Stomata• A stomate is a pore in the leaf that a leaf can open or shut• When a stomate is open – CO2 can diffuse into the leaf – H2O can diffuse out of the leaf – Which do you think occurs faster? Why?
Rates of Diffusion Through the Stomata• The concentration gradient between the concentration of water vapor inside a leaf (essentially 100%) and outside of a leaf is much great than the concentration gradient between CO2 inside and outside the leaf• Water is a smaller molecule than carbon dioxide• Thus rate diffusion of CO2 out of the leaf is much faster than rate of diffusion of H2O into the leaf
When Do Plants Need CO2?• Carbon dioxide is needed in the Calvin Cycle which only occurs during the day?• Thus, plants should be expected to open their stomata during the day
Stomata Open During Daytime• Because water is lost faster from the leaf when temperatures are higher, plants risk losing a lot of water when they open their stomata during the daytime.• In most plants, the benefits of opening their stomata in the daytime to allow CO2 to be used in Calvin Cycle is greater than the costs of losing water.
How Do You Know it is Time to Water Your Plants?
Plants Rely on Turgor Pressure to Provide Support
Plants Respond to Water Stress (toolittle water ) by Closing Their Stomata • When stomata are closed – No water can diffuse out of the plant – No CO2 can enter the plant – Therefore , Calvin Cycle stops and no photosynthesis occurs
Problem Facing Desert Plants• Experience water stress most of the time – If they kept their stomata closed all of the time then they would never be able to conduct photosynthesis!!!
Desert Plants Possible Solution• In deserts the costs of water loss are much greater!!• What happens if desert plants tried to open their stomata at night when it is cooler so they will not lose as much water.
Possible Solution Open Stomata at Night• Open stomata at night – CO2 diffuses in at night when rates of water loss are lower• Close stomata in the daytime and conduct photosythesis as normal.
Problem With This Solution• Because diffusion requires a concentration gradient and the concentration of CO2 in the atmosphere is so low, a plant keeping its stomata open at night would quickly reach equilibrium so no more CO2 would enter• Thus, the plant would run out of CO2 early the next day so photosynthesis would cease.
A Better Solution• In order to keep CO2 diffusing into a leaf all night the concentration gradient must be maintained.• Can maintain the concentration gradient by removing the CO2 from inside of the leaf – Use the CO2 in a reaction to form a new product and CO2 can diffuse in all night
CAM Photosynthesis• CAM stands for Crassulacean Acid Metabolism
CAM Photosynthesis• CAM plants open their stomata at night – When CO2 enters the cell it enters a reaction that converts it to Malate (an acid) – Malate build up in the plant cell over night • Stored in a vacuole• Plants close their stomata in the daytime – Malate is broken down to release which enters the Calvin Cycle – All other details of photosynthesis exactly the same in CAM plants as in regular C3 plants
CAM Plants• Temporally separate the uptake of CO2 from the use of CO2• C3 plants – CO2 uptake in daytime, Calvin Cycle in daytime• CAM plants – CO2 uptake at night, Calvin Cycle in daytime
What a Great Solution!Why Don’t All Plants Do This?
Limitation of CAM Photosynthesis• Because there is limited space inside the vacuole, there is a limit to how much malate can be stored at night• Once that malate is used up the next day, there is no more CO2 so photosynthesis stops – Thus, CAM plants have lower rates of photosynthesis during the day than C3 plants
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