C3.1 - Body systems

System integration

Process by which different components of a living system work together to perform an overall function. Communications occur between nervous system, endocrine system, and local feedback loops

Cell

Basic unit of life with a specific function eg. Muscle cell

Tissue

Group of similar cells working together. eg. Muscle tissue

Organ

Structure of different tissues with a specific role. eg. heart

Organ system

Group of organs working together for a function eg. circulatory system

Organism

A complete living thing made of multiple systems eg. Cheetah

Emergent properties

Abilities that arise from the interaction of simpler parts. eg. A cheetah hunts well due to the musculoskeletal system, nervous system, and Circulatory/respiratory system.

Nervous system

Very fast electrical impulses across neurons for short lived effects eg. Reflect withdrawal from hot surface.

Endocrine system

Slower chemical messengers secreted into blood for longer lasting effects eg. Adrenaline release during fight or flight

Examples of transport in the blood system

Oxygen from lungs to muscles.

Nutrients from gut to tissues.

Waste to lungs/kidneys

Role of brain

Gathers info from multiple sensory inputs, processes and compares inputs to form an understanding, coordinates appropriate responses based on processed info.

Learning

Modifying behaviour through experience

Memory

Storing info for future use

Conscious process

Controlled by the brain. Involves awareness, thinking, and decision making. eg. Waving at a friend

Unconscious process

Controlled without active thought. Involves spinal cord as an integrating centre. eg. Rapid withdrawal of hand from something hot.

Role of spinal cord

Receives sensory input, processes it, sends motor output without using the brain.

Sensory neuron

specialised nerve cells that carry info from receptors to the central nervous system (CNS). Sensory neurons transmit electrical impulses generated by stimuli.

Process of sensory neuron to CNS

Stimulus detected by a receptor cell.

Eg. Light (eye), sound (ear), temperature (skin), chemicals (nose/tongue).

Conversion to electrical signal (generator potential → action potential).

Sensory neuron carries impulse to:

Spinal cord – for rapid reflex responses.

Cerebral hemispheres – for conscious perception and decision-making.

Motor neurons

Specialised nerve cells that carry instructions from the CNS to effectors.

Process from CNS to movement

Decision made in the cerebral hemispheres.

Example: Deciding to wave at a friend.

Motor commands sent down through descending pathways in the spinal cord.

Motor neuron activation → electrical impulse travels to muscle fibres.

Muscle contraction occurs → producing movement.

Protective sheath

Outer covering that protects and supports the nerve

Nerve fibres

Individual axons, surrounded by protective sheath

Myelinated fibres

Have a fatty myelin sheath that increases impulse speed.

Unmyelinated fibres

Lack a myelin sheath → slower conduction.

Process of reflex arc

Free sensory nerve detects tissue damage, signal is sent to sensory neuron, which carries the impulse towards the spinal shord. Interneuron in grey matter of spinal cord connects sensory to motor neuron, processes signal locally. Motor neuron sends impulse to effector. Effector contracts, pulling hand away from danger.

Role of Cerebellum

Coordinates movements in the body. Works with other brain regions to make movements smooth and coordinated.

Circadian rhythm

Daily biological cycles lasting 24 hours. Regulate processes like sleep, hormone release, and body temperatures.

Suprachiasmatic nucleus (SCN)

Cluster of neurons in the hypothalamus. Acts as a pacemaker for circadian rhythms and adjusts to environmental light using signals from specialised retinal cells.

