photosynthesis Flashcards
What are autotrophs
- organisms that produce their own food from inorganic substances (e.g. CO2)
- harness light energy through photosynthesis, converting it into chemical energy
What are heterotrophs?
- cannot produce their own food
- need to ingest nutrition from other sources
What is photosynthesis/ what is its purpose
- biochemical pathway
- energy from sunlight cannot be directly used for cellular processes (cannot be transported, stored)
- photosynthesis transforms light energy into chemical energy in the form of glucose
Biochemical pathway of photosynthesis (balanced equation)
and word equation
6CO2 + 12H20 —-(light, chlorophyll) —- C6H12O6 +6O2 +6H20
carbon dioxide + water (light, chlorophyll) glucose + oxygen
word equation- where does each part come from/go
CO2: enters from atmosphere through stomata in the leaves
oxygen: released through stomata
water: taken up from roots, transported to leaves
light: sunlight hits the leaves and is absorbed by chlorophyll (not a reactant)
Stomata
- pores that allow the exhange of gases, and water to leave the plant
- surrounded by specialised cells (guard cells) which regulate opening and closing of pore
Light-dependent stage- where, and brief description
- in thylakoid membranes
involves:
- absorption of light energy by chlorophyll
- splitting of water
- production of NADPH and ATP
light-independent stage- what is it and purpose, when can it occur
- Calvin cycle
- occurs in stroma
- involves formation of glucose from carbon dioxide
- if NADPH and ATP are present, it can occur without light
Chloroplast, what are they and structure
Chloroplast:
- membrane bound organelles
- site of photosynthesis
- green due to chlorophyll
Structure:
- inner and outer membrane
- thylakoids (think individual pancake) (contain chlorophyll)
- granum (stack of thylakoids)
- stroma (gel like fluid, fills inner space, contains enzymes for light independent reaction)
Structure of chloroplast:
- outer and inner membrane
- thylakoids: membrane bound discs, contain chlorophyll, provide large surface area to capture sunlight
- thylakoid grouped into stacks called grana (singular- granum)
- enzymes in light-dependent stage located in thylakoid membranes
-stroma: fluid inside chloroplast, contains enzymes for calvin cycle
Photosynthetic pigments
- molecules that capture light energy
- many photosynthetic pigments- only need to know chlorophyll
- each photosynthetic pigment absorbs different wavelengths (colours) of light
Chlorophyll
- green photosynthetic pigment
- absorbs light energy
- found in thylakoid membrane
- absorbs violet wavelengths the best (then red, blue, yellow), reflects green
Light dependent stage
- Chlorophyll absorbs light energy from sun
- Energy trapped by chlorophyll is used to split water molecules into oxygen (O2), hydrogen ions (H+), high energy electrons
- Oxygen is a waste product- released into atmosphere via stomata.
- electron transport chain moves hydrogen ions from stroma into thylakoid to create a higher h+ concentration. - hydrogen ions pass through ATP synthase. This enzyme uses energy from H+ to synthesise a bond between ADP and Pi, creating ATP
- NADP+ accepts H+ that exit ATP synthase as well as electrons created by splitting of water, forming NADPH
Detailed process (don’t need to know everything)
- sunlight hits photosystem 2, energy is absorbed by chlorophyll which is used to excite electrons
- electrons become high energy
- when they have high energy, they have a tendency to lose energy to become less excited. they do this by being leaving photosytem 2 to an electron acceptor
- photosystem 2 has lost its electrons so it breaks the hydrogens off the oxygen (from H2o) and the electrons from H2O are released and go into photsystem 2, replacing electrons
- electrons release energy as they go, which is used to pump H+ into thylakoid
- electrons reach NADP+ reductase. negatively charged so attract H+ and NADP+, electrons go into NADP+ and hydrogen gets added to become NADPH
- higher H+ gradient inside membrane. facilitated diffusion through ATP synthase. releases energy, causing ATP synthase to rotate. as it rotates, it undergoes a conformational change so a phosphate ion and ADP can bind to form ATP
ATP and ADP +Pi
ATP:
- coenzyme
- transfers energy between reactions
- joining ADP and Pi requires energy which is stored in the bond created
ADP:
- the breakdown of ADP + Pi releases energy stored in the bond between 3rd phophate and ADP molecule
NADP+, NADPH
coenzyme
- transfer of electrons and protons between reactions, supporting anabolic reactions (e.g photosynthesis)
Inputs and outputs of light dependent stage
Inputs/reactants:
H20
NADP+
ADP + Pi
Outputs/products:
O2
NADPH
ATP
Light independent stage / calvin cycle
- Carbon fixation: (inorganic to organic)
Rubisco fixes CO2 from atmosphere to 5 carbon compound RuBP (inorganic carbon captured and attached to organic RuBP) This molecule is unstable and splits into 2 3 carbon molecules of PGA - Carbon reduction
- through multiple processes, PGA converted into glucose
- energy provided by breaking down ATP into ADP + Pi
- protons required donated by NADPH, which turns into NADP+ - Molecules not used in creation of glucose, used to regenerate RuBP (energy provided by ATP)
- H2O is also created from oxygen released from left over oxygen of CO2 with H+ provided by NADPH
2 cycles are required to make 1 glucose molecule
How can the rate of photosynthesis be measured?
