Integration of Body Systems (DP IB Biology)

The brain, along with the spinal cord, forms the central nervous system and is responsible for controlling complex behaviours, both conscious and unconscious, by processing and coordinating information.

The cerebral cortex, the highly folded outer layer of the brain, is responsible for higher-order processes such as intelligence, memory, consciousness, and personality.

The cerebellum, located underneath the cerebral cortex, is responsible for balance, muscle coordination, and movement.

The brainstem relays messages between the cerebral cortex, cerebellum, and spinal cord, with the medulla, a key part of the brainstem, controlling unconscious activities like heart rate and breathing.

The pituitary gland produces hormones, including those that control the menstrual cycle (FSH and LH), while the hypothalamus regulates body temperature and produces hormones that control the pituitary gland.

Conscious input to the brain is provided by photoreceptors in the eyes (for vision), chemoreceptors in the tongue (for taste), thermoreceptors in the skin (for temperature), and mechanoreceptors in the inner ear (for sound vibrations).

Unconscious input is provided by osmoreceptors (detecting blood water content), baroreceptors (sensing blood pressure changes), and proprioceptors (providing information on balance and movement).

The spinal cord acts as a neural pathway between the body and the brain, processing sensory information and enabling motor responses. It also serves as an integration centre for unconscious processes and reflex reactions.

White matter contains the axons of neurons that carry information to and from the brain, while grey matter contains neurons and synapses involved in spinal cord integration processes, creating reflex responses.

A receptor is a specialised cell that detects changes in the environment, which cause a stimulus.

Receptor cells convert energy in one form (e.g. light, heat, or sound) into electrical impulses within a sensory neurone, allowing the nervous system to process environmental changes.

Receptor cells are often found in sense organs, such as light receptors in the eyes or chemoreceptors in the taste buds.

When receptor cells are stimulated, they become depolarised. If the stimulus is strong enough, this depolarisation triggers an action potential in the sensory neurone, transmitting the impulse to the CNS.

When chemoreceptors in the taste buds detect salt:

• Sodium ions from the salt diffuse through channel proteins in the microvilli of chemoreceptor cells, leading to membrane depolarisation

• This depolarisation triggers calcium ion channels to open, causing neurotransmitter release

• This then initiates an action potential in the sensory neurone

• Transmitting the impulse to the brain

Motor neurones carry action potentials from the CNS to effectors (muscles and glands), initiating the required response e.g. hormone release or movement.

Motor neurones can terminate at neuromuscular junctions, which are connections between the motor neurone and a muscle cell.

When an impulse arrives at the motor neurone's presynaptic membrane, it causes calcium ions to enter the neurone. This leads to the release of acetylcholine (ACh), which diffuses across the junction, binds to receptor proteins on the muscle cell, and triggers an action potential that leads to muscle contraction.

Calcium ions, released from the sarcoplasmic reticulum in response to an action potential, bind to troponin molecules on the actin filaments. This causes the troponin-tropomyosin complex to shift, exposing binding sites on the actin for myosin, thus initiating muscle contraction.

Myelination refers to the presence of a myelin sheath around the axons of neurones, which speeds up the transmission of nerve impulses along the neurone. Myelinated neurones transmit impulses faster than unmyelinated ones.

A reflex is an action of the body that occurs without conscious thought.

True.

Reflexes are automatic and rapid, minimising damage to the body.

A reflex arc is a pathway along which impulses are transmitted from a receptor to an effector without involving conscious regions of the brain.

A nociceptor is a pain receptor that detects harmful stimuli like a sharp pin or hot flame.

The order of a pain reflex arc is: nociceptor → sensory neurone → relay neurone → motor neurone → effector muscle.

The pupil reflex prevents damage to the eye from bright light.

The cerebellum coordinates movement, including balance, posture, walking, hand and finger movements, eye movements, and speech.

False.

The motor cortex of the cerebrum initiates movement, not the cerebellum.

Melatonin is a hormone secreted by the pineal gland that regulates circadian rhythms, including the sleep-wake cycle.

True. Circadian rhythms in humans are influenced by melatonin.

A circadian rhythm is a daily cycle of physiological and behavioural patterns that occur in a 24-hour period.

Increasing melatonin levels lead to feelings of tiredness and promote sleep.

Light resets the melatonin system daily, keeping the circadian rhythm aligned with daylight hours.

The pineal gland secretes melatonin, which regulates circadian rhythms by responding to light and dark signals.

Epinephrine prepares the body for reacting to a stressful situation, often called the "fight or flight" response.

False.

Epinephrine increases heart rate during stressful situations.

The "fight or flight" response is the body's reaction to stress, fear, or excitement, preparing it for vigorous activity.

Blood vessels in less important organs constrict to divert more blood to organs involved in the "fight or flight" response.

A hormone is a chemical messenger produced by an endocrine gland and carried by the blood.

