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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)