Lecture 5-6 Flashcards
What are the interactions of light with matter?
Emission, reflection and refraction
Absorption, scattering and transmittance
What is Beer-Lambert’s Law?
Describes amount to which scattering, transmission and absorption occur based on material light is interacting with.
Transmittance = I / I0
I = intensities
Example: absorbance = log10 (I0/I) = epsilon * l * c
l = length of solution light passes through
c = concentration of solution
epsilon = absorptivity of material
What kind of scattering is sub-10 microns?
Elastic scattering – Mie or Rayleigh
Raman also happens but is less important
What kind of tissue scatters the most amount of light in biological systems?
Lipids
What are absorbance and scattering defined by?
Cross-sectional area of material that is interacting with light
Scattering coefficient = density * effective cross section
Absorption coefficient = density * effective cross section
What is transmittance?
Measure of how much light from light source is able to pass through material to the other side:
How much light is NOT absorbed
How much light is NOT scattered
What do scattering and absorption cause?
Exponential decay of transmittance or ratio of light being able to pass through object
T = exp(-usL)
T = exp(-uaL)
What is the mean free path?
Average distance over which a moving particle travels before changing its direction or energy as a result of collisions with other particles.
Greater mean free path = greater penetration = deeper imaging
What does the mean free path of a photon travelling through tissue depend on?
- Refractive index variations
- Size of scatterer (mie or rayleigh)
- Wavelength (longer wavelength = greater mean free path)
What is the Earth’s Energy Budget?
Describes the balance between radiant energy that reaches Earth from the sun and energy that flows from Earth back out to space.
Energy from the sun is captured by photosynthesis
What is photosynthesis?
Converts carbon dioxide from air and water from soil into sugars (used to grow) and oxygen that is released back into the air.
Light is needed as an energy source for this reaction to occur.
Key differences between photosynthesis and cellular respiration?
Photosynthesis:
Reactants = H20 and CO2
Products = sugars and O2
- Captures energy from sunlight to produce sugars
- Occurs only in plant cells and autotrophs
- Occurs in chloroplasts
Cellular respiration:
Reactants = sugars and O2
Products = H2O, CO2, and energy
- Captures energy from sugars and releases energy
- Occurs in all eukaryotes, autotrophs and heterotrophs
- Occurs in cytoplasm and mitochondria
What is the light-dependent reaction?
Occurs in thylakoid membrane in the stroma of chloroplast
Electron transport chain produces: ATP, NADPH and O2
- Photoexcitation of PSI and PSII
Chlorophyll molecules (on each photosystem) have different absorbance profiles = antennae complex
Chlorophyll molecules excited by light at the peripheries transfer excited electron to reaction center which then goes to the ETC
Reaction center of PSII: P680
Reaction center of PSI: P700
- Electron transfer chain (ETC)
Excited electron from PSII is passed down ETC - reducing molecules of lower energy (PSII and PSI)
Electron is re-excited by light at PSI + passed down ETC + reduced from NADP+ to NADPH
Photolysis = water is split into hydrogen (adding to H+ gradient) and O2 product
- Electron used to replenish PSI and PSII
- Chemiosmosis & ATP synthase
Membrane proteins use energy dissipated by ETC to pump hydrogen ions into thylakoid lumen and create H+ gradient
H+ ions pass through membrane protein ATP synthase and drive reduction of ADP to ATP
Light-dependent reaction: Calvin-Benson Cycle
Create energy (ATP and NADPH) that fuels the cycle allowing organism to produce sugars from CO2
