survival and response

selection pressure

- the environmental force altering the frequency of alleles in a population
- plants and animals need to respond to their environment to increase their chances of survival, by obtaining nutrition and reproducing
- organisms that survive and reproduce have a greater chance of passing on their alleles
- there is always selection pressure that favours organisms with the most appropriate responses

mammalian hormones

- stimulate their target cells via the blood system
- specific to the tertiary structure of receptors on their target cells and produce responses that are usually slow, long-lasting and widespread

examples of responses

tropisms, taxis, kinesis, reflexes, homeostasis

response

an action due to a change in environment

stimulus

a detectable change in the internal or external environment of an organism that produces a response in an organism

describe how moving away from harm (e.g. moving away from extreme temperature) can aid an organism/species survival?

greater chance of surviving to reproduce and pass on advantageous alleles to next generation

sequence of events that lead from a stimulus to a response

stimulus → receptor → co-ordinator → effector → response

positive taxis

organism moves towards the stimulus

negative taxis

organism moves away from the stimulus

positive tropism

organism grows towards the stimulus

negative tropism

organism grows away from the stimulus

tropism

- a directional response

- growth response of part of the plant

- to a directional stimulus

IAA - indoleacetic acid

- an example of an auxin
- involved in phototropism and plant cell elongation
- produced in shoot and root tips
- transported by diffusing into shoot

phototropism and IAA

1. shoot tip cells produce IAA, which diffuses down all sides of shoot
2. light causes the movement of IAA away from light to the shaded side
3. greater concentration of IAA causes cells elongation
4. the shaded side elongates faster than the light side, so the shoot bends towards the light

gravitropism and IAA

1. cells in the tip of the root produce IAA
2. gravity influences the movement of IAA from the upper to lower side of the root
3. a greater concentration of IAA in roots leads to inhibition of root cell elongation
4. cells on the upper side elongate faster than the lower side, causing roots to bend towards the force of gravity

similarities between taxis and kinesis

• both a response to a stimulus

• both use movement to find a favourable environment

similarity between a taxis and a tropism

both directional response to a stimulus

difference between a taxis and a tropism

taxis whole organism moves and tropism a growth response

what are the two main types of simple response?

taxis and kinesis

taxis

a directional response to a stimulus

single celled algae response to light

- move towards light - positive phototaxis
- because they photosynthesise more so produce more glucose

some bacteria that need to absorb glucose response to glucose

- move towards areas where glucose is more concentrated - positive chemotaxis
- because they use glucose for respiration to release more energy for cellular processes

kinesis

- a non-directional response to a stimulus
- it doesn’t move towards or away from a stimulus but instead changes its: rate of movement and the number of turns an animal makes - this varies with the intensity of the stimulus

kinesis in favourable conditions

• slow movement

• lots of turns

kinesis in unfavourable conditions

• fast movement

• less turns

woodlice prefer damp conditions, survival advantages of woodlice moving away from dry conditions

• stops them drying out

• helps them avoid predators

receptors

- cells or proteins that detect stimuli
- are specific to one type of stimulus
- converts energy from stimulus → form that can be processed by an organism: e.g. nerve impulse

effector

cells, tissues, organs or organ systems that carry out a response to a stimulus

nerve impulse

- used to respond to stimuli by passing info from receptors → effectors
- specific to a target cell only because it releases a chemical messenger directly onto it, producing a response that is usually rapid, short-lived and localised

autonomic nervous system

controls involuntary subconscious activities of internal muscles and glands, i.e. reflexes

spinal cord

• a column of nervous tissue

• it runs along the back and lies inside the vertebral column for protection

how do organisms respond to stimuli?

- neurones are specialised cells that transmit nerve impulses
- 3 types: sensory, relay, motor neurones

how do reflexes increase humans’ survival chances?

reflexes:
- are rapid
- reduce the amount of damage to human body tissues
- do not have to be learned (which would be too costly)
- allow humans to escape predators
- enable homeostatic control

nervous system splits into

central NS and peripheral NS

sympathetic NS (fight or flight)

• stimulates effectors to speed up activity

• helps us cope with stressful situations

parasympathetic NS (rest and digest)

• inhibits effectors to slow down activity

• conserving energy and replenishing body reserves

reflex

involuntary response to a sensory stimulus

reflex arc

the pathway neurones, involves 3 neurones: sensory, relay, motor neurones

nervous system

- electrical impulses pass along nerve cells
- secrete neurotransmitters directly onto target cells - enables rapid communication
- rapid, short lived, localised, specific

hormonal system

- chemical ‘hormones’ transported in blood plasma → stimulate target cells

- slower, long lasting, widespread, less specific

describe how you could compare the IAA concentration in shoot tips from two different plant species

1. size of shoot
2. number of shoot tips
3. size of agar block
4. shoots at same stage of development