✿Green plants make the carbohydrate glucose from carbon dioxide
and water.
✿At the same time, oxygen is produced.
✿If you just mix carbon dioxide and water together, they will not
make glucose.
✿They have to be given energy before they will combine.
✿ Green plants use the energy of sunlight for this. The reaction
This equations summarize photosynthesis
✱To show the number of molecules involved in the reaction, a
balanced equation needs to be written.
✱ Carbon dioxide contains two atoms of oxygen, and one of carbon,
so its molecular formula is 𝑪𝒐𝟐 .
✱ Water has the formula 𝑯𝟐 𝐨.
✱Glucose has the formula 𝑪𝟔 𝑯𝟏𝟐 𝑶𝟔 .
✱ Oxygen molecules contain two atoms of oxygen, and so they are
written 𝑶𝟐 .
✱ The balanced equation for photosynthesis is this:
✲The chlorophyll is present in the
chloroplasts of the leaf cells and it is
inside the chloroplasts that the reaction
takes place.
✲The energy from light is transferred by
chlorophyll as chemical energy to drive the
reactions that form carbohydrates from water
and carbon dioxide.
✲During this process the energy is used to split
water into hydrogen ions and oxygen.
✭The hydrogen ions are used to reduce carbon dioxide to carbohydrate.
✭As a result, the light energy absorbed by chlorophyll becomes the
chemical bond energy in the simple sugars that are produced.
✭ Glucose is one simple sugar that is produced; it is converted to starch as
a store of energy in leaves and other parts of plants.
✭ Photosynthesis provides energy for plants and for all other organisms
that feed on plants directly or indirectly.
✭The equations show that oxygen is produced as a by-product.
✭ The plant may use oxygen in its own respiration or it may diffuse out into
the atmosphere where it is used by other organisms.
✿Chloroplasts act as
energy transducers,
converting light energy
into chemical energy.
✿They are small, green
structures present in the
cytoplasm of the leaf
cells.
✿Chlorophyll is the
substance that gives
leaves and stems their
green color.
One of the first carbohydrates to be made in photosynthesis is
glucose. There are several things that may then happen to it.
❁ A molecule has to be small and soluble to be transported
easily.
❁ Glucose has both of these properties, but it is also rather
reactive.
❁ It is therefore converted to the complex sugar sucrose to
be transported to other parts of the plant.
❁ Sucrose molecules are also quite small and soluble, but
less reactive than glucose.
❁ The sucrose may later be turned back into glucose again,
to be broken down to release energy, or turned into starch
and stored, or used to make other substances which are
needed for growth.
❃
Green plants need the following in order to carry out photosynthesis:
❃ Light, which provides energy for the process.
❃ Chlorophyll, a green pigment that absorbs the energy from light.
❃ Carbon dioxide, which diffuses into the leaves from the air •
❃Water, which is absorbed by the plant's roots from the soil.
❃Water and carbon dioxide are the raw materials for photosynthesis.
❃If a plant carries out photosynthesis it will make simple sugars and store them as
starch.
❃If it cannot photosynthesize then it will not make starch.
✱If the production of starch is your evidence that photosynthesis is taking place,
then you must make sure that the leaf does not contain any starch at the
beginning of the experiment.
✱This is done by de starching the leaves.
✱ It is not possible to remove the starch chemically, without damaging the leaves,
so a plant is de starched simply by leaving it in darkness for 2 or 3 days.
✱ Potted plants are de starched by leaving them in a dark cupboard for a few days.
✱In the darkness, any starch in the leaves will be changed to sugar and carried
away from the leaves to other parts of the plant.
✱ For plants in the open, the experiment is set up on the day before the test.
✱During the night, most of the starch will be removed from the leaves.
✱Better still, wrap the leaves in aluminium foil for 2 days while they are still on the
plant.
Take a de-starched, variegated plant such as a
geranium.
Variegated' means some parts of the leaves are
white because there is no chlorophyll there.
Place the plant in sunlight for about 6 hours.
✺Now test this variegated leaf for starch using the starch test.
✺You should find that only the green parts of the leaf go blue-black.
✺The green parts contain chlorophyll which is needed for photosynthesis to
make starch.
