Photosynthesis: Crash Course Biology #8

Hank explains the extremely complex series of reactions whereby plants feed themselves on sunlight, carbon dioxide and water, and also create some by products we're pretty fond of as well.

Citations:

Table of Contents:
1) Water 1:16
2) Carbon Dioxide 1:32
3) Sunlight/Photons 1:43
4) Chloroplasts 1:57
5) Light Reaction/Light-Dependent 2:42
a. Photosystem II 3:33
b. Cytochrome Complex 5:54
c. ATP Synthase 6:16
d. Photosystem I 7:06
6) Dark Reactions/Light-Independent 7:55
a. Phase 1 - Carbon Fixation 8:50
b. Phase 2 - Reduction 11:31
c. Phase 3 - Regeneration 12:02

Want to find Crash Course elsewhere on the internet?

[stereomaster]

I remembered having to learn about this when I was in high school Biology so I came back to see if it would come back to me at age 37. Nope, I ended up rocking back and forth in the corner of my office in the feeble position. Then I snapped out of it... I now embrace this truth - plants turn water, sunlight, and CO2 into food using magic

[moriahsun]

remember in elementary school when all you needed to know is: photosynthesis needs the sun, water and carbon dioxide to make food? Now it has come to this.

[SenatorDishTowel]

These are the notes I took. Hopefully this can further your understanding

LIGHT DEPENDENT REACTIONS (STAGE 1):
-Photons strike the chlorophyll in Photosystem II (PSII) and an electron absorbs the energy and gets excited (photoexcitation)
-The electron travels from PSII (first protein complex of 4) along the electron transport chain (via mobile electron carriers) to harness energy.
*mobile electron carriers are proteins that carry electrons along a membrane
[-The chlorophyll splits a stable molecule of H2O, stealing an electron to replace the old one. Oxygen is then released from the plant. This allows us to breathe.]
-The energized electrons from PSII that were picked up by the electron carriers are transported to the next protein complex, the Cytochrome Complex.
-The Cytochrome Complex serves stands between PSII and PSI (all embedded in the thylakoid membrane) and uses energy from the electrons to pump hydrogen ions into the thylakoid.
-Protons accumulated from the splitting of H2O and via the Cytochrome Complex now in the thylakoid then move toward the enzyme ATP synthase, which uses the energy to pack an inorganic phosphate onto ADP to create ATP.
-As electrons move along the Electron Transport Chain, they lose energy
-The ‘tired’ electrons become re-energized as it reaches Photosystem I and travels on another electron carrier, where all the energy is used to make NADPH.
*Products of Light Dependent reactions: chemical energy in the form of ATP and NADPH, and the by-product oxygen.

LIGHT INDEPENDENT REACTIONS (STAGE 2):
-First phase of the Calvin Cycle is known as Carbon Fixation
-Begins in the stroma (empty space of chloroplast). Involves fixing a CO2 molecule onto Ribulose Bisphosphate (RuBP) with the help of an enzyme RuBisCo. This causes the RuBP to become unstable.
-This new 6-carbon chain has to break apart, creating two molecules of 3-phosphoglycerate, which occurs to three molecules of RuBP.
-Second phase of the Calvin Cycle is called Reduction.
-ATP combines a phosphate group onto the 3-phosphoglycerate and NADPH packs some electrons, creating 2 molecules of Glyceraldehyde 3-Phosphate (G3P).
-G3P is a high energy, 3-carbon compound that plants can convert into many carbohydrates such as glucose, cellulose, or starch. (glucose=short term energy / cellulose=structure / starch=long term energy)
-The last phase of the Calvin Cycle is call Regeneration.
-9 ATPs and 6 NADPHs are required to convert the 3 RuBPs we started with into 6 G3Ps. However, only one G3P can leave the cycle... the other 5 G3Ps are needed to regenerate the original RuBPs, completing one round of the cycle.

Hope this helps!

[silentofthewind]

Mitochondria are the powerhouse of the cell.


That's what I've learned so far.

[chloeholtz]

I appreciate the fact that he admits this process is stupidly hard for no reason except... science😂

[Mylkzi]

Everyone here is cramming for AP biology but I’m just cramming for normal biology how is this fair

[beuyhs]

Summary for those whom it may concern:
We begin in PS2 (photosystem 2), light energy absorbed by the cholorophyll exciting electrons in it.
Excited electrons move up to a higher energy level.
Excited electrons move to electrons carriers forming electron transport chain (redox reactions occur).
Energy is lost from electrons moving down the electron transport chain.
Water is split into H+, e- and 1/2 O2 by photolysis (light energy used to break it down).
Electron from the water being split is given back to PS2.
Energy lost from electrons moving down electron transport chain is used to pump H+ into thylakoid from the stroma creating proton gradient.
Protons move out of thylakoid down concentration gradient through ATP Synthase.
Energy from this movement is used to add inorganic phosphate to ADP forming ATP.


PS1: Light energy absorbed by chlorophyll exciting electrons to a higher energy level.
Electron transferred to NADP along with a H+ from stroma to form reduced NADP (NADPH).
- Cyclic photophosphorylation:
Electrons excited by energy absorbed and passed onto electron carriers.
Electrons passed back to PS1 (hence 'Cyclic')- no electrons lost)).
H+ pumped into thylakoid from stroma by energy lost from electrons moving down energy levels.
Proton gradient used causes high concentration of H+ in thylakoid to reduce by H+ leaving through ATP Synthase.
Energy from this movement allows ---> ADP+P(i)-->ATP.

NADPH and ATP used in light independant reactions.

