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, including transport, signaling, and structural support.

• Transport Proteins: Facilitate movement of substances across the membrane (e.g., 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.

The Fluid-Mosaic Model

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 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 and hydrophobic tails.

• 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 maintaining cellular homeostasis.

Predicting Net Diffusion

• Solute: Moves from high to low concentration.

• Water (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 (diffusion, osmosis, facilitated diffusion).

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

Facilitated Diffusion

Transport proteins (channels, carriers) help molecules cross the membrane without energy input.

Sodium-Potassium Pump

Maintains electrochemical gradients by pumping Na+ out and K+ in, using ATP.

• Role of ATP: Provides energy for conformational changes.

Electrochemical Gradient

Combination of concentration gradient and electrical charge difference across the membrane.

Cotransport

Pumps and channels work together; one molecule'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 signal 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.

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 bound by ligands.

G Protein-Coupled Receptor Pathway

• Ligand binds receptor.

• G protein activated.

• G protein triggers downstream effectors (e.g., enzymes).

Ligand-Gated Ion Channel Pathway

• Ligand binds channel.

• Channel opens, ions flow, changing membrane potential.

Signal Transduction

Process of converting a signal from outside to a functional response inside the cell.

Second Messengers

• cAMP: Cyclic AMP, amplifies signals inside cell.

• Ca2+: Calcium ions, trigger various responses.

Cascades and Amplification

Series of steps where each activates the next, amplifying the original signal.

Cellular Responses

• Gene expression changes.

• Metabolic pathway activation.

• Cell movement or division.

Membrane vs. Intracellular Receptors

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

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

Differential Responses

The same signaling molecule can cause different responses in different cell types due to receptor and pathway differences.

Chapter 45: Hormones and the Endocrine System

Cell Signaling in the Endocrine System

• Reception: Hormone binds receptor.

• Transduction: Signal relayed inside cell.

• Response: Change in cell activity (e.g., metabolism, growth).

Hormones: Definition and Function

Hormones are chemical messengers produced by endocrine glands, regulating physiological processes.

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

• Pituitary Gland

• Thyroid Gland

• Adrenal Glands

• Pancreas

• Gonads (Testes, Ovaries)

Neurosecretory Cells and Neurohormones

Neurosecretory cells release neurohormones, linking nervous and endocrine systems (e.g., hypothalamus).

Hormones and Homeostasis

Hormones regulate internal balance (homeostasis) by feedback mechanisms.

Positive vs. Negative Feedback

• Negative Feedback: Response reduces stimulus (e.g., blood glucose regulation).

• Positive Feedback: Response amplifies stimulus (e.g., oxytocin in 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 hormones.

Antagonistic Hormones

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

Chapter 48: Neurons, Synapses, and Signaling

Structure and Function of Neurons

Neurons are specialized cells for transmitting electrical signals.

• 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).

Sodium-Potassium Pump in Neurons

Maintains resting potential and enables action potentials.

Leak Channels

Allow passive movement of ions, helping maintain resting potential.

Ligand-Gated Ion Channels and Graded Potentials

Open in response to neurotransmitters, causing small 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 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

1. Resting State: All channels closed;

2. Depolarization: Na+ channels open; rises.

3. Repolarization: K+ channels open; falls.

4. Hyperpolarization: K+ channels remain open; below resting.

5. Return to Resting: Pump restores original state.

Graph Interpretation

Membrane potential changes from resting, through graded, to 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

• Afferent Neurons: Carry signal to CNS.

• Integration: CNS processes signal.

• Efferent Neurons: Carry response to muscle.

CNS and PNS Connections

• PNS: Sensory and motor divisions.

• ANS: Autonomic Nervous System (involuntary).

• Motor: Voluntary control.

• ANS Subdivisions:

  • Parasympathetic: Rest and digest.

  • Sympathetic: Fight or flight.

  • Enteric: Gut regulation.

ANS Subdivision Roles

Glial Cells in CNS vs. PNS

Grey vs. White Matter

• Grey Matter: Cell bodies, dendrites.

• 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

Release neurotransmitters to activate neurons, initiating action potentials.

Stimulus Intensity and Action Potential Frequency

Stronger stimuli increase action potential frequency, not amplitude.

Types of Sensory Receptors

Transduction in the 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.