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 mechanisms by which molecules move in and out of cells.

• 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 active transport (e.g., Na+/K+ pump) and secondary active transport (uses energy from another gradient).

• Secondary Active Transport: Uses the energy stored in the gradient of one molecule to drive the transport of another molecule against its gradient.

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

Key Equation:

$\text{Rate of diffusion} = k \times [\text{Concentration gradient}]$

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.

• Diffusion: Driven by concentration gradients; does not require energy.

• Osmosis: Movement of water from low solute concentration to high solute concentration.

Example: Oxygen diffuses from alveoli into blood due to concentration differences.

Facilitated Diffusion and Channel Proteins

Facilitated diffusion involves carrier or channel proteins that help move substances across the membrane without energy input.

• Channel Proteins: Form pores for specific ions or molecules.

• Carrier Proteins: Bind and transport specific molecules.

Equilibrium and Gradients

Equilibrium is reached when the concentration of a substance is equal on both sides of the membrane. Gradients (chemical, electrical, and electrochemical) drive transport.

• Chemical Gradient: Difference in concentration of a substance.

• Electrical Gradient: Difference in charge across the membrane.

• Electrochemical Gradient: Combined effect of chemical and electrical gradients.

Key Equation:

$\text{Electrochemical gradient} = \text{chemical gradient} + \text{electrical gradient}$

Membrane Potentials

Resting Membrane Potential

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

• Generated by: Unequal distribution of ions (Na+, K+, Cl-) and selective permeability of the membrane.

• Maintained by: Na+/K+ ATPase pump and leak channels.

Key Equation:

$E_{m} = \frac{RT}{zF} \ln \frac{[\text{ion outside}]}{[\text{ion inside}]}$

(Nernst equation for equilibrium potential)

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.

• Depolarization: Na+ channels open, Na+ enters the cell.

• Repolarization: K+ channels open, K+ leaves the cell.

• Hyperpolarization: Membrane potential becomes more negative than resting.

Example: The action potential travels from the axon hillock to the axon terminal, triggering neurotransmitter release.

Graded Potentials

Graded potentials are small changes in membrane potential that can summate to trigger an action potential.

• Temporal Summation: Multiple signals in quick succession.

• Spatial Summation: Multiple signals from different locations.

Synaptic Transmission

Chemical and Electrical Synapses

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

• Chemical Synapse: Neurotransmitters are released from the presynaptic neuron and bind to receptors on the postsynaptic cell.

• Electrical Synapse: Direct flow of ions through gap junctions.

Example: Acetylcholine release at the neuromuscular junction.

Neurotransmitters and Receptors

Neurotransmitters are chemical messengers that transmit signals across the synaptic cleft.

• Excitatory neurotransmitters: Cause depolarization (e.g., glutamate).

• Inhibitory neurotransmitters: Cause hyperpolarization (e.g., GABA).

• Reuptake: Neurotransmitters are taken back into the presynaptic neuron, terminating the signal.

Example: SSRIs inhibit serotonin reuptake, increasing serotonin levels in the synaptic cleft.

Bones and Bone Tissue

Types and Functions of Bone Tissue

Bone tissue provides structural support, protection, and aids in movement. There are two main types: compact bone and spongy bone.

• Compact Bone: Dense, forms the outer layer of bones.

• Spongy Bone: Porous, found at the ends of long bones and inside flat bones.

Example: The femur has a shaft of compact bone and ends of spongy bone.

Bone Cells

Bone tissue contains several types of cells, each with specific functions.

• Osteoblasts: Build new bone matrix.

• Osteocytes: Mature bone cells, maintain bone tissue.

• Osteoclasts: Break down bone matrix.

Bone Structure and Growth

Bones grow and remodel throughout life via ossification and bone repair processes.

• Long Bone Structure: Diaphysis (shaft), epiphysis (ends), metaphysis (between shaft and ends), epiphyseal plate (growth plate).

• Ossification: Formation of bone tissue. Two types: intramembranous and endochondral.

Characteristic	Intramembranous Ossification	Endochondral Ossification
Bones formed by this type of ossification	Flat bones (skull, clavicle)	Long bones (femur, humerus)
Model used	Membrane	Cartilage
Site where ossification begins	Ossification center	Primary ossification center in cartilage
Type of bone formed first	Spongy bone	Spongy bone

Bone Remodeling and Repair

Bone remodeling is a continuous process involving bone resorption and formation. Bone repair occurs after fractures and involves several stages.

• Stages of Bone Repair: Hematoma formation, callus formation, bone formation, bone remodeling.

• Factors Influencing Remodeling: Mechanical stress, hormones, nutrition.

Classification of Bones

Bones are classified by shape: long, short, flat, irregular, and sesamoid.

• Long Bones: Femur, humerus.

• Short Bones: Carpals, tarsals.

• Flat Bones: Skull, sternum.

• Irregular Bones: Vertebrae.

• Sesamoid Bones: Patella.

Bone Markings and Structures

Bones have various markings for muscle attachment, passage of nerves and blood vessels.

• Foramen: Opening for nerves/vessels.

• Process: Projection for muscle attachment.

• Canal: Passageway within bone.

Additional Info

• Osmolarity: The concentration of solute particles in a solution. Hypotonic solutions have lower osmolarity than the cell, hypertonic have higher, and isotonic are equal.

• Periosteum: Dense connective tissue covering the bone surface, anchoring via Sharpey's fibers.

• Epiphyseal Plate: Growth plate in long bones, site of lengthwise growth during development.