OCR A2 Biology

2. As well as responding to biotic (living) and
abiotic (non-living) factors, plants need to
respond to external stimuli. This is to...
Avoid abiotic stress
Avoid being eaten
Maximise photosynthesis
Cope with changing conditions
Survive long enough to reproduce

3. Tropism is a directional growth response in which the direction
of the response is determined by the direction of the external
stimulus...
 Phototropism  shoots grow towards the light to
photosynthesise
 Geotropism  roots grow towards the pull of gravity
 Chemotropism  attracted to chemicals (e.g. Pollen tubes
grow down the style to the ovary for fertilisation)
 Thigmotropism  shoots of climbing plants wind around
other plants or solid structures to gain support
Responding towards a stimulus is known as positive tropism
and vice versa.

4. Abscisic Acid:
 Stimulates stomata closure under water stress
 Inhibits shoot growth
 Induces seeds to synthesise storage proteins
 Induces gene transcription
Auxins (IAA)
 Stimulate cell division and cell elongation
 Stimulate differentiation of xylem and phloem
 Delays leaf senescence and fruit ripening
 Increases stretchiness of cell wall
 Inhibit or promote leaf and fruit abscission
 Stimulates growth of flower parts
 Stimulates ethylene production at high concentrations

5. Cytokinin
 Stimulates cell division
 Stimulates morphogenesis in tissue culture
 Stimulates growth of lateral buds – release of apical
dominance
 Stimulates leaf expansion resulting from cell enlargement
Ethylene
 Stimulates release of dormancy
 Stimulates shoot and root growth and differentiation
 Stimulate leaf and fruit abscission
 Stimulate flower and leaf senescence
 Stimulates flower opening/fruit ripening

6. Gibberellins
Stimulates cell division and elongation
which leads to cell elongation
Stimulates enzyme production
Can delay leaf and citrus fruit senescence

7. The hormones are actually referred to as plant growth
regulators by scientists. The presence of auxin promotes
the active transport of hydrogen ions through the ATPase
enzyme into the cell wall. The resulting low pH allows
optimum conditions for the wall-loosening enzymes to
work. These enzymes break bonds within the cellulose
so the walls become less rigid and can expand as the
cells take in water.

8. A shoot bends towards a light source because auxin is
transported to the tip of the shoot to the cells in the
shade so the cells take up more water and elongate. As
the cells elongate more on the shaded side, the shoot
bends towards the light source

9. Cytokinins stop the leaves of a deciduous tree senescing
by making sure the leaves act as a sink for phloem
transport. The leaf is therefore guaranteed a good supply
of nutrients. If cytokinin production drops, the supply
dwindles and senescence begins and is usually followed
by abscission. Also, the auxin production drops so
ethene production increases and cells in the abscission
zone are more sensitive to ethene. This increases
cellulase production which digests the walls of the cells in
the abscission zone, eventually separating the petiole
from the stem.

10. The growing apical bud at the tip of the shoot inhibits growth of
lateral buds further down the shoot.
Auxins are produced in the tip of the main shoot and inhibit the
growth of side shoots. When the tip of the main shoot is
removed, or an auxin transport inhibitor is applied below the
apex of the main shoot, the side shoots grow. This shows
that auxin is produced in the apex of the main shoot and
transported to lateral buds to inhibit their growth. In low
concentrations of auxin, side shoot growth isn’t inhibited. This
is also shown where, as the plant grows taller, the lateral
buds at the bottom of the plant start to grow larger – they are
further away from the main shoot (there is a lower
concentration of auxin so their growth is less inhibited). Cells
of genetically dwarf plants grow considerably when treated
with Gibberellic acid.

11. Growing cuttings
Producing fruit without seeds
Ripening fruit
Increasing the size of fruit
Controlling weeds (selective weed killers)

12. Animals need to respond to the environment
to stay alive. This is done using nerves and
hormones to control responses ranging
from muscle actions to running away from
a predator to fine control of balance,
posture and temperature regulation.

