Responding to the environment

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Why plants respond to the environment

Tropism - a directional growth response in which the direction of the response is determined by the direction of the external stimulus.

Responding helps plants to avoid abiotic (non-living) stress or being eaten (predation), and to survive long enough to reproduce.


  • Phototropism - shoots grow toward light (+vely phototrophic)
  • Geotropism - roots grow toward pull of gravity, to anchor them into soil to take up water for support, photosynthesis and to keep the plant cool. There are also nitrates in the water (needed to make amino acids).
  • Chemotropism - on a flower, pollen tubes grow down the style, attracted by chemicals to the ovary where fertilisation can take place.
  • Thigmotropism - shoots of climing plants wind round other plants or solid structures and gain support.

Plants control responses using hormones, which move around the plant by active transport, diffusion or mass flow in the phloem sap or in xylem vessels. Some can amplify eachothers effects (synergy) and some can cancel out each others effects (antagonism). 

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Hormone Effects

Hormones are produced by cells in a variety of tissues in the plant and act on target cells or tissues. They bind to receptors on the plasma membrane. They have specific shapes - so bind to specific receptors with complementary shapes, ensuring the hormones only act on the correct tissues.

Hormone  Effects

Auxins Promote cell elongation; inhibit growth of side shoots; inhibit leaf abscission (leaf fall)

Cytokinins Promote cell division

Gibberellins Promote seed germination and growth of stems

Abscisic acid Inhibits seed germination and growth; causes stomatal closure when plant is stressed by low water availability

Ethene Promotes fruit ripening

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Plant Growth

  • Apical meristems - at tips of roots and shoots and make them get longer
  • Lateral bud meristems - in buds, giving rise to side shoots.
  • Lateral meristems - in a cylinder near the outside of roots and shoots - make them wider.
  • Intercalary meristems - between nodes (where leaves + buds branch off), making shoot longer.


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What causes phototropisms?

Phototropic response in shoots (

The shaded side elongates faster than the illuminated side, pushing the end towards the light. Auxins act on the shaded side, promoting an increase in the rate of elongation, making the shoots bend towards the light.

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Shedding leaves

Cytokinins stop the leaves of deciduous trees senescing (ageing - turning brown and dying) by making sure the leaf acts as a sink for phloem transport, so it has a good supply of nutrients. If supply of cytokinins is low, senescence begins and leaves are shed. Usually auxin inhibits abscission by acting on cells in the abscission zone.

  • Leaf senescence causes auxin production at leaf tip to drop
  • Cells in abscission zone are more sensitive to ethene
  • Drop in auxin levels = an increase in ethene production
  • Increased production of the enzyme cellulase, which digests the walls of cells in the abscission zone, separating the petiole from the stem

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Apical Dominance

If the apex is broken off a plant, side branches start to grow from lateral buds which were dormant - - auxins prevent lateral bud growth. To test - a paste containing auxins is applied to cut shoot and the lateral buds did not grow.

Auxin inhibitor added to near apex of shoot - - lateral buds grew. Therefore normal concentrations inhibit lateral growth, whereas low levels promote growth.

There is not a direct link, however:

  • Abscisic acid inhibits bud growth. High auxin conc in shoot keeps levels of abscisic acid high in the bud. When shoot is removed, abscisic acid concentrations drop and buds grow.
  • Cytokinins promote bud growth - directly applying can override the apical dominance effect. When auxin levels are high, the shoot apex is made a sink for cytokinins. When that is removed, cytokinins spread more evenly around the plant, promoting bud growth.
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Gibberellins and stem elongation

When gibberellins was applied to dwarf varieties of plants (eg peas), the plants grew taller. Suggests that it is responsible for stem growth.

In a comparison of concentrations of gibberellins in tall plants and dwarf plants, concentrations were higher in tall plants as the tall plants possess the dominant allele for an enzyme which converts one form of gibberellins to the active form.

Gibberellins cause growth of the internodes by stimulating cell elongation (by loosening cell walls) and cell division (by stimulating production of a protein which controls the cell cycle). Internodes of dwarf peas have fewer and shorter cells than those in tall plants.

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Commercial uses of plant hormones


  • Taking cuttings - dipping the end in rooting powder (containing auxins) promotes root growth
  • Seedless fruit - treating unpollinated flowers with auxin can promote growth of seedless fruit as it promotes ovule growth, triggering production of auxin by tissue in the fruit, completing development
  • Herbicides - used to kill weeds as they promote shoot growth so much so the stem cannot support itself, buckles and dies


  • Fruit production - delays senescence in citrus fruit, extending the time they can be left unpicked and their shelf life. Improve apple shape with cytokinins. Elongate grape stalks so they are less compacted and the grapes are bigger.
  • Brewing- can speed the process of making maltose, which is turned into malt
  • Sugar production - makes sugar cane stems lower, so more sugar is available, increasing yield.
  • Plant breeding - speeds growth so trees are reproductively active, speeds seed production and they can harvest seeds.
  • Inhibition makes flowers short and stocky, preventing lodging (stems bend as of weight of water, making crops difficult to harvest)
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Commercial uses of plant hormones 2


Delay lead senescence so can be used to prevent yellowing of lettice leaves.Used to mass produce plants - they promote bud growth so there are many branches which can be split into lots of small plants in tissue culture.


