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Review Question
• Which mode of nutrition do the green
plants carry out?
A.
Autotrophic nutrition
B.
Heterotrophic nutrition
Sorry! You’re wrong!
• Heterotrophic nutrition is the mode of
nutrition in which organisms have to depend
on other organisms or dead organic matters
as their food sources. Green plants, however,
can make organic food by themselves using
simple inorganic substances.
Very Good!
• Autotrophic nutrition is the mode of
nutrition in which organisms can make
organic food by themselves using simple
inorganic substances.
• The process by which the green plants
obtain nutrients is called :
Photosynthesis
Nature of photosynthesis
• Occurs in Chloroplast
• Necessary factors :
 Carbon dioxide
 Water
 Sunlight
 Chlorophyll
Experiments on
photosynthesis
Destarching – removing starch
• Reason:
– To show that any starch found after the
experiment is produced during the period
of investigation
• Method:
– By placing the plant in dark for for 2 days
To test a leaf for the presence
of Starch
boiling
water
turn off
Bunsen
burner
boiling
alcohol
hot water
iodine
turn off
Bunsen
burner
Why is the leaf first boiled in water ?
Ans: Because the process can soften the leaf, break
down the cuticle and kill the leaf.
white tile
boiling
water
turn off
Bunsen
burner
boiling
alcohol
hot water
iodine
turn off
Bunsen
burner
Why is the leaf then boiled in alcohol ?
Ans: To decolourize the leaf (to remove chlorophyll).
white tile
boiling
water
turn off
Bunsen
burner
boiling
alcohol
hot water
iodine
turn off
Bunsen
burner
white tile
Why is it important to turn off the Bunsen burner when you
are heating the alcohol ?
Ans: Because alcohol catches fire easily.
boiling
water
turn off
Bunsen
burner
boiling
alcohol
hot water
iodine
turn off
Bunsen
burner
white tile
Why is the leaf put in hot water after being boiled in
alcohol ?
Ans: To soften the leaf.
(The leaf becomes brittle after boiling in alcohol.)
boiling
water
turn off
Bunsen
burner
boiling
alcohol
hot water
iodine
turn off
Bunsen
burner
white tile
What colour change can be observed if starch is present ?
Ans: The leaf becomes blue-black when iodine solution
is added.
To show that Chlorophyll is
necessary for Photosynthesis
purple part
green part
Which part of the leaf turns blue-black when treated with
iodine solution ?
Ans: Only the green part.
purple part
green part
Is chlorophyll necessary for photosynthesis ?
Ans: Yes.
purple part
green part
What is the control in this experiment ?
Ans: The green part of the leaf acts as the control.
To show that Carbon
Dioxide is necessary for
Photosynthesis
sunlight
A
water
B
potassium
hydroxide solution
Why is potassium hydroxide solution put in one of the
flasks ?
Ans: To absorb all the carbon dioxide in flask B.
sunlight
A
water
B
potassium
hydroxide solution
What happens to each leaf after testing the presence of
starch at the end of the experiment ?
Ans: The leaf in flask A changes to blue-black while the
leaf in flask B stains brown.
sunlight
A
water
B
potassium
hydroxide solution
What do your results suggest about the relationship
between carbon dioxide and photosynthesis ?
Ans: Carbon dioxide is necessary for photosynthesis.
Summary of Photosynthesis
6 CO2 + 6 H20
Carbon Dioxide + water
(reactants)
sunlight
chlorophyll
C6H12O6 + 6 O2
Glucose + oxygen
(products)
By photosynthesis, green plants convert
solar energy into chemical energy
stored in glucose
Route of Carbon Dioxide
for Photosynthesis
Chloroplast
Spongy Mesophyll
Cell
H2O
Air Space
Stoma
CO2
Chloroplast
Palisade
Mesophyll Cell
H2O
Air Space
Stoma
CO2
Mechanism of photosynthesis
•
•
-
A series of enzyme-controlled reactions
Involved in 2 stages:
Light-dependent stage (Light reaction)
Light-independent stage (Dark reaction)
Light reaction (Light-dependent stage)
light
water
 hydrogen  oxygen
chlorophyll