13. Central Nervous System, CNS (brain and spinal cord)
Peripheral Nervous System, PNS (neurones carry impulses
in/out of CNS)
PNS:
 Sensory Neurones (impulses from receptor to CNS)
 Motor Neurones (from CNS to effector)
Motor Neurones:
 Somatic Motor Neurones (CNS  skeletal)
 Autonomic Motor Neurones (CNS  cardiac muscle 
smooth muscle in gut  glands)

14. How does it differ?
Most neurones are non-myelinated
Conntection to effector consists of at least
2 neurones which connect at a ganglion
(somatic consists of 1 neurone)

15. Sympathetic Parasympathetic
Most active in times of stress Most active in sleep and relaxation
Neurones are linked at a ganglion just
outside the spinal cord – so pre-
ganglion neurones are very short
Neurones are linked at a ganglion
within target tissues, so pre-ganglion
neurones vary in length
Post-ganglion neurones secrete
noradrenaline
Post-ganglion neurones secrete
acetylcholine
Increases heart rate and ventilation
rate, dilates pupils
Decreases heart rate and ventilation
rate, constricts pupils

17.  Cerebellum  control of all higher order processes (e.g.
Memory, language, emotions, thinking)
 Cerebral Cortex  outermost layer, folded to give large
surface area
 Cerebellum  control and coordination of movement and
posture
 Medulla Oblongata  control heart rate, breathing rate and
smooth muscle of gut
 Hypothalamus  control of autonomic nervous system and
some endocrine glands

18. The conscious decision to move voluntarily
is initiated in the cerebellum. Neurones
from the cerebellum carry impulses to the
motor areas so the motor output to the
effectors can be adjusted appropriately.

20. Ligaments hold
the bones
together and
prevent
dislocation.
The synovial
membrane
produces synovial
fluid.
The synovial fluid lubricates joints and provides food and oxygen to the cartilage
The cartilage allows easy movement, absorbs shock and distributes load

21. Impulses at the NMJ cause vesicles to fuse with the pre-
synaptic membrane and release acetylcholine (ACh) into the
gap. ACh binds to receptors on the sacrolemma causing
depolarisation – the depolarisation wave travels down
tubules (T system). T system depolarisation leads to the
release of calcium ions from stores in the sarcoplasmic
reticulum. Calcium ions bind to protein in the muscle, causing
contraction. Acetylcholinesterase in the gap rapidly breaks
down ACh so contraction only occurs when impulses arrive
continuously.

22. The Brain controls the strength of contraction as
many motor neurones stimulate a single muscle.
Each one branches to the NMJ causing the
contraction of a motor unit. The more motor units
stimulated, the greater the force of contraction.
This is known as gradation of response.

23. Synapse Neuromuscular Junction
Post-synaptic membrane is the cell
surface membrane of a neurone
Post-synaptic membrane is the cell
surface membrane of a neurone
Neurotransmitter may be ACh or
noradrenaline
Neurotransmitter is ACh
Depolarisation of post-synaptic
membrane may be stimulatory or
inhibitory
Depolarisation of post-synaptic
membrane is stimulatory
In both, the neurotransmitter is secreted, diffuses across the
cleft, binds to receptors in the postsynaptic membrane and is
finally broken down

24. Support
Protection
Movement
Produce blood cells
Stores minerals

25. During
contraction,
the H band
shortens
and the A
band
doesn’t
change

26. Voluntary Muscle Involuntary Muscle Cardiac Muscle
Striated Unstriated Semi-striated
Contracts and fatigues
quickly
Contracts and fatigues
slowly
Contracts
spontaneously, doesn’t
fatigue
Cylindrical cells are
multinucleate
Short, spindle-shaped
cells with a single
nucleus
Cylindrical cells form
branched fibres with
intercalated discs joined
at ends
Involved in movements
of bones of skeleton
about the joints
Involved in movements
of materials along
internal tubes (e.g.
Peristalsis)
Involved in pumping
blood around the body
Controlled by somatic
nervous system
Controlled by autonomic
nervous system
Myogenic. Controlled by
autonomic nervous
system

27. An action potential arrives at the NMJ causing depolarisation
and the release of calcium ions from the sarcoplasmic
reticulum. Calcium ions diffuse through the sarcoplasm and
binds to troponin, which is covering the myosin binding site.
This removes troponin, thereby exposing the site. The
myosin head then binds to actin, forming a cross bridge. The
head group then bends causing thin filaments to slide past
thick filaments and overlap it more. This is the power stroke.
Also, ADP and Pi are released. ATP then binds to myosin,
breaking the cross bridge and causing the release of actin.
ATP is then hydrolysed and the myosin head resets back to
the resting position. It can now attach to the next binding site
along the actin filament.

