Chapter 11: Photosynthesis Flashcards
There are 3 main sequences in photosynthesis. What are they?
The absorption of light energy by the chlorophyll molecules in a plant cell
The light-dependent stage where light energy is directly used to form oxygen, ATP and reduced NADP (NADPH)
The light-independent stage where the products of the light-dependent stage are used to form triose-phosphate (TP) which goes on to form the products of photosynthesis (lipids, glucose)
What do plants need energy for?
- Photosynthesis
- Active transport (uptake of minerals from soil via roots)
- DNA replication
- Cell division
- Protein synthesis
What do animals need energy for?
Muscle contraction
Maintain a constant body temperature
Active transport (e.g. absorption of glucose)
DNA replication
Cell division
Protein synthesis
Where is the site of photosynthesis?
The site of photosynthesis is the leaves of a plant. The leaf is the main photosynthetic structure in eukaryotic plants, while the chloroplasts are the cellular organelles where photosynthesis actually occurs.
How is a leaf adaptated for effective photosynthesis?
- A large surface area for absorbing sunlight
- A thin structure so that diffusion pathways are short and because light is absorbed in the first few micrometres of the leaf
- A transparent epidermis and waxy cuticle that lessens water loss through evaporation and lets sunlight through to the mesophyll cells
- Long narrow upper mesophyll cells so that more of them can fit at the top of the leaf and packed with chloroplasts to absorb sunlight
- Stomata are able to open and close depending on light intensity, which helps reduce water loss when the plant is not photosynthesising
- Lots of air spaces in the lower mesophyll layer that allow quick diffusion of gases
Extras
- Many stomata for gaseous exchange so that no mesophyll cell is too far away from one (short diffusion pathway)
- A special arrangement of leaves makes leaves non-overlapping and stops leaves been overshadowed by each other
- A network of xylem vessels that transport water to leaf cells and a network of phloem vessels that carry away the sugars produced during photosynthesis
The raw materials for photosynthesis are…
Carbon dioxide
- colourless gas
- 0.04% of atmosphere
- enters microscopic pores/stomata in leaves
Water
- root hairs absorb water passively - no energy needed
- keeps plant tissue turgid
- maximising SA for light
- source of electrons
- solvent - all chemical reactions must occur in solution
Light
- catalyses the reaction
- absorbed by chloryphyll
The products of photosynthesis are:
Glucose
- used to make other substances such as fats and proteins
Oxygen
- may be used in respiration
- may diffuse out of plant via stomata
NADPH
- gives hydrogen to independent stage
ATP
- gives energy to independent stage
What is the chemical formula for photosynthesis?
6CO2 + 6H2O (+ energy) —- C6H12O6 + 6O2
What is reduction?
Reduction is when:
- a molecule gains electrons (e-) or hydrogen (which is essentially an electron and a proton)
- a molecule loses oxygen
What is oxidisation?
Oxidisation is when
- a molecule loses electrons (e-) or hydrogen
- a molecule gains oxygen
What is the mnemonic for oxdiation and reduction?
Oxygen
Is
Loss… of electrons
Reduction
Is
Gain… of electrons
Chlorophyll absorbs light energy from which part of the electromagnetic spectrum.
Chlorophyll absorbs light energy from the visible part of the electromagnetic spectrum
Why do plants have different pigments?
Each pigment absorbs a different wavelength of light so that the total amount of light absorbed is greater than if just one pigment was used
Having more than one type of pigment increases the range of wavelengths of light that a plant can absorb
Why do leaves appear green?
Only certain wavelengths of light are used for photosynthesis: 60nm and 700nm
The photosynthetic pigments chlorophyl a, chlorophyll b and carotene can only absorb red and blue light
Other wavelengths/green light is mostly reflected back, which is why plants look green.
