Download PhotosynI

Transport in Plants
or
Why do they need so much durn water?
To answer this we need a Quick
Review of Photosynthesis
The source of all metabolic energy on earth
(this is why plants are way cooler than animals!)
What does photosynthesis do?
• PS is a 2-step process
• FIRST STEP:
– turns light energy into chemical
energy
– (your book calls these the
“light-dependent reactions”)
• SECOND STEP:
– uses that chemical energy to
“fix” atmospheric carbon into
sugar
– (your book calls these the
“light-independent reactions, or
the “Calvin Cycle”)
• Where does
photosynthesis take
place?
• In chloroplasts
• In higher plants these
are located primarily in
leaf cells
A closer look at thylakoids
• These are internal membranes in the chloroplast
• They create an “inner thylakoid space” similar to the inter
membrane space in mitochondria
Chlorophyll
• The main energy receiving
molecule in chloroplasts
• Light can easily excite its
electrons
• Electron excitation makes
chlorophyll become a good
electron donor (i.e. a reducer
in a red/ox reaction)
• These energetic electrons
can now enter an electron
transport system similar to
that in mitochondria
• Other pigments contribute to light absorption:
– Carotenes (absorb in the blue: appear orange-yellow)
– Xanthophylls (absorb in the blue-green: appear reddish)
• These pigments pass their energy to chlorophyll
All higher land plants have the same
photosynthetic pigments
• The things we call “algae” have different
photosynthetic pigments
– Red
– Brown
– “Blue-green”
– Green
• Some unicellular marine algae can change their
photosynthetic pigments rather quickly (why?)
ATP is made by “chemiosmosis”
(Remember that?)
OK, that was step #1 of photosynthesis
• The second step uses the energy (ATP and NADPH) created in
step one to fix atmospheric carbon in a process called the
“Light-independent reactions” or “Calvin Cycle”
• This occurs in the stroma of the chloroplast
• CO2 is “sucked” out of the air by enzymatically attaching it to
a 5C sugar called ribulose bisphosphate (RuBP)
• The enzyme that does this is called RuBP Carboxylase, aka
RuBisCO (the most abundant protein in the world)
• The product is… what would you guess? (5+1=?)
• … Two 3C sugars called phosphoglycerate
• Six turns of the CC ultimately produce one glucose molecule
for export
• These glucoses are the basis for ALL metabolic life on earth
So plants get ALL their carbon from
the atmosphere
• That means their cells must be in contact with the
atmosphere
• That means their cells (which are full of water)
lose water through evaporation
• This is more or less important for different plants:
– Aquatic plants – no problem
– Plants in Oneonta – usually no problem
– Plants in very dry places – BIG problem!
Cross section of a typical leaf
Stomates
Comprised of two opposing “guard cells”
How do plants solve this CO2/H20
conflict?
• There are two main ways:
–Morphoplogical
–Biochemical
Morphological adaptations to dry
conditions (xerophytes)
• Strategies:
– Avoid dry weather
• Only grow/flower etc. when it is wet (be dormant the
rest of the time)
– Get more water
• Have really extensive root systems
• Be a really good water extracter (actually a
biochemical solution)
– Lose less water from leaves
• Several strategies for this
What are the major environmental factors
that cause the loss of water from leaves?
• Humidity
• Temperature
– Lower temperatures = lower humidity
• Wind
• Which of these can plants alter?
Ways to cut down on transpiration
• Have a low surface/area ratio
– Thicker – water is further from the surface
• Reduce the size of leaves (sometimes to “spines”)
– Stems are perpendicular to the sun - cooler
• Have a waxy cuticle
– Some xerophytes have 10X the wax on their surface
• Have fewer stomates
– Good and bad as a solution to this problem
• Curl leaves
– Stomates are more plentiful on the bottoms of leaves maintains higher local humidity
• Have “sunken” stomates
– Creates a “still” area – maintains a higher local humidity
• Have hairy leaves
– Same…
Sunken stomates
Leaf hairs
Here’s another glitch...
• PS evolved when there was tons of CO2 and very
little O2 in the atmosphere
• The modern atmosphere is 21% O2, .035% CO2
• RuBisCO sometimes “accidentally” fixes O2
instead of CO2 (called “photorespiration”)
• Very, very energy inefficient
• Worse where it is hot and dry
• “Tropical grasses” have evolved a “pre-fixation”
strategy which minimizes photorespiration
• The first product is a 4C compound, so the
process is called “C4 photosynthesis”
The End.