Gas Exchange in Plants (SL IB Biology)

• Gas exchange in plants occur through the leaf
• The leaf contains the following tissues:
  • Epidermal tissue forming the outer boundary of the leaf
  • Mesophyll tissue that make up the bulk of internal structure of the leaf
  • Vascular tissue which transports substances between the leaf and the rest of the plant

Epidermis

• This is formed by a single layer of tightly packed cells 
  • The leaf has an upper and lower epidermis which protects the inner parts of the leaf
• The lower epidermis contains tiny pores called stomata (singular stoma)
  • Each stoma is surrounded by two guard cells which controls the opening and closure of the pore
    • When water moves into the guard cells they become turgid and change shape which opens the stomata
    • They become flaccid when water is lost and this causes the stomata to close
  • Stomata are the structures through which gas exchange occur in a leaf
    • They allow for the diffusion of oxygen and carbon dioxide into and out of the leaf
• The epidermis is often covered by a waxy layer called the cuticle
  • This forms an impermeable barrier

Mesophyll tissue

• These are formed by parenchyma cells which contain chloroplasts
  • This is where photosynthesis occurs in the leaf
• Two types of mesophyll tissue are found in the leaf:
  • Palisade mesophyll forms a layer beneath the upper epidermis and contain many chloroplasts for maximum photosynthesis
  • Spongy mesophyll contains large air spaces between the cells for gas exchange to occur

Vascular tissue

• Vascular tissue is arranged in vascular bundles and is responsible for the transport of substances around the plant
  • Vascular bundles form the veins in leaves 
  • Xylem transports water and mineral ions from the roots to the leaves
  • Phloem transports the products of photosynthesis from the leaves to other parts of the plant

Structure of a Leaf Diagram

The structure of a leaf has distinct layers each with their own function

Adaptations for gas exchange

• The leaf has several adaptations that facilitate gas exchange

Leaf Adaptations for Gas Exchange Table

• The majority of photosynthesis takes place in the leaves of plants
  • Some plants are able to carry out photosynthesis in the cells of their stems 
• During photosynthesis, carbon dioxide is taken in by the leaf and oxygen is released 
  • The pores in the epidermis of the leaf through which this gas exchange takes place are known as stomata (singular stoma)
  • The stomata need to be open all the time in order for gas exchange, and therefore photosynthesis, to continue
• The problem for plants is that as the stomata open to allow gas exchange to occur, water in the form of water vapour is also lost through the stomata
  • This water loss is known as transpiration
  • Most plants can use cells called guard cells to close their stomata in order to reduce water loss, but this will also reduce gas exchange and therefore their rate of photosynthesis
  • Transpiration is the inevitable consequence of gas exchange in the leaf
• There are some advantages to the process of transpiration
  • It provides a means of cooling the plant via evaporation
  • The transpiration stream is helpful in the uptake of mineral ions
  • The turgor pressure of the cells, due to the presence of water as it moves up the plant, provides support to the leaves and to the stems of non-woody plants
    • Leaves with high turgor pressure do not wilt and therefore have an increased surface area for photosynthesis

Transpiration in the Leaf Diagram

The loss of water vapour from leaves by evaporation through the stomata is unavoidable as gas exchange for photosynthesis can only occur when the stomata are open

Factors affecting the rate of transpiration

• Air movement
  • More air movement leads to increased rates of transpiration 
    • The air outside a leaf usually contains a lower concentration of water vapour than the air spaces inside a leaf, causing water vapour to diffuse out of the leaf
    • When the air is relatively still, water molecules can accumulate just outside the stomata, creating a local area of high humidity
    • Less water vapour will diffuse out into the air due to the reduced concentration gradient
    • Air currents, or wind, can carry water molecules away from the leaf surface, increasing the concentration gradient and causing more water vapour to diffuse out
• Temperature
  • Higher temperatures lead to higher rates of transpiration, up to a point at which transpiration rates will slow 
    • An increase in temperature results in an increase in the kinetic energy of molecules
    • This increases the rate of transpiration as water molecules evaporate out of the leaf at a faster rate
    • If the temperature gets too high the stomata close to prevent excess water loss
    • This dramatically reduces the rate of transpiration
• Light intensity
  • Higher light intensities will increase the rate of transpiration up to a point at which transpiration rates will level off 
    • Stomata close in the dark and their closure greatly reduces the rate of transpiration
    • Stomata open when it is light to enable gas exchange for photosynthesis; this increases the rate of transpiration
    • Once the stomata are all open any increase in light intensity has no effect on the rate of transpiration
• Humidity
  • Higher humidity levels reduce the rate of transpiration 
    • If the humidity is high that means the air surrounding the leaf surface is saturated with water vapour
    • This causes the rate of transpiration to decrease as there is no concentration gradient between the inside of the leaf and the outside
      • At a certain level of humidity, an equilibrium is reached; water vapour levels inside and outside the leaf are the same, so there is no net loss of water vapour from the leaves

Several environmental factors affect the rate of transpiration in plants

Measuring the rate of transpiration

• The effect of environmental factors on the rate of transpiration in plants can be measured using a piece of equipment called a potometer
  • Note that while potometers are used to measure transpiration rates, they technically measure the rate of water uptake rather than the rate of transpiration, as a small amount of the water taken up by a plant will be used in photosynthesis
    
    • Because the amount of water used in photosynthesis is so small in relation to the total amount of water that passes through a plant, the rate of water uptake can reasonably be used to represent the rate of transpiration
• Different types of potometer exist
  • Bubble potometers measure the movement of an air bubble along a water-filled tube connected to a plant shoot as water is drawn up by the shoot
    • The position of the air bubble is recorded at the start of an experiment, and then a researcher can either measure how far the bubble moves in a set amount of time, or time how long it takes for the bubble to move a certain distance
  • Mass potometers measure the change in mass of a water-filled test tube connected to a plant shoot as it loses water over a set amount of time
• The effect of various environmental factors on transpiration can be measured by placing the potometer in different conditions e.g.
  • Wind speed
  • Humidity
  • Light intensity
  • Temperature

A bubble potometer uses the movement of an air bubble to measure the rate at which water is drawn up by a plant shoot. In this image the air bubble will move to the left along the tube as the plant transpires

• Environmental factors can be investigated in the following ways
  • Air movement
    • A fan on different settings could be used to vary the flow of air around a plant shoot
  • Humidity
    • Enclosing the plant shoot in a plastic bag can increase the humidity
    • A humidifier or dehumidifier could be used to give a measurable variation in humidiy
  • Light intensity
    • A lamp at different distances or with different types of light bulb can be used to vary light intensity
  • Temperature
    • A thermometer or temperature probe can be used to find surroundings with different air temperatures
    • A heater or air conditioner can be used to give a measurable variation in temperature
• A researcher would need to be aware of the importance of controlling any variables other than the variable being investigated to ensure that any results are valid e.g. placing a plant shoot in different rooms could be a way of varying temperature, but might bring the risk of also varying light levels and humidity; these variables would need to be controlled