Gas exchange in plants

Palisade Mesophyll

• main photosynthetic tissue

• cells are deep, packed full of chloroplasts

• chloroplasts able to move to give maximum light absorption

Upper Epidermis

• single layer of cells

• transparent, no chloroplasts —> light can pass straight through to tissues below

• not many stomata (heat would cause excessive evaporation)

Waxy Cuticle

• reduces water loss significantly

Spongy Mesophyll

• fewer chloroplasts so less important for photosynthesis

• cells are loosely packed with a lot of AIR SPACES

• gas exchange happens on moist surface

• co2 diffuses out of air spaces and into cells and o2 the opposite

Lower Epidermis

• no waxy cuticle

• many stomata to allow diffusion of gases in and out

• water diffuses out as water vapour

Vascular Bundle

• in the centre of the leaf

• contains xylem and phloem

Label the vascular tissue in a leaf:

Adaptations of the leaf for gas exchange and photosynthesis:

• large SA - many stomata to capture maximum light energy

• thin - short diffusion pathway, light penetrates through leaf

• transparent - light passes through to the main site of photosynthesis

• palisade cells ELONGATED + packed with chloroplasts - maximises light absorption

• air spaces - free diffusion of gases, allows co2 to diffuse in

• stomata - gas exchange in and out, allows co2 into the leaf

What happens during the process of stomatal OPENING?

1. chloroplasts photosynthesis, producing ATP

2. ATP provides energy for active transport of K+ ions into guard cells from epidermal cells

3. stored starch converted into MALATE

4. K+ ions & malate are soluble - lower the ψ in guard cell

5. water enters via OSMOSIS

6. guard cell becomes turgid

7. thicker inner wall than outer so bend less & pore (stomata) created

What happens during the process of stomatal closure?

1. K+ ions diffuse out PASSIVELY

2. water moves out of guard cell via OSMOSIS

3. guard cell is plasmolysed

4. stoma closes

How to calculate field of view:

• size of square/rectangle given with specimen in mm

• divided by magnification

• to give actual

• then divided by size of field of view in mm2

Equation for standard deviation:

How to apply this method: (example)

Method for determination of stomatal density:

1. pick a large leaf, paint the lower epidermis between the veins with clear nail polish. wait until dry

2. use sellotape to rip of the portion from the leaf

3. place under microscope and count the number of stomata in field of view

State two other environmental factors that should have been controlled:

OPTIONS

• humidity

• wind speed/air movement

• concentrations of different minerals in soil

• pH of soil

• water availability

• concentration of co2

Describe how structure of Mesophytes allow the plant to carry out photosynthesis and how the adaptations in the leaves in Xerophytes and Hydrophytes enable to the plant to survive in different environmental conditions

9QER

Mesophyte - adequate water supply

Xerophyte - low water supply

Hydrophyte - high water supply