Melatonin secretion

Follows dinural pattern. Night: High levels Morning/day: Low

Main effects of epinephrine on body

Muscles & Liver: Breakdown of glycogen to glucose, more fuel for ATP production

Blood: Glucose levels rise, Instant energy supply

Lungs: Bronchiole dilation, more oxygen intake

Heart: Faster heart rate, more oxygenated blood to muscles

Circulation: Blood diverted to muscles and liver, prioritises vital tissues for action

Hypothalamus

Acts as a link between nervous sytem and endocrine system. Monitors blood temperature, hormone levels, nutrient levels

Pituitary gland

Releases hormones that control other endocrine glands, controlled by hypothalamus

How pituitary gland and hypothalamus work

Hypothalamus detects change in the body (e.g., low thyroid hormone). Sends releasing/inhibiting signals to pituitary. Pituitary releases hormones into the blood. These hormones target other endocrine glands (thyroid, adrenal, gonads). Target glands produce their own hormones to restore balance

Barorecepctors

Located in walls of aorta and carotid arteries. Monitors blood pressure, if BP rises it sends signals to reduce heart rate, if BP falls it sends signals to increase heart rate.

Chemoreceptors

Same as baroreceptors, monitors blood pH, Oxygen and carbon dioxide concentration.

Role of medulla

Processes signals from baroreceptors and chemoreceptors. Sends nerve impulses to either increase or decrease heart rate.

Feedback mechanism in chemoreceptors

Condition	Detected by Chemoreceptors	Response
↑ CO₂ (↓ pH)	More acidic blood	Medulla sends nerve impulses to diaphragm & intercostal muscles → ↑ ventilation rate → more CO₂ exhaled → pH returns to normal
↓ CO₂ (↑ pH)	More alkaline blood	Medulla reduces stimulation of breathing muscles → ↓ ventilation rate → CO₂ levels rise back to normal
Low O₂ (hypoxia)	Especially in carotid/aortic receptors	Increases ventilation rate even if pH is normal to ensure enough O₂ reaches brain

Peristalsis

Wave like, involuntary contractions that move food and waste along the digestive tracts

CNS in digestive tract

Swallowing and egestion, both are voluntary

Enteric nervous system in digestive tract

Peristalsis (Oesophagus to large intestine), it is involuntary

Phototropism

Growth towards light (Positive/negative)

Gravitotropism

Growth towards gravity (Positive/negative)

Precision

how close repeated measurements are to each other. Increase by using finer tools & consistent technique

Accuracy

how close measurements are to the true value. Increase by aligning protractor carefully & avoiding parallax errors

Reliability

consistency of results when repeated. Increase by repeating experiment & averaging results

How phototropism works

Light stimulus from one side of the shoot. Auxin hormone (produced in shoot tip) moves to the shaded side. Cell elongation increases more on the shaded side than the lit side. Shoot bends towards the light.

Phytohormones

Chemical messengers in plants that regulate growth, development, and responses to environmental stimuli

How auxin efflux carriers work

Passive Entry: Auxin moves into the cell by simple diffusion.

Directional Exit: Auxin efflux carriers are positioned only on one side of the cell membrane.

Coordinated Cell Action: If all neighbouring cells align their carriers on the same side, auxin is moved cell-to-cell in one direction.

Result: Auxin becomes concentrated in a specific part of the plant (e.g., shaded side of a shoot in phototropism).

Promotion of cell growth by auxin

Hydrogen Ion Secretion: Auxin stimulates H⁺ ion pumps in the plasma membrane.

H⁺ ions are actively transported into the apoplast (space between cell wall and membrane).

Cell Wall Acidification: Lower pH breaks cross-links between cellulose microfibrils.

Loosening of Cell Wall: Cellulose structure becomes more flexible.

Cell Elongation: Water uptake via osmosis increases cell volume.

Flexible walls allow the cell to stretch and grow.

Interactions between auxin and cytokin in regulating root and shoot growth

Root tips produce cytokinin, which is transported up to shoots.

Shoot tips produce auxin, which is transported down to roots.

These hormones influence each other’s growth-promoting effects:

Auxin promotes root growth and can inhibit shoot growth in some cases.

Cytokinin promotes shoot growth and can inhibit root growth.

Ethene

Acts as a signalling chemical that regulates many processes, including fruit ripening

How ethene works in ripening

When fruit starts to ripen, it produces ethylene, which triggers changes like softening of fruit, color changes, development of aroma and sweetness.