Chapter 6 // Pt1: photosynthesis overview 

Chapter 6

Intro to Photosynthesis

The Sun dictates all energy on Earth. .00000005% of the sun’s energy reaches Earth, 1% of that energy is used for photosynthesis.

• All life is sustained by energy but not all energy sustains life
• Sunlight energy must be converted to chemical energy before it can be used.

Sun’s Energy Flows Through the Ecosystem

• All organisms need Carbon to build molecules of life
• Autotrophs get the carbon they need from inorganic molecules - ex. CO₂
• Autotrophs use photosynthesis to harvest the sun’s energy
• Photosynthesis is a metabolic pathway

Photosynthesis Products - Sugar

1. Stored as polysaccharides for later
2. Remodeled into other compounds
3. Broken down to release energy held in bonds

Photosynthesis Products - Oxygen

• ~30% comes from terrestrial plants
• ~70% comes from marine plants, algae, and bacteria

Two Types of Autotrophy

Photoautotrophs

Get their energy from the sun (photosynthesis)

Chemoautotrophs

Get their energy from other inorganic compounds Ex: bacteria and archaea; use nitrogen, iron, and sulfur; found around hydrothermal vents and in soil

Photosynthesis feeds most life on Earth

• Consumers

• Get carbon by breaking down organic molecules assembled by other organisms

• Get carbon from organic molecules assembled by photosynthesizers

  

Photosynthesis is a metabolic pathway with many reactions in 2 stages.

First stage needs light to run

Light Dependent Reactions

• Conversion of light energy to chemical energy

Two Main Goals:

• Make ATP (chemical energy)
• Splits H₂O to release oxygen
• Hydrogen ions and electrons from broken down H₂O added to and NADP+
• Forms NADPH
• Powers second stage of reactions

Second stage doesn’t need light to run

Light Independent Reactions

• Runs on ATP and NADPH from first stage

Main Goal:

• Build sugars from CO₂ and water
• AKA Calvin Cycle

Photosynthesis is a cyclic metabolic pathway

• After light independent reactions run using NADPH and ATP
• Back to NADP+ and ADP
• Recycled to start light dependent reactions again

Chloroplasts

• Eukaryotic photosynthetic organisms have chloroplasts
• A chloroplast is a plastid that contains chlorophyll and in which photosynthesis takes place
• Plants have many chloroplasts per cell
• Plants and algae both have chloroplasts

Chloroplast anatomy

• Thylakoids - carry out light dependent reactions; membranous disks inside chloroplasts

• Stroma- carry out light independent actions; fluid inside chloroplast, suspends thylakoids

  

Photosynthesis in Prokaryotes

• Cyanobacteria don’t have chloroplasts
• But they do have thylakoids
• Light independent reactions are carried out in the cytoplasm

Photosynthesis: Sunlight as Energy

Energy Flows Through Ecosystem

• Trophic Levels

• Autotroph make own food/energy

  • AKA producers

  sun’s Energy

• Photoautotrophs make own food using sun’s energy thru photosynthesis

• Photosynthesis is cyclic metabolic pathway (many reactions)

• Occurs in two stages

First Stage:

Light-Dependent Reactions

Convert sunlight energy to chemical energy

Powered by sunlight

Maked products to power second stage

• ATP and NADPH

Oxygen is accidental byproduct

• Their trash is our treasure

Second Stage:

Light-Independent Reactions

Not powered by light

Powered by ATP and NADPH

Builds sugar

Calvin Cycle

Structures for Photosynthesis

• Prokaryotes
  • Thylakoids and cytoplasm (cyanobacteria)
• Eukaryotes
  • Chloroplasts, thylakoids, and stroma (plants and algae)

What is Sunlight and How does it Work?

Theodor Engelman

• Discovered sunlight is driver for photosynthesis
• Hypothesized color of light affects photosynthesis
• Tested effects of light on photosynthesis by measuring oxygen levels

Spectrum of light

• Used spectrum of light on green algae in water
• Different colors of light used in different parts of water
• Used oxygen-requiring bacteria to measure oxygen levels from algae

Engelman’s Results

• Bacteria clustered in blue and red areas with algae
• Photosynthetic cells illuminated by different colors released more or less oxygen
• Blue and red are best for driving photosynthesis
• Found to be true to this day with actual oxygen-detecting equipment

Why Blue and Red?

Light

• Electromagnetic reaction
• Type of energy
• Moves like waves in an ocean = wavelength
  • Short wavelength = short distance between crests, high frequency
  • Long wavelength = long distance between crests, low frequency

Spectrum of Electromagnetic Radiation

• Tiny portion is visible to human eye

• 380-750 nm

  • AKA visible light
  • Humans can’t see most of the spectrum

  

Colors

• Different wavelengths = different colors
  • Violet has shortest wavelength
  • Red has longest wavelength

Prisms

• Light separates when passed through a prism
• Shows all colors of visible light
• Rainbows act as a prism = rainbow in sky

Light is a wave and a particle

• Light acts as wavelength and photon
• Photon is a particle of light
  • Packet of energy

Energy is inverse (opposite) to wavelength

• Photons with lots of energy = shorter wavelengths
• Photons with little energy = longer wavelengths

Pigments

• Pigment is organic molecule that selectively absorbs light of specific wavelengths
  • ‘Antenna’ for light
• Unabsorbed wavelengths give us color
  • Plants don’t absorb green wavelengths = appears green
  • Black absorbs all colors, reflecting nothing
  • White reflects all colors, absorbing nothing

Chlorophyll

• Chlorophyll a Reflects green, making plants green
• Accessory pigments works alongside chlorophyll a
• Orange carrots have beta-carotene
• Red roses and blue violet have anthocyanin

Photosynthesizers use pigments based on environment

• Deep seawater reflects green and blue-green light
• Deep sea algae evolved to absorb that blue and blue-green light
• Instead, colors like red algae

Leaves changing in Fall

• Chlorophyll (green) masks other pigments in plants
• Plants preparing for fall by conserving nutrients - move chlorophyll away from leaves first
• Leaving yellow and orange pigments to be moved later
• Some instead make anthocyanin - red and purple leaves
• Then die in winter

Plants easily absorb blue and red light - reflects green light

• Chlorophyll in chloroplasts absorbs red/blue
• Reflects green
• Makes plants green

Why are fruits and flowers not green?

• Attract pollinators
• Tell differences from leaves and flowers (pollen)