Plant Metabolism (Photosynthesis and Respiration)

Topic: Photosynthesis and Respiration

Learning Goals

• Understand plant metabolism
• Examine how plants convert light into biological energy, ATP
• Examine how plants store energy
• Examine how plants access that energy later (respiration)

Image Details: "Photosynthesis" by dmott9 is licensed under CC BY-ND 2.0.

Review Chemistry

You will notice that as you go through the learn assignments this week they are will be heavier on the chemistry terms than our last several weeks. Don't recall our chemistry overview please review the Intro to Chemistry section in our Physical and Chemical Features Learn Assignment. I have embedded the presentation below for your convienience. If you feel comfortable with chemistry feel free to move onto the next slide.

Plant Metabolism

What is metabolism

Metabolism is all the chemical processes required for an organism to be alive. It focuses heavily on energy needs and the pathways of biological energy (the movement of electrons between bonds, especially ATP).

Examples of plant metabolic processes

Anything a plant does that is required for life is part of it's metabolism. That includes things like:

• Growth
• Defenses (sap production, toxins, etc.)
• Storing energy
• Reproduction

Image Details: "Photosynthesis" by dmott9 is licensed under CC BY-ND 2.0.

Acquiring Energy

Autotrophs

Plants are autotrophs, meaning they produce their own biological energy (ATP) using non-living resources. This resource for plants is sunlight. We refer to them as photosynthesizers (photo: light synthesis: builder) Note: Other types of autotrophs exist, called chemotrophs. These use other chemicals (such as sulfur from deep sea vents) to create ATP. Plant create energy and store it in chemical bonds, typically in the form of glucose (recall the molecule we built in lab) using the process of photosynthesis. They access that energy when they need it through the process of cellular respiration. We will look at photosynthesis first, then respiration.

Image Details: "plants" by Schnittke is licensed under CC BY 2.0.

Photosynthesis

"When life hands you light, make sugar!"

• Photosynthesis is a series of metabolic pathways uses the energy from the light, converts it into ATP (biological energy storage molecule) and NADPH (a different energy molecule that is a temporary transport), and then stores the energy in glucose or other sugar molecules.
• Used by most autotrophs
• Requires:
• Light
• Water
• Carbon Dioxide

Steps of Photosynthesis

• Overall Chemical Formula: 6CO2 + 6H2O → C6H12O6 + 6O2
• Two major steps:
• Light Dependent
• converts light to biological energy
• ATP
• NADPH
• Light Independent (Calvin Cycle)
• converts biological energy to sugar

Properties of Light

The electromagnetic spectrum

• What is light?
• formed when an excited electron "drops down" to a stable orbital giving off it's excess energy
• a packet of energy, called a photon, with a particular wavelength
• Different wavelengths make-up different parts of the electromagnetic spectrum

Properties of Light

How we see

• Color vision
• Our eyes have color-receptive cones that detect when a photon of light impacts it
• Our brain then interprets that as color
• Object color
• Light photons that bounce off of pigment (a type of protein)

Note: Color blindness isn't uncommon. The images to the right might look the same if you have a common type of color blindness.

Properties of Light

The electromagnetic spectrum

• Wavelength
• measured by the distance of cycle
• Spectral Colors (Visible Light)
• Wavelengths between 700nm (red) and 400nm (violet)
• This is range humans can see, but this isnt true for all life:
• Most mammals: 2 of color-receptive cones
• Humans: 3 types of cones
• Birds, Reptiles, many fish: 4 types of cones
• Mantis shrimp: 16 types of cones
• Yellow light is the most abundant visible light (yellow sun)

Properties of Light

The electromagnetic spectrum

• Non-spectral Colors
• Ultraviolet, X-ray, Gamma-rays
• under 400nm
• Infrared (heat), microwaves, radiowaves, TV waves, etc.
• over 700nm

Properties of Light

Light and Photosynthesis

• Photosynthesis uses photoreceptors to capture light energy
• Some wavelegnths of light are used more than others

Chloroplasts

Stroma (fluid inside)

Outer membrane

Inner membrane

Intermembrane space

Thylakoid (single disk) Granum (stack of disks) Lumen (fluid inside the thylakoid)

• Evolutionary history - cyanobacteria symbiote that became an organelle.
• We know this because it has a two layers of membrane and it's own DNA that resembles a bacteria
• Role: produces glucose which is moved to other parts inside and outside the cell

