Cell Membrane Composition and Structure

Cell Membranes

• Biological membranes are composed of lipids, proteins, and carbohydrates.
• The fluid mosaic model describes the phospholipid bilayer, where membrane proteins can move laterally.
• Membrane fluidity: decreases when temperatures are lower
• Organisms are able to adjust the lipid composition to remain fluid using saturated vs unsaturated fatty adics

Membrane Composition & Structure

• Carbohydrates attach to proteins, forming glycoproteins, or lipids, forming glycolipids, on the cell's outer surface.
• In animal cells, some membrane proteins connect with filaments in the extracellular matrix.
• Peripheral membrane proteins do not penetrate the bilayer.
• Cholesterol molecules interspersed among phospholipid tails influence the fluidity of fatty acids in the membrane.
• Some membrane proteins interact with the interior cytoskeleton.
• Some integral proteins cross the entire phospholipid bilayer while others only penetrate partially.
• Each phospholipid has a hydrophilic (water-attracting) head and hydrophobic (water-repelling) tails.
• Cholesterol is interspersed between the phospholipid bilayer, contributing to membrane fluidity and stability.
• Integral proteins are transmembrane and involved in transport, signaling, and cell adhesion.
• Peripheral proteins are involved in signaling and maintaining the cell's shape.
• Nonpolar, hydrophobic fatty acid "tails" interact with each other in the bilayer's interior.
• Charged, or polar, hydrophilic "head" portions interact with polar water.
• Peripheral membrane proteins attach to the phospholipid bilayer's surface with ionic bonds.
• Integral membrane proteins are inserted into the phospholipid bilayer.
• An integral protein that crosses the bilayer completely is a transmembrane protein.

Membrane Properties

• The inner and outer surfaces of a membrane have varying properties because of different phospholipid compositions, exposed domains of integral membrane proteins, and peripheral membrane proteins.
• Regions of a plasma membrane can have different membrane proteins based on their function.
• Membranes are dynamic and constantly being formed, transformed, fused, and broken down.
• Membranes also have carbohydrates, usually oligosaccharides, on the outer surface.
• These carbohydrates serve as recognition sites for cells and molecules.
• Glycolipids are carbohydrates + lipid.
• Glycoproteins are carbohydrates + protein.

Cell Adhesion

• Cells in an organism or tissue recognize and bind to each other using membrane proteins that protrude from the cell surface.
• Cell junctions are specialized structures that hold cells together.
• Types of cell junctions: Tight junctions, desmosomes, and gap junctions.
• Tight junctions form a quilted seal, restricting the movement of dissolved materials in the intercellular space, and ensuring a "directed" movement of materials.
• Desmosomes allow cells to strongly adhere to one another, but permit materials to move around them in the intercellular space.
• Gap junctions provide channels for chemical and electrical communication between cells.

Membrane Transport and Permeability

• Membranes have selective permeability.
• Permeable membranes allow some substances to pass through, while impermeable membranes do not.
• Membrane transport is divided into passive and active transport.
• Passive transport does not require outside energy.
• Passive transport uses different types of diffusion.
• The energy for passive transport comes from the concentration gradient.
• The concentration gradient is the difference in concentration between one side of the membrane and the other.
• Active transport requires outside energy in the form of ATP.
• Active transport is necessary for substances that does not naturally occur through diffusion.
• Passive transport across a membrane occurs by simple or facilitated diffusion.
• Simple diffusion goes directly through the phospholipid bilayer.
• Facilitated diffusion goes through a channel protein, or, by means of a carrier protein
• Solutes diffuse across a membrane from a region with a greater solute concentration to a region with less.
• Equilibrium is reached when concentrations are identical on both sides.
• Particles continue to move, but there is no net change in distribution.

