Biology OCR Gateway A - B3
Organism Level Systems
B3.1
The Nervous System
The nervous system is made up of neurons and it works by detecting changes in your external environment. The body has sensory receptors
that detect these changes. These changes are called a stimulus.
There are three main stages to a nervous response:
1. There is a change in the environment. This is the stimulus.
2. Groups of cells detect the stimulus. These are the sensory receptors.
3. A response occurs from the effectors. These are muscles or glands. Muscles respond to an impulse by contracting, which causes
movement. Glands respond by releasing hormones.
Central Nervous System (CNS) And Receptor Cells
The central nervous system or CNS consists of the brain and spinal cord. It coordinates the response and decides what to do. CNS sends
information to an effector which was mentioned before and the effector responds accordingly.
Receptor cells are found in your sense organs. DIfferent receptors detect different stimuli. They change the stimulus into electrical impulses that
travel along neurones to the CNS.
Neurones
There are three different types of neurones:
1. Sensory Neurones: Carries electrical impulses from receptor cells to the CNS.
2. Relay Neurones: Carries electrical impulses from sensory neurones to motor
neurones.
3. Motor Neurones: Carries electrical impulses from the CNS to effectors.
● Neurons transmit information as electrical impulses. These impulses are
passed along the axon of a neurone
● Neurones have branched endings called dendrites that allow them to
connect with other neuornes.
● Some axons are surrounded by a myelin sheath which acts as an electrical
insulator, speeding up the impulse/
● The connection between two neurones is called a synapse.
The Reflex Arc
B3.2
Hormonal and Negative Feedback System
Other than neurons, using hormones is another way to send information around the body.
Hormones are chemical messengers produced in various glands called endocrine glands. These glands make up the endocrine system.
Hormones are released directly into the blood. The blood then carries them to the other parts of the body. Hormones are slow compared to
nervous impulses but they have longer-lasting effects.
Even though hormones travel all over the body, they only affect particular cells in particular places of the body. These cells are called target
cells - they have the right receptors to respond to that hormone.
1
, Adrenaline
Adrenaline is a hormone released by the adrenal glands. It is commonly known to be the ‘fight or flight’ response.
In the body adrenaline is done by activating processes that increase the supply of oxygen and glucose to cells. When the brain detects a
stressful situation, it sends nervous impulses to the glands which respond by secreting adrenaline. This gets the body ready for action.
Thyroxine
Thyroxine is a hormone released by the thyroid gland. Its role is to regulate
metabolic rate - the speed at which chemical reactions in the body occur. It’s important
for loads of processes in the body, such as growth and protein synthesis. It is released
in response to TSH (thyroid stimulating hormone), which is released from the
pituitary gland.
A Negative feedback system keeps the amount of thyroxine in the blood at the right
level. When the level is too high, the secretion of TSH from the pituitary gland is
inhibited. This reduces the amount of thyroxine released so the level in the blood falls
back towards normal
Negative Feedback
The body can control the levels of hormones in the blood using negative feedback systems. When the body detects that a level of a substance
has gone above or below the normal level, it triggers a response to bring the level back to normal again. This can be seen in the example above
using thyroxine.
Hormones in Reproduction
Sex Hormones
Testosterone: The main male sex hormone - produced in the testes. It stimulates sperm production and is important for the development of the
male reproductive system.
Oestrogen: The main female sex hormone - produced in the ovaries. It’s involved in the menstrual cycle and promotes female sexual
characteristics.
Progesterone: Produced by the ovaries and helps to support pregnancy and is involved in the menstrual cycle.
FSH (follicle-stimulating hormone and LH (luteinising hormone): Hormones released from the pituitary gland in the brain. They help control
the menstrual cycle.
The Menstrual Cycle
The menstrual cycle is the monthly sequence of events in which the female body releases an egg and prepares the uterus (womb) in case it
receives a fertilised egg.
Stage 1 - Day 1 is when menstruation begins. The uterus lining breaks down and is released.
Stage 2 - The lining of the uterus builds up again. This stage happens from day 4 to 14. It builds up into a thick spongy
layer full of blood vessels ready to receive a fertilised egg.
Stage 3 - An egg develops and is released. This stage is called ovulation. It is also the stage in which the woman
is the most fertile. This happens at about Day 14.
Stage 4 - The lining is maintained. The lining is maintained for approximately 14 days, until day 28. If no
fertilised egg has implanted into the uterus wall by day 28, the lining
starts to break down again and the whole cycle restarts.
The menstrual cycle is controlled by four hormones:
1. FSH (follicle- stimulating hormone)
● It causes the egg to mature in one of the ovaries.
● Stimulates the ovaries to produce oestrogen.
2. Oestrogen
● The hormone responsible for thickening and growing the lining of the uterus.
● Stimulate the production of LH.
● Inhibits the production of FSH so that only one egg is released in each cycle.
