Site author Richard Steane
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- the most important process on earth?

It is important to get the process of photosynthesis into perspective, especially in relation to the processes covered in previous topics: respiration, and digestion in animals. There are many misconceptions to do with the nature of energy and its interconversion, and the differences between plants and animals.
The statement below needs some points of explanation and amplification. The margins have spaces to make notes which explain the highlighted words (or you can click on them using your mouse!).
Green plants are extremely significant because they are the only organisms with the biochemical ability to "make" chemically complex organic food for themselves, starting with simple inorganic substances.
The process is called photosynthesis, and it uses the energy of light. Some of the sun's light energy is absorbed by the quite remarkable green pigment chlorophyll, and converted into chemical energy, stored in the molecular structure of the sugars and other compounds derived from them.
In the process, green plants use up carbon dioxide and produce oxygen, which is needed by most living organisms especially animals for respiration . Green plants therefore have a major impact on the air around us, and in fact on the whole planet.

In addition, all animals depend on plants for their supply of food. All the major components of diet are synthesised by plants in the first place.

The equation for photosynthesis

Complete the word equation for photosynthesis below:

Reactants gives Products
symbolic equation:
6 CO2 + 6 H2O gives C6H12O6+ 6 O2
word equation:
carbon dioxide + water gives glucose + oxygen        

Why are 2 words written above the arrow?

> they are not reactants or products - essential but chlorophyll is not used up

What are the similarities and differences between the (word) equations for photosynthesis and aerobic respiration?

>same substances, different direction (respiration needs no light/chlorophyll)

What are the main parts of a plant where photosynthesis takes place?

> leaves

What does the word photosynthesis mean?

> putting together with light

Where is the chlorophyll? Be as specific as possible.

> inside chloroplasts inside mesophyll cells of leaf

What happens to photosynthesis at night?

> it stops

Taking each of the 4 main ways that plants use glucose from photosynthesis in turn, say whether this could be passed on to you if you were to eat the plant. Explain why, and answer the supplementary question in each case.

Use to which plant put glucose Possible use to Man when plant is eaten?
1 (gives plant energy) no - already used up
2 (stored for later use) yes - roots/seeds/fruits are edible
3 (converted into cellulose) no - forms indigestible fibre
4 (used for synthesis of proteins) yes - digested back into amino acids

By what process does the plant obtain energy? > respiration

In what chemical form is glucose/food stored for later use?> starch/oil

For what purpose is glucose converted into cellulose? > cell walls

By the addition of which element can carbohydrates such as glucose be used for the synthesis of proteins?> nitrogen

Limiting factors in photosynthesis

What do you think would be the effect of increasing the following variables?

carbon dioxide concentration > proportionate increase in photosynthesis

light intensity > proportionate increase in photosynthesis up to plateau

temperature > proportionate increase in photosynthesis in certain range

Which of these would be a limiting factor in a normal warm day?

> carbon dioxide concentration

The principles of manipulating the previously mentioned factors required for photosynthesis can be used in order to provide optimum conditions for plant growth in glasshouses, and in an ecological context to improve the efficiency of energy tranfer in the production of food. For instance, with some crops, horticulturists find it is worthwhile installing gas heating, and releasing the exhaust gases directly into the glasshouses where they grow. Give a logical chain of explanation for this finding.

> Extra warmth speeds up metabolism, and the extra carbon dioxide results in extra photosynthesis, extra food, extra growth, extra yield, extra PROFIT!

Photosynthesis in the garden...

Why is it a good idea to rake the lawn to remove (tree) leaves from the surface in autumn?

> This allows more light (and water etc?) to reach the grass plants below

Some gardeners like to grow varieties of plants with leaves which are not as green as the usual plant,e.g. "golden" privet.

Why do they sometimes find that these plants are overtaken by green sections?

> Greener plants grow more efficiently

... and in the cereal bowl

There are many aspects of the process of photosynthesis which are easily confused, because we tend to think of plants as very different from us. Biologists see the same principles in operation!

This food produced by plants is the same sort of food which we might eat (carbohydrates, fats, proteins, etc.).
See the side panel of the Cornflakes packet (data from an old packet - modern data values differ slightly in detail), and answer the questions below.
Refer to the units on the respiration process to see what might happen to it!

What biological justification is there for the claim that this breakfast cereal has a link with sunshine?

> the corn plant makes food by photosynthesis


Typical value
per 100g
For 30g Serving with
125ml of
Semi-Skimmed Milk
(of which sugars) g
(starch) g
(of which saturates) g
FIBRE g 3 0.9
SODIUM g 1.1 0.4
4.2 (85)
1.2 (85)
1.3 (85)
15 (85)
1.7 (85)
333 (165)
0.85 (85)
1.3 (25)
0.4 (30)
0.6 (40)
4.6 (25)
0.6 (30)
110 (55)
0.75 (75)
IRON mg 7.9 (55) 2.4 (17)
What specific chemical substance (not an added ingredient) is the main dietary component of cornflakes? How much does this make up, as a percentage?

> starch > 75%

Plant Mineral Nutrition

Plants are also sometimes said to take in "food" in the form of mineral(inorganic) "salts" like nitrate, phosphate, magnesium and potassium ions, mostly from the soil. In fact, plants use these substances in quite small amounts. These are better described as plant mineral nutrients, and we shall return to them as a topic later. Some of these are the same as "minerals" required in animal diets.
Refer to the units on animal nutrition to see how this need for different sorts of food is universal in plants and animals.

In fact the main inorganic nutrients used by plants are carbon dioxide gas from air (how much in the atmosphere? > 0.04 %) and waterfrom soil.

The distinction between organic and inorganic compounds is so important that it is worthwhile sorting some substances into groups, with formulae if possible:

( List of examples: carbon dioxide, glucose, nitrate ion, oxygen, protein, starch, water - add more if you can)

Inorganic formula Organic formula
carbon dioxide CO2 glucose C6H12O6
nitrate ion NO3- protein -
oxygen O2 starch (C6H10O5)n
water H2O    

Virtually all other forms of life - including Man - depend on plants for these complex organic substances, either as building blocks for their growth, or as fuel - recovering the energy taken from the sun in the first place. As such they are hijacking the products of plants" activities!

It is a curious fact that oxygen is produced by green plants as a waste by- product of the process, and because not much of it is needed by plants, they release it into the atmosphere!

What is this process (getting rid of waste) called?

> excretion

What process in living organisms uses up oxygen? (2 words)

> aerobic respiration

What sort of organisms carry out this process?

> almost all

What is the general name for the process which uses up oxygen, not in living organisms?

> combustion/burning

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