Cell Membrane: Structure and Function

Transition from Cell Membrane

• Focuses on cell membrane communication and second messengers.

Cell Membrane Components

• Composed of phospholipids, proteins, carbohydrates, and cholesterol.
• Phospholipids form a bilayer with polar heads facing outwards and nonpolar tails inwards.
• Proteins are either peripheral or integral, serving various functions like transport and signaling.
• Carbohydrates form a glycocalyx on the cell surface for cell recognition.
• Cholesterol is interspersed within the phospholipid bilayer, contributing to membrane fluidity.

Cell Membrane Functions

• Involved in transport, intercellular joining, enzymatic activity, cell-cell recognition, and signal transduction.
• Transports substances across the membrane.
• Facilitates cell adhesion and communication.
• Catalyzes chemical reactions.
• Recognizes other cells.
• Converts extracellular signals into intracellular responses.

Protein Channels

• Water and water-soluble molecules utilize protein channels to traverse the cell membrane.
• Water channels are also known as aquaporins.
• Molecules that do not dissolve in lipids can pass through protein channels if they are small enough
• Ions like Na, K, Cl, and Ca pass rapidly through protein channels in the plasma membrane.
• Some protein channels have gates that control permeability.

Ion Channels

• Some channels are always open (doorless), while others open and close (gated).
• Membrane permeability to ions can change rapidly because of gated channels.
• Gated channels are voltage-gated (sensitive to voltage changes) or ligand-gated (sensitive to chemical changes).

Membrane Connections

• Gap junctions are for communication.
• Tight junctions are for creating barriers.
• Desmosomes are for adhesion.

Transport from Membrane

• Passive transport relies on kinetic energy, including diffusion and osmosis, without using cellular energy.
• Active transport requires cellular energy, encompassing active transport, endocytosis, exocytosis, and phagocytosis.

Diffusion and Osmosis

• Diffusion moves ions or molecules from high to low concentration.
• Body fluids involve constant motion of water, dissolved substances, molecules, and ions.
• Osmosis involves the passage of water across a selectively permeable membrane from a less dense to a denser area.

Tonicity Concepts

• Isotonic environments indicate the liquid inside and outside the cells has equal osmotic activity.
• Hypertonic solutions = 1% NaCl.
• Isotonic solutions = 0.9% NaCl or 5% Glikoz.
• Hypotonic solutions = 0.3% NaCl.

Active Transport

• Active transport moves ions and molecules from low to high concentration.
• This process needs energy to overcome the concentration difference.

Active Transport Types

• Primary active transport uses ATP directly.
• Secondary active transport uses ATP indirectly.

Endocytosis and Exocytosis

• Endocytosis transports substances from outside to inside of the cell.
• Exocytosis transports substances from inside to outside.
• Large molecules like proteins and polynucleotides use these mechanisms.

Sodium-Potassium Pump

• Hydrolyzes one ATP molecule to expel 3 Na++ out of the cell while bringing 2 K+ into the cell.
• This pump is vital for maintaining cell volume, and without it, cells would swell and burst.
• Sodium ions are expelled, pulling water out to counter osmotic effects.
• An osmotically activated Na-K pump will expel excess ions and water if the cell starts to swell.

Chemical Messengers

• Cells communicate via intercellular chemical messengers

Types of Chemical Messengers

• Paracrines: Local signals, inactivated by local enzymes.
• Autocrines: self-regulating.
• Neurotransmitters: Act locally at synapses.
• Neurohormones.
• Hormones: Secreted by endocrine cells, transported in the bloodstream
• Cytokines: Secreted by cells (often white blood cells), for local and distant signaling.

Chemical Messengers Action

• Released into the Extracellular Fluid (ECF) upon stimulation and act on target cells.
• Differ in origin and how they reach target cells.

Cell Signaling Overview

• Involves reception, transduction, and response.
• Includes a signal-transduction pathway.

Signal-Target Interaction

• Includes three basic cell membrane receptors.
• G-Protein dependent.
• Tirosin-kinase.
• Ion channels.

Chemical Messengers and Receptors

• Chemical messengers attach to protein receptors on the cell membrane's outer surface.
• The combination of messenger and receptor triggers a series of events that control cellular activity.
• This results in the opening or closing of specific ion channels, & signal transfer to a second intracellular messenger.
• The "signal transduction mechanism" refers to this phenomenon.

First Messengers

• The hormone or neurotransmitter is the first messenger and binds to specialized protein receptors on the outer surface of the plasma membrane.
• This triggers membrane transport, secretion, metabolism, and contraction.

G-Protein Dependent Receptors

• They use a receptor called the G-protein.
• The G-Protein is loosely attached to the cytoplasmic side of the membrane and can be active or inactive.

Activation of G-Protein

• A signal molecule binds to the receptor, causing it to change shape and activate the G-protein.
• GDP is replaced by GTP on the protein.
• The active G-protein binds to an enzyme on the membrane, activating it and triggering cellular responses.

Receptor Response

• The binding leads to the opening or closing of specific channels in the membrane.
• Triggers intracellular chemical messenger responses.

Types of Reactions: Ligand-Gated Channels versus G-Protein Coupled Receptor

• This involves neurotransmitters binding to receptors.
• This results in channel opening or uses G-proteins to indirectly open channels with intracellular messengers.

Tyrosine-Kinase Receptors

• Typically used with growth factors.
• They are monomers that become dimers on activation.

Ion Channel Receptors

• Ligands bind to a specific site on the receptors.
• The shape of the channel proteins will change and therefore alter the concentration of a particular ion in the cell.
• They happen in synapses to stimulate electrical impulses.

Intracellular Receptors

• Hydrophobic molecules pass through the membrane and bind to receptors inside the cell.
• Steroids, thyroid hormones, and NO use this method.

Signal Transmission

• Protein phosphorylation is the basic mechanism.

Second Messengers

• Extracellular messengers cannot enter target cells directly.
• They bind to membrane receptors, which then trigger the activation of intracellular proteins.
• Cyclic adenosine monophosphate (cAMP) and Ca2+ are two major pathways for this process.

Cyclic AMP Pathway

• The effector protein (adenylyl cyclase) converts intracellular ATP to cAMP.
• Cyclic AMP Pathway converts intracellular ATP to cAMP
• G-protein acts as an intermediary.
• G proteins bind to guanine nucleotides (GTP or GDP) on the inner surface of the plasma membrane.

Cyclic AMP Pathway Action

• The effector protein is the enzyme adenylyl cyclase, which converts intracellular ATP to cAMP.
• Activation of the cAMP system modifies heart rate, female sex hormone synthesis, breakdown of stored glucose in the liver, and control of water conservation.

Ca2+ As A Second Messenger

• The first messenger binds to a surface receptor and G proteins, which leads to the activation of the enzyme phospholipase C
• This produces diacylglycerol (DAG) and inositol triphosphate (IP3).
• Inositol Triphosphate is responsible for mobilizing intracellular Ca2+ stores to increase cytosolic Ca2+.
• Calmodulin mediates Ca2+-dependent cell events.
• Cellular proteins are either activated or inhibited.

cAMP and CA2+ Pathways

• cAMP and Ca2+ pathways frequently overlap in cell activity.
• cAMP can both influence Ca2+ and vice versa.
• Ca2+-activated calmodulin and cAMP-dependent kinase alter the activity of adenylyl cyclase or carrier channels.
• In certain instances, both Ca2+ and cAMP regulate certain intracellular proteins.