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Photoreceptors in plants
Photoreceptors - A structure in a living organism, esp. a sensory cell or sense organ, that responds to light falling on it
2.1.1 Coordination
It is essential in all multicellular organisms that all cells are able to communicate with each other ­ those may be plants
or animals. The interaction of cells enables them to coordinate their activities appropriately. Organisms have
specialised cells or molecules that are sensitive to changes in the environment (stimulus), called receptors.…read more

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Long-day plants need an abundance of PFR, which only happens when they have short nights because not all of the PFR
has been converted and the sun begins the conversion of PR back to PFR again.
Phytochromes exert their effects by activating other molecules in plant cells which affect various metabolic
pathways. The phytochromes also act as transcription factors in the nucleus, switching genes on and off.
2.1.…read more

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Information picked up by a receptor is transmitted to the central nervous system (CNS), which compiles of the brain
and the spinal cord, as action potentials along a sensory neurone. These neurones have their cell bodies in small
swellings, called ganglia, just outside the spinal cord. The impulse is then transmitted to a relay neurone which lies
entirely within the brain or spinal cord. The impulse is then transmitted to many other neurones, one of which may be
a motor neurone.…read more

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When a receptor receives a stimulus it can cause a different set of sodium channels to open. This allows sodium ions
to flood into the cell, down an electrochemical gradient (electro ­ difference in charge across the membrane, the
chemical gradient ­ the difference in the concentration of sodium ions). This rapidly reverses the potential difference
across the cell membrane, making it much less negative inside. The neurone is said to be depolarised.…read more

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The calcium ions affect tiny vesicles inside the neurone,
which contain a transmitter substance such as
acetylcholine. These vesicles move towards the
presynaptic membrane and fuse with it, releasing their
contents into the cleft
The transmitter substance diffuses across the cleft and
slots into receptor molecules in the postsynaptic
This causes sodium-ion channels to open, so sodium
ions flood into the cytoplasm of the neurone,
depolarising it
The depolarisation sets up an action potential in the
postsynaptic neurone
2.2.…read more

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When no light falls onto a rod cell, and the potential difference is normal, it
constantly releases transmitter substances, which diffuses to a neurone next
to it. This transmitter substance stops that neurone generating action
potentials. When light falls on the rod cell and membranes become
hyperpolarised, this causes it to stop releasing transmitter substances. Now
the neighbouring neurone can generate action potentials. These are
transmitted along axons to the optic nerve, which carry them to the visual centre
in the brain.…read more

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Animal hormones are almost all small protein molecules No protein or steroid plant hormones have been found
or steroids
2.4 The Human Brain
The brain is part of the CNS. Its role is to initiate and coordinate activities in different parts of the body. It receives
input from receptors both inside and at the surface of the body, and the information from these is integrated to
produce appropriate actions in response.
The brain is made up of neurones, and other cells called glial cells.…read more

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We `see' because our brain processes the image formed from the retina, using past experience and other sensory
inputs to enhance the information obtained. If a tiny image of a person falls onto your retina, your brain doesn't see this
as an extremely small person standing just in front of you, but as a normal-sized person standing far away.
The capacity of the brain to process and interpret the action potentials that arrive along the optic nerve is acquired
during early childhood.…read more

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Newborn babies show various reflex reactions, such as the startle reflex. This happens when a
baby hears a sudden loud noise, or is dropped a short distance. The baby responds by flinging out arms and legs
and contracting the neck muscles. This response is largely innate but it may also be influenced by the experiences of
the baby while it was a fetus in the uterus.…read more



thanks so much

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