Layers within a Leaf

  • Upper epidermis: Covered by a waxy cuticle which provides protection
  • Palisade Mesophyll cells: Many chloroplasts for photosynthesis
  • Spongy Mesophyll cells: Air spaces for diffusion of gases
  • Lower epidermis: Protects the underside of a leaf and has stomata (pores)
  • Chloroplast: Light is absorbed for photosynthesis
  • Stomata: Small pores allowing gases to diffuse in and out of the leaf
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The Relation between Gas Exchange and Transpiratio

Gas Exchange in Plants:

  • Exchange of CO2 and O2 between the leaf and atmosphere
  • CO2 diffuses into leaf and O2 diffuses out due to photosynthesis

Transpiration in Plants:

  • Loss of water vapour from the leaves and stems of plants
  • Inevitable concequence of gas exchange due to photosynthesis
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Why Transpiration is an inevitable Consequence of

1. Leaves must absorb CO2 and excrete O2 in photosynthesis so that Gas Exchage occurs

2. Adaptation for effcient Gas Exchange having moist cell walls and large surface area

3. Spongy Mesophyll layer has many air spaces to increase the surface area of moist cell walls exposed to air

4. Water evaporates from the moist cell walls as air spaces have a high relative humidity

5. Water vapour diffuses from air spaces to the air/atmosphere outside the leaf

6. Hence leading to Transpiration

7. Expidermis secretes wax forming a waxy cuticle (waterproof coating to leaf) this prevents excessive Transpiration but also blocks Gas Exchange

8. Stomata are needed to open and allow gas exchange of CO2 and O2

9. Inevitablely water vapour also escapes during Transpiration

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Why is Transpiration Important?

  • Provides Photosynthesis with a needed raw material
  • Minerals are transported in the leaves to synthesise important molecules
  • Provide cooling effects to the leaves
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Transport Systems: Xylem

Xylem: Carries water and minerals from the roots around the plant

Phloem: Transports dissolved sugars from the leaves around the plant


  • Long continuous tubes running through the plant, stems and roots
  • Walls are thickened with lignin in the form of wood, rings or spirals
  • Lignin prevents the walls of the xylem vessels from collapsing
  • Adds strength to the woody material of older plants


  • No plasma membranes so water can move in and out freely
  • Lumen is filled with sap as the nuclei of the original cells break down. End walls also break down forming a continuous tube
  • Cell wall is impregrated with lignin in the form or helical or ring-shaped cellulose thickenings - resist inward pressures
  • Pores in the outer cellulose walls conduct water out of xylem into cell walls of adjacent leaves
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How does Water flow through a Plant?

1. Water enters through the roots

2. Root hair cells have hair-like projections growing between the soil particles to absorb water

3. Water goes up the steam till it reaches the leaves through a transpiration stream

Transpiration stream: Flow of water in xylem from roots to leaves to replace the water lost by Transpiration

How does the Transpiration Stream happen? 

  • Tension
  • Adhesion
  • Cohesion
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How does the Transpiration Steam happen?

1) Main movement of water in the xylem is from the roots to the leaves to replace the water lost by Transpiration

2) Tension is generated in the leaves by Transpiration due to the adhesive property of water to the cell wall

3) Tension causes the water to move up to the leaves

4) When water evaporates from the mesophyll cell walls, more water is drawn from the xylem through cellulose pores in the cell walls to replace it, this generates tension which is transmitted from one water molecule to the next due to the cohesive property of water

5) Tension generated in the leaves is transmitted down the coloumn of water in the xylem to the roots

What if there is maximum Transpiration?

