Homeostasis & Physiological Control Summary & Study Notes

🧭 Homeostasis and Control Systems

Essential components: Sensors (receptors for temperature, pH, touch), Effectors (muscles, sweat glands), and Response (heat production). These parts are linked in a negative feedback loop that returns a system to its set point and maintains stability. A conformer follows environmental changes along a linear line of conformity, while a regulator maintains stability with an s-shaped line of conformity; the straight portion represents the zone of stability.

🧪 Environmental Changes and Acclimatization

Environmental changes occurring over weeks to months (such as altitude or day length) lead to acclimatization, which is slower, occurs over many days, and is reversible once the environment returns to previous conditions.

🧫 Membranes: Structure and Amphipathicity

Cell membranes contain two phospholipid layers and are amphipathic. The membrane has a fatty acid tail that is nonpolar (neutral charge) and a polar head containing glycerol, phosphate, and other polar groups.

🌀 Membrane Dynamics and Fluidity

Membrane phospholipids move within the lipid bilayer, making the membrane fluid. Fatty acid chains can flex or bend, and fluidity depends on tail length and the degree of unsaturation (double bonds). More double bonds increase fluidity, while longer tails tend to reduce it due to more interactions.

🧱 Plasma Membrane: Boundary and Homeostasis

The plasma membrane is a defining feature of all cells, establishing the cell boundary and separating internal contents from the surrounding environment. It is critical for maintaining homeostasis by controlling what enters and leaves the cell.

🧭 Selective Permeability of the Plasma Membrane

Permeability is selective: some substances cross freely, others under certain conditions, and others cannot cross. The hydrophobic core of the lipid bilayer blocks ions; many macromolecules are too large. Gases, lipids, and small polar molecules can cross; selective permeability is essential for homeostasis.

⚡ Active Transport: Against the Gradient

Active transport moves substances against their concentration gradient. While passive transport requires gradient direction, active transport uses energy to move nutrients (often from low to high outside to inside) and wastes (high inside to outside).

Primary Active Transport

Primary active transport uses energy from ATP. A classic example is the sodium-potassium pump ($\text{Na}^+/ \text{K}^+$ pump$)$, an antiporter that moves ions in opposite directions. A symporter (or co-transporter) moves ions in the same direction.

Secondary Active Transport

Secondary active transport does not use ATP directly. It relies on an electrochemical gradient created by primary transport (often a proton gradient) to drive the movement of another molecule against its gradient.

🧬 Endocrine System and Negative Feedback

The endocrine system can operate via negative feedback. Example: after a meal, high blood glucose triggers the sensor (pancreas) to release insulin, which promotes uptake of glucose and reduces blood glucose levels, restoring homeostasis.

🔬 Neurons: The Fundamental Unit

Neurons receive stimuli at dendrites and the cell body. The axon hillock sums all inputs; if the combined signal reaches threshold, an action potential is fired and travels to the axon terminal to release neurotransmitters. Neurotransmitters bind receptors on the postsynaptic membrane, generating a new signal in the postsynaptic neuron.

⚡ Resting Membrane Potential

Neurons are electrically excitable, with a resting membrane potential ($V$) due to ion distributions inside versus outside the cell. Leak K+ through channels is a major determinant of $V$, making the inside of the neuron more negative relative to the outside. The pump and leaks maintain this imbalance, which allows neurons to respond to stimuli.

🔬 Depolarization and Repolarization

During depolarization, the inside becomes less negative when voltage-gated Na+ channels open. Repolarization follows when Na+ channels close and K+ channels open, causing K+ to exit and the membrane potential to fall. A brief undershoot can occur below the resting potential before the system returns to baseline.

🧭 Saltatory Propagation

In myelinated (vertebrate) axons, myelin sheaths speed up conduction. At nodes of Ranvier, voltage-gated Na+ and K+ channels are concentrated, allowing action potentials to jump from node to node and increasing transmission speed.

