Membrane Transport, Membrane Potentials, Synaptic Transmission, and 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).

• Endocytosis and Exocytosis: Bulk transport mechanisms for large molecules or particles.

Key Terms:

• Concentration Gradient: Difference in concentration of a substance across a space.

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

• Carrier Proteins: Proteins that facilitate the movement of substances across the membrane.

• Channel Proteins: Proteins that form pores for specific ions or molecules to pass through.

Example: The Na+/K+ ATPase pump actively transports sodium out of the cell and potassium into the cell, maintaining electrochemical gradients.

Osmolarity and Tonicity

Osmolarity and tonicity describe the concentration of solutes in solutions and their effects on cell volume.

• Osmolarity: Total concentration of all solute particles in a solution.

• Tonicity: The effect of a solution on cell volume (isotonic, hypotonic, hypertonic).

Example: Placing a cell in a hypotonic solution causes water to enter the cell, leading to swelling.

Diffusion and Facilitated Diffusion

Diffusion is the movement of molecules from an area of higher concentration to lower concentration. Facilitated diffusion uses membrane proteins to help substances cross the membrane.

• Simple Diffusion: Direct movement through the lipid bilayer.

• Facilitated Diffusion: Movement via channel or carrier proteins.

Equation:

$J = -D \frac{dC}{dx}$

Where J is the flux, D is the diffusion coefficient, and $\frac{dC}{dx}$ is the concentration gradient.

Membrane Potentials

Resting Membrane Potential

The resting membrane potential is the electrical potential difference across the cell membrane when the cell is not actively sending signals.

• Typical Value: -70 mV in neurons.

• Key Contributors: Na+/K+ pump, K+ leak channels, and negatively charged proteins inside the cell.

Equation:

$E_{m} = \frac{RT}{F} \ln \left( \frac{[K^+]_{out}}{[K^+]_{in}} \right)$

Where Em is the membrane potential, R is the gas constant, T is temperature, F is Faraday's constant, and [K+] are potassium concentrations.

Action Potentials

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

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

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

• Threshold: The membrane potential at which an action potential is triggered.

Example: During depolarization, Na+ channels open, allowing Na+ to enter the cell.

Refractory Periods

Refractory periods are times during which a neuron cannot fire another action potential.

• 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: Release of neurotransmitters into the synaptic cleft.

Key Steps:

1. Action potential arrives at axon terminal.

2. Ca2+ channels open, Ca2+ enters the cell.

3. Neurotransmitter vesicles fuse with membrane and release contents.

4. Neurotransmitter binds to receptors on postsynaptic cell.

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

Example: Acetylcholine is released at neuromuscular junctions to stimulate muscle contraction.

Summation and Termination

Summation refers to the additive effects of multiple synaptic inputs.

• Temporal Summation: Multiple signals in quick succession.

• Spatial Summation: Signals from multiple synapses at once.

Termination of synaptic transmission can occur via reuptake, enzymatic degradation, or diffusion away from the synapse.

Bones and Bone Tissue

Bone Structure and Types

Bones are classified by shape and structure, and serve as the framework for the body.

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

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

• Flat Bones: Thin and broad (e.g., skull).

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

• Sesamoid Bones: Embedded in tendons (e.g., patella).

Key Structures:

• Diaphysis: Shaft of long bone.

• Epiphysis: Ends of long bone.

• Metaphysis: Region between diaphysis and epiphysis.

• Periosteum: Outer covering of bone.

• Endosteum: Lining of the medullary cavity.

• Spongy Bone: Contains trabeculae; found in epiphyses.

• Compact Bone: Dense outer layer; contains osteons.

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, maintain bone tissue.

• Osteoclasts: Bone-resorbing cells.

Bone Formation and Growth

Bones develop through two main processes:

• Intramembranous Ossification: Bone develops directly from mesenchymal tissue (e.g., flat bones of the skull).

• Endochondral Ossification: Bone develops from a cartilage model (e.g., long bones).

Characteristic	Intramembranous Ossification	Endochondral Ossification
Bones formed by this type	Flat bones (skull, clavicle)	Long bones (femur, humerus)
Model used	Mesenchymal tissue	Cartilage model
When ossification begins	Early fetal development	Later fetal development
Type of bone formed	Spongy and compact	Spongy and compact

Bone Remodeling and Repair

Bone remodeling is a continuous process involving bone resorption and formation. Bone repair follows injury and involves several stages:

1. Hematoma formation

2. Fibrocartilaginous callus formation

3. Bony callus formation

4. Bone remodeling

Extracellular Matrix of Bone

The bone matrix consists of organic and inorganic components:

• Organic: Collagen fibers, proteoglycans

• Inorganic: Hydroxyapatite (calcium phosphate crystals)

Types of Bone Fractures

Bones can break in various ways, classified by the pattern and severity:

• Simple (closed) fracture

• Compound (open) fracture

• Comminuted fracture

• Spiral fracture

• Epiphyseal plate fracture

Practice and Study Questions

• How do diffusion and osmosis differ?

• What drives active transport?

• What is the effect of placing a cell in a hypotonic solution?

• Describe the phases of an action potential and the role of ion channels.

• What are the functions of osteoblasts, osteoclasts, and osteocytes?

• Compare intramembranous and endochondral ossification.

• List the steps of bone repair.

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