IGCSE
Biology
4.2 Plant nutrition
Define photosynthesis as the fundamental process by which plants
manufacture carbohydrates from raw materials using energy from
light.
Plants use water from the soil, carbon dioxide from the air and energy from
sunlight to make carbohydrates & oxygen.
Green plants absorb light energy using chlorophyll in their leaves.
They use it to react carbon dioxide with water to make a sugar called glucose.
The glucose is used in respiration, or converted into starch and stored.
Oxygen is produced as a by-product.
Explain that chlorophyll traps light energy and converts it into chemical
energy for the formation of carbohydrates and their subsequent
storage.
Chloroplasts turn some of the sugar they make into starch, as this is a good
way to store it.
Starch is not soluble in water, and must be converted into other substances
before it can be translocated.
State the word equation for the production of simple sugars and oxygen.
carbon dioxide + light energy ( in the presence
of chlorophyll )
glucose + oxygen
State the balanced equation for photosynthesis in symbols
6CO2 + 6H2O light (chlorophyll )
C6H12O6 + 6O2
Investigate the necessity for chlorophyll, light and carbon dioxide for
photosynthesis, using appropriate controls.
Without enough light, a plant cannot photosynthesise very quickly, even if
there is plenty of water and carbon dioxide.
chlorophyll
Photosynthesis occurs in chloroplasts.
Chloroplasts are located in the palisade mesophyll tissue of leaves.
Chlorophyll is the pigment that absorbs light energy that is used for
photosynthesis.
Chloroplasts can move around inside cells.
Chloroplasts need a good supply of water, carbon dioxide and sunlight in order
to carry out photosynthesis efficiently.
chloroplasts as seen
through a microscope
a leaf section as seen through a microscope
chlorophyll
A variegated plant is used to test for the relationship between the green
pigment in plants & starch production.
1. A destarched variegated plant was left in the sunlight for four hours.
2. A leaf is removed and tested for starch.
The green parts of the leaf contain starch.
The white do not.
3. This suggests that chlorophyll is needed for starch production.
light
A leaf usually has a large surface area, so that it can absorb a lot of light.
Its top surface is protected from water loss, disease and weather damage by
a waxy layer.
The upper part of the leaf is where the light falls, and it contains a type of
cell called a palisade cell.
This is adapted to absorb a lot of light.
It has lots of chloroplasts and is shaped like a tall box.
Testing a leaf for starch.
If you want to see if starch has been produced in a plant, you have to start
with a plant that does not have starch.
1. Test the plant for starch.
2. Place the plant in the dark for 2 or 3 days.
3. Test again for starch.
Using iodine to test for the presence of starch.
Iodine solution ( an amber colored liquid ) turns dark ( blue/black ) in the
presence of starch.
If leaves are to be tested for the presence of starch, the cell walls in the
leaf must be broken, and the chlorophyll removed.
The cell walls must be broken because to make it easier for substances to
leave or enter the cell.
The chlorophyll must be removed so that the color change in the iodine can be
seen.
light
Showing how light is needed to make starch.
1. Two leaves of a de-starched plant are to be tested for the presence of
starch.
2. One is completely covered with aluminum foil, the other with clear plastic
wrap.
3. The plant is put in the sun for 4 hours, and both leaves are tested for
starch.
The leaf in the plastic contains starch.
The leaf in the foil does not.
This suggests that light is needed for the production of starch.
carbon dioxide
Sometimes photosynthesis is limited by the concentration of carbon dioxide
in the air.
Even if there is plenty of light, a plant cannot photosynthesize if there is
insufficient carbon dioxide.
Investigate and state the effect of varying light intensity on the rate
of photosynthesis (e.g. in submerged aquatic plants).
Increasing the light intensity will boost the speed of photosynthesis.
However, once the light intensity has reached a certain level, giving the plant
more light has no effect. The extra light cannot be used for
photosynthesis because the plant is photosynthesizing as fast as it can.
