Chapter 8 - Photosynthesis

Photosynthesis

• process by which light energy is converted to chemical energy and stored in sugar or other organic molecules

• series of reactions which convert light energy, carbon dioxide, and water → sugar and oxygen

• involves a series of redox reactions and the passing of electrons

autotroph

organism that makes its own food to grow and does not have to feed off of other organisms

photoautotroph

uses light energy to synthesize organic molecules

• excites electrons causing environment to fuel organic molecules

mixotrophic

can go through autotrophic pathways but also consumes other organisms like heterotrophs

chemotrophic

uses heat energy to generate organic molecules

what organisms are capable of photosynthesis (eukaryotes and prokaryotes)

Eukaryotes

• plants

• multicellular algae

• unicellular eukaryptes

Prokaryotes

• cyanobacteria

• purple sulfur bacteria

heterotrophs

organisms which must feed off of other living things

• animals consume plants and/or other animals

• decomposers feed off of dead organisms or organic litter

  • ex. fungi and most prokaryotes

• rely on the oxygen produced as a by-product of photosynthesis from autotrophs in order to breathe

photosynthesis vs. cellular respiration

simplified equation for photosynthesis is the reverse of the equation for cellular respiration BUT they are not the direct reverse reaction

leaf structure

has chloropalsts, veins, and stomata

mesophyll

interior tissue layer of the leaf composed of photosynthetic cells

• each mesophyll cell contains an average 30-40 chloroplasts

stomata

opens and closes which allows CO2 to enter and O2 to exit the leaf

chloroplasts (+ chlorophyll)

• double membrane bound organelles

• filled with a dense fluid called stroma

• in the stroma, there are a series of membrane bound sacs called thylakoids

• chlorophyll - green pigment which absorbs light energy and is found in the thylakoid membranes

overview of photosynthesis

• an anabolic pathway which creates complex sugars from simple building blocks

• requires the input of energy by utilizing energy

• made up of two distinct parts:

• Light reactions

• Calvin cycle (AKA dark reactions/light-independent reactions)

steps of light reactions

1. Light is absorbed by chlorophyll in the thylakoid membranes

2. Light (electromagnetic energy) interacts with water which splits to produce electrons, protons, and oxygen (leaves the cell through stomata or goes to chloroplasts for cellular respiration)

3. electrons are temporarily stored in NADP+

4. solar energy from light is used to reduce the electron carrier (NADP+ → NADPH)

5. chemiosmosis results in phosphorylation by using light energy to phosphorylate ADP → ATP

• products created in the light reactions (NADPH and ATP) power the calvin cycle

steps of calvin cycle

6. carbon fixation occurs to attach CO2 (from environment) into organic compounds

7. Uses NADPH and ATP to help reduce the fixed carbon into carbohydrates

8. simple sugars can be used as fuel in respiration or in any other synthesis reactions

• NADP+ and ADP are recycled back into light reactions

The Light Reactions

AKA Hill reactions/light-dependent reactions

• utilizes a series of protein systems bound to the thylakoid membrane

• uses light energy to split water and create chemical energy (NADPH and ATP)

Photosynthetic Pigments

• substance which absorbs visible light

• different pigments absorb different wavelengths of light

Types of photosynthetic pigments and their functions

Chlorophyll a - main light capturing pigment

• absorbs violet-blue and red light

• reflects green light which is why it looks green

Chlorophyll b - accessory pigment

• absorbs blue and yellow-orange light

• reflects green which is why it looks green

Carotenoids - accessory pigments which are photoprotective

• absorbs violet and blue-green light

• reflects orange, yellow, red colour

• pulls and absorbs light away from vulnerable tissues to protect them for UV

What’s special about carotenoids

they pull and absorb light away from vulnerable tissues to protect them from UV

Melanin

another pigment which makes up the brown colour of things

excitation of electrons

• when a pigment absorbs light, one of the pigment’s electrons are excited to a higher electron shell

• in an isolated state, this potential energy is transformed into heat and released as photons of light, as the electron falls back down to a lower shell

Photosystems

• Complex of pigments, small organic molecules, and proteins.

