Membrane Structure and Function

Membrane Proteins: Types and Functions

Membrane proteins are essential for various cellular processes, including transport, communication, and structural support. They are classified based on their location and function:

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

• Enzymatic Proteins: Catalyze reactions at the membrane surface.

• Signal Transduction Proteins: Transmit signals from outside to inside the cell.

• Cell Recognition Proteins: Allow cells to identify each other (often glycoproteins).

• Intercellular Joining Proteins: Connect adjacent cells.

• Attachment Proteins: Anchor the membrane to the cytoskeleton or extracellular matrix.

Fluid-Mosaic Model of Cell Membranes

The fluid-mosaic model describes the cell membrane as a dynamic structure with proteins embedded in or attached to a fluid lipid bilayer. Lipids and proteins can move laterally, allowing flexibility and self-healing.

• Phospholipid Bilayer: Provides a semi-permeable barrier.

• Cholesterol: Modulates fluidity and stability.

• Proteins: Serve various functions as described above.

• Carbohydrates: Attached to proteins/lipids for cell recognition.

Major Components of Eukaryotic Plasma Membrane

• Phospholipids: Form the basic structure; hydrophilic heads face outward, hydrophobic tails inward.

• Cholesterol: Maintains membrane fluidity.

• Proteins: Integral and peripheral; perform transport, signaling, and structural roles.

• Carbohydrates: Glycoproteins and glycolipids for cell recognition.

Diffusion Across the Lipid Bilayer

Small, nonpolar molecules (e.g., O2, CO2) can diffuse freely. Polar or charged molecules require transport proteins.

• Simple Diffusion: Passive movement of molecules down their concentration gradient.

• Facilitated Diffusion: Uses transport proteins for polar/charged molecules.

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

Key Terms and Definitions

• Solvent: The substance in which solutes are dissolved (e.g., water).

• Solute: The substance dissolved in a solvent.

• Concentration: Amount of solute per unit volume.

• Gradient: Difference in concentration across a space.

• Osmolarity: Total solute concentration in a solution.

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

• Osmosis: Diffusion of water across a semipermeable membrane.

Semipermeable Membrane

A semipermeable membrane allows certain molecules (usually small and nonpolar) to pass while blocking others. This property is crucial for maintaining cellular homeostasis.

Predicting Net Diffusion

• Solute Diffusion: Moves from high to low concentration across the membrane.

• Water Diffusion (Osmosis): Water moves toward higher solute concentration.

Tonicity and Water Movement

Tonicity describes the effect of a solution on cell volume:

• Hypertonic: Higher solute outside; water leaves cell, cell shrinks.

• Isotonic: Equal solute; no net water movement.

• Hypotonic: Lower solute outside; water enters cell, cell swells.

Passive vs. Active Transport

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

• Active Transport: Requires energy (ATP); moves substances against gradient.

Facilitated Diffusion

Transport proteins (channels, carriers) enable movement of specific molecules across the membrane without energy input.

Sodium-Potassium Pump

The sodium-potassium pump maintains membrane potential by moving Na+ out and K+ in, using ATP:

• 3 Na+ out, 2 K+ in per ATP.

• Creates an electrochemical gradient (charge and concentration difference).

Cotransport

Pumps and channels can work together, allowing one molecule to move down its gradient and another to move against its gradient (e.g., glucose/Na+ cotransport).

Exocytosis and Endocytosis

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

• Endocytosis: Membrane engulfs material to bring it inside.

Cell Communication

Reasons for Cell Signaling

• Coordinate activities (growth, division, metabolism).

• Respond to environmental changes.

• Maintain homeostasis.

Types of Signaling

• Paracrine: Local signaling between nearby cells.

• Synaptic: Local signaling via neurotransmitters (nervous system).

• Endocrine: Long-distance signaling via hormones in the bloodstream.

Stages of Cell Signaling

• Reception: Cell detects signal molecule.

• Transduction: Signal is relayed and amplified inside the cell.

• Response: Cell changes activity (gene expression, metabolism, etc.).

Transmembrane Proteins in Signaling

Transmembrane proteins (receptors) bind extracellular signals and initiate intracellular responses.

G Protein-Coupled Receptors (GPCRs) and Ligand-Gated Ion Channels

• GPCRs: Signal binds receptor, activates G protein, triggers downstream effects.

• Ligand-Gated Ion Channels: Signal opens channel, ions flow, changes membrane potential.

Signal Transduction and Second Messengers

Signal transduction involves cascades of molecular events, often using second messengers like cAMP to amplify signals.

• cAMP: Cyclic AMP, a common second messenger.

• Cascade: Series of steps amplifying the signal.

Cellular Responses

• Gene expression changes.

• Enzyme activation.

• Cell movement or division.

Membrane vs. Intracellular Receptors

• Membrane Receptors: Bind hydrophilic signals; trigger cascades.

• Intracellular Receptors: Bind hydrophobic signals; directly affect gene expression.

Differential Responses

The same signaling molecule can cause different responses in different cells due to receptor types and downstream pathways.

Hormones and the Endocrine System

Stages of Cell Signaling in Endocrine Context

• Reception: Hormone binds receptor.

• Transduction: Signal relayed/amplified.

• Response: Physiological change (e.g., metabolism).

Hormones: Definition and Function

Hormones are signaling molecules produced by endocrine glands, regulating processes like growth, metabolism, and homeostasis.

