Membrane Transport, Membrane Potentials, Synaptic Transmission, and Bone & Bone Tissue: Study Guide

Membrane Transport

Types of Membrane Transport

Membrane transport refers to the movement of substances across the cell membrane, which is essential for maintaining cellular homeostasis. There are several types of membrane transport, each with distinct mechanisms and energy requirements.

• Passive Transport: Movement of substances down their concentration gradient without energy input. Includes simple diffusion, facilitated diffusion, and osmosis.

• Active Transport: Movement of substances against their concentration gradient, requiring energy (usually ATP). Includes primary active transport (e.g., Na+/K+ pump) and secondary active transport (e.g., symport, antiport).

• Transport Proteins: Includes channels, carriers, and pumps that facilitate movement of molecules across membranes.

Example: The Na+/K+ ATPase pump actively transports Na+ out of and K+ into the cell, maintaining membrane potential.

Diffusion and Osmosis

Diffusion is the movement of molecules from an area of higher concentration to lower concentration. Osmosis is the diffusion of water across a selectively permeable membrane.

• Factors Affecting Diffusion: Concentration gradient, molecular size, distance, and temperature.

• Osmosis: Water moves toward higher solute concentration.

• Osmolarity: Measure of solute concentration; affects water movement.

• Example: Red blood cells in a hypotonic solution swell due to water influx.

Plasma Membrane Proteins

Proteins embedded in the plasma membrane facilitate transport and communication.

• Channel Proteins: Allow specific ions or molecules to pass through by diffusion.

• Carrier Proteins: Bind and transport substances across the membrane.

• Pumps: Use energy to move substances against their gradient.

Classification of Solutions

Solutions are classified based on their osmolarity relative to the cell:

• Isotonic: Same osmolarity as the cell; no net water movement.

• Hypertonic: Higher osmolarity than the cell; water moves out, cell shrinks.

• Hypotonic: Lower osmolarity than the cell; water moves in, cell swells.

Membrane Potentials

Resting Membrane Potential

The resting membrane potential is the electrical potential difference across the plasma membrane when the cell is at rest, typically around -70 mV in neurons.

• Key Factors: Ion concentration gradients (Na+, K+), selective permeability, and activity of the Na+/K+ ATPase pump.

• Equation:

$V_m = \frac{RT}{F} \ln \left( \frac{[K^+]_{out}}{[K^+]_{in}} \right)$ (Nernst equation for K+)

• Depolarization: Membrane potential becomes less negative.

• Hyperpolarization: Membrane potential becomes more negative.

Action Potentials

An action potential is a rapid change in membrane potential that propagates along the neuron, allowing for signal transmission.

• Phases: Depolarization, repolarization, hyperpolarization, and return to resting potential.

• Channels Involved: Voltage-gated Na+ and K+ channels.

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

Example: The opening of Na+ channels causes rapid depolarization.

Refractory Periods

Refractory periods ensure unidirectional propagation of action potentials and limit their frequency.

• Absolute Refractory Period: No new action potential can be initiated.

• Relative Refractory Period: A stronger stimulus can initiate another action potential.

Synaptic Transmission

Types of Synapses and Transmission

Synaptic transmission is the process by which neurons communicate with each other or with effector cells.

• Electrical Synapse: Direct passage of ions via gap junctions.

• Chemical Synapse: Neurotransmitter release from presynaptic neuron binds to receptors on postsynaptic cell.

• Key Structures: Synaptic vesicles, neurotransmitter, synaptic cleft, postsynaptic receptor.

Excitatory and Inhibitory Postsynaptic Potentials

Postsynaptic potentials determine whether a neuron will fire an action potential.

• Excitatory Postsynaptic Potential (EPSP): Depolarizes the postsynaptic membrane.

• Inhibitory Postsynaptic Potential (IPSP): Hyperpolarizes the postsynaptic membrane.

Termination of Synaptic Transmission

Synaptic transmission can be terminated by:

• Reuptake of neurotransmitter

• Enzymatic degradation

• Diffusion away from the synaptic cleft

Bone and Bone Tissue

Functions and Types of Bone Tissue

Bones provide structural support, protect organs, enable movement, and serve as sites for mineral storage and blood cell formation.

• Skeletal System Components: Bone, cartilage, tendons, ligaments.

• Bone Types: Long, short, flat, irregular, sesamoid.

• Bone Tissue Types: Compact bone (dense), spongy bone (porous).

Bone Cells

Bone tissue contains several specialized cell types:

• Osteogenic Cells: Stem cells that differentiate into osteoblasts.

• Osteoblasts: Bone-forming cells.

• Osteocytes: Mature bone cells maintaining bone tissue.

• Osteoclasts: Bone-resorbing cells.

Bone Structure

Bones have complex structures that support their functions.

• Long Bone Anatomy: Diaphysis (shaft), epiphysis (ends), metaphysis (between shaft and ends), periosteum (outer covering), endosteum (inner lining).

• Compact Bone: Contains osteons (Haversian systems), lamellae, canaliculi, lacunae with osteocytes.

• Spongy Bone: Contains trabeculae and spaces for marrow.

Bone Formation and Growth

Bones develop through two main processes:

• Intramembranous Ossification: Bone develops directly from mesenchymal tissue; forms flat bones.

• Endochondral Ossification: Bone develops from cartilage; forms most bones of the body.

Characteristic	Intramembranous Ossification	Endochondral Ossification
Bones formed by this type	Flat bones (e.g., skull, clavicle)	Long bones (e.g., femur, humerus)
Model used	Mesenchymal tissue	Hyaline cartilage
Where ossification begins	Ossification center in membrane	Primary ossification center in cartilage
Type of bone formed	Spongy and compact bone	Spongy and compact bone

Bone Remodeling and Repair

Bone remodeling is a continuous process involving bone resorption and formation, regulated by hormones and mechanical stress.

• Bone Deposition: Addition of new bone by osteoblasts.

• Bone Resorption: Removal of bone by osteoclasts.

• Repair Stages: Hematoma formation, fibrocartilaginous callus, bony callus formation, bone remodeling.

Bone Fractures

Bones can break in various ways, classified by the nature of the fracture.

• Types: Simple, compound, comminuted, avulsion, compression, spiral, epiphyseal plate, contaminated.

Bone Matrix

The bone matrix consists of organic and inorganic components.

• Organic: Collagen fibers, ground substance.

• Inorganic: Hydroxyapatite (calcium phosphate crystals).

Additional Info

• Summation in Neurons: Temporal summation (multiple signals in quick succession) and spatial summation (multiple signals from different locations) can trigger action potentials.

• Neurotransmitter Reuptake: Drugs like SSRIs inhibit reuptake of serotonin, increasing its synaptic concentration.

• ADHD and Dopamine: Reduced dopamine levels affect movement and impulse control.