Facts: Light dependent stage
First stage of photosynthesis
Needs light energy
Site: thylakoid membrane
Products:
- ATP
- NADPH
- oxygen
Facts: Light independent stage/Calvin cycle
Does not need light directly
Needs the products of light dependent stage
Products of light-dependent used to reduce/fix CO2 into carbohydrate
Site: stroma of chloroplast
Simple sugars formed from CO2
ATP supplies energy
NADPH supplies hydrogen/protons
6 cycles needed to form 1 hexose 6C sugar
Describe what happens in the light-dependent stage of photosynthesis
Light energy is absorbed by chlorophyll in photosystem II
- Chlorophyll becomes photoionised
- An electron is excited to a higher energy level and released from the chlorophyll molecule
- Electron passed down electron transport chain
- Energy released as it moves down
- Energy used to:
make ATP – photophosphorylation
ATP provides energy for light independent stage
reduce NADP – form reduced NADP or NADPH
NADPH carries hydrogen to light independent stage/has reducing power
Reduced NADP (NADPH)
Coenzyme/electron carrier used in photosynthesis
Main product of light dependent reaction
Protons produced from photolysis of water
Pass through ATP synthase channel
Into thylakoid space
Protons taken up by NADP
NADP is reduced when it picks up the protons
Has gained OILRIG
What is a coenzyme?
A coenzyme is a molecule that aids the functioning of an enzyme by transferring a chemical group from one molecule to another, i.e. it is a carrier
What is photolysis?
Photolysis is the splitting of a molecule using light energy.
Water H2O is split into
- oxygen O2
- protons H+
- electron e-
Replaces the electrons lost from chlorophyll
Describe the chemiosmotic theory…
The process of electrons flowing down the electron transport chain, creating a proton gradient across the thylakoid membrane to drive ATP synthesis
How is ATP made in the light dependent stage of photosynthesis?
Energy released from electrons passing down the electron transport chain is used to create an electrochemical/proton gradient across the thylakoid membrane
Protons transported/pumped into thylakoid
Protons build up in the thylakoid and move down their concentration gradient back into the stroma
Thylakoid membrane impermeable to protons
Have to pass through the ATP synthase enzyme embedded in the thylakoid membrane
The energy from this movement combines ADP and Pi to form ATP
This process of making ATP is photophoshorylation
What is cyclic photophosphorylation?
Cyclic photophosphorylation is a type of photosynthesis that occurs in bacteria.
It only involves PSI
The electrons released from chrolophyll return to PSI via an electron transport chain/electron carriers
The energy released when the electron passes down the transport chain is used to make small amounts of ATP
No oxygen or reducing power in the form of reduced NADP is formed
What happens to the rate of photosynthesis at different light intensities:
At high light intensity:
more light energy for the light-dependent stage meaning there is more light energy for ATP production and the production of reduced NADP
This means that the light-independent reaction can happen much more quickly as the products needed for it are being quickly supplied
As a result more sugars are produced that can be used for respiration
And as respiration increases, there is more energy for growth and there is a faster synthesis of new organic substance
What is the importance of reduced NADP in photosynthesis?
Reduced NADP is needed for biosynthesis
What is non-cyclic photophosphorylation?
Non-cyclic photophosphorylation is a type of photosynthesis that occurs in plants and cyanobacteria.
Two photosystems are involved: PSI and PSII
An electron from PSII is boosted to a higher energy level, released/emitted/ejected and passed down an electron transport chain to PSI and this electron transport chain generates a proton force used to make ATP
The electrons lost are replaced by the photolysis of water (releasing oxygen) and does not return to PSII
Hence this way of making ATP is called non-cyclic
Light energy is absorbed by PSI and an electron is excited and emitted, going towards making reduce NADP to NADPH
The ejected electron is replaced by an electron from PSII
How is reduced NADP formed in the light dependent stage of photosysnthesis?
Reduced NADP/NADPH is formed when an electron released from PSI is transferred to NADP along with a proton/H+ ion from the stroma
Describe what happens in the light independent stage of photosynthesis?
Carbon dioxide diffuses into the stroma from atmoshpere
- CO2 combines with 5-carbon molecule RuBP ribulose biphosphate
- reaction catalysed by enzyme rubisco
- results in a 6-carbon molecule
- very unstable so splits/breaks down quickly
- to give 2x 3-carbon molecules of GP glycerate 3 phosphate
ATP and NADPH are used to convert GP to 3-carbon molecule TP triose phosphate
- ATP is hydrolysed to provide energy to convert GP to TP
- NADPH donates protons needed to reduce GP/for the reaction to happen
For every 1 molecule of CO2 2 TP molecules are produced
How many cycles of the Calvin cycle are needed to produce 1 hexose sugar and why.
