Transport of water and Transpiration

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Photosynthesis is one of the most fundamental processes in life.  Without it, carbon in the atmosphere could not be incorporated into organic molecules such as glucose, and oxygen would not be produced.  These products of photosynthesis fuel respiration;  another fundamental life process.

 Overall photosynthesis equation: 6CO2   +   6H2O ------ C6H12O6   +   6O2

 Q. How does carbon dioxide enter plant cells?

By diffusion.

From the atmosphere into the leaves.

Through the stomata.

Plant roots feature specialised cells called root hair cells.  These have specialised features in order to maximize the uptake of water.

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Purpose of Caspirian *****

When water moves from the soil to the xylem by the apoplast pathway-it never crosses through a plasma membrane. This would mean the plant has no control over what enters the xylem – weed killer, heavy metal ions (e.g. lead) etc.

The Casparian ***** exists to prevent this problem. The endodermis cells secrete a waxy substance called suberin onto their cell walls. These waterproofed cell walls form the Casparian *****.

Now, when substances moving via the apoplast pathway arrive at the endodermis, they are blocked and forced to cross the selectively permeable plasma membrane.Therefore the Casparian ***** allows the plant to select what enters the xylem.

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Root Pressure

Root pressure -The action of the endodermis moving mineral ions into the xylem by active transport drives water into the xylem by osmosis. This forces water into the xylem and pushes the water up the xylem. Root pressure can push water a few metres up a stem, but cannot account for water getting to the top of tall trees. This is done by evaporation of water from the leaf.

Transpiration- Evaporation / loss of water vapour  from leaves  via stomata.

The Stem: How water moves up the stem from the root into the leaf

  • Water does not move up the xylem by osmosis.
  • The water molecules move as one, by mass flow.

Mass Flow- The movement of a body of water from an area of high pressure to an area of lower pressure.

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Cohesion & Adhesion

Water therefore moves up the stem in the xylem from the area of high pressure (roots) to an area of lower pressure (leaf) by Mass Flow . Two forces help to move the water in the xylem by Mass Flow, keeping it as one long unbroken column of water from the root all the way up to the leaf.

Cohesion- Water molecules stick together with Hydrogen Bonds. This means when one water molecule evaporates into the leaf, it drags the water molecules behind it up the xylem.

 Adhesion- Water molecules stick to the Lignin in the walls of the xylem, this supports the weight of the water column-preventing it breaking. 

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How water moves out of Xylem and into atomsphere

1. Water moves out of the xylem by osmosis.

2. The water moves into the spongy mesophyll cells via the Symplast and Apoplast pathways

3. The water evaporates into the air spaces, down the water potential gradient

 4. The water vapour diffuses out of the air spaces through the stomata into the outside air, down the water potential gradient

 This movement of water out of the xylem and evaporating off the Spongy Mesophyll creates a low pressure in the leaf-this drives the mass flow of water up the stem from the roots to the leaf

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A Potometer

A potometer is used to estimate the rate of water loss.  The potometer is set up as shown in the diagram below making sure that there are no air bubbles inside the apparatus. Water lost by the leaf is replaced from the water in the capillary tube. The movement of the meniscus at the end of the water column can be measured.

  • Cut the shoot under water (this prevents air getting into the stem and blocking the xylem vessels)
  •  Allow time for the plant to adjust to the surroundings, and keep environmental conditions constant (light intensity, humidity, temperature, air movement)
  •  Time how long it takes the meniscus to move a set distance.
  • Reset the meniscus with water from the syringe, and take more readings so that you can calculate the mean rate of uptake.

Why do Potometers not technically measure the rate of transpiration?

  • Potometer measures water uptake

  • Not all water taken up is lost

  • Some water used in photosynthesis / making cells turgid

 The study the effect of different environmental conditions on the rate of transpiration, the Potometer can be placed in different environmental situations.

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Transpiration rate

Number of leaves- More leaves means a larger surface area over which water vapour can be lost.

Number, size and position of stomata- Many, large stomata means water vapour is lost quickly. Stomata on lower surface, water vapour loss is slower

Presence of cuticle- A waxy cuticle reduces evaporation from leaf surface

Light- Stomata open in daylight to allow gas exchange for photosynthesis

Temperature- A higher temperature will increase the rate of water loss. It decreases the relative water vapour potential in the air, increases the rate of evaporation and rate of diffusion (water molecules will have more kinetic energy)

Humidity- Higher humidity in the air will reduce the water vapour potential gradient  so decrease the rate of water loss

Air movement or wind- Air movements carry away water vapour and so maintain a high water vapour potential gradient

Water avalibility- Stomata will close if there is little water in the soil reducing rate of water loss

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Why plants transpire

Why does a plant bother allowing transpiration? Water is required in the leaf for photosynthesis & keeps the cells turgid. The flow of water transports useful mineral  ions (Mg2+, Na+, Cl-) through the plant. Evaporation of water cools the plant.

Q. Why is the loss of water by transpiration unavoidable?

Stomata are open to allow gaseous exchange / entry of carbon dioxide / exit of oxygen for photosynthesis / Water vapour leaves the leaf/ Down a water vapour potential gradient / Higher temperatures during the day/ Causes greater evaporation through leaf surface all the time

 Q. Describe and explain how water is moved up the xylem from the roots to the leaves.

Water moves into xylem down water potential gradient / Root pressure / high hydrostatic pressure at bottom of xylem / Water vapour loss / transpiration at leaves  / Creating low hydrostatic pressure at top of xylem / Water under tension as it is pulled up in a continuous column / Cohesion between water molecules due to hydrogen bonding/ Adhesion of water molecules to xylem / Capillary action / Water moves up xylem by mass flow / From higher hydrostatic pressure to lower hydrostatic pressure down hydrostatic

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Xerophytes are plants that are well adapted to living in very dry or arid conditions with little rainfall (Cacti are a good example).  They have a number of adaptations to reduce water loss.

Smaller leaves, particularly leaves shaped like needles- Less surface area over which water vapour can be lost

Stems, not leaves, used for photosynthesis (leaves may be reduced to spines)-  The fewer the leaves, the less transpiration from them

Leaves can roll up- Less surface area exposed to air, stomata are hidden inside rolled up leaf, reducing the rate of diffusion of water vapour from leaf

Thick waxy cuticle- Reduces evaporation

Hairs on surface of leaf- Hairs trap moist air next to stomata which reduces water potential gradient

Suken Stomata- Layer of moist air is trapped next to the stomata which reduces water potential gradient

Low water potential inside cell- reduces evaporation of water from surface

Densely packed spongy mesophyll- reduces surface area over which water is evaporated

Swollen stems- can store water for use when supplys run out.

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Plants which grow in water either partially or completely are known as aquatic plants or hydrophytes.  They may be fresh water or marine water plants.  Plants which grow in water systems like in ponds, lakes, streams, rivers, pools, etc are known as fresh water plants.  The plants which grow in salt water are known as salt water plants or marine plants.

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