Photosynthesis C1.3

• What is photosynthesis?


  The process by which cells synthesise organic compounds (glucose) from inorganic molecules (carbon dioxide and water) in the presence of sunlight. It is anabolic, as it is the synthesis of organic compounds that are then used either for cellular structure or catabolic digestion during respiration. This requires photosynthetic pigment (chlorophyll) and can only occur in plants and certian bacteria.
• Why is photosynthesis referred to as an energy conversion?

  Photosynthesis is an energy conversion, as light energy is converted into chemical energy in carbon compounds. This transformation supplies most of the chemical energy needed for life processes in ecosystems.
• What are the two steps of photosynthesis, and where do they occur?


  It is a two-step process:

  Light dependent reactions convert light energy to chemical energy (ATP) Occur in the thylakoids, arranged into grana

  Light independent reactions use the chemical energy to synthesise organic compounds (glucose) Occur in the fluid component of a chloroplast, the stroma
• What do the light-dependent reactions step consist of?

  Chlorophyll absorbs light energy (mostly red or blue wavelengths)

  Light energy is used to split water molecules into oxygen, (H+) protons and electrons. This is known as photolysis

  ATP and NADPH are produced during this photolysis reaction, NAPH carrying the oxygen and the hydrogen.
• What do the light-independent reactions consist of?


  ATP and hydrogen (carried by NADPH) are transferred to the site of the light independent reactions

  The hydrogen is combined with carbon dioxide to form complex organic compounds (e.g. carbohydrates, amino acids, etc.) during the Calvin cycle

  The carbon is fixed by the enzyme Rubisco, with ATP providing the chemical energy required to join the molecules together (anabolic reaction)

  The energy is stored within the Carbon compound
• How is energy converted in organisms?

  Sunlight - photosynthesis (plants) - chemical energy in glucose - cellular respiration - ATP - Cell activities - Heat (often lost to environment)

  
  

  Therefore, sunlight is the principal energy source in most ecosystems. It drives photosynthesis, which supports producers at the base of food chains.
• Why is photosynthesis a reduction reaction?

  Photosynthesis is a reduction reaction, meaning that it involves the gaining of electrons. Hydrogen is needed for the reduction that converts carbon dioxide into glucose, and this is sourced from photolysis.
• What is photolysis?


  Photolysis is the process of splitting molecules of water, and can only occur if sufficient energy is provided from light.

  Hydrogen is released during photolysis as water is separated into protons (hydrogen ions) and electrons. Oxygen is a waste product of photolysis, and diffuses from photosynthesising cells. Here is the equation for photolysis:

  2H2O+light→4H++4e−+O2
• Where are the carbons, hydrogens and oxygens forming glucose sourced from?

  Carbon is sourced from the carbon dioxide in the air

  Hydrogen is sourced form the photolysis of water

  Oxygen is sourced both from the photolysis of water and from carbon dioxide
• What does it mean that oxygen is a by-product of photosynthesis?

  Oxygen is a by-product of photosynthesis.

  It is produced as a result of photolysis, the splitting of water into hydrogen atoms (protons), electrons and oxygen.

  The increased concentration of oxygen inside chloroplasts causes it to diffuse from them and out of leaf cells to air spaces within the leaf. The oxygen diffuses from these air spaces through the stomata into the outside air.
• What are chloroplasts?


  Chloroplasts contain different types of chlorophyll, alongside other pigments called accessory pigments.

  The accessory pigments absorb different light wavelengths, so appear as different colours to the human eye.
• What is thin layer chromatography?

  A plastic strip is coated with a thin layer of porous material

  A spot containing leaf pigments is extracted from leaf tissue and placed near one end of the strip

  A solvent runs up the strip, separating the different types of pigment
• What is the procedure for the separation and identification of photosynthetic pigments through chromatography?

  Tear a leaf into pieces and place in the mortar

  Add sand for grinding

  Add propanone (acetone) to dissolve the pigments, that contain a non-polar and hydrophobic tail.