✺The white parts contain no chlorophyll, so no photosynthesis occurs here.
✺Therefore the white parts of the leaf give a negative result with the starch
test.
Based on this experiment it seems reasonable to suppose that
chlorophyll is needed for photosynthesis
✲Take a de-starched plant.
✲Enclose it in a plastic bag with a chemical
that absorbs carbon dioxide.
✲Soda lime absorbs carbon dioxide.
✲Leave the plant in the light for a few
hours.
✲ Test a leaf for starch.
✲The leaf should show a negative result
for the starch test.
✲ Deprived of carbon dioxide the leaf is
unable to photosynthesize and make
starch.
✸ Cut a simple shape from a piece of aluminium foil
to make a stencil and attach it to a de starched.
✸ After 4 to 6 hours of daylight, remove the leaf and
test it for starch.
✸
Result:
Only the areas which had received
light go blue with iodine.
✸ As starch has not formed in the areas that
received no light, it seems that light is needed for
starch formation and thus for photosynthesis.
✵The products of photosynthesis are simple
sugars, such as glucose, and oxygen.
✵Most of the sugars which a plant needs for food
are made in the leaves.
✵ Some of the glucose is used for respiration in
the leaf.
✵Some of the glucose is changed into starch and
stored in the leaves for use in the future, e.g. at
night.
✵ Some of the glucose is used to make cellulose,
which is needed to make cell
walls.
✵Glucose is converted to sucrose and
transported to other parts of the plant in
the phloem.
✵ Glucose can also be converted to other
substances:
✵Plants get nitrogen by absorbing nitrate
ions from the soil.
✵ Glucose and nitrate are used to form
amino acids, which are built up into
proteins.
✵ Plants need proteins for growth and cell
repair and for making enzymes and
hormones.
✵ Sugars are converted to oils, which are an
efficient way to store energy in seeds.
☞Think about the food you have eaten in the last 24 hours.
☞A lot of what you eat comes from plants like rice, maize, wheat,
potatoes, soya beans and nuts.
☞If you eat meat or fish then that comes from animals that have
eaten plants.
☞Products like cooking oil and margarine are made from plants.
☞Oxygen is a by-product of photosynthesis. It is not the main product.
☞Some of the oxygen produced is used
by the plant's respiration, but usually
there is more than is needed, so most
diffuses out of the leaves into the
atmosphere.
☞That is why it is called a by-product.
☞Plant products Plants provide us with
our food and also food for our
livestock.
☞Plants also provide a range of raw
materials for industry, such as timber
and cotton.
☞Many medicines have been
discovered in plants including digitalis,
which is a heart drug, and two anticancer drugs extracted from the rosy
periwinkle from Madagascar.
✱Plants are the dominant organisms in almost all environments.
✱ As such they provide habitats for animals and microorganisms.
✱The rainforests make up only 6% of the Earth's land surface, but they
support more than half the world's species of animals and plants.
✱The concentrations of gases in the atmosphere are kept constant by
photosynthesis.
✱Without green plants, the concentration of carbon dioxide in
the air would increase and the concentration of oxygen
would decrease.
Oxygen is produced in photosynthesis
❋ Place a short-stemmed funnel over some Canadian
pondweed in a beaker of water.
❋ Fill a test-tube with water and place it upside-down
over the funnel stem.
❋ The funnel is raised above the bottom of the beaker
to allow the water to circulate.
❋Place the apparatus in sunlight.
❋Bubbles of gas should appear from the cut stems and
collect in the test-tube.
❋Set up a control in a similar way but place it in a dark
cupboard.
❋When sufficient gas has collected from the plant in the
light, remove the test-tube and insert a glowing splint.
Result: The glowing
splint bursts into flames.
✦The relighting of a glowing splint does not
prove that the gas collected in the test-tube
is pure oxygen, but it does show that it
contains extra oxygen and this must have
come from the plant.
✦ The oxygen is given off only in the light.
✦ Note that water contains dissolved
oxygen, carbon dioxide and nitrogen.
✦These gases may diffuse in or out of the
bubbles as they pass through the water and
collect in the test-tube.
