System integration is the coordination process where the component parts of a living system communicate and interact, allowing them to perform an overall function together. It sounds abstract, but it’s the daily work of staying alive: muscles, nerves, blood vessels, glands and organs aren’t separate little machines.

A subsystem is a smaller working part within a larger system, contributing to that system’s function. In a body system, subsystems may be organs; inside an organ, they may be tissues; within a cell, they may be organelles.

Integration depends on communication. Sometimes the interaction is a simple feedback loop, where the result of a process affects the process itself. A negative feedback mechanism is a regulatory loop that reduces deviation from a set condition. A positive feedback mechanism is a regulatory loop that amplifies a change until a particular endpoint is reached. In real organisms, feedback often sits inside a larger network with several inputs and outputs, rather than one neat circle.

Two guiding ideas keep coming back in this topic. Nerves and hormones integrate body systems by sending information between parts of the body; feedback mechanisms regulate body systems by changing activity when monitored conditions move away from, or toward, important endpoints.

Multicellular organisms are arranged as nested systems. A cell is the smallest living unit that can carry out life processes. A tissue is a group of specialised cells working together to perform a particular function. An organ is a body structure made of several tissues that cooperate to perform a specific function. An organ system is a group of organs that interact to carry out a broad life function. An organism is an individual living thing made of integrated systems.

Hierarchy of biological organisation from cell to organism, showing the subsystem each level is built from and how integration produces function.

This hierarchy matters. Each level relies on the one below it, yet it can show features that the smaller parts do not have by themselves. An emergent property is a feature of a system that appears from interactions among its parts and is not obvious from the parts considered separately. Think of a cheetah hunting: muscle contraction, vision, respiration, circulation, balance, learning and motivation all need to work together. A list of organs would not predict the chase.

Cells in a tissue are specialised, physically connected and chemically coordinated. In animal tissues, membrane proteins help neighbouring cells adhere. In plant tissues, the middle lamella between cell walls helps cells stick together. Many tissues contain more than one cell type, because they divide the work; one cell type may form a thin exchange surface while another secretes a useful substance.

Organs bring in another layer of interdependence. In a leaf, photosynthetic tissue depends on gas-exchange tissue for carbon dioxide supply, while gas-exchange tissue depends on photosynthetic tissue to maintain concentration gradients. At organ-system level, organs may be physically linked, as in the digestive tract, or spread through the body, as in the endocrine system.

Animal organs work together in three main ways: nerve messages, hormone messages, and transport in the blood.

A hormone is a signalling chemical released by cells in one part of an organism that changes the activity of target cells elsewhere. The endocrine system is a communication system made of hormone-secreting glands and cells that release hormones into the blood. Hormonal signalling is usually slower than nervous signalling, but its effects can spread widely and last longer. Only cells with the correct receptor respond, so a hormone may circulate through the body without affecting every cell.

A nerve impulse is an electrical signal that travels along a neuron membrane due to ion movement. The nervous system is a communication system made of neurons and supporting cells that transmits rapid signals to specific cells. Nervous signalling is quick and targeted. It is especially useful when one exact muscle or gland needs to respond straight away.

Comparison of hormonal and nervous signalling in animal organ integration.

The blood system is not just plumbing; it is the transport link that makes organ integration possible. Blood carries hormones from endocrine glands to target organs. It also delivers oxygen and glucose to respiring tissues, carries carbon dioxide and other wastes away from tissues, moves water and solutes around the body, and takes absorbed nutrients from the gut to other organs. Signalling tells organs what to do; transport supplies the materials and energy to do it.

The brain is the central nervous system organ that receives information from many inputs, processes it, stores some information and sends instructions to coordinate responses. Keep the syllabus boundary clear here: you need the broad role of the brain in combining inputs, learning and memory, not detailed neurotransmitter pathways.

Sensory information reaches the brain from specialised sense organs, and also from receptors inside the body. The brain then compares and combines these inputs. Movement, for example, depends on visual information, balance information and sensory information from muscles and joints being processed together.

Memory is the capacity of the nervous system to store information so it can influence future responses. Learning is a change in behaviour or response based on experience. Both are integration processes. They depend on many neurons interacting, not a single “memory cell”.

After it has processed information, the brain sends output to effectors. An effector is a muscle or gland that carries out a response after receiving a signal. Muscles contract; glands secrete. So the brain is not just an information store — it is a control centre that links input to action.

