System integration is coordination: the component parts of a living system communicate and interact so they can carry out an overall function together. It sounds abstract, but it’s the everyday business of being alive. Muscles, nerves, blood vessels, glands and organs don’t work as separate little machines.

A subsystem is a smaller working part within a larger system that contributes to the system’s function. In a body system, that might mean organs; within an organ, tissues; within a cell, organelles.

Integration needs 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 usually sits inside a wider network with several inputs and outputs, rather than a neat single 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 level below it, but it can also 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. A cheetah’s ability to hunt is a good example: 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, held together physically and coordinated chemically. 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 level 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 are integrated in three main ways: messages through nerves, messages through hormones and transport through blood.

A hormone is a signalling chemical released by cells in one part of an organism, changing the activity of target cells somewhere else. 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 it can spread widely and last longer. Because only cells with the correct receptor respond, a hormone can 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’s 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 more than plumbing; it’s the transport link that lets organs work together. Blood carries hormones from endocrine glands to target organs. It also moves oxygen and glucose to respiring tissues, takes carbon dioxide and other wastes away from tissues, carries water and solutes around the body, and transports absorbed nutrients from the gut to other organs. Signalling tells organs what to do; transport supplies the materials and energy needed 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. For the syllabus, keep the focus broad: how the brain combines inputs, supports learning and memory, rather than detailed neurotransmitter pathways.

Sensory information reaches the brain from specialised sense organs and 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 require many neurons interacting, not a single “memory cell”.

After processing 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, linking the brain with the body and coordinating 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, which 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 occurs without deliberate thought and can happen 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 with free nerve endings, for example heat, touch or pain in the skin. Others get their input from specialised receptor cells, such as the light-sensitive cells in the retina. The body also uses internal receptors for essential feedback: stretch receptors monitor muscle length and blood pressure, while chemoreceptors monitor the chemical condition of blood.

From the head, signals often enter the brain through cranial nerves. From much of the body, signals enter the spinal cord through spinal nerves and may then pass to the brain. Sensory input reaching the cerebral hemispheres is processed in specialised regions, such as the visual cortex for information from the eyes.

The two-point discrimination test is a good way to picture how sensory input is organised. 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 easier to detect as separate on a fingertip.

A motor neuron is a neuron that carries nerve impulses from the central nervous system to an effector. The cerebral hemispheres, especially the primary motor cortex, send signals 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 areas of the cortex signal to different body parts. The amount of cortex given to each body part depends on how finely its movements are controlled. Hands and face need precise control, so they have strong representation.

Motor output is usually a chain, not 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 body organs. A nerve fibre is a long extension of a neuron, usually an axon, that conducts nerve impulses.

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

A transverse section of a nerve is worth learning as a picture, not just as words. Look for the outer protective sheath, 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 also links neatly to the question about patterns of organisation. Neurons have branching dendrites, giving a dendritic pattern because one structure keeps dividing into smaller branches. Nerves, blood vessels and airways show branching organisation too. A food web, however, is reticulate because many connections form a net-like pattern rather than a simple one-way branch.

A reflex action is a rapid, involuntary response to a specific stimulus. A reflex arc is the route through receptors, neurons and effectors that coordinates a reflex. Reflexes happen quickly because they involve 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 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 have consciously analysed what happened. 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 it helps maintain balance.

Usually, the cerebellum doesn’t decide the aim of a movement. It acts more like a fine-tuner. The cerebral hemispheres may start the movement, while the cerebellum makes 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 working out every shift in body position.

A circadian rhythm is an internal biological rhythm with a cycle of about 24 hours. In humans, sleep and waking are strongly influenced by a circadian system linked to light-dark cycles.

Melatonin is a hormone secreted by the pineal gland that promotes drowsiness and helps establish 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 reads 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, focus on the pattern: high melatonin supports sleeping; falling melatonin supports waking.

Epinephrine

Epinephrine is a hormone secreted by the adrenal glands. It prepares the body for vigorous activity and 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 effects of epinephrine need to be wide-ranging. Skeletal muscles use more ATP, so they need extra oxygen and more respiratory substrate. Epinephrine helps get both to where they're needed.

Important 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 effect is to upgrade supply and redirect it: more blood per minute reaches active muscles, and that blood carries more glucose and oxygen. It’s a good example of integration, with endocrine signalling, circulation, gas exchange and muscle metabolism shifting 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 brain region that connects information from the nervous system with endocrine control. The pituitary gland sits below the hypothalamus and secretes hormones under hypothalamic control.

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

That link lets the brain 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. The chemoreceptors involved in heart-rate control are also located in the aorta and carotid arteries.

When blood pressure is too low, baroreceptors send input to the medulla, which increases heart rate and stroke volume. Blood pressure rises. When blood pressure is too high, the medulla reduces heart rate, so blood pressure falls. This is negative feedback because the response opposes the change.

