Lecture 2: Photosynthesis and Water Relations Flashcards
Photosynthesis (Overall)
Process of converting light energy into chemical energy, energy storing process.
6CO2+H2O–> C6H12O6+6O2+ATP/NADPH
-Photosynthesis occurs in chloroplasts: in the storm are stacks of thylakoids called granum, which contain chlorophyll in the membrane.
-In the mesophyll: 2 external membranes, storm, thylakoids, thylakoid space, granum. Gas exchange occurs in the stomata
Ecological considerations (adaptations+modifications due to the environment)
Affected by light intensity, temperature and external/air CO2 concentration:
-Higher light absorption for leaves that receive unfiltered sunlight at the top the canopy
-Leaves in shade absorb little of the spectrum EXCEPT for the red wavelengths (700-800nm)
-Leaves grown in the sun have a thicker structure: longer palisade cells, increased spongy mesophyll cells.
-Photosynthetic leaves have a light compensation point (where CO2 uptake = CO2 evolution, the amount of light needed to start photosynthesis)
–saturation point= max CO2 uptake
–shade plants have a lower dark respiration point, whereas sun plants have a higher light compensation point.
Transpiration
Passing of water in and out of stomata:
-entry point for CO2 and exit point for H2O
Why do plants need water?
-Constitutes 90% of the weight of young cells
-Important part of enzyme actions and other chemical reactions
-Photosynthesis
-Cell turgor (shape and volume)
-Transpiration (cooling system)
-Absorption of nutrients
In what ways does water move?
Diffusion, osmosis, water potential, imhibition
Diffusion
random movement of cells across a concentration gradient, from a region of high concentration to a region of low concentration.
Osmosis
diffusion of water across a concentration gradient, from a region of high concentration to a region of low concentration across a semipermeable membrane.
-stops after reaching an equilibrium
Water potential
Water has free energy/potential:
MPa of pure water @ standard temp 25C, pressure 0
-Consists of osmotic potential and turgor (pressure) potential
-Water potential in plants is always negative: looks at the tendency of water to move from regions of h.w.p to l.w.p
H.w.p: closer to 0
L.w.p: further from 0 (more negative)
e.g.: roots+soil have a very high w.p compared to leaves+atmosphere.
Osmotic potential
The effect solutes (ions, sugars) have on water potential
-Adding solutes decreases water potential
-Osmotic potential is always negative (water potential starts at 0)
Pressure (turgor) potential
Develops against the cell wall as a result of water entering the vacuole (always positive)
How does water get transported to the xylem?
Cohesion-tension theory:
-H2O molecules are polar and adhere to the xylem due to capillary forces (charges on the wall)
-Through transpiration from stomata there’s a lower w.p than in adjacent cells.
-Causing replacement water to be transported
-Transpiration creates a tension in the water columns contained in the xylem.
Wilting
Soil dehydration causes soil w.p to be low, plants cannot absorb water from the soil anymore.
How does phloem transport work?
Pressure-flow hypothesis:
-Translocation of phloem loading and unloading with ATP
Pressure-flow hypothesis: Source
Where photosynthates are produced (leaves):
1. Sugars enter sieve tubes through companion cells (active transport)
2. Water from xylem follows by osmosis
3. Turgor pressure develops, which is the driving force for fluids to rush towards the sink
Pressure-flow hypothesis: Sink
Where sugars are used (growing cells) or sugars are stored (fruits, stems, roots):
1. Sugars are actively transported out of the phloem
2. Decreases water potential in the sieve tubes (osmotic potential becomes less negative)
3. Water flows out and turgor pressure drops.
