Transport in Plants Water flow in plants

Water and Minerals Transported in xylem Transpiration Evaporation, adhesion, and cohesion Relies on negative pressure Sugars Transported in phloem Bulk flow Calvin cycle in leaves loads sucrose into phloem Relies on positive pressure Gas Exchange Photosynthesis CO2 in and O2 out Cellular Respiration O2 in and CO2

The Importance of Transpiration Helps to keep the plant cool in hot weather – a method of evaporative cooling. It supports photosynthesis and encourages the exchange of gases, helping maintain levels of CO2 and O2 in the atmosphere. It also- Plays a significant role in the water cycle and releases approx. 10% of water back into the environment. Produces 200 to 1,000 lbs of water for each pint of solid material produced by the plant Creates water vapor that forms into fog and clouds

Water enters into the root hair cells by osmosis. The root hair cell is hypertonic (higher osmotic pressure) to the surrounding soil water, ie the cell has a lower water molecule concentration. Water moves from cell to cell through the root cortex by osmosis along a concentration gradient; this means that each cell is hypertonic to the one before it. In the center of the root, the water enters the xylem vessels. Water may move by diffusion through cell walls and intercellular spaces. 4

Cohesion and adhesion in the transpiration stream

Hydrogen Bonds and Cohesion Each water molecule is hydrogen bonded to four others. H O The hydrogen bond has ~ 5% of the strength of a covalent bond. However, when many hydrogen bonds form, the resulting union can be sufficiently strong and quite stable. Adhesion is the tendency of molecules of different kinds to stick together. Water sticks to the cellulose molecules in the walls of the xylem, counteracting the force of gravity. http://www.ultranet.com/~jkimball/BiologyPages/H/HydrogenBonds.html

As water molecules evaporate out the stomata in the leaves, cohesion pulls the water molecules up from the roots to replace the lost water Adhesion of water molecules to the cell wall fights gravity and aids in the pull of the water molecules up the xylem.

6 Environmental Factors Affecting Transpiration Relative humidity:- air inside leaf is saturated (RH=100%). The lower the relative humidity outside the leaf the faster the rate of transpiration as the  gradient is steeper Air Movement:- increase air movement increases the rate of transpiration as it moves the saturated air from around the leaf so the  gradient is steeper. Temperature:- increase in temperature increases the rate of transpiration as higher temperature Provides the latent heat of vaporization Increases the kinetic energy so faster diffusion Warms the air so lowers the  of the air, so  gradient is steeper

4. Atmospheric pressure:- decrease in atmospheric pressure increases the rate of transpiration. 5. Water supply:- transpiration rate is lower if there is little water available as transpiration depends on the mesophyll cell walls being wet (dry cell walls have a lower ). When cells are flaccid the stomata close. 6. Light intensity :- greater light intensity increases the rate of transpiration because it causes the stomata to open, so increasing evaporation through the stomata.

Moving Air Removes the Boundary Layer of Water Vapour From the Leaf Still air Moving air Saturated air accumulates around leaf Water vapour is removed from the leaf surface cross section through a leaf Lower  Boundary layer the  gradient is increased, so faster rate of water evaporation via the stomata

Intrinsic Factors Affecting the Rate of Transpiration. Leaf surface area Thickness of epidermis and cuticle Stomatal frequency Stomatal size Stomatal position

Movement of Water Through the Stomata Water moves from a higher (less negative) water potential to a lower (more negative) water potential H2O Diffusion shells Always a compromise between photosynthesis and transpiration A leaf may transpire more than its weight in water in a day…this loss must balance a plant’s need for CO2 for photosynthesis

Wilting If water lost by transpiration is greater than water uptake via the roots the plant cells become flaccid and the plant wilts. Subject to copyright clearance a suitable image showing plants wilting could be inserted here. e.g. one similar to that found at: http://pdc.unl.edu/sugarbeet/RhizochtoniaRootAndCrownRot/suddenwilt.htm When the guard cells are flaccid the stomata close

Leaf section The upper epidermis has no stomata Subject to copyright clearance a suitable image showing a leaf section could be inserted here. e.g. one similar to that found at: http://www.bbc.co.uk/schools/gcsebitesize/biology/greenplantsasorganisms/0photosynthesisrev2.shtml The lower epidermis has stomata. The guard cells control the opening and closing of the stomata

Lower Epidermis of Tradescantia

Surface view of leaf epidermis showing the guard cells which are flaccid and the stoma closed.