Chemoreceptors add 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 that speed it up and signals that slow it down. A useful classroom image is an accelerator and a brake: 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 control it mainly 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. Chemoreceptors in the aorta and carotid arteries also play a role, especially in monitoring blood oxygen. When carbon dioxide rises and pH falls, these chemoreceptors stimulate the respiratory centres to increase ventilation rate. More carbon dioxide is exhaled, blood pH moves back toward normal, and ventilation rate can fall again. That’s negative feedback.

The effectors are the breathing muscles. The diaphragm is a sheet of muscle below the lungs that contracts during inhalation to increase 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.

A practical investigation can use spirometer or sensor data. Tidal volume, ventilation rate and air flow can be estimated from volume-time traces; exercise should generally increase ventilation rate and often tidal volume because carbon dioxide production rises.

Peristalsis

Peristalsis is a wave of smooth muscle contraction that pushes material along the gut. The gut wall has circular muscle, arranged around the gut, and longitudinal muscle, arranged along its length. Behind a bolus, circular muscle contracts so the material cannot move backwards; longitudinal muscle helps carry it forward.

The enteric nervous system

The enteric nervous system is a network of neurons in the wall of the digestive tract. It coordinates gut movement without conscious control. Between swallowing and egestion, peristalsis is mainly controlled by this enteric system, not by deliberate thought. So digestion carries on while you’re busy doing something else.

The central nervous system

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

Egestion of faeces

Egestion of faeces is also partly under voluntary control after early childhood. The rectum and anal sphincters are involved. Control shifts 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 directional stimulus. 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. Focus on the word “growth”. A tropism isn’t 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 comfortable collecting qualitative and quantitative observations. Qualitative data are descriptive observations that do not involve numerical measurement, such as a labelled drawing showing that shoots curved toward a lamp. Quantitative data are numerical observations obtained by counting or measuring, such as the angle of curvature of a shoot.

A strong tropism record might use diagrams or photographs taken at set intervals. For quantitative work, you could measure curvature angle with a protractor or 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 where light intensity is greater.

When light arrives from one side, the shoot tip detects the direction it comes from. Cells on the shaded side elongate faster than cells on the illuminated side, which makes the shoot bend toward the light. Once the tip is facing the light source, growth evens out again.

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

You don’t need to learn other named tropisms for this statement. Focus on the causal pattern: lateral light $\to$ unequal growth $\to$ curvature toward light.

A phytohormone is a plant signalling chemical that controls growth, development or responses to stimuli, often at very low concentrations. Plants don’t rely on just one “plant hormone”; they use a variety of phytohormones.

Phytohormones affect growth by changing how quickly cells divide and enlarge. They shape development too, such as whether buds grow, flowers form or fruits ripen. They also help plants respond to stimuli, including the directional stimuli involved in tropisms.

At this level, you should recognise 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 key idea is that plants integrate body systems chemically too.

Auxin efflux carriers

are membrane transport proteins that move auxin out of plant cells and help build auxin concentration gradients in tissues. Their position 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 cannot 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 shared polar placement creates a concentration gradient across a tissue. Plants 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, where 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 changes in this space.

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

The sequence runs like this: auxin concentration increases $\to$ hydrogen ions are pumped into the apoplast $\to$ cell wall becomes more acidic $\to$ cross-links loosen $\to$ turgor pressure drives elongation. Where auxin concentration is higher, cells elongate faster. Where auxin concentration is lower, they elongate more slowly.

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

Root and shoot growth have to stay coordinated. If a plant puts on a huge shoot without enough root, it can’t take up enough water and mineral ions. If it grows 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 the shoots and into the roots. Root tips produce cytokinin, which moves upward into the shoots. In some processes, these two phytohormones work together; in others, they push in opposite directions.

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.

Ethylene

Ethylene is a gaseous phytohormone, also called ethene, that stimulates fruit ripening. In many fruits, ripening triggers more ethylene production, which then pushes ripening further: a positive feedback loop.

Ripening involves changes you can see, along with chemical changes inside the fruit. The green colour is lost or masked, cell walls partly break down so the fruit softens, acids and starch are converted into sugars, and volatile scent molecules are produced. By the time the seeds are ready for dispersal, these changes make the fruit attractive to animals.

The positive feedback is simple but powerful. Ethylene stimulates ripening, and ripening fruit produces more ethylene. That extra ethylene stimulates more ripening. Since ethylene is a gas, it can diffuse from one fruit to nearby fruits, so ripening can happen rapidly and synchronously.

The benefit is synchronisation. If a plant has many ripe fruits at once, it is more likely to attract seed-dispersing animals. Agriculture uses the same biology when ripening fruit is placed near unripe fruit to speed ripening.

This answers the linking question about positive feedback: its consequence is amplification. In childbirth, fruit ripening and some cell-signalling pathways, positive feedback drives a process quickly toward an endpoint instead of holding a variable steady. That is the opposite logic from negative feedback.