The central nervous system is the part of the nervous system made up of the brain and spinal cord. It integrates information and coordinates responses. The spinal cord is the central nervous system organ inside the vertebral column; it links the brain with the body and coordinates some unconscious responses.

In a transverse section of the spinal cord, two main regions stand out. White matter is nervous tissue rich in myelinated nerve fibres that carry signals up and down the spinal cord. Grey matter is nervous tissue rich in neuron cell bodies, dendrites and synapses, where information is processed.

A conscious process is a response that can be voluntarily initiated or controlled while awake. An unconscious process is a response that happens without deliberate thought and can occur when awake or asleep. The spinal cord matters especially in unconscious reflexes, because processing in spinal grey matter can produce a response without waiting for full processing in the brain.

Don’t make the mistake of saying “skeletal muscle always means conscious”. Skeletal muscle can be used consciously, as when you choose to stand up, but it also takes part in unconscious posture reflexes. Biology is often less tidy than tables make it look.

A sensory neuron carries nerve impulses from a receptor or sensory nerve ending to the central nervous system. A receptor is a cell or nerve ending that detects a specific stimulus and starts a signal. A stimulus is a detectable change in the internal or external environment.

Some sensory neurons detect stimuli directly through free nerve endings, for example heat, touch or pain in the skin. Others pick up signals from specialised receptor cells, such as light-sensitive cells in the retina. The body also relies on internal receptors: stretch receptors help monitor muscle length and blood pressure, while chemoreceptors monitor the chemical condition of blood.

Signals from the head often reach the brain through cranial nerves. Signals from much of the body enter the spinal cord through spinal nerves and may then pass to the brain. In the cerebral hemispheres, sensory inputs are handled by specialised regions, such as the visual cortex for information from the eyes.

The two-point discrimination test gives a useful way to picture sensory input. A receptive field is the area from which sensory information is treated by a central nervous system neuron as coming from one location. Fingertips have smaller receptive fields than the shoulder, so two close points are more easily detected as separate on the fingertip.

A motor neuron carries nerve impulses from the central nervous system to an effector. The cerebral hemispheres, especially the primary motor cortex, send output through motor pathways to skeletal muscles.

Skeletal muscle is muscle tissue attached to bones that contracts to move parts of the skeleton. When a nerve impulse reaches the end of a motor neuron at a muscle, it stimulates the muscle fibres to contract. That’s the key idea here: output from the cerebral hemispheres becomes movement because motor neurons activate muscles.

Motor control is mapped across the primary motor cortex. Different regions of the cortex signal to different body parts, and the amount of cortex given to a body part reflects how finely its movements are controlled. Hands and face need precise control, so they have a large representation.

Motor output often works as a chain, not as one long neuron running from cortex to toe. One motor pathway may descend from the brain into the spinal cord and synapse with another motor neuron, which then leaves the spinal cord to reach a specific muscle.

A nerve is a bundle of nerve fibres wrapped in a protective sheath, carrying impulses between the central nervous system and the body organs. A nerve fibre is the long extension of a neuron, usually an axon, that conducts nerve impulses.

Most nerves include both sensory and motor fibres. So one nerve can carry input from receptors towards the central nervous system, then carry output from the central nervous system towards muscles or glands. There are exceptions, but mixed nerves are the usual pattern in body regions.

It helps to learn a transverse section of a nerve by sight. Look for the outer protective sheath, the many circular nerve fibres inside, myelinated fibres with a clear or stained ring of myelin around an axon, and unmyelinated fibres without a thick myelin sheath.

This is a useful spot to link back to the question about patterns of organisation. Neurons have branching dendrites, giving a dendritic pattern because one structure divides again and again into smaller branches. Nerves, blood vessels and airways show branching organisation too. A food web, however, is reticulate: its many connections make a net-like pattern, not a simple one-way branch.

A reflex action is a rapid, involuntary response to a specific stimulus. A reflex arc is the pathway of receptors, neurons and effectors that coordinates a reflex. Reflexes are fast because they use few neurons and can be processed in the spinal cord.

Use the hand-withdrawal pain reflex as your main example. A free sensory nerve ending in the skin acts as the pain receptor. When the hand touches something damaging, positively charged ions enter the sensory nerve ending, threshold is reached, and impulses pass along the sensory neuron into the spinal cord.

