Membrane Structure and Function

Fluid-Mosaic Model of Cell Membranes

The fluid-mosaic model describes the structure of cell membranes as a mosaic of diverse protein molecules embedded in a fluid bilayer of phospholipids. This model explains both the flexibility and selective permeability of biological membranes.

• Phospholipid bilayer: Forms the fundamental structure, with hydrophilic heads facing outward and hydrophobic tails inward.

• Proteins: Embedded within or attached to the bilayer, serving various functions (see below).

• Cholesterol: Interspersed among phospholipids, modulates membrane fluidity and stability.

• Carbohydrates: Attached to proteins (glycoproteins) or lipids (glycolipids), involved in cell recognition and signaling.

Functions of Membrane Proteins

• Transport: Channels and carriers move substances across the membrane.

• Enzymatic activity: Some proteins catalyze reactions at the membrane surface.

• Signal transduction: Receptors transmit signals from outside to inside the cell.

• Cell-cell recognition: Glycoproteins serve as identification tags.

• Intercellular joining: Proteins help cells adhere to each other.

• Attachment to cytoskeleton and extracellular matrix: Maintains cell shape and stabilizes membrane proteins.

Membrane Permeability and Transport

• Semi-permeable membrane: Allows some substances to cross more easily than others.

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

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

• Facilitated diffusion: Passive transport aided by proteins (channels or carriers).

• Active transport: Movement against concentration gradient, requires energy (ATP).

• Example: Oxygen (O2) and carbon dioxide (CO2) diffuse freely; ions and large polar molecules require transport proteins.

Key Terms

• Solvent: The substance in which a solute is dissolved (e.g., water).

• Solute: The substance dissolved in a solvent.

• Concentration: Amount of solute per unit volume of solution.

• Gradient: A difference in concentration across a space or membrane.

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

Tonicity and Water Movement

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

• Hypertonic: Higher solute concentration outside; water leaves cell.

• Hypotonic: Lower solute concentration outside; water enters cell.

Transport Mechanisms

• Passive transport: No energy required; moves substances down their gradient.

• Active transport: Requires energy; moves substances against their gradient.

• Sodium-potassium pump: Uses ATP to move 3 Na+ out and 2 K+ in, maintaining electrochemical gradients.

• Electrochemical gradient: Combined effect of concentration gradient and electrical charge difference across membrane.

• Cotransport: Coupled transport of two substances via a single protein (e.g., symporters, antiporters).

• Exocytosis: Vesicles fuse with membrane to release contents outside cell.

• Endocytosis: Cell engulfs material by forming vesicles from membrane.

Cell Communication

Reasons for Cell Signaling

Cells communicate to coordinate activities, respond to environmental changes, and maintain homeostasis.

• Growth and development

• Immune responses

• Metabolic regulation

Types of Cell Signaling

• Paracrine signaling: Local signaling; affects nearby cells.

• Synaptic signaling: Local; neurotransmitters cross synapses between neurons.

• Endocrine signaling: Long-distance; hormones travel via bloodstream.

Stages of Cell Signaling

1. Reception: Signal molecule binds to receptor.

2. Transduction: Signal is converted to a form that can bring about a response, often via a cascade of molecular events.

3. Response: Cellular activity is triggered (e.g., gene expression, enzyme activation).

Signal Transduction Pathways

• Transmembrane proteins: Receptors that span the membrane and transmit signals inside.

• G protein-coupled receptors (GPCRs): Activate G proteins, which then trigger downstream effects.

• Ligand-gated ion channels: Open or close in response to binding of a signaling molecule, allowing ions to flow.

• Second messengers: Small molecules (e.g., cAMP) that relay signals inside the cell.

• Cascade: Series of steps amplifying the signal.

Signal Amplification and Response

• Amplification: One signal molecule can cause a large cellular response via a cascade.

• Responses: Changes in gene expression, metabolism, cell movement, etc.

• Pathways: Membrane receptors act at the cell surface; intracellular receptors act within the cell (e.g., steroid hormones).

• Same signal, different responses: Due to different receptors or pathways in different cell types.

Hormones and the Endocrine System

Hormones: Definition and Function

• Hormones: Chemical messengers secreted by endocrine glands, travel through blood to target cells.

• Functions: Regulate growth, metabolism, development, reproduction, and homeostasis.

Types of Hormones

• Water-soluble hormones: (e.g., peptides, amines) Bind to cell surface receptors; cannot cross membrane.

• Lipid-soluble hormones: (e.g., steroids) Cross membrane and bind to intracellular receptors.

Hormone Pathways and Organs

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

• Neurohormones: Hormones released by neurosecretory cells.