3. LH (luteinising hormone)
● Stimulates the release of an egg at day 14 (ovulation)
2
Organism Level Systems
B3.1
The Nervous System
The nervous system is made up of neurons and it works by detecting changes in your external environment. The body has sensory receptors
that detect these changes. These changes are called a stimulus.
There are three main stages to a nervous response:
1. There is a change in the environment. This is the stimulus.
2. Groups of cells detect the stimulus. These are the sensory receptors.
3. A response occurs from the effectors. These are muscles or glands. Muscles respond to an impulse by contracting, which causes
movement. Glands respond by releasing hormones.
Central Nervous System (CNS) And Receptor Cells
The central nervous system or CNS consists of the brain and spinal cord. It coordinates the response and decides what to do. CNS sends
information to an effector which was mentioned before and the effector responds accordingly.
Receptor cells are found in your sense organs. DIfferent receptors detect different stimuli. They change the stimulus into electrical impulses that
travel along neurones to the CNS.
Neurones
There are three different types of neurones:
1. Sensory Neurones: Carries electrical impulses from receptor cells to the CNS.
2. Relay Neurones: Carries electrical impulses from sensory neurones to motor
neurones.
3. Motor Neurones: Carries electrical impulses from the CNS to effectors.
● Neurons transmit information as electrical impulses. These impulses are
passed along the axon of a neurone
● Neurones have branched endings called dendrites that allow them to
connect with other neuornes.
● Some axons are surrounded by a myelin sheath which acts as an electrical
insulator, speeding up the impulse/
● The connection between two neurones is called a synapse.
The Reflex Arc
B3.2
Hormonal and Negative Feedback System
Other than neurons, using hormones is another way to send information around the body.
Hormones are chemical messengers produced in various glands called endocrine glands. These glands make up the endocrine system.
Hormones are released directly into the blood. The blood then carries them to the other parts of the body. Hormones are slow compared to
nervous impulses but they have longer-lasting effects.
Even though hormones travel all over the body, they only affect particular cells in particular places of the body. These cells are called target
cells - they have the right receptors to respond to that hormone.
1
, Adrenaline
Adrenaline is a hormone released by the adrenal glands. It is commonly known to be the ‘fight or flight’ response.
In the body adrenaline is done by activating processes that increase the supply of oxygen and glucose to cells. When the brain detects a
stressful situation, it sends nervous impulses to the glands which respond by secreting adrenaline. This gets the body ready for action.
Thyroxine
Thyroxine is a hormone released by the thyroid gland. Its role is to regulate
metabolic rate - the speed at which chemical reactions in the body occur. It’s important
for loads of processes in the body, such as growth and protein synthesis. It is released
in response to TSH (thyroid stimulating hormone), which is released from the
pituitary gland.
A Negative feedback system keeps the amount of thyroxine in the blood at the right
level. When the level is too high, the secretion of TSH from the pituitary gland is
inhibited. This reduces the amount of thyroxine released so the level in the blood falls
back towards normal
Negative Feedback
The body can control the levels of hormones in the blood using negative feedback systems. When the body detects that a level of a substance
has gone above or below the normal level, it triggers a response to bring the level back to normal again. This can be seen in the example above
using thyroxine.
Hormones in Reproduction
Sex Hormones
Testosterone: The main male sex hormone - produced in the testes. It stimulates sperm production and is important for the development of the
male reproductive system.
Oestrogen: The main female sex hormone - produced in the ovaries. It’s involved in the menstrual cycle and promotes female sexual
characteristics.
Progesterone: Produced by the ovaries and helps to support pregnancy and is involved in the menstrual cycle.
FSH (follicle-stimulating hormone and LH (luteinising hormone): Hormones released from the pituitary gland in the brain. They help control
the menstrual cycle.
The Menstrual Cycle
The menstrual cycle is the monthly sequence of events in which the female body releases an egg and prepares the uterus (womb) in case it
receives a fertilised egg.
Stage 1 - Day 1 is when menstruation begins. The uterus lining breaks down and is released.
Stage 2 - The lining of the uterus builds up again. This stage happens from day 4 to 14. It builds up into a thick spongy
layer full of blood vessels ready to receive a fertilised egg.
Stage 3 - An egg develops and is released. This stage is called ovulation. It is also the stage in which the woman
is the most fertile. This happens at about Day 14.
Stage 4 - The lining is maintained. The lining is maintained for approximately 14 days, until day 28. If no
fertilised egg has implanted into the uterus wall by day 28, the lining
starts to break down again and the whole cycle restarts.
The menstrual cycle is controlled by four hormones:
1. FSH (follicle- stimulating hormone)
● It causes the egg to mature in one of the ovaries.
● Stimulates the ovaries to produce oestrogen.
2. Oestrogen
● The hormone responsible for thickening and growing the lining of the uterus.
● Stimulate the production of LH.
● Inhibits the production of FSH so that only one egg is released in each cycle.
3. LH (luteinising hormone)
● Stimulates the release of an egg at day 14 (ovulation)
2