Optional Reading, before we move onto the steps

Photosynthesis is a fun example of discovering things in "odd" ways. Science doesn't always work in the order or expected way (not all discoveries were intentional). The discovery of how photosynthesis works and its pathway is a whole series of right answer found the wrong way or just odd ways of thinking. This quote showcases this well: "The history of science is never written by the scientists involved in making discoveries. That would be too painful, too embarrassing..." - Andy Benson (1977)A couple examples for photosynthesis:

• Realizing that plants used CO2 because the room felt better with plants
• The way the scientists figured out how it works was figured out by algae lollypops and the scientist overheating in his wife's car while she was shopping.

The first article linked below tells the story of how photosynthesis was discovered. It isn't required, but I do highly recommend reading it. It tells more about the examples above.

Optional Reading: Discoveries in oxygenic photosynthesis (1727–2003): a perspective

Steps of Photosynthesis

You Are Here Membrane of the thylkoid

Light Dependent Reaction Steps

Photosystem II

Before we begin the steps, please note:

• I am going to describe the steps and then there will be a video that shows the whole thing through.
• Don't stress is some of the details feel confusing. Just notice that it uses light to move things. That movement works like a current that generates energy.
• Don't memorize protien names. Just notice that CO2 is used, water is used, and O2 is made. Notice that the O2 isn't made from the CO2 (it comes from the water) and that all the CO2 is put into sugar.

H+

H+

Stroma Side

e-

Lumen Side

e-

H20

H+

H+

H+

H+

H20

O2

Steps of Photosynthesis

You Are Here Membrane of the thylkoid

Light Dependent Reaction - Step 1

Photosystem II

• Moves through a chain of proteins called photosystems.

• Starts with photosystem 2
• It was discovered second and so is called photosystem 2 (represented with II)

• Captures light energy and uses it to break water and excite (energize) electrons

H+

H+

Stroma Side

e-

Lumen Side

e-

H20

H+

H+

H+

H+

H20

O2

hydrogen pump proteins

Steps of Photosynthesis

H+

plastoquinone

Stroma Side

Light Dependent Reaction - Step 2

H+

cytochrome

H+

• Electrons used in next two proteins in the sequence to pump hydrogens
• These are literally pumps. They move the hydrogens from one side to the other to build up a concentration gradient (like filling a dam with water)
• A third protein carries it to the next photosystem

e-

e-

e-

PC

e-

H+

H+

H+

H+

Lumen Side

H+

H+

H+

Steps of Photosynthesis

NADP

NADPH

Light Dependent Reaction - Step 3

photosystem I

e-

• Photosystem I Complex
• Photosystem I
• Two additional protiens that sit on top.
• The photosystem uses light to re-excite the electron and make NADPH (a temporary energy storage molecule)

Stroma Side

e-

e-

PC

Lumen Side

H+

H+

H+

H+

H+

H+

H+

H+

ATP Synthase

Steps of Photosynthesis

ATP

ATD

Light Dependent Reaction - Step 4

H+

• ATP Synthase
• Uses the concentration gradient of H+ to make ATP
• H+ ions either:
• used to make NADPH
• re-pumped through by the hydrogen pump proteins

Stroma Side

H+

Lumen Side

H+

H+

H+

H+

H+

H+

H+

Steps of Photosynthesis

H+

Diagram of all the Light Dependent Reaction Parts

ATP

H+

NADPH

H+

Stroma Side

e-

e-

Thylakoid Membrane

e-

H+

e-

Lumen Side

e-

H20

H+

H+

H+

H+

H+

H+

H+

O2

H+

Steps of Photosynthesis

You Are Here The Stroma

Light Independent Reaction (Calvin Cycle)

• This part doesn't require light and can happen at night.
• It is a three-phase cycle.
• Its a cycle because the starting enzyme replenishes in the cycle.

Steps of Photosynthesis

You Are Here The Stroma

Light Independent Reaction (Calvin Cycle)

• Phase 1 : Carbon Fixation
• Uses CO2 and ATP to make temporary molecules
• Phase 2: Reduction
• Uses NADPH and makes one glucose.