In the grey matter of the spinal cord, the sensory neuron synapses with a single interneuron. An interneuron is a neuron within the central nervous system that connects sensory and motor pathways and processes information. The interneuron then synapses with a motor neuron. Impulses travel out along the motor neuron to skeletal muscle, the effector, which contracts and pulls the hand away.

The brain perceives the pain, but withdrawal starts before you’ve consciously analysed the situation. That’s the advantage: limit damage first, work out the details second.

The cerebellum sits at the back and lower part of the brain. It coordinates the timing and precision of skeletal muscle contraction, and helps maintain balance.

The cerebellum does not usually decide the aim of a movement. It works more like a fine-tuner. The cerebral hemispheres may start the movement, while the cerebellum helps keep the contractions smooth, accurately timed and balanced. Walking across a room, catching a ball, standing upright and typing all rely on this constant adjustment.

To keep balance, the brain has to combine information from the eyes, inner ear, muscles and joints. The cerebellum uses that information to adjust posture and muscle contraction without you consciously calculating every shift in body position.

A circadian rhythm is an internal biological rhythm with a cycle of about 24 hours. In humans, the circadian system strongly affects when we sleep and wake, and it is linked to light-dark cycles.

Melatonin is a hormone secreted by the pineal gland. It promotes drowsiness and helps set the sleep-wake cycle. Its secretion follows a diurnal pattern: levels rise during the evening and night, then fall toward dawn and stay low during the day.

The pineal gland is an endocrine gland in the brain that secretes melatonin. Timing signals from the brain’s internal clock influence its secretion, and light detected in the retina adjusts that clock. That’s why light exposure late at night can shift sleep patterns: the body treats light as timing information.

Melatonin doesn’t “knock you out” like an anaesthetic. It nudges the body toward night physiology: drowsiness, sleep readiness and changes such as a night-time fall in core body temperature. For IB, the key pattern is simple: high melatonin supports sleeping; falling melatonin supports waking.

Epinephrine is a hormone secreted by the adrenal glands that prepares the body for vigorous activity. It is also called adrenaline. The adrenal glands are endocrine glands above the kidneys that secrete hormones involved in stress and activity responses.

Before intense muscle contraction, the body needs this kind of wide-reaching signal. Skeletal muscles will need more ATP, which means they need more oxygen and more respiratory substrate. Epinephrine helps get both to where they’re needed.

Key effects include:

• glycogen breakdown in muscle cells, providing glucose for respiration
• glycogen breakdown in liver cells, releasing glucose into the blood
• dilation of bronchi and bronchioles, making ventilation easier
• increased ventilation rate
• increased heart rate through effects on the sinoatrial node
• vasodilation of arterioles supplying skeletal muscles and liver
• vasoconstriction of arterioles supplying the gut, kidneys, skin and extremities

The result is a shift in supply. More blood per minute reaches active muscles, and that blood contains more glucose and oxygen. It’s a neat example of integration: endocrine signalling, circulation, gas exchange and muscle metabolism all change together.

In a threatening or exciting situation, epinephrine can also override slower feedback adjustments. For a short time, the body prioritises rapid action over normal resting regulation.

The hypothalamus is a region of the brain that connects information from the nervous system with endocrine control. The pituitary gland sits below the hypothalamus and secretes hormones under its control.

You don’t need the detailed differences between anterior and posterior pituitary mechanisms here. At syllabus level, keep it simple: the hypothalamus receives many kinds of information, including signals about blood composition, body temperature, osmolarity, hormone levels and emotional state. It then influences pituitary hormone secretion.

Through this link, the brain can control endocrine responses. For example, osmoreceptors in the hypothalamus help regulate secretion of antidiuretic hormone, which affects water balance. During puberty, hypothalamic signals stimulate pituitary secretion of hormones that act on the gonads, leading to changes in sex hormone secretion.

The hypothalamus-pituitary link is a strong example of body-system integration: nervous information is converted into hormonal output, and that hormonal output can affect organs across the body.

Heart rate is the number of heartbeats per minute. A cardiovascular centre in the medulla oblongata adjusts it by sending nerve impulses to the heart. The medulla oblongata is a region of the brainstem that coordinates involuntary processes, including cardiovascular and respiratory regulation.

A baroreceptor is a pressure receptor in the wall of a blood vessel that detects blood pressure. Baroreceptors are found in the aorta and carotid arteries. A chemoreceptor is a receptor that detects chemical conditions, such as blood pH or concentrations of oxygen and carbon dioxide. Chemoreceptors involved in heart-rate control are also located in the aorta and carotid arteries.