28. The role of ATP is required to break the cross bridge connection
and re-set the myosin head forwards.
Maintenance
 Anaerobic Respiration in Sarcoplasm  production of lactic
acid which increases blood supply to the muscles
 Aerobic Respiration in Mitochondria  oxygen and a
respiratory substrate is needed to regenerate ATP
 The transfer of a phosphate group from Creatine Phosphate
to ADP and Sarcoplasm

29. This is the full range of coordinated responses of animals to
situations of perceived danger. The combined nervous and
hormonal response makes the organism ready for actions that lead
to confrontation of the danger or escape from it. the cerebral
understanding of a threat activates the hypothalamus, stimulating
activity in the sympathetic nervous system. This triggers the
release of adrenaline from the adrenal medulla into the blood. The
hypothalamus also releases corticotropin releasing factor (CRF)
into the pituitary gland, stimulating the release of adreno-
corticotropic hormone (ACTH) from the anterior pituitary gland.
This then stimulated a number of hormones from the adrenal
cortex which can help the body resist stressors. On perception of
threat, there is a period of heightened awareness where an animal
assesses (an autonomic response)

30.  Pupils dilate, so more light enters with more detailed visual information
 Heart rate and blood pressure increases, increasing the flow of blood
directed to muscles in case of explosive and sustained action
 Blood glucose levels increase, which supplies respiratory substrate to the
muscles to regenerate ATP
 Metabolic rate increases to mobilise nutrients
 Arterioles to the digestive system and skin constrict; those to the liver and
muscle dilate
 Increased sweat production increases metabolism so more heat is
generated to be removed
 Endorphins released act as ‘natural painkillers’
 Ventilation rate and depth increase so there is an increased removal of
carbon dioxide and supply of oxygen for heightened activity
 Erector pili muscles in the skin contract so hairs stand up. this helps an
animal look bigger

31.  Doesn’t need to be learned
 Immediate survival value for a young, inexperienced
animal in a dangerous situation
 Appropriate for invertebrates with a short life span
who don’t have time to learn
 Requires few neurones
 Likely to be appropriate for the animal’s habitat as
the alleles controlling it will have been subject to
natural selection

32. Innate Learned
Genetically determined Determined by relationship between
genetic make-up of the individual and
environmental influences
Rigid, inflexible, stereotypical in all
members of a species
Altered by experience so there is a
variety within a species
Unintelligent Forms basis of all intelligent and
intellectual activity

33.  Escape Reflexes  a particular stimulus brings about an
automatic response to avoid predators (e.g. Earthworms
withdraw underground in response to vibrations)
 Taxes  a directional movement in response to an external
stimulus (e.g. Woodlice move away from light to be less
visible to predators; negative phototaxis)
 Kineses  a movement in response to an external stimulus.
The rate of movement is related to the intensity but not the
direction of a stimulus (e.g. Woodlice in dry/bright conditions,
they will move around rapiddly and randomly until they are
more suitably placed)
The Waggle Dance  worker honey bees perform this dance to
communicate the direction and distance of a food source to
others

34. Animal responses that change or adapt with
experience. It is adapted to respond to
changing circumstances of environments.

35.  Habituation  animals learn to ignore certain stimuli because
repeated exposure results in neither reward nor punishment.
This avoids wasting energy in making escape responses to
non-harmful stimuli
 Imprinting  young animals associate with another (usually a
parent). It only occurs during the sensitive (receptive) period
which helps learn skills from parents. After, they will only
follow and learn from objects that look like the first objects
 Latent (exploratory)  animals will explore new surroundings
and learn information that is not of immediate value but it will
be at some point in the future

36.  Classical Conditioning  a form of adaptive learning in which
the innate response is modified. The animal learns to
respond to a stimulus that is different from the usual stimulus
 Operant Conditioning  a form of adaptive learning in which
an animal learns to carry out a particular action in order to
receive a reward or avoid an unpleasant experience
 Insight  the ability to think and reason in order to solve
problems or deal with situations in ways that don’t resemble
simple, fixed, reflex responses or the need for repeated trial
and error

37. There is hierarchy within gorilla troops. There is
usually a silverback (dominant male) which protects
numerous females and mates with them to produce
offspring. Male offspring eventually leave the group
once they have reached a mature age to avoid
conflict with the silverback and inbreeding which
could lead to genetic defects. This is known as
social organisation.
Communication systems also exist within troops, such
as grunts to signal danger or issue threats and facial
expressions for recognition. Grooming reinforces
relationships.

38.  Greater ability to detect and deter predators is achieved
by groups of individuals working together
 Knowledge and protection of food is shared with the
group
 Their brain is large which slows maturity. The security of
a group enhances survival and learning of immature
young
 Females give birth to one (or few) infant(s) at a time, so
maternal care and group protection enhances survival
 The young learn through observation and play with other
members

39. There are a range of dopamine receptors in the brain.
Depending on how effective the receptors are, there will be
different levels of dopamine in the brain. The different levels
are linked to a range of conditions (e.g. Schizophrenia, ADHD
and Parkinsons). The DRD4 receptor is one of the most
variable receptors. By studying the levels of dopamine in the
brain and the genotype of the individual, the alleles which
may influence different conditions can be investigated and
different drugs for the conditions can be developed.