Photosynthesis Flashcards
Photosynthesis
Process by which green plants make their own food:
Autotrophs
Make sure of inorganic compounds such as water and carbon dioxide to make complex compounds, hence plants are photoautotrophs
Photosynthesis products
Glucose but its derivatives are starch, cellulose, proteins, DNA, lipids, RNA
Features of light
Wave length
intensity
Duration
- To be used as energy source for organisms light is first converted to chemical energy: transduction
Quanta and Photon
Radiant energy comes in discrete pockets called quanta
A single quantum of light is called a photon
Chemical equation for photosynthesis
6CO2 + 6H2O –> C6H1206 + 6O2
Reduction process
Carbon dioxide is chemically reduced by the addition of hydrogen.
Carbon dioxide has been fixed in photosynthesis - incorporated of carbon dioxide into glucose
Oxidised
Water is oxidised
Stages
Light dependent and Light Independent
Light dependent: location + products
Thylakoid membrane of chloroplast
Cyclic and non-cyclic stages
Light energy is absorbed by chlorophyll to produce ATP, and NADPH (reduced NADP)
Using H atoms from water and releasing oxygen gas
Light Independent: location + products
Stroma of the chloroplast
Calvin Cycle
Products: reduced carbon dioxide to carbohydrates, NADP bring hydrogen to reduce carbon dioxide
Driven by energy from the hydrolysis of ATP (not direct sunlight)
Co-enzymes
- A molecule that aids the function of an enzyme
Work by transferring a chemical group from one molecule to another
Involved in photosynthesis - as NADP (transfer of hydrogen to one molecule to another
Redox reactions: Reduction
If something is reduced it has gained electrons and hydrogen or lost oxygen
Redox reactions: Oxidation
If something is oxidised it has lost hydrogen and electrons or gained oxygen
Oxidation of one molecule always involves the reductions of another
Features of leaves
Large surface area to volume with chlorophyll located close to the top of the surface
Thin structure for rapid light penetration
Network of air spaces for gases to reach the photosynthetic tissue
Vessels to transport water and take away to photosynthates - via the phloem
Photosynthates
Sucrose and amino acids
Chloroplast: structure
A type of organelle known as plastids (with colour)
Double membrane bounded / biconvex disc shape (5mm diameter)
Has internal membranes - thylakoid membrane, stack=granum + stroma
Thylakoid membrane - granum
Site of Light dependent stage
Contain PS l and PS ll (a form of chlorophyll)
electron carriers
ATP synthase (catalyse ATP formation)
Stroma
Jelly like substance
location of Light Independent stage
Chloroplast adaptations
Envelope: keeps reactants for photosynthesis close to reaction sites
Thylakoid: offers large S>A for light absorption
ATP synthase: an enzyme that makes ATP in LDR
Stroma: contains enzymes, sugars and organic acids for the LIR
Chlorophyll
Absorbs light of all wave lengths other than green(reflected)
IS a mixture of chemicals - chlorophyll a / chlorophyll b / carotenoids (carotenes +xanthophylls)
Carotenoids
called accessory pigments. They absorb light of different wave lengths from chlorophyll and than pass on the energy to the chlorophyll molecule. More pigments = leaf able to to absorb a greater range of wave lengths
Absorption spectrum
It shows the amount of light absorbed by chlorophyll or its component pigments at different wave lengths
Action spectrum
Shows the rate of photosynthesis of a plant at different wave lengths i.e. it shows how effective these pigments are in stimulating photosynthesis.
absorption + action spectrum relationship
Blue + red wave lengths which are strongly absorbed are also wave lengths which stimulate photosynthesis.
Photosynthesis depends on the effectiveness of different wave lengths in absorbing light
LDS: excitation of an electron
- Photon of light its chlorophyll molecule
- Energy is transferred to the other electrons in the molecule
- Electrons raised to a higher energy
If they are raised to a high enough level they will dissociate - Electrons are picked up by an electron acceptor - ATP is formed as the electron is passed along the electron transport chain
- As electrons are going down the electron transport chain energy is released which is used to combine ADP+Pi to make ATP
LDS: Cyclic photophosphorylation
Only uses PS l
- The excited electron is passed along a series of electron carriers. As it moves from one carrier to the next, each carrier becomes reduced then re-oxidised in a series of redox reactions.
