Optical Absorption in Biological Tissue
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Questions and Answers

What defines the specific wavelengths of light that a chromophore can absorb?

The specific wavelengths are defined by the molecular or atomic structure of the chromophore.

Explain the relationship between kinetic energy and internal energy in the context of light absorption by biological tissue.

When light is absorbed by biological tissue, the kinetic energy of the photons is converted into the internal energy of the molecules.

What types of energy can a molecule store, and how do these relate to its absorption capabilities?

A molecule can store rotational kinetic energy, vibrational energy, translational kinetic energy, and electronic potential energy; these energy levels determine its absorption spectrum.

Why can't molecules store arbitrary amounts of energy, and what is the consequence of this limitation?

<p>Molecules can only store fixed amounts of energy due to quantization; this limits the range of light wavelengths they can absorb.</p> Signup and view all the answers

Describe how Boltzmann’s Distribution relates to the concept of thermal equilibrium in molecular energy levels.

<p>Boltzmann’s Distribution describes the probability of a molecule occupying various energy levels in thermal equilibrium based on their energy differences and temperature.</p> Signup and view all the answers

What is the sum of all types of molecular energy called?

<p>Internal energy.</p> Signup and view all the answers

How is the kinetic part of internal energy related to temperature?

<p>It is associated with thermal motion.</p> Signup and view all the answers

Which type of molecular energy allows for rotational motion?

<p>Rotational kinetic energy.</p> Signup and view all the answers

What type of energy is required to excite vibrational modes in molecules?

<p>More energy than required for rotational modes.</p> Signup and view all the answers

What do electronic energy levels refer to in terms of molecular structure?

<p>The potential energy of electrons relative to their distance from the nucleus.</p> Signup and view all the answers

Which type of light is necessary to excite valence electrons in molecules?

<p>Blue or ultraviolet (UV) light.</p> Signup and view all the answers

Rank the types of molecular energy transitions from lowest to highest energy required.

<p>Rotational &lt; Vibrational &lt; Electronic.</p> Signup and view all the answers

What causes absorption lines in the emission spectrum of a gas, and why are they not infinitely thin?

<p>Absorption lines are caused by gases absorbing light at specific frequencies corresponding to their emission spectrum. They are not infinitely thin due to spectral broadening, where absorption occurs at frequencies slightly different from the resonance frequency.</p> Signup and view all the answers

Describe the line shape function and its significance in spectral absorption.

<p>The line shape function describes the absorption spectrum near the resonance frequency, peaking at that frequency and remaining non-zero at nearby frequencies. Its significance lies in showing that absorption is not confined to a single line but spans a small range of wavelengths.</p> Signup and view all the answers

In biological tissue, how do absorption spectra differ from those of gases?

<p>Absorption spectra in biological tissue are much more smoothly varying and continuous due to the complex structure of biomolecules and their numerous vibrational energy levels. This contrasts with the discrete peaks typical of gases.</p> Signup and view all the answers

Explain why biological tissue is often not homogeneous and its impact on absorption spectra.

<p>Biological tissue is rarely homogeneous because it contains many different types of molecules, each contributing to the absorption spectrum. This heterogeneity results in a complex and varied absorption profile rather than a simple spectrum.</p> Signup and view all the answers

What are the implications of spectral broadening for the study of gas absorption spectra?

<p>Spectral broadening implies that gas absorption spectra are not strictly defined and occur over a range of wavelengths, complicating precise measurements. It affects the interpretation of absorption data, necessitating consideration of nearby frequencies.</p> Signup and view all the answers

What is the significance of photon absorption in molecules?

<p>Photon absorption excites electrons in a molecule, making it more likely to react with neighboring molecules.</p> Signup and view all the answers

Define resonance absorption in the context of molecular interactions.

<p>Resonance absorption occurs when the frequency of light matches the natural frequency of a vibrating system, facilitating energy transfer.</p> Signup and view all the answers

How quickly does light absorption occur in molecules?

<p>Light absorption occurs very quickly, typically on the timescale of femtoseconds.</p> Signup and view all the answers

What determines the likelihood of a molecule absorbing light?

<p>The likelihood of absorption depends on the photon energy matching the energy difference between the current and excited state of the molecule.</p> Signup and view all the answers

Discuss the relationship between absorption and emission spectra in gases.

<p>The emission spectrum of a gas contains narrow peaks at specific frequencies, which correspond to the frequencies at which the gas can absorb light.</p> Signup and view all the answers

What is meant by the term 'natural frequencies' in vibrating systems?

