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. It is essential for maintaining cellular homeostasis and involves several mechanisms:

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

• Active Transport: Movement of substances against their concentration gradient, requiring energy (usually ATP). Includes primary and secondary active transport.

• Other Types: Includes bulk transport mechanisms such as endocytosis and exocytosis.

Key Terms:

• Diffusion: Movement of molecules from an area of higher concentration to lower concentration.

• Osmosis: Diffusion of water across a selectively permeable membrane.

• Facilitated Diffusion: Passive movement of molecules via membrane proteins.

• Primary Active Transport: Direct use of ATP to transport molecules (e.g., Na+/K+ pump).

• Secondary Active Transport: Uses the energy from the movement of one substance down its gradient to move another substance against its gradient (e.g., symporters and antiporters).

Example: The Na+/K+ ATPase pump moves 3 Na+ ions out of the cell and 2 K+ ions into the cell, maintaining electrochemical gradients.

Plasma Membrane Proteins in Transport

• Channel Proteins: Provide corridors for specific molecules or ions to cross the membrane.

• Carrier Proteins: Bind to molecules and change shape to shuttle them across the membrane.

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

Osmosis and Tonicity

• Osmosis: Water moves from areas of low solute concentration to high solute concentration.

• Tonicity: The ability of a solution to cause a cell to gain or lose water.

• Types of Solutions:

  • Isotonic: No net water movement.

  • Hypertonic: Cell loses water and shrinks.

  • Hypotonic: Cell gains water and swells.

Formula for Osmolarity:

$\text{Osmolarity} = \sum_{i} c_i \cdot n_i$ Where $c_i$ is the molar concentration of solute $i$, and $n_i$ is the number of particles into which the solute dissociates.

Bulk Transport

• Endocytosis: Uptake of materials via vesicle formation (includes phagocytosis and pinocytosis).

• Exocytosis: Release of materials from the cell via vesicles.

Membrane Potentials

Resting Membrane Potential

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

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

• Goldman-Hodgkin-Katz Equation: Used to calculate the membrane potential considering multiple ions:

$V_m = \frac{RT}{F} \ln \left( \frac{P_{K^+}[K^+]_{out} + P_{Na^+}[Na^+]_{out} + P_{Cl^-}[Cl^-]_{in}}{P_{K^+}[K^+]_{in} + P_{Na^+}[Na^+]_{in} + P_{Cl^-}[Cl^-]_{out}} \right)$

• Depolarization: Membrane potential becomes less negative.

• Hyperpolarization: Membrane potential becomes more negative.

• Repolarization: Return to resting potential after depolarization.

Action Potentials

Action potentials are rapid, temporary changes in membrane potential that propagate along neurons.

• Phases: Resting state, depolarization, repolarization, hyperpolarization, return to rest.

• Threshold: The critical level to which the membrane potential must be depolarized to initiate an action potential.

• Refractory Periods: Times during which a neuron cannot fire another action potential (absolute and relative).

Example: During depolarization, voltage-gated Na+ channels open, allowing Na+ influx.

Synaptic Transmission

Types and Mechanisms

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

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

• Chemical Synapses: Use neurotransmitters to transmit signals across a synaptic cleft.

Key Steps in Chemical Synaptic Transmission:

1. Action potential arrives at axon terminal.

2. Voltage-gated Ca2+ channels open; Ca2+ enters the terminal.

3. Neurotransmitter vesicles fuse with the membrane and release contents.

4. Neurotransmitter binds to receptors on the postsynaptic cell.

5. Postsynaptic potential is generated (excitatory or inhibitory).

6. Neurotransmitter is removed from the synaptic cleft (reuptake, degradation, or diffusion).

Summation: The process by which multiple synaptic potentials combine within one postsynaptic neuron (spatial and temporal summation).

Termination of Synaptic Transmission

• Reuptake of neurotransmitter into presynaptic neuron.

• Enzymatic degradation in the synaptic cleft.

• Diffusion away from the synapse.

Bone and Bone Tissue

Structure and Function

Bones provide support, protection, movement, mineral storage, blood cell formation, and energy storage.

• Types of Bone Cells:

  • Osteogenic cells: Stem cells that differentiate into osteoblasts.

  • Osteoblasts: Bone-forming cells.

  • Osteocytes: Mature bone cells that maintain bone tissue.

  • Osteoclasts: Bone-resorbing cells.

• Bone Matrix: Composed of organic (collagen fibers) and inorganic (hydroxyapatite) components.

Bone Classification

• Long bones: Longer than they are wide (e.g., femur).

• Short bones: Cube-shaped (e.g., carpals).

• Flat bones: Thin and broad (e.g., sternum).

• Irregular bones: Complex shapes (e.g., vertebrae).

• Sutural and sesamoid bones: Additional categories based on location and development.

Bone Structure

• Compact Bone: Dense outer layer; contains osteons (Haversian systems).

• Spongy Bone: Lattice-like structure; contains trabeculae and red marrow.

• Periosteum: Membrane covering the outer surface of bones.

• Endosteum: Membrane lining the medullary cavity.

Bone Formation (Ossification)

Bone develops via two main processes:

Characteristic	Intramembranous Ossification	Endochondral Ossification
Bones formed by this type	Flat bones of skull, clavicle	Long bones, most bones of skeleton
Model used	Fibrous membrane	Hyaline cartilage
When ossification begins	Early fetal development	Later fetal development
Type of bone compact or spongy formed first	Spongy bone	Spongy bone

Bone Growth and Remodeling

• Appositional Growth: Increase in bone thickness.

• Interstitial Growth: Increase in bone length (at epiphyseal plate).

• Bone Remodeling: Ongoing replacement of old bone tissue by new bone tissue.

• Bone Repair: Involves hematoma formation, fibrocartilaginous callus formation, bony callus formation, and bone remodeling.

Bone Disorders

• Osteoporosis: Decreased bone mass and increased fracture risk.

• Osteomalacia: Softening of bones due to vitamin D deficiency.

• Fracture Types: Simple, compound, comminuted, avulsion, compression, spiral, etc.

Practice and Study Questions

• Define and give examples of passive and active transport.

• Explain the difference between osmosis and diffusion.

• Describe the events of an action potential and label a diagram.

• Compare absolute and relative refractory periods.

• Describe the steps of synaptic transmission and how it can be terminated.

• List and describe the functions of osteogenic cells, osteoblasts, osteocytes, and osteoclasts.

• Compare intramembranous and endochondral ossification using the table above.

• Describe the process of bone repair and the stages involved.

Additional info: Some content and context were inferred and expanded for clarity and completeness based on standard Anatomy & Physiology curriculum.