General Biology Study Guide: Membranes, Cell Signaling, Endocrine and Nervous Systems, and the Senses

Membrane Structure and Function

Functions and Components of Membrane Proteins

Cell membranes are dynamic structures composed of lipids, proteins, and carbohydrates. Membrane proteins play diverse roles essential for cellular function.

• Transport: Facilitate movement of substances across the membrane (channels, carriers).

• Enzymatic Activity: Catalyze specific reactions at the membrane surface.

• Signal Transduction: Transmit signals from outside to inside the cell via receptor proteins.

• Cell-Cell Recognition: Glycoproteins serve as identification tags for cellular interactions.

• Intercellular Joining: Membrane proteins connect adjacent cells (e.g., gap junctions, tight junctions).

• Attachment: Anchor the membrane to the cytoskeleton and extracellular matrix.

Example: Aquaporins are channel proteins that facilitate water transport across the membrane.

The Fluid-Mosaic Model

The fluid-mosaic model describes the structure of cell membranes as a mosaic of protein molecules drifting laterally in a fluid bilayer of phospholipids.

• Phospholipid Bilayer: Provides fluidity and a semi-permeable barrier.

• Proteins: Embedded or attached to the bilayer, performing various functions.

• Carbohydrates: Attached to proteins or lipids, involved in cell recognition.

Transport Across Membranes

Transport mechanisms regulate the movement of substances into and out of cells.

• Passive Transport: Movement down a concentration gradient without energy input (e.g., diffusion, osmosis, facilitated diffusion).

• Active Transport: Movement against a concentration gradient, requiring energy (e.g., sodium-potassium pump).

Key Terms:

• Solute: Substance dissolved in a solvent.

• Solvent: The dissolving medium (usually water in biological systems).

• Concentration Gradient: Difference in concentration of a substance across a space.

• Osmolarity: Total concentration of solute particles in a solution.

• Diffusion: Movement of molecules from high to low concentration.

• Osmosis: Diffusion of water across a selectively permeable membrane.

Example: Water moves from areas of low solute concentration to high solute concentration via osmosis.

Predicting Water Movement

Water movement across membranes depends on the relative concentrations of solutes inside and outside the cell.

• Hypertonic Solution: Higher solute concentration outside the cell; water moves out, cell shrinks.

• Hypotonic Solution: Lower solute concentration outside; water moves in, cell swells.

• Isotonic Solution: Equal solute concentration; no net water movement.

Facilitated Diffusion and Active Transport

Transport proteins assist in moving substances across the membrane.

• Facilitated Diffusion: Passive movement via channel or carrier proteins.

• Active Transport: Uses energy (usually ATP) to move substances against their gradient.

Example: The sodium-potassium pump moves 3 Na+ ions out and 2 K+ ions into the cell per ATP hydrolyzed.

$\text{Na}^+/\text{K}^+ \text{ pump:} \\ 3\ \text{Na}^+_{\text{in}} + 2\ \text{K}^+_{\text{out}} + \text{ATP} \rightarrow 3\ \text{Na}^+_{\text{out}} + 2\ \text{K}^+_{\text{in}} + \text{ADP} + \text{P}_i$

Endocytosis and Exocytosis

Cells transport large molecules via vesicles.

• Endocytosis: Uptake of materials by engulfing them in vesicles.

• Exocytosis: Release of materials by fusion of vesicles with the plasma membrane.

Cellular Communication

Cell Signaling Overview

Cells communicate using chemical signals to coordinate activities.

• Reception: Signal molecule binds to a receptor protein.

• Transduction: Signal is converted to a form that can bring about a cellular response, often via a signal transduction pathway.

• Response: The cell takes action, such as activating an enzyme or turning on a gene.

Types of Cell Signaling

• Paracrine: Signals act on nearby cells.

• Synaptic: Nerve cells release neurotransmitters at synapses.

• Endocrine: Hormones travel through the bloodstream to distant cells.

Membrane Receptors and Signal Transduction

Receptors can be membrane-bound or intracellular. Membrane receptors include G protein-coupled receptors (GPCRs), ligand-gated ion channels, and enzyme-linked receptors.

• GPCRs: Activate G proteins, which then trigger intracellular signaling cascades.

