Chapter 7: Membrane Structure and Function

Membrane Proteins and Their Functions

The plasma membrane contains various proteins that perform essential functions for the cell. These proteins are embedded within or attached to the lipid bilayer.

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

• Enzymatic Proteins: Catalyze specific 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 (often glycoproteins).

• Intercellular Joining Proteins: Connect adjacent cells.

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

The Fluid-Mosaic Model

The fluid-mosaic model describes the structure of cell membranes as a dynamic combination of lipids and proteins.

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

• 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; hydrophilic heads face outward, hydrophobic tails inward.

• Proteins: Integral (span the 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. Large or charged molecules require transport proteins.

• Can Diffuse: Gases, small hydrophobic molecules.

• Require Transport: Ions, polar molecules, large molecules.

Key Terms

• 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 to pass while blocking others, crucial for cell function.

Predicting Direction of Diffusion and Osmosis

• Solute Diffusion: Moves from high to low concentration.

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

Tonicity and Water Movement

• 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 (ATP); moves against gradient.

Facilitated Diffusion

Transport proteins (channels, carriers) enable movement of substances that cannot diffuse directly.

Sodium-Potassium Pump

Maintains cell membrane potential by moving Na+ out and K+ in, using ATP.

• Equation: per ATP hydrolyzed

Electrochemical Gradient

Combination of concentration gradient and electrical charge difference across membrane.

Cotransport

Pumps and channels work together; one substance's movement drives another's transport.

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 communicate to coordinate activities, respond to environment, and regulate growth and development.

Types of Signaling

• Paracrine: Local signaling between nearby cells.

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

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

Three 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 upon ligand binding.

G Protein-Coupled Receptor Pathway

• Ligand binds receptor.

• G protein activated.

• Triggers downstream effectors (e.g., enzymes).

Ligand-Gated Ion Channel Pathway

• Ligand binds channel.

• Channel opens, ions flow, changing membrane potential.

Signal Transduction and Second Messengers

• Signal Transduction: Conversion of signal to cellular response.

• Second Messengers: Small molecules (e.g., cAMP) amplify and relay signals.

Cascades and Signal Amplification

Series of steps (cascade) amplify the signal, allowing a small stimulus to produce a large response.

Cellular Responses

• Gene expression changes.

• Enzyme activity altered.

• Cell movement or division.

Membrane vs. Intracellular Receptors

• Membrane Receptors: Bind hydrophilic ligands; initiate signal transduction.

• Intracellular Receptors: Bind hydrophobic ligands; 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

Stages of Cell Signaling in Endocrine System

• Reception: Hormone binds receptor.

• Transduction: Signal relayed inside cell.

• Response: Cellular activity changes (e.g., metabolism).

Hormones: Definition and Function

• Hormones: Chemical messengers secreted by endocrine glands; regulate physiology and behavior.

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; activate second messengers.

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

Organs Involved in Hormone Production

• Hypothalamus, Pituitary, Thyroid, Adrenal, Pancreas, Gonads

Neurosecretory Cells and Neurohormones

• Neurosecretory Cells: Neurons that release hormones.

• Neurohormones: Hormones released into blood by neurons.

• Hypothalamus: Coordinates nervous and endocrine systems.

Hormones and Homeostasis

Hormones regulate internal balance (homeostasis) via feedback mechanisms.

Positive vs. Negative Feedback

• Negative Feedback: Inhibits further hormone release; maintains stability.

• Positive Feedback: Enhances hormone release; amplifies response.

Examples of Negative Feedback

• Regulation of blood glucose by insulin and glucagon.

• Thyroid hormone regulation.

Tropic vs. Non-Tropic Hormones

• Tropic: Stimulate other glands to release hormones (e.g., TSH).

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

Adrenal Gland and Stress Response

• Adrenal Cortex: Releases corticosteroids (long-term stress).

• Adrenal Medulla: Releases epinephrine/norepinephrine (short-term stress).

Thyroid Hormone Regulation Pathway

• Hypothalamus releases TRH → Pituitary releases TSH → Thyroid releases T3/T4

Antagonistic Hormones

• Pairs with opposite effects (e.g., insulin lowers blood glucose, glucagon raises it).

• Maintain homeostasis by balancing actions.

Chapter 48: Neurons, Synapses, and Signaling

Structure and Function of Neurons

Neurons are specialized cells for transmitting electrical and chemical signals.

• Dendrites: Receive signals.

• Cell Body: Integrates signals.

• Axon: Transmits signals to 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 in Nerve Conduction

Maintains ion gradients essential for action potentials.

• Uses ATP to move Na+ out, K+ in.

Leak Channels and Resting Potential

Allow passive movement of ions, maintaining resting potential.

Ligand-Gated Ion Channels and Graded Potentials

Open in response to neurotransmitters, causing small changes in membrane potential (graded potentials).

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 axon hillock, an action potential is triggered.

Integration of Stimuli at Axon Hillock

Neuron sums excitatory and inhibitory inputs to decide whether to fire.

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.

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 potential.

Graph Interpretation

Membrane potential changes from resting, through graded potentials, to action potential spike.

Chapter 49: Nervous Systems

Core Functions of Nervous System

• Input: Sensory information (PNS).

• Integration: Processing (CNS).

• Output: Motor response (PNS).

Knee-Jerk Reflex Pathway

• Afferent neurons carry signal to CNS.

• Integration occurs in spinal cord.

• Efferent neurons carry response to muscle.

CNS and PNS Connections

• CNS: Brain and spinal cord.

• PNS: Nerves outside CNS.

• Autonomic Nervous System (ANS): Involuntary functions.

• Motor: Voluntary movement.

• 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.

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

Components of Sensory System

• Reception: Detecting stimulus.

• Transduction: Converting stimulus to electrical signal.

• Transmission: Sending signal to CNS.

• Perception: Interpreting signal in brain.

• Amplification: Increasing signal strength.

• Adaptation: Reduced response to constant stimulus.

Sensory Receptor Cells

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

Non-Neuronal Sensory Receptors

Release neurotransmitters to activate neurons, initiating action potentials.

Stimulus Intensity and Action Potential Frequency

Stronger stimuli produce higher frequency of action potentials.

Types of Sensory Receptors

• Mechanoreceptors: Touch, pressure, hearing.

• Photoreceptors: Vision.

• Chemoreceptors: Taste, smell.

• Thermoreceptors: Temperature.

• Nociceptors: Pain.

Transduction in Five Senses

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

• Vision: Photoreceptors; transduction in retina.

• Taste: Chemoreceptors; transduction in taste buds.

• Smell: Chemoreceptors; transduction in olfactory epithelium.

• Touch: Mechanoreceptors; transduction in skin.

Additional info: Academic context and definitions have been expanded for clarity and completeness. Tables have been inferred and reconstructed for comparison and classification purposes.