9.1: Plants
The importance of plants
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make carbohydrates using energy from the Sun & CO₂ from atmosphere
producers of the food chains & food webs (oxygen & glucose)
(plant crops) source of food
part of water cycle
Other uses eg. medicines, clothing, building, biofuels and etc
The main structure of a plant include:
Leaves
 capture energy from the Sun using green colour chlorophyll
 for photosynthesis to make carbohydrates
Stems
 support the leaves by holding them out so that they are able to capture sunlight
 support flowers & fruits
Roots
 supply water & mineral nutrients from the soil
Flowers (only appear at certain time in the life cycle of plant)
 reproductive structures of plants
 form seeds & fruits
9.2: Photosynthesis
Biomass: material living organisms are made of 生物物质
Animals have to eat plants to get the biomass they need. However, plants can make their
own biomass using sunlight, air and water in photosynthesis.
“ Most of the biomass on Earth has come from photosynthesis (plants) ”
Where do plants get both of the small molecules to make carbohydrates in
photosynthesis? (carbon dioxide & water)
 CO₂ get from the air and also can be made during aerobic respiration
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H₂O get from the soil
Plants capture light energy using the green pigment chlorophyll.
Plants can also photosynthesis with artificial light, just that they grow better in sunlight.
The importance/function of chloroplasts in photosynthesis
 contain the green chlorophyll needed to capture light energy for photosynthesis
 carry out photosynthesis since most of the reactions take place inside it
The number of chloroplasts in a plant cell tells you about the amount of photosynthesis
it carries out.
Uses of glucose in photosynthesis
 aerobic respiration
 store as starch
 make other substances/molecules
9.3: Evidence for photosynthesis
Testing for starch
As we know, there must be some of the glucose made in a leaf by photosynthesis is
turned into starch to be stored. To prove that the plant has been photosynthesising, we
will just need to do an experiment on testing a leaf for starch.
Using iodine to test a leaf for starch
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Take a leaf from a plant and it must have been in the light for at least 12 hours to
demonstrate photosynthesis.
Drop the leaf into boiling water approximately 1-2 minutes to remove the waterproof
layer, stop all chemical reactions and destroy the cell membrane
Place the leaf in a test tube of ethanol and put the test tube into the hot water to
remove the colour of the leaf (to decolourize)
Spread the leaf on a white tile and add a few drops of iodine solution
If starch is present, the leaf will turn blue-black. This shows it contains starch and has been
photosynthesising.
Besides that, we know that a leaf uses its starch stores for respiration when it is in the
dark and it has enough starch to keep respiring for 2-3 days only. If we are going to
investigate photosynthesis by looking for starch in an opposite way, we should destarch
it first.
How is a plant destarched? (preventing photosynthesis)
Place the plant in the dark for at least 3 days, so that we know it has no starch left.
9.4: Evidence of photosynthesis
Oxygen bubbles
How does light intensity affect the rate of photosynthesis?
Independent variable: Distance between the lamp (light source) and the plant
Dependent variable: Number of bubbles (of oxygen) being produced
Constant/Control variable(s):
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Type of water plants (such as Hydrilla, Cabomba and Elodea)
Temperature of the water in the test tube
Amount of the water in the test tube
The effect of light intensity on photosynthesis can be investigated in water plants by counting
the number of bubbles produced in a given amount of time to show how quickly
photosynthesis is taking place.
Risks
 Ensure that your hands are dry when handling the lamp.
 Water plant must be destroyed after used and not released into natural water courses.
Results
As the distance between the lamp and plant increases, less bubbles of oxygen being produced.
(Explanation) The higher the light intensity, the more photosynthesis the plant occurs.
Instead of counting the number of bubbles being produced, the volume of oxygen
produced could be measured by collecting the gas produced in a gas syringe.
9.5: The need of minerals
Plants need minerals to maintain healthy growth and life processes for a long time, just
like humans need iron and calcium. Otherwise, it will look sickly and eventually die.