• Ligand-Gated Ion Channels: Open or close in response to ligand binding, allowing ions to flow across the membrane.

Second Messengers: Small molecules like cAMP relay signals inside the cell.

Example: Epinephrine binding to a GPCR leads to cAMP production, which activates protein kinase A.

Signal Specificity and Response Diversity

The same signaling molecule can produce different responses in different cell types, depending on receptor type and intracellular pathways.

Hormones and the Endocrine System

Hormone Function and Types

Hormones are chemical messengers secreted by endocrine glands, regulating physiology and behavior.

• Water-Soluble Hormones: Bind to membrane receptors (e.g., insulin).

• Lipid-Soluble Hormones: Pass through membranes and bind to intracellular receptors (e.g., steroid hormones).

Hormone Pathways and Feedback

• Neurosecretory Cells: Neurons that release hormones into the blood (e.g., hypothalamus).

• Negative Feedback: A process where the output reduces the original stimulus, maintaining homeostasis.

• Positive Feedback: Output enhances the original stimulus (less common).

Example: Regulation of blood glucose by insulin and glucagon is a negative feedback loop.

Tropic vs. Non-Tropic Hormones

• Tropic Hormones: Stimulate other endocrine glands (e.g., TSH stimulates the thyroid gland).

• Non-Tropic Hormones: Directly affect target tissues (e.g., insulin acts on muscle and fat cells).

Neurons and Nervous System Function

Neuron Structure and Glial Cells

Neurons are specialized for communication. Glial cells support and protect neurons.

• Schwann Cells: Form myelin sheath in the peripheral nervous system.

• Oligodendrocytes: Form myelin sheath in the central nervous system.

• Astrocytes: Support and nourish neurons, maintain the blood-brain barrier.

Membrane Potential and Ion Channels

The resting membrane potential is the voltage difference across the neuron's plasma membrane, typically about -70 mV.

• Sodium-Potassium Pump: Maintains ion gradients by pumping Na+ out and K+ in.

• Leak Channels: Allow passive movement of ions, contributing to resting potential.

• Voltage-Gated Channels: Open or close in response to changes in membrane potential, crucial for action potentials.

$\text{Resting potential:} \\ V_m \approx -70\ \text{mV}$

Action Potentials

Action potentials are rapid changes in membrane potential that transmit signals along neurons.

• Depolarization: Na+ channels open, Na+ enters the cell.

• Repolarization: K+ channels open, K+ leaves the cell.

• Hyperpolarization: Membrane potential becomes more negative than resting.

Phases of Action Potential: Resting, depolarization, repolarization, hyperpolarization, return to resting.

The Nervous System

Organization and Function

The nervous system is divided into central (CNS) and peripheral (PNS) components, each with specialized functions.

• CNS: Brain and spinal cord; processes information.

• PNS: Nerves and ganglia; transmits signals to and from the CNS.

Peripheral Nervous System Subdivisions

• Somatic Nervous System: Controls voluntary movements.

• Autonomic Nervous System (ANS): Regulates involuntary functions.

• ANS Subdivisions:

  • Sympathetic: "Fight or flight" responses.

  • Parasympathetic: "Rest and digest" responses.

Reflex Arcs and Integration

Reflexes are rapid, involuntary responses to stimuli, involving sensory and motor neurons.

The Senses

Overview of Sensory Systems

Sensory systems detect and process environmental stimuli.

• Reception: Detection of stimulus by sensory receptors.

• Transduction: Conversion of stimulus energy into electrical signals.

• Transmission: Sending signals to the CNS.

• Perception: Interpretation of signals by the brain.

Types of Senses and Transduction Events

• Hearing/Equilibrium: Hair cells transduce mechanical vibrations into electrical signals.

• Vision: Photoreceptors transduce light into electrical signals.

• Taste: Taste receptor cells transduce chemical signals.

• Smell: Olfactory receptors transduce odorant molecules.

• Touch: Mechanoreceptors transduce pressure or vibration.

Table: Comparison of Hormone Types

Hormone Type	Solubility	Receptor Location	Example
Water-Soluble	Hydrophilic	Cell membrane	Insulin
Lipid-Soluble	Hydrophobic	Intracellular	Cortisol

Additional info: Some explanations and examples were expanded for clarity and completeness based on standard biology curricula.