<p>Natural frequencies are the frequencies at which a system naturally vibrates, allowing for optimal energy coupling.</p> Signup and view all the answers

Why do gas absorption spectra not have infinitely thin spectral lines?

<p>Absorption spectra lines have finite widths, which means absorption can occur at wavelengths slightly different from the resonance frequency.</p> Signup and view all the answers

Explain the concept of absorption spectra and line broadening.

<p>Absorption spectra display distinct lines where light is absorbed, while line broadening refers to the finite width of these lines due to various factors.</p> Signup and view all the answers

What role do chromophores play in optical absorption?

<p>Chromophores are parts of molecules that absorb light at specific wavelengths, often corresponding to their natural frequencies.</p> Signup and view all the answers

Describe the importance of the near-infrared window in optical absorption.

<p>The near-infrared window is important because it allows for deeper tissue penetration and less scattering in biological samples.</p> Signup and view all the answers

Study Notes

Optical Absorption

  • Biological tissue is composed of molecules such as water, proteins, lipids, DNA, etc.
  • When light interacts with tissue, it's absorbed by specific molecules called chromophores.
  • Different chromophores have different absorption spectra, meaning they absorb different wavelengths of light with varying strengths.
  • These differences in absorption spectra are due to differences in the molecular or atomic structure of the chromophores.
  • Optical absorption in tissue is important. In therapeutic applications of tissue optics, light absorption leads to changes in the internal energy of the molecule, which can cause thermal or chemical effects.

Molecular Energy Levels

  • Energy exists in many forms.
  • Two primary forms are kinetic energy (energy of motion) and potential energy (energy of position).
  • Molecules can only hold specific amounts of energy, called energy levels.
  • Internal energy is the sum of all types of molecular energies (rotational, vibrational, translational, and electronic).
  • The kinetic part of internal energy is related to thermal motion and temperature.

Rotational Kinetic Energy

  • Molecules in a system that are not bound to each other can rotate and store energy in the form of rotational motion.
  • Two parts of a molecule, held together by a bond that allows them to rotate, can store rotational energy
  • Rotational modes are easily excited with low energy, leading to absorption in the microwave region of the EM spectrum.

Vibrational Energy

  • Different parts of molecules can also vibrate relative to each other.
  • The bonds holding the atoms together are not perfectly rigid, allowing for vibrations.
  • Vibrational modes require more energy to excite than rotational modes, which results in absorption in the near-infrared or red part of the visible spectrum.

Electronic Energy Levels

  • Electronic energy levels relate to the potential energy of electrons relative to the nucleus.
  • These energy transitions typically require higher energies (Ultraviolet or X-ray) than vibrational or rotational ones, and are associated with the highest energy changes.
  • Excitation of electrons can lead to chemical reactions.

Internal Energy and Absorption Spectra

  • The structure of the energy levels of a molecule determines its absorption spectrum.
  • The total absorption spectra is often a combination of many spectra of the different molecules present in biological tissue.

Light Absorption / Resonance Absorption

  • Light absorption is typical in vibrating systems when the driving frequency (of the light) matches the natural frequency.
  • This condition is called resonance.
  • The likelihood of absorption is influenced by the absorption cross section. The absorption cross-section is the ratio of the power absorbed by the molecule to the incident power per unit area.

Absorption Spectra

  • Absorption spectra in complex biological matter, like tissue, exhibit smooth variations rather than distinct peaks.
  • This is due to combinations of multiple vibrational levels transitions between the various chromophores, and broadening across multiple vibrational states.
  • The measured absorption spectrum of a complex material, like tissue, is a blend of the constituent molecules present. Substances like water, hemoglobin, melanin, and lipids all have distinct absorption ranges.

Near Infrared Window

  • The wavelength range between 650-1300 nm is known as the near-infrared window.
  • Light in this range penetrates tissue more deeply than light at shorter wavelengths due to lower absorption of endogenous chromophores.

Absorption Coefficient

  • The absorption coefficient describes the likelihood of a transition from one energy state to another due to photon absorption.
  • It is often expressed as a molar absorption coefficient which represents the absorption by a mole of molecules.
  • The total absorption coefficient of a material is the sum of the molar absorption coefficients for each of its components weighted by the concentrations of those components.
  • Nonlinear effects like multiphoton absorption, ground state depletion and saturation and bleaching are further factors in energy absorption and transfer in biological substances.

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Description

Explore how light interacts with biological tissues through optical absorption. This quiz covers the concept of chromophores, their unique absorption spectra, and the implications of these interactions in therapeutic applications. Understand the role of energy levels in molecules as they relate to therapeutic effects.

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