✦ The composition of the gas in the testtube may not be the same as that in the
bubbles leaving the plant.
★There are a number of factors that interact to make the process of
photosynthesis work.
★Any one of these could become a limiting factor.
★ This means that the factor directly affects the rate of
photosynthesis on its own, regardless of the level of the other
factors.
★The major limiting factors for photosynthesis are light intensity,
temperature, and carbon dioxide levels.
✱The law of limiting factors states that when a chemical process depends
on more than one essential condition being favorable, the rate of
reaction will be limited by the factor that is nearest its minimum value
✸The process of photosynthesis depends upon:
❉1 - The availability of light
❉2 - The presence of a pigment to absorb the light
❉3 - A supply of carbon dioxide and water
❉4 - A temperature suitable for enzyme activity
✸ If any of these factors is in short supply, the rate of photosynthesis will be less than its
maximum possible rate.
✸ The limiting factor is the one which controls the rate of the overall process
because it is the factor nearest to its minimum value.
✸ The limiting factor varies at different times and under different conditions.
✱To measure the effects of light intensity on the rate of photosynthesis:
✱1. Cut a piece of pondweed about 5 cm in length.
✱2. Put a paperclip on the pondweed to stop it floating to the surface.
✱3. Put a lamp close to the plant and measure the distance between the
plant and the lamp.
✱4. Count the number of bubbles released over 5 minutes. Repeat several
times and calculate the average.
✱5. Repeat this procedure with the lamp at different distances from the
plant.
✺The number of bubbles should decrease
as the distance between the lamp and the
plant increases.
✺ When the lamp is close to the plant the
light intensity is high.
✺ This gives the plant lots of energy so the
rate of photosynthesis is high.
✺ As the lamp is moved further away the
light intensity decreases; there is less
energy for the plant and the rate of
photosynthesis decreases.
✤The graph shows some results of an experiment to measure the
effect of increasing light intensity on the rate of photosynthesis.
✤ In the dark there is no photosynthesis.
✤At low light intensities there are very few bubbles of oxygen
produced.
✤The rate of photosynthesis increases as the light intensity
increases until at a certain light intensity the rate does not
increase any further.
✤ Increasing the light intensity increases the energy available to
the plant for photosynthesis.
✤When investigating the effect of different light intensities it is
important to keep the other environmental conditions the same.
✤Temperature can be kept constant by putting a thermometer into
the beaker of water and adding hot or cold water if the
temperature changes.
✤ The lamp may give off heat, so the temperature may increase.
✤The concentration of carbon dioxide in the water can be kept
constant by adding sodium hydrogen carbonate to the water and
letting it dissolve.
✤ This ensures that the plant will not run out of carbon dioxide.
❉To find the effect of temperature pieces of pondweed are placed into testtubes which are put inside beakers of water at different temperatures.
❉ The minimum is five different temperatures across a range of 10°C to
about 40°C.
❉ The lamp is kept at one distance all the time so that the light intensity is
constant.
❉Sodium hydrogen carbonate is added to the water in the test-tubes to
make sure carbon dioxide concentration is constant.
❉The number of bubbles of oxygen produced at different temperatures can
be counted in the same way.
✱The maximum rate of photosynthesis
occurs at the optimum temperature.
Which is 20C and 30C for land plants.
✱The actual temperature depends on the
plant that is used.
✱Plants that grow in temperate latitudes
have optimum temperatures that are lower
than those that grow in the tropics.
✱At temperatures above 40°C the rate slows
down.
✱ This is because the enzymes involved in
the chemical reactions of photosynthesis
are temperature sensitive and destroyed
(denatured) at higher temperatures.
★Adding different quantities of sodium hydrogen carbonate to the water
increases the concentration of carbon dioxide.
★ At least five different test-tubes can be used each with a different mass of
sodium hydrogen carbonate added.
★ The light intensity and the temperature must be kept constant.
★You can see that the effect of increasing carbon dioxide on the rate of
photosynthesis is the same as the effect of increasing the light intensity.
★ If there is no carbon dioxide there can be no photosynthesis.
★As the carbon dioxide concentration increases there is an increase in the
rate of photosynthesis.