Diffusion Rate

• Diffusion rate depends on the size and mass of the solute molecules/ions.
• Diffusion rate is impacted by the lipid solubility/polarity of the solute.
• Diffusion rate is impacted by the charge of the solute.
• Diffusion rate increases depending on the temperature and concentration gradient of solution
• Diffusion rate decreases depending on the density/viscosity of the solution
• Higher density harder for molecules to go across
• With higher concentration gradient there is a higher rate of diffusion
• Phospholipid heads carry a negative charge due to the phophate group

Simple Diffusion

• Small, nonpolar molecules have high permeability
• Small, uncharged polar molecules have high permeability, but less than small, nonpolar
• Large, uncharged polar molecules have low permeability
• Ions have low permeability
• The movement of water (and other solvents) is a special case of diffusion, called osmosis.
• Water diffuses from a region of higher water concentration/lower, to a region of lower water concentration/higher solute concentration.
• Isotonic solution: equal solute concentration (and equal water concentration).
• An example of isotonic is physiological saline at 0.9% NaCl.
• Hypertonic solution: higher solute concentration.
• An example of hypertonic saline is >0.9% NaCl.
• Hypotonic solution: lower solute concentration.
• An example of hypotonic saline is <0.9% NaCl.
• In hypotonic solutions, cells tend to take up water.
• In hypertonic solutions, cells tend to lose water.
• Animal cells must remain isotonic to the environment to prevent destructive loss (crenation) or destructive gain of water (lysis).
• Plants and other cell walls organism prevent cells from bursting under hypotonic conditions.
• Turgor pressure/turgidity develops under these conditions, keeps plants upright, and stretches the cell wall during growth.
• In hypertonic conditions, they lose their water, which results in plasmolysis.

Facilitated Diffusion

• Help to to time better like a water pipe
• Channel proteins have a central pore, lined with polar amino acids, for example: Na+ channel, K+ channel, Cl- channel, or, water channel (aka aquaporin).
• Carrier proteins are membrane proteins that bind some substances and speed their diffusion through the bilayer.
• Carrier proteins: K+ carrier proteins, glucose transporters, Na+/K+ pump.
• The rate of carrier-mediated facilitated diffusion is at maximum when solute concentration saturates the carrier proteins.
• No rate increase is observed with further solute concentration increase.

Active Transport

• Active transport moves substances across a membrane against a concentration gradient.
• It requires “outside" energy.
• This is energy other than the potential energy created by a concentration gradient.
• The energy source is often adenosine triphosphate (ATP).
• Active transport is directional, and it involves three kinds of carrier proteins: Uniports, Symports, and Antiports.
• In primary active transport, energy from the direct hydrolysis of ATP moves ions into or out of cells against their concentration gradients.
• The sodium-potassium (Na+-K+) pump is an example of primary active transport.
• Secondary active transport couples the passive movement of one solute with its concentration gradient (Na+) to the movement of another solute against its concentration gradient (glucose).
• Energy from ATP is used indirectly to establish the concentration gradient resulting in movement of the first solute.

Membrane Transport Mechanisms

• Simple diffusion does not require cellular energy, drives by concentration gradient, does not require membrane protein, and is not specific
• Facilitated diffusion does not require cellular energy, driven by concentration gradient, requires membrane protein, and is specific
• Active transport requires cellular energy, driven by ATP hydrolysis against its concentration gradient, requires membrane protein, and is specific

Endocytosis and Exocytosis

• Endocytosis transports macromolecules, large particles, and small cells into eukaryotic cells by engulfment and by vesicle formation from the plasma membrane.
• Phagocytosis refers molecules or entire cells engulfed, this results in a phagosome.
• Pinocytosis creates vesicle forms that bring small dissolved substances or fluids into a cell.
• Exocytosis secretes materials in vesicles from the cell when the vesicles fuse with the plasma membrane.
• This results in indigestible materials being expelled.
• Exocytosis allows other materials to leave the cell, like digestive enzymes and neurotransmitters.

Functionality and Dynamism

• Membranes function to give recognition and initial processing of extracellular signals and energy transformations.
• Membranes also function to organize chemical reactions.
• Although not all cellular membranes are identical, ordered modifications in membrane composition accompany the conversions of one type of membrane into another type.