Adaptations for Gas Exchange in Plants

How the adaptations of a leaf increase its rate of gas exchange 

• Thin: makes the diffusion pathway for gases entering and leaving short 

• Contains air spaces: allow oxygen and carbon dioxide to diffuse between the stomata and the cells 

• Contains stomata: contain pores that allow gas exchange in and out of the leaf 

The structure of guard cells that surround a stomata 

Guard cells have a thin outer cell wall and a thick inner cell wall, both made up of cellulose. They contain chloroplasts and a vacuole; that shrinks when the pore of the stoma closes. The guard cells control the opening and closing of the stomata.  

How guard cells open to form the stomata  

During the day:  

• If water enters the guard cells, they become turgid and swell, and the stomatal pore opens 

• If water leaves the guard cells, they become flaccid and the stomatal pore closes.  

Angiosperms 

They can be classified as monocots or dicots. 

 The stomata in dicot leaves are at the lower epidermis (abaxial) and the stomata in monocot leaves are at the lower epidermis (abaxial) and upper epidermis (adaxial).  

Collenchyma 

A tissue often associated with leaves. It gives support to short-lived structures 

Functions of leaf features 

Cuticle 

• reduces excess water loss (waxy and waterproof) 

• Enables light to penetrate mesophyll cells (transparent) 

Upper and lower epidermis 

• Protection of tissue layers inside leaf 

• Enables light to penetrate (transparent) 

Palisade mesophyll 

• Many cells can fit in the layer to maximise absorption of light 

• Increase absorbtion of light and the rate of photosynthesis (lots of chloroplasts)  

Spongy mesophyll 

• Cells contain some chloroplasts 

• Large number of air spaces for gas exchange within leaf 

•  No ventialtion mechanism so relies on diffusion 

Air spaces 

• Gas exchange of CO2 and O2 for respiration and photosynthesis 

• Water vapour evaporates into these spaces to maintain transport of water from roots to leaves 

Xylem 

• transport of water and ions from roots to all other parts of the plant 

Phloem 

• transport of carbon compounds (sucrose and amino acids) from the site of photosynthesis and storage organs to the rest of the plant 

Bundle sheath cells 

• Provides support and protection to the vascular bundles (in some plants these enable more efficient photosynthesis) 

Stomatal pores 

• Sites of gas exchange (CO2 and O2) with the external environment  

• Water vapour lost from the stomata when open 

Guard cells 

• Control the opening and closing of the stomata 

• Contain chloroplasts 

Chloroplasts 

• Site of photosynthesis 

• Involved in the mechanism for stomatal opening and closing 

Veins 

• Transport water, nutrients and food 

Monocot leaves vs. Dicot leaves 

• Monocot leaves have stomata on both the upper and lower epidermis, dicot only on one which is normally the lower epidermis. 

• Leaves in monocots are rolled to reduce the exposed SA to sunlight, whereas the distribution of stomata in the lower epidermis of dicots reduced the water loss by transpiration. 

• Stomata on monocot leaves are surrounded by dumbbell shaped guard cells, whereas in dicot they’re bean shaped guard cells. 

• The arrangement of stomata in monocot are highly ordered rows, whereas dicot leaves are more irregular. 

Xerophytes (dry, arid environments) 

• Stomata located on the lower underside of leaf (abaxial) --> cooler, less exposed to sunlight means less water loss thus reducing rate of transpiration. 

• Sunken stomata --> helps to trap humid air around stomata {reducing water potential gradient}  

• {fewer/smaller} stomata + {fewer/smaller} gaps for water loss 

• Tiny hairs – stiff, trap water vapour and reduce the {diffusion/concentration/water potential} gradient between the inside of the lead and the outside (to reduce air flow and evaporation) 

• Lower density of stomata reduced the overall SA in which water can be lost. 

• Thick cuticle/epidermis + reduce evaporation/water loss 

• Fibres provides support and prevents wilting 

Hydrophytes (partially or fully submerged in water) 

• Stomata on {upper/adaxial} surface of floating leaves as there is no stomata on {lower/abaxial} surface for gas exchange with air. 

• Surrounded by water so, water provides support, there is little need for transport so {xylem} is poorly developed. 

• Leaves have little or no cuticle because there is no need to reduce water loss 

• Leaves and stems have large air spaces which {act as gas reservoir for gas exchange} provide {buoyancy} for {light/photosynthesis}