Chapter 7: Membrane Structure and Function

Membrane Proteins: Types and Functions

Membrane proteins are essential for the structure and function of cell membranes. They perform a variety of roles:

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

• Enzymatic Proteins: Catalyze reactions at the membrane surface.

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

• Cell Recognition Proteins: Allow cells to identify each other (important in immune response).

• 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.

• Phospholipid Bilayer: Provides fluidity and barrier function.

• Proteins: Scattered throughout, performing various functions.

• Cholesterol: Modulates membrane fluidity and stability.

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

Major Components of Eukaryotic Plasma Membrane

• Phospholipids: Form the basic structure; amphipathic (hydrophilic head, hydrophobic tail).

• Proteins: Integral (span membrane) and peripheral (attached to surface).

• Cholesterol: Maintains membrane fluidity.

• Carbohydrates: Glycoproteins and glycolipids for cell recognition.

Diffusion Across the Lipid Bilayer

Small, nonpolar molecules (e.g., O2, CO2) can diffuse freely; ions and large polar molecules require transport proteins.

• Simple Diffusion: Direct movement through bilayer.

• Facilitated Diffusion: Via channels or carriers.

• Active Transport: Requires energy (ATP).

Key Terms and Definitions

• Solvent: The substance in which solutes are dissolved (usually 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 of 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 or nonpolar) to pass while blocking others.

• Predicting Diffusion: Solutes move down their concentration gradient.

• Predicting 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.

• Isotonic: Equal solute; no net water movement.

• Hypotonic: Lower solute outside; water enters cell.

Passive vs. Active Transport

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

• Active Transport: Requires energy; moves against gradient.

Facilitated Diffusion

Transport proteins (channels, carriers) enable movement of substances that cannot cross the bilayer directly.

Sodium-Potassium Pump

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

• Role of ATP: Provides energy for transport against gradient.

Electrochemical Gradient

Combination of chemical (concentration) and electrical (charge) gradients across the membrane.

Cotransport

Pumps and channels work together; one substance moves down its gradient, driving another against its gradient.

Exocytosis and Endocytosis

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

• Endocytosis: Membrane engulfs material to bring it inside.

Chapter 11: Cell Communication

Reasons for Cell Signaling

Cells signal to coordinate activities, respond to environment, and regulate growth, development, and metabolism.

Types of Signaling

• Paracrine: Local signaling between nearby cells.

• Synaptic: Local signaling via neurotransmitters at synapses.

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

Stages of Cell Signaling

• Reception: Signal detected by receptor.

• Transduction: Signal relayed and amplified inside cell.

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

Transmembrane Proteins in Signaling

Act as receptors, initiating signal transduction when a ligand binds.

G Protein-Coupled Receptor Pathway

• Ligand binds receptor.

• G protein activated.

• G protein activates enzyme or channel.

• Cellular response triggered.

Ligand-Gated Ion Channel Pathway

• Ligand binds channel.

• Channel opens, ions flow.

• Change in membrane potential leads to response.

Signal Transduction and Second Messengers

Signal transduction is the process of converting a signal to a cellular response. Second messengers (e.g., cAMP) amplify and relay signals.

Cascades and Amplification

Cascades involve sequential activation of proteins, amplifying the signal.

Cellular Responses

• Gene expression changes.

• Metabolic pathway activation.

• Cell movement or division.

Membrane vs. Intracellular Receptors

• Membrane Receptors: Bind polar signals; trigger cascades.

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

Differential Responses to Same Signal

Different cell types may respond differently to the same signaling molecule due to distinct receptor types or pathways.

Chapter 45: Hormones and the Endocrine System

Cell Signaling in the Endocrine System

• Reception: Hormone binds receptor.

• Transduction: Signal relayed/amplified.

• Response: Change in cell activity.

Hormones: Definition and Function

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

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; activate second messengers.

• 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, linking nervous and endocrine systems (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.

• Non-Tropic: Target non-endocrine tissues.

Adrenal Gland and Stress Response

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

Thyroid Hormone Regulation

Hypothalamus and pituitary control thyroid hormone production via feedback loops.

Antagonistic Hormones

Pairs of hormones with opposite effects (e.g., insulin and glucagon) maintain homeostasis.

Chapter 48: Neurons, Synapses, and Signaling

Structure and Function of Neurons

Neurons have specialized structures for signal transmission:

• 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, regulate environment.

Membrane Potential and Voltage

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

• Resting Potential: Typically -70 mV.

• Measured: Using microelectrodes.

Sodium-Potassium Pump

Maintains resting potential by moving 3 Na+ out and 2 K+ in per ATP used.

Leak Channels

Allow passive movement of ions, helping maintain resting potential.

Ligand-Gated Ion Channels and Graded Potentials

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 and Action Potential

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

Integration at Axon Hillock

Neuron sums all incoming signals to determine if threshold is reached.

Voltage-Gated Ion Channels

Open in response to voltage changes, essential for action potential propagation.

Steps of Action Potential

1. Resting State: All channels closed.

2. Depolarization: Na+ channels open, Na+ enters.

3. Repolarization: K+ channels open, K+ exits.

4. Hyperpolarization: K+ channels remain open briefly.

5. Return to Resting: Pump restores original state.

Membrane Potential Graph Interpretation

Graph shows changes in membrane potential during rest, graded potentials, and action potential.

Chapter 49: Nervous Systems

Core Functions of the Nervous System

• Input: Sensory information (PNS).

• Integration: Processing (CNS).

• Output: Motor response (PNS).

Knee-Jerk Reflex Pathway

Involves afferent (sensory) and efferent (motor) neurons, with integration in the spinal cord.

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).

Roles of ANS Subdivisions

• Parasympathetic: Slows heart, stimulates digestion.

• Sympathetic: Increases heart rate, inhibits digestion.

• Enteric: Controls gastrointestinal function.

Glial Cells in CNS vs. PNS

• Schwann Cells: PNS.

• Oligodendrocytes: CNS.

• Astrocytes: CNS.

Grey and White Matter Distribution

• Grey Matter: Cell bodies.

• White Matter: Myelinated axons (connections).

Chapter 50: Sensory and Motor Mechanisms

Basic Components of Sensory Systems

• Reception: Detecting stimulus.

• Transduction: Converting stimulus to electrical signal.

• Transmission: Sending signal to CNS.

• Perception: Interpreting signal in brain.

• Amplification: Increasing signal strength.

• Adaptation: Decreased response to constant stimulus.

Sensory Receptor Cells

Specialized cells detect stimuli and initiate transduction, often by changing membrane potential.

Non-Neuronal Sensory Receptors

Can trigger action potentials in adjacent neurons via neurotransmitter release.

Stimulus Intensity and Action Potential Frequency

Stronger stimuli increase action potential frequency, not amplitude.

Types of Sensory Receptors

• Mechanoreceptors: Touch, pressure, hearing.

• Photoreceptors: Vision.

• Chemoreceptors: Taste, smell.

• Thermoreceptors: Temperature.

• Nociceptors: Pain.

Transduction in the Five Senses

Additional info: Where original notes were brief, academic context and definitions were added for clarity and completeness.