• Phase 3: Regeneration of RuBP
• Uses more ATP and the rest of the carbon to resets the Rubisco enzyme to be reused

All Steps of Photosynthesis

Cellular Respiration

"When life hands you sugar, make energy!"

• Cellular Respiration
• a series of metabolic pathways extracts the energy from the bonds in glucose and converts it into ATP and similar energy storing molecules.
• Full process varies on organism
• Eukaryotic cellular respiration contains anaerobic (without oxygen) and aerobic (with oxygen) parts
• Some (not all) prokaryotic cellular respiration is strictly anaerobic (called obligate anerobes)

History of Respiration and Oxygen

Oxygen and the First Mass Extinction

• Oxygen occurred at very low levels early in Earth's history
• This is due to the temperature of earth's formation. The heat present causes oxygen to bond into CO2 or into other minerals, but not as O2.
• Interesting note: The presence of O2 blocks particular colors of infrared. This has the potential of being observed when viewing planets outside of our solar system. This is how astronomers identify planets that may have life.

History of Respiration and Oxygen

Oxygen and the First Mass Extinction

• Early life
• Earliest prokaryotic cells show in the fossil record around 3.8bya
• Oxygen wasn't present in measurable concentrations until 2.7bya
• Early respiration was anaerobic.
• Early autotrophs were using chemosynthesis

History of Respiration and Oxygen

Oxygen and the First Mass Extinction

• Cyanobacteria
• Early photosynthesizers
• Produced oxygen as a waste product
• Their respiration was anaerobic
• Resulted in the "Great Oxidation Event"
• Resulted in the first mass extinction
• Oxygen is highly toxic without mechanisms to keep it from "stealing" electrons
• It caused their respiration to run backwards and they couldn't get energy once oxygen reached a certain level

History of Respiration and Oxygen

Optional Reading: Oxygen and Anaerobes

Glycolysis

The Breaking of Glucose

• All forms of cellular respiration using sugars begin with glycolysis.
• Occurs in the cytoplasm, not the mitochondria
• Glycolysis
• "gluco" means sugar, "lysis" means breaking
• anerobic: it doesn't use oxygen
• used by many types of cells
• prokaryotic
• mammalian red blood cells

Glycolysis

All Steps Visualized. Don't worry about the details, just notice how it moves through a series of steps.

Pyruvate (and NADH)

The next steps of energy acquisition

• Glycolysis alone is a very inefficient process
• only two net ATP produced (four made, two used)
• It is inefficient for most eukaryotic cells
• Pyruvate is a waste product of glycolysis that either needs to be removed (through fermentation) or further broken down to extract more energy (aerobic respiration).
• Anaerobic (not covered in this class)
• Fermentation
• Aerobic:
• breakdown of pyruvate
• Citric Acid Cycle

Citric Acid Cycle

Aerobic processing of pyruvate

• Occurs in the Mitochondria. Pyruvate is moved from the cytoplasm into the mitochondria.
• Pre-step: The breakdown of pyruvate
• Results in the creation of Acetyl CoA and additional NADH
• Acetyl CoA is also part of the breakdown of lipids and proteins (in addition to carbohydrates)
• This allows for efficiency in energy production

Citric Acid Cycle

You are here

Aerobic processing of pyruvate

• Citric Acid Cycle (also known as the TCA cycle and the Krebs cycle)
• Occurs in the matrix of the mitochondria
• Produces ATP (two), NADH, and FADH2
• Provides the material for the Electron Transport Chain
• produces large amounts of ATP using NADH and FADH2

Citric Acid Cycle

All Steps Visualized. Like before, focus on big picture details, not individual chemical names

Electron Transport Chain

All Steps Visualized. Notice how mechanical protiens are. Notice how electron movement is harnessed, just like in photosynthesis.

Wrap-Up

Draw the pathway of photosythesis and into respiration as if it were happening in a single plant cell. Start with the cholorplast, show it go into the cytoplasm, and then into the mitochondria. Label the following in the diagram:

• where CO2 is produced
• where CO2 is used
• where O2 is produced
• where O2 is used
• where water is produced
• where water is used
• where ATP is produced
• where ATP is used

You will need to post before you can see other diagrams.

Image Details: A sketch of a plant cell as a starting reference. Chloroplast is on the left (circle with oval stacks), cytoplasm is the space in the middle, mitochondria (oval with the inner squiggle) is on the right.