★Above a certain concentration the rate remains constant.
✥The rate of
photosynthesis increases
linearly with
increasing CO2 concentration
(from point A to B ).
✥The rate falls gradually, and at a
certain CO2 concentration it
stays constant (from point B to
C).
✥ Here a rise in CO2 levels has no
effect as the other factors such
as light intensity become
limiting.
 When plants are growing outside, we cannot do
much about changing the conditions that they
need for photosynthesis.
 But if crops are grown in glasshouses, then it is
possible to control the conditions so that they are
photosynthesizing as fast as possible.
✤ Growers try to improve the yield of their
crops by giving them the best possible
conditions for photosynthesis to take place.
✤ Conditions inside a glasshouse allow plants
to:
✤ 1. Grow earlier in the year
✤ 2. Grow in places where they would
not normally grow well.
✱The following conditions inside glasshouses are controlled.
✱Temperature: Sunlight heats up the inside of the glasshouse.
The glass stops a lot of this heat from escaping. Electric heaters are
used in cold weather. Ventilator flaps are opened to cool the
glasshouse on hot days.
✱ Light: The glass lets in sunlight. Artificial lighting can be used to
grow plants when light intensity gets too low. Blinds keep out very
strong light and shading lowers the temperature in tropical countries.
✲Carbon dioxide: Growers can pump
carbon dioxide into glasshouses to increase
carbon dioxide concentration. They can also
burn butane or natural gas which provides
carbon dioxide and also heat to raise the
temperature of glasshouses in cold weather.
✲Water: Many glasshouses have automatic
watering systems using sprinklers and
humidifiers which ensure plants always get
enough water.
Carrot plants grown in increasing concentrations
of carbon dioxide from left to right
 All these factors are monitored and controlled by computers so few staff are needed.
 Sensors for carbon dioxide concentration, humidity, light intensity and temperature detect
changes in these limiting factors.
 Computers process data from the sensors and control all heating, ventilation, lighting and
shading in the glasshouse.
A leaf is a factory for making carbohydrates.
Leaves are therefore specially adapted to allow photosynthesis
to take place as quickly and efficiently as possible.
✯A leaf consists of a broad, flat part
called the lamina , which is joined to
the rest of the plant by a leaf stalk or
petiole.
✯Running through the petiole are
vascular bundles, which then form
the veins in the leaf.
✯These contain tubes which carry
substances to and from the leaf.
✯Although a leaf looks thin, it is in fact
made up of several layers of cells.
Leaves have
✷ A large surface area — to absorb light rays.
✷ A thin shape — so gases can diffuse in and out easily.
✷ Many chloroplasts — to absorb light for the reactions that take
place in photosynthesis.
✷ Veins — to support the leaf surface and to carry water and ions to
the leaf cells, and to take sucrose and amino acids away from the leaf
to all other parts of the plant.
★The top and bottom of the leaf are covered
with a layer of closely fitting cells called the
epidermis.
★These cells do not contain chloroplasts.
★ Their function is to protect the inner layers of
cells in the leaf.
★ The cells of the upper epidermis often secrete
a waxy substance, that lies on top of them. It is
called the cuticle, and it helps to stop water
evaporating from the leaf.
★ There is sometimes a cuticle on the underside
of the leaf as well.
✫In the lower epidermis, there are
small openings called stomata
(singular: stoma).
✫ Each stoma is surrounded by a pair
of sausage-shaped guard cells which
can open or close the hole.
✫ Guard cells, unlike other cells in the
epidermis, do contain chloroplasts.
✱The lower epidermis has many
stomata.
✱Usually the upper epidermis contains
fewer or no stomata.
✱The stomata allow carbon dioxide to
diffuse into the leaf, to reach the
photosynthetic tissues.
✱ They also allow oxygen and water
vapor to diffuse out.
The lower surface of a leaf, showing the closely
fitting cells of the epidermis.
The oval openings are stomata, and the two
curved cells around each stoma are guard cells
(x 450).
✵ Stomata are opened and closed by guard cells.
✵ Stomata usually open during the day.
✵ Water passes into the guard cells by osmosis.
