Photosynthesis Flashcards
Light-dependent reaction, light-independent reaction and the chloroplast
What is oxidation?
- loss of electrons or hydrogen
- gain of oxygen
What is reduction?
- gain of electrons or hydrogen
- loss of oxygen
Where does photosynthesis take place?
Chloroplasts within leaves
Why are leaves adapted?
to bring together 3 raw materials of photosynthesis
- H2O, CO2 & light & remove O2 and C6H12O6
Adaptations of leaves for photosynthesis
- large SA to absorb lots of light
- arrangement of leaves on plant to minimize overlapping (avoids shadowing of 1 leaf by another)
- Thin so DD for gases kept short
- numerous stomata for gaseous exchange so all mesophyll cells only short DD from one
- transparent cuticle & epidermis to let light through to photosynthetic mesophyll cells below
- long, narrow upper mesophyll cells packed with chloroplasts that collect sunlight
- stomata that open & close in response to changes in light intensity
- network of xylem that bring water to leaf cells & phloem that carries sugars away produced during photosynthesis
- many air spaces in lower mesophyll level to allow rapid diffusion in the gas phase of O2 & CO2
Photosynthesis equation
6CO2 + 6H2O –>(light on arrow) C6H12O6 + 6O2
What is photosynthesis?
- metabolic pathway with many intermediate reactions:
(process of energy transferral where some of energy is conserved in form of chemical bonds
1 capturing of light energy
2 light-dependent reaction
3 light-independent reaction
Shape of chloroplast (where photosynthesis takes place)
- disc-shaped & 2-10 micrometers long & 1 micrometer in diameter
Structure of Chloroplast?
- surrounded by double membrane
- inside are 2 distinct structures: grana & stroma
What is the grana?
stacks of thylakoids (disc-like structure) (where light dependent stage happens)
- within thylakoids is chlorophyll
What is intergranal lamellae ?
tubular extensions that join up with thylakoids in adjacent grana in some thylakoids
What is the stroma?
fluid filled matrix (where light-independent stage happens)
-within stoma are other structures e.g. starch grains
Uses of glucose
- storage molecule (starch) stores chemical energy
- used in respiration
- cellulose
- used to make lipids (glycerol)
- used to make AAs
How does water enter plants?
Roots in transpiration
How does carbon dioxide come into plants?
Diffuses into leaves
The light-dependent reaction of photosynthesis involves what?
- capture of light whose energy is used for 2 reasons:
- add Pi molecule to ADP making ATP
- split water into H+ ions (protons) & OH- ions, splitting caused by light (PHOTOLYSIS)
3 Equations involved in light-dependent reaction of photosynthesis
ADP + Pi -> ATP
NADP + H+ + 2e- –> NADPH (reduced NADP)
H20 –> 2H+ + 2e- + 1/2 O2
Model Answer: Describe the events of the light-dependent reaction of photosynthesis
1 photon of light hits chlorophyll in photosystem 2 (PS2) causing photoexcitation - raises 2e- up an energy level, they leave PS2 (photoionisation) in an Electron transport chain (ETC)
2 they travel through electron carriers in a series of redox reactions releasing energy as they go
3 energy released from electron carriers in ETC is used as energy to move H+ ions against their conc gradient from stroma into thylakoid lumen, H+ ions move through ATP synthase down their conc gradient which catalyses ADP and Pi to form ATP (this is chemiosmosis)
ADP + Pi –> ATP
4 another photon of light hits chlorophyll in PS1 & re-excites electrons and they leave PS1 and ETC and join with NADP
H+ ions that moved down their conc gradient combine with NADP also to form reduced NADP in stroma
NADP + H+ + 2e- –> NADPH
5 Photolysis of water provides e- to reduce the chlorophyll in PS2 and replace the H+ used in the reduction of NADP.
H20 –> 2H+ + 2e- + 1/2O2
What function is chloroplast adapted to?
capturing sunlight and carrying out light dependent reaction of photosynthesis
Site of the light-dependent reaction
thylakoids of chloroplasts
- disc-like structures stacked together in groups (called grana)
State 4 ways Chloroplast’s structure is adapted to its function
1 thylakoid membranes provide large SA for attachment of chlorophyll, electron carriers and enzymes that carry out LD reaction
2 chlorophyll arranged in photosystems to maximise absorption of light
3 granal membranes have ATP synthase channels within them (catalyses production of ATP), also selectively permeable which allows establishment of proton gradient
4 chloroplasts contain both DNA & ribosomes so they can quickly & easily manufacture some of the proteins involved in the light-dependent reaction
Where does the Light-independent reaction of photosynthesis take place?
stroma of chloroplasts
Another term for the light independent reaction?
