B4 Photosynthesis (2)

Greek scientists including Aristotle investigated plant growth.

They only saw soil touching plants so they decided planta grow and gain mass from minerals in the soil. 

Thy couldn't see air touching plant.

• Dried soil, weighed, put in pot.
• Planted willow tree weighing 2.2kg in soil.
• Added rainwater whenever dry.

~5 years later~

• Removed tree form pot.
• Tree weighed 76.7kg - gained 74.5kg.
• Dried soil and weighed, mass changed very little.
• Because so little soil gone, must have been other source causing tree to gain mass.
• He only added water so he concluded it was water.

Now we know plants also gain mass from CO2 in the air but this experiment showed it wasn't just minerals from the soil.

• Burning candle in a sealed container and observed flame went out. Candle couldn't be relit while in container.
• Then placed a burning candle and a living plant in container. Flame went out but after a few weeks, candle could be re-lit.
• Burning candle used up something to make flame go out and living plant restored te air so candle could burn again.

Sealed container with exhaled air and a mouse. Mouse dies after a few seconds.

Fills another sealed container with exhaled air and a living plant. Waits a few days. This time mouse survives for a few minutes.

Mouse couldn't survive long in exhaled air because breathing took something out of air.

Living plant restored the air allowing mouse to live longer.

Plants restore something (oxygen) to the air and burning and breathing takes it out.

Oxygen is product of photosynthesis.

Scientists knew oxygen was released during photosynthesis but did not know whether it came from carbon dioxide or water.

A scientist supplied plants with water containing oxygen isotope-18 and carbon dioxide with normal oxygen isotope-16.

When plants photosynthesised, oxygen-18 released, so oxygen came from water not carbon dioxide.

Light provides energy for photosynthesis. If light level is raised, rate of photosynthesis increases to a certain level.

Beyond that, it won't make a difference because temperature or CO2 becomes limiting factor.

CO2 is one of the raw materials needed for photosynthesis.

Only 0.04% of air is CO2, pretty scarce.

CO2 will increase rate of photosynthesis to a certain point.

After that, it is no longer the limiting factor and temperature or light becomes limiting factor.

Photosynthesis likes warm but not too hot.

As temperature increases, so does rate of photosynthesis but too high and plant's enzymes denature so rate rapidly decreases.

Usually at about 45 degrees Celsius which is hot for outdoors.

If temperature is limiting factor, usually it's too low.