Photosynthesis
Part I
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
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)
2 Steps
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 System ( 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
• Requires 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, Chlorophyll
using 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 that will use the
energy form the electrons to transport H+
across the membrane – result high
concentration of H+ inside the thylakoid
• H+ ions build up inside the thylakoid from the
break down of water and the ETC
ATP formation
• H+ wants to move from high to low
concentration (Diffusion) but it needs help
from a protein in the cell membrane called
ATP synthetase
• 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 electrons energy
again
Then they go through another ETC
Here they are used to make NADPH which
goes 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 ATP and NADPH (energy) from light
dependent reactions and carbon dioxide to
make carbohydrates/glucose
Dark Side of Photosynthesis
To be Continued.............