Photosynthesis Flashcards
Describe the method of studying the density of stomata
- Apply clear nail polish to the lower epidermis of a leaf between the veins and allow it to dry
- Using forceps, peel the nail polish from the leaf to produce a replica of the lower epidermis
- Place the replica on a microscope slide and count the number of stomata using the microscope
Other than the presence of stomata, give 3 adaptations of the leaf for photosynthesis
- Large surafce area: absorbs as much light as possible
- Thin: light penetrates all levels
- Air spaces in the spongey mesophyll: allows CO2 to diffuse into photosynthesising cells
Biochemical reactions in cells often involve cyclic metabolic pathways which occur in compartments (such as chloroplasts) inside cells. Why is this important?
- Reactions can occur in small volume/ isolation of enzymes/reactants
- Membranes isolate reactions within organelles, keeping products seperate from the cytoplasm
- Mitochondria can be positioned close to areas needing ATP, and organelles ca move within the cytoplasm
State the location and function of the light dependent stage of photosynthesis
Thylakoid membrane and thylakoid space of chloroplast
Involves the conversion of light energy into chemical energy (ATP and NADPH)
State the location and function of the light independent stage of photosynthesis
Stroma of chloroplast
Uses the prducts of the light dependent reaction (ATP and NADPH) to produce organic molecules such as glucose
Where are photosynthetic pigments located?
Within the thylakoid membranes of chloroplasts
During photosynthesis, different wavelengths of light strike the leaf and are absorbed by varous pigments. What is the role of the pigments?
Capture light energy from the sun and convert it into chemical energy
What are the 2 main types of photosynthetic pigments in flowering plants?
Primary pigments and accessory pigments
What is the primary pigment and which wavelengths does it absorb?
Chlorophyll a absorbs blue and red wavelengths of light
What are the secondary pigments and which wavelengths do they absorb?
- Chlorophyll b absorbs blue and red wavelengths of light
- Carotenoids absorb violet/blue light (xanthophyll & carotene)
Why are accessory pigments important?
They absorb wavelengths of light not absorbed by the primary pigments
This ensures a wider range of wavelengths are absorbed, increasing the efficiency of photosynthesis
What macronutrient is needed to produce chlorophyll a?
Magnesium ions
Describe how chromatography can be used to separate photosynthetic pigments
- Tear up leaves and grind with acetone to form a dark green pigment solution
- Use capillary tube to spot the pigment on to chromatography paper
- Place the chromatography paper into a solvent
- Remove the chromatography paper once the solvent has travelled to the top of the strip. Observe the different distances the pigments have travelled
- Calculate the Rf value for each pigment
- Identify each pigment by comparing Rf values to known values
Why do we add acetone when grinding leaves during chromatography?
Acetone dissolves the phospholipid membrane to extract the pigment
How do you calculate the Rf value?
distance travelled by the pigment/distance travelled by solvent
Where are photosystems located?
On the thylakoid membranes of chloroplasts
What is the role of photosystems?
- Absorb photons of light energy
- Act as transducers, transferring this light energy to high energy electrons
- This energy is then used to fuel proton pumps and synthesise ATP
Describe the photosystem structure
Accesssory pigments are grouped into clusters with associated proteins, forming an antenna complex
These pigments absorb photons of light energy and funnel this energy down the antenna complex to the reaction centre
Chlorophyll a molecules are found in the reaction centre, below the antenna complex
When photons of light energy reach chlorophyll a, electrons are excited to become high energy electrons
At what wavelength does photosystem I best absorb light?
700nm
At what wavelength does photosystem II best absorb light?
680nm
What 6 things does the light dependent reaction involve?
- Requires light energy
- Takes place in the thylakoid membranes and thyakoid space
- Involves the synthesis of ATP from ADP and Pi (via photophosphorylation)
- Involves the splitting of water using light (photolysis)
- Involves the reduction of NADP to form redNADP
- Releases oxygen gas as a by product
What are the 2 light dependent reactions in higher plants?
