6: Organisms Response to Changes

Question 1

How do organisms increase their chances of survival?

Answer 1

• living organisms (plants and animals) increase survival chance by responding to changes in their internal/ external environment

Question 2

How will organisms react in different environments?

Answer 2

• either move away from harmful environments or towards favourable environments
• ensure their conditions are always optimal for metabolism

Question 3

How do plants respond to changes in the environment?

Answer 3

tropisms and auxins

Question 4

What is a tropism and auxins?

Answer 4

a plant response to a stimulus coming from a certain direction
- they do this by regulating their growth
- towards stimulus: positive tropism
- away stimulus: negative tropism
Auxins- a group of naturally occurring and artificially synthesised plant hormones. They play an important role in the regulation of plant growth. (e.g. IAA)

Question 5

What is phototropism?

Answer 5

shoots of plants grow towards the light, as they need sunlight to photosynthesise (response to light)
- shoots show positive phototropism
- roots show negative phototropism

Question 6

What is gravitropism/ geotropism?

Answer 6

roots of plants grow down to anchor plants in the soil (response to gravitational pull)
- roots show positive geotropism,
- shoots show negative geotropism

Question 7

What does IAA (indoleacetic acid) do?

Answer 7

controls cell elongation in plants, produced in tips of shoots, transported down the shoot causing cells to elongate and plant growth

Question 8

What happens to the shoots and roots in phototropism?

Answer 8

SHOOTS- positive phototropism:
- initially, IAA evenly distributes throughout the shoot region
- when light intensity changes, IAA moves to shaded side of the shoot
- greater concentration of IAA builds on shaded side, causing cells on this side to elongate more than those on the light side
- cells elongate faster causing shoot tip to bend towards the light
ROOTS- negative phototropism:
- initially, IAA evenly distributes throughout the shoot region
- when light intensity changes, IAA moves to shaded side of the shoot
- high concentration of IAA inhibits cell elongation on the shaded side of the roots
- cells on shaded side grow slower than the light side, root bends and grows away from the light

Question 9

What happens to the shoots and roots in gravitropism/geotropism?

Answer 9

SHOOTS- negative geotropism:
- IAA diffuses from upper to lower side of shoot
- cell elongation causes plant to grow upwards
ROOTS- positive geotropism:
- IAA diffuses to lower side of roots
- inhibits cell elongation causing cells to elongate at a slower rate compared to the top causing roots to grow downwards with gravity

Question 10

How do different factors affect IAA?

Answer 10

• tip is removed- light can’t be detected, no IAA produced, shoot won’t bend in any direction
• tip of shoot covered- light can’t be detected, no IAA produced, no cell elongation, shoot won’t bend in any direction
• agar block- plant shoot grows naturally towards light as agar block permeable to IAA
• agar on half block- IAA only diffuse down one side, only that side elongates.
• impermeable barrier- no IAA diffuse, plant doesn’t grow
• impermeable barrier on part- IAA diffuse onto only one side, that cell elongates despite postition of light

Question 11

What is nervous communication?

Answer 11

Response to a stimulus (change in environment) coordinated by the nervous system
made of: sensory neurones, CNS, motor neurones, effector

Question 12

How does simple reflex work and triggered?

Answer 12

to respond to a stimulus, it must be detected first by receptors (cells/ proteins on surface membrane )
- each receptor is specific to 1 type of stimulus
- when a stimulus is detected by receptor cells an electrical impulse is sent along the sensory neurone
- electrical impulse transmitted to central nervous system
- when the electrical impulse reaches the end of the sensory neurone a chemical (neurotransmitter) is released into the **synapse ** (a gap between 2 neurones)
- this passes on information and the new electrical impulse is generated in the relay neurone in the cns
- cns processes the information and sends impulses along the motor neurone to the effector (muscle/ gland)

Question 13

What is the simple reflex?

Answer 13

rapid involuntary response to a stimulus
- as it doesn’t involve brain, we don’t waste time thinking (fast response)
- simple reflexes produce a protective effect

Question 14

why do species have simple responses?

Answer 14

• simple mobile organisms
• to keep them in a favourable environment

Question 15

What are the 2 types of simple responses?

Answer 15

tactic & kinetic

Question 16

What is a tactic response?