✵ This makes them bend so the stoma opens.
✵ Carbon dioxide diffuses into the leaf for
photosynthesis and oxygen and water vapor
diffuses out.
✵At night the stomata close.
✵ Water passes out of the guard cells by osmosis
and they straighten and move closer together so
closing the stomata pores.
✵ The stomata also close in hot, dry
weather to help prevent the plant wilting.
✫The middle layers of the leaf are called the mesophyll (`meso' means
middle, and `phyll' means leaf).
✫ These cells all contain chloroplasts.
✫The cells nearer to the top of the leaf are arranged like a fence or
palisade, and they form the palisade layer.
✫This is a tissue made of elongated cells, each containing hundreds
of chloroplasts, and is the main site of photosynthesis.
✫ The palisade cells are close to the source of light, and the upper
epidermis is relatively transparent, allowing light to pass through to
the enormous numbers of chloroplasts which lie below.
✫ The cells beneath them are rounder, and arranged quite loosely, with
large air spaces between them. They form the spongy layer
Running through the
mesophyll are veins or
vascular bundles.
 Each vein contains large,
thick-walled xylem
vessels for carrying water.
 There are also smaller,
thin-walled phloem
tubes for carrying away
sucrose and other
substances that the leaf has
made.
✲The structure of the leaf is adapted for
gas exchange.
✲The cells in the spongy
mesophyll (lower layer) are loosely
packed, and covered by a thin film of
water.
✲These large spaces allow these layers
to help carbon dioxide move around
the leaf.
✲ The spongy mesophyll also allows the
plant to bend and move in the wind,
which itself helps move gases around
the leaf's cells.
✲The stomata control gas
exchange in the leaf.
✲The gases diffuse into the
intercellular spaces of the
leaf through stomata.
✲During the day,
photosynthesis can be
going 10 or even 20 times
faster than respiration
(depending on light
intensity), so the stomata
must stay open so that the
plant has enough carbon
dioxide, most of which
diffuses in from the
external atmosphere.
✸ Plants need a source of nitrate ions 𝑵𝒐𝟑 − for making amino acids.
✸ Amino acids are important because they are joined together to make proteins,
needed to form the enzymes and cytoplasm of the cell.
✸ Nitrates are absorbed from the soil by the roots.
✸Magnesium ions (Mg2+) are needed to form chlorophyll, the
photosynthetic pigment in chloroplasts.
✸ This metallic element is also obtained in salts from the soil.
Nitrates and magnesium are often referred to as ‘mineral salts’ or
‘mineral elements’. If any mineral element is lacking, or deficient, in
the soil then the plants may show visible deficiency symptoms.
✸Magnesium is absorbed from the soil
as magnesium ions (Mg2+).
✸Magnesium forms part of the
chlorophyll molecule.
✸ Deficiency causes chlorosis – the
leaves turn yellow, usually from the
bottom of the plant first.
✸Plants need magnesium to complete
enzyme reactions, including those
involved in photosynthesis.
✸ Without it, a plant can't make
chlorophyll, which is responsible for
green leaf color.
✸ Magnesium also makes it possible for
plants to absorb and use other essential
elements, such as the macronutrients
nitrogen, sulfur and phosphorus.
✱Nitrogen is absorbed from the soil
as nitrate ions (NO3 –) or
ammonium ions (NH4 +).
✱ Because nitrogen is required for so
many food molecules, especially
proteins (including enzymes),
deficiency causes severe symptoms.
✱The whole plat is stunted, with a
weak stem and yellowing, dying
leaves.
✱Nitrogen is essential for the growth
of leaves and stems, and it helps to
give plants a rich, green color.
❋Nitrogen is a
significant
component of
nucleic acids such
as DNA, the
genetic material
that allows plant
cells to grow and
reproduce.
❋ Also Nitrogen is a
key elemental
component in
chlorophyll and in
all the proteins in
plant body.
☞Legume plants, such as
peas and beans, have
bacteria in their roots that
are able to 'fix'
atmospheric nitrogen into
a form that the plants can
use to make proteins.
☞ Plants such as clover are
grown and ploughed back
into the soil to increase
the nitrogen content for
crops.