• Composed of a reaction-center complex (which houses chlorophyll a) surrounded by light harvesting complexes

what do light-harvesting complexes contain

• Light harvesting-complexes contain variety of pigments (chlorophyll a and b, and carotenoids)

  • broadens area and spectrum of absorption

2 types of photosystems

• Photosystem 2 (P680)

• Photosystem 1 (P700)

Number represents the wavelength of light they absorb at the highest frequency

how is light moved around in photosystems

they are hot potatoed between pigment molecules

Reaction-center Complex

Allows the conversion of light energy to potential energy to chemical energy

• done by transferring the electron to the primary electron acceptor

• loss of energy as heat or light is prevented

steps of reaction-center complex

1. photon of light is absorbed by a pigment in the light-harvesting complexes

2. energy is eventually transferred to special pair of chlorophyll a in the reaction-center complex

3. The special pair of chlorophyll a molecules transfer the excited electron to the primary electron acceptor

how do the light reactions use Photosystems 1 and 2

• the light reactions utilize two photosystems in order to provide the energy required for carbon fixation in the Calvin Cycle

• Each photosystem is followed by an ETC allow for the production of ATP in PS 2 and NADPH in PS 1 through the use of linear electron flow

Steps of Linear electron flow

1. photon stimulates an electron from pigment in light-harvesting complex of photosystem 2 into a higher energy electron shell

• energy continues to be passed on as it excites and passes electrons then goes back to the ground state

• process is repeated until P680 pair reaches the reaction-center complex

2. electron is transferred from P680 to primary electron acceptor

• P680 → P680+

3. Enzyme catalyzes splitting of water into electrons and photons

• electrons are passed on to the P680 pair

• protons are released into the thylakoid space

• oxygen is released but leaves the cell as a by-product

4. Electrons are transferred from the primary electron acceptor through an ETC

• electrons are transferred from ETC to PS 1

• flow is always PS 2 → PS 1

5. Energy from the ETC is used by cytochrome complex to pump protons into the thylakoid space

• generates an electrochemical gradient used by chemiosmosis and ATP production

6. Pigments in light-harvesting complexes energy of PS 1 are stimulated

• electrons are passed around through pigments to P700 pair in reaction-center complex and then passed onto he primary acceptor

• P700+ accepts electrons from ETC

7. electrons are passed from primary acceptor of photosystem 1 into another ETC

8. NADP+ reductase catalyzes the production of NADPH using electrons from the ETC

• NADPH can now be used in the Calvin Cycle to power carbon fixation

Chemiosmosis and ATP production (how it is connected to this chapter)

• protons are pumped into the thylakoid space from the cytochrome complex of ETC following PS2

• diffusion through ATP synthase drives ATP production

Light reactions summary

1. Light activates PS2, H2O is split, and electrons are fed into the ETC, producing O2 and H+

2. ETC pumps H+ into thylakoid space and moves electrons to PS1

3. proton gradient allows for chemiosmosis and ATP synthesis

4. light stimulates PS 1 and electrons are passed from the 1st ETC to the 2nd ETC

5. NADP+ reductase catalyzes production of NADPH

6. ATP and NADPH generated in the light reactions are used to fuel the calvin cycle

Calvin Cycle

• occurs in the stroma of the chloroplast or cytoplast in unicellular organisms

• 3 phases:

  • carbon fixation

  • reduction

  • regeneration

Glyceraldehyde-3-phosphate (G3P)

• simple sugar produced is G3P, not glucose

• produced in energy investment phase of glycolysis

• 1 G3P glucose = 3 turns of the Calvin Cycle

Steps of the calvin cycle more in depth

1. Carbon fixation

• rubisco catalyzes the conversion of ribulose biphosphate (RuBP) to an unstable 6-carbon molecule by adding CO2

• this molecule immediately splits into 2 molecules of 3-phosphoglycerate

2. Reduction

• Phosphorylation of 3 phosphoglycerate by ATP

• Reduction by NADPH and loss of a phosphate group which produces G3P

• 1 of the 6 G3P molecules produced exits the cycle

• for every 3 turns of the cycle, there is a net output of 1 G3P

3. Regeneration

• regeneration of ribulose biphosphate (RuBP) is necessary in order to restart the cycle

• 5 molecules of G3P are turned into 3 molecules of RuBP through a series of enzymatic reactions

  • reactions require the input of 3 molecules of ATP

How many ADP and NADP+ molecules are produced per cycle of the Calvin Cycle

1 glucose = 2 G3P = 6 turns of the calvin cycle

(3 ADP and 2 NADPH) x 2

Photorespiration

• biochemical pathway which uses some elements of photosynthesis and cellular respiration

• occurs when availability of CO2 is low due to hot and dry climates

• instead of binding CO2, rubisco binds O2

outcomes of photorespiration

• production of CO2

• use of ATP

• less energy efficient

alternative modes of carbon fixation

C4 Plants

• CO2 is fixed into a 4-Carbon compound

• physical compartmentalization for separation into steps

CAM plants

• open stomata during the night and close them during the day

• store intermediate organic acids in vacuoles

• no physical compartmentalization

• temporal separation of steps

what percent of organic materials stored created through photosynthesis is used as fuel in cellular respiration of the organism

50%

how is excess organic materials stored in plants and where

stored as starch

locations stored in:

• seeds

• tubers

• fruits

G3P structure (draw it)

What does photosynthesis create

oxygen in our atmosphere and 150 billion metric tons of carbohydrates each year