Topic 6: Immunity, Infection and Forensics Flashcards
6.1 How can you determine the time of death of a mammal by examining the extent of decomposition?
After death, tissues in the body begin to break down due to the action of enzymes and bacteria.
Within a few hours to a few days: Autolysis occurs first - the body’s own enzymes (from lysosomes) break down the cells. Bacteria (from both within the body and the external environment) invade the issues after death, releasing enzymes that result in decomposition. There is a greenish discolouration of the skin (which will later spread across the body, darkening to a reddish-green and then a purple-black colour).
Within a few days to a few weeks: Due to the action of bacteria, gases (such as CH4, CO2, NH3, H2) are produced. The body becomes bloated, and the skin begins to blister and fall off.
Within a few weeks: As the tissues further decompose, the gas is released and the body deflates. Tissues begin to liquefy and shrink, seeping out into the area around the body.
Different conditions affect the rate of decomposition, e.g temperature (affecting enzyme action, and the rate of reaction), oxygen availability (affecting aerobic microorganisms, and the rate of respiration) and injuries to the body (allowing the entry of bacteria that aid decomposition).
6.1 How can you determine the time of death of a mammal from forensic entomology?
A dead body is quickly colonised by a variety of insects - the study of this is forensic entomology. The time of death can be estimated by identifying the type of insect present on the body and the stage of life cycle the insect is in. Different conditions will affect an insect’s life cycle, such as drugs (.e.g. some accelerate development), humidity, oxygen and temperature (e.g. the higher the temperature, the faster the metabolic rate and the shorter the life cycle).
6.1 How can you determine the time of death of a mammal by examining the stages of succession?
The type of organism found in a body changes over time, going through a number of stages - this is known as succession. As a group of organisms feed on a decomposing body, conditions change and become favourable to another group of organisms. As the corpse continues to change, it attracts different organisms that feed on it until only the skeleton remains. This predictable sequence of organisms can be used to determine the time of death.
The length of each stage in the succession depends on the condition of the body, as well as environmental factors (e.g. location). While similar to plant succession, most of the early insects remain on the body while other insects colonise it, until the advanced stage of decay (in plants, many of the early species are replaced as conditions change).
6.1 How can you determine the time of death of a mammal by examining body temperature?
All mammals produce heat from metabolic reactions (e.g. a human’s internal temperature should be around 37). In a dead body, the metabolic reactions slow down and eventually stop, resulting in the body temperature decreasing until it equals the temperature of the surroundings (following a sigmoid curve).
Forensic scientists know the cooling rate of a human body, and with the temperature of a dead body, they can work out the approximate time of death . Conditions such as air temperature, clothing and body weight can affect the cooling rate of the body e.g. the cooling rate of a clothed body will be slower than one without the clothing, because it’s insulated, or if the body is immersed in water it will cool much more rapidly, as water is a better conductor of heat than air.
6.1 How can you determine the time of death of a mammal by examining the degree of muscle contraction?
After death, the muscles in a dead body usually relax, and then start to contract and become stiff 4-6 hours later - this is known as rigor mortis (and it wears off 24-36 hours after death).
1. Muscle cells become deprived of oxygen, and oxygen-dependent reactions stop.
2. Anaerobic respiration takes place, producing lactic acid.
3. The pH of the cells decrease, inhibiting enzymes and thus inhibiting anaerobic respiration. ATP is no longer produced.
4. No ATP means that bonds between the myosin and the actin in muscle cells become fixed, and the body stiffens.
Smaller muscles in the head contract first, with larger muscles in the body being the last to contract.
Rigor mortis is affected by degree of muscle development and temperature (e.g. it occurs faster at higher temperatures because the metabolic reactions in the body are faster).
6.2 Explain the role of microorganism in the carbon cycle.
Microorganisms (such as bacteria and fungi), both within a dead body and from the external environment, secrete enzymes to decompose the dead organic matter. Small molecules are produced and used in aerobic and anaerobic respiration. Carbon dioxide (and methane) are as a byproducts of respiration - this recycles the carbon back into the atmosphere.
