light intensity and photosynthesis (required practical)

Photosynthetic rate

The rate at which plants take in carbon dioxide, water and the energy from sunlight to produce glucose and oxygen

Light intensity

A factor that typically increases the rate of photosynthesis

Distance from light source

The independent variable or factor that is changed in this experiment to investigate photosynthesis

LED light source

Is used as it releases less heat compared with other light bulbs

Temperature

A variable that should be controlled in this experiment to ensure the results are valid

Sodium hydrogen carbonate

Added to the water to supply the pondweed with a controlled source of carbon dioxide

Pondweed

A species of aquatic plant such as Cabomba or Elodea

Acclimatise

Allow the pondweed time to settle during each test before recording data to ensure results are valid

Bubbles of gas

The pondweed should release oxygen bubbles that can be counted as a measure of photosynthetic rate

Number of oxygen bubbles

The dependent variable or factor that is measured in this experiment to investigate photosynthesis

Mean average

The total sum of the data values divided by the number of times the data was recorded

Volume of oxygen gas

A more accurate experiment would be to collect the oxygen in an inverted measuring cylinder and record the volume

Expected results

As distance from the light source doubles, the number of bubbles per minute should decrease by a factor of four

Inverse-square law

Light intensity is inversely proportional to the square of the distance from a light source

Inversely proportional relationship

As the distance of light from a plant increases, the light intensity decreases

Water bath

A container of water heated to a given temperature for control purposes

Examples of other control variables

To ensure the results are valid the volume, concentration, external light and pondweed variety should be controlled