Cell Membrane: Structure, Function, and Permeability

• Cell membranes are critical for cells to function as they define cell boundaries
• A cell membrane, while biological, can be compared to something non-biological
• It helps to understand the function better

Selective Permeability

• Cell membranes must be selectively permeable
• Only selectively permeable membranes can facilitate key functions
• Processes include osmosis, water balance, maintaining membrane potential, waste removal, hormone and signal transmission, and fulfilling energy requirements

Membrane Composition

• Human bodies are composed of over a hundred trillion cells
• Cells are building blocks of life that can also thought of as microscopic human beings
• These have miniature brains (DNA) and energy sources like mitochondria
• Cells communicate and produce proteins and hormones, acting as small factories

Cell Membrane Structure

• The cell membrane is a thin elastic sheet 7.5-10 nanometers thick, which is essentially a lipid (fat) material
• It contains primarily phospholipids and cholesterol, and interspersed proteins
• 1 nanometer = 1x10-9 m = one billionth of a meter
• The cell membrane surrounds all living cells
• It is the cell's most important organelle because it surrounds other delicate organelles
• Membranes surrounding the nucleus and other organelles are nearly identical to the cell membrane
• Phospholipids form a thin, flexible sheet
• Proteins "float" in the phospholipid sheet like icebergs
• Carbohydrates extend out from the proteins

Cell Membrane Components

• The cell membrane consists of cholesterol, phospholipids/sphingolipids, carbohydrates, and proteins

Lipid Double Layer

• Plasma and organelle membranes are composed of 40-80% lipid
• Hydrophobic lipids are water-insoluble/fat-soluble with "fatty acid tails" on the inside
• Hydrophilic lipids are polar, water-soluble/fat-insoluble with "heads" on the outside
• Phospholipids are the most abundant lipid in cell membranes
• Cholesterol influences membrane fluidity

Membrane Proteins

• Membrane proteins float in a "sea" of membrane lipids
• Protein count depends on membrane function, with protein:lipid ratios ranging from 1:4 to 4:1
• Protein components are asymmetrically situated in the membrane
• Classified according to structure or function, membrane proteins are critical for most membrane behavior

Membrane Protein Function

• Can function in different ways
• Membrane transporters move molecules
• Structural proteins provide stability
• Membrane enzymes act on substrates
• Membrane receptors bind ligands

Glycocalyx

• Glycocalyx is an outer layer or "coat" of glycoproteins embedded in the cell membrane
• It protrudes and covers the outer surface
• It features carbohydrate ("sugar") side chains of glycoproteins, providing a binding surface for components of the extracellular matrix (ECM)
• The glycocalyx gives the cell membrane a negative charge
• It maintains cell-to-cell contact

Membrane Dynamics Depend On

• Membrane properties and structures like solubility, membrane proteins, channels, and receptors
• Properties of transported molecules and substances such as their size, electrical charge, and solubility
• Availability of energy (ATP, kinetic)

Transport is Driven By Requirements

• Energy (passive or active)
• Physical/chemical properties

Diffusion

• Diffusion includes simple and facilitated versions

Simple Diffusion

• Solubility affects transport across the cell membrane, which is passive
• Nonpolar, fat-soluble substances dissolve across the membrane
• Polar, water-soluble substances require membrane proteins for transport in simple diffusion

Fick's Law

• Fick’s Law defines simple diffusion
• The movement of gases across a membrane is an example including O2, CO2, and NO and are lipophilic

Facilitated Diffusion

• It doesn't use ATP energy
• Movement is driven by a concentration gradient
• Substances cannot dissolve in the cell membrane alone
• Needs membrane proteins

Voltage-Gated Ion Channels

• Protein channels can act as voltage-gated
• These channels respond to electrical events
• A chemical signal also can open a channel, and these are called ligand gated
• In ligand-gated, a chemical binds

Primary Active Transport

• Chemical energy (ATP hydrolysis) drives transport against concentration gradients

Primary Active Transport

• An example of this is the Na+/K+ pump, an enzyme (Na+/K+-ATPase) and a carrier protein
• Hydrolysis of 1 ATP molecule pumps 3 Na+ ions out and 2 K+ ions into the cell

ATP Hydrolysis

• ATP + ATPase = ADP + Pi
• The phosphate group can react with a protein, called phosphorylation, modifying the protein's function

Sodium Potassium Pump

• The Nat- K+- ATPase pump is a carrier protein actively pumps Nat out of cells and K+ into cells
• Does this against their concentration gradients for primary active membrane transport

Co-Transport

• The process uses a Na+-Glucose Co-Transporter
• Glucose crosses the intestinal/renal epithelium against its gradient
• Na+-glucose co-transporter protein transports glucose

Endocytosis and Exocytosis

• Endo = into the cell
• Exo = out of the cell

Receptor-Intracellular Signal Transduction Cascade

• Water-soluble (protein) ligands usually bind to cell membrane receptors
• Fat-soluble ligands typically bind to intracellular
• All cell membrane receptors can be grouped into different classes

Lock and Key Model

• Hormone binds a receptor
• This activates a process that leads to an effect on the cell

4 Membrane Receptor Categories

• Channel
• Enzyme
• G-Protein-coupled receptor
• Integrin receptor

G-Protein Coupled Receptors

• 1st Class of membrane receptors
• G protein-coupled receptors (GPCRs) are proteins that span the membrane
• Ligand bonding opens ion channel or alters enzyme activity
• Common type in the body

GPCR Mechanism

• Ligand bond to G-protein coupled receptor (“GPCR”)
• Ligand-receptor complex which prompts G-protein to trigger an amplifier which then triggers a second signal

Intracellular Molecules Include

• Signal Transducers (G-proteins)
• Amplifying Enzymes (ΑΕ)
• Second Messengers

Signal Transduction

• Transduction defined as changing signal into different form
• Used at the cell membrane to cause effect

Receptor-Enzyme Receptors

• 2nd Membrane receptor class
• Ligand bonds causing an intracellular enzyme to active.

Receptor-Enzyme Class Mechanism

• Receptor connected with an enzyme on inner membrane, forming “RECEPTOR-ENZYME” complex
• With receptor-tyrosine kinase receptor (TK). The complex leads to activation of proteins by phosphorylating proteins
• Insulin-receptor (found on muscle, liver and fat cells)

Ligand-Gated Ion Channels

• 3rd membrane receptor class
• The channel acts as a receptor

Ligand-Gated Mechanism

• An example of is nicotinic acetylcholine receptor (nAChR) in neuromuscular junction
• Receptor binds, causing a opening of gate of ion channel
• Causing a polarization of membrane.

Fat Soluble Hormone Functions

• Leading to the synthesis of new proteins
• Blocking the synthesis of existing proteins

The Na/K Pump Restores Ion balance

Resting Potential

• Resting Membrane Potential
• The negative or positive state due to imbalance

Action Potential

• Action Potential is driven by a shift from negative to positive, creating a charge
• A stimulus disturbs membrane, ion channels disturb which shifts and creates action potential