Understanding graphs and variables

DO YOU THINK ALL PLANTS PHOTOSYNTHESIZE AT THE SAME RATE?

Discuss with your partners and find reasons and factors that would influence photosynthesis.

PHOTOSYNTHESIS RATE

•The factors that affect the rate of photosynthesis are:

•Light

•Temperature

•Concentration of carbon dioxide.

These factors, in scientific terms, are known as variables, and they can vary in nature. The rate of photosynthesis varies with them.

In the lab, when we do experiments, we will usually deal with two types of variables:

- INDEPENDENT variable: what is varied during the experiment – WHAT I CHANGE

- DEPENDENT variable: what is being measured – WHAT I OBSERVE

HOW DO THESE VARIABLES RELATE WITH EACH OTHER?

We must focus our observations on the dependent variable to see how it responds to the change made to the independent variable. The new value of the dependent variable is caused by and depends on the value of the independent variable.

EXAMPLES:

QUESTION WHAT I CHANGE WHAT I OBSERVEDoes heating a cup of water allow it to dissolve more sugar?

Temperature of the water measured in degrees Centigrade

Amount of sugar that dissolves completely measured in grams

Does fertilizer make a plant grow bigger?

Amount of fertilizer used

Growth of the plant measured by its height or weight

Imagine you have designed an experiment to test the rate of photosynthesis with different factors such as light intensity, temperature and carbon dioxide concentration.

What would the rate of photosynthesis be, the dependent or independent variable? And the factors?

MAKING GRAPHSAfter scientists record their data in tables, they usually make graphs to display it for analysis.

Graphs show the relationship between two variables. The pattern of curve or line that is drawn will help you to form conclusions about your data.

Now you can put all this in practice!BUILD A GRAPH:

Imagine you have done an experiment to solve the following problem:

DOES LIGHT INTENSITY AFFECT THE RATE OF PHOTOSYNTHESIS IN ELODEA PLANTS?

Your Hypothesis: Yes, light intensity does affect the rate of photosynthesis.  Too much light may cause plants to "burn out" while too little light will not provide enough light for optimal photosynthesis activity.

Procedure This experiment will look at the affect of light intensity on the rate of

photosynthesis on Elodea plants.  -  The rate of photosynthesis is determined by the number of bubbles

formed by the Elodea plants. The oxygen bubbles are a product of photosynthesis. Plants have shown to produce more bubbles when placed closer to a light source (a lamp) than further away. The efficiency of photosynthesis can be measured by the amount of oxygen bubbles formed during a given amount of time. The bubbles counted were  only the ones that floated to the top of the test tube.  Baking soda is added to the test tubes to provide  adequate amounts of carbon dioxide for photosynthesis to occur.

Independent variableTime and distance to the light are the independent variables.

Dependent variablesThe dependent variable will be the number of bubbles formed and floated to the top of the test tube.  

Data

Distance of the light source/ Time

10 seconds 30 seconds 60 seconds

50 cm 13 45 11440 cm 15 54 12130 cm 16 54 12420 cm 20 71 15010 cm 29 92 206

The number of bubbles formed (dependent variable) as a function of time (independent variable) at various light distances from the test tube.

Your task:You have to build a graph using the data of the table. - Time goes in the x-axis

- Number of bubbles goes in the y-axis.

You plot a line with a different color for each distance.

Based on the data collected and the graph:- interpret the results and outline the conclusions of the

experiment- discuss the validity of the hypothesis