Photosynthesis Flashcards
Define photosynthesis
The production of carbon compounds in cells using light energy
What is the word and symbol equation for photosynthesis?
- Carbon dioxide + Water (+ Light energy) → Oxygen + Glucose
- 6CO2 + 6H2O → 6O2 + C6H12O6
Where does photosynthesis take place?
Chloroplasts
Describe the structure of chloroplasts
- Thylakoids - large surface area for light absorbing photosystems - Provides site for electron transport chain and chemiosmosis
- Thylakoid space - very small volume so a steep proton gradient builds up quickly
- Granum - stack of thylakoid membranes to maximise light absorption
- Stroma - contains enzymes needed for Calvin cycle (e.g. rubisco)
- Starch grain - storage of carbohydrate
- Outer and inner membranes
- 70S ribosomes & DNA
Describe how the structure of a chloroplast is related to its function
- Large surface area of thylakoids/grana for light absorption
- Space inside thylakoids for accumulation of protons
- Stroma contains enzymes used in Calvin cycle/light independent reactions
- Arrangement of photosystems to allow electron transport to take place
- Double membrane on the outside allows separation from rest of cell
- Presence of DNA & ribosomes for protein synthesis
- Starch grains store carbohydrates (from photosynthesis)
What is the electromagnetic (EM) spectrum?
- Range of all possible frequencies of electromagnetic radiation
- Visible spectrum is one region of the EM spectrum
What is the wavelength of visible light?
Between 400 and 700 nm
Compare wavelengths of light in the visible part of the EM spectrum
- Red - longest wavelength
- Violet - shortest wavelength
What is a pigment?
- A substance that absorbs light
- Colour of pigment comes from the wavelengths of light reflected (not absorbed)
What is chlorophyll?
- Pigment responsible for absorbing light
- Releases electrons used to produce ATP
Which photosynthetic pigments are found in the chloroplast?
- Chlorophyll a (main pigment)
- Chlorophyll b (accessory pigment)
- Carotenoids (accessory pigment)
- Xanthophylls (accessory pigment)
Where are the photosynthetic pigments found in the chloroplasts?
In photosystems
Where in a photosystem are accessory pigments found?
Light harvesting system (antenna complex)
What is the role of the pigments in the light harvesting system?
- Absorb light energy of different wavelengths
- Transfer energy to reaction centre
Where in a photosystem is chlorophyll a found?
Reaction centre
What is the role of the reaction centre?
- Releases high energy electrons
- Used in the light-dependent stage of photosynthesis
Outline the role of plant pigments in the process of photosynthesis
- Pigments in photosystems absorb photons of light
- Light energy excites electrons
- Energy passed from pigment to pigment
- Energy reaches reaction centre where chlorophyll a is found
- Accessory pigments allow for wider range of wavelengths to be absorbed
Compare chlorophyll’s absorption in the different parts of the visible spectrum
- Absorbs blue light most strongly, followed by red
- Reflects green light most strongly
Describe what an absorption spectrum shows
The wavelengths of light absorbed by each pigment (e.g. chlorophyll)
Define action spectrum
Graph showing overall rate of photosynthesis at each wavelength of light
What is an action spectrum of photosynthesis?
- Graph showing overall rate of photosynthesis at each wavelength of light
- Maximum rates occur in blue light, followed by red light
- Lowest rates occur in green light
Distinguish between absorption spectrum and action spectrum
- Absorption spectrum shows wavelengths of light which are absorbed by each pigment
- Action spectrum shows overall rate of photosynthesis at each wavelength of light
Chlorophyll reflects most green light yet there is still use of green light on the action spectrum.
Explain how this is possible
Accessory pigments which absorb green light
How may mixtures of photosynthetic pigments be separated?
Using thin layer chromatography
Describe the process of thin layer chromatography (TLC)
- Stationary phase - small strip of TLC plate used
- Grind leaves with organic solvent (mobile phase)
- e.g. propanone
- Apply drop of extract to TLC strip near to one end
- Repeat until concentrated spot produced
- Place TLC strip in test tube
- Put solvent into test tube so level of solvent below spot
- Ensure strip not touching sides of tube
- Leave solvent to run up strip
- Remove TLC strip before solvent reaches top of strip
How are different pigments identified after carrying out TLC?
- Different components of the mixture travel at different speeds, causing them to separate
- An Rf value can then be calculated and compared to data tables
- Rf value = distance component travels
distance solvent travels
Give a suitable solvent for extracting photosynthetic pigments from plant tissue
Propanone, alcohol, ether
What are the two steps involved in photosynthesis?
- Light-dependent stage
- Light-independent stage
Where do light-dependent reactions take place?
Thylakoids
What are the products of the light-dependent reactions?
- Reduced NADP (NADPH)
- ATP
- Oxygen
Define photolysis
- Splitting of water molecules
- Using energy from light
- Electrons and protons formed used in light-dependent stage
- Oxygen is a waste product
How is oxygen produced in photosynthesis?
Photolysis of water
Where are photosystems located?
Thylakoid membrane
Explain the processes involved in light absorption by photosystems
- Pigments in light harvesting system absorb certain wavelengths of light
- Light energy transferred to chlorophyll a in reaction centre
- Causes an electron in the pigment to be raised to a higher energy level
- Chlorophyll a passes excited electron pair to electron acceptors in thylakoid membrane
- Two types of photosystem - I and II
Define photophosphorylation
Production of ATP in chloroplasts
How is a proton gradient built up in the chloroplast?
- Pair of excited electrons from reaction centre of photosystem II passed to a chain of
electron carriers - Electrons give up energy as they pass from one carrier to next
- Energy released used to pump protons across thylakoid membrane from stroma into
thylakoid lumen - Establishes proton gradient
Describe the process of chemiosmosis
- ATP synthase located in thylakoid membranes
- Allows protons to diffuse back across the membrane to the stroma
- Uses energy that protons release as they diffuse down concentration gradient to produce ATP
What is NADP+?
- Co-enzyme
- Reduced by electrons and H+ ions to form reduced NADP (NADPH)
What is used to reduce NADP+ in the light-dependent reactions of photosynthesis?
- Protons from the thylakoid space
- Electrons from electron transport chain
What is the role of NADPH?
Used as reducing agent in light-independent stage
Outline the light-dependent reactions of photosynthesis
- Light produces an excited electron from photolysis of water
- Photosynthetic pigments in photosystem II absorb light
- Excited electron pair pass along electron transport chain
- Protons from photolysis of water pumped into thylakoid space
- ATP produced by the light dependent reactions
- ATP production by chemiosmosis by ATP synthase
- Electrons from photosystem II passed to photosystem I
- Light excites electrons in photosystem I to higher energy level
- Leads to production of NADPH
- In non-cyclic photophosphorylation electrons from photolysis needed for photosystem II
- In cyclic photophosphorylation electrons from photosystem I return to it
- Oxygen from photolysis is a waste product