Light is required

Chlorophyll in chloroplast traps Light energy

Light energy absorbed by chlorophyll splits water
molecules into hydrogen and oxygen
(photolysis)
light
water
 hydrogen  oxygen
chlorophyll
 Oxygen is formed as a by-product &
released as a gas through stoma to
outside
 Hydrogen enters light-independent stage
Dark reaction (Light-independent stage)
carbon' dioxide  hydrogen  carbohydrate  water
 Light is not required; can take place either
in light or darkness
 Hydrogen produced in light reaction
combines with CO2 to form carbohydrates
 Water is formed as a by-product
Summary of Photosynthesis
C6H12O6 + 6 O2
6 CO2 + 6 H20
Light
Carbon Dioxide
Water
H
Light Reaction
Oxygen
Dark Reaction
Glucose
(C6H12O6)
Fate of carbohydrate
products in the plant
carbon dioxide and water
photosynthesis
carbohydrates (e.g. glucose)
release energy by respiration
carbon dioxide and water
photosynthesis
carbohydrates (e.g. glucose)
convert into other products for growth
carbon dioxide and water
photosynthesis
carbohydrates (e.g. glucose)
fatty acids
glycerol
combine to form lipids for use or as a food
store
carbon dioxide and water
photosynthesis
carbohydrates (e.g. glucose)
mineral salts from soil
(e.g. NO3-, SO42-)
amino acids
join together to become
protein molecules
carbon dioxide and water
photosynthesis
carbohydrates (e.g. glucose)
convert into starch / lipid for storage
Glucose is not a good storage molecule because
Glucose is a soluble substance
- it can diffuse out & lost
- it affects water potential
carbon dioxide and water
photosynthesis
carbohydrates (e.g. glucose)
change into sucrose and is transported
to other parts through phloem
Factors affecting
the rate of
photosynthesis
 Light intensity
 Rate of Photosynthesis
 Carbon dioxide
concentration
 Rate of Photosynthesis
 Temperature
 Rate of Photosynthesis
Temperature > 40℃
Rate  & Stop soon
If no water, stomata close
no gaseous exchange
photosynthesis ceases
Why is Photosynthesis
important ?
Green plants: the only organisms
capable of trapping light
to manufacture food from
simple inorganic
substances
Why is Photosynthesis
important ?
• Green plants are producers
• It provides food for other
organisms & starts the food chain
Why is Photosynthesis
important ?
Atmospheric carbon dioxide is absorbed
& oxygen is released into the
atmosphere which maintains the
composition of the atmosphere constant
Parts of plant where
photosynthesis takes place
Mainly in the leaf because
– it contains a lot of chloroplasts
– it is well adapted for performing
photosynthesis
Internal
Structure of Leaf
cuticle
upper epidermis
palisade mesophyll
chloroplasts
spongy mesophyll
intercellular space
(air space)
guard cell
lower epidermis
stoma
cuticle
Cross-section
of a leaf of
dicotyledon
upper
epidermis
protect internal
tissues from
mechanical
damage and
bacterial &
fungal invasion
Cuticle
• a waxy layer
• prevent
water loss
from the leaf
surface
• protect the
leaf
mesophyll
palisade
mesophyll
(layer)
columnar cells
contains
many
closely packed
chloroplasts
together
absorb light
more efficiently
irregular cells
loosely packed
together to leave
numerous
large air
some (fewer
spaces
than palisade
allow
rapid
mesophyll)
diffusion
of gases
chloroplasts
for
throughout
the leaf
photosynthesis
spongy
mesophyll
(layer)
same as
upper
epidermis
except the
cuticle is
thinner
lower
epidermis
opening which
allows gases
to pass
through it to go
into or out of
the leaf
stoma
(stomata)
guard cells
control the
size of stoma
Stoma
Guard cells
thinner outer wall
thicker inner wall
stoma
vascular
bundle (vein)
Internal Structure of Leaf
xylem
phloem
xylem
• to transport water
and mineral salts
towards the leaf
• for mechanical
support
phloem
• to transport
organic
substances
(food/sugar/
glucose/
sucrose) away
from the leaf
Adaptation of leaf
to photosynthesis
The leaf is thin
decrease diffusion
distance for gases &
light
The leaf is broad &
flattened
increase surface area
to absorb more
sunlight
Cuticle in upper
epidermis is
transparent
Allows most light to
pass into
photosynthetic
mesophyll tissues
Palisade mesophyll
cells are closely
packed and contain
many chloroplasts
To carry out
photosynthesis more
efficiently
Spongy mesophyll
cells are loosely
packed with numerous
large air spaces
To allow rapid diffusion
of gases throughout
the leaf
Numerous stomata
on lower epidermis
To allow rapid
gaseous exchange
with the
atmosphere
Extensive vein system
• Allow sufficient
water to reach the
cells in the leaf
• To carry food away to
other parts of the
plant
Mineral requirements
in plants
• In order to synthesize amino acids, nitrate ions
which must be taken into the plant from the soil
through the root
• Other minerals are also necessary to maintain
the life of the plant (N, Mg, P, K, S, etc)
The importance of nitrogen
• For synthesis of proteins, chlorophyll, etc
• Taken in form of nitrate ions
• Deficiency symptoms:
– Little growth ( - no protein made)
– Yellowing of leaves ( - no chlorophyll made)
The importance of magnesium
• Essential component of chlorophyll
• Deficiency symptoms:
– Yellowing of leaves (no chlorophyll made)
– Poor growth (no food manufactured
because of lack of chlorophyll)
Use of fertilizers in
agriculture
• Continuous harvesting crops removes
valuable mineral salts from soil
Fertilizers are added to replace such loss
• Two kinds of fertilizers:
– Natural fertilizers
– Chemical fertilizers
Natural fertilizers
• From manure (animal waste)
• Organic compounds in it are
decomposed by the bacteria in
soil to form mineral salts
Chemical fertilizers
• Mainly nitrogenous and phosphorous
compounds manufactured artificially
Comparison between natural
and chemical fertilizers
Natural fertilizers
Chemical fertilizers
Contain humus which can
improve soil texture
No humus so cannot improve
soil texture
Less soluble in water so less
likely to be washed away
Very soluble in water so more
likely to be washed away
Comparison between natural
and chemical fertilizers
Natural fertilizers
Chemical fertilizers
Much cheaper
Very expensive
Less soluble in water so more
difficult to be absorbed
Very soluble in water so
easier to be absorbed
Time is needed for the
decomposition to complete
before nutrients are available
to plants
More readily to be used by
the plants
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