Photosynthesis Flashcards
What is the chemical equation for photosynthesis?
6CO2 + 6H2O ➡️ C6H12O6 + 6O2
What are two types reactions involved in photosynthesis?
- Light dependent reaction
*Trapping of light energy by photosynthetic pigments in chloroplast
*Use light energy to excite electrons in chlorophyll (photoactivation) and split water (photolysis)
*Energy produced as ATP and NADPH (reduced NADP)
*Energy transferred to light-independent reaction
2.Light-independent reaction
*AKA the Calvin cycle
*energy from light-dependent reactions used for…
*Fixation of carbon dioxide/ carbon fixation to produce of complex organic molecules (such as carbs, amino acids and lipids)
What are photosynthetic pigments?
*Trap light energy
*Found on thylakoid membrane of chloroplasts
*pigments are arranged in light-harvesting clusters =photosystems
*Diff pigments absorb different wavelengths of light
Two groups of photosynthetic pigments:
1.Chlorophylls
e.g. chlorophyll a,chlorophyll b
- Carotenoids
e.g. Beta-carotene,xanthophyll
Chlorophyll a is a primary pigment
*The rest are accessory pigments
How are photosynthetic pigments arranged?
- pigments are arranged in light harvesting clusters = photosystems/antenna complex
*Primary pigments absorb light and act as reaction centres in the light-dependent reactions of photosynthesis
*Accessory pigments surround a primary pigment
➡️Accessory pigments may absorb diff wavelengths of light
➡️Light energy are then passed to primary pigments.
Two types of photosystems:
*Photosystem II (PSII)
*Photosystem I (PSI)
What is the colour of the photosynthetic pigments?
Chlorophyll:
Absorbs mainly red and blue-violet lights
Reflects green light
Carotenoids:
Absorb mainly blue-violet lights
Reflects red light
Chlorophylls:
Chlorophyll a=yellow-green
Chlorophyll b=blue-green
Carotenoids:
Beta-carotene=orange
Xanthophyll=yellow
What is the light absorption spectrum?
*Graph of light absorbance by pigments at different wavelengths of light.
What is photosynthetic action spectrum?
*Graph of rate of photosynthesis at different wavelengths of light.
*Related to absorption spectrum (but different)
*Also dependent of wavelength of light
(Shorter wavelength, more energy!)
What is chromatography used for?
Used to separate and identify different pigments
What are the steps in chromatography?
1) grind/blend leaves
* form suspension of plant extract with mixture of different pigments
2) place extract on a marked line of the chromatography paper using a small dropper. Dry and repeat.
3) place paper into solvent so that solvent is just below the extract
4) allow solvent to run (rise up paper) for a fixed time duration
*Solvent carry different pigments at different speed, so diff distance from loading line
*Depends on pigments mass
*Pigments separated➡️ chromatogram
5) Measure distance travelled by solvent and pigment
6) Calculate Rf value
7)Compare Rf values against published values to identify pigments
How do we calculate Rf value?
Rf= distance travelled by pigment/distance travelled by solvent
What are the two pathways of light-dependent reaction?
Two pathways:
1. Non-cyclic photophosphorylation
2. Cyclic photophosphorylation
What are the steps in light dependent reaction: Non-cyclic photophosphorylation
Step 1: Photoactivation
Step 2: Electron transport chain and ATP synthesis
Step 3: photolysis and reduction of NADP
Describe the first step in non cyclic photophosphorylation: photoactivation
Step 1: Photoactivation
*Light energy absorbed by both photosystems (PSII-p680 and PSI-p700)
*Passed to primary pigment at reaction centre
*Electrons excited to a higher energy level
*Electrons emitted from reaction centres
*Electrons captured by electron acceptors
Describe the second step of non-cyclic photophosphorylation: electron transport chain and ATP synthesis.
