Cell Membranes: Amphipathic Molecules & Fluid Mosaic Model

Amphipathic Molecules

• In a cell membranes, this type of molecules has both hydrophilic and hydrophobic properties.
• Phospholipid molecule
• This molecule contains a phosphate functional group carrying a negative electric charge, making it water-loving or hydrophilic.
• Hydrophilic parts attract polar water molecules.
• Has two fatty acid tails comprised of a CH2 chain, being hydrophobic.
• Hydrophobic parts avoid water and aggregate with similar substances.

Fluid Mosaic Model

• Lipids maintain a membrane's physical integrity.
• It acts as a barrier to the rapid movement of hydrophilic materials, like polar molecules and ions.
• The phospholipid bilayer functions similarly to a lipid sea, where many proteins can float.
• The model describes the components of a membrane move freely.
• Membranes vary, with some having more protein than lipids, some being lipid-rich, some containing cholesterol or other sterols, and some being rich in carbohydrates.
• Carbohydrates attach to the exterior of proteins, forming glycoproteins, or to lipids, forming glycolypids.
• Peripheral membrane proteins attach noncovalently to the membrane surface.
• Membrane proteins consist of multiple polypeptide subunits and some integral membrane proteins span the membrane.
• Some integral membrane proteins are partly embedded in the bilayer
• Anchored membrane proteins are linked to lipids inserted into the membrane.

Membrane Fluidity

• Phospholipids constitute a major part of membranes and arrange into a lipid bilayer.
• Phospholipids can vaiy in fatty acid chain length, degree of saturation, and polar head groups.
• More fluid membranes allow more movement.
• A phospholipid moves from one end of the membrane to the other in one second.
• Movement primarily occurs within the plane of the membrane.
• It's rare for movement between layers.
• Lipid composition influences membrane fluidity.
• Shorter chains results in weaker interaction and result in more fluid membranes.
• Longer chains result in stronger interaction and result in less fluid membranes.
• Cholesterol makes them less fluid.
• Less cholesterol makes them more fluid.
• Fluidity increases with higher temperatures and decreases with lower temperatures.
• In cold weather, plants, bacteria, and hibernating animals adapt with less fluid membranes.
• Lipid composition changes for fluidity with more short chain lipids and more unsaturated lipids containing double bonds for more fluidity.

Membrane Proteins

• The amount of lipids per protein varies.
• Normal membranes contains approximately 25 lipids per protein.
• Mitochondrial membranes contain about 15 lipids per protein.
• Myelin membranes have about 70 lipids per protein.
• 25% of all proteins associate with membranes.
• Integral membrane proteins exist, being partly embedded in the phospholipid bilayer.
• These proteins possess both hydrophilic and hydrophobic regions, or domains.
• Hydrophilic domains feature stretches of amino acids with hydrophilic side chains.
• They exhibit a polar nature allowing them engage and protrude into aqueous area in and out of a cell, or organelle.
• Hydrophobic domains feature of amino acids with hydrophobic side chains.
• They exhibit a nonpolar nature avoiding engagement with-fatty acids inside phospholipid bilayer, distant to water. Peripheral membrane proteins exist but don't embed in the bilayer.
• They interact with the integral membrane proteins' exposed parts and polar heads of phospholipid molecules.
• Anchored membrane proteins attach covalently to fatty acids, with hydrophobic lipid components inserted into the phospholipid bilayer.
• Anchored membrane proteins maintain their association with the membrane.
• Transmembrane proteins span the phospholipid bilayer and protrude on either side
• Transmembrane proteins can have specialized domains on each side of the membrane.
• Membrane proteins are not fixed.
• Membrane proteins can move within the membrane.
• Proteins distribute asymmetrically on the inner and outer membrane surfaces, resulting in distinct surface characteristics.
• The position of a membrane protein can be confined to a specific area.
• An example of a membrane proteins is the one in muscle cell membranes, responsible for recognizing neuron signals.
• Cytoskeletal components can restrict their movement as its components sit just below the inner face of the membrane and are attached to membrane proteins intruding into the cytoplasm.

Membrane Carbohydrates

• The cell membrane contains carbohydrates on its outer surface which serve as recognition sites.
• Glycolipids have carbohydrates covalently linked to a lipid, providing a recognition signal between cells.
• Carbohydrates on certain glycolipids transform when a cell becomes cancerous, making them targets to immune system cells.
• Glycoproteins feature at least one carbohydrate chain linked covalently to a protein, comprised of oligosaccharides.
• These oligosaccharides do not exceed 20 monosaccharide units.
• Glycoproteins function in cell recognition and adhesion.
• Proteoglycans, being heavily glycosylated proteins, possess a higher number of carbohydrate.
• Their carbohydrate chains are usually longer than in glycoproteins and play a part in cellular recognition and adhesion.

Cell Junctions

• These forms as when binding cells provide materials to additional membrane structures that hold them together.
• Tight junctions limit movement of substances through cell spaces.
• Cells that line the bladder have tight junctions prevent leaks into the body cavity.
• Tight junctions maintain distinct faces of a cell within a tissue, reducing movement of membrane proteins across its surface.
• Tight junctions hold particular function within parts of its surface like endocytosis.
• Desmosomes hold neighboring cells firmly, acting as spot welds or rivets and attach to intermediary filaments.
• They promote movement of materials in the extracellular matrix.
• Desmosomes provide structure for tissues subject to physical stress, like skin.
• Gap junctions channels connect membrane pores in adjacent cells, allowing movement of substances and quick current spread, enabling heart muscles to jointly beat.