  • Speed fruit ripening in apples, tomatoes and citrus fruits
  • Promote fruit drop
  • Promote female sex expression, reducing the chance of self pollination (making cucumbers taste bitter) and increasing yield
  • promote lateral growth - compact flowering stems.

Inhibiting ethene means fruits can also be stored for longer (useful in shipping and improving shelf life).

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The Structure of the Brain


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Function of brain parts


  • conscious thought and emotional responses
  • the ability to override some reflexes
  • features associated with intelligence, such as reasoning and judgement.

Cerebellum -Nonconscious operations, the fine control of muscular movements.

  • balance
  • sensory activities - judging the position of objects and limbs
  • tensing muscles to manipulate objects
  • feedback position on muscle position
  • operation of antagonistic muscles for contraction and relaxation

Medulla Oblongata - Controls non-skeletal muscles (eg heart). Cardiac centre regulates heart rate. Respiratory centre controls breathing rate and depth.

Hypothalamus - Controls homeostatic mechanisms. Sensory input from receptors is recieved and automatic responses are initiated which regulate temperature and blood water potential. It regulates the pituitary gland so controls much of the endocrine function of the body.

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Organising the nervous system

Living things need to respond to changes in the internal and external environment in order to stay alive. From running away from a predator, to balance, posture and temperature regulation.

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Parasympathetic and Sympathetic nervous systems


Most active in sleep and relacation. The neurones of a pathway are linked at a ganglion within the target tissue. So pre-ganglionic neurones vary considerably in length. Post-ganglionic neurones secrete acetylcholine as the neurotransmitter at the synapse between the neurone and effector.

Effects: decreased heart rate, pupil constriction, decreased ventilation rate and sexual arousal.


Most active in times of stress. The neurones of a pathway are linked at a ganglion outside of the spinal chord. So pre-ganglionic neurones are very short. Post-ganglionic neurones secrete noradrenaline at the synapse between neurone and effector.

Effects: increased heart rate, pupil dilation, increased ventilation rate, and ******.

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Movement at the elbow joint

It is a synovial joint, with the triceps and biceps working antagonistically.Synovial fluid is a lubricant, easing the movement of bones at the joint, produced by the synovial membrane. Bones are held together by ligaments and cartilage reduces friction where bones meet.

1.  Impulses arriving at the neuromuscular junction cause vesicles to fuse with the pre-synaptic membrane and release acetylcholine into the gap.

2.  Acetylcholine binds to receptors on the muscle fibre memrane (SARCOLEMMA) causing depolarisation.

3.  Depolarisation travels down tubules (T system)

4.  T system depolarisation leads to Ca2+ release from stores in sarcoplasmic reticulum.

5.  Ca2+ binds to proteins in the muscle, which leads to contraction

6.  Acetylcholinesterase in the gap rapidly breaks down acetylcholine so that contraction only occurs when impulses arrive continuously.

The motor unit - each motor neurone branches to neuromuscular junctions, causing the contraction of a cluster of musicle cells. The more motor units stimulated - the greater the force of contration - gradation of response.

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Three types of muscle

Involuntary (smooth) muscle: stimulated by autonomic nervous system - Spindle shaped cells which contain bundles of actin and myosin and a single nucleus. Slow contration, but tires slowly.

  • Walls of intestine - moves food
  • Iris of eye - controls LI in eye, contation of radial muscle dilates pupil, circular muscle constricts pupil.
  • Walls of arteries and cervix - contration reduces blood flow.

Cardiac muscle - atrial, ventricular and specialised excitatory and conductive muscle fibres.

Myogenic - capable of stimulating contraction without a nerve impulse. Sympathetic stimulation increases its rate, whereas parasympathetic stimulation decreases the rate. Muscle cells are made of many individual cells connected in rows. Dark areas are intercalated discs, which are membranes. There are gap junctions with free diffusion and so action potentials pass quick and easy between muscle fibres. Under a microscope it is straited. It contracts powerfully and without fatigue.

Voluntary (skeletal or striated) muscle - movement of skeleton joints

Cells form fibres, containing several nuclei, each is surrounded by a cell surface membreane called the sarcolemma. Sarcoplasm contains many mitochondria, an extensive sarcoplamic reticulum and a number of myofibrils (contractile units made up of sarcomeres and thick actin and myosin myofilaments). Contracts fast but tires easily.

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The Sarcomere

Thin filaments are in two strands, made of actin, coiled around each other like a twisted double ring of beads. Tropomyosin molecules coil around, reinforcing it. A troponin complex is attached to each tropomyosin molecule, each made up of 3 polypeptides. One binds to actin, one to tropomyosin to keep it in place and one to calcium ions.

