8/3/2019 B7.2

1/4

1

B7.2 PHOTOSYNTHESIS (NOT AGAIN??!!)

6H2O + 6CO2 ----------> C6H12O6+ 6O2

As stated in the last section, this is a simplified equation for

photosynthesis. Light energy is absorbed by the chemical chlorophyll.

This energy is used to convert the carbon dioxide and water into

glucose and oxygen.

Figure 3 shows the structure of glucose

Glucose is a sugar which can be used for 3 purposes

y It can be changed into chemicals needed for the growthof plant cells. These chemicals include cellulose, proteins

and chlorophyll.

y It can be used in respiration to release energy.y It can be converted into starch for storage. Starch is a

better storage molecule than glucose, because it is

insoluble and doesnt mess up the osmotic balance in thecell. (If there is a difference in sugar levels, osmosis can

take place either bursting or shrivelling plant cells.

NASTY!)

8/3/2019 B7.2

2/4

2

Figure 4 shows the structure of starch it is a polymer of glucose!!

DONT LEARN THIS FORMULA!!!!!! ON SECOND THOUGHTS!!

THE COMPENSATION POINT

The compensation point is where the rate of photosynthesis exactly

matches the rate of respiration. At this point, the uptake of carbon

dioxide through photosynthesis matches the respiratory release of

carbon dioxide, and the uptake of oxygen by respiration is exactly

matched to the release of oxygen in photosynthesis. This point is

reached during early mornings and late evenings.

At the compensation point, the rate of photosynthesis is balanced to

the rate of respiration so that the plant is neither consuming nor

building biomass.

For aquatic plants where the level of light at any given depth is roughly

constant for most of the day, the compensation point is the depth at

which light penetrating the water creates the same balanced effect.

The energy released during respiration in plants can be used to convert

glucose to starch and cellulose (another polymer), and also to convert

glucose and nitrates to amino acids (which are the building blocks of

proteins.

ACTIVE TRANSPORT (BUSES OR TRAINS??!)

This is the movement of molecules or ions across a cell membrane using

energy provided by respiration. Examples of substances that can be

8/3/2019 B7.2

3/4

3

actively transported across membranes are sodium ions and glucose.

Active transport occurs in plant roots, which absorb nitrates this way.

Energy is needed because the movement occurs against a concentration

gradient, with substances being moved from an area of lowconcentration to an area where there is a higher concentration. (the

opposite of the situation in diffusion, where molecules move from high

to low concentration.)

RATE OF PHOTOSYNTHESIS

This is affected by the light intensity, the concentration of carbon

dioxide and the temperature as shown in figures 5, 6 and 7 below.

How The Light Intensity Affects The Rate Of Photosynthesis

Figure 5

At high light intensity, the rate of photosynthesis levels out, not due to

light intensity but due to other limiting factors, including competition

between oxygen and carbon dioxide..

How Carbon Dioxide Concentration Affects The Rate OfPhotosynthesis

8/3/2019 B7.2

4/4

4

Figure 6

As carbon dioxide concentration increases, the rate of photosynthesis

increases until the enzymes are working flat out!!

How The Temperature Affects The Rate Of Photosynthesis

Figure 7

As the temperature goes up, the rate of photosynthesis increases until

you get to the optimum temperature. Above this temperature, the

enzymes start to become denatured.