Step 2: Electron transport chain and ATP synthesis
*Electrons passed along the electron carriers of ETC
*Electrons release energy to produce ATP using chemiosmosis
*Energy used to pump H+ across membrane into the thylakoid membrane
*Proton gradient formed across the thylakoid membrane
*H+ move down gradient into stroma
*VIA ATP synthase
➡️To synthesise ATP (from ADP and Pi)
*ATP made is passed to light-independent reaction
*Electrons passed to PS I
Describe what happens in the third step of non-cyclic photophosphorylation: photolysis and reduction of NADP?
Step 3: Photolysis and reduction of NADP
*Occurs at PS II only
*Requires enzyme
*Splitting of water to H+ and OH-
*Electrons are removed from OH-
H20 ➡️ 2H+ + 1/2O2 + 2e-
*Electrons-replace electrons lost from PS II
*Oxygen-waste gas, released
*H+ ion-combine with de-energized electrons from PS I to reduce NADP
NADP + 2H+ +2e- ➡️ NADPH
How are electrons lost replaced in non cyclic photophosphorylation?
Electrons lost from:
1) PS II
➡️ Replaced by electrons from photolysis of water
H2O➡️2H+ + 1/2O2 +2e-
2) PS I
➡️ Replenished by electrons from PS II, after passing through the ETC
*Electron donor=H2O
*Final electron acceptor=NADP
*NADP reduced to NADPH
What can be seen in the “Z scheme” diagram of non-cyclic photophosphorylation?
Z-scheme
*Involves both photosystems
*Electrons flow from PS II ➡️ PS I
*Involves the photolysis of water
*Carried along electron carriers
➡️ATP is synthesized
*Final electron acceptor=NADP
Describe cyclic photophosphorylation of the light dependent reaction.
- involves only PS I
- reaction centre of PS I photoactivated
*Electrons excited and emitted from chlorophyll - captured by an electron acceptor
- passed along ETC
- energy released by electrons
➡️ATP synthesis by chemiosmosis
*Electrons returned to original Photosystem, PS I
*No photolysis of water involved
*No reduced NADP formed
*Fins electron acceptor= PS I
What is the hill reaction used for?
Used to determine the effect of light intensity/ wavelength of light on chloroplast activity
Based on photolysis of water and reduction of NADP in chloroplast
How does the Hill reaction work?
1)Chloroplasts are isolated
2)Presence of light and water
3) Redox agents (e.g. DCPIP)
*DCPIP=dichlorophenolindophenol
*Substitute for NADP
*Acts as an electron acceptor/oxidising agent
When DCPIP reduced:
Blue➡️colourless
O2 is produced
Rate of loss of=measure of chloroplast activity Based
What are the steps in the light independent reaction
*Occurs in the stroma of chloroplasts
*Does NOT require light
Steps:
1) Fixation of carbon dioxide
2) Reduction
3) Regeneration
What is the first step of light-independent reaction?
AKA Calvin Cycle
Step 1: Fixation of carbon dioxide
*Carbon dioxide (1C) is combined with ribulose biphosphate (RuBP) (5C)
*To produce 2 x glycerate-3-phosphate (GP or PGA) (3C)
*Catalysed by enzyme, ribulose biphosphate carboxylase (rubisco)
What is the second step of light-independent reaction?
AKA The calvin cycle
Step 2: Reduction
*The reduction of 2 x GP (Glycerate-3-phosphate) (3C)
*Uses ATP and reduced NADP
*Some NADP regenerated
*Produced 2 x triose phosphate (TP) (3C)
1/6 molecules of TP is converted to other molecules (e.g. carbs, amino acids, lipids) for uses in the plant cell.
What is the third step of the light independent reaction?
Step 3: Regeneration of RuBP
*5/6 molecules of TP is used to regenerate RuBP(Ribulose biphosphate)
*Uses ATP
How does the structure of chloroplast relate to its function?
*Main photosynthetic organelle
*Can be seen with a light microscope
*Second largest organelle in the cell (after nucleus)
*Appear as biconvex discs
*2 phospholipid membranes
*Stroma inside
*Stack of thylakoids=grana (singular granum)
How does the structure of grana relate its function in chloroplast?
