Topic 5: Photosynthesis Flashcards
Where does photosynthesis occur?
In chloroplasts, mainly in the leaf of eukaryotic plants. Can be found in bacteria too
Give some adaptations of the leaf for photosynthesis
- Large surface area to absorb sunlight
- A leaf arrangement that minimises overlapping
- Thin - most light absorbed on surface so short diffusion pathway for gases
- Transparent cuticle + epidermis - lets light into photosynthetic mesophyll cells
- Long narrow upper mesophyll cells packed with chloroplasts to collect sunlight
- Numerous stomata - all mesophyll cells a short diffusion pathway to gases
- Stomata open/close in response to light intensity changes
- Many air spaces in lower mesophyll layer - rapid diffusion in gas phase of CO2 + O2
- Network of xylem - brings water to leaf cell, phloem takes away sugars produced
What is the overall equation for photosynthesis?
6 carbon dioxide + 6 water –> glucose + 6 oxygen
What are the main stages of photosynthesis?
- Capturing of light energy by chloroplast pigments
- Light-dependent reaction
- Light-independent reaction
What is oxidation?
- Loss of electrons
- Loss of hydrogen
- Gain of oxygen
- Energy given out
What is reduction?
- Gain of electrons
- Gain of hydrogen
- Loss of oxygen
- Energy taken in
What are the stages of the light dependent reaction?
- Photoionisation
- Electron transfer chain
- Chemiosmotic theory
- Photolysis of water
Describe the photoionisation stage of the light dependent reaction
Chlorophyll absorbs light energy, exciting a pair of electrons (raises them to a higher energy level), which then leave the molecule. Chlorophyll has been oxidised
Describe the electron transfer chain of the light dependent reaction
The electrons released from photoionisation get passed along electron carriers in the thylakoid membrane, losing energy at each step (this is a series of redox reactions).
Some of the energy released is used for the production of ATP from ADP + Pi
Describe the role of Chemiosmotic theory in the light dependent reaction
- The mechanism by which ATP is produced
- H+ are pumped from stroma into thylakoid by proton pumps (protein carriers)
- The energy for this comes from electrons released by the photolysis of water (also produces H+)
- Concentration of H+ inside thylakoid increases, maintaining a concentration gradient compared to the low concentration in the stroma
- Protons move by facilitated diffusion into the stroma through ATP synthase channel proteins.
- As they pass through, the structure of the enzyme changes, so it catalyses ADP + Pi –> ATP
Describe the photolysis of water in the light dependent reaction
- Water molecules split using light
- 2H2O –> 4H+ + 4e- + O2
- The electrons produces replace the lost electrons in chlorophyll molecules
- Protons move out the thylakoid membranes through ATP synthase and are taken up by NADP to produce reduced NADP.
- Oxygen is either used in respiration or diffuses out the leaf
How are chloroplasts adapted for the light dependent reaction?
- Thylakoid membranes have a large surface area for chlorophyll attachment, electron carriers and enzymes
- Network of proteins in grana hold chlorophyll in manner allowing maximum light absorption
- Granal membranes have ATP synthase channels - selectively permeable to establish a proton gradient
- Chloroplasts have their own DNA and ribosomes to produce proteins needed for photosynthesis quickly and easily
Where does the light dependent reaction occur?
Thylakoid membranes in chloroplasts
What is the light independent reaction also known as and where does it occur?
Calvin cycle
Stroma of chloroplasts
Describe the stages of the light independent reaction
- CO2 diffuses through stomata, ultimately into the stroma of chloroplasts
- In the stroma, CO2 reacts with ribulose bisphosphate (RuBP - [5C]), catalysed by the enzyme rubisco
- This reaction produces 2 glycerate-3-phosphate (GP - [3C])
- Reduced NADP from the light-dependent reaction is used to reduce GP to triose phosphate (TP), using energy supplied by ATP
- NADP is re-formed and goes back to the light-dependent reaction to be reduced again by accepting H+
- Some TP molecules are converted to organic substances e.g glucose, starch etc
- Most TP molecules are used to regenerate RuBP using ATP from the light-dependent reaction
