B3.1 - gas exchange in plants (6g) Flashcards
What are the 2 ways we can measure stomatal density?
- Obtaining an image
- Calculating stomatal density
How do we obtain the image for calculating stomatal density?
An image can be obtained by:
- Directly observing a leaf, or using a micrograph of a leaf
- Performing leaf casts
- Paint the lower epidermis of a leaf with clear nail varnish and let it dry.
- Cover the varnish with clear tape and smooth it down.
- Peel the tape to remove the varnish.
- Transfer the tape to a microscope slide to view.
How do we determine stomatal density?
- Determine the area of the image. This could be in mm² or μm².
- Using a scale bar on the micrograph to calculate the area of the rectangular picture.
- Using a stage micrometer to measure the diameter of the field of view and calculate the area of that circle. - Count the whole number of stomata that are visible in the image or on the cast.
- Where partial stomata are visible, count any where at least half of the stoma is visible. - Divide the number of stomata by the area. Unless otherwise instructed, round to the nearest whole number and express this as a number of stomata per mm² or μm².
What is a plan diagram?
Plan diagrams do not show individual cells, rather they show the boundaries of each tissue type.
What is the transverse section?
A cross-section that cuts through the object, separating the top and bottom.
What is a dicotyledonous leaf?
A plant that develops from a seed that has two seed leaves.
What is transpiration?
Transpiration is the loss of water vapour from leaves by evaporation and diffusion.
What is the process of transpiration?
- Water evaporates from the cell walls of spongy mesophyll cells and diffuses into the air spaces.
- Water vapour passes out through the stomata by diffusion, with guard cells controlling the size of the stomata and therefore the rate of transpiration.
- The lows water is replaced from the xylem by mass flow through cell walls of spongy mesophyll cells.
Why does transpiration occur in a plant?
Transpiration is a consequence of gas exchange in a leaf.
- Gas exchange requires open stomata, which allows water vapour to move out of the leaf, down its concentration gradient into the atmosphere.
What are the 4 factors that affect the rate of transpiration?
- Light intensity
- Temperature
- Humidity
- Wind speed
Additional: Leaf surface area
How does light intensity affect rate of transpiration?
There is more transpiration in higher light intensities.
- Light causes stomata to open because the increased rate of photosynthesis means the carbon dioxide concentration in leaves is lower, requiring gas exchange with the atmosphere.
How does temperature affect rate of transpiration?
There is more transpiration as the temperature rises.
- More heat leads to faster evaporation of water, while more kinetic energy means there is faster diffusion of water vapour as it exits the leaf.
How does humidity affect rate of transpiration?
There is less transpiration as the atmospheric humidity rises.
- The air spaces inside the leaves are saturated with water vapour therefore higher atmospheric humidity means there is a smaller concentration/diffusion gradient between the leaf and the atmosphere, meaning less water vapour is lost through the stomata.
How does wind speed affect rate of transpiration?
There is more transpiration as wind speed increases.
- This is because humid air is blown away from the lead, increasing the concentration gradient of the water vapour, meaning more water vapour is lost.
How can leaf surface area affect rate of transpiration?
There is more transpiration as the leaf surface area increases.
- The larger surface contains more stomata, meaning more water vapour is lost.
- Transpiration does not only occur through the leaves, a small amount occurs through the stem.
How can reliability of quantitative data be measured?
Reliability of quantitative data is increased by repeating measurements.
- In this case, repeated counts of the number of stomata visible in the field of view at high power illustrate the variability of biological material and the need for replicate trials.
What are the 7 adaptations of leaves for gas exchange?
- Waxy cuticle
- Epidermis
- Palisade mesophyll
- Spongy mesophyll
- Air spaces
- Stomatal guard cells
- Veins
How does a waxy cuticle aid a leaf to be adapted for gas exchange?
- This wax lipid layer prevents uncontrolled gas exchange.
- Instead, this ensures the vast majority of gas exchange occurs through the stomata, which can be controlled.
How does an epidermis aid a leaf to be adapted for gas exchange?
- The upper epidermis usually has no stomata, or fewer, than the lower epidermis.
- This ensures the majority of gas exchange occurs through the stomata, which can be controlled.
How does the palisade mesophyll aid a leaf to be adapted for gas exchange?
- This tightly packed region of cells with numerous chloroplasts is a major site of photosynthesis.
- This region of the leaf therefore often requires high concentrations of carbon dioxide and needs to remove high concentrations of oxygen.
How does the spongy mesophyll aid a leaf to be adapted for gas exchange?
- The loosely packed, roughly spherical cells have large surface areas for effective gas exchange into and out of the cells.
How does air spaces aid a leaf to be adapted for gas exchange?
- The air spaces between the cells in the spongy mesophyll allow gases to diffuse relatively quickly between the cells and the atmosphere, via the stomata.
How does stomatal guard cells aid a leaf to be adapted for gas exchange?
- The pairs of guard cells in the lower epidermis can create a pore between them, opening or closing these as required.
- When open, gas exchange between the air spaces and atmosphere can occur, with oxygen and carbon dioxide diffusing down their concentration gradients.
- When rates of photosynthesis are high, carbon dioxide diffuses into the leaf, and oxygen diffuses out.
How do veins aid a leaf to be adapted for gas exchange?
- These vascular bundles contain xylem and phloem. Xylem brings water, allowing photosynthesis to occur.
- Phloem removes the photosynthetic products, converted to sucrose.
- Both these events are required for photosynthesis to continue, thereby maintaining the need for gas exchange.