- Grana
*Membrane has large surface area
*Holds pigments, enzymes, electron carriers needed for light-dep.reactions
*Holds ATP synthase needed for ATP synthesis by chemiosmosis
*Large no. of pigment molecules arranged to absorb light
*Arranged in light-harvesting clusters (photosystems)
*For efficient light absorption
*Diff pigments arranged in funnel-like structures
*Each pigments passes energy to next pigment
*Till it reaches the reaction centre
How does the structure of stroma relate to its function in chloroplast?
*Site of light-indep. reactions of the calvin cycle
*Contains enzymes of the calvin cycle,sugars and organic acids
*Bathes grana membranes so received products of light-dep. reactions.
*Also has 70S ribosomes, loops of DNA, lipid droplets and starch grains
*Ribosomes: produce chloroplast proteins (via translation)
How does the structure of a palisade mesophyll cell relate to its function?
- Overall cell arrangement and shape
*Cells are closely packed to absorb maximum light
*Light surface area for diffusion of gases
*Verticle to surface of leaf to reduce number of cross walls
*Cell walls thin for maximum light penetration/diffusion of gases
*Moist cell surfaces for diffusion of gases
*Near air spaces to circulate gases/provide a reservoir of CO2 - Chloroplasts
*Large number of chloroplasts to absorb maximum light
*Large vacoule pushes chloroplasts to edge of cell
→Short diffusion path for carbon dioxide
→Can absorb maximum light
*Chloroplasts can move
→Towards light
→Away from high light intensity to avoid damage
How does the structure of dicotyledonous leaf relates to its function?
*Thin/flat to give large surface area to volume ratio
*Held at right angles to sun to allow max.light absorption.
*Has cuticle on upper surface to prevent water loss via cuticular transpiration
*Has closely packed palisade mesophyll arranged vertical to the surface of leaf
*Has spongy mesophyll that provides large surface area for CO2 uptake/gas exchange
*Mesophyll cells have moist surfaces for diffusion of gases
*Has stomata/guard cell for entry of CO2
*Has xylem to supply water/mineral ions and act as support
*Has phloem for translocation of products of photosynthesis
What is a limiting factor?
A factor that restricts a process’s rate
What are the limiting factors affecting the rate of photosynthesis?
- Light intensity
- Carbon dioxide concentration
- Temperature
Describe how light intensity acts a limiting factor affecting the rate of photosynthesis?
Rate of photosynthesis increases with light intensity
But levels off due to other limiting facrs such as temperature and [CO2]
Describe how carbon dioxide acts a limiting factor affecting the rate of photosynthesis?
Rate of photosythesis increases with [CO2]
But levels off due to limiting factors: temp and light intensity
Describe how temperature acts a limiting factor affecting the rate of photosynthesis?
*Rate of photosynthesis increases with temperature
*But decreases after the optimum temp because: *Rubisco has higher tendency to catalyze reaction with O2 with RuBP, instead of CO2 (photorespiration) at high temp and high light intensity
*Enzymes also start to denature
How can we measure the rate of photosynthesis?
*Use aquatics plants such as Elodea and Cabomba
*Use gas syringe/microburette/photosynthometer
1. Cut shoot of aquatic plant
*Must cut step cleanly,under water
*Plant must be well illuminated b4 use
2. Place shoot in tube of hydrogen carbonate solution (to provide CO2) and water bath to mantain temp
*Water needs to be aerated beforehand by bubbling air through it, to prevent other gases from dissolving in water
3. Place in different conditions to investigate its effect on the rate of photosynthesis
*Light intensity-Place light source 5 min different distances, d from plant
*Wavelength of light-Use min 5 different colour filters
*Concentration of cafrbon dioxide-Add min 5 diff concentrations of hydrogen carbonate solution to water surrounding plant
*Temperature-use water bath of min 5 diff temperatures
- Alllow aclimatization
- Count no. of bubbles of oxygen gas produced per unit time OR collect oxygen gas and measure length/volume produced per unit time
- Repeat at least three times+mean
