5.1 Membrane Models

    A. Early Observations

        1. At turn of the century, researchers noted lipid-soluble molecules entered cells more rapidly than
            water-soluble molecules, suggesting lipids are component of plasma membrane.
        2. Later chemical analysis revealed the membrane contained phospholipids.
        3. Gorter and Grendel (1925) found amount of phospholipid extracted from a red blood cell
            was just enough to form one bilayer; suggested nonpolar tails directed inward, polar heads outward.
        4. To account for permeability of membrane to non-lipid substances, Danielli and Davson proposed
            sandwich model (later proved wrong) with phospholipid bilayer between layers of protein.
        5. With the electron microscope available, Robertson proposed proteins were embedded in outer
            membrane and all membranes in cells had similar compositions—the unit membrane model.

    B. In 1972, Singer and Nicolson introduced the currently accepted fluid-mosaic model of membrane structure.

        1. Plasma membrane is phospholipid bilayer in which protein molecules are partially or wholly embedded.
        2. Embedded proteins are scattered throughout membrane in irregular pattern; varies among membranes.
        3. Electron micrographs of freeze-fractured membrane supports fluid-mosaic model.

5.2 Plasma Membrane Structure and Function

    A. Fluid-mosaic Model

        1. Membrane structure has two components, lipids and proteins.
        2. Lipids are arranged into a bilayer
            a. Most plasma membrane lipids are phospholipids, which spontaneously arrange themselves into a bilayer.
            b. Nonpolar tails are hydrophobic and directed inward; polar heads are hydrophilic and are
                directed outward to face extracellular and intracellular fluids.
            c. Glycolipids have a structure similar to phospholipids except the hydrophilic head is a
                variety of sugar; they are protective and assist in various functions.
            d. Cholesterol is a lipid found in animal plasma membranes; reduces the permeability of membrane.
            e. Glycoproteins have an attached carbohydrate chain of sugar that projects externally.
            f. The plasma membrane is asymmetrical; glycolipids and proteins occur only on outside and
                cytoskeletal filaments attach to proteins only on the inside surface.

    B. Fluidity of the Plasma Membrane

        1. At body temperature, the phospholipid bilayer has consistency of olive oil.
        2. The greater the concentration of unsaturated fatty acid residues, the more fluid the bilayer.
        3. In each monolayer, the hydrocarbon tails wiggle, and entire phospholipid molecules can
            move sideways at a rate of about 2 µm—the length of a prokaryotic cell—per second.
        4. Phospholipid molecules rarely flip-flop from one layer to the other.
        5. Fluidity of the phospholipid bilayer allows cells to be pliable.
        6. Some proteins are held in place by cytoskeletal filaments; most drift in fluid bilayer.

    C. The Membrane Is a Mosaic

        1. Plasma membrane and organelle membranes have unique proteins; RBC plasma membrane
            contains 50+ types of proteins.
        2. Membrane proteins determine most of the membrane’s functions.
        3. Channel proteins allow a particular molecule to cross membrane freely (e.g., Cl- channels).
        4. Carrier proteins selectively interact with a specific molecule so it can cross the plasma
            membrane (e.g., Na+ - K+ pump, sodium potassium pump).
        5. Receptor proteins are shaped so a specific molecule (e.g., hormone or other molecule)
            can bind to it.
        6. Enzymatic proteins catalyze specific metabolic reactions.

    D. Cell-Cell Recognition

        1. Carbohydrate chains of glycolipids and glycoproteins identify cell; diversity of the
            chains is enormous.
            a. Chains vary by number of sugars (from 15 to several hundred).
            b. Chains vary in branching.
            c. Sequence of sugars in chains varies.
        2. Glycolipids and glycoproteins vary from species to species, from individual to individual of
            same species, and even from cell to cell in same individual.
        3. In development, different type cells in embryo develop their own carbohydrate chains; these
            chains allow tissues and cells of the embryo to sort themselves out.
        4. Immune system rejection of transplanted tissues is due to recognition of unique glycolipids and
            glycoproteins; blood types are due to unique glycoproteins on the membranes of red blood cells (RBC).