When blood pressure drops too low, baroreceptor input to the medulla increases heart rate and stroke volume, which raises blood pressure. When blood pressure rises too high, heart rate is reduced, so blood pressure falls. This is negative feedback because the response opposes the change.

Chemoreceptors provide chemical monitoring. Low blood oxygen, high carbon dioxide or low blood pH signals that tissues need improved blood flow. The medulla responds by sending impulses that increase heart rate and stroke volume, delivering more oxygen and removing more carbon dioxide. If oxygen and pH are high enough, heart activity can be reduced.

The heart receives signals to speed up and signals to slow down. In class, the accelerator-and-brake image works well: sympathetic input increases heart rate; vagus nerve input decreases it.

Ventilation rate is the number of breathing cycles per minute. Respiratory centres in the brainstem mainly control it by sending nerve impulses to the diaphragm and intercostal muscles.

Carbon dioxide drives most of the change in blood pH. Cells produce carbon dioxide during respiration, and carbon dioxide in the blood adds to acidity, so a rise in carbon dioxide lowers blood pH. If pH drops too far, enzymes and cells are affected; normal blood pH stays within a narrow range.

Chemoreceptors in the brainstem monitor pH changes. Those in the aorta and carotid arteries contribute too, especially by monitoring blood oxygen. When carbon dioxide rises and pH falls, chemoreceptors stimulate the respiratory centres to increase ventilation rate. More carbon dioxide is exhaled, blood pH rises back toward normal, and ventilation rate can fall again. This is negative feedback.

The breathing muscles act as the effectors. The diaphragm is a sheet of muscle below the lungs that contracts during inhalation, increasing thoracic volume. Intercostal muscles are muscles between the ribs that help move the ribcage during ventilation. Signals sent to these muscles change the timing and depth of breathing.

Spirometer or sensor data give a practical way to investigate this. From volume-time traces, tidal volume, ventilation rate and air flow can be estimated; exercise should generally increase ventilation rate and often tidal volume because carbon dioxide production rises.

Peristalsis is a wave of smooth muscle contraction that pushes material along the gut. In the gut wall, circular muscle is arranged around the gut, while longitudinal muscle runs along its length. When circular muscle contracts behind a bolus, it stops the material moving backwards; longitudinal muscle helps push it forwards.

The enteric nervous system is a network of neurons in the wall of the digestive tract that coordinates gut movement independently of conscious control. From swallowing to egestion, peristalsis is controlled mainly by this enteric system rather than by deliberate thought. So digestion carries on while you’re doing something else.

The central nervous system plays a role at the two ends of the process. Swallowing begins voluntarily: the tongue, made of skeletal muscle, pushes food to the back of the mouth. Then receptors in the pharynx trigger involuntary actions, and the enteric nervous system controls movement through the oesophagus and gut.

Egestion of faeces also comes partly under voluntary control after early childhood. The rectum and anal sphincters are involved, with control shifting from purely involuntary in babies to voluntary regulation in older children and adults. For the exam, keep the boundary clear: voluntary control at the beginning and end; enteric involuntary coordination between them.

A tropism is a directional growth response: a plant organ grows toward or away from a stimulus coming from one direction. A positive tropism is growth toward a stimulus. A negative tropism is growth away from a stimulus.

Seedlings work well for observing tropisms because their shoots and roots grow quickly. Shoots often grow toward light, while roots often grow in the direction of gravity. The key word is “growth”. A tropism is not a quick movement like an animal reflex; it happens when one side grows faster than the other.

For the application of skills, you need to be able to collect both qualitative and quantitative observations. Qualitative data are descriptive observations without numerical measurement, for example a labelled drawing showing that shoots curved toward a lamp. Quantitative data are numerical observations collected by counting or measuring, such as the angle of curvature of a shoot.

A useful tropism record might include diagrams or photographs taken at set intervals. For quantitative work, you could measure curvature angle with a protractor or with image-analysis software. Accuracy is closeness of a measured value to the true value. Precision is the fineness or repeatability of measurement. Reliability is confidence in results because methods reduce random variation and repeated measurements agree.

Precision and accuracy can be limited by unclear start and end points for the angle, uneven seedling age, inconsistent light intensity, different seed orientation, parallax when reading a ruler or protractor, and too few repeats. To improve the experiment, use seedlings of similar age, a fixed light source and distance, a dark control where useful, repeated trials, calibrated measuring tools, consistent timing and the same method for measuring all seedlings.

