Photosynthesis Flashcards
What is photosynthesis?
The process where glucose is formed from carbon dioxide and water using light energy, the light energy is converted into chemical energy.
6CO2 + 6H2O → C6H12O6 + 6O2
Adaptations of a leaf for photosynthesis
● Large surface area-absorbs as much sunlight as possible
● Arrangement of leaves on the plant that minimises overlapping - so avoids the shadowing of one leaf by another
● Thin - most light is absorbed in the first few micrometres of the leaf and the diffusion distance for gases is kept short
● Transparent cuticle and epidermis - let light through to the photosynthetic mesophyll cells beneath
● Long, narrow upper mesophyll cells - packed with chloroplasts that collect sunlight
● Numerous stomata - gaseous exchange so chat all mesophyll cells are only a short diffusion pathway from one
● Stomata - open and close in response to changes in light intensity
● Many air spaces in spongy lower mesophyll layer - allow rapid diffusion in the gas phase of carbon dioxide and oxygen
● Network of xylem - brings water to the leaf cells, and phloem that carries away the sugars produced during
photosynthesis.
3 main stages of photosynthesis
- CAPTURING OF LIGHT ENERGY - by chloroplast pigments such as chlorophyll
- THE LIGHT-DEPENDENT REACTION
- THE LIGHT INDEPENDENT REACTION - in which these protons (hydrogen ions) are used to produce sugars and
other organic molecules.
Structure of chloroplasts
- they are surrounded by a double membrane
- two distinct regions in the chloroplast membranes: grana (thylakoids) and stroma
- grana: stacks of up to 100 thylakoids
- stroma: gel like substance which contains enzymes, sugars and organic acids
Photosynthetic pigments
- Chlorophyll a
- Chlorophyll b
- Carotene
These are coloured substances that absorb light energy required for photosynthesis.
They are found in the thylakoid membranes attached to proteins.
Photo systems
A complex in which photosynthetic pigments are arranged in the form of clusters for the efficient absorption and utilization of sunlight energy in thylakoid membranes.
How are chloroplasts adapted to carry out light-dependent reactions?
● Thylakoid membranes provide a large surface area for the attachment of chlorophyll electron carriers and enzymes that carry out the light dependent reaction
● A network of proteins in the grana hold the chlorophyll in a very precise manner that allows maximum absorption of light.
● The granal membranes have ATP synthase channels within them, which catalyse the production of ATP. They
are also selectively permeable which allows establishment of a proton gradient.
● Chloroplasts contain both DNA and ribosomes so they can quickly and easily manufacture some of the proteins involved in the light dependent reaction
Light dependent reaction
-Occurs in THYLAKOID MEMBRANES and GRANA in chloroplast
OVERVIEW:
Light energy and water are used to create ATP and reduced NADP which are needed for the light-independent reaction
● Non cyclic phosphorylation:
○ Photoionisation of chlorophyll
○ Photolysis of water
○ Production of ATP and reduced NADP (NADPH2)
○ Chemiosmosis
● Cyclic phosphorylation
RESULTS FROM LDR FROM NON-CYCLIC PHOSPHORYLATION:
- NADPH2 - reduces CO2 to carbohydrate in light-independent reaction
- ATP - energy to build carbohydrate
- OXYGEN - waste product which comes from splitting of water by light
PHOTOIONISATION OF CHLOROPHYLL
The chlorophyll is ionised (lost an electron) by the light:
1.Light energy passes through photosystem 2.
2. When a chlorophyll molecule absorbs light energy, it boosts the energy, excites of a pair of electrons within this
chlorophyll molecule, raising them to a higher energy level.
3. They break free and leave the chlorophyll molecule
4. As result the chlorophyll molecule becomes ionised (photoionisation).
5. The electrons that leave the chlorophyll are taken up by a molecule called an electron carrier.
6. Chlorophyll molecule loses electrons - oxidised.
7. Electron carrier gained electrons - reduced.
8. Some of the energy from the released electrons is used to make ATP and reduced NADP in chemiosmosis
PHOTOLYSIS OF WATER
Light energy is absorbed by chlorophyll and splits water into oxygen, hydrogen and electrons.
H2O → ½ O2 + 2H+ + 2e-
● Water is taken in by the roots.
● Electrons removed from chlorophyll during ionisation are replaced by the electrons formed in photolysis.
● This is catalysed by an enzyme present in PSII which capture light energy for the reaction.
USES OF PRODUCTS OF PHOTOLYSIS OF WATER
➔ H+ ion - picked up by NADP which is reduced, forms NADPH which is used in light-independent reaction
➔ e- replace the electrons in the chlorophyll which were removed during photoionisation
➔ Oxygen produced is a waste product - used in respiration or released out of stomata by diffusion
CHEMIOSMOSIS, NON-CYCLIC PHOSPHORYLATION
AND - PRODUCTION OF ATP
- Electrons released from photoionisation of chlorophyll pass down electron transfer chain in electron carriers
(proteins) embedded within the thylakoid membranes from PSII to PSI via redox reactions. - As they move down the chain, they lose and release energy at each step which is used to actively transport H+ ions from the chloroplast stroma into the thylakoid through proton pumps (protein carriers) in the thylakoid membrane - raising concentration of protons in thylakoid above stroma.
- The photolysis of water also produces protons which further increases their concentration inside the thylakoid space.
- This creates and maintains a concentration/electrochemical proton gradient.