Membrane Adhesion

• It ensures integrity of tissue by letting a cell attach to extracellular matrix.
• Cells can detach to move and reattach to other cells.
• Integrin transmembrane, epithelial cells to extracellular matrix.
• There is 24+ known human integrins, all attaching to proteins in the extracellular matrix and microfilaments .
• Integrin maintains cell structure through cytoskeleton interaction and its binding to the extracellular matrix is noncovalent and reversible.
• New integrin synthesizes to help a moving cell interact with its new tissue environment.

Homeostasis

• The body's cells share an internal environment formed by extracellular fluids.
• Cells take in nutrients and expel waste into surrounding fluid.
• For cell's survival and function, the environment physical and chemical composition must stay within a tight range.
• Homeostasis maintains the body's condition.
• A constant internal state enables cells to work efficiently, even if external conditions would not usually support them.
• The cells and systems must coordinate to ensure homeostasis.
• Homeostasis refers to cellular environment that involves membranes actions and eukaryotic organelles
• Homeostasis provides constant internal environment.

Selective Permeability

• Biological membranes allow certain substances to pass through them, not all.
• Membranes dictate what enters or exits. There exists 2 types for substance movements:
• Passive transport without bond energy input
• Active transport driven with bond energy

Diffusion and Osmosis

• Concentration gradient is defined as the concentration variation on opposite sides of membrane, thus, energy is derived for passive transport.
• There exists two forms of diffusions:
• Simple diffusion- movement through phospholipid bilayer.
• Facilitated diffusion -requires a channel protein or carrier proteins..
• Diffusion describes random movement to equilibrium where, although all particles individually move randomly, net motion happens directionally until equilibrium.
• Diffusion means NET movement from high to low concentration.
• In a solution , diffusion of solute is independent.
• The rate of diffusion is affected by the molecules sizes and temperature of the soultion.
• Diffusion relies on a number of factors:
• Smaller= faster diffusion.
• Higher temperature = faster molecules or ion energy, that speeds up the rate of transfer with higher temperature.
• High density of solution = decreases the diffusion.
• High concentration gradient = fast diffusion of substance.
• Greater the surface area = rapid diffusion.
• Diffusion functions efficiently as the components automatically are distributed to equilibrium.
• Biological membranes divide different compartments such as cytoplasm and extracellular environment where it will either be permeable or impermeable to solutes.
• Impermeable molecules stay divided.
• Solutes that membrane can pass through then diffuses from compartment until concentration is equally spread as individual solute continue to passively move through the membrane.
• Osmosis drives water flow via membrane, coming from a region with less solutes into a region with more solute.
• In water there exists amount of dissolved solute.
• Driven by water's drive toward higher solute concentrations.

Movement Over Membranes

• Small molecules pass through phospholipids and hydrophobic molecules can move through the lipid area readily to pass through
• Polar / electrically charged molecules are unable to pass easily as charged cannot be hydrophilic, in an environment which is the reverse, leading to a hinderance.

Solution Tonicity

• Tonicity terms refer to the concentration of solute in comparison to a solution, such as isotonic, hypertonic or, hypotonic.
• Isotonic solutions have even concentrations as solutions.
• Hypotonic soultions have a lower concetration in contrast with a second soltuion.
• Hypertonic soultions have a higher concentration in contrast with a soultion.
• Cells would lose water when in a hypertonic soultion in net outside and thus water wants to reach equilibrium by going to this area.
• Water goes to hypertonic areas from hypotonic soultions across a membrane thus.

Carrier vs Channel Proteins

• These integral membrane proteins possess pores spanning the membrane, permitting molecules through: facilitated diffusion.
• Several forms contain ion channels. These ion channels frequently posses a gated channel to allow movement by stimulus from ligand/chemical or, sound, light to change charge over membrane by opening the channel.
• Aquaporins are there for h20 only
• Carrier molecules bind to assist a molecule over: facilitated diffusion, typically for polar compounds.

Active Transport

• It involves AGAINST concentration gradient which requires energy as well as transport being derectional.
• This involves 1 of 3 classes of transporters:
• Uniporter- single substance / single direction
• Simporter- two substances / same direction
• Antiporter - two substances / opposite direction
• As for source, primary directly uses ATP while secondary relies on established gradients.
• Na+/K+ pump exists across all animal cells.
• It is transmembrane and antiporter in transport.
• With a molecule ATP, it splits with 3 Na+ going from inside to net outside and 2 K+ net inside
• Creates high [Na+] outside, and lower [K+] inside cells, creating membrane pontential.

Active Transport Types

• Primary form pump/channel directly uses energy from ATP in gradient formation.
• Secondary transport involves passive process, uses no ATP where a "channel" created gradients.

Endocytosis and Exocytosis

• Large concentration differences over cell membrane is very important.
• Cells that are eukaryotic secrete molecules or intakes large ones for vesicles formation by
• Endocytosis ( cell intake ) and Exocytosis ( secretion out).
• In it exists also several forms of endocytosis like phagocytosis which part of cell membrane envelop large cells and/or parts, common in protists and white blood cells.
• Food vacoules usually end up fusing to lysosome for digestion, where with smaller vesicles it is "cellular drinking". Receptors that transport trigger these uptakes such as pit forms. Also exists "kiss and run" where vesicle opens and closes but is short.