L17 - Photosynthesis Flashcards
Photosynthesis
Converts light, CO2, and water into carbohydrates
Glucose: precursor for all Carbon containing compounds
Oxygen essential for cellular respiration for (almost) all terrestrial life
Leaves and photosynthesis
Mesophyll
• Palisade parenchyma
• Spongy parenchyma
Stomata
• Pores for gas exchange
• Typically located on both sides of leaf
Carbon fixation: CO2 enters mesophyll through stomata
• Stomata open during day, CO2entry towards chloroplasts
• Chloroplasts in parenchyma cells transform light energy into chemical energy
Chloroplast
Look at slide 6
Light reactions
- Light is harvested by the pigments
- A series of oxidation-reduction reactions converts light energy into chemical energy
- Water is oxidized:
- Providing electrons
- Protons, proton gradient used to allow for ATP formation
- Oxygen is released as side product
In a nutshell: the energy from light is used to generate NADPH and ATP, two storage forms of energy
Carbon reactions - Calvin cycle
- Uses the NADPH and ATP generated by electron & proton transport for the fixation of CO2into carbohydrates
- Rubisco fixes carbon in the Calvin cycle
- This happens when light ‘hits’ the leaf, and stomata are open to allow CO2 entry to photosynthetic cells
Environmental factors
The three most important factors for photosynthesis are:
- Light intensity
- CO2concentration in the leaf (limited by the stomata)
- Temperature
Photosynthesis and light intensity
1) Photosynthesis is a rate
Net photosynthesis rate = μmol m-2s-1
2) Net photosynthesis rate is the rate of CO2uptake minusthe rate of CO2evolution through mitochondrial respiration.
!!!This is why at low light intensity, net photosynthesis rate can become negative!!!
Photosynthesis and Temperature
Enzyme turnover rate doubles as temperature increases by 10 degrees.
T increases -> rate of Photoresp. & rate of mitochondrial respirt. increase -> Net photosynthesis rate decreases
Mitochondrial respiration and temperature
- Low at low temperatures and increases strongly at higher temperatures
- The intercept of the light response curve of net photosynthesis rate is strongly dependent on temperature
- At low temperatures, net photosynthesis rate could be positive even at very low light intensities, while at higher temperatures, net photosynthesis rate would be negative at low light intensities
Photosynthesis in fruits
- During fruit/flower development, carbon demand is very high
- For fruits to do photosynthesis, they need chloroplasts, chlorophyll, CBB enzymes
- During ripening in many fruits, chlorophyll is broken down, and photosynthesis stops
- In green fruits and vegetables, photosynthesis can continue after harvest!
- Chlorophyll fluorescence can be used to predict damage, and stress
Postharvest storage and distribution
During storage in the dark chlorophyll, protein and carbohydrates (starch, sugars) generally decrease
● Dark induced senescence (PCD)
Also other (presumed health promoting) compounds
may decrease
● Glucosinolates , Flavonoids, Carotenoids,, Lycopene
● Vitamins
Examples of postharvest of fruits and storage
See slides 20-26
Light during storage (1/2)
Light during postharvest can improve quality of (green) vegetables
Light preserves Vitamin C (Less pinking and brown coloration due to Vitamin C)
Often very low light intensities already work well
Carbohydrates go down in the dark!
(Lettuce example):
- Less browning on cut edges
- Less senescence
- Longer shelf life
Light during storage (2/2)
What light intensity?
Which colours work?
What happens to Vitamin C and sugars?
Light levels from 5 30 µmol/m 2 /s improve visual quality
and shelf life of fresh cut lettuce
All colors work (red, green, blue, white)
Increases in Vitamin C and sugars
What is the Light Compensation Point (LCP) ?
The amount of light required for the rate of photosynthesis to be equivalent to the rate of respiration.
Light below LCP: Respiration is higher than photosynthesis
(e.g in darkness): there is no photosynthetic sugar production
