radiation

radiation

the transmission of energy as electromagnetic waves or as moving subatomic particles

radiation is broadly divided into two types

ionising and non-ionising

non-ionising radiation

• Long wavelength radiation (low frequency)

• May have enough energy to excite molecules

• But does not have enough energy to break chemical bonds

• e.g. radio waves, microwaves, infrared radiation, visible light and some UV

 

ionising radiation

• Short wavelength radiation (high frequency)

• Sufficient energy to break chemical bonds

• e.g. X-rays, gamma rays, some UV

radiowaves

• Longest wavelengths and lowest frequency

• Most commonly artificially generated by a transmitter

• Used for communication over long distances

• Exposure is not harmful

effects of radiowaves

• Can cause the oscillation of polar molecules

• Can generate heat

• Can be used for muscle relaxation as radio waves can penetrate tissue

• Can kill cancer cells

microwaves

• Shorter wavelengths and higher frequency compared with radio waves

• Most commonly artificially generated by a transmitter

• Used for communication

• Used for cooking food as waves are absorbed by water

• Can generate heat in tissues

effects of microwaves

• Microwaves can denature proteins and amino acids

• Denaturing is the process by which proteins lose their normal folded structure via an external stress

infrared radiation

• Generated by anything that produces heat

• Main source of electromagnetic radiation from the sun

• Widely produced and used by humans

effects of infrared radiation

• Causes heating which can cause burns

• Can also denature proteins

visible light

• Light humans can see between 400-790 THz

• Can damage the eye if bright and prolonged enough

• Damage can be due to heating

photochemical toxicity

• Bright light in the blue (and ultraviolet) spectrum can damage the retinal pigment epithelium and photoreceptor cells

• Through a photochemical cascade involving oxidative stress and free radical generation

 

1. High energy photons absorbed by chromophores (molecules that absorb light) in the retinal pigment epithelium

2. This energy excites the molecules which transfer energy to O2, forming reactive oxygen species

3. Reactive oxygen species overwhelm antioxidant defenses

4. Leads to death of retinal pigment epithelial cells and loss of vision

ultraviolet radiation

• Invisible radiation produced naturally by the sun

• Can be produced artificially

• Contains both non-ionising and ionising wavelengths

• Non ionising → UVA and UVB → longer wavelengths

• Ionising → UVC → shorter wavelengths

• Physiological effects can be beneficial of harmful

beneficial effects of UV radiation

• UVB is required for humans to make vitamin D

• Usually just a few minutes of exposure is sufficient

vitamin D formation

• 7-dehydrocholesterol in the skin absorbs UVB from sunlight

• this energy cleaves the steroid structure (7-dehydrocholesterol) creating previtamin D → unstable and undergoes non-enzymatic thermal isomerisation in vitamin D3

• vitamin D3 diffuses into the blood, binds to vitamin D-binding protein for transport to the liver for further activation

harmful effects of UV radiation (UVA)

• All forms of UV can be harmful

• UVA exposure generates reactive oxygen species (ROS)

• ROS can cause DNA single strand breakage → reaction with the deoxyribose sugar in the DNA structure

harmful effects of UV radiation (UVB and UVC)

• UVB and UVC exposure can result in double strand DNA breakages

• Formation of dimers -> covalent bonds that form between adjacent pairs

• Most are promptly fixed by DNA repair mechanisms

• However an overwhelming number can lead to mutations and cancer

• UVB and UVC can directly damage mRNA

  Triggering an immune and inflammatory response

what is photokeratitis and what type of UV is it caused by?

sunburn of the eye → caused by UVC

ionising radiation

• Has sufficient energy to ionise a molecule (break chemical bonds)

• Ions created are generally unstable

• Ions want to become stable via radioactive decay → shedding of particles and energy

ionising radiation can have different types of (a) and varying degrees of (b)

(a) charge

(b) penetrance

alpha radiation

• Lowest end of ionising spectrum

• A particle consisting of two protons and two neutrons

• For example radium 226 (solid) becomes radon 222 (gas) → alpha radiation emitted from radium to form radon

• Rapidly lose energy in matter and can be stopped by a sheet of thick paper or human skin

• If inhaled or ingested can cause severe tissue damage and cancer

beta radiation

• Electrons shed from a radioactive particle

• Beta particles can travel through metres of air and through millimetres of skin or tissue

• Can be stopped by 1-3mm aluminium

• Sufficient intensity of beta-radiation can cause burns

• If inhaled or ingested can cause severe tissue damage (acute radiation syndrome) and cancer

gamma rays and x rays

• Photons emitted by the nucleus of some radionucleotides

• Often the 'daughter product' of alpha and beta decay

• Can easily pass into the human body and can damage bone marrow and internal organs

• Can be stopped by thick lead or concrete

x-rays

• High energy photons produced when electrons strike a target or when electron are rearranged within an atom

• Largely indistinguishable from gamma rays

• Called 'X-rays' when used for medical purposes

• Called 'gamma-rays' otherwise

• Able to penetrate many substances including biological tissue

neutron radiation

• Emission of neutrons

• Typically during nuclear fusion and nuclear fission

• Most dangerous and severe type of radiation

• High kinetic energy

• Readily passes through most substances

• Free neutrons are very unstable and decay into a proton + electron

• Can create radioactivity in bodily tissues by transforming nuclei to radionuclides

effects of ionising radiation can be divided into two types:

1. deterministic effects

2. stochastic effects

deterministic effects

• Reliably occur above a threshold dose

• Severity increases with increasing dose

• High doses of radiation delivered in a short period of time (e.g. atomic bombings, radiation therapy for cancer treatment)

• Effects are most pronounced on cells that rapidly divide

• e.g. skin, bone marrow, cancer cells

deterministic effects mechanisms (list 3)

1. Causes direct and indirect cellular damage

• Radiation causes DNA strand breaks, cross-linking, dimers → direct

• Radiation ionises water in tissues producing ROS which amplify damage → indirect

 

2. Cell death

• Apoptosis triggered in response to direct DNA damage

• Damaged cells try to replicate leading to cell death

• Radiation directly damages cell membranes causing lysis

 

3. Immune response

• Radiation triggers an inflammatory response

• Causes fever, tissue damage, etc

• Bone marrow fails to produce new lymphocytes in response to radiation

ionising radiation effects on bone marrow, GI tract, skin, and reproductive organs

stochastic effects

• occur with increasing probability as dose increases

• severity is independent of dose

• includes cancers

• the most common mechanism is via the production of double-strand breaks in DNA → most difficult to repair

biological repair of DNA due to ionising radiation (single stranded breaks)

• DNA polymerase uses complimentary strand to copy repair portion

• DNA ligase joins backbones

name the two genes that inhibit progression of the cell cycle when damaged DNA is detected

• cells can also detect DNA damage

• Rad9 gene detects DNA damage and stops cells from entering the M-phase (mitotic phase → DNA divided between daughter cells)

• p53 detects damaged DNA and stops cells from entering the S-phase

cancer risks of ionising radiation

summary

radiation → the emission of energy in the form of electromagnetic waves

non- ionising radiation → low wavelength and frequency

• can cause excitation of ions

• cannot break chemical bonds

• radiowaves, microwaves, infrared radiation, visible light, some UV

ionising radiation → high wavelength and frequency

• can break chemical bonds

• some UV, a-radiation, b-radiation, X-rays, gamma rays, neutron radiation

radiowaves

• artificially generating for communication

• generally does not cause harm

microwaves

• artificially generated for communication

• used for heating

• generally does not cause harm

infrared

• produced naturally by biological organisms

• generates heat

• can be artificially generated

• denatures proteins

•