The Magic of Turning Sunlight and Air into a Carrot

Plants both conduct photosynthesis (combine energy from sunlight, carbon from the atmosphere, and water/nutrients from the soil to form starches) as well as respiration (breaking down of these starches to unlock energy for vital functions to keep the plant alive). Let's look into both of these processes and the structures involved - starting with photosynthesis

Basics of photosynthesis and respiration

These are the simplified chemical reactions for cellular respiration and photosynthesis (plants do both). Notice that they are basically the inverse of each other.

This diagram represents photosynthesis and respiration in a more graphical form. Please notice the parallels between this diagram and the above equations. Photosynthesis is how plants make their food and building materials, respiration is how they convert their food to energy for cellular work, etc.

Basic Steps of Photosynthesis

Light dependent reactions
- Produce ATP and NADPH via activation of an ETC (use water and light, waste = O2)
Light independent reactions (Calvin Cycle)
- Gather inorganic Carbon (CO2) and synthesize longer chain organic carbon (glucose) using NADPH and ATP created in the Light dependent reactions

photosynthesis: light dependent reactions

Crash Course - Photosynthesis - Light Dependent Reactions - Look to "Ted Ed" video further down the page for a better description of Light Independent Reactions

How does each of these contribute to Photosynthesis

Roots
Water
Xylem
Carbon Dioxide
Oxygen
Stomata
Photons
Chloroplast (a plastid)
- Chlorophyll
- Thylacoids
- Grana
- Lumen
- Stroma
- Phospholipid Bilayer

Light-Dependent Reactions
Photoexitation
Electrons
Photosystem II
Electron Transport Chain (ETC)
Water splitting
Hydrogen ions
Electron carrier
Cytochrome complex
Concentration Gradient
ATP Synthase
ADP
ATP
Photosystem I
NADP+
NADPH
Calvin Cycle
Carbon Fixation

RuBisCo
glucose
cellulose
starch

Light Dependent Reactions - some guiding questions and a few notes

Where are the protons coming from and where are they going?
Why are we pumping protons?
Make sure you pay attention to WHERE all of this is happening
Solar fusion → photon → photosystem II [chlorophyll(s), proteins, etc.] → ETC →split H20 to replenish electron [H+ and O2 produced] → Cytochrome complex → continue ETC & pump H+ →
- - ADP → ATP
  - NAD+ → NADPH
  - H2O → Oxygen

If the Crash Course video goes too fast feel free to stop and rewatch portions. You are also welcome to watch this video series which is VERY detailed and goes slowly:

Above is a basic gif of the light dependent reactions to aid in your understanding of the Crash Course video.

This gif is an actual image of a stoma (plural stomata) which is sped up. Stomata are openings in the leaves that allow regulated intake of carbon dioxide and release of water and oxygen. The diagram labeled "Stomata" illustrates the structures in stomata and how the guard cells are controlled by the water pressure in the central vacuoles.

Parts of the Plant Cell - Thylacoid, Grana, and Chloroplasts

*Note - turn on Closed Captions (CC) to get explanations in the video

This video is a deep dive into the plant cell. Here you can enjoy a visualization of chloroplasts, thylacoid, grana, etc.

The above diagrams are meant to further your understanding of chloroplasts. Please study them insomuch as they are useful to understanding where and how photosynthesis takes place.

photosynthesis: Light independent reactions

TED Ed - Light Independent Reactions (i.e. Calvin Cycle)

How does each relate to the Calvin Cycle?

RuBP
RuBisCo
PGAs
NADPH
ATP
Energy
Hydrogen ion
G3P
Glucose
6 "production lines" working together
cycles (i.e. non-linear production)
feedback loops

What is the deal with RuBisCO? Why do folks say it is inefficient?

Non-Cyclic Pathway: Under certain conditions, the photoexcited electrons take an alternative path called cyclic electron flow, which uses photosystem I (P700) but not photosystem II (P680). This process produces no NADPH and no O_2,but it does make ATP. This is called cyclic photophosphorylation. The chloroplast shifts to this process when the ATP supply drops and the level of NADPH rises. Often the amount of ATP needed to drive the Calvin cycle exceeds what is produced in non-cyclic photophosphorylation. Without sufficient ATP, the Calvin cycle will slow or even stop. The chloroplast will continue cyclic photophosphorylation until the ATP supply has been replenished. ATP is produced through chemiosmosis in both cyclic and non-cyclic photophosphorylation.

Types of Photosynthesis - C3, C4, CAM - a matter of water loss

C4 Pathway

Adds Mesophyll unit on top of C3 machinery to pre-concentrate carbon before being taken into the plant
CO2 stored as malate
stomata open less often than C3
many grasses, grains, sugar cane, corn, etc.

CAM Pathway

Called CAM after the first family within which it was found (Crassulaceae).
CO2 is stored in the form of an acid before used by the plant
Stomata open at night only
Succulents, agave, pineapple, cacti, orchids

Photorespiration is a process that occurs in plants where oxygen is added to RuBP instead of CO2. This occurs when oxygen levels are very high and reduces the overall efficiency of photosynthesis. C4 plants avoid this by moving the CO2 molecules into the sheath bundles (a structure unique to C4 plants) where CO2 concentration is considerably much higher than oxygen. This is where the Calvin cycle occurs where the efficiency is very high.

CAM (Crassulacean Acid Metabolism) plants have a very different but more effective approach to conserving water. They collect CO2 at night when the environment is much cooler and stores the concentrated CO2 as malate. This is then released back during the day and is consumed for photosynthesis. This process allows CAM plants to keep their stomata closed during the day where water evaporation is more likely due to the heat.

respiration - process and structures - briefly

Steps of Aerobic Respiration

Glycolysis
- Break down glucose, make pyruvate, get a little ATP
Citric Acid Cycle
- Uses pyruvate, reduces NAD+ to NADH (electron carrier), also produces FADH2 (electron carrier), get a bit more ATP
Electron Transport Chain
- Uses the electrons on NADH and FADH2, uses O2 as an electron “mop”, produces A LOT of ATP

Cellular Respiration

This is a quick video detailing cellular respiration. The main thing is to notice that the ETC is doing pretty much the inverse of photosynthesis. Pay attention to where energy comes from, where it gets produced, and what the byproducts are. Remember that plants perform both photosynthesis and cellular respiration!

Electron Transport Chain

Here is a quick gif of the ETC. Please use it to cement your understanding of the above video on cellular respiration.

Application of knowledge of cellular respiration to botanical questions

Respiration slowed by inundated soils leading to lack of oxygen at roots - leading to reduced aerobic respiration
Glyphosate and Cellular Respiration - “[Glyphosate and other herbicides like it]...induce the less-efficient, ATP-producing pathways, namely fermentation and alternative respiration, by increasing the key metabolite, pyruvate.”

Here is an interesting application of shifting photosynthetic pathways to hopefully grow more resilient rice.

other resources...

Great Detailed YouTube Series on Photosynthesis

https://www.youtube.com/watch?v=Jk8Lo8uqww8

https://www.youtube.com/watch?v=isyksgQPnVY

https://www.youtube.com/watch?v=Ce9JikM1mSo

https://www.youtube.com/watch?v=pSBLInoOxmI&t=86s

https://www.youtube.com/watch?v=Dq38MpYOb8w

C3, C4, CAM explained

This goes into more detail than we will be doing in class...but feel free to watch if you would like to learn more about the different photosynthetic pathways.

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