Photosynthesis Flashcards
How does energy enter plants
- Energy in light absorbed by chlorophyll
- transferred into chemical energy of molecules formed during photosynthesis
- molecules used by plants to produce atp during respiration
What is the site of photosynthesis
- leaf is the main photosynthetic structure
- chloroplasts are the cellular organelles
What are the three raw materials of photosynthesis?
Water, carbon dioxide and light
What are the products of photosynthesis
Oxygen and glucose
How are leaves adapted to bring together raw materials of photosynthesis and remove waste products? (9)
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- large surface area= absorb as much light as possible
- arrangement of leaves= minimise overlapping so avoids shadowing of one leaf by another
- thin= light absorbed by by first few micrometers of leaf so short diffusion distance
- transparent cuticle & epidermis = let light through to photosynthetic mesophyll cells beneath
- long, narrow upper mesophyll cells packed with chloroplasts= collect as much sunlight as possible
- numerous stomata= gaseous exchange, all mesophyll cells have short diffusion pathway
- stomata open and close= response to changes in light intensity
- many air spaces in lower mesophyll layer= allow rapid diffusion in gas phase of CO2 & O2
- network of xylem & phloem= brings water to leaf cells, carries away sugars produced in photosynthesis
What is the equation for photosynthesis
6CO2+6H2O—(light)—> C6H12O6 + 6O2
What are the three main stages of photosynthesis?
- capturing of light energy
- the light-dependent reaction
- the light-independent reaction
What happens in the first main stage of photosynthesis?
Light energy captured by chloroplast pigment, chlorophyll
What happens in the second main stage of photosynthesis?
Some of the light energy absorbed is conserved in chemical bonds; during the process an electron flow is created by the effect of light on chlorophyll, causing photolysis to take place, creating NADPH, ATP and oxygen
What is photolysis
The light energy absorbed into the chlorophyll causing the splitting of water into protons, electrons and oxygen
What are the products of photolysis
NADPH, ATP and oxygen
What is the third main stage of photosynthesis
- Light-independent reaction
- The protons (hydrogen ions) are used to produce sugars and other organic molecules
What are the typical features of chloroplasts?
Disc-shaped, 2-10 nm long and 1nm in diameter, they are surrounded by a double membrane and inside contain two distinct regions; the grana and stroma
What is the grana
- stacks of up to 100 disc-like structures called thylakoids
What are thylakoids
-makes up grana
-where the light-dependent reaction takes place
-contains photosynthetic pigment, chlorophyll
- some have tubular extensions, that join up with adjacent thylakoids, called intergranal lamellae
What is the stroma
- fluid filled matrix where the light-independent reaction takes place
-contains a number of other structures such as starch
In the light-dependent reaction, what is light energy used for?
- to make ATP by adding inorganic phosphate to ADP
- to carry out photolysis and split water into H+ ions (protons) and OH- ions
What is oxidation and reduction?
OILRIG
- oxidation= loss of electrons/loses hydrogen
- reduction= gaining electrons/ gains hydrogen
What are excited electrons?
A pair of electrons boosted to a higher energy level from light energy absorbed by chlorophyll
What happens when electrons get excited?
They leave the chlorophyll molecule and are taken up by an electron carrier
What is photoionisation?
When a pair of excited electrons leave the chlorophyll molecule, therefore making the chlorophyll molecule ionised
In photoionisation what is oxidised and what is reduced?
Reduced= electron carrier
Chlorophyll molecule= oxidised
What are the stages that happen when electrons leave the chlorophyll molecule?
- Passed along electron carriers in a series of oxidation-reduction reactions,
- the electron carrier forms a transfer chain in the thylakoid membrane
- Each electron carrier is a slightly lower energy level than the last, so electrons lose energy at each stage
- Some of the energy is used to combine inorganic phosphate and ADP to make ATP
What is the chemiosmatic theory?
