Study Guide MT 1

Plasma Membrane

Plasma Membrane

The phospholipid bilayer plus associated proteins and molecules (transmembrane proteins) that confer selective permeability to ions, glucose, and other molecules.

Nucleus

The cellular structure that hosts the genome and is the site of mRNA transcription.

Ribosomes

Sites of protein synthesis (translation) found on the endoplasmic reticulum or free in the cytoplasm.

ER/Golgi Complex

A vesicle-based system (budding and fusion) that sorts new proteins to various destinations within or outside the cell such as the PM, the out side of the cell (exocytosis), or lysosomes. The cytoplasm and other organelles get their proteins from free ribosomes.

Mitochondria

Organelles that produce ATP from glucose or fatty acids, essential for energy metabolism through oxidative phosphorylation.

Lysosomes

Organelles that digest cellular debris by fusing with intracellular vesicles derived from endocytosis.

Peroxisomes

Organelles that detoxify free radicals.

Cytoplasm

Consists of the semi-liquid cytosol, organelles, cytokeleton, and where intermediate metabolism occurs.

Microtubules

Dynamic polymers of tubulin that form highways for movement of
transport vesicles via kinesin and dynein motor proteins, and cilia and flagella for generating
movements.

Microfilaments

Dynamic polymers of actin that work with myosin (motor protein) to produce cellular contraction.

Intermediate Filaments

Longer proteins produced by various genes that provide structural support.

Differential Gene Expression

The process by which different cell types express unique subsets of genes despite containing the same DNA.

Levels of Organization

The basic definitions of cell, tissue, organ, organ system, and organism.

cell: the basic unit of life enclosed by a membrane that can obtain fuel, exchange materials, intracellular transport, metabolize, synthesize proteins, and reproduce

tissue: a group of cells that possess a similar structure and perform a specific function including 4 types — muscle (contraction), nervous (initiate/transmitting electrical impulses), epithelial (exchange across barriers and to secrete substances), connective (support).

organ: body structure that consists of different tissue types

organ system: collection of organs that preform related functions

organism: the highest level of biological complexity, where all the different parts of a living being, including cells, tissues, organs, and organ systems, work together to form a complete, independent individual capable of carrying out life functions.

Homeostasis

The tendency of a system to maintain internal stability by having a coordinated response to any situation or stimulus that would disturb its normal condition or function.

Factors that must be maintained — conc. of nutrients, conc, of O2 and CO2, conc, of waste products, conc, of water and electrolytes, pH, temperature, volume/pressure, defense against foreign invaders.

Intrinsic — local control systems “built in” to an organ or tissue.

Extrinsic — external control system outside of an organ permitting coordination regulation of several organs.

Negative Feedback — the control action decreases the effect of any disturbance (the response opposes the change).

Sensor: mechanism to detect the controlled variable.

Integrator: compares the sensor’s input with the set point.

Set Point: The desired range of the controlled variable.

Effector: Adjusts the value of the controlled variable.

Extracellular Communication

The three main types of signaling (hormonal, paracrine, synaptic) that differ in spatial range.

Hormonal: Hormones travel through the circulatory system to reach their distant target cells.

Paracrine: Cell secretes a signaling molecule into the extracellular fluid that affects nearby cells. The signaling molecule, called a paracrine factor, diffuses over a short distance.

Synaptic: A chemical signal travels between nerve cells at a synapse in response to electrical signal. The presynaptic cell releases neurotransmitters into the synaptic cleft, which are then transported across the synapse to bind to receptors on the postsynaptic cell.

Receptor Types

Basic features of nuclear receptors, GPCRs, enzyme-linked receptors, and ionotropic receptors.

Nuclear Receptor: (intracellular) that activates gene expression.

GPCRs: (cell surface) G protein coupled receptor that detect extracellular signals like hormones, neurotransmitters, and light, and then transmit this information inside the cell by activating intracellular signaling pathways through the interaction with G proteins; essentially acting as a communication channel between the cell's external environment and its internal functions.

Enzyme: (cell surface) linked receptors.

Ionotropic Receptors: (cell surface) ion channels.

