Co-ordination & Response (Edexcel IGCSE Biology)

False.

Receptor cells detect changes (stimuli) and transmit this information to the coordination centre, which then processes the information and sends signals to effectors to carry out responses.

An effector is a muscle or gland that brings about a response in order to restore normal conditions in the body.

True.

Coordinated responses require a stimulus (environmental change), receptor cells (to detect stimuli) and an effector (to carry out responses and restore optimal conditions). There will also be a coordination centre to process information.

A coordination centre, such as the brain or spinal cord, receives and processes information from receptors and initiates responses that restore optimal conditions in the body.

The effector that releases oestrogen is the ovaries. Oestrogen is released in the menstrual cycle in response to the hormone FSH.

Plants need to respond to stimuli to ensure that they can maximise their growth, e.g.

• Responding to light ensures their leaves can absorb maximum light energy for photosynthesis

• Responding to gravity ensures that shoots grow upwards towards light and roots grow downwards towards water and mineral ions

Tropisms are directional growth responses to stimuli, such as light and gravity, in plants.

A positive tropism occurs when a plant grows towards a stimulus, e.g. growing towards light.

A negative tropism occurs when a plant grows away from a stimulus, e.g. growing against gravity.

False.

Positive geotropism will result in downward growth that is towards gravity; this would occur in roots. Growth away from gravity occurs in shoots and is known as negative geotropism.

Phototropism is the directional growth of plants in response to light, allowing shoots to grow towards light to optimise photosynthesis.

Geotropism, also known as gravitropism, is the directional growth response of plants in relation to gravity; roots grow downwards and shoots grow upwards.

True.

Shoots exhibit positive phototropism by growing towards light, and negative geotropism by growing away from gravity.

True.

Roots demonstrate negative phototropism by growing away from light, and positive geotropism by growing towards gravity.

True.

Auxin is a growth regulator that alters the growth of plants; it is sometimes referred to as a plant hormone.

Auxin stimulates cell elongation in plant shoots.

When light shines on one side of a shoot the auxin produced in the tip accumulates on the shaded side, causing faster elongation and growth of cells on the shaded side compared to the sunny side.

Auxin accumulation on the shaded side of a shoot causes cells on the shady side to elongate at a faster rate than cells on the light side; this leads to bending of the shoot towards the light.

A shoot that receives even light on all sides will grow straight upwards. This is because auxin would be evenly distributed and so cell elongation would occur at the same rate in all cells.

If a plant was placed in a dark room auxin would be evenly distributed and so the plant shoots would continue to grow straight upwards.

Information is transmitted through the nervous system along neurones in the form of electrical impulses.

Information is transmitted by the endocrine system when hormones are carried by the blood to target cells.

False.

Hormones are used to control functions that do not require instant responses; this is because hormonal transmission is slow in comparison to nervous transmission.

Examples of hormone, or endocrine, glands include:

• Ovaries

• Testes

• Pituitary gland

• Pancreas

• Adrenal gland

False.

Hormones only affect specific target cells because they bind to receptors which are only present on the surface of these target cells.

True.

Hormonal responses can be very long-lived, while the effect of a nerve impulse stops as soon as impulse transmission stops.

The CNS is made up of the brain and the spinal cord.

False.

A bundle of neurones is known as a nerve.

The role of the central nervous system is to receive information from the senses and coordinate a response.

Sensory neurones carry impulses from sense organs to the central nervous system (CNS).

False.

Motor neurones carry impulses from the CNS to effectors (muscles or glands).

False.

The myelin sheath insulates axons in order to increase the speed at which nerve impulses are transmitted.

The pathway of an impulse is as follows:

stimulus → receptor → sensory neurone → CNS → motor neurone → effector → response

Examples of stimuli which would trigger a response via the nervous system include:

• Light

• Sound

• Touch / pressure / pain

• Temperature

• Chemicals, e.g. in the air or in food/drink

When a receptor is stimulated an electrical impulse is sent along sensory neurones towards the central nervous system (CNS).

