GCSE Biology Edexcel B1 Topic 2

I made these revision notes myself. I got an A* in Biology GCSE and 72 UMS in the B1 exam for your note.

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B1 Topic 2 ­ Responding to change
Homeostasis
Conditions in our body need to be maintained and controlled so cells can function properly. For example:
o Osmoregulation: This is the regulation of water content. A balance needs to be kept between water
gained and water lost.
o Thermoregulation: This is the regulation of body temperature. Excess body heat must be lost when
you get too hot, but retained when you get too cold.
o Blood glucose regulation: Blood glucose must be kept at a steady level.
Negative feedback
Negative feedback is the mechanism which keeps conditions in our body steady:
o Changes in the environment trigger a response that counteracts the changes e.g. a rise in body
temperature causes a response that lowers body temperature.
o As a result, the internal environment stays around the norm.
o This only works within limits ­ if the environment changes too must it might not be possible to
counteract it.
Thermoregulation
When our body temperature gets too low, enzyme-catalysed reactions don't work properly or fast enough.
Cells cannot survive and enzymes are at the risk of denaturing.
Body temperature is monitored by the hypothalamus in our brain. It receives impulses from receptors in the
skin that provide information about skin temperature.
When the hypothalamus detects a change, it causes a response in the dermis of the skin.
When you're too hot: When you're too cold:
Erector muscles relax, so hairs lie flat. Erector muscles contract so hairs stand upright.
Sweat is produced. When sweat evaporates, it This traps an insulating layer of air which keeps
transfers heat from the skin to the environment the body warm.
which cools the body down. Little to no sweat is produced.
Blood vessels (shunt valves) close to the surface Blood vessels constrict near the surface of the
of the skin dilate. This allows more blood to flow skin. This means less blood flows near the
close to the surface of the skin, meaning more surface, so less heat is lost to the surroundings.
heat is transferred into the surroundings. This is This is called vasoconstriction.
called vasodilation.
Hormones and Nerves
Hormones
Hormones are chemical messengers which travel the blood to activate target cells.
Hormones are released directly into the blood stream and are carried around the body until they reach their
target organ/cells.
The target cells have the right receptors to respond to the hormone. Similarly, organs being affected are
called target organs.
Hormones travel at "the speed of blood". Their effects are long lasting.
Neurones

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Features of neurones:
Dendrons & Dendrites Fine cytoplasmic extensions They create junctions with
from the cell body. Dendrites other neurones.
extend from Dendrons.
Axon A fibre which is longer than any
These carry impulses along to
other extensions of the cell the effector organ. It does this
body. by forming a synapse which is a
minute gap between two
neurones.
Myelin Sheath Myelin is a fatty material which The myelin sheath insulates the
covers the axon. axon which prevents short
circuits of the nerve impulses.…read more

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Muscles contract in response to a nervous impulse whereas glands secrete substances as a response.
Sensory neurones
Long Dendrons and short axons carry nerve impulses from the receptors in the sense organs to the CNS.
Motor Neurones
Many short dendrons and one long axon to carry nerve impulses from the CNS to the effectors.
Relay Neurones
Many short dendrons and axons carry nerve impulses from sensory neurones to motor neurones.
Reflexes
Reflexes have to be quick and they are so because they are automatic.…read more

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The neurones in the reflex arc go through the spinal cord or through an unconscious part of the brain (you
don't have to think).
2. When a stimulus (e.g. pin) is detected by the receptors, impulses are sent along a sensory neurone to the
CNS.
3. In the CNS the sensory neurone passes on the message to a relay neurone.
4. Relay neurones relay the impulses to a motor neurone.
5. The impulses then travel along the motor neurone to the effector (e.g. muscle)
6.…read more

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This occurs often with overweight or obesity. This condition is where the pancreas doesn't produce enough
insulin or when a person becomes resistant to insulin (their body cells don't respond properly).
In both of these cases blood sugar levels can rise to a dangerous level.
Obese people have an increased risk of Type 2 Diabetes. People are classified as obese if they have a body
mass index (BMI) of over 30.…read more

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Other plant hormones
Cytokinins ­ induces mitosis (causes plants to grow and flower and stimulates seed germination).
Ethylene ­ Ripening of fruit and dropping of leaves.
Abscisic Acid ­ inhibits growth during winter so plants can hibernate and not waste energy
Uses of plant hormones
Selective weedkillers
Artificial auxin is used as a selective weedkiller as it makes plants with broad leaves, like dandelions and
daisies grow out of control and die.
This means plants with narrow leaves, such as wheat and grass, are unaffected.…read more

Comments

Isaacbam

very useful sums up topic 2 very well thanks

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