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 CO 2 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 CO 2 /H 2 0 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 CO 2 and very little O 2 in the atmosphere The modern atmosphere is 21% O 2,.035% CO 2 RuBisCO sometimes “accidentally” fixes O 2 instead of CO 2 (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.