  Use the pestle to grind the leaf tissue and dissolve out the pigments

  If the propanone evaporates, add more

  When the propanone has turned dark green, allow the solids to settle. Pour the propanone into a watch glass

  Evaporate the propanone and water from the propanone in the watch glass, using a hairdryer

  When the watch glass contains a smear of dry pigment, add more propanone and use a paint brush to dissolve the pigments

  Use the paintbrush to transfer a small amount of this to the TLC (thin-layer chromatography) strip. This should be in the middle, and very dark, achieved through repeated application

  Place the strip into a test tube, the strip extending nearly to the bottom of the strip without touching it

  Pour solvent into the test tube until it very nearly reaches the base of the TLC strip. It must not touch the pigment spot

  Leave the tube alone, without touching it, and watch the pigments separate

  Mark the area that the solvent reached, and the area the pigment reached, measuring the distance moved by the solvent and the distance moved by each pigment using a ruler.

  Calculate the Rf (retardation factor) value pigment/solvent
• What are pigments?


  The first stage of photosynthesis is the absorption of sunlight, which is facilitated by chemical substances known as pigments such as chlorophyll. Pigments contain a light-absorbing region and a membrane-anchoring region, which is a non-polar and hydrophobic tail. Pigments appear as different colours to humans because they absorb and transmit different wavelengths of light.
• What are the wavelengths of visible light?

  Visible light is from 400nm to 700 nm, and ranges from the longest wavelength (red) to the shortest wavelength (violet)
• What are the different pigments?

  
  

  White substances absorb/reflect all visible light wavelengths

  Transparent substances allow all visible light wavelengths to pass through

  Pigments that absorb all wavelengths of light appear black, and they transform the large amounts of light energy into other forms of energy, generally heat

  Other pigments absorb some visible light wavelengths but not others.

  For example, a flower that appears blue actually absorbs and reflects all wavelengths of visible light apart from blue.
• What are photons?


  A photon is a particle or unit of light. They are discrete quantities of energy.

  The energy of a photon is related to its wavelength.

  The longer the wavelength, the less energy the photon holds
• How do pigments absorb photons, making ATP?

  Each pigment contains electrons at discrete and specific energy levels (according to the pigment's atomic configuration). The electrons can absorb light at some wavelengths (photons) and become energised, delocalised and ionised, causing them to jump to a higher energy level (excitation)

  The energy from the excited electrons can be harnessed by the cell to make chemical energy (ATP through photophosphorylation)

  The amount of energy required for this to happen is only supplied by certian wavelengths of light (photons)
• What is chlorophyll, and and why are most ecosystems green?

  The main photosynthetic pigment is chlorophyll. All chlorophyll appears green to the human eye, because the photons of red and blue visible light can excite chlorophyll electrons, but green visible light cannot. So, most green light is reflected, making plants appear green to humans. This is why most plants in ecosystems appear green.
• What is the absorption spectrum?

  The wavelengths of light absorbed by a pigment are shown on a graph known as the absorption spectrum.

  The x axis shows the wavelength of light in nanometres

  The y axis shows absorption, often as a percentage.

  This may be quantitively measured using a spectrophotometer.
• What is an action spectrum?

  An action spectrum shows the rate of photosynthesis at each wavelength of light. this may be measured through the rate of carbon dioxide consumption or the rate of oxygen production.

  The y axis should represent the relative amount of photosynthesis, as a percentage of the maximum rate.

  When plotting both, the x axis should show wavelength of light, in nanometres. This should be from 400nm (violet) to 700nm (red)
• What does an action spectrum look like?

  Peaks in blue (~450 nm) and red (~660 nm):

  These wavelengths are strongly absorbed by chlorophyll, so the rate of photosynthesis is high.

  Dip in green (~500-550 nm):

  Green light is reflected, not absorbed well, so the rate of photosynthesis is lower.
• How do you investigate how carbon dioxide concentration influences the rate of photosynthesis?