6 turns/cycles of the Calvin cycle are needed to produce 1 hexose sugar.
Because…
For every 1 cycle you get 2 molecules of TP
For 3 cycles you get 6 molecules of TP (2 x 3)
But for every 6 molecules of TP
5 are used to regenerate RUBP
Only 1 goes towards making a hexose sugar
ratio of 5:1
Carbohydrates or hexose sugars are made by…
joining 2 molecules of TP triose phosphate molecules together
Lipids are made using glycerol and fatty acids.
- Glycerol is sysnthesised from…
- Fatty acids are synthesised from…
Glycerol is sysnthesised from TP while fatty acids are synthesised from GP
Some amino acids are synthesised from…
GP
Name 3 photosynthetic pigments in the chloroplasts of plants.
chlorophyll a
chlorophyll b
carotene
At what wavelength does photosystem I absorb light best?
Photosystem I absorb light best at a wavelength of 700 nm
At what wavelength does photosystem II absorb light best?
Photosystem II absorbs light best at a wavelength of 680 nm
What are photosynthetic pigments?
Photosynthetic pigments are coloured substances within chloroplasts that absorb light energy at different wavelengths
Briefly explain how ribulose biphosphate RuBP is regenerated in the Calvin cycle. (2 marks)
For every 6 molecules of TP produced, 5 of them are used to regenerate RuBP
The rest of the ATP from the light-dependent stage is needed to convert TP to regenerate RuBP
Why is the Clavin cycle important?
It produces TP and GP
TP and GP are converted into useful substances
Such as: glucose, sucrose, starch, lipids, proteins
That are needed for growth of the plant
What are the optimum conditions for photosynthesis?
High light intensity
- light provides energy for light dependent stage
- higher light intensity = more energy
Temperature of 25 °C
- enzymes denature at above 45 °C and become inactive at below 10 °C
- stomata close to avoid losing water = less CO2
Carbon dioxide at 0.4%
- higher % of carbon = higher rate of photsynthesis
- hgher than 0.4% = stomata start to close
Constant supply of water
- not enough = photosynthesis stops
- too much = waterlogged cells
- reducing uptake of minerals needed to make minerals fro making chlorophyll a
What is the saturation point?
The saturation point is the point at which a factor is no longer the limiting the reaction
Something else has become the limiting factor
Increasing a variable makes no change to the dependent variable
?
Describe what is going on in this graph. Light intensity is plotted against rate of photosynthesis.
At the start:
- Light is the limiting factor/the rate of photosynthesis is limited by the light intensity
- as light intensity increases the rate of photosynthesis increases
- the graph reaches saturation point
- increasing light intensity after this point makes no difference
- something else has become the limiting factor
- The graph levels off
Describe what is going on in this graph. Temperature is plotted against rate of photosynthesis.
Name 2 factors that can limit plant growth.
- Temperature
- Carbon dioxide
- Light intensity
- Water
Why is it important that argicultural growers know about factors that limit photosynthesis?
They are able to use the information to create optimum environments/conditions with the right levels of everything that plants need
This increases growth
So increases yield
- more to eat
- more income
How do managment techniques in glasshouses help growers to create optimum conditions?
CO2 is added to the air by burning small amounts of propane
Light gets through the glass while lamps provide light at night
Glasshouses trap heat energy which keeps the air warm
Heaters and coolers used to maintain a constant optimum temp
Air cirulation used so that temperature is even throughout
Why would plants in a greenhouse with added CO2 grow faster on average and be larger than plants grown in a greenhouse with no added CO2?
CO2 is used by the plant for making glucose
glucose provides energy
the more CO2 the more glucose
more respiration
more energy/ATP
for cell division, DNA replication, protein synthesis
i.e. growth
Desribe 2 ways in which conditions can controlled in glasshouses to increase yields. (2 marks)
- Burning
2.