💡 Excitatory and Inhibitory Signals

Neurotransmitter binding can open ion channels, creating a postsynaptic potential. If the postsynaptic cell is driven toward depolarization it is excitatory (EPSP); if towards hyperpolarization it is inhibitory (IPSP), shaping the neuron's response.

👁️ Sensory Receptor Cells and Transduction

Sensory receptor cells detect physical stimuli (e.g., light, sound) and may fire action potentials themselves or synapse with neurons to propagate signals. The stimulus opens ion channels, causing depolarization and triggering action potentials to encode information about the environment.

🧬 Encoding Stimulus Strength

Action potentials are all-or-none, but information is often encoded in the rate and timing of spikes. Higher firing frequency generally signals a stronger stimulus, such as brighter light or louder sound.

🔬 Diffusion and Gas Exchange

Diffusion supports cellular oxygen delivery in thin organisms or those with few cell layers. In many animals, diffusion limits O2 delivery to tissues that are near capillaries or specialized ventilation systems.

🫁 Respiratory Strategies in Vertebrates

Most land vertebrates use tidal ventilation, expanding the thoracic cavity to draw in air and relying on elastic recoil to exhale. Alveoli are the sites of gas exchange with a surrounding capillary network; surfactants reduce surface tension to aid inflation and uniform ventilation.

🐦 Birds: Unidirectional Ventilation

Birds breathe using unidirectional ventilation with cross-current flow. Fresh air moves through posterior air sacs into the lungs, then into anterior air sacs, ensuring continuous gas exchange during both inhalation and exhalation.

🔬 Circulation: Arteries and Veins

Arteries carry oxygen-rich blood away from the heart (pulmonary arteries are an exception). Veins return oxygen-poor blood to the heart (pulmonary veins are an exception). The typical flow is artery → arteriole → capillary bed → venule → vein.

🩸 Open Circulatory Systems

In open circulatory systems, blood bathes tissues directly in a body cavity and is then returned to circulation. They have fewer vessels and, in many cases, limited control of fluid movement; insects are common examples.

🔥 Metabolic Rates and Exercise

Metabolic rate measures energy use, often via $\mathrm{O}_2$ consumption. It rises with exercise due to ATP demand and initially relies on anaerobic pathways and stored energy; recovery gradually repays the O2 debt.

🧭 Metabolic Rates, Body Size, and Temperature

At rest, larger animals typically consume more energy, but mass-specific metabolic rate decreases with size. Temperature strongly influences reaction rates, leading to various thermal strategies.

🛡 Brown Adipose Tissue and Thermogenesis

Brown adipose tissue is specialized for thermogenesis, rich in mitochondria and highly vascularized. It helps generate internal heat, especially in small mammals and young individuals.

🧬 Immune System: Innate and Adaptive Arms

The immune system has two major arms: the innate (non-specific, ancient) and the adaptive (specific, remembers past infections). Immunity is gradually acquired after exposure to pathogens.

🧫 Host-Pathogen Recognition and Inflammation

When barriers fail, white blood cells (WBCs) respond. Toll-like receptors (TLRs) on phagocytes recognize conserved pathogen molecules, triggering phagocytosis and cytokine signaling to coordinate defense. Inflammation involves mast cells releasing histamine, increasing blood flow and permeability, and recruiting phagocytes.

💠 Complement System and Antibody Roles

The complement system activates against pathogens through MAC formation, opsonization to tag pathogens for phagocytes, and cell signaling. Antibodies can initiate complement activation and coordinate adaptive responses.

🧬 Antibodies, B Cells, and Clonal Selection

B cells produce antibodies with variable specificities. When an antigen binds best to a B cell receptor, that cell clones into plasma cells and memory B cells; this clonal selection underpins the immune response and memory.

🧠 Secondary Immune Response and Memory

Memory B cells enable a faster and stronger response upon subsequent antigen exposure, compared with the primary response, due to a larger memory pool and quicker differentiation into antibody-secreting cells.