Topic 3: Exchange: Plant gas exchange

Dicotyledonous leaf

• veins run parallel to the vessel

• Broad in flat shape of leaf maximise concentration gradient because large surface area = maximum diffusion of carbon dioxide constantly diffuses in. And oxygen out = maintains concentration gradient as levels decrease and increase.

• Oxygen buildup is prevented as it is efficiently being removed due to large surface area and thin/short diffusion distance

• Waxy cuticle on bottom and top of leaf. Bottom waxy cuticle is thinner, because it has more exchanged properties on the bottom and protection from the evaporation on top.

Gas exchange in plants

• Plants need carbon dioxide for photosynthesis and also oxygen for respiration. Both processes produce the other as a waste product.

• Depend depending on the time of day the balance of photosynthesis to respiration will create different concentration gradient which caused gases to diffuse in or out

Mesophyll cells

These are cells within the mesophyll tissue, located between the upper and lower epidermis

Upper epidermis

With waxy cuticle, this reduces water loss from the leaf surface, reduces transpiration

Palisade chlorophyll

Many chloroplasts

Spongy mesophyll

Large surface area and therefore many need to mesophyll cells

Stoma

These are small pores surrounded by guard cells on underside of leaves that can open and close

Air spaces – allow fast diffusion of gases into the cell

Interconnecting spaces that run throughout the mesophyll layer

Vascular tissue (xylem and phloem)

Transport water and nutrients

1st adaptation for gas exchange

Airspaces – provide a network for gases to quickly diffuse in and out of the leaf and access photosynthesising cells

2nd adaptation for gas exchange

Mesophyll cells - dispersed around leaf, providing a large surface area across which gases can diffuse

3rd adaptation for gas exchange

Stomata - these open when conditions are suitable for photosynthesis, allowing inward diffusion of carbon dioxide and outward diffusion of oxygen, and close to minimise water loss. (Many stomata - high stomatal density - short diffusion distance to cells = more gas exchange)

Guard cell swollen (open stomata-turgid)

In a wall is thickened more than the tips of guard cell, causes the curve shape

Guard cell shrunken (stoma closed - flaccid)

Happens during night because of no photosynthesis as no sunlight therefore no need carbon dioxide therefore prevents water being lost

Why does water move into guard cells?

• osmosis (driven by a difference in water potential)

• Chloroplast, but Guard cells, do not do photosynthesis but do produce ATP

• ATP used to active transport potassium ions into the cells = reducing water potential inside so water enters the cells via osmosis (this is triggered by sunlight)

First stimuli that opens stomata

Light – light receptors on plasma membrane of God cells trigger active transport proteins to pump potassium into guard cells

Second stimuli that opens stomata

Carbon dioxide – low carbon dioxide concentrations in the leaf as photosynthesis uses it up

Structural adaptations to prevent water loss

• Waxy cuticle

• Stomata found on underneath surface of leaf

Behavioural adaptations to prevent water loss

• Stomata close at night

• Deciduous trees lose their leaves in winter (enzymes for photosynthesis can’t function therefore stomata lost and frozen ground = low water availability)

Xerophytes

• Plants adapted to restricted supply of water

• Adapted to reduce water loss through transpiration

• Prevents desiccation and death

Thicker leaves

Lower surface area to volume ratio. Succulent leaves store water for dry periods.

Waxy cuticle

Sticker and on both sides of leaf. Reduces water loss through evaporation. Waterproof barrier. 10% water loss through it via evaporation. Thicker cuticle = less water loss.

Sunken stomata in pits

Traps still moist air next to the leaf equals reduces the water potential gradient (therefore less movement of water out of stomata). Reduce airflow and evaporation of water.

Cacti

• No leaves only succulent and stem

• Spines and hairs trapped moist air next to stomata

• Stomata open at night

• Water storage in stems

• Shallow roots absorbed rainwater before evaporates

• Deep roots access water deep below surface

Hairy leaves

• Trap still moist air near leaf surface

• Reduces water potential gradient between inside and outside of leaf

• Less water lost through evaporation

Spines

• The leaves are reduced to spines to reduce surface area for water loss

• Spines covered in waterproof wax

• Sunken stomata, lowest water potential gradient = less water loss

Lowest stomatal density than normal plants

Reduces water loss by transpiration

Water storage organs

Conserve water for when it is in low supply

Xerophytic adaptations

• Rolling up leaves – the leaves can roll up so that the tough waxy cuticle is on the outside of the leaf

• The stomata open onto an enclosed space in the middle where the air is very humid

• There are hairs to trap a layer of moist air too

• Encloses the stomata on the lower surface to reduce airflow and evaporation of water

Maram grass

• grow on sand dunes in British beaches

• Large variation in water availability in sand

• Salt concentration of sand is high reducing ability of roots to absorb water

• Longer root network to reach more water