Two of the most important minerals for plants
Nitrates (contains 78% of nitrogen)(mixed as salts)
 are produced by decomposers 分解体 eg. fungi, bacteria, protozoa and etc
 to make proteins that are required for many different processing including
photosynthesis
Magnesium (pure substance)
 to make chlorophyll
 to replace the chlorophyll when chloroplasts get worn out 耗尽 or damaged
Deficiencies in plants
Lack of nitrates
Lack of magnesium
Mineral deficiency
Symptoms
Nitrate
Magnesium
poor growth, older leaves turn brown, short & small
(roots) pale yellow
Where do plants get their minerals? Soil
Nitrate
 from soil except some plants called legumes 荚豆 eg. peas, beans and clover
Magnesium
 from soil since the minerals are dissolved in the water in the soil
For legumes, they have root nodules full of special bacteria that makes nitrates from the
nitrogen in the air. The plants get some of these nitrates in return for sugar from
photosynthesis. Legumes…
 are able to grow very well in nitrate-poor soil
 even add nitrates to the soil and make it more fertile
 are often grown by the farmers to improve the soil for crops that needs lots of nitrogen
9.6: The use of fertilisers
Fertilisers are substances that replace minerals such as nitrates in the soil. There are
two main types of fertilisers.
Natural fertilisers eg. manure
 made from decomposed animal droppings & compost from decomposed plant
material
 cheap and improve the structure of soil
 release their nutrients slowly
 limited supply
Artificial fertilisers (already for 100 years)
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made in huge quantities in industrial processes
work fast and are always available
amount of nitrates added can be controlled
expensive and do not improve the structure of soil
The impact of science applied to industry
By developing artificial nitrate fertilisers, billions of lives have saved. Artificial nitrate helps
more people to have enough food to live and raise children.
A German chemist, Fritz Haber developed a way of making a compound called ammonia
which acts as a nitrate-rich fertiliser. However, his laboratory method only made small
amounts of ammonia, so it was not much use to farmers.
Then, Carl Bosch, a chemist and engineer, developed a way to use Haber’s reaction on
industrial scale. Therefore, this Haber-Bosch process has produced around 120 million
tonnes of nitrate fertiliser for farmers all over the world.
9.7: Water & mineral transport in plants
Plants need water for photosynthesis, transport (carry substances to the different parts
of plant body) and support. If a plant does not get enough water, its stems and leaves
droop.
The water moves from the soil into the root hair cells by diffusion.
Root hair cells have long microscopic hairs that give plants a big surface area. It allows
a lot of water and minerals such as nitrates to move by diffusion through the surface
into the root of the plant.
Transporting water & minerals around the plants
Xylem (a specialised plant tissue)
 tube in the plant stem
 carries water and dissolved minerals sources to the cells where they are needed from
the roots
Stomata
 tiny holes on the underside of leaves’
 surrounded by special cells which open & close it
 carbon dioxide moves in & spare oxygen moves out through it
Whenever the stomata are opened, water evaporates from cells of the leaves and moves
out through the stomata by diffusion. This process is called transpiration. As water
vapour evaporates from the leaves, more water is pulled up through the plant.
When it is hot/windy, more transpiration takes place because more water evaporates
from the leaves.
9.8: Xylem, phloem & plant pests
Phloem (a living tissue)
 transport food and dissolved sugars in it
 use energy to move the water & dissolved sugars up to the buds & flowers and down
to the roots
Difference between xylem & phloem
Xylem transports soluble mineral nutrients and water molecules from the roots to the other
parts of the plant. However, phloem transports food and other nutrients including sugar and
amino acids from leaves to storage organs.
Every plant has phloem tubes full of sugar-rich liquid in their stems, leaves and roots.
Therefore, many different insects target this sugary water as easy food but this is only
for them but not the plants.
Plant pests eg. aphids, plant hoppers, leaffhoppers, scale insects, shield bugs and etc
 always attack the phloem
 small but in large numbers
 destroy billions of plants every year
Plant pests have specialised mouth parts called stylets to get the sugary liquid they
want. Stylets are sharp, pointed, hollow tubes. The insect sticks its stylet into the phloem
of a plant and feeds on the sugary liquid inside.
Plant pests often carry plant pathogens. When they stick their mouthparts deep into the
transport tissues of the plant, they may also carry pathogens into the xylem/phloem.
Pathogens will be carried all over the plant and spread into all of the cells of the plants.
For example, if pathogens block the xylem tissue of a plant, water cannot reach the
stems and leaves. The plant will wilt 枯萎 and die.