Photosynthesis (DP IB Biology)

Carotene produces an R_f value of 0.95.

The two types of chromatography are paper chromatography and thin-layer chromatography.

A pencil is used as the ink in pens will separate into pigments within the experiment and obscure the results

An absorption spectrum is a graph that shows the absorbance of different wavelengths of light by a particular pigment in the chlorophyll.

Plants absorb only some wavelengths of light because each pigment absorbs only certain wavelengths (colours) of visible light. Pigments reflect the colour of the wavelengths that they cannot absorb. For example, chlorophyll a, is seen as green by humans because green light is reflected by chlorophyll a.

Carotenoids absorb wavelengths of light mainly in the blue-violet region of the spectrum

Light energy results in the excitation of electrons within a pigment molecule (chlorophyll) which triggers the transfer of electrons leading to a series of reactions which make up the process of photosynthesis.

During photosynthesis, light energy is transformed into chemical energy when glucose is formed.

Chlorophylls mainly aborbs light between 400 and 500 nm (blue) and around 650 to 680 nm (red).

An action spectrum is a graph that shows the rate of photosynthesis at different wavelengths of light.

Both absorbance spectra and action spectra have:

• two main peaks – at the blue-violet region and the red region of the light spectrum

• a trough in the green-yellow region of the light spectrum

The rate of photosynthesis is highest at the blue-violet and red regions of the light spectrum. This is because these are the wavelengths of light that photosynthetic pigments can absorb.

The rate of photosynthesis can be determined by measuring the volume of oxygen produced or the carbon dioxide consumption at different wavelengths of light.

A control variable is that the light source should be kept the same distance from the plant.

Aquatic plants are often used to determine the rate of photosynthesis because the volume of oxygen produced can be easily measured using a gas syringe or bubbles of oxygen can be counted within the water.

The independent variable when determining the rate of photosynthesis for varying wavelengths of light are different wavelengths (colours) of light.

The data required to plot an action spectrum are:

• Wavelength of light (nm) on the x-axis

• Rate of photosynthesis on the y-axis

True.

The rate of photosynthesis can be measured by counting oxygen bubbles from a cutting of Elodea.

A hypothesis is a proposed explanation for an idea which may be true or false. A hypothesis can be proposed before experimentation or based on evidence from an experiment already carried out.

The independent variable is the factor that is deliberately changed throughout the experiment. For example, when investigating the effect of light intensity on the rate of photosynthesis, the independent variable would be the distance of the plant.

The dependent variable is the factor that is measured to see if it is affected by the independent variable. For example, when investigating the effect of temperature on the rate of photosynthesis, the dependent variable would be the rate of photosynthesis, measured (usually) by the volume of oxygen produced.

As light intensity increases, the rate of photosynthesis also increases, up to a certain point where it levels off. After this point, another factor is limiting the rate of photosynthesis.

True.

Temperature must be controlled during photosynthesis experiments to prevent it from affecting the results. This can be done using a heat shield in front of a light source (which generates heat) or a water bath.

The CO₂ concentration can be controlled by boiling and re-cooling water and then adding a set mass of sodium hydrogen carbonate.

The rate of photosynthesis decreases because enzymes begin to denature and cannot catalyse the reaction.

A limiting factor is a condition or resource that limits the rate of photosynthesis when it is in short supply. Limiting factors that affect the rate of photosynthesis are the concentration of carbon dioxide, light intensity or temperature.

A gas syringe can be used to measure the volume of gas produced in a photosynthesis experiment. Alternatively, a less accurate method is to use an inverted measuring cylinder placed over an aquatic plant.

Light intensity can be varied by using a light source and placing it at regular distances from a plant, e.g. 5 cm, 10 cm, 15 cm, 20 cm, 25 cm.

Suggested hypotheses for the effect of limiting the concentration of carbon dioxide on the rate of photosynthesis include:

• Carbon dioxide concentration will affect the rate of photosynthesis

• As carbon dioxide concentration increases the rate of photosynthesis will increase

Carbon dioxide enrichment experiments aim to study the effects of increased CO₂ levels on plant growth and photosynthesis to predict future impacts.

Free air carbon dioxide enrichment (FACE) experiments are conducted in natural ecosystems where CO₂ is pumped in to study its effects on larger plants and trees.

False.

In FACE experiments, variables cannot be controlled because they are carried out in natural ecosystems, but variables can be monitored and taken into account when analysing results.

True.

Enclosed greenhouse experiments allow for the control and manipulation of variables. They should be controlled to ensure that the effect of only one variable is being considered at any one time.

Controlled variables are any factors that may affect the experiment's results and need to be controlled or monitored. This may include temperature, light intensity, light wavelength, carbon dioxide concentration, and water availability. The control variables will depend on the variable that is being manipulated (the independent variable).

Enclosed greenhouse experiments are used to control and manipulate variables like light, CO₂, and temperature in photosynthesis studies.

The types of plants used in enclosed greenhouse experiments are small species that can be contained in a greenhouse can be studied using enclosed greenhouse experiments.

Controlling variables is essential to ensure valid results by isolating the effect of only the independent variable on the dependent variables (rate of photosynthesis).