(2) Membrane Structure and Function

COLOR CODE MEANINGS:

  • BLUE TEXT = Overall meaning of something/Need to know

  • PURPLE TEXT = small meanings within that something

  • I don’t define any color text at gated-ion channels cuz it’s too complicated but everything from ligand to light falls under the gated-ion channels category.



    ***THE CELL MEMBRANE***

  • Plasma membrane - semi-permeable outer cell membrane made of a phospholipid bilayer with embedded proteins

    • Transport of molecules into and out of the cell.

    • Facilitate enzymatic activity.

    • Signal transduction between cells.

    • Cell to cell recognition and communication.

    • Intercellular joining.

    • Serving as an attachment point for the cytoskeleton and extracellular matrix.

    • 4 components that make it up: phospholipids, cholesterol, membrane carbohydrates, and membrane proteins


***BILAYER OF PHOSPHOLIPIDS***

  • Phospholipids has two features: Hydrophillic head and Hydrophobic tail

    • Hydrophillic head hydrogen bonds with molecules, interact with charged molecules and polar molecules

  • The bilayer of phosopholipids are oriented towards the aqueous cytoplasm (intracellular side) or the aqueous extracellular space (extracellular side)

  • The fluidity of the membrane is vital to the functioning of the cell.

    • Saturated phospholipids are less fluid

    • Unsaturated phosholipids are more fluid

    • Factors contributing to fluidity:

      • Length and saturation of fatty acids

      • Presence of cholesterol

      • Protein density




*** FLUID MOSAIC MODEL***

  • Fluid Mosaic Model - new model of the plasma membrane. best accounts for the structure and functions of the plasma membrane as we now understand them.

    • Plasma membrane is made up of a bilayer of phospholipids with embedded proteins, carbohydrates, glycolipids, and glycoproteins, and, in animal cells, cholesterol.

    • Proteins:

      • Integral proteins serve as channels or pumps to move materials into or out of the cell.

      • Peripheral proteins on the exterior or interior surfaces of membranes, attached either to integral proteins or to phospholipid molecules.

        • Provide structural support, facilitate intracellular signaling, and carry out enzymatic activities

      • Lipid-anchored proteins proteins that are covalently attached to lipid molecules embedded within biological membranes

        • carry out essential functions such as signal transduction, cell recognition, and enzymatic activity

      • Glycoproteins can be exploited by viruses to gain entry into the cell

        • Viruses densely coat their outer glycoproteins with host-derived sugars, which hides vulnerable protein regions from the host's neutralizing antibodies and immune detection

    • Carbohydrates:

      • found on the exterior surface of cells and are bound either to proteins (forming glycoproteins) or to lipids (forming glycolipids) for cell recognition and communication

        • Glycoproteins: mediate cell-to-cell recognition, cell adhesion, and signaling

        • Glycolipids: act as surface markers for cell-to-cell recognition, help stabilize the cell membrane, and serve as receptors for signaling molecules

    • Cholesterol:

      • keeps it stable by preventing it from becoming either too rigid or too fluid as temperatures change

        • At high temperatures: Cholesterol decreases fluidity

        • At low temperatures: Cholesterol increases fluidity

      • It breaks up hydrophobic interactions between fatty acids.

      • The ratio of cholesterol to saturated versus unsaturated lipids impacts membrane fluidity.


***MEMBRANE TRANSPORT***

  • All movement across a membrane requires a driving force, such as a concentration gradient

    • Cells utilize existing gradients to passively move molecules across

    • Cells can use ATP and specialized proteins to create a concentration gradient during active transport

      • Happens in 3 steps: solute binds to protein pump → hydrolysis of ATP changes its shape → solute molecule moved into area of high concentration


***Passive Transport***

  • Simple Diffusion - movement of molecules from an area of high concentration to an area of low concentration until equilibrium is reached.

