosphorylation?
cyclic photophosphorylation involves the use of only one photosystem (PS I) to generate ATP. the energised electrons released from PS I are recycled after passing through the ETC. this cyclic process can be used to produce a steady supply of ATP in the presence of sunlight, but cannot be used for organic synthesis.
what is non-cyclic photophosphorylation?
non-cyclic photophosphorylation involves the use of both photosystems to generate ATP and reduced NADP. the energised electrons from PS I are used to reduce NADP. the lost electrons are replaced by electrons from PS II that have passed through the ETC. the electrons lost from PS II are replaced by electrons generated by the photolysis of water. this process is non-cyclic, as the reduction of NADP requires the oxidation of a water molecule.
what are thylakoids?
the light dependent reactions occur within specialised membrane discs called thylakoids. in plants, the thylakoid discs are organised into stacks called grana within the chloroplasts. in cyanobacteria and single-celled algae, the thylakoids do not form grana and are free-floating.
what is step one of the calvin cycle?
carbon fixation
the calvin cycle begins with a 5C compound called RuBP. an enzyme, rubisco, catalyses the attachment of a CO2 molecule to RuBP. the resulting 6C compound is unstable and breaks down into two 3C compounds called GP. a single cycle involves three molecules of RuBP combining with three molecules of CO2 to make six molecules of GP.
what is step two of the calvin cycle?
reduction of GP
GP is converted into TP using reduced NADP and ATP. the reduced NADP and ATP are generated by the light dependent reactions. reduction by reduced NADP transfers hydrogen atoms to the compound, while the hydrolysis of ATP provides energy. as six molecules of GP were produced via carbon fixation, six molecules of TP are similarly produced per cycle.
what is step 3 of the calvin cycle?
regeneration of RuBP
of the six molecules of TP produced per cycle, one TP molecule may be used to form half a sugar molecule. hence two cycles are required to produce a single glucose monomer, and more to produce polysaccharides like starch. the remaining five TP molecules are recombined to regenerate stocks of RuBP. the regeneration of RuBP requires energy derived from the hydrolysis of ATP.
what other carbon compounds does the calvin cycle produce?
the calvin cycle is responsible for the fixation of carbon from inorganic sources to form the different types of organic compounds required by the cell. synthesis of organic molecules happens via a variety of metabolic reactions that take place within the cytoplasm. the fraction of those TPs that are not used to regenerate RuBP can be converted in the cytosol to form vital biomacromolecules.
how are the light dependent and independent reactions interdependent?
the two photosynthetic reactions are interdependent – each requires completion of the other in order to occur. the light independent reaction cannot occur without the products of the light dependent reaction (ATP and reduced NADP). the light dependent reaction requires an unloaded coenzyme (NADP) to carry the hydrogen atoms – this coenzyme is unloaded by the light independent reaction. hence, a lack of carbon dioxide in the light independent reaction will prevent the production of NADPH by photosystem II in the light dependent stage.
