Cell Membrane Transport and Nerve Cell Function

Flashcards about cell membrane transport and nerve cell function.

Permeability of lipid bilayer

Largely depends on size, polarity, and charge of the substance. Small, nonpolar, and hydrophobic molecules diffuse freely. Charged substances are generally impermeable.

Major Types of Transport Proteins

Channel proteins (facilitated diffusion, no energy) and carrier proteins (facilitated diffusion without energy or active transport with energy).

Uniporter

Moves a single type of molecule in one direction, down its concentration gradient, without energy.

Symporter

Transports two different molecules simultaneously in the same direction, using the movement of one molecule down its gradient to drive the other.

Antiporter

Moves two molecules in opposite directions, often relying on energy stored in ion gradients (secondary active transport).

Potassium (K⁺) concentration

Highest inside the cell.

Sodium (Na⁺) concentration

Highest outside the cell.

Chloride (Cl⁻) concentration

Highest outside the cell.

Calcium (Ca²⁺) concentration

Highest outside the cell.

P-type ATPases

Phosphorylated during transport, move ions like Na⁺, K⁺, and Ca²⁺ (e.g., Na⁺/K⁺ pump).

V-type ATPases

Pump protons (H⁺) to acidify organelles like lysosomes, without phosphorylation.

F-type ATPases

Use a proton gradient to generate ATP (ATP synthase), located in mitochondria and chloroplasts.

ABC transporters

Use ATP to transport lipids, drugs, and metabolic waste (e.g., CFTR for chloride ions).

Na⁺/K⁺-ATPase function

P-type ATPase that moves sodium, potassium, and calcium ions across cell membranes.

V-class H⁺-ATPases function

Use ATP hydrolysis to actively transport hydrogen ions (H⁺) into organelles to acidify their interior, found in lysosomes and Golgi apparatus.

ABC superfamily function

Uses ATP to move a wide variety of molecules across cell membranes, including ions, sugars, lipids, and drugs.

Na⁺-linked symporters function

Use the sodium ion gradient to transport another molecule (e.g., glucose, amino acids) into the cell.

Maintaining membrane potential

Ion gradients and selective permeability; Na⁺/K⁺-ATPase pumps 3 Na⁺ out, 2 K⁺ in; K⁺ leak channels increase negative charge inside.

Patch clamp

Technique to study electrical activity of individual ion channels by forming a tight seal on a small patch of cell membrane.

pH regulation

Primarily achieved by proton (H⁺) pumps like V-type H⁺-ATPases. Sucrose import into vacuole uses proton-sucrose antiporters.

Water movement across membrane

Primarily through osmosis and aquaporins, specialized channel proteins.

Glucose transport across epithelium

Na⁺-linked symporters on apical side, GLUT uniporters on basolateral side.

Acidic pH in stomach

Maintained by parietal cells secreting HCl via H⁺/K⁺-ATPase proton pump exchanging H⁺ for K⁺.

Nerve Cell Function

Receive, process, and transmit electrical and chemical signals.

Three Types of Nerve Cells

Sensory neurons, motor neurons, and interneurons.

Four Parts of a Nerve Cell

Cell body (soma), dendrites, axon, and axon terminals (synaptic terminals).

Action Potential

Rapid, temporary electrical signal that travels along a neuron or muscle cell membrane due to opening/closing of voltage-gated ion channels.

Direction of Action Potential

From axon hillock down the axon toward axon terminals (unidirectional).

Synapses

Specialized junctions where neurons communicate with other neurons, muscle cells, or gland cells by transmitting electrical or chemical signals.

Parts of a Synapse

Presynaptic terminal, synaptic cleft, postsynaptic membrane, synaptic vesicles.

Reflex Arc

Simplest neural pathway controlling an automatic, rapid response to a stimulus.

Stimulatory Signals in Reflex Arcs

Activate the effector, causing a response like muscle contraction.

Inhibitory Signals in Reflex Arcs

Prevent or reduce activity in a neuron or muscle, coordinating movement and preventing overreaction.

Electric Potential Regulation

Movement of ions across the neuronal membrane through ion channels and pumps.

Resting Membrane Potential

Voltage difference between inside and outside of cell when not firing (~ -70mV), maintained by Na+/K+ pump & K+ leak channels

Role of Channels in Neuronal Potential

Leak channels maintain resting potential. Voltage-gated Na⁺ channels cause depolarization, K⁺ channels repolarize. Ca²⁺ channels release neurotransmitters. Ligand-gated channels initiate signals.

Action Potential Generation

Neuron is stimulated enough to reach a critical threshold, causing a rapid reversal of the membrane potential.

Steps of Action Potential Generation

Resting state, depolarization, peak, repolarization, hyperpolarization, return to resting potential.

Role of Voltage-Gated Channels in Action Potential

Na⁺ channels initiate AP. K⁺ channels return membrane to resting. Channels open/close in response to voltage making AP directional and rapid.

Myelination

Process by which myelin wraps around axons to increase speed and efficiency of electrical signal transmission.

How Myelin is Formed

Oligodendrocytes (CNS) and Schwann cells (PNS) wrap membranes around axons, forming myelin sheaths.

Node of Ranvier

Unmyelinated gaps between myelin segments packed with voltage-gated channels, critical for saltatory conduction.

Role of Glial Cells (CNS)

Oligodendrocytes (myelin), astrocytes (support), microglia (immune), ependymal cells (CSF).

Role of Glial Cells (PNS)

Schwann cells (myelin) and satellite cells (support).

Neurotransmitters

Chemical messengers used by neurons to communicate with other cells across a synapse.

Function of Neurotransmitters

Release triggered by action potential, bind to receptors causing excitatory or inhibitory effects.

How Neurotransmitters Are Released

AP reaches terminal, Ca²⁺ enters, vesicles fuse with membrane (exocytosis), neurotransmitters released, bind to receptors, and are then cleared.

V-SNARE

Located on synaptic vesicle membrane (e.g., synaptobrevin).

T-SNARE

Located on presynaptic plasma membrane (e.g., syntaxin and SNAP-25).

Synaptotagmin

Calcium-sensing protein on vesicle, triggers fusion when Ca²⁺ enters axon terminal.

Botulinum Toxins

Block neurotransmitter release by cleaving SNARE proteins, inhibiting acetylcholine release, leading to muscle paralysis.

Acetylcholine (ACh)

Key neurotransmitter in somatic and autonomic nervous systems. Broken down by acetylcholinesterase (AChE).

Nerve Gas Mechanism

Inhibits acetylcholinesterase, causing ACh buildup, leading to muscle spasms, paralysis, and respiratory failure.

Enzymatic Degradation

Neurotransmitters broken down by enzymes in the synaptic cleft (e.g., AChE breaking down acetylcholine).

Reuptake into Presynaptic Neuron

Neurotransmitters taken back up into the presynaptic cell via transport proteins (e.g., serotonin reuptake by SERT).

Diffusion Away from the Synaptic Cleft

Neurotransmitters passively diffuse out and are cleared by glial cells (e.g., glutamate taken up by astrocytes).

Muscle Contraction Initiation

Nerve impulse leading to shortening of muscle fibers, involving communication at the neuromuscular junction (NMJ).

Steps of Muscle Contraction

AP to axon terminal, ACh released, Na⁺ enters, depolarization, Ca²⁺ released, binds to troponin, exposes myosin sites, and myosin binds to actin