    • small non-polar molecules only (carbon dioxide, oxygen, ethanol, steroids, fatty acids, and nitric oxide)

    • influenced by temperature of system, size of particle, & concentration gradient of system

      • increased temp = faster diffusion

      • decreased temp = slower diffusion

  • Osmosis - movement of water molecules across a semi-permeable membrane

    • Aquaporin channel proteins aid this movement

    • Dependent on the surrounding solutes inside or outside the cell

    • flow of water from a region of low solute concentration to a region of high solute concentration
      Osmosis focuses only the movement of water, while tonicity is focused on what (so the solutes) directs the flow of this water

  • Tonicity - ability of an extracellular solution to make water move into or out of a cell by osmosis

    • Isotonic - extracellular fluid has the same concentration of non-permeating solutes as the cell interior

    • Hypotonic - extracellular fluid has a lower concentration of non-permeating solutes than the cell interior

      • Effect: Water rushes into the cell. Animal cells can swell and burst, while plant cells build up turgor pressure

    • Hypertonic - extracellular fluid has a higher concentration of non-permeating solutes than the cell interior.

      • Effect: Water flows out of the cell. Animal cells shrink

You can easily remember tonicity terms if you know that 1. they focus on extracellular solutes, and 2. hypo means low, and 3. hyper means high

  • Facilitated Diffusion - passive movement of molecules across the cell membrane, from an area of high concentration to an area of low concentration, facilitated by specialized transport proteins.

    • Carrier Proteins - Transmembrane proteins which bind to a solute and undergo a change in shape to translocate the solute across the membrane.

    • Channel Proteins - Transmembrane proteins which form a channel in the membrane to allow rapid diffusion of particles.  

      • Gated Ion Channels - allow for the flow of ions to occur through the channel. 

        • Ligand-Gated Ion Channels - use small molecules to trigger the channel to open and allow for ions to pass.

        • Voltage-Gated Ion Channels - open in response to a change in charge across the membrane.

        • Mechanically Gated Ion Channels - open in response to a physical stimulus.

        • Light-Gated Ion Channels - open in response to light wavelengths


***ACTIVE TRANSPORT***

  • Sodium-Potassium Pump - protein that exchanges 3 Na+ ions (moves out of the cell) with two K+ ions (moves into the cell).

    • Three Na+ ions bind to the protein pump → ATP is hydrolyzed, releasing a phosphate group to the protein → protein changes shape and the sodium ion is moved across the membrane → change in the conformation of the protein exposes the binding sites for two K+ ions → phosphate group is released which returns the protein to its original shape → The K+ ions are moved across the membrane, completing the exchange of ions.

  • Protein Pumps

    • proteins that generate a proton gradient (H+) across a cell membrane.  A proton gradient is also known as a pH difference.

  • Secondary Active Transport

    • Creates a concentration gradient using ATP and then coupling the driving force of the gradient to move solutes into a cell

      • 1. “Using ATP to create a concentration gradient”

        • A primary pump (like the Sodium-Potassium pump) uses ATP energy to aggressively push ions (usually Sodium, Na) out of the cell. This builds up a massive crowd of sodium ions outside, creating a high-to-low concentration gradient

      • 2. “The driving force of the gradient”

        • Because there is a massive crowd of sodium outside and very few inside, those sodium ions strongly "want" to diffuse back inside the cell. This structural gradient acts like a charged battery or stored potential energy

      • 3. “Coupling … to move solutes into the cell”

        • A neighboring transport protein (a cotransporter) provides a doorway. It allows a sodium ion to snap back inside the cell ("downhill"). However, the doorway will only open if another solute—like a glucose molecule or amino acid—hitches a ride alongside it. The energy released by sodium moving down its gradient forces the cargo molecule inside, even if that cargo is moving "uphill" from low to high concentration



***BULK TRANSPORT***

  • Endocytosis - active transport of large particles into a cell

    • Phagocytosis - cell takes in large particles


    • Pinocyotsis - small molecules dissolved in extracellular fluid are brought into the cell

    • Receptor-mediated Endocytosis - employs receptor proteins in the plasma membrane that have a specific binding affinity for certain substances

  • Exocytosis - expels material from the cell into the extracellular fluid.