[sabrinajusino]

I failed Biology 3 times already, this is my 4th time taking the class. First time actually trying to seriously understand it. I’ve been watching your videos that relate to what we’re learning in class, I want to thank you for helping me earn a 92% on my first exam. I’m so grateful for you!

[AG-sjvt]

Who else zones out during the video and then has to rewatch it again...and again...

[jayy]

Basically the video in text, hopefully to further your understanding.

Requires water (H2O), carbon dioxide (CO2) and light.
Roots of plants absorb water and transport it to the leaves through xylem, CO2 enters the plant (while O2 exits) through microscopic pores in leaves called stomata, and individual photons are absorbed from the sun in the plant by the pigment chlorophyll.
**It is important that O2 levels remain low (explained later) for the Calvin Cycle (Stage 2)
Inside the cell of plants are plastids called chloroplasts, which contain thylakoids (which store chlorophyll), which contain lumen inside and Stroma outside; stacks of thylakoids are called grana.


Light Dependent Reactions (Stage 1)
PS II (wavelength p680)
-Photons strike the chlorophyll in Photosystem 2 (PS II) and an electron absorbs the energy and becomes excited (photoexcitation), but has nothing to do with this energy.
-The electron travels from PS II (1st protein complex of 4) along the Electron Transport Chain (via mobile electron carriers) to harness the energy; across a series of reactions the electrons lose their energy.
-The energised electrons from PS II that were picked up by electron carriers are transported to the 2nd protein complex, the Cytochrome Complex.
-The cytochrome complex stands in between PS II and PS I (which are all embedded in the thylakoid membrane), and uses the lost energy from the electrons to pump hydrogen ions into the thylakoid.-Now missing an electron, the chlorophyll molecule will split a H20 molecule in cooperation with PS II to steal its electron and replenish the one it lost, the by-products of this process are hydrogen ions (single protons) and oxygen.
**By-product of oxygen is the reason behind the survival of living organisms.
-Protons accumulated from the splitting of H2O and via the cytochrome complex now in the thylakoid then move towards the enzyme ATP Synthase, which uses that energy to pack an inorganic phosphate (Pi) onto ADP to create ATP.

PS I (wavelength p700)
The 'tired' electrons becomes reenergised by some photons as it reaches Photosystem 1 (PS I) and travels on another electron carrier, where all of the energy is used to help make NADPH (similar to ATP) by an enzyme, which combines 2 electrons and 1 Hydrogen ion with NADP+. (NADP+ + 2 e- + 1 H+ = NADPH).
**Products of light dependent reactions: Chemical energy in the form of ATP and NADPH, as well as the by-product O2.



Light Independent Reactions/Calvin Cycle (Stage 2)
**Doesn't require light energy from photons, uses energy derived from ATP and NADPH to fuel processes
Carbon Fixation (1st phase of Calvin Cycle)
-Begins in the Stroma (empty space of chloroplast). Involves fixing a CO2 molecule onto Ribulose-Biphosphate or RuBP (the starting and end point of the 'cycle'), with the help of an enzyme called RuBisCo (which converts inorganic carbon into organic carbon), which causes it to become unstable.
-So this new 6-Carbon chain has to break apart, creating 2 molecules of 3-Phopshoglycerate, which occurs to 3 molecules of RuBP (explained later).
**If RuBisCo reacts with O2 instead of CO2, it creates a toxic by-product known as phosphogycolate (process of photorespiration) believed to alter enzymatic functions.

Reduction (2nd phase)
-ATP combines a phosphate group onto the 3-Phosphoglycerate molecules and then NADPH packs some electrons, creating 2 molecules of Glyceraldehyde 3-Phosphate (G3P).
-G3P is a high-energy, 3-carbon compound that plants can convert into many carbohydrates such as glucose, or cellulose or starch.

Regeneration (3rd and last phase)
-9 ATP molecules and 6 NADPHs are required to convert the 3 RuBPs started with into 6 G3Ps, however only 1 G3P can leave the cycle - with the other 5 G3Ps needed to regenerate the original 3 RuBPs, completing 1 round of the cycle.
**G3P is considered the ultimate product of photosynthesis as the glucose gained from it is the energy used by all cells.

[meganriley]

*hears a bunch of complicated words*

*zones out*

*freaks out and restarts the video*

[notwanheda]

This is probably the first crash course video that I've watched and then felt MORE nervous for my exam afterward

[roganvanaswegen]

RIP to the people who are watching these videos who have tests the next day...

[bagginn]

For the lazy :D GL on tests!

Photosystem II captures and transfers energy.
1) chlorophyll absorbs
energy from sunlight

2) energized electrons
enter electron
transport chain

3) water molecules are
split

4) oxygen is released as
waste

5 )hydrogen ions are
transported across
thylakoid membrane

6) Photosystem I captures energy and produces
energy-carrying molecules.


7) chlorophyll absorbs
energy from sunlight


8) energized electrons are
used to make NADPH


9) NADPH is transferred
to light-independent
reactions

[ria-qjhz]

"Or more likely you want to do well on your test." Pffffttt. At this point I just want to pass.

[jadaok]

Watching this the day before my bio exam and realizing that I’m so gonna fail

[arkexemption]

Good luck to everyone on their exam tomorrow! Now stop reading comments and get back to revision

[alamisc]

currently crying in bed at 3am trying to study for my AP Bio test but every single drop of will and motivation has been drained out of me.

[undergroundskeptic]

*Replays video more than 1000 times*

*Still doesn't get photosynthesis*

[quantumcrash]

I have my test in an hour. So far I know electrons are involved somehow.