PhotosynthesisWhat are the word and chemical equations for

photosynthesis?

PhotosynthesisObjectives

Outline the key features and process involved In

photosynthesis.

Explain the factors affecting the rate of photosynthesis.

Outcomes

3: Outline key information about photosynthesis.

5: Explain the basics of how photosynthesis takes place and

factors affecting its rate.

7: Analyse how changing conditions will affect

photosynthesis and the reasons why.

Key terms: Photosynthesis, ATP, chlorophyll, pigment.

White light is made up of all the colours of the spectrum

Why do objects appear different colours?White light strikes

Green is reflected.

All other colours are absorbed by the object

The observer sees green

Or red

Or shades of colours

Leaves absorb all colours other than green, which is reflected

Although they also transmit greenSo we see green light coming through the leaf

The main photosynthetic pigment is chlorophyll

It absorbs the light mainly at the blue and red ends of the spectrum

This is called and Absorption spectrum. It outlines the differences in absorption of red, green and blue light by chlorophyll

So most photosynthetic activity occurs at the blue and red ends of the spectrum

This is the absorption spectrum

This is the action spectrum

The chemical reactions of photosynthesis – in 2 steps

1) Light splits water:

H2O

To form… Hydrogen

ions

H+

Oxygen ions

O-

This is called

photolysis

The use of hydrogen

2) ATP and hydrogen from photolysis are used to fix carbon dioxide

H+ + CO2 → C6H12O6 (Organicmolecules)

(Not balanced)

,

Leaves absorb CO2, root hairs absorb H2O and leaves excrete O2

Measuring photosynthetic ratesThe rate of photosynthesisThe rate can be measured directly by measuring how much oxygen is being released as a waste product, or by the rate of uptake of carbon dioxide. The diagram to the right shows a suitable set-up using elodea (pond weed).

As the elodea photosynthesizes, bubbles of oxygen gas are produced. These rise and can either be counted for a fixed period of time, or collected in an upturned water-filled measuring cylinder and the volume of water displaced recorded.

Photosynthetic rate can also be measured indirectly by measuring biomass. In other words, measuring the mass of the plant before and after photosynthesis has occurred. It is important to ensure that any change in mass is not the result of water loss or any other variable.

Factors that effect photosynthetic rate

• Light intensity As light intensity increases, rate of photosynthesis

increases. This is because the increase in light intensity initially increased the rate at which water is split as well as increasing the energy within the chlorophyll. Ultimately, this increases the rate of glucose production. The plateau is the result of other factors (such as CO2 concentration), limiting any further increase.

• Carbon dioxide As carbon dioxide concentration increases, the rate of

photosynthesis increases until it plateaus. The initial increase is caused by the rapid uptake of CO2 which is used to form glucose molecules. The carbon is “fixed”. ATP is needed to provide the energy, and hydrogen from the photolysis of water is also needed. In low light intensities, carbon fixation is less since there is s shortage of ATP and hydrogen.The plateau is the result of other factors (such as light intensity), limiting any further increase.

• Temperature As temperature increases, the rate of photosynthesis

increases, until it drops off at a similar rate. The increase initially is caused by the increase in kinetic energy of the reacting particles, which makes them more likely to collide with the enzymes of photosynthesis. However, once the optimum temperature for enzyme activity is reached, the enzyme activity decreases, and with it the rate of photosynthesis.