gas exchange

GAS EXCHANGE IN SINGLE-CELLED ORGANISMS

SIMPLE DIFFUSION OF GASES THROUGH CELL SURFACE MEMBRANE

adaptions of gas exchange in single celled organisms

• They have a relatively large surface area 

• They have a thin surface

• They have a short diffusion pathway

GAS EXCHANGE IN DICOTYLEDONOUS PLANTS

DIFFUSION OF OXYGEN AND CARBON DIOXIDE DURING PHOTOSYNTHESIS AND RESPIRATION 

At times the gases produced in one process can be used for the other. 

• This reduces gas exchange with the external air. 

• Overall, this means that the volumes and types of gases that are being exchanged by a plant leaf change. 

• This depends on the balance between the rates of photosynthesis and respiration.

The main gas exchange surface is the surface of the mesophyll cells in the leaf.

There is no specific transport system for gases, which simply move in and through the plant by diffusion.

Diffusion takes place in the gas phase (air), which makes it more rapid than if it were in water.

ADAPTATIONS FOR EFFICIENT GAS EXCHANGE in plants

large surface area

short diffusion pathway

maintained concentration gradient

LARGE SURFACE AREA TO VOLUME RATIO

Air spaces inside a leaf have a very large surface area compared with the volume of living tissue.

Large surface area of mesophyll cells for rapid diffusion

SHORT DIFFUSION PATHWAY 

Many stomata so no living cell is far from the external air, and therefore a source of oxygen and carbon dioxide.

Numerous interconnecting air-spaces that occur throughout the mesophyll so that gases can readily come in contact with mesophyll cells

MAINTAINED CONCENTRATION GRADIENT 

When stomata are open, gases enter and exit so there is always a favourable diffusion gradient for CO₂ and O₂. 

Stomata

• OPEN: in bright light when photosynthesis and respiration occurs, so gas exchange is needed. Water moves into the guard cells  by osmosis making them turgud and they open. 

• CLOSED: In dim light -when photosynthesis cannot occur therefore no gases need to be exchanged, the stomata close to limit transpiration and save water.  They also close when the plant is dehydrated and water moves out of the guard cells and they become flaccid, causing them to close.