PHOTOSYNTHESIS - (PS12)

Question 1

State the equation of photosynthesis in words and state the necessity for chlorophyll and light

Answer 1

Energy from sunlight
Co2 + water—————————
Absorbed by chlorophyll

Photosynthesis is the process by which plants manufacture sugars (glucose) from carbon dioxide (CO2) and water (H2O). The energy for this process is obtained from sunlight, which is absorbed by chlorophyll. Oxygen is given off as a by-product .

Question 2

Photosynthesis details

Answer 2

Photosynthesis occurs principally in the leaves, in cell organelles called chloroplasts, although any green part of the plant can photosynthesise.

Plants absorb water from soil solution through root hairs into xylem vessels and is transported upwards into the leaves.

Stomata on the leaf surface allows the exchange of co2 and o2 with the atmosphere.

• in chloroplasts, molecules of co2 and water combine in a series of chemical reactions
• the energy required for the chemical changes comes from light energy from the sun which is absorbed by the green pigment chlorophyll
• the new molecule formed is a simple sugar (c6 H12 o6) the elements of carbon, hydrogen and oxygen that make up the simple sugar are held together by bonds. These bonds store the energy from sunlight until it is required.
• the sugar molecules can be used immediately, or converted to starch which is a complex sugar and stored by the plant.
• oxygen is released back into the atmosphere via the stomata as a by product of the process
• the molecules of sugar and starch can be used as energy to fuel other cell processes such as cellular respiration.

Question 3

List the environmental Factors effecting the rate of photosynthesis

Answer 3

TEMPERATURE
Photosynthesis is enzyme driven. Rate increases with temperature. As enzymes more active in higher temps
Below 25•c photosynthesis slows down
Above 35•c photosynthesis can stop

LIGHT
Duration
Length of natural daylight plant exposed to. Longer daylight higher photosynthesis

Quality
Measured in PAR (photosynthetically active radiation)
Solar radiation in the wave band 400 (blue light) to 700 manometers (red light) is used in photosynthesis.

Intensity
Measured in LUX (lumens per square meter)
Min intensity for photosynthesis is approx 500-1000 lux
Max amount plants can use 30,000 lux
Most grow well in 10,000 - 15,000 lux
Mid day sunlight on sunny summers day 110,000 lux approx
Mid day sunlight overcast summers day 10,000-25,000 lux.

In low levels plants may use more glucose in cellular respiration than they produce by photosynthesis.
If this continues for long time plant will die.
Too high light (intensity) can damage chlorophyll- but is uncommon in nature.

Light Intensity can be increased by using supplementary lighting ( most common high pressure sodium and mercury vapour which provide correct quality of light for optimum growth.
Clear glass important to allow high level natural light

CO2 LEVELS
Level in atmosphere is usually around 0.03%
This is limiting
Increased co2 = increased rates of photosynthesis but too high can inhibit respiration and damage plant
Co2 level can fall in densely planted greenhouses
Increased ventilation can help
Growers frequently use paraffin heaters or propane gas burners to enrich co2 levels

WATER
Lack of water leads to stomata closing and wilting, thereby effecting gaseous exchange and photosynthesis

CHLOROPHYLL
As photosynthesis requires chlorophyll, variegated leaves that contain less chlorophyll are photosynthetically less efficient and therefore grow slower
Iron, magnesium and nitrogen are constituents of chlorophyll- if in short supply chlorophyll production is reduced - therefore reduced photosynthetic capability

Question 4

Law of limiting factors

Answer 4

If any of the external factors required for photosynthesis are in short supply or reduced (light, co2, water, temperature, chlorophyll)- no matter how plentiful the other factors, then this factor will reduce the rate of photosynthesis or even stop the process of photosynthesis
Which ever factor is in short supply becomes the LIMITING FACTOR

Question 5

List leaf adaption to maximise photosynthesis

Answer 5

A- BROAD FLAT SHAPE
Large surface area to absorb sunlight and co2

A- THIN STRUCTURE
Allows greater gaseous exchange

A- LARGE INTERCELLULAR SPACES IN
MESOPHYLL
Easy passage of co2

A- NUMEROUS STOMATA
For exchange of co2 and o2

A- CONCENTRATION OF CHLOROPLASTS IN UPPER LEAF SURFACE
Chloroplasts receive more light closer to the light source

NETWORK OF VEINS
Provides ready supply of water

ARRANGEMENT OF LEAVES ON STEMS
Allows light to reach maximum number of leaves