• Major organs: Hypothalamus, pituitary, thyroid, adrenal glands, pancreas, gonads.

Feedback Regulation

• Negative feedback: Response reduces the initial stimulus (e.g., insulin lowers blood glucose).

• Positive feedback: Response amplifies the stimulus (e.g., oxytocin in childbirth).

• Antagonistic hormones: Hormones with opposite effects (e.g., insulin and glucagon).

Examples of Hormonal Regulation

• Adrenal gland: Releases hormones in response to stress (e.g., adrenaline, cortisol).

• Thyroid hormone pathway: Regulated by hypothalamus and pituitary via negative feedback.

Neurons: Structure and Function

Neuron Structure

• Cell body (soma): Contains nucleus and organelles.

• Dendrites: Receive signals from other neurons.

• Axon: Transmits signals to other cells.

• Axon hillock: Integrates signals and initiates action potentials.

Glial Cells

• Schwann cells: Form myelin sheath in PNS.

• Oligodendrocytes: Form myelin sheath in CNS.

• Astrocytes: Support neurons, regulate extracellular environment.

Membrane Potential and Ion Channels

• Resting potential: Voltage across membrane at rest, typically -70 mV.

• Measured using: Microelectrodes and voltmeters.

• Sodium-potassium pump: Maintains gradients by pumping 3 Na+ out and 2 K+ in, using ATP.

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

• Ligand-gated ion channels: Open in response to neurotransmitter binding, causing graded potentials.

• Voltage-gated ion channels: Open in response to changes in membrane potential, crucial for action potentials.

Action Potentials

• Excitatory (depolarizing) stimuli: Make membrane potential less negative.

• Inhibitory (hyperpolarizing) stimuli: Make membrane potential more negative.

• Threshold potential: Level needed to trigger action potential, usually at axon hillock.

• Integration: Neuron sums all inputs at axon hillock to decide whether to fire.

• Phases: Depolarization (Na+ in), repolarization (K+ out), hyperpolarization, return to resting potential.

Example graph: Membrane potential vs. time shows resting, graded, and action potentials.+

The Nervous System

Core Functions and Organization

• Input: Sensory neurons detect stimuli (PNS).

• Integration: Processing and interpretation (CNS: brain and spinal cord).

• Output: Motor neurons elicit responses (PNS).

Signal Pathways and Reflexes

• Knee-jerk reflex: Involves afferent (sensory) and efferent (motor) neurons, with integration in spinal cord.

• Pathway: Stimulus → sensory neuron (afferent) → CNS (integration) → motor neuron (efferent) → response.

Peripheral Nervous System (PNS) Subdivisions

• Autonomic Nervous System (ANS): Involuntary control.

  • Parasympathetic: "Rest and digest" (e.g., slows heart rate).

  • Sympathetic: "Fight or flight" (e.g., increases heart rate, inhibits pancreas).

  • Enteric: Controls digestive tract.

• Motor (Somatic) System: Voluntary muscle control.

Glial Cells in CNS vs. PNS

• Schwann cells: PNS

• Oligodendrocytes: CNS

• Astrocytes: CNS

Grey and White Matter

• Grey matter: Neuron cell bodies, dendrites, unmyelinated axons.

• White matter: Myelinated axons connecting different brain/spinal cord regions.

The Senses

Components of Sensory Systems

• Reception: Detection of stimulus by sensory receptor.

• Transduction: Conversion of stimulus energy to a change in membrane potential.

• Transmission: Sending of action potentials to CNS.

• Perception: Interpretation of signals by the brain.

• Amplification: Strengthening of sensory signals.

• Adaptation: Decreased response to sustained stimulus.

Sensory Receptor Cells

• Role: Detect specific stimuli and initiate transduction.

• Non-neuronal receptors: Release neurotransmitters to activate neurons.

• Stimulus intensity: Encoded by frequency of action potentials.

Types of Sensory Receptors

• Mechanoreceptors: Sense touch, pressure, vibration.

• Photoreceptors: Detect light (vision).

• Chemoreceptors: Detect chemicals (taste, smell).

• Thermoreceptors: Sense temperature.

• Nociceptors: Detect pain.

Transduction in the Five Senses

• Hearing/Equilibrium: Mechanoreceptors in ear; transduction occurs in hair cells.

• Vision: Photoreceptors in retina; transduction occurs in rods and cones.

• Taste: Chemoreceptors in taste buds; transduction occurs in taste cells.

• Smell: Chemoreceptors in olfactory epithelium; transduction in olfactory receptor neurons.

• Touch: Mechanoreceptors in skin; transduction in sensory nerve endings.

Additional info: Where original content was brief, standard academic context and definitions have been added for clarity and completeness.