Thick filaments are bundles of the protein myosin. Each made of a tail and two protruding heads. Each consists of many myosin molecules whose heads stick out from opposite ends of the filament.

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The Power Stroke

1. Myosin heads attach to actin filaments forming a cross bridge

2. Head group bends, causing the thin filament to be pulled along and so more overlap with myosin. ADP and Pi are released in the power stroke.

3. Cross bridge is then broken as new ATP attaches to the head.

4. The head moves backwards as the new ATP is hydrolysed to ADP and Pi. It can then form a cross-bridge with the thin actin filament further along.

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The binding sites for myosin head groups on the actin fibre are covered up by the tropomyosin subunits. This meant that a myosin head group cannot attach, so crossbridges cannot form and contraction cannot occur. When an action potential arrives- calcium ions are released from the sarcoplasmic reticulum. They bind to troponin molecules, changing their shape. The tropomyosin moves away from the binding sites on the actin as a result, and allows the attachment of myosin head groups. 

When stimulation stops, calcium ions are actively transported back to the sarcoplasmic reticulum by carrier proteins on the membrane - - relaxation.

Maintaining a supply of ATP - 

  • Aerobic respiration
  • Anaerobic respiration
  • Transfer from creatine phostphate in the muscle cell sarcoplasm: the phosphate group can be transferred to ADP to form ATP. (short term supply)
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Fight or Flight Response

Physiological changes: pupils dilate, heart rate and blood pressure increases, arterioles to the digestive system and skin are constricted while those to muscle and liver are dilated, blood glucose levels increase, metabolic rate increases, erector pili muscles in skin contract so hairs raise, ventilation rate and depth increases, endorphins (natural painkillers) are released and sweat production increases.

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Fight or Flight Response 2

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Innate Behaviour

Innate - inherited response, similar in all members of the same species. Genetically determined so environment has no impact. Rigid and inflexible. Patterns of behaviour are universal in species. Unintelligent - no sense of the purpose of the behaviour.

Invertebrates rely for their survival on 3 types of innate behaviour to escape predation, locate a suitable habitat and locate food:

Reflexes - to avoid predators.

Kineses - orientation behaviour where the rate of movement increases when the organism is in unfavourable conditions. It is non-directional, so movement is random and organisms do not actively seek out favourable conditions.

Taxes - is a directional orientation response. Direction is in relation to a stimulus which triggers a reponse. Eg positive phototaxis is movement to light and positive chemotaxis is towards a chemical.

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Learned Behaviour

Learned behaviour - responses that change or adapt with experience. Benefits organisms as they have a longer lifespan in particular environments.

Habituation - learn to ignore stimuli as repeated exposure results in neither reward or punishment. It avoids wasting energy in making escape reponses to non-harmful stimuli.

Imprinting - young animals become associated with another organism. It occurs in a sensitive period.

Classical conditioning -  where animals can learn to relate a pair of events and respond to the first in anticipation of the second. This type of learning is passive and involuntary.

Operant conditioning - where animals learn to associate an operation with a reward or punishment (known as reinforcers). It is active and to an extent voluntary, it is also known as trial and error learning.

Latent (exploratory) learning - animals explore surroundings and retain useful information.

Insight learning - ability to think and reason in order to solve problems/unfamiliar situations.

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Primate Behaviour

Advantages of social behaviour in primates:

  • females give birth to only one or few infants at a time. Maternal care and group protection enhances the survival rate of the young.
  • the young learn through observation of and play with the other members of the group - learned behaviour
  • the final relatively large brain size slows the maturation of primates. The security of a group enhances the survival and learning of immature young
  • knowledge and protection of food sources is shared with the group
  • greater ability to detect and deter predators is achieved by groups of individuals working together
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Human Behaviour, Dopamine and DNA

Dopamine - acts as a neurotransmitter and a hormone, a precursor molecule in the production of noradrenaline and adrenaline. Low levels=parkinsons disease. Raised levels=schizophrenia.Dopamine increases general arousal and decreases inhibition,leading to an increase in creativity in conjunction with cerebral activity. There are 5 receptors (DRD1 to DRD5). 

Inheritance of varients of the DRD4 gene affects the levels and action of dopamine in the brain.

Attention-deficient hyperactivity disorder (ADHD) - a particular variant is more frequent in those with ADHD.

Addictive and risk behaviours - variant associated with addictive behaviours such as smoking and gambling and risk-taking behaviours.

The variants of the receptor bind with dopamine and a number of processes are involved, including control of motivation and learning, and is linked to regulatory effects on other neurotransmitter release. Antisychotic drugs work by blocking dopamine receptors.

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What a great contribution to the revision cards on the topics of plant responses, brain structure and behaviour. These are well written, accurate, and  have clear colour images to illustrate the main processes. Teaming these up with a set of flashcards of the key terms and their definitions would make a good set of resources.

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