- Protons (H+) pumped into thylakoid from the stroma using protein pumps in thylakoid membrane
- Energy to drive this comes from electrons released during photolysis
- Protons produced during photolysis further increase concentration during thylakoid space
- Overall creates and maintains a concentration gradient of protons across thylakoid membrane with high conc inside thylakoid space and low conc in stroma
- Protons can only cross thylakoid membrane through ATP synthase channel proteins (rest of membrane impermeable to proteins)
—Channel proteins form small granules on membrane surface- also known as stalked granules - As proteins pass through synthase channels, cause changes to structure of enzyme which then catalyses combustion of inorganic phosphate and ADP to make ATP
What is the equation for photolysis?
2H2O —> 4H+ + 4e- + O2
What happens to the protons produced during photolysis?
- Protons pass out thylakoid space through ATP synthase channels and are taken up by electron carrier NADP
- NADP becomes NADPH (reduced)
- NADPH is main product of light-dependent stage and it enters light-independent reaction, taking with it electrons from chlorophyll molecules
How are chloroplasts structurally adapted to their function of capturing sunlight and carrying out light-dependent reaction of photosynthesis?
- Thylakoid membranes provide large surface area for attachment of chlorophyll, electron carriers and enzymes that carry out light-dependant reaction.
- Network of proteins in grana hold chlorophyll in very precise manner that allows maximum light absorption
- Granal membranes have ATP synthase channels which catalyse production of ATP, also selectively permeable which allows establishment of protein gradient
- Chloroplasts contain DNA and ribosomes so they can quickly and easily manufacture some of the proteins involved in light-dependent reaction
What are the main products of the light-dependent reaction in photosynthesis?
ATP
NADPH
What do ATP and NADPH do in the third stage of photosynthesis?
Reduce glycerate-3-phosphate
Where does the light-independent reaction take place?
Stroma
Who worked out the details of the light-independent reaction?
Melvin Calvin
What is the light-independent reaction sometimes referred to as?
The Calvin cycle
What happens in the first two stages of the Calvin cycle?
- Carbon dioxide from the atmosphere diffuses into the leaf through the stomata & dissolves in the water around the mesophyll cells—then diffuses through cell surface membrane, cytoplasm and chloroplast membranes into stroma of chloroplasts
- In the stroma, carbon dioxide reacts with the 5-carbon compound ribulose biphosphate (RuPB), reaction catalysed by ribulose biphosphate carboxylase (rubisco)
What happens in stages 3&4 of the Calvin cycle?
- Reaction between carbon dioxide and Rubisco produces two molecules of 3-carbon glycerate 3-phosphate (GP)
- NADPH from light-dependent reaction used to reduce glycerate 3-phosphate to triose phosphate (TP) using energy supplied by ATP
What happens in stages 5,6&7 of the Calvin cycle?
- The NADP is reformed and goes back to light-dependent reaction to be reduced again by accepting more protons.
- Some triose phosphate molecules are converted to organic substances plants require like starch, cellulose, lipids, amino acids and nucleotides.
- Most triose phosphate molecules are used to regenerate ribulose biphosphate using ATP from light-dependent reaction.
How are chloroplasts adapted for carrying out the light-independent reaction?
- fluid in stroma contains enzymes needed to carry out light-independent reaction, stromal fluid membrane bound which means chemical environment with high concentration of enzymes and substrates can be maintained within it.
- stromal fluid surrounds grana, so products of light-dependent reaction can readily diffuse into stroma
- contains both DNA and ribosomes so can easily create proteins involved in light-independent reaction
Describe the role of RuPB in the Calvin cycle
Reacts with carbon dioxide, catalysed by rubisco, to produce two molecules of 3-carbon glycerate 3-phosphate (GP)
Describe the role of RuPB in the Calvin cycle
Reacts with carbon dioxide, catalysed by rubisco, to produce two molecules of 3-carbon glycerate 3-phosphate (GP)
How is NADPH formed in the light-dependant reaction used in the light-independent reaction?
Reduces glycerate 3- phosphate to triose phosphate (TP)