  Boil water to fill a large beaker, allowing it to cool. This removed carbon dioxide and other dissolved gasses

  Pour the water from one beaker to another, to oxygenate it, repeatedly

  Cut the stem of pondweed and place it upside down in water. No bubbles are expected to emerge, due to the lack of carbon dioxide in the water

  Keep the temperature of the water stable, at 25 degrees celsius. Make sure that it is also brightly illuminated

  Add sodium hydrogen carbonate to the beaker, enough to raise the carbon dioxide concentration by 0.01 mol/dm

  Count any bubbles that emerge for 30 seconds

  Repeat the last two steps until further increases in carbon dioxide do not affect the rate of bubble production
• How do you investigate how light intensity influences the rate of photosynthesis?

  Fill a syringe with 10cm of 0.2 mol/dm sodium hydrogen carbonate. This is the supply of carbon dioxide.

  Place 10 leaf discs into the sodium hydrogen carbonate solution, replacing the plunger of the syringe afterwards.

  Hold the syringe vertically and squeeze excess air out. Place a finger over the nozzle and pull on the syringe's barrel, creating suction that draws air from the air spaces in the leaf discs.

  Tap the syringes so that bubbles of gas rise into the nozzle

  Repeat the last two steps several times, until the leaf discs become denser than the sodium hydrogen carbonate solution and sink to the bottom

  Place the syringe vertically at a measured distance from a light source, the discs fully illuminated

  As the discs photosynthesise, they produce oxygen, causing them to become less dense and rise to the top of the syringe. Time how long it takes for each disc to rise to the surface of the syringe

  Measure the light intensity using a lux meter

  Repeat with the syringe at different distances from the light source.

  Suitable distances from the light source are: 22 mm 41 mm 50 mm 71 mm
• How do you investigate how temperature influences the rate of photosynthesis?

  Temperature may be varied in different ways:

  Water bath

  Heat blocks

  Fermenter Data logger and oxygen electrode pH monitor (as carbon dioxide is absorbed, pH increases)
• What is a limiting factor, in regards to photosynthesis?

  A limiting factor is any condition that directly restricts the rate of a process. It is the factor furthest from its optimum, and even if other factors are ideal, the process cannot proceed faster than the rate allowed by the limiting factor.

  Limiting factors of photosynthesis are:

  Temperature/pH Alters the functionality of photosynthetic enzymes Low/high temperatures= slow rate Extreme pH= slow rate Rate highest at conditions optimum for Rubisco and other enzymes

  Light intensity Determines the activation of photosynthetic pigments

  Carbon dioxide concentration Levels of availability influences reaction rates as it is a required substrate Increases until a plateau (bcs of enzymes)
• What does the increasing carbon dioxide in the atmosphere mean for plant growth?

  Increasing carbon dioxide levels above current atmospheric levels of about 400ppm increases rates of photosynthesis and plant growth.
• What are greenhouse experiments with carbon dioxide?

  Carbon dioxide levels can be artificially increased in indoor greenhouses by adding CO2 from 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 FACE experiments?

  FACE experimentation may be within agricultural crops and young tree plantations, or natural/semi-natural forests. The first of these was an Eucalyptus forest in Australia, and the second and Oak forest in England.

  Free air carbon dioxide enrichment experiments involve the placement of pipes which emit CO2 around an experimental area

  The concentration of carbon dioxide is monitored by sensors which then adjust the flow of CO2 from the pipes

  FACE experiments represent open systems which incorporate natural conditions like rainfall and temperature fluctuations

  FACE experiments can also measure the effects of CO2 enrichment 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
• Is photosynthesis endo or exothermic?

  Endothermic (absorbs light energy)
• What are the carbon compounds that are produced by photosynthesis used for?

  SCARF

  S- Starch

  C- Cellulose

  A- Amino acids

  R- Respiration

  F- Fats