A2 Responding to the Environment Help

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Plants Respond to Stimuli
Tropism: a directional growth response in which the direction of the response is determined by
the direction of the external stimulus.
PhototroPism: shoots grow towards light ­ enables them to photosynthesise.
GeotroPism: roots grow towards the pull of gravity.
ChemotroPism: attracted towards chemicals.
ThigmotroPism: shoots of plants gain structure and support by growing towards objects that
can stabilise the plant.
What controls the plant responses?
HORMONES/PLANT GROWTH SUBSTANCES: chemical messengers that can be transported
away from their site of manufacture to other areas in the plants i.e. target cells.
Produced by cells in a variety of tissues in the plant.
When hormones reach their target cells they bind to receptors on the plasma membrane.
Hormones move around the plant in the following ways:
-Active transport
-Mass flow (phloem)
Auxins ­ produced Promotes cell
at apex. elongation; inhibits
growth of
side-shoots; inhibits
leaf abscission.
Cytokinins ­ Promotes cell
produced in the division.
Gibberellins Promotes seed
germination and
growth of stems.
Abscisic Acid Inhibits seed
germination and
growth; causes
stomatal closure
when the plant is
stressed by low
water availability.
Ethene Promotes fruit
Non-directional responses are called nastic responses.
How plants respond to the environment
Plant growth

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The cell wall is highly restrictive, so therefore growth only happens in particular places in
-Apical meristems: located at the tips (apices) ­ responsible for the roots and shoots
getting longer.
-Lateral bud meristems: are found in buds. These could give rise to side shoots.
-Lateral meristems: are found in the cylinder near the outside of roots and shoots and are
responsible for the roots and shoots getting wider.
-(in some plants) intercalary meristems: are located between the nodes.…read more

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ABSCISIC acid (AA) inhibits bud growth. High concentrations of auxin in the shoot may keep AA
levels high in the bud. When auxin is removed, AA drops ­ bud grows.
-Cytokinins (CK) promote bud growth. High conc of auxins make the shoot apex a sink for CK ­
produced in the roots. When the tip is removed, CK spreads evenly throughout the plant;
promoting growth in the buds.
Gibberellins and Stem Elongation
Read Book pg.…read more

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CYTOKININS: can delay yellowing of lettuce leaves after being picked.
Used in tissue culture to help mass-production ­ promote bud and shoot growth from small
pieces of tissue taken from parent plants.
ETHENE: cannot be sprayed directly gas.
2-chloroethylphosphonic acids ­ can be sprayed in a solution.
Speeds up fruit ripening
Promotes fruit dropping
Promotes female sex expression in cucumbers
Promoting lateral growth in some plants.
Restricting ethene's effects can help fruits stay fresh for longer.…read more

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Medulla Oblongata: controls action of smooth muscle in the gut wall, and controls breathing
movements and heart rate.
Organising the Nervous System
Subdividing the Nervous System
The nervous system coordinates actions of the body through electrical impulses. It works in
conjunction with the endocrine system.…read more

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The Elbow ­ A closer look
An example of a synovial joint ­ a lot of movement is needed. Synovial fluid is a lubricant, eases
movement of the bone at the joint. The biceps and triceps act antagonistically in order to move
the forearm at the elbow.
Impulses arriving at the neuromuscular junction cause vesicles to fuse with the pre-synaptic
membrane release of acetylcholine into the gap.
AC binds to receptors on the muscle fibre membrane (sarcolemma) causes depolarisation.…read more

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Contractions are relatively slow, tires slowly.
Cardiac Muscle:
Atrial muscle
Specialised excitatory and conductive muscle fibres. ­ Contract feebly, but conduct electrical
impulses and control the rhythmic heartbeat. The SINOATRIAL NODE is made up of specialised
E&CMF ­ has the greatest ability for self-excitation, the electrical impulse spreads from here
over the heart.
CM is made up of many individual cells, connected in rows ­ dark areas = intercalated disks.…read more

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Many mitochondria
Extensive sarcoplasmic reticulum
A number of myofibrils; contractile element ­ each consists of a chain of smaller
contractile units called sarcomeres. Within the myofibrils there are two types of
myofilaments: THIN actin and THICK myosin ­ these run the length of the cell.
This type of muscle contracts powerfully and quickly, but fatigues rapidly.
The Sliding Filament Model
The Sarcomere
From one z-line to another ­ 2.5um, z-lines are closer together during contraction.…read more

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Ca ions
travel through the sarcoplasm and bind to the troponin molecules ­ this changes the shape of
the troponin, which in turn moves the tropomyosin away from the binding sites on the actin.…read more

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Innate Behaviour
Innate behaviour is any animal response that occurs without the need for learning.
Innate Behaviours Learned Behaviours
Genetically Determined by the
determined ­ not relationship between
impacted on by the the genetic make-up
environment. Passed of the individual and
onto offspring. environmental
influences. May be
passed on by
Inflexible Can be altered by
Patterns of Considerable variety
behaviour are the is shown between
same (stereotypical) members of a
in all members of the species.
same species.…read more


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