University of Louisiana, Lafayette

BIOL 112

Photosynthesis Lab Report
Abstract:
Photosynthesis is a natural process all plants undergo to create energy from light. In a
plant, photosynthesis creates oxygen as a byproduct, and cellular respiration uses oxygen to
make water. The speed or rate of both these processes combined is called “net photosynthesis”.
We wanted to see if light intensity affected the rate of photosynthesis. Our hypothesis was that
the net rate of photosynthesis will be proportional with the intensity of the light. We cut out disks
from a leaf and saturated them with a solution using a vacuum method until they all sank. After
placing them is a shallow petri dish with a lamp directly above the dish, we observed how fast
and how many leaves underwent photosynthesis and rose to the top of the solution. Our two
trials with different lamp distances showed that the closer the lamp is to the petri dish, the
quicker the percent floating increased. While these results supported my hypothesis, there was
not enough evidence to fully confirm the hypothesis. More trials with different variables [e.g.
lamp height] would have to be performed to conclude my hypothesis.
Introduction:
Photosynthesis is an extensive process that utilizes light to create energy. Initially, this
energy is captured in the form of ATP and NADPH in the Light Reaction. Then the stored
energy from ATP and NADPH is used by the Calvin Cycle produce high energy sugar for further
storage and use. In the Light Reaction, water is broken down to its elemental forms, oxygen and
hydrogen, and they are released as byproducts. The thylakoid membrane produces ATP using the
electron transport chain and must use the free electron to bind oxygen to hydrogen and make
water. We have established that photosynthesis produces oxygen and that respiration uses
oxygen. These two rates make up the net photosynthesis.
The leaf, where photosynthesis occurs, is comprised of many layers. The main layers of
the middle leaf or mesophyll are the palisade layer and the spongy layer (Weed, 2011) The large
spongy layers of the mesophyll have cells with spacious cavities. This allows the spongy layer to
diffuse atmospheric gas with the cells of the plant easily. The epidermis of the leaf contains many
pores called stoma. The stoma allows carbon dioxide to enter the inner leaf and be absorbed into
the spongy layer. Also, excess oxygen produced as a byproduct of photosynthesis leave the leaf
through the spongy layer.
For our experiment, we are going to determine the rate at which oxygen is produced
during photosynthesis. We are going to saturate the spongy layer of leaves with a solution. This
will make the leaves sink to the bottom of a water solution. When the leaves start photosynthesis,
they are going to produce oxygen and fill their spongy layer cells with oxygen. This will make
the leaves float in the water solution. The “time to float” is inversely proportional to
photosynthetic rate in the leaf tissue. (Bio Lab Manual, 2011) We can then set up more
experiments with different variables such as light intensity to see if the variables have an effect
on the rate of photosynthesis in the leaves. We have to understand that watching leaves float is a
qualitative judgment, and the values are estimates. It is hypothesized that the net rate of
photosynthesis will be proportional with the intensity of the light. For example, the closer a light
source is; the quicker the leaves will rise. If this hypothesis is true, then the time it takes for the