Describe the intake of carbon dioxide and water by plants.
intake of carbon dioxide
Plants get carbon dioxide from the air through their leaves.
Only about 0.04% of the atmosphere consists of carbon dioxide
Carbon dioxide molecules diffuse through the stomata, through the air spaces
between the spongy mesophyll cells, through the cell walls, cell
membranes and cytoplasm of the spongy and palisade cells and into the
chloroplasts.
The carbon dioxide diffuses through small holes in the underside of the leaf
called stomata. (One of these holes is called a stoma. The plural is stomata.)
The lower part of the leaf has loose-fitting cells, to allow carbon dioxide to
reach the other cells in the leaf.
This also allow the oxygen produced in photosynthesis to leave the leaf easily.
intake of water
Plants need water:
 to help keep cells turgid, so that soft parts of the plant ( petals &
leaves ) are held in shape.
 to be used as a solvent; substances need to be dissolved in water inside
cells so that metabolic reactions can take place.
 for photosynthesis
Plants get their water from the soil.
Water is taken up by root hairs and moves across the root into the xylem.
In the xylem it flows up through the root and stem into the leaves.
Inside the leaf, the water moves out of the xylem vessels, and some of it
goes into the chloroplasts in the mesophyll cells.
Most of the water turns to water vapor and evaporates.
The water passes through the leaf and out the stomata in a process called
transpiration.
The movement of water along this pathway ( from soil to the atmosphere ) is
called the transpiration stream.
root hairs
Plants get the water they need for
photosynthesis through their roots.
The roots have a type of cell called a root hair
cell - these project out from the root into
the soil.
Roots have a big surface area and thin walls,
which allow water to pass into them
easily by osmosis.
Corn root with root hairs for uptake of
water & nutrients from the soil.
Note that root cells do not contain chloroplasts, as they are normally in the dark
and cannot photosynthesize.
The water absorbed by the root hair cells passes through the plant in xylem
tubes, and eventually reaches the leaves.
If a plant does not absorb enough water, it will wilt or go floppy.
Without water it may also not photosynthesise quickly enough, and it may die.
Turgid plant that's watered regularly & a flaccid plant without enough water.
Identify and label the cuticle, cellular and tissue structure of a
dicotyledonous leaf, as seen in cross-section under the light
microscope and describe the significance of the features of a leaf in
terms of functions, to include:
•
distribution of chloroplasts – photosynthesis,
•
stomata and mesophyll cells – gas exchange
•
vascular bundles (xylem and phloem) – transport and support.
name of
tissue
how these structures help it perform
its function more efficiently
function
palisade
mesophyll
packed with chloroplasts ( contain chlorophyll);
regular shaped, closely packed cells
form continuous layer for efficient absorption of
sunlight
photosynthesis
spongy
mesophyll
cells
irregular shaped, loosely arranged cells
provides large surface area
for efficient gas exchange
gas exchange
vascular
bundles
phloem – live cells
xylem – dead cells
arranged end-to-end
hollow
walls contain lignin
no cytoplasm, cell membrane, nucleus
phloem –
transport glucose
xylem –transport
of water,
dissolved
minerals;
support
Leaves are adapted in several ways to help them perform their
function.
Features of leaves
Adaption
Purpose
Large surface area
To absorb more light
Thin
Short distance for carbon dioxide to diffuse into leaf cells
Chlorophyll
Absorbs sunlight to transfer energy into chemicals
Network of veins
To support the leaf and transport water and carbohydrates
Stomata
Allow carbon dioxide to diffuse into the leaf
Inside the leaf
Describe the importance of:
• nitrate ions for protein synthesis,
• magnesium ions for chlorophyll synthesis.
vascular
bundle
Explain the effects of nitrate ion and magnesium ion deficiency on plant
growth.
Plants need to take in a number of elements to stay alive.