Phototropism is directional growth in response to light. In shoots, positive phototropism means the shoot grows toward the side with greater light intensity.

When light reaches a shoot from one side, the shoot tip detects its direction. Cells on the shaded side elongate faster than those on the illuminated side, so the shoot bends toward the light. Once the tip faces the light source, growth evens out again.

This helps the plant capture more light for photosynthesis, especially when nearby plants create shade. A seedling that bends toward a gap in the canopy gets more light, giving it a better chance of making enough sugars to survive.

You are not required to learn other named tropisms for this statement. Focus on the causal pattern: lateral light → unequal growth → curvature toward light.

A phytohormone is a plant signalling chemical that controls growth, development or responses to stimuli, often at very low concentrations. Plants use several phytohormones, not one single “plant hormone”.

Phytohormones change growth by altering rates of cell division and cell enlargement. They also shape development: buds may grow, flowers may form and fruits may ripen. Some help plants respond to stimuli, including the directional stimuli involved in tropisms.

Examples you should recognise at this level include auxin, a phytohormone involved in cell elongation and phototropism; cytokinin, a phytohormone involved in cell division and shoot growth; and ethylene, a gaseous phytohormone involved in fruit ripening. Other phytohormones exist, but the main idea is simple: plants integrate body systems chemically too.

Auxin efflux carriers are membrane transport proteins that move auxin out of plant cells, helping to build auxin concentration gradients in tissues. Where they sit in the plasma membrane matters.

Auxin can enter plant cells by diffusion when it is in an uncharged form. Once inside, the cytoplasm makes auxin more likely to be charged, so it can’t simply diffuse back out through the membrane. Efflux carriers get around this by actively transporting auxin out of the cell.

When efflux carriers are concentrated on one side of each cell, auxin is pumped again and again in the same direction, from cell to cell. That polar placement across many cells creates a concentration gradient across a tissue. The plant can then use the gradient to produce different growth rates, as in phototropism.

This is another pattern-of-organisation link. The transport pathway is not a random soup of molecules. It is a coordinated cellular pattern: many cells align their carriers, and a tissue-level gradient emerges.

Auxin helps cells grow by loosening the cell wall, which lets turgor pressure stretch the cell. The apoplast is the cell-wall and extracellular space outside the plasma membrane through which substances can move. Cell elongation depends on the changes that happen in this space.

Plant cell walls contain cellulose microfibrils cross-linked by other wall molecules. Those cross-links make the wall strong, but they also limit expansion. Auxin stimulates hydrogen ion secretion into the apoplast. As pH falls, cross-links in the wall weaken, so cellulose microfibrils can slide apart more easily and the cell elongates.

The sequence is: auxin concentration increases → hydrogen ions are pumped into the apoplast → cell wall becomes more acidic → cross-links loosen → turgor pressure drives elongation. Cells elongate faster where auxin concentration is higher. Where auxin concentration is lower, they elongate more slowly.

This difference in growth rate bends shoots during phototropism. Auxin doesn’t “push” the shoot toward light; it causes unequal cell elongation, and the shape of the shoot takes care of the rest.

Root and shoot growth have to work together. If a plant makes a huge shoot without enough root, it can't take up enough water and mineral ions. If it makes plenty of roots but very little shoot, it can't photosynthesise enough. Auxin and cytokinin help keep the two in balance.

Shoot tips produce auxin, which moves down shoots and into roots. Root tips produce cytokinin, which moves upward into shoots. In some processes these two phytohormones work together; in others, they act against each other.

Auxin and cytokinin transport and how their interactions balance root and shoot growth.

A synergistic interaction is an interaction in which two factors act together to produce a stronger or shared effect. Auxin and cytokinin can act synergistically in cell division and cell enlargement. An antagonistic interaction is an interaction in which two factors have opposing effects. Auxin and cytokinin can act antagonistically in the formation of roots and side shoots.

Apical dominance is a useful example. Apical dominance is inhibition of lateral bud growth by signals from the shoot tip. Auxin from the shoot tip inhibits lateral buds, while cytokinins tend to promote their growth. Remove the shoot tip and auxin falls, so lateral shoots can grow more strongly. Gardeners use this when they pinch out shoot tips to make a plant bushier.