  • Pressure in xylem is very low
  • Side walls are very strong preventing inward collapse
  • The walls are strengthened by depositing more cellulose and impregnanting the thickening with lignin
  • Thickened cell walls with lignin are much harder and woody
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Measuring the rate of Transpiration

A Potometer can be used to measure the rate of water uptake


  • Fresh shoot is cut under water and is transferred to the apparatus under water to aviod air bubbles
  • Air bubble moves along tube as water is absorbed by shoot
  • A resrvior is set up that allows water to flow into the capilliary tube, pushing the air bubble back to the start of the tube
  • As the plant transpires it draws water out of the capilliary tube to replace the losses
  • The capilliary tube is narrow so the losses of water from the plant gives mesurable movements of the air bubble
  • Repeat measurements of the distance moved in one minute to ensure the data gained is reliable
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Factors to consider when setting up a Potometer

  • Use plants that have shoots with thin waxy cuticles
  • Shoots must be cut under water - prevent air from entering xylem vessels which will block water flow. Thouroughly seal the contact point
  • Leaves that are going to be used must be dry
  • Use a mass potometer (plant with complete root system is immersed in container)
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Temperature and Transpiration

  • Higher temperature increases evaporation and therefore transpiration
  • Increased rate of diffusion through air spaces and humidity of outside air is lowered
  • In very high temperatures stomata may close
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Wind Speed and Transpiration

  • Wind moves evaporated water molecules away from leaf
  • The facter the air movement the quicker the water will be moved - this keeps a high concentration gradient of water between the inside and outside of leaf. Transpiration rate increases
  • Very high wind speed causes the stomata to close which lowers the overall transpiration rate
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Humidity and Transpiration and Comparison

  • The more humid the air the more water is inside it
  • This reduces the concentration gradient between the air spaces inside the leaf and the air outside
  • Water molecules will diffuse out more slowly therefore decreasing the transpiration rate


Warmer temperature increases the transpiration rate

Transpiration rate is higher on a Windy Day

Transpiration rate is higher when the air is less humid

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Models of Water Transport in Xylem

Water has Adhesive Properties

  • Water adheres to glass and so rises up in capilary tube
  • Mercury does not adhere to glass and does not rise up in capilary tube

Water is drawn through capiliaries in cell walls

  • Paper is made of cellulose so water rises up the filter paper

Evaporation of Water can cause Tension

  • Porous pot is similar to a leaf cell. Water evaporates from the surface of the pot
  • More water is drawn into the pot through a tube to replace lost water
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Absorption of Water in the Roots

  • Higher concentration of mineral ions in root hair cells than in soil. Plants still absorb more from the soil using Active Transport
  • Root hair cells have mitochondria and protein pumps for Active Transport
  • The root hair cells have a higher solute concentration than soil therefore water moves into the plant by osmosis
  • The branching of roots and growth of root hair cells allows for an increase in surface area
  • Roots only absorb if oxygen is present so that ATP can be produced for Active Transport
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Adaptations of Desert Plants to absorb Water

Xerophytes - Desert Plants

  • Plants in dry habitats are adapted to reduce transpiration for water conservation
  • Verical Stems: Allow sunlight to be absobed early and late in the day
  • Thick Cuticle: Thick waxy cuticle which covers the stem
  • Spines (not leaves): Reduces surface area of transpiration
  • Open stomata at night: Opens at night and not during day when heat is most intense

EXAMPLE: Marram Grass

  • Thick waxy cuticle
  • Smaller air spaces in spongy mesophyll
  • Few stomata and stomata are within pits
  • Cells change shape: Leaves roll up so that the lower epidermis and stomata are on the inside
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Adaptations of plants in Saline Soils to absorb Wa

Halophytes - Plants in Saline Soils

In coastal habitats and arid (no water) areas

  • Water moves up the soil and then evaporates this leaves dissolved ions at the surface
  • Most plants cannot grow in these conditions only Halophytes are able to

Keep in Water

  • Solute concentration is higher in plant than soil
  • DO NOT rasie  only maintain sodium concentration in vacuole - affects cell activities
  • Raise concentrations of other solutes (potassium) maintained in cytoplasm
  • Water can also be conserved in water storing tissues: Succulents

Get rid of Sodium

  • Active Transport
  • Excretion from special glands within the leaves
  • Accumulate the ion in certain leaves and then shed the leaf
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