Timed Responses: Rhythms of Life
The 2 main foci of this topic is about how the response occurs...
1. What are the processes involved?
2. How does this kind of a response give the organism an ecological/adaptive advantage over other organisms?
1. What are the processes involved?
2. How does this kind of a response give the organism an ecological/adaptive advantage over other organisms?
Some key notes to read:
Timed responses: responses to cycles and seasons: 4 main cycles
- Day/night: Circadian Rhythms. (Earth spins)
- Tides: Tidal Rhythms (Moon/sun gravity pull and earth spin)
- Monthly or lunar rhythms (Moon orbit of earth)
- Annual or seasonal Rhythms. (earth orbit of sun)
Rhythms may be either endogenous or exogenous.
Exogenous rhythms: Rhythms which are controlled by the external environment. Eg the way a plant germinates when there is water etc. These alter according to a “cue” from the environment.
Endogenous rhythms: Rhythms which are under internal control. Most living organisms have an internal clock that allows them to respond to these cycles. The internal clocks are “endogenous” – they are genetically inherited. Most are reasonably accurate, but do need to be synchronised daily usually using light. These rhythms repeat when the organisms is kept in constant conditions eg in a lab with no light, temp change etc.
Circadian Rhythms: “Around a day”; Repeated each day:
3 main kinds of circadian rhythms:
- Diurnal: active during the daylight hours
- Nocturnal: active during the night. (Dark)
- Crepuscular: Has 2 periods of activity in 24 hours: usually active at dusk and at dawn.
“Photoperiod”: The time of the light. - How long the day is. Organisms use photoperiod to synchronise their internal clock to the actual day length.
Actogram: a graph which plots activity of an organism over time to show rhythms.
“Zeitgebers”: (Time keepers) also known as environmental cues: These are abiotic factors which tell the organism what is happening interms of the earth cycles. The main zeitgebers are photoperiod (Daylight hours) and temperature.
To investigate rhythms the zeitgebers are kept in constant conditions. If the rhythm continues in lab conditions when the temperature and light are kept constant the rhythm is proved to be endogenous.
When the rhythm continues in constant conditions it is called “free-running”, because it is not being controlled by the environment.
The rhythm will run according to the internal endogenous clock and is no longer synchronised by the zeitgebers. Most endogenous clocks are a little inaccurate, so over a few days the rhythm will drift out of sequence with the actual day length and show “Phase shift”.
Entrainment: If an organism is kept in controlled conditions and it’s light regime is manipulated, it is able to be trained to commence it’s circadian rhythm at a different time of the day. Note the rhythm will still have the same period…. repeating pattern, but it will start at the time set by the experimenter. This is how organisms adjust to different time zones.
Evidence of Endogenous Rhythms/ presence of an endogenous clock:
A rhythm is endogenous if it is continued/repeated in constant conditions.
The rhythm will revert to its Free Running period in constant conditions
The rhythm may be able to be entrained to a different time, but it will still continue
The Clip below is an explanation of how to read an actogram. The narrator shows you exactly how to do this quite well and is a new Zealand NCEA example,
For a bit of light relief try the game below:
Plant clocks
A 3 minute example of the advantage of rhythms in plants... very interesting research:
Below is a quick 5 minute clip from TEDED on rhythms in plants. It is quite a good summary to view.
Information in this section is mainly for wide reading and extension information.
Advanced information made by Auckland LENS Science programm
Extension information:
The latest on how plants actually use sugars to tell time: the control of their Circadian Rhythms. This is a short article from Science Daily written in 2013.
image (and full scientific article) from
http://journal.frontiersin.org/article/10.3389/fpls.2014.00564/full
http://journal.frontiersin.org/article/10.3389/fpls.2014.00564/full