The Calvin Cycle
Model answer: Describe the process of the Calvin Cycle
1 RuBP (Ribulose Biphosphate) combines with CO2 to form a 6-carbon intermediate
2 catalysed by rubisco
3 6 carbon molecule breaks down into 2 x GP (glycerate-3-phosphate)
4 GP reduced to TP (triose phosphate) using reduced NADP - using energy from ATP & (both from light-dependent reaction)
5 some TP is used to manafacture organic substances/hexose sugars, some used to regenerate RuBP
note: organic substances produced from TP could be lipids, glucose, cellulose, amino acids, starch, nucleotides i.e. what the plant requires
Why does when RuBP combines with CO2 it breaks down into 2 3-carbon molecules?
it’s an unstable intermediate molecule
How is Glycerate-3-phospahte reduced to Triose phosphate?
reduced NADP from LD reaction is used to reduce GP to TP - using energy supplied by ATP
State 2 uses of the triose phospahte produced
1 some of the triose phosphate is used to regenerate RuBP in the Calvin cycle
2 some of the triose phosphate is converted to useful organic substances
How does the NADP go back into the LD reaction after its use in the calvin cycle?
NADP re-formed and goes back into light-dependent reaction to be reduced again by accepting more protons
What are some of the useful organic substances TP (triose phosphate) is converted into?
- starch
- cellulose
- lipids
- AAs
- nucleotides
How is the chloroplast adapted to carrying out the light-independent reaction of photosynthesis?
1 fluid of stroma contains all enzymes needed for this reaction. Stromal fluid is membrane-bound in chloroplast so a chemical environment which has a high conc of enzymes and substrates can be maintained there - distinct from environment of cytoplasm
2 stromal fluid surrounds grana and so products of light-dependent reaction in the grana can readily diffuse into stroma
3 contains both DNA and ribosomes so it can quickly and easily manafacture some of the proteins involved in the light-independent reaction
Identify environmental factors that limit the rate of photosynthesis
Light intensity
Temperature
CO2 concentration
State the importance of sufficient water in photosynthesis
plants need a constant supply of water
too little - photosynthesis stops
too much - soil becomes waterlogged (reducing uptake of minerals)
State all optimum conditions for photosynthesis
- high light intensity of a certain wavelength
(higher light intensity = more energy provided) - temp around 25 C
(too low would make enzymes inactive, too high would denature them e.g. rubisco and ATP synthase) - Carbon dioxide at 0.4%
(gives a higher rate of photosynthesis but higher and stomata start to close)
How agricultural growers (farmers) counteract limiting factors that limit plant growth?
they create optimum conditions in glasshouses:
CO2 conc - CO2 added to air (by burning small amount of propane in CO2 generator)
Light - lights can get in through glass, lamps provide light at night
temp - glasshouses trap heat energy from sunlight, which warms air - heaters & cooling systems used to maintain optimum temp, air circulation makes sure temp is even throughout glasshouse
What is the function of the light-dependent reaction?
- to convert solar energy into chemical energy in the form of NADPH and ATP, which are used in calvin cycle and fuel assembly of sugar molecules
What is the function of the calvin cycle?
- making simple sugars from carbon dioxide and water
state products of the light-dependent reaction
reduced NADP, ATP, Oxygen
state products of the light-independent reaction
sugars and other organic molecules
suggest why measuring the volume of gas produced by a plant (while investigating photosynthesis) may not be an accurate representation of rate of photosynthesis
- vol of O2 produced will be less than that produced by photosynthesis as some of the O2 will be used in cellular respiration
Do plants respire or just photosynthesize?
- plants respire in the normal way i.e. using glycolysis, Krebs cycle, oxidative phosphorylation etc.
- but respiration is masked by the fact that photosynthesis produces oxygen faster than respiration takes it up and photosynthesis uses carbon dioxide faster than respiration produces it
The concentrations of Carbon dioxide in the air at different heights above the ground in a forest changes over a period of 24 hours. Use your knowledge of photosynthesis to describe these changes and explain why they occur [5 MARKS]
- High concentration of carbon dioxide linked with night/darkness;
- No photosynthesis in dark/night / light
required for photosynthesis/light dependent reaction; - (In dark) plants (and other organisms)
respire; - In light net uptake of carbon dioxide by
plants/plants use more carbon dioxide than they produce/ rate of photosynthesis greater than
rate of respiration; - Decrease in carbon dioxide concentration with height;
- At ground level fewer leaves/less
photosynthesising tissue/more animals/less light;
microorganisms make the carbon in polymers in a dead worm available to cells in a leaf. Describe how [5 MARKS]
- Microorganisms are saprobionts
- Secrete enzymes (onto dead tissue) /
extracellular digestion; - Absorb products of digestion/smaller
molecules - Respiration (by microorganisms) produces carbon dioxide;
- Carbon dioxide taken into leaves;
- Through stomata;
