# Examining The Rate Of Photosynthesis Using Leaf Discs Biology Essay

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The rate of photosynthesis can be measured by calculating the quantity of carbon dioxide consumed per unit time, or by calculating the quantity of oxygen consumed per unit time. The rate at which photosynthesis occurs is determined by rate-limiting step. The rate limiting step is the step that photosynthesis occurring the most slowly. The factors which affect the rate of photosynthesis include, light intensity, temperature and carbon dioxide concentration. Each factor affects a different rate-limiting step.

One of the factors is light intensity. As light intensity increases, the photosynthetic rate increases until a point is reached where the rate begins to level off. At low light intensity, photosynthesis occurs slowly because only a small quantity of ATP and NADPH is created by the light dependent reactions. As light intensity increases, more ATP and NADPH are created, thus increasing the photosynthetic rate. At high light intensity, photosynthetic rate levels out, not due to light intensity but due to other limiting factors, including competition between oxygen and carbon dioxide for the active site on RUBP carboxylase.

## RESEARCH QUESTION

Does the different distance of leaf disc from the light source affect the rate of photosynthesis in leaf disc?

## HYPOTHESIS

As the distance of the leaf disc from the light source become closer, the rate of photosynthesis becomes faster. As the distance of the leaf disc from the light source become further, the rate of photosynthesis becomes slower.

This is because at the high light intensity, chloroplast absorb high amount of light and it will produce oxygen at higher rate thus the time taken for leaf disc to rise is shorter. At the low light intensity, chloroplast absorb low amount of light and it will produce oxygen at lower rate thus the time taken for leaf disc to rise is longer.

## INDEPENDENT VARIABLE

The distance between leaf disc and light source

cm

0-50

## DEPENDENT VARIABLE

Time taken for leaf disc to rise on the surface of sodium hydrogen carbonate

s

## -

TABLE 1 : Independent and Dependent Variable

## WAYS TO CONTROL

The temperature

&deg;C

Use the same temperature for each distance which is room temperature.

Humidity

## -

Place the entire beaker in the same place for every distance.

Volume of 3% of sodium hydrogen carbonate used

cm³

Used 20ml of sodium hydrogen carbonate for each distances.

Amount of leaf disc used in each distance

## -

Use only 5 leaf disc in each distances.

The size of each leaf disc

Mm

Use cork borer to get an approximately 8mm of leaf disc.

Type of leaf disc used

## -

Use the same type of leaf in every distance.

TABLE 2 : Controlled Variables

## 1.

Pineapple leaf discs

90

## 2.

3% sodium hydrogen carbonate solution

1 bottle reagent

## -

TABLE 3 : Materials

## VOLUME / SIZE

1.

Lamp

1

Standard size

2.

syringes

1

10ml

3.

Cork borer

1

8mm size

4.

beaker

6

100ml

5.

Forceps

1

Standard size

6.

Thermometer

1

Standard size

7.

Stopwatch

1

Standard size

8.

Long ruler

1

Standard size

TABLE 4 : Apparatus

## METHOD

Refer to the attachment.

## Leaf disc 5

10.00

285

285

310

320

330

270

270

290

310

320

285

290

305

310

320

20.00

350

352

355

363

370

350

356

358

366

379

352

354

363

368

370

30.00

686

689

693

702

734

698

712

751

788

803

670

679

703

722

758

40.00

1304

1387

1395

1521

1632

1421

1456

1534

1567

1638

1380

1470

1499

1632

1721

50.00

3601

3621

3675

3777

3820

3256

3345

3457

3578

3699

3525

3612

3702

3756

3896

60.00

4603

4639

4689

4781

4902

4720

4790

4886

4904

5056

4589

4690

4844

4981

5021

TABLE 5 : Quantitative Data

## QUALITATIVE DATA

The pineapple leaf is green in colour and thick.

The 3% sodium hydrogen carbonate is colourless.

Bubbles are formed around the leaf discs when immersed in the sodium hydrogen carbonate.

The leaf discs sink at the bottom of the sodium hydrogen carbonate at first, and started to float to the surface of the indicator when the oxygen gas produced.

## AVERAGE TIME TAKEN FOR LEAF DISC TO RISE

The average time taken for leaf disc to rise is calculated by using this formula :

Time taken for each leaf disc to rise Number of trials

For example, the average time taken for leaf disc to rise at the 10.00cm of distance from light source is calculated ,

Time taken for each leaf disc to rise Number of trials

= 285+285+310+320+330+270+270+290+310+320+285+290+305+310+320 15

= 300s

The average times taken for leaf disc to rise for other distance are calculated by using the same formula.

## The average time taken for leaf discs to rise on the surface of sodium hydrogen carbonate solution, minutes, min (± 0.5min)

10.00

300.000

20.00

360.400

30.00

719.200

40.00

1503.800

50.00

3621.333

60.00

4806.333

TABLE 6 : The Average Time Taken for Leaf Discs to Rise

## RATE OF PHOTOSYNTHESIS

The rate of photosynthesis is calculated by using this formula

1 ______

Average time taken for leaf disc to rise

For example, rate of photosynthesis at the 10.00cm of distance from light source is calculated ,

1 ______

Average time taken for leaf disc to rise

= 1 300

= 0.00333 s-1

The rates of photosynthesis for other distance are calculated by using the same formula.