Answer 16

• directional movement in response to a stimulus and involves the organism either moving towards something (positive tatic response) or away from something (negative tactic response)
• eg.phototaxis (light), thermotaxis (temperature) and chemotaxis (chemicals)

Question 17

What is a kinetic response?

Answer 17

• non-directional movement in response to stimulus
• woodlice show kinetic response to humidity
• in high humidity, they’ll move slowly and turn less often so they’ll stay where they are
• when air gets drier, woodlice will move faster and will turn more often do they will move into a new area
• this increses their chances of survival with a high humidity as this will reduce water loss and conceal them from predators

Question 18

What is a receptor?

Answer 18

• can either be cells or proteins on cell surface membrane of cells that detect different stimuli
• CNS detects changes to internal and external environments through receptors
• receptors specific to different specific stimuli
• transducers: change in stimulus detected by sensory neurone, converting change in energy to nervous impulses (generator potential)

Question 19

What is resting potential?

Answer 19

• if no stimulus, then receptors in nervous system are in their resting state
• membrane of receptors has ion channels and ion pumps, allowing ions to move in and out of the cell
• inside of cell more negative than the outside (as more +ions outside)
• difference in charge means there’s a potential difference across the membrane

Question 20

What is generator potential?

Answer 20

• if a stimulus is detected, membrane of receptor becomes excited and more permeable
• allows more ions to enter the cell, changing the potential difference across the membrane so generator potential generated

Question 21

What is an action potential?

Answer 21

• generator potential must reach threshold to be passed onto sensory neurone
• if generator potential big enough, it triggers an action potential (nervous impulse)
• ACTION POTENTIALS ARE THE SAME SIZE SO STRNGTH OF STIMULUS MEASURED BY FREQUENCY

Question 22

What is the pacinian corpuscle?

Answer 22

mechanoreceptors that detect mechanical stimuli (e.g. pressure and vibration)
- found in skin and soles of feet
- contain end of sensory neurone (sensory nerve ending) wrapped in layers of connective tissue (lamellae)
- plasma membrane of sensory neurone ending has special type of sodium ion (stretch mediated sodium ion channels)

Question 23

How does the pacinian corpuscle detect changes?

Answer 23

• when stimulus detected, lamellae become deformed and press on sensory nerve ending (no longer in resting potential)
• causes cell membrane of sensory neurone to stretch, deforming ion channels
• causes stretch mediated sodium ion channels to open so sodium ions diffuse in
• influx of sodium ions changes potential difference of membrane and depolarises it, creating generator potential
• if reached threshold level, it’ll trigger an action potential, which is passed onto the central nervous system

Question 24

What are photoreceptors?

Answer 24

receptors in the eye that detect changes in light
- located on the retina (innermost layer of the eye)
- light enters eye through pupil and focused onto retina
- amount of light entering eye controlled by muscle of iris
- fovea is area of retina contaning lots of photoreceptors
- nerve impulses from photoreceptors cells are carried from retina to brain by the optic nerve (a bundle of neurones
- where optic nerve leaves the eye is the blind spot (as no receptors there)

Question 25

What are the 2 types of photoreceptors?

Answer 25

• Rods- sensitive to light intensity, lead to images being seen in black and white (monochromatic)
• Cones- respond to different wavelengths of light, allowing us to perceive images in full colour
• both photoreceptors connected to the optic nerve by bipolar neurones

Question 26

How do we see?

Answer 26

• when light enters the eye, it hits the photoreceptors, the light energy is absorbed and converted into electrical energy (generator potential)
• light bleaches the pigment in these cells (rhodopsin in rods, iodopsin in cones) breaking them down and causing a chemical change,
• altering permeability of cell surface membrane to sodium ions
• threshold level generates action potential
• action potential sent along bipolar neurone, which connects to photoreceptor to the optic nerve, sending impulses to the brain

Question 27

What do rod cells do?

Answer 27

• don’t distinguish between wavelengths of light, only intensity (monochromatic)
• more numerous than cone cells in retina around peripheral (outside)
• used to detect light at very low light intensities, very sensitive to light intensity
• light detected must exceed threshold to trigger generator and action potential
• as many rod cells connected to single sensory neurone

Question 28

What i