- rate of CO2 consumption
- rate of O2 produced
- rate of glucose /starch produced
- rate of plant growth
How does light availability affect rate of photosynthesis?
- at low light intensity: photosynthetic rate is absent or slow
- with increasing light intensity, photosynthetic rate increases due to more light availablity for light dependent reactions
- beyond optimal light intensity (where it first reaches greatest rate), rate plateaus.
- this point is the light saturation point
- another factor is limiting (e.g. enzymes involved in LD are saturated, availability of CO2, number of chloroplasts)
How does water availability affect rate of photosynthesis?
too little:
- soil dries out, rate declines then stops
- stomata close to prevent further water loss, preventing uptake of CO2 needed for calvin cycle
- prolonged period of water deficit can cause plant death
too much water: waterlogging
- rate declines then stops
- waterlogged soil prevents cells in roots from being able to respire
- this in turn stops the roots from taking up water, limiting its availablity as an input for LD reactions
How does temperature affect rate of photosynthesis?
low temp:
- rate is slow
- low collision rates between reactants and eznyme active sites
increasing temp:
- increased rate
- more collisions between reactants and active sites
- reaches optimal point
beyond opitmal:
- rate declines
- due to heat denaturing enzymes (altered shape of enzymes means active site is no longer complementary to substrate)
How does CO2 concentration affect rate of photosynthesis?
low co2:
- rate is slow
- less glucose can be created
increasing co2:
- rate increases as more glucose can be created
- reaches optimal
beyond optimal:
- plateaus
may be due to:
- enzymes involved in carbon fixation become saturated
- availability of coenzymes (e.g NADPH) become a limiting factor
Describe the role played by each of the coenzymes NADPH and ATP in photosynthesis.
- NADPH transfers hydrogen ions and electrons
- ATP transfers energy
what type of molecule is glucose?
monosaccharide (simple sugar)
comparing C3,C4 and CAM: temp, water availability, stomata open when, examples
C3:
temp: low to moderate (15-25 degrees)
water availability: plentiful water
stomata open: during day
majority of plants (rice, evergreen trees)
C4:
temp: warm/humid/tropical (30-40)
water availability: tropical (low water)
stomata open: during day
e.g corn, sugar cane
CAM:
temp: hot/arid (40+)
water availability: drought (low water)
stomata open: during night
e.g cacti
Carbon fixation in C3 plants: Rubisco works most efficiently when:
- high CO2 levels in leaves
- low oxygen levels (occurs when water is freely available)
- moderate temperatures
C3, C4, CAM: differences:
1. Enzyme to fix CO2 from air
2. Product of carbon fixation
3. Location of carbon fixation events
4. Location of Calvin cycle
5. Enzyme starting Calvin cycle
Also, who is more prone to photorespiration, C3 or C4?
C3:
1. Rubisco
2. PGA
3. mesophyll cells
4. mesophyll cells
5. Rubisco
C4:
1. PEP carboxylase
2. Oxaloacetic / malate
3. first in mesophyll, second in bundle sheath
4. bundle sheath cells
5. rubisco
CAM:
1. PEP carboxylase
2. Oxaloacetic /malate
3. Both in Mesophyll cells (but different timing)
4. mesophyll cells
5. rubisco
C3 is more prone to photrespiration as C4 have adaptations to avoid this
Why are C3 plants called C3, C4 called C4
C3: when they fix CO2, the immediate product (PGA) has 3 carbon atoms
C4: when they fix Co2, immediate product has 4 carbons
Photorespiration:
- temperatures rise, low water availability: stomata close to prevent water loss
- blocks entry of CO2 and exit of O2, creating high O2 and low CO2 environment in mesophyll cells
- O2 is a competitive inhibitor: RUbsico binds to O2 rather than CO2
- When Rubisco binds to O2, photorespiration occurs: CO2 is produced
- less glucose produced, less plant growth
purpose of C4 and CAM plants
- have adaptations to prevent photorespiration and maximise photosynthesis
C4 plants:
(spatial)
Carbon fixation: mesophyll cells
- PEP carboxylase (instead of Rubsico) fixes CO2 to Malic acid (4 carbon compound)
- PEP carboxylase can only bind to CO2, so no photorespiration
Glucose production:
- malic acid transported to bundle sheath cells
- malic acid broken down into CO2, maintaining high CO2 concentration for calvin cycle
- Rubisco preferentially binds to CO2, allowing calvin cycle to occur
CAM plants:
- mesophyll cells
Carbon fixation (at night)
- stomata open, CO2 enters
- Co2 fixed by PEP carboxylase, forming malic acid (PEP carboxylase can only bind to Co2 so no photorespiration)
- malic acid stored in vacuoles in mesophyll cells
Calvin cycle (during day- stomata closed)
- malate transported from vacuoles, broken down to release CO2, maintains high CO2 concentration
- RUbisco preferentially binds to CO2, calvin cycle occurs, no photorespiration
- glucose production maximised, providing energy for plant growth
How do C4 plants assist in water preservation?
- capture more CO2 in less time
- reducing time stomata are open