True. The hypothalamus controls the release of hormones from the pituitary gland.

The hypothalamus monitors the water balance of the blood and releases ADH if the blood becomes too concentrated.

The pituitary gland produces and releases hormones that regulate various bodily processes and stimulates other endocrine glands.

False.

The medulla controls the heart rate through unconscious activities.

A baroreceptor is a pressure receptor that monitors changes in blood pressure, located in the aorta and carotid arteries.

Norepinephrine is released by sympathetic neurones to increase heart rate.

When the acceleratory centre is activated, impulses are sent to the SAN, leading to an increased heart rate.

A chemoreceptor is a sensor that detects changes in blood pH, oxygen, and carbon dioxide levels.

Ventilation rates in the body are controlled by respiratory centres located in the medulla of the brain.

True.

During exercise, the production of carbon dioxide increases due to increased respiration.

Carbonic anhydrase is an enzyme in red blood cells that catalyses the reaction between carbon dioxide and water to form carbonic acid.

Chemoreceptors detect changes in blood pH caused by hydrogen ions and send action potentials to increase ventilation rates.

The equation is:

• CO₂ + H₂O ⇌ H₂CO₃

• carbon dioxide + water ⇌ carbonic acid

True.

The enteric nervous system (ENS) controls peristalsis in the digestive system.

Peristalsis is a series of muscle contractions that pass like a wave along the alimentary canal, moving food through the digestive system.

The peristaltic reflex is triggered by the detection of the bolus of food by stretch receptors in the alimentary canal.

Circular muscles contract to reduce the diameter of the alimentary canal, preventing food from moving backwards.

Smooth muscle ahead of the bolus relaxes, opening the lumen of the alimentary canal to allow food to move forward.

Tropisms are growth responses in plants that are affected by external stimuli such as light, gravity, and the presence of objects.

Examples of tropisms include phototropism (response to light) and gravitropism/geotropism (response to gravity).

Tropisms can be investigated through qualitative diagrams of seedling growth or quantitative measurements of the angle of curvature.

Qualitative observations record observations without numerical data, while quantitative observations collect numerical data that can be processed mathematically.

A qualitative observation could be describing the colour change in a starch test using iodine.

There are other valid answers.

A quantitative observation could be recording the percentage cover of a plant species using a quadrat.

There are other valid answers.

Precise measurements have little spread around the mean value, reducing the impact of random errors.

Accurate measurements are close to the true value, reducing the impact of systematic errors.

Reliability can be increased by taking careful and accurate measurements, using appropriate precision, and performing multiple trials.

Both qualitative and quantitative observations can provide useful information, and the value depends on the purpose of the experiment.

Positive phototropism is the growth response of plant shoots towards light.

Phototropism ensures plants maximise the amount of light they can absorb for photosynthesis, which is crucial due to their immobility.

Positive phototropism affects the growth of plant shoots and the tops of stems.

Phytohormones are plant hormones that regulate growth, development, reproductive processes, longevity, and even death in plants.

Examples of phytohormones include auxins, abscisic acid, cytokinins, ethylene, gibberellins, and brassinosteroids.

Auxin causes cells to elongate, leading to stem growth in shoots.

Auxin inhibits cell growth in roots, which is the opposite effect to that in shoots.

Auxin efflux carriers are membrane proteins (called PIN3 proteins) that actively pump auxin out of cells, allowing it to move to the next cell.

Light affects the expression of genes coding for PIN3 efflux pumps, creating an uneven distribution and leading to an auxin gradient across the stem or root.

Auxin binds to receptors, stimulating proton pumps to acidify the cell wall, activating expansins to loosen the cell wall, and increasing water uptake to cause cell elongation.

Auxin stimulates potassium ion channels to open, increasing potassium ion concentration and decreasing the water potential in the cytoplasm, leading to water uptake and cell elongation.

Light affects the expression of PIN3 efflux pumps, creating an auxin gradient with more auxin on the shaded side, leading to faster cell elongation and bending towards the light.

Auxin has a key role in regulating plant growth, with different effects in shoots (promoting elongation) compared to roots (inhibiting growth).

Auxin, produced in shoots, promotes cell elongation, while cytokinin, produced in roots, promotes cell division. They are transported to each other's production sites and work together to integrate root and shoot growth.

At low concentrations, auxin limits the action of cytokinin, but increased cytokinin counteracts this inhibitory effect. At higher concentrations, the interactions between auxin and cytokinin are prevented.

Cytokinin is transported from the roots to the shoots through the xylem, while auxin is transported from the shoots to the roots through the phloem sap.

The production of ethylene during fruit ripening is an example of positive feedback. Ethylene gas can diffuse between fruits, triggering further ethylene release and accelerating the ripening process.

The original stimulus of ethylene production causes a response that leads to more ethylene being produced, further enhancing the ripening process in a positive feedback loop.