False.

The effector is the part of the body that carries out the response, so it is the last component to receive information about a stimulus. The first component would be the receptor.

Receptors are specialised cells that detect specific stimuli and convert them into electrical impulses, initiating the transmission of signals through the nervous system.

True.

Relay neurones are found in the central nervous system. They connect sensory neurones to motor neurones.

A synapse is a junction between two neurones where a very small gap exists between one neurone and the next.

Neurotransmitters are chemical signalling molecules used to transfer signals between neurones at synapses.

False.

Electrical impulses cannot directly jump the gap at synapses. Electrical signals are converted into chemical signals in the form of neurotransmitters.

An electrical impulse travels across the gap at a synapse as follows:

• Neurotransmitters are released and diffuse across the gap between neurones

• Neurotransmitters bind to receptor molecules on the second neurone

• A new electrical impulse is generated in the second neurone

False.

Neurotransmitters move across the gap at a synapse by diffusion from a high concentration to a low concentration.

After impulse transmission has finished the neurotransmitters are broken down by enzymes to prevent continuous stimulation of the second neurone.

False.

Reflex responses do not involve conscious processing in the brain; they are automatic and rapid, meaning that the brain may not become consciously aware of the stimulus until after the body has responded.

A reflex is an involuntary reaction that does not involve conscious processing in the brain. It is automatic, rapid, and helps minimise damage to the body.

Examples of reflex responses include:

• Pain-withdrawal, e.g. removing a hand from a hot or sharp object

• Blinking

• The pupil reflex

• Coughing

• The knee-jerk reflex

In a reflex arc for withdrawing from a hot object the pathway is as follows:

Hot object (stimulus) → sensory neurone → relay neurone in spinal cord → motor neurone → muscle (effector) → contraction to move away from object (response)

The three types of neurone in a reflex arc are:

• Sensory neurones carry impulses to the CNS

• Relay neurones connect sensory and motor neurones within the CNS

• Motor neurones carry impulses from the CNS to effectors

The spinal cord acts as the coordinator in a reflex arc, receiving sensory input and generating motor output without involving the conscious brain.

Synapse occur where signals are transmitted from one neurone to another; in a reflex arc this would be:

• Between sensory and relay neurones

• Between relay and motor neurones

Reflex responses are important because they are quick and automatic, allowing the body to avoid injury.

The receptor cells in the retina which are sensitive to light are rods and cones.

The optic nerve is responsible for transmitting electrical impulses from the receptor cells in the retina to the brain.

The fovea is a region of the retina with the highest density of cones; light focused here results in detailed, colour vision.

True.

The circular and radial muscles of the iris contract and relax to alter the light that can enter the eye through the pupil.

The cornea is a transparent layer located on the surface of the eye. It provides protection and plays a role in focusing light onto the retina.

The blind spot is the area where the optic nerve exits the retina; it lacks receptor cells, thus creating a spot where no vision occurs.

The eye is a highly specialised sense organ which detects light stimuli.

False.

The optic nerve is made up of sensory neurones which transmits electrical impulses from the receptors in the retina to the brain.

The suspensory ligaments connect the ciliary muscle to the lens, facilitating changes in lens shape during accommodation. This allows the eye to focus on near or distant objects.

The receptor cells that are affected in colour blindness are the cone cells. These are the receptors which are responsible for detecting light wavelength, so they provide colour vision.

Accommodation is the process by which the lens of the eye changes its shape to focus on objects at different distances. It is facilitated by the contraction or relaxation of the ciliary muscles and adjustment of the suspensory ligaments.

Refraction is the bending of light as it enters a medium of a different density, e.g. the lens of the eye.

Two structures of the eye which are responsible for refraction of light are:

• Cornea

• Lens

The ciliary muscle controls the shape of the lens by contracting to thicken it during close vision and relaxing to flatten it during distant vision. This is known as accommodation.