5.3 Permeability of the Plasma Membrane

    A. Types of Membranes and Transport

        1. The plasma membrane is differentially permeable; only certain molecules can pass
            through freely.
        2. A permeable membrane allows all molecules to pass through; an impermeable membrane
            allows no molecules to pass through; a semipermeable membrane allows some molecules
            to pass through.
            a. Small non-charged lipid molecules (alcohol, oxygen) pass through the membrane freely.
            b. Small polar molecules (carbon dioxide, water) easily pass following their concentration gradient.
            c. Macromolecules cannot freely cross a plasma membrane.
            d. Ions and charged molecules have difficulty crossing the hydrophobic phase of the bilayer.
        3. Both passive and active mechanisms move molecules across membrane.
            a. Passive transport moves molecules across membrane without expenditure of energy
                by cell; includes diffusion and facilitated transport.
            b. Active transport uses energy (ATP) to move molecules across a plasma membrane; includes
             active transport, exocytosis, endocytosis, and pinocytosis.

    B. Diffusion and Osmosis

        1. In diffusion, molecules move from higher to lower concentration (i.e., down their
            concentration gradient).
            a. A solution contains a solute, usually a solid, and a solvent, usually a liquid.
            b. In the case of a dye diffusing in water, dye is a solute and water is the solvent.
        2. Membrane chemical and physical properties allow only a few types of molecules to
            cross by diffusion.
            a. Lipid-soluble molecules (e.g., alcohols) diffuse; lipids are membrane’s main
                structural components.
            b. Gases readily diffuse through lipid bilayer. Movement of oxygen from air sacs (alveoli)
                to blood in lung capillaries depends on concentration of oxygen in alveoli.
        3. Osmosis is the diffusion of water across a differentially permeable membrane.
            a. Osmotic pressure is hydrostatic pressure, on side of membrane with higher solute
                concentration, produced by water diffusing to that side of membrane; thistle tube example:
                1) A differentially permeable membrane separates two solutions.
                2) Beaker has more water (lower percentage of solute) and thistle tube has less water.
                3) The membrane does not permit passage of the solute.
                4) Membrane permits passage of water with net movement of water from beaker to
                    inside of tube.
                5) Osmotic pressure allows liquid increase on side of membrane with greater
                    percent of solute.
            b. Osmotic pressure is pressure that develops in a system due to osmosis.
            c. Osmosis is constant process in life: for example, water is absorbed in large intestine,
                retained by kidneys, and taken up by blood.
        4. Tonicity is strength of a solution in relationship to osmosis; determines movement of water
            into or out of cells.
            a. Isotonic is where the relative solute concentration of two solutions are equal.
            b. Hypotonic is where a relative solute concentration of one solution is less than another solution.
            c. Hypertonic is where relative solute concentration of one solution is greater than another solution.
            d. Swelling of plant cell in hypotonic solution creates turgor pressure; how plants maintain erect position.
            e. Solutions that cause cells to shrink are hypertonic solutions; red blood cells placed in salt
                solution above 0.9% shrink and wrinkle, a condition called crenation.