## (±

10.00

300.000

0.003333

20.00

360.400

0.002775

30.00

719.200

0.001390

40.00

1503.800

0.000665

50.00

3621.333

0.000276

60.00

4806.333

0.000208

TABLE 7 : Rate of photosynthesis

## STANDARD DEVIATION OF AVERAGE TIME TAKEN FOR LEAF DISC TO RISE

The standard deviation of average time taken for leaf disc to rise is calculated by using this formula

Where : ∑x2 = Sum of square of the time taken for leaf discs to rise

∑f = Total leaf discs involve in reading collections (15)

2 = Square of mean of time taken

For example, the standard deviation of average time taken for leaf disc to rise at the 10.00cm of distance from light source is calculated ,

= 18.439089

The standard deviations of average time taken for leaf disc to rise for other distance are calculated by using the same formula.

## The standard deviations of average time taken for leaf disc to rise

10.00

300.000

18.439089

20.00

360.400

8.507252

30.00

719.200

38.554204

40.00

1503.800

113.809900

50.00

3621.333

167.568759

60.00

4806.333

145.515482

TABLE 8 : The Standard Deviation

## THE UNCERTAINTIES OF RATE OF PHOTOSYNTHESIS

The uncertainties of the rate of photosynthesis is calculated by using this formula

ΔROP = Δ time x ROP

Time taken

For example, uncertainties of the rate of photosynthesis at the 10.00cm of distance from light source is calculated ,

ΔROP = Δ time x ROP

Time taken

= 18.439089 X 0.003333

300.000

= ±0.000205

The uncertainties of the rate of photosynthesis for other distance are calculated by using the same formula.

## The uncertainties of the rate of photosynthesis

10.00

0.003333

± 0.000205

20.00

0.002775

±0.000065

30.00

0.001390

±0.000075

40.00

0.000665

±0.000050

50.00

0.000276

±0.000013

60.00

0.000208

±0.000006

TABLE 9 : The Uncertainty of ROP

## LIGHT INTENSITY

The light intensity is calculated by using this formula :

L = 1

(distance2)

For example, the light intensity at the 10.00cm of distance from light source is calculated ,

1 (distance2)

= 1 102

= 0.01 cm-2

The light intensities for other distance are calculated by using the same formula.

## The light intensity

10.00

0.010000

20.00

0.002500

30.00

0.001111

40.00

0.000625

50.00

0.000400

60.00

0.000278

TABLE 10 : The Light Intensity

10.00

0.003333

± 0.000205

0.010000

20.00

0.002775

±0.000065

0.002500

30.00

0.001390

±0.000075

0.001111

40.00

0.000665

±0.000050

0.000625

50.00

0.000276

±0.000013

0.000400

60.00

0.000208

±0.000006

0.000278

TABLE 11 :

## DISCUSSION

Photosynthesis converts carbon dioxide and water into glucose and oxygen with the help of light energy from the sunlight.

In light dependent reaction, light energy from the sunlight absorbed by the chlorophyll to activate the electron in the reaction centre to higher level.

In this experiment, the effect of different light intensity to the rate of photosynthesis is investigated.

At low light intensity, which is 0.000278cm-2 , the product of the light-dependent reactions - NADPH and ATP is in shortages. The rate limiting step in the Calvin cycle is the point where glycerate 3-phosphate is reduced.

At high light intensity, which is 0.0025cm-2 and the 0.01cm-2, the carbon dioxide concentration is limiting.

From the graph, the rate of photosynthesis increase with increasing light intensity. However, as light intensity become higher, the rate of photosynthesis only increases at low rate. Theoretically, the rate of photosynthesis will increase directly proportional to the light intensity until it become constant at X, where other factor had become limiting factors. Then, increasing light intensity will give no effect on rate of photosynthesis.

At the beginning of the experiment, air is expelled from the pineapple leaf discs to make each discs will be free from any gas. When the pineapple leaf discs immersed in 3% sodium hydrogen carbonate, the carbon dioxide would dissolve in the mesophyll cell before diffusing in the cell.

The leaf discs will start float on the surface of the solutionwhen photosynthesis have occurred.

## SUGGESTION

The distance between the light source and the leaf discs is hard to measure by using a ruler.

Use measuring tape is the better apparatus to measure the distance and give the accurate distances.

Some of leaf discs is in upside down position when the leaf discs is immersed in the sodium hydrogen carbonate solution

Make sure all the leaf discs showing their surface of the leaf Before the leaf discs are exposed to the light by using forceps.

The diameter of the leaf discs is too small an allow a small amount of chlorophyll absorbs the light energy. Less oxygen will produced and it will take a longer time for the leaf discs to rise.

Increase diameter by using a bigger cork borer. When the diameter is wider, then more leaf discs surface can expose to the light source and increases the rate of photosynthesis.

When we expel the air from the syringe, there are some of the 3% of sodium hydrogen carbonate also expelled together with the air, thus reduce the volume of the sodium hydrogen carbonate.

Make sure there is not sodium hydrogen carbonate expelled together with the air.

## CONCLUSION

The increase in light intensity will result in increase in rate of photosynthesis until certain light intensity where increase in light intensity brings no effect on rate of photosynthesis. This is simply because the higher light intensity will boost more electrons to rise to the orbital of higher energy level and thus produces more NADPH and ATP that will increase the rate of photosynthesis.