Each carrier in the sequence has a lower energy level than the preceding one, so the electrons are passed along, enough energy is released to synthesise ATP from ADP+Pi.
The Electron is passed along a series of carriers the last one handing it back to the chlorophyll molecule, so it may be excited once again.
LDS: Non cyclic photophosphorylation
- An excited electron from PS ll passes to an electron acceptor and down an electron transfer chain to PS l which is at a lower level than PS 11
- This loss of energy allows the synthesis of ATP from ADP+Pi
- The loss an electron from PS ll stimulates loss of electrons from water (photolysis)
- This electron released by photolysis enters PS ll to replace the electron lost by chlorophyll after absorption of light.
- light energy can then excite an electron from PS l and this electron passes to an electron acceptor NADP
- NADP also takes up a hydrogen ion from water = NADPH
Photolysis of water
2H2o –> 4H+ + O2 + 4e-
Produces: hydrogen ions / electrons / oxygen
Each electron is recombined with a proton to form a hydrogen atom which is taken up by the hydrogen carrier NADP to form NADPH
Photolysis of water: protons
(H+ions) build up in the thylakoid lumen and then transfers too hydrogen acceptor
Photolysis of water: electrons
(e-)replace the excited chlorophyll electrons that moved up the chain. These must be replaced for the molecule to become stable and thus absorb light.
Photolysis of water: oxygen
diffuses out of the chloroplast and eventually into the air as waste product
Chemiosis
Formation of ATP
As light excited electrons move along the electron transport chain it releases energy. The energy released is used to transport protons to the thylakoid membrane
The build up of H+ ions generates a proton gradient across the thylakoid membrane.
This gradient is used to make ATP using ATP synthase enzyme as the H+ ions move through the spinning molecule
Energy released from this aids the combining of ADP+Pi = ATP
LIS: Calvin cycle
Calvin cycle is made up of three steps
- carbon fixing
- reduction
- regeneration
LIS: Calvin cycle - Carbon fixing
CO2 binds to a 5 carbon (5c) sugar ribulose bisphosphate (RuBP) to form 2 molecules of the 3-carbon compound (3c) glycerate phosphate (GP)
Uses the enzyme Rubisco
LIS: Calvin cycle - Reduction
Each GP molecule is reduced to form (3c) triose phosphate (GALP or TP)
ATP and NADPH from LDS are used here
ATP - provides the energy for this step
NADPH - acts as a reducing agent and provides hydrogen
The ADP and NADP return to the thylakoid membrane for recycling
LIS: Calvin cycle - Regeneration
Two GALP molecules turn into RuBP through the hydrolysis of ATP into ADP+Pi
Synthesis of organic substances occurs after 6 turns
Regeneration: GALP –> cellulose
After 6 turns the GALP (3c) is converted into a hexose sugar (beta glucose). Enzyme catalysed reaction
The B-glucose molecules chemically and repeatedly join up in condensation reactions to form 1, 4 glycosidic bonds between them
The product is a long unbranched chain a glucose polymers.
Neighbouring chains of these glucose polymers are held firmly together by cross-linking
Regeneration: GALP–> starch
After 6 turns GALP is turned into a hexose sugar (alpha glucose)
The a-glucose molecules join up through condensation reactions form 1,4 and 1,6 glycosidic bonds
Amylose: 1,4 glycosidic bonds - straight unbranched
Amylopectin: 1,4 + 1,6 glycosidic bonds - branched
Regeneration: GALP –> DNA
After 6 turns GALP is turned into a hexose sugar glucose
The glucose is then changed into a deoxyribose sugar
With help of Nitrates, Phosphates (actively absorbed) and GALP, DNA bases are also synthesised and DNA formation process starts