- Non-cyclic photophosphorylation
- Cyclic photophosphorylation
Describe what happens at photosystem II during non-cyclic photophosphorylation
- Photons of light are absorbed by PSII and passed to chloropyll a in the reaction centre
- This excites 2 electrons on chlorophyll a to a higher energy level, causing them to be emitted, passed to electron acceptors and then transferred along a chain of carriers (including a proton pump) to photosystem I
- As the electrons move along the chain, they release energy, which is used to pump hydrogen ions (protons) from the stroma across the thylakoid membrane into the thylakoid space
- This creates an elecrochemical gradient due to the high H+ concentration in the thylakoid space and low concentration in the stroma
- H+ flow down this proton gradient, through a proton channel connected to ATP synthase providing energy for the formation of ATP from ADP and Pi
Describe when happens at photosystem I during non-cyclic photophosphorylation
- Photons of light energy are absorbed by PSI, causing 2 electrons in chlorophyll a to be raised to a higher energy level, emitted, and passed to another electron acceptor
- The elecron acceptor transfers the electrons to protons outside the thylakoid membrane into the stroma, which then reduce NADP to NADPH
- The use of H+ to reduce NADP lowers their concentration in the stroma, helping to maintain the electrochemical gradient between the thylakoid space and the stroma
- The emitted electrons do not return to their original source so this process is known as non-cyclic photophosphorylation
Describe photolysis (happens during non-cyclic photophosphorylation)
- Photolysis is the splitting of water using light
- It occurs in the thylakoid space
- Water splits into oxygen, protons and electrons
- The electrons replace those lost by photosystem II, making the chlorophyll molecule stable
- The protons help to maintain the electrochemical gradient between the thylakoid space and the stroma
Where/when does cyclic photophosphorylation occur?
Occurs in bacteria and primitive plants. Higher plants also use this pathway to provide extra ATP , especially when CO2 is in short supply, or if levels of reduced NADP are high
Describe cyclic photophosphorylation
- Only photosystem I is involved. Light energy is absorbed at PSI and channelled to chlorophyll a
- Electrons in the chlorophyll a become excited, energised and released. They are accepted by an electron acceptor and eventually passed along the same tranfer chain as that used by the electrons from PSII
- Protons are pumped into the thylakoid space and ATP is produced from ADP + Pi
- The same chhemiosmotic theory as that used to describe the Z scheme is believed to be involved in the productionn of ATP
- Electrons excited and released form PSI return to PSI
How is the light independent stage linked to the light dependent stage?
The light independent stage (Calvin cycle) uses ATP and reduced NADP from the light dependent stage to fix carbon dioxide into organic molecules
Describe the light independent stage (Calvin Cycle)
- CO2 from the atmosphere is fixed with RuBP (5C)
- The enzyme RuBisCo catalyses this reaction
- Results in the formation of an unstable 6C compound which splits into two molecules of Glycerate-3-phosphate (3C)
- Glycerate-3-phosphate is then reduced using reduced NADP produced during the light dependent stage
- ATP from the light dependent stage is also required. ATP is hydrolysed and the phosphate transferrred to Glycerate-3-phosphate, resulting in the formation of 2 molecules of triose phosphate (3C)
- One carbon atom of the 6 available is removed from the Calvin cycle and can be used to produce organic molecules such as glucose
- Five carbon atoms remain in the Calvin cycle and are used to regenerate RuBP so the cycle can begin again
Name 2 symptoms of a deficiency of nitrates
- Reduced growth of organs
- Chlorosis - yellowing leaves due to inadequate chlorophyll production so plant no longer able to absorb light energy
Name a symptom of a deficiency of magnesium
Pronounced chlorosis between veins of older leaves as existing magnesium is moved and transported to new leaves
Name a symptom of deficiency of phosphates
Stunting of plant growth
What are the 3 limiting factors of photosynthesis?
- Temperature
- CO2 concentration
- Light intensity
How does an increase in temperature affect the rate of photosynthesis?
An increase in temperature generally enhances enzyme activity, thereby increasing the rate of photosythesis
The light dependent stage is much less dependent on enzyme activity than the light independent stage. Explain the implications of this on the overall rate of photosynthesis
An increase in temperature speeds up the LIS more than it speeds up the LDS. The supply of products from the LDS will not be able to keep up with the demand from the LIS. The overall rate of photosynthesis will become limited by the supply of reduced NADP and ATP
Describe a physical change that could occur in the leaf that might result in the rate pf photosynthesis decreasing at very high carbon dioxide concentrations
- Carbon dioxide diffuses into leaves and dissolves in water
- The enzyme carbonic anhydrase catalyses this reaction
- Carbonic acid is formed which dissociates to form H+ ions and HCO3- ions
- The accumulation of H+ ions causes cells to become more acidic
- The acidity denatures enzymes in mesophyll cells in the leaves
Explain the effect that an increase in light intensity will have on the yield of photosynthates
As the light dependent reaction speeds up, ATP and reduced NADP are produced at a faster rate. They enter the Calvin cycle and organic molecules are produced at a faster rate
Describe a physical change that could occur in a leaf that might result in the plateau at high light intensity. Explain the advantage of this to the plant
At high light intensity, temperatures are also high so stomata close to reduce transpiration/wilting