The most important are:
 carbon
 hydrogen
 oxygen
Plants get hydrogen and oxygen from water in the soil, and carbon and oxygen
from carbon dioxide and oxygen in the atmosphere.
Water and carbon dioxide are used to synthesize food during photosynthesis.
Oxygen is used to release energy from food during respiration.
In addition to these three elements, plants need a number of minerals for
healthy growth.
These are absorbed through the roots as mineral ions dissolved in the soil
water.
Two important mineral ions needed by plants are:
 nitrate - for making amino acids, which are needed to make proteins
 magnesium - for making chlorophyll
If a plant does not get enough minerals, its growth will be poor.
It will suffer from deficiency symptoms:
deficient in nitrate - it will suffer from stunted growth
deficient in magnesium - it's leaves will turn yellow
TOMATO PLANT
Nitrogen deficiency
TOMATO PLANT
Magnesium deficiency
Plants and minerals
Plants use minerals from the soil to build the complex molecules they need to
survive and grow.
Poor plant growth may be due to a deficiency of one or more minerals.
Tests can be done to establish which mineral is deficient or a general
purpose NPK fertilizer added.
Plants need to build the complex molecules they need from minerals (elements
and simple compounds), which they get from the soil.
Animals on the other hand get their complex molecules ready made from
their food – they need very few minerals to supplement their diet.
If a plant shows poor growth it may be due to a lack of one or more minerals
in the soil.
It's possible to do chemical tests to find out which minerals are deficient,
but it's often more convenient to add a general fertiliser.
This is sometimes called NPK fertiliser because it contains the elements
nitrogen (N), phosphorus (P) and potassium (K).
These elements, plus magnesium (Mg), are the minerals that plants need in
the greatest quantities.
Uses of different minerals
Mineral
Use
Deficiency
symptoms
Explanation
Nitrate ions
(NO3)
Building proteins
and growth
Poor growth and All amino acids contain
yellow leaves
nitrogen. Amino acids are
the building blocks of
protein.
Phosphate
ions (PO4)
Respiration and
growth
Poor root growth A component of DNA
and discolored
molecules and cell
leaves
membranes
Potassium
ions (K+)
Respiration and
photosynthesis
Poor flower and
fruit growth,
discolored
leaves
Must be present for
photosynthesis and
respiration enzymes to work
Magnesium
ions (Mg2+)
Photosynthesis
Yellow leaves
Chlorophyll molecules
contain magnesium ions. It's
the magnesium that makes
chlorophyll green.
How minerals are absorbed
The concentration of minerals in the soil is very low.
They dissolve in water and move around the soil in solution.
Root hair cells are adapted to absorb the water out of soil by osmosis – they
have a large surface area, thin walls and are close to the xylem cells
used for transporting water up the plant.
Minerals cannot be absorbed by osmosis (which is the movement of water
only) or diffusion (because the minerals are in very low concentration).
The root hair cells have carrier molecules on their surface that pick up the
minerals and move them into the cell against the concentration gradient.
This requires energy, and is called active transport.
Describe the uses, & the dangers of overuse, of nitrogen-containing
fertilizers.
As crops grow they remove nutrients, such as compounds of nitrogen,
potassium and phosphorus, from the soil.
These crops are harvested & taken away from the fields where they were
grown.
If this process is repeated for a number of years the supply of nutrients in
the soil is exhausted.
The land becomes less fertile, so plants grow poorly & produce only a small
yield.
Farmers use fertilizers to put back into the soil the nutrients that have been
removed by growing crops.
Fertilizers are chemicals used by farmers to help their crops grow well.
Synthetic fertilizers are cheaper, easier to store, transport & apply than
organic fertilizers.
Rain can wash fertilizers off the fields and into rivers.
This causes water plants to overgrow and block out the light.
Other plants die as a result.
They use up oxygen as they rot away, and fish and other animals suffocate.
This process is called eutrophication.