Responses to External Environments

Essential Knowledge 2.C.2

Organisms Respond to Changes in Their External Environments

• Respond to changes through behavioral (group of organisms) and physiological mechanisms (within an organism)

• Examples include hibernation and migration in animals, nocturnal and diurnal activity, shivering and sweating in humans

Less Common Examples

• Phototropism and photoperiodism in plants

• Taxis and kinesis in animals

• Chemotaxis in bacteria

Phototropism in Plants

• Growth of a plant shoot toward or away from light

• Triggered by a class of plant hormones called auxins

• Auxins are produced in the cells located on the dark side of the shoot, stimulates stem elongation which bends the stem toward the light source

Phototropism in Plants

Phototropism in Plants

Taxis and Kinesis

• Movements observed in animals

• Taxis is a directional response to a stimulus

• Kinesis is a non-directional response to a stimulus (movement is random)

Phototaxis in Euglena

• Euglena are photosynthetic animal-like protists

• Move towards a light source – positive phototaxis

Kinesis in Cockroaches

• Cockroaches scatter when you turn on a light

• Do not run in a positive or negative direction, movements are random

Chemotaxis in Bacteria

• Occurs when an organism directs its movements according to certain chemicals in their environment

• Examples: Locate high concentration of food or avoid the source of a poison

Physiological Mechanisms in Animals

• Thermoregulation is the process by which animals maintain an internal temperature within a normal range

Importance of Body Temperature

• Body temperatures below or above an animal’s normal range can reduce the efficiency of cellular enzymes and other temperature sensitive reactions within an organism

• May have fatal results

Endotherms

• Examples include humans, other mammals and birds

• Body temp is warmed mostly by heat generated by metabolism (internal reactions)

• Body temp remains relatively stable despite large fluctuations in environmental temperature

Endotherms

Ectotherms

• Examples include amphibians, reptiles and many fish

• Body temp is warmed through heat gained from external sources

• Body temp changes with environmental temperature, use behavioral means to heat or cool the body

Ectotherms