Chemical Types of Hormones

• Peptide/Protein Hormones: Water-soluble.

• Steroid Hormones: Lipid-soluble.

• Amino Acid Derivatives: Can be water- or lipid-soluble.

Hormone Pathways: Water vs. Lipid-Soluble

• Water-Soluble: Bind membrane receptors; trigger signal transduction.

• Lipid-Soluble: Cross membrane; bind intracellular receptors.

Organs Involved in Hormone Production

• Hypothalamus

• Pituitary Gland

• Thyroid

• Adrenal Glands

• Pancreas

• Gonads

Neurosecretory Cells and Neurohormones

Neurosecretory cells release neurohormones, integrating nervous and endocrine functions (e.g., hypothalamus).

Hormones and Homeostasis

Hormones regulate internal balance (homeostasis) via feedback mechanisms.

Positive vs. Negative Feedback

• Negative Feedback: Inhibits further hormone release (e.g., blood glucose regulation).

• Positive Feedback: Enhances hormone release (e.g., oxytocin during childbirth).

Tropic vs. Non-Tropic Hormones

• Tropic: Target other endocrine glands (e.g., TSH).

• Non-Tropic: Directly affect target tissues (e.g., insulin).

Adrenal Gland and Stress Response

Adrenal glands release hormones (e.g., cortisol, adrenaline) to regulate stress responses.

Thyroid Hormone Regulation

Hypothalamus releases TRH → Pituitary releases TSH → Thyroid releases thyroid hormone.

Antagonistic Hormones

Pairs of hormones with opposite effects (e.g., insulin lowers blood glucose, glucagon raises it) maintain homeostasis.

Neurons: Structure and Function

Neuron Structure and Function

• Dendrites: Receive signals.

• Cell Body: Integrates signals.

• Axon: Transmits signals.

• Axon Terminals: Communicate with other cells.

Glial Cells

• Schwann Cells: Myelinate axons in PNS.

• Oligodendrocytes: Myelinate axons in CNS.

• Astrocytes: Support neurons, maintain environment.

Membrane Potential and Voltage

Membrane potential is the voltage difference across the cell membrane, measured in millivolts (mV).

Sodium-Potassium Pump in Neurons

Maintains resting potential and enables action potentials by creating ion gradients.

Leak Channels and Resting Potential

Leak channels allow passive movement of ions, stabilizing resting potential.

Ligand-Gated Ion Channels and Graded Potentials

Ligand-gated channels open in response to neurotransmitters, causing graded changes in membrane potential.

Excitatory vs. Inhibitory Stimuli

• Excitatory (Depolarizing): Make membrane potential less negative.

• Inhibitory (Hyperpolarizing): Make membrane potential more negative.

Threshold Potential and Action Potential

If graded potentials reach threshold at the axon hillock, an action potential is triggered.

Integration at Axon Hillock

Neuron sums all incoming signals; if threshold is reached, fires action potential.

Voltage-Gated Ion Channels

Open in response to changes in membrane potential, crucial for action potential propagation.

Steps of Action Potential Generation

1. Resting State: All channels closed; membrane at -70 mV.

2. Depolarization: Na+ channels open; membrane potential rises.

3. Repolarization: K+ channels open; membrane potential falls.

4. Hyperpolarization: K+ channels remain open briefly; membrane potential drops below resting.

5. Return to Resting: Channels reset; pump restores gradients.

Graph Interpretation

Membrane potential graphs show changes during rest, graded potentials, and action potentials.

The Nervous System

Core Functions

• Input: Sensory information (PNS).

• Integration: Processing (CNS).

• Output: Motor response (PNS).

Knee-Jerk Reflex Pathway

• Afferent Neurons: Carry sensory input to CNS.

• Integration: CNS processes signal.

• Efferent Neurons: Carry motor output to effector.

CNS and PNS Connections

• PNS: Sensory and motor divisions.

• ANS: Autonomic Nervous System (involuntary).

• Motor: Voluntary control.

• ANS Subdivisions: Parasympathetic (rest/digest), Sympathetic (fight/flight), Enteric (gut).

ANS Subdivision Roles

• Parasympathetic: Slows heart, stimulates digestion.

• Sympathetic: Increases heart rate, inhibits digestion.

Glial Cells in CNS vs. PNS

• Schwann Cells: PNS.

• Oligodendrocytes: CNS.

• Astrocytes: CNS.

Grey vs. White Matter

• Grey Matter: Cell bodies.

• White Matter: Axons (connections).

Sensory and Motor Mechanisms

Basic Components of Sensory System

• Reception: Detect stimulus.

• Transduction: Convert stimulus to electrical signal.

• Transmission: Send signal to CNS.

• Perception: CNS interprets signal.

• Amplification: Increase signal strength.

• Adaptation: Decrease response to constant stimulus.

Sensory Receptor Cells

Receptor cells detect stimuli and initiate transduction, changing membrane potential.

Non-Neuronal Sensory Receptors

These cells release neurotransmitters to activate neurons, initiating action potentials.

Stimulus Intensity and Action Potential Frequency

Stronger stimuli increase action potential frequency, not amplitude.

Receptor Types and Stimuli

• Mechanoreceptors: Touch, pressure, hearing.

• Photoreceptors: Vision.

• Chemoreceptors: Taste, smell.

• Thermoreceptors: Temperature.

• Nociceptors: Pain.

Transduction in the Five Senses