Paper 2 - Photosynthesis Flashcards
Properties of ATP
Releases small, manageable amounts of energy — Energy is not wasted as heat
Small and soluble molecule — Can easily diffuse around cell to where it’s needed
Easily hydrolysed — Energy released instantaneously
Quickly re synthesised— Constant supply of energy to the cell
Can phosphorylate other molecules when hydrolysed — Makes other molecules more reactive
Cannot pass out of the cell through membrane — Cell always has immediate supply of energy
Light dependent reaction - photoionisation
- When a chlorophyll in PSII absorbs light energy and electrons become excited
- The electrons leave the chlorophyll molecule = oxidation
- These electrons are taken up by an electron carrier = reduction
Light dependent reaction - Making ATP
- A chain of protein electron carriers in a membrane through which excited electrons flow = electron transport chain
- Electrons lose energy at each stage
- This energy is used to transport protons (H+) into the thylakoid
- Protons diffuse back into the stromatolites via the channel by ATP synthase
- ADP + Pi — ATP
- Light is used for this process = photophosphorylation
- This is known as chemiosmosis
LDR - Making NADPH
- Light energy is absorbed by PSI = excitation of electrons
- Electrons are transferred to NADP (coenzyme) along with H+ from storma = NADPH
- Used in light independent reaction
LDR - Photolysis of water
- Loss of electrons when light strikes a chlorophyll molecule = shortage of electrons
- If chlorophyll continues to absorbs knight energy, electrons must be replaced
- Replacement electrons are provided from H2O molecules that are split using light energy
- Photolysis of H2O = H+ and O2
- Oxygen is either used in respiration or diffused out as a waste product
Non cyclic phosphorylation
Form of LDR which produces both ATP and NADPH and uses both PSI and PSII
Cyclic Phosphorylation
Form of LDR which produces small amounts of ATP, doesn’t produce NADPH and only uses PSI
Light Independent Reaction - The Calvin Cycle
- Occurs in the storma
- CO2 combines with ribulose biphosphate (5C) - catalysed by rubisco enzyme
- CO2 and RuBP produces 2 molecules of glycerate phosphate (3C)
- ATP produced in LDR provides energy to reduce GP to triode phosphate
- Reduced NADP from the LDR is used to reduce GP to TP (provides hydrogen)
- Some TP molecules (1/6) are converted to organic substances e.g. glucose, sucrose and amino acids
- Most TP molecules (5/6) are used to regenerate RuBP, using ATP from the LDR, which supplies the phosphate.
Products of The Calvin Cycle
Explain why the apparatus needs to be airtight
So that they’re not artefacts such as air bubbles, which would increase the volume of gas measured and so, provide an unreliable result
Explain why the temperature of the water bath needs to be kept constant
Soo that it is only the light intensity that is affecting the rate of photosynthesis, not temperature
Suggest an advantage of providing an additional source of CO2
To ensure that there’s sufficient CO2 and O2 and they don’t limit the rate of photosynthesis
Suggest a reason for carrying toy the experiment in a dark room
To prevent other light falling on the plant as thus may fluctuate and will affect the light intensity and so the rate of photosynthesis
Suggest why the plant is kept in the dark before the experiment begins
In order to prevent starch from interfering, we remove the starch by keeping the plant in the dark. When in the dark, the photosynthesis doesn’t occur and the stored starch is sued up
Suggest why measuring the volume of gas produced by the plant in this experiment and may not be an accurate measure of photosynthesis
Gases are often fluid and nice subtle and can come out quickly as bubbles so may be difficult to count
Optimum conditions for plants - light intensity
High light intensity - the higher the light intensity the more energy there’s provided for LDR of photosynthesis and so the more glucose is produced and so faster the rate of photosynthesis. If too high, chlorophyll could become damaged.
Optimum conditions - wavelength of light
The photosynthetic pigments chlorophyll a &b and carotene only absorb red and blue light from sunlight - so by suing the optimum wavelength the rate of photosynthesis will be increases.
Optimum conditions - temperature
Optimum temp for enzymes is 25 degrees. If the temp falls below 10 degrees, the enzymes become inactive. If the temp rises above 45 degrees, the enzymes denature,. Stomata start to closer in high temps to avoid water loss and so less CO2 can diffuse into the plant.
Optimum conditions - water
If the water was too little, photosynthesis would stop. If too much the plant becomes waterlogged and the lack of O2 reaching roots results in less aerobic respiration and so there would be less ATP for the active transport of minerals.
Commercials glasshouses
- Artificial lighting with specific wavelengths
- Pump CO2 into glasshouse/use propane/ paraffin burners
- Ventilation
- Glass panels - stops heat from escaping as well as allowing light through
- Thermostat
- Humidifier
Structure of a chloroplast
What is the compensation point ?
The point at which the volume of oxygen and carbon dioxide absorbed during respiration is exactly the same as the oxygen and CO2 used given out during respiration.
Required Practical 8 - The effect of a factor on the rate of dehydrogenase activity
METHOD
- Remove stalks from leaf samples. Grind sample using pestle and mortar then place into a chilled isolation solution.
- Use a muslin cloth and funnel to filter the sample into a beaker
- Suspend the beaker in an ice water to keep sample chilled.
- Transfer to centrifuge tubes and centrifuge at high speed for 10 mins = chloroplast separated into the pellet
- Remove supernatant and add pellet to the fresh isolation medium.
- Set the colorimeter to the red filter. Zero using a cuvette containing chloroplast extract and distilled water.
- Place test tube 30 cm from light source and add DCPIP.
- Take a sample and add to cuvette. Then measure its absorbance using the colorimeter
- Take a sample and measure its absorbnace every 2 mins for 20 minutes
- Repeat for different distances from lamp up to 100 cm (vary light intensity)
Required Practical 8 - The effect of a factor on the rate of dehydrogenase activity
HAZARDS
- DCPIP - irritant to skin and eyes so wear eye protection
- Biohazard - contamination so wash hands after use
- Lamps - damage to eyes so don’t look directly at lamp
- Broken glass - cuts from sharp object so take care when handling glassware and keep away from edge of the desk
Required Practical 8 - The effect of a factor on the rate of dehydrogenase activity
CONCLUSION
As the light intensity decreases the rate of photosynthesis also decreases. This is because the rate of photoionisation is slowed down so the overall rate of the light dependent reaction will be slower.
Less electrons are realised by the chlorophyll, so DCPIP accepts less electrons. It will take longer to turn from blue to colourless.
When DCPIP is blue, absorbance is higher. Higher rate of decrease, shown by a steep gradient on the graph, means that the dehydrogenase is highly active.
Required Practical 8 - The effect of a factor on the rate of dehydrogenase activity
CONTROL VARIABLES
Source of chloroplast - spinach bought from same store
Volume of chloroplast suspension and DCPIP - use graduated cylinder
Concentration of DCPIP - 5cm^3 DCPIP solution was combined with 1cm^3 of water
