Coordination, Response & Homeostasis (Cambridge (CIE) IGCSE Biology)

False.

Sensory neurones in a reflex arc connect receptor cells with relay neurones in the CNS.

While there are some reflex arcs that involve the brain, the brain regions involved are not connected with conscious thought.

A relay neurone is a neurone within the CNS that connects sensory and motor neurones.

Motor neurones in reflex arcs connect relay neurones with effectors.

False.

Effects are muscles and glands.

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 which diffuse across a synapse gap.

The events at a synapse are as follows:

• neurotransmitters are released from vesicles

• neurotransmitters 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.

True.

One role of a synapse is to ensure that impulses travel in one direction only.

Sense organs are groups of receptor cells that respond to specific stimuli.

The sense organs and stimuli are:

• eyes respond to light

• ears respond to sound

• skin responds to touch and temperature

• nose responds to chemicals in the air

• tongue responds to chemicals in food and drink

False.

Receptor cells in the sense organs connect to sensory neurones.

False.

The sense organs are not part of the central nervous system. The CNS is made up of the brain and spinal cord.

The sense organs contain receptor cells that are connected to the peripheral nervous system (PNS).

The structures labelled A-E are:

• A = iris

• B = cornea

• C = lens

• D = retina

• E = optic nerve

  

The function of the cornea is to refract light into the eye.

False.

The optic nerve carries impulses to the brain.

In dim light the pupil dilates in order to allow as much light into the eye as possible.

Extended Tier Only: the radial muscles in the iris contract and the circular muscles relax.

The iris is responding to bright light; this can be seen because the pupil is constricted.

Extended Tier Only: the pupil is constricted due to the radial muscles relaxing and the circular muscles contracting.

Accommodation is the process by which the lens of the eye changes its shape to focus on objects at different distances.

The ciliary muscles contract or relax to change the shape of the lens during 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; this reduces refraction of light for distant vision.

Cone cells absorb light of different colours/wavelengths, providing colour vision.

Rod and cone cells are distributed as follows:

• rod cells are found all over the retina, with the exception of the blind spot

• cone cells are concentrated in the fovea

The fovea is a region of the retina with the highest density of cones cells. Its function is to form sharp, coloured images.

A hormone is a chemical substance, produced by a gland and carried by the blood, which alters the activity of one or more target organs.

Hormones are carried around the body in the blood.

The endocrine system is composed of glands that secrete hormones directly into the bloodstream.

The glands are:

• A = pancreas

• B = adrenal gland

• C = testes

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

• increased pupil diameter

False.

Compared to the nervous system, the effects of hormones are slower.

Glucagon is secreted by the pancreas.

The role of adrenaline in the control of metabolic activity is:

• increasing blood glucose concentration

• increasing heart rate

This helps to deliver more glucose to muscles for respiration to ensure that the body is well prepared for high levels of activity.

Homeostasis is the maintenance of a constant internal environment within narrow limits.

The function of insulin is to lower blood sugar concentration.

Negative feedback describes a control mechanism that functions to reverse any change in conditions to return to the set point.

Events A-D are:

• A = release of insulin from the pancreas

• B = uptake of glucose from the blood by the cells

• C = release of glucagon from the pancreas

• D = glycogen is broken down into glucose

The liver is involved with glucose regulation as follows:

• glucose is converted into glycogen and stored in liver cells in response to insulin

• glycogen in liver cells is broken down into glucose in response to glucagon

True.

Type 1 diabetes can be treated by injecting insulin. Monitoring and controlling diet allows a correct insulin dose to be administered.

Structures A-D are:

• A = hair erector muscle

• B = sweat gland

• C = hair

• D = blood vessel

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

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

True.

Contraction of the hair erector muscles causes the hairs on the skin surface to stand up, providing a layer of insulation in cold temperatures.

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.

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.

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.

Gravitropism, also known as geotropism, is the directional growth response of plants towards or away from gravity; roots grow downwards and shoots grow upwards.

Phototropism is the directional growth of plants towards or away from light, allowing shoots to grow towards light to optimise photosynthesis.

False.

In an investigation into the effect of gravity on newly germinated seedlings the dependent variable is the length and/or direction of growth of newly grown roots and shoots.

The direction of gravity is the independent variable in this investigation.

Variables that need to be controlled in an investigation into the effect of light on newly germinated seedlings include:

• direction of gravity

• temperature

• water availability / how damp the cotton wool is

• type / size / age of seedlings

True.

Phototropism is a growth response controlled by the plant chemical auxin.

False.

Auxin is produced in the shoot tip before diffusing to the shady part of the plant.

When light shines on one side of a shoot the auxin produced in the tip accumulates on the shaded side.

Auxin stimulates cell elongation in plant shoots.

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.