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Lecture 9

BIOL 1902 Lecture Notes - Lecture 9: Stoma, Photosynthesis, General Idea

Course Code
BIOL 1902
Michael Runtz

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Lecture 9
Continuing on with plants dealing with subzero temperatures:
For some plants, the upper part freezes off and dies while the main part of the
plant is in the soil and survives.
however, the main part still needs to be cold hardy.
excess water withdrawn and evaporated from leaves and twigs
water is drawn out of the cells, which increases solute
protective sugars are also absorbed into the cell,
increasing solute concentration even more.
as a result, the freezing point gets lower.
the cell membrane becomes more flexible by the addition
of unsaturated fatty acids
antifreeze proteins to suppress ice formation and dehydration
are produced
ice can form inside, between the gaps of cells and can go through the
cell wall. however, the cell membrane is more flexible so they resist ice
penetration and just deflect inward.
plants become cold hardy through acclimation, which has several steps.
the first stage of acclimation is triggered by a change in the
photoperiod (the ratio from daylight to darkness).
plants react to the change with their phytochromes.
phytochromes are the light sensitive photopigments that get the
plant into action to prepare for cold temperature.
also causes the cells to go dormant.
makes them more responsive to low temperatures.

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the second stage is triggered by cold (not sub-zero) temperatures (10-0
in the second stage, the plants go in high gear and the cell
membranes start getting more flexible and the plant goes into a
state of dormancy (?).
In the far north, some trees are cold hardy to -80 degrees celcius.
For coniferous trees/plants, (and other trees that have needles) retaining needles
creates new problems:
damage by solar radiation
solution: they create new pigments: xanthophyll pigments that protects
them from radiation
they enable chlorophyll to use sun’s energy to create heat, not
Skunk Cabbage has an unusual adaptation for the cold:
it turns up the heat
desiccation (drying out) - (on calm sunny days, not windy days)
the solution is within their size. size is important: the farther north you
go, the smaller trees are.
small surface area is important, leaf surface area is small
leafs close their stomata (pores) so water can’t escape
needles have thick cuticles to lock in water.
plants that retain their leaves have hairs on the underside of the leaves.
hairs trap moisture.
hairs also break up the wind that can dry out the plant.
some of the plants that are evergreens (rock polypody for examples)
reduce leaf surface area by curling up to reduce desiccation.
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