When focusing on a near object:

• The ciliary muscles contract

• The suspensory ligaments loosen

This causes the lens to become thicker (more curved), increasing refraction of light for close vision.

False.

When focusing on a distant object:

• The ciliary muscles relax

• The suspensory ligaments tighten

This pulls on the lens, making it thinner and less curved; this reduces refraction of light for distant vision.

The pupil reflex protects the retina from damage in bright light or improves vision in dim light. It does this by adjusting the size of the pupil to control the light entering the eye.

The antagonistic muscle pair responsible for the pupil reflex is the:

• Circular muscles

• Radial muscles

Both of these muscles are found in the iris of the eye.

True.

Radial muscles contract and the circular muscles relax in response to dim light. This results in dilation of the pupil to allow more light to reach the retina.

In bright light the circular muscles contract and the radial muscles relax. This causes the pupil to constrict, reducing the light entering the eye and protecting the retina from damage.

Temperature regulation ensures that core temperature stays at around 37 °C. This maintains optimal enzyme activity and therefore efficient physiological function.

Sweating increases heat loss when sweat evaporates from the skin surface. The evaporation of sweat transfers heat energy away from the skin.

Vasodilation is a cooling mechanism in which the diameter of skin arterioles increases. This increases blood flow to capillaries in the skin surface, transferring heat to the environment by radiation.

An increase in body temperature leads to the following changes:

• Sweat glands release sweat which cools the skin by evaporation

• Skin arterioles undergo vasodilation to increase blood flow to the capillaries at the surface of the skin

True.

Exercise raises core body temperature due to increased metabolic activity in the contracting muscles. Sweating and vasodilation occur as part of the temperature regulation mechanisms of the skin.

True.

Vasoconstriction is a heat retention mechanism. The diameter of skin arterioles decreases to reduce blood flow to capillaries in the surface of the skin. This minimises heat loss by radiation.

Shivering aids temperature regulation because the rapid muscle contractions generate heat through respiration; this helps to raise the core body temperature.

False.

Capillaries do not have a muscle layer and therefore cannot constrict. Constriction occurs in the arterioles that supply the skin capillaries, reducing blood flow to the skin surface.

A hormone is a chemical messenger molecule released from glands and transported in the blood. Hormones bring about changes in specific target cells.

Adrenaline is known as the 'fight or flight' hormone and its role is to prepare the body for action. Its effects include:

• Increased heart rate

• Increased breathing rate

• Diversion of blood flow to muscles

• Breaking down glycogen to release glucose

The pancreas is the gland that produces and secretes insulin.

False.

Insulin reduces blood glucose levels by increasing the uptake of glucose from the blood into the body cells, where it is converted into glycogen. Blood glucose levels increase in response to adrenaline.

Testosterone, produced in the testes, is responsible for the development of secondary sexual characteristics in males.

False.

Progesterone, produced in the ovaries, maintains the uterine lining during parts of the menstrual cycle and during pregnancy. Secondary sexual characteristics develop in females due to the action of oestrogen.

Oestrogen, produced by the ovaries, is responsible for the development of secondary sexual characteristics in females. It is also involved with regulation of the menstrual cycle.

Osmoregulation is the homeostatic mechanism regulated by ADH. It involves maintaining the balance of water and solutes in the blood.

The secretion of ADH changes in response to blood water content as follows:

• ADH secretion increases when blood water content is low

• ADH secretion decreases when blood water content is high

True.

Increased secretion of ADH increases water reabsorption in the nephrons when blood water content is low, resulting in production of concentrated urine.

The menstrual cycle is regulated by:

• FSH

• LH

• Oestrogen

• Progesterone

The roles of FSH and LH in the menstrual cycle are:

• FSH stimulates egg maturation in the ovary and stimulates the release of oestrogen

• LH triggers ovulation

False.

ADH, LH and FSH are produced by the pituitary gland. Oestrogen and progesterone are produced by the ovaries.