A single stoma and guard cells Guard cells flaccid Guard cells turgid Stoma open Stoma closed

The guard cells control the opening and closing of the stomata Guard cells flaccid Guard cells turgid Thin outer wall Thick inner wall Stoma closed Stoma open

Regulating Stomatal Opening:-the potassium ion pump hypothesis Guard cells flaccid K+ K+ ions have the same concentration in guard cells and epidermal cells K+ K+ K+ K+ Light activates K+ pumps which actively transport K+ from the epidermal cells into the guard cells K+ K+ K+ K+ K+ K+ K+ Stoma closed

Regulating Stomatal Opening:-the potassium ion pump hypothesis H2O Increased concentration of K+ in guard cells K+ K+ Lowers the  in the guard cells K+ K+ K+ K+ K+ K+ K+ Water moves in by osmosis, down  gradient K+ K+ K+

Guard cells turgid Stoma open Increased concentration of K+ in guard cells H2O H2O K+ K+ Lowers the  in the guard cells K+ H2O H2O K+ K+ K+ K+ K+ K+ H2O H2O K+ Water moves in by osmosis, down  gradient K+ K+ Stoma open

Xerophytes Have Special Adaptations to Reduce the Rate of Transpiration Xerophytes live in hot, dry environments These cacti have reduced leaf area as the leaves are reduced to spines Fleshy leaves to hold water Silver surface to reflect sun

Adaptations to Reduce Water Loss in Xerophytes Thick waxy cuticle to reduce evaporation Reduced leaf area e.g.needles Hairy leaves:- the hairs trap a layer of saturated air Sunken stomata:- the pits above the stomata become saturated Rolled leaves:- this reduces the area exposed to the air and keeps the stomata on the inside so increasing the water vapour inside the roll Increasing the water vapour around the stomata reduces the water potential gradient so slows water loss

Cross Section of Marram Grass Leaf Leaf is rolled with sunken stomata on the inside Hairs trap water vapour Water evaporating via the stomata collects in the air trapped in the rolled leaf This reduces the  gradient so reduces water loss from the plant

Adaptation to Increase Water Uptake in Xerophytes Deep extensive root system to maximise water uptake Accumulation of solutes in the root system to reduce the , so making the  gradient from the soil to the root cells steeper Some very shallow roots to absorb dew which condenses on the soil at night

Graph to show stomatal opening over 24 hours Some plants close stomata during hottest time-saving water 100 Increased light intensity causes more stomata to open stomatal opening/% An adaptation to hot dry environments Stomata close as the sun sets Dawn-stomata begin to open 12 2 4 6 8 10 12 2 4 6 8 10 12

The End

Click on the marks above Questions What is transpiration? Give three environmental factors which will increase transpiration rate. (2marks) Explain how potassium ions are moved into the guard cells in light, and how this affects the guard cells and stomata. (6marks) Give three adaptations a xerophyte may have to reduce transpiration and explain how they do this. (4marks) Plants close their stomata at night and some also close their stomata around mid day. Explain why this is advantageous to the plant (2marks) finish Click on the marks above to check your answer

Transpiration is the loss of water from a plant by evaporation Answer Q 1 Transpiration is the loss of water from a plant by evaporation Higher temperature, increased air movement, lower humidity Back to question

Potassium ions are pumped into the guard cells by active transport Answer Q 2 Potassium ions are pumped into the guard cells by active transport against the concentration gradient this lowers the water potential inside the guard cells water is drawn in by osmosis from the surrounding cells which have a higher water potential/down the water potential gradient guard cells swell and become turgid guard cells bend causing the stomata to open any 6 from the above Back to question

Thick waxy cuticle on leaves reduces evaporation Answer Q3 Any three from:- Thick waxy cuticle on leaves reduces evaporation Curled leaves reduce evaporation by trapping humid air inside the curl so reducing the water potential gradient Reduced leaf area, e.g. spines, reduces the area from which evaporation can occur Hairy leaves -trap a layer of humid air around the leaf,so reducing the water potential gradient Sunken stomata – moist air trapped above stomata, so reducing the water potential gradient Back to question

Answer Q4 Stomata closed at night when there is no light for photosynthesis, so reducing water loss by evaporation/transpiration via the stomata Closing stomata at mid day, which is the hottest part of the day, is an advantage in hot dry environments, as transpiration is reduced. Back to question

Can you think of any synoptic links? Water potential Osmosis Active transport Respiration (energy required for active transport) Photosynthesis (light and CO2 required for photosynthesis, CO2 enters via stomata, water used in photosynthesis)