    C. Transport by Carrier Proteins

        1. Plasma membrane impedes passage of most substances but many molecules enter or leave at rapid rates.
        2. Carrier proteins are membrane proteins that combine with and transport only one type of
            molecule; are believed to undergo a change in shape to move molecule across in active and
            facilitated transport.
        3. Facilitated transport is passive transport of specific solutes down their concentration gradient,
            facilitated by a carrier protein; glucose and amino acids move although not lipid-soluble.
        4. Active transport is transport of specific solutes across plasma membranes against the
           concentration gradient through use of cellular energy (ATP).
            a. Iodine is concentrated in cells of thyroid gland, glucose is completely absorbed into lining
                of digestive tract, and sodium is mostly reabsorbed by kidney tubule lining.
            b. Active transport requires ATP and have high number of mitochondria near membranes.
            c. Proteins involved in active transport are often called "pumps"; the sodium-potassium pump
                is an important carrier system in nerve and muscle cells.
            d. Salt (NaCl) crosses a plasma membrane because sodium ions are pumped across and the
                chloride ion is attracted to the sodium ion and simply diffuses across.
        5. Membrane-Assisted Transport
            a. In exocytosis, a vesicle often formed by Golgi apparatus fuses with the plasma membrane as
                secretion occurs; method by which insulin leaves insulin-secreting cells.
            b. During endocytosis, cells take in substances by vesicle formation as plasma membrane pinches off.
            c. In phagocytosis, cells engulf large particles forming an endocytic vesicle
                1. Phagocytosis is commonly performed by ameboid-type cells (e.g., amoebas and macrophages).
                2. When the endocytic vesicle fuses with a lysosome, digestion occurs.
            d. Pinocytosis occurs when vesicles form around a liquid or very small particles.
            e. Receptor-mediated endocytosis occurs when specific macromolecules bind to plasma
                membrane receptors.
                1) This allows cells to receive specific molecules and then sort them within the cell.
                2) A macromolecule that binds to receptor is called a ligand; binding of ligands to receptors
                    causes receptors to gather at one location.
                3) This location is a coated pit with a layer of fibrous protein called clathrin, on cytoplasmic side.
                4) Pits appear associated with exchange of substances between cells (e.g., maternal and fetal blood).
                5) When cholesterol enters a cell, membrane and receptors are returned to the plasma membrane.

5.4 Modification of Cell Surfaces

    A. Plasma Membrane

        1. The plasma membrane is outer living boundary of a cell.
        2. Many cells have an extracellular component formed outside of membrane; plant, fungi, algae, and
            bacteria form cell walls; animal cells have an extracellular matrix.

    B. Plant Cell Walls

        1. Plant cells are surrounded by a porous cell wall that varies in thickness, depending on function of cell.
        2. Plant cells have primary cell wall composed of cellulose polymers united into threadlike microfibrils
            that for fibrils.
        3. Cellulose fibrils form a framework whose spaces are filled by non-cellulose molecules:
            a. Pectins allow cell wall to stretch; are abundant in the middle lamella that holds cells together.
            b. Non-cellulose polysaccharides harden the wall of mature cells.
        4. Lignin adds strength and is a common ingredient of secondary cell walls in woody plants.
        5. Plasmodesmata are narrow channels that pass through cell walls of neighboring cells and connect their
            cytoplasms, allowing direct exchange of molecules and ions between neighboring plant cells.

    C. Extracellular Matrix of Animal Cells

        1. Extracellular matrix is meshwork of insoluble proteins with carbohydrate chains that are produced and
            secreted by animal cells; fills spaces between animal cells.
        2. This matrix most likely influences the development, migration, shape and function of cells.
        3. Collagen gives the matrix strength and elastin gives it resilience.
        4. Fibronectins and laminins bind to membrane receptors; permit communication between matrix and cytoplasm.
        5. Fibronectins and laminins form pathways that direct the migration of cells during development.
        6. Proteoglycans are glycoproteins that provide a packing gel that joins the various proteins in matrix and most
            likely regulate signaling proteins that bind to receptors in the plasma protein.

    D. Junctions Between Cells

        1. Cell junctions are points of contact that physically link neighboring cells or provide
            functional links; animal cells have three types: adhesion junctions, tight junctions,
            and gap junctions.
        2. In adhesion junctions (desmosomes), internal cytoplasmic plaques, firmly attached
            to cytoskeleton within each cell are joined by intercellular filaments; hold cells together
            where tissues stretch (e.g. in heart, stomach, bladder).
        3. In tight junctions, plasma membrane proteins attach to each other, producing zipper-like
            fastenings; hold cells together so tightly that tissues (e.g., epithelial lining of stomach,
            kidney tubules) are barriers.
        4. A gap junction allows cells to communicate.
            a. They are formed by the joining of two identical plasma membrane channels; channel of
                each cell is lined by six plasma proteins.
            b. They provide strength to the cells involved and allow the movement of small molecules
                and ions from the cytoplasm of one cell to the cytoplasm of the other cell.
            c. Gap junctions are important to function of heart muscle and smooth muscle because they
                permit diffusion of ions required for cells to contract.