Neurons

Specialized cells for chemical and electrical signaling in the brain that are connected by synapses and they release chemical neurotransmitters that are detected by other neurons which helps the brain process sensory signals and produce motor output.

Ion Movement

The basis of electrical signaling in neurons, involving the movement of ions across the plasma membrane.

Transmembrane Proteins

Two types for ion/molecule movement: carriers and channels.

Carriers: mediated transport that escorts molecules across the membrane that needs assistance.

Channels: permeable to specific ions such as Na+ or K+

Carriers

Proteins that have binding site for the molecule to be transported: (1) Facilitated diffusion uses a fixed affinity site and transports down the concentration gradient. (2) Pumps have variable affinity sites and transport uphill, against the concentration gradient.

Na/K ATPase

A pump that maintains concentration gradients by transporting 3 sodium out and 2 potassium in.

Ion Channels

Proteins with pores that allow ions to permeate the membrane without binding sites.

Driving Forces

chemical driving, which is diffusion down a concentration gradient, and the electrical driving force which results from electrostatic interactions at a distance. At any moment, each driving force can be represented as a vector which has direction and magnitude. At any moment, ions experience a net driving force which is the vector sum.

Membrane Potential

The charge separation across a neuron's membrane, typically around -70 mV at rest (excess of negative charges on the inside and excess of positive charges on the outside). Polarity is referenced from inside relative to outside. The amount of charge separation underlying biologically meaningful electrical signaling is extremely small compared to the total number of ions in bulk solution on both sides of the membrane. Therefore, Na and K concentration gradients do not run down during normal physiological operation.

Equilibrium Potential

The membrane potential at which there is no net charge movement for a specific ion, calculated using the Nernst equation.

Resting Membrane Potential (RMP)

The potential of a cell at rest, influenced by the permeability of various ions. At rest, K permeability dominates as there are more K leak channels than Na leak channels. It is calculated using the GHK equation.

Graded Potentials

Changes in membrane potential that decrease in size as they move away from the source and caused by passive dissapation.

Action Potentials (APs) (Spikes)

All-or-none electrical signals that are initiated at the axon hillock and propagate along axons to trigger neurotransmitter release.

Voltage-Gated Channels

Channels that open in response to changes in membrane potential, crucial for AP generation. These channels produce voltage-dependent, time variant changes in membrane permeability to Na+ and K+. Net driving force on Na is strong at AP onset but weak at AP peak. The opposite is true for K.

Refractory Periods

The absolute and relative periods during which a neuron cannot or can fire another AP, respectively.

Absolute: It begins when all Na channels have opened (occurs just after threshold is reached) and ends when Na inactivation is removed.

Relative: It begins when Na inactivation is removed and ends when the resting potential is restored following K channel deactivation.

AP Propagation

The speed and reliability of action potential transmission influenced by axonal diameter, membrane resistance and myelination (internal resistance and the presence or absence of myelin).

Contiguous Conduction

AP propagation relying on a continuous distribution of v-gated Na and v-gated K
channels along the length of the axonal membrane. This is an active process in the sense that it is not self-limited in time and space.

Saltatory Conduction

AP propagation that relies on myelin (insulator) and clusters of v-gated Na and v-gated K
channels found at the Nodes of Ranvier. This is an active process at the sites of initiation
(axon hillock) and nodes of Ranvier, and a passive process (graded potential) underneath
the myelinated stretches of axon.

Membrane Resistance

The resistance of the membrane that affects how far current can flow down an axon before leaking out

In giant axons, internal resistance low which favors AP propagation. In narrow axons like those in our brains, internal resistance is high which favors leak out and poor propagation. Myelin increases membrane resistance such that the axial path is now the lower resistance path.

Myelin

An insulator that increases membrane resistance and decreases capacitance, speeding up AP propagation.

Na Channel Inactivation

A mechanism that ensures unidirectional AP spread and the annihilation of APs experimentally induced at either end of an axon when they collide.

Demyelinating Diseases

result in slow and unreliable AP propagation. The autoimmune disease multiple sclerosis commonly affects the cerebellum, a brain structure which plays an important role in calibrating ongoing movements. The symptoms = ‘action tremors’.