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Biology
B1 - Cell-level systems
B1.4 - Photosynthesis
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Created by
Sohaib Azhar
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What is photosynthesis?
To make food plants need:
CO2
- diffuses from air into plant through
stomata
Water - Enters roots through root hair cells by
osmosis
These substances react to make
glucose
, which is used by plants for food
Oxygen is also made and used for
respiration
Where does photosynthesis happen?
Happens in
chloroplasts
Mainly in the
leaf
but some happens in the stem
Light transfers energy from Sun to
Chlorophyll
where
CO2
+ H2O makes
Glucose
Stage 1 (light dependent) - Energy transferred from lights splits water into
Oxygen
and H+ ions
Stage 2 (light
independent
) -
CO2
combines with H+ ions to make glucose
Photosynthesis
is an
endothermic
reaction
How is the glucose used
Used immediately in
respiration
Converted into sugar molecules e.g
fructose
/
sucrose
Glucose that is not needed straight away turns into
starch
which can be used at
night
for respiration
Cellulose to form cell
walls
Testing for starch
You can show light,
chlorophyll
and
CO2
are needed for
photosynthesis
by testing presence of starch in a leaf
Steps:
Place leaf
in
boiling
water to kill it
The place it in tube of boiling
ethanol
to remove chlorophyll
Wash
leaf to remove ethanol
Drop
iodine
onto leaf
If it turns
blue-black
then starch is present
If
starch
is not present then it is unable to photosynthesise
Proving chlorophyll is needed for photosynthesis
Variegated leaves only have
chlorophyll
on some parts which means only parts are green
To prove chlorophyll is needed place destarched
variegated
plant in sunlight for several hours
Then test for starch
Proving light is needed for photosynthesis
Take a
destarched
plant and cover part of it with
black card
Place plant in sunlight for
several
hours and test for presence of starch
Proving CO2 is needed for photosynthesis
Take destarched plant and place it in a polythene bag with
soda lime
(which absorbs water + CO2)
Place in sunlight for
several
hours before testing for starch
Proving Oxygen is given off in photosynthesis
Place upside down tube above an aquatic plant
Put it in
light
for maximum photosynthesis
Then put a
glowing
split in the tube and if it relights oxygen is present
Factors affecting the rate of photosynthesis
Light Intensity:
The higher the light intensity the
greater
the rate of photosynthesis
This continues until light reaches
maximum
rate
CO2:
Greater the CO2 concentration, the
greater
the rate of photosynethesis
But there is little CO2 in atmosphere so it is a
limiting
factor
Temperature:
As it increases so does rate of photosynthesis
But it is controlled by
enzymes
so once optimum rate passes enzymes
denature
slowing it down
Interaction of limiting factors
To check effects on
photosynthesis
Light intensity - place light sources at diff
distances
from plant
CO2
- Add diff masses of KHCO3 to H2O as it releases
CO2
Temperature - Place apparatus in
water
bath
at diff temperatures