Biology

BIO100A Midterm/Final Report example

<Note: This report is based on a technique that you will not do in the class. You can watch it on Youtube https://www.youtube.com/watch?v=XV9FOWleErA (5 minutes). In the experiment, we will be comparing 3 groups of leaf disks: exposed to direct sunlight, exposed to sunlight behind a window, and away from the window.>

Introduction

The purpose of the lab is to observe and become more familiar with photosynthesis. We will be comparing the effect of light intensity on photosynthesis using spinach leaf disks. Photosynthesis is a process by which plants and other producers are able to convert solar energy to chemical energy, and build up organic molecules such as glucose using CO2 from the air as source of carbon (Phelan, 2015). In addition, photosynthesis also releases oxygen as a byproduct, which is used by all aerobic organisms. In this experiment we will be using spinach leaves to explore the effect of more or less sunlight on photosynthesis (Steucek et al, 1985). Small disks from the spinach leaves cut using a hole puncher will be first depleted from oxygen, which will make them sink, then placed in water with sodium bicarbonate (Na2HCO3) at different locations. Bicarbonate is used as carbon source leaves will not be in touch with air when the experiment starts. As photosynthesis takes place, using the sunlight as source of energy and the bicarbonate as source of carbon (CO2), the leaves produce oxygen, which makes them float. We will be measuring the time that it takes for half (50%) of the disks to float. As photosynthesis depends on sunlight, it is my hypothesis that it will take longer to float when the leaves are not exposed to direct sunlight. I also believe that the leaves in direct sunlight will float faster compared to the ones behind the windowsill, as glass absorbs UV light.

Methods

The experiment took place in-house, between 11 am and 1 pm. The temperature in the sun (outside and behind the window) was 75 oF, while inside the room in the shade it was 71 oC. We placed spinach leaf disks in 3 syringes, soaked them with a bicarbonate solution, and eliminated the air contained in them. Once the syringes were set up I placed them in 3 different locations: on a windowsill exposed to direct sunlight, behind the same window on a table, and inside another room, away from windows. The actual experiment started at noon, and the sunshine was bright and steady (no clouds). The photosynthetic process was monitored by observing the disks starting to float, due to production of oxygen. Time necessary to have half of the disks (5 of 10) floating above the 5 ml mark on the syringe was recorded.

Results

After preparing the experiment as described in the Methods section, we compared the time that took for each group to have half of the leaf disks floating. It was shorter for the leaves in direct sunlight and the group behind the window, and longer for the leaves away from the window. Figure 1 shows the comparison of the time needed for 50% of the disks to float. While there was virtually no difference in the time needed for the samples exposed to sunlight (outside vs behind the window, 25 and 26 minutes, respectively), the leaves not exposed to sunlight, in the living room, required a longer time (35 minutes).

Figure 1. Time needed for 50% leaves to float.

(Below is the original table, which is not required for the report).

Discussion & Conclusion

Electrons in chlorophyll, the photosynthetic pigment in the chloroplasts, become excited by the energy of sunlight. The electrons enter an electron transport chain where high-energy molecules such as NADPH and ATP are produced. In the process, water is split and oxygen gas is released. Bicarbonate in water releases CO2, which is fixed by the leaf in the Calvin cycle. This means the carbon of the CO2 is incorporated into organic molecules, resulting in the production of glucose.