Membrane Structure and Function

Fluid-Mosaic Model of Cell Membranes

The fluid-mosaic model describes the structure of cell membranes as a dynamic combination of lipids, proteins, and carbohydrates. This model explains how membranes maintain flexibility and allow for the movement of embedded molecules.

• Major Components: Phospholipids, proteins, cholesterol, and carbohydrates.

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

• Membrane Proteins: Integral and peripheral proteins serve functions such as transport, signaling, and structural support.

• Cholesterol: Modulates membrane fluidity and stability.

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

Example: Glycoproteins on red blood cells determine blood type.

Functions of Membrane Proteins

Membrane proteins perform a variety of essential functions for the cell.

• Transport: Channels and carriers facilitate movement of substances across the membrane.

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

• Signal Transduction: Receptors transmit signals from the extracellular environment to the cell interior.

• Cell Recognition: Glycoproteins serve as identification tags.

• Intercellular Joining: Proteins help connect adjacent cells.

• Attachment: Proteins anchor the membrane to the cytoskeleton and extracellular matrix.

Transport Across Membranes

Cells regulate the movement of substances using various transport mechanisms.

• Passive Transport: Movement down a concentration gradient without energy input (e.g., diffusion, osmosis).

• Active Transport: Movement against a gradient, requiring energy (e.g., sodium-potassium pump).

• Facilitated Diffusion: Passive transport via membrane proteins.

• Osmosis: Diffusion of water across a semipermeable membrane.

Key Terms: Solvent, solute, concentration, gradient, osmolarity, diffusion, osmosis.

Equation:

Where is the flux, is the diffusion coefficient, and is the concentration gradient.

Sodium-Potassium Pump

The sodium-potassium pump is an active transport protein that maintains cellular ion gradients.

• Function: Pumps 3 Na+ ions out and 2 K+ ions into the cell per ATP hydrolyzed.

• Role: Maintains resting membrane potential and cell volume.

Equation:

Cellular Communication

Types of Cell Signaling

Cells communicate using chemical signals that can act locally or at a distance.

• Paracrine Signaling: Signals affect nearby cells.

• Synaptic Signaling: Neurotransmitters cross synapses between neurons.

• Endocrine Signaling: Hormones travel through the bloodstream to distant targets.

• Autocrine Signaling: Cells respond to signals they produce themselves.

Stages of Cell Signaling

Cell signaling involves three main stages:

• Reception: Signal molecule binds to a receptor.

• Transduction: Signal is relayed and amplified inside the cell, often via a cascade of molecular events.

• Response: The cell carries out a specific activity, such as gene expression or metabolic change.

Second Messengers

Small molecules such as cAMP, Ca2+, and IP3 act as second messengers in signal transduction pathways.

• Function: Amplify and distribute signals within the cell.

• Example: cAMP activates protein kinase A, leading to cellular responses.

Hormones and the Endocrine System

Hormones: Definition and Types

Hormones are chemical messengers secreted by endocrine glands that regulate physiological processes.

• Water-Soluble Hormones: Peptides and amines; bind to cell surface receptors.

• Lipid-Soluble Hormones: Steroids; pass through membranes and bind to intracellular receptors.

Hormone Pathways and Feedback

Hormone secretion is regulated by feedback mechanisms.

• Negative Feedback: Reduces hormone output when levels are sufficient.

• Positive Feedback: Enhances hormone output in response to a stimulus.

Example: Regulation of blood glucose by insulin (negative feedback).

Endocrine Organs and Neurosecretory Cells

Major endocrine organs include the pituitary, thyroid, adrenal glands, and pancreas. Neurosecretory cells release neurohormones that act on distant targets.

• Hypothalamus: Integrates nervous and endocrine functions.

• Pituitary Gland: Releases tropic and non-tropic hormones.

Nervous System Structure and Function

Neurons and Glial Cells

The nervous system consists of neurons and supporting glial cells.

• Neurons: Transmit electrical signals.

• Glial Cells: Support and protect neurons (e.g., Schwann cells, oligodendrocytes, astrocytes).

Membrane Potential and Ion Channels

Neurons maintain a resting membrane potential using ion pumps and channels.

• Resting Potential: Typically -70 mV, maintained by the sodium-potassium pump.

• Ion Channels: Allow selective movement of ions; voltage-gated channels are crucial for action potentials.

Equation:

Where is the equilibrium potential, is the gas constant, is temperature, is ion charge, is Faraday's constant.

Action Potentials

Action potentials are rapid changes in membrane potential that transmit signals along neurons.

• Phases: Depolarization, repolarization, hyperpolarization.

• Threshold: Minimum stimulus required to trigger an action potential.

• Propagation: Movement of the action potential along the axon.

The Nervous System: Organization and Integration

Core Functions

The nervous system integrates sensory input, processes information, and coordinates responses.

• Central Nervous System (CNS): Brain and spinal cord.

• Peripheral Nervous System (PNS): Nerves outside the CNS.

Autonomic Nervous System (ANS)

The ANS regulates involuntary functions and is divided into sympathetic and parasympathetic branches.

• Sympathetic: Prepares body for 'fight or flight'.

• Parasympathetic: Promotes 'rest and digest'.

Integration and Reflexes

Reflex arcs and integration centers coordinate rapid responses to stimuli.

• Example: Knee-jerk reflex involves sensory and motor neurons.

The Senses

Basic Components of Sensory Systems

Sensory systems detect and process environmental stimuli through specialized receptors.

• Reception: Detection of stimulus by sensory receptors.

• Transduction: Conversion of stimulus energy into electrical signals.

• Transmission: Propagation of signals to the CNS.

• Perception: Interpretation of signals by the brain.

Types of Senses and Transduction

The five senses—hearing/equilibrium, vision, taste, smell, and touch—each have unique transduction mechanisms.

• Hearing/Equilibrium: Hair cells transduce mechanical vibrations.

• Vision: Photoreceptors transduce light energy.

• Taste: Chemoreceptors transduce chemical signals.

• Smell: Olfactory receptors transduce airborne chemicals.

• Touch: Mechanoreceptors transduce pressure and vibration.

Relationship Between Stimulus and Response

The intensity and duration of a stimulus affect the frequency and pattern of action potentials generated by sensory neurons.

Summary Table: Types of Cell Signaling

Summary Table: Comparison of Water-Soluble and Lipid-Soluble Hormones

Additional info: Some explanations and examples have been expanded for clarity and completeness based on standard General Biology curriculum.