Photosynthesis
The process that makes life of Earth
possible
Chloroplasts
• Where Photosynthesis takes place
• Holds many flat bag shaped membranes
called thylakoids
– Thylakoid – collect light using chlorophyll
– Connected to one another and arranged in stacks
called grana (stacks of pancakes)
– Stroma (pancake syrup)- liquid filled space around
the thylakoids
Photo (light)-synthesis (putting
things together)
Remember:
- To make a large molecule (glucose) you need Energy
- When you break down a large molecule (glucose) you
generate Energy
How do Plants get Water?
How do Plants get water?
• Water from the soil gets brought up from the
roots to the leaves
• Water gets pulled up as water evaporates
from leaves because of heat from the sun –
think of how liquid gets from the bottom of a
cup to your mouth with a straw
How do Plants get Carbon Dioxide?
How do plants get carbon dioxide?
• Stomata or tiny openings in the bottom of the
leaf let carbon dioxide in and oxygen out
How do plants get light energy from
the sun?
• We already know this! Plants trap light energy
from the sun using the pigment chlorophyll
which is found in the chloroplast
Now we have all 3 ingredients:
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Why is photosynthesis so important?
• All the energy on the planet comes from the sun
• Autotrophs are the only organisms that can harness
that energy
• All the energy for living things starts with the
autotrophs who capture it from the sun!
• Every food chain starts with an autotroph and all the
energy in that food chain originated with the sun!!
2 Steps of Photosynthesis
1. Light-dependent reactions: convert the sun's
energy into chemical energy - generate the
energy needed for the next step
2. Light-independent reactions: use the energy
from step 1. and Carbon dioxide to make
glucose (food)
Step 1
• Occurs in two different areas of the thylakoid
– Photosystem I and Photosystem II
• Photosystem – light collecting unit of the
chloroplast
• Photosystem has Chlorophyll and Electron
Transport Chain (a chain of Carrier Proteins)
Light
electrons
- The more Energy an electron has , the further
away from the protons it can get;
- Given enough energy it can escape
altogether leaving behind a (+) charged
particle – ion
- When an electron falls closer to protons, it
releases energy
- When Hydrogen (picture) looses 1 electron, it
becomes a H+ ion
Step 1 – getting that energy from
the sun
• Light-dependent – reactions need light
• Require water and sunlight
• Chlorophyll absorbs the energy from the sun –
(high energy particles called photons) – with
enough energy 2 electrons will pop off from
chlorophyll
• To replace these lost electrons from Chlorophyll
- photons will split water into oxygen, hydrogen
ions and electrons – O2 H+ e-
Step 1 – Photosystem II
• The 2 electrons from Chlorophyll will travel
down the electron transport chain (ETC) – a
chain of carrier proteins
• Each time the electrons jump from 1 carrier
protein to another they will release some
energy
• The energy from the electrons is then used to
transport H+ across the membrane – result
high concentration of H+ inside the thylakoid
ATP formation
• H+ ions build up inside the thylakoid
• H+ wants to move from high to low
concentration (Diffusion) but it needs help
from a protein in the cell membrane called
ATP synthetase (channel protein + enzyme)
• As H+ passes through, ATP synthetase is forced
to turn and when it does it adds a phosphate
group to ADP to make ATP
Step 1 – Photosystem I
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The electrons from photosystem II lost energy
when they pumped H+ across the membrane
In photosystem I light gives the electrons
energy again
Then they go through another ETC
Here they are used to make NADPH which go
on to the next set of reactions
Summary of Light dependent reactions
• Require water and sunlight, have 2 parts:
photosystem II and photosystem I
• As light shines on Chlorophyll it looses 2
electrons, and replaces them with 2 electrons
from water
• As the plant uses the electrons from water H+
and O2 are left behind
• Oxygen gas is given off in photosystem II and ATP
is made
• NADPH is made in photosystem I
Step 2 – Light-independent
reactions
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Also called Dark reactions or the Calvin cycle
Takes place in the stroma of the chloroplast
Need carbon dioxide to build carbohydrates
Use hydrogen, ATP and NADPH (energy) from
light dependent reactions and carbon dioxide
to make carbohydrates/glucose
Dark Side of Photosynthesis
To be Continued.............
Step 2 - Light-Independent
Reactions........
Glucose
.........Also known as The Calvin
cycle
From Air by way of Stomata
From Step 1 –
Light-dependent
Reactions
The Calvin Cycle
• Called a cycle because the starting material is
regenerated each time the process occurs
• Enzymes join carbon dioxide molecules with
molecules that are already in the cell - these
molecules will regenerate each time
• The energy from ATP and NADPH are used in
this process
3 X 5-C molecules ( 15 C)
+ 3 CO2
6 x 3-C molecules (18 C)
5 x 3-C molecules
1X 3-C
molecule
product/leaves the cycle
2 of these make 1 glucose
Sugar Production
With each turn of the Calvin cycle a few 3C
sugars leave the cycle
These molecules become the building blocks
that the plant cell uses to make sugars, lipids,
amino acids and other compounds
The plant will use these compounds as the
food and building materials it needs to grow
Summary of Light Independent
Reactions

Calvin cycle uses ATP and NADPH from light
dependent reactions and carbon dioxide from
the air to make sugars
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Plant uses the sugar to meet its energy needs
and to build molecules needed for growth
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When other living things eat these plants they
too will get the energy they need.
Factors that Affect Photosynthesis
• The process can speed up or slow down
depending on several factors
• Temperature: enzymes work best between 1535 degrees Celsius
• Light: very bright light speeds up
photosynthesis
• Water levels
Water Loss and Photosynthesis
• Water is lost through the stomata where
carbon dioxide enters
• If its hot the plants will close the stomata to
keep from loosing water but photosynthesis
will slow down due to the lack of carbon
dioxide
• Some plants that live in dry areas have special
ways to save water and carry out
photosynthesis
C4 Plants
• Have a special chemical pathway that gets
carbon dioxide into the Calvin cycle
• It uses extra ATP but it allows the plant to
carry out photosynthesis when its hot
• Examples: Corn, sugar cane
CAM Plants
• Save water by taking air into their leaves only
at night
• At night carbon dioxide is used to make acids
• During the day these acids are turned back
into carbon dioxide for photosynthesis
• Example: Pineapples trees, desert cacti