Chapter 11: Photosynthesis Flashcards
(47 cards)
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