○ Higher concentration of protons inside the thylakoid space and a low concentration in the stroma. - The protons move down their concentration gradient from the thylakoid space into the stroma via the enzyme ATP synthase, which is embedded into the thylakoid membrane.
- As the protons pass through these ATP synthase channels they cause changes to the structure of the enzyme and provide the energy which catalyses the combination of ADP + Pi t→ATP.
NADPH PRODUCTION
- Electrons moving down the electron transport chain are losing energy.
- However when they are in the thylakoid space they appear near PSI
- Light energy is absorbed by PSI which excites the electrons
- They move from the thylakoid space up the photosystem into the stroma
- A hydrogen from the stroma and the electron come together to reduce NADP
- This process directly reduces NADP to NADPH (reduced NADP)
WHY DO PROTONS ACROSS MEMBRANE IN ATP SYNTHASE CHANNELS?
- Protons can only cross the thylakoid membrane through ATP synthase channel proteins
- The rest of the membrane is impermeable to protons.
-These channels form small granules on the membrane surface and so are also known as stalked granules.
COENZYMES
A molecule that aids the function of an enzyme.
- They work by transferring a chemical group from one molecules to another.
- NADP transfers hydrogen from one molecule to another to reduce a molecule
- or take hydrogen from a molecule to oxidise it
HOW ARE CHLOROPLASTS ADAPTED TO CARRY OUT
LIGHT-INDEPENDENT REACTIONS?
● The fluid of the stroma contains all the enzymes needed to carry out the light-independent reaction.
● The stroma fluid surrounds the grana and so the products of the light-dependent reaction in the grana can readily diffuse into the stroma.
● It contains both DNA and ribosomes so it can quickly and easily manufacture some of the proteins involved in the
light-independent reaction.
LIGHT-INDEPENDENT REACTION / CALVIN CYCLE
-Occurs in STROMA
-The products of the light-dependent reaction of photosynthesis - ATP and reduced NADP, are used to reduce glycerate
3-phosphate in the second stage of photosynthesis.
-Temperature affects this reaction as it involves ENZYMES.
OVERVIEW:
CO2 combines with RuBP to make TP. TP can then be used to make glucose (and other organic substances for plant)
1. Formation of GP
2. Formation of TP
3. Regeneration of RuBP
CALVIN CYCLE: Step 1
CO2 + RuBP → unstable 6 C compound →2 X GP (3-carbon glycerate-3-phosphate)
1. CO2 enters through stomata into leaf and diffuses into stroma.
2. Combines with ribulose biphosphate (RuBP), a 5 carbon compound.
3. Reaction catalysed by rubisco (ribulose bisphosphate carboxylase) enzyme.
4. This forms an unstable 6 carbon compound which quickly breaks down.
5. Forms 2 molecules of 3-carbon glycerate-3-phosphate.
CALVIN CYCLE: Step 2
ATP and reduced NADP used in reduction of GP to TP (triose phosphate)
6. Hydrolysis of ATP → ADP + Pi (from LDR) provides energy to reduce 3C GP into 3C triose phosphate
7. Reaction requires H+ ions which come from reduced NADP (NADPH) which is oxidised and recycled to form NADP.
8. The NADP goes back to the LDR to be reduced again by accepting more protons
9. ⅙ TP is then converted into useful organic hexose sugars e.g. glucose and ⅚ continues to regenerate RuBP
10. Each cycle releases 1 carbon from TP, the other 5 carbon go to next step
CALVIN CYCLE: Step 3
Regeneration of RuBP
11. Five out of every 6 molecules of TP produced in the cycle are used to regenerate RuBP
12. This uses the rest of the ATP produced by the LDR
INEFFICIENCY OF THE CALVIN CYCLE
The calvin cycle needs to turn 6 times to make 1 hexose sugar
● 1 CO2 cycle produces 2 TP
● 3 cycles produces 6 TP
● ⅙ of TP produced is actually used for sugar formation
● So for 3 turns, 1 useful TP is produced
● Sp 6 turns, 2 useful TP is produced
● 2x TP = hexose sugar
● So need 6 turns of the cycle
● 6 turns of the cycle needs from the LDR:
○ 18 ATP
○ 12 reduced NADP
LAW OF LIMITING FACTORS
At any given moment, the rate of a physiological process is limited by the factor that is at its least favourable value.
LIGHT INTENSITY AS LIMITING FACTOR
● Light provides the energy for the LDR - the higher the light intensity, the more energy it provides.
○ For photolysis of water
○ For photoionisation of chlorophyll
● When light is limiting factor, the rate of photosynthesis is directly proportional to light intensity.
● As light intensity is increased, the volume of O2 produced and CO2 absorbed due to photosynthesis will increase to the light compensation point:
○ The volume of oxygen produced and carbon dioxide absorbed due to photosynthesis is balanced by the oxygen absorbed and the carbon dioxide produced by cellular respiration.
○ At this point there will be no net exchange of gases into or out of the plant.
● Further increases in light intensity will cause a proportional increase in the rate of photosynthesis and increasing
volumes of oxygen will be given off and carbon dioxide taken up
TEMPERATURE AS LIMITING FACTOR
● Photosynthesis involves enzymes (ATP synthase and rubisco)
● If the temperature falls below 10oC, the enzymes become inactive.
● If temperature increases above 45oC, the enzymes denature.
● At high temperatures stomata have to close to limit water loss, causing photosynthesis to slow down as less carbon dioxide can enter the leaf when stomata are closed.
