Cell Membrane

• Amphipathic 


  Has both hydrophilic (heads) and hydrophobic (tails) parts
• Selective Permeability

  The ability of the membranes to regulate the substances that enter and exit
• Fluid Mosaic Model

  A model to describe the structure of cell membranes. Fluid: membranes are held together by weak hydrophobic interactions, meaning it can move and shift.
• Cholesterol

  High temp: reduces movement. Low temp: reduces tight packing of phospholipids
• Mosaic

  Comprised of many macromolecules
• Integral proteins

  Proteins embedded into the lipid bilayer.
• Peripheral proteins

  Proteins not embedded into the lipid bilayer. They are loosely bonded to the surface.
• Glycolipids and glycoproteins

  Glycolipids are carbs bonded to lipids. Glycoproteins are carbs bonded to proteins. (they are the most abundant) But both are important for cell-to-cell-recognition.
• Cell wall

  Composed of cellulose. It is thicker than plasma membranes and contains plasmodesmata. (hole-like structures in walls filled with cytosol that connect adjacent cells)
• Movement/flexibility

  When the membrane is flexible, it is able to move materials across the membrane
• What would happen if proteins did not fold correctly?

  Solutes and molecules couldn't cross the membrane because the locations of hydrophobic and hydrophilic regions would be altered.
• Different types of molecules during diffusion

  Large polar molecules: can't interact with hydrophobic tails, they need channel proteins or active transport

  Charged molecules: hydrophobic tails won't allow diffusion

  Gases: small, nonpolar, diatomics allowed

  Small polar molecules: possible, but slow and not efficient (water)

  Hydrophobic molecules: tails allow movement
• Passive transport

  Does not require energy because a solute is moving with its concentration or electrochemical gradient.

  Import of material/export of waste. Ex: diffusion, osmosis, facilitated diffusion
• Diffusion

  Spontaneous process via constant motion of molecules.

  Substances move from high to low concentrations (DOWN/WITH the concentration gradient)

  Directly across the membrane. But different molecules do it at different rates
• Osmosis


  Diffusion of water down its concentration gradient across a selectively permeable membrane.

  Diffusion of water from areas of low solute concentration to high solute concentration
• Facilitated diffusion

  Diffusion of molecules through the membrane via transport proteins.

  Increases rate of diffusion for: Small ions, water, carbohydrates

  2 types: Channel and carrier
• Channel proteins

  Provide a channel for molecules and ions to pass

  Channel is hydrophilic and usually gated (only allow passage when there's a stimulus)

  Ions, and small molecules use these via hydrophilic pores
• Carrier Proteins

  Undergo conformational changes for substances to pass

  Lipid insoluble, hydrophilic molecules uses these
• Active transport

  Transport of a molecule that requires energy (ATP) because it moves a solute AGAINST its concentration gradient.

  Ex: pumps, cotransport, exocytosis, endocytosis
• Pumps

  Maintain membrane potential (unequal concentrations of ions across the membrane results in an electrical charge "electrochemical gradient"

  Cytoplasm is negative, while the extracellular fluid is positive

  Energy is stored in electrochemical gradient
• Examples of pumps

  Electrogenic pump: proteins that generate voltage across membranes, which can be used later as an energy source for cellular processes

  Sodium potassium pump: a way to regulate concentrations of Na and K. 3 Na gets pumped out, and 2 K gets pumped in. +1 net charge in the extracellular fluid

  Proton pump: integral membrane protein that builds up a proton gradient across the membrane. Used by plants, fungi, and bacteria. H+ gets pumped out
• Cotransport

  Coupling of a favorable transport (downhill) of one substance with an unfavorable transport (uphill) of another. This uses energy stored by electrochemical gradients (generated by pumps) to move substances against concentration gradient

  Plants use this for sugars and amino acids. Ex: sucrose can only travel into a plant against its concentration gradient ONLY if it is with a H that is diffusing WITH/DOWN its electrochemical gradient
• Exocytosis (out of cell)


  Secretion of molecules via vesicles that fuse to the plasma membrane by forming a bilayer. After fusion, the contents are released into the extracellular fluid. Ex: nerve cells releasing neurotransmitters
• Endocytosis (into the cell) 


  The uptake of molecules from vesicles fused from the plasma membrane

  3 types: phagocytosis, pinocytosis, receptor-mediated
• Phagocytosis

  When a cell engulfs particles to be later digested by lysosomes. Cell surrounds particle with pseudopodia, packages particles into a food vacuole, and then the vacuole fuses with a lysosome to get digested
• Pinocytosis

  Nonspecific uptake of extracellular fluid containing dissolved molecules. Cell takes in dissolved molecules in a protein-coated vesicle, with the protein coat helping mediate the transport of molecules
• Receptor-mediated endocytosis


  Specific uptake of molecules via solute binding to receptors on the plasma membrane. Allows the cell to gather large quantities of a specific substance. When solutes bind to receptors, they cluster in a coated vesicle to be taken into the cell
• Tonicity

  The ability of an extracellular solution to cause a cell to gain or lose water. Depends on the concentration of solutes that cannot pass through the cell membrane.

  3 types: Isotonic, hypertonic, hypotonic
• Osmoregulation

  Cells must be able to regulate their solute concentrations and maintain water balance. Animal cells will react differently than cells with plant walls
• Isotonic solutions

  No net movement of water. The concentration of nonpenetrating solutes inside and outside of the cell are equal. Water diffuses in and out of cell at same rate
• Hypertonic solutions


  Lose water to their extracellular surroundings. Concentration of nonpenetrating solutes is higher outside of cell. Water will move to the extracellular fluid. cells shrivel and die.

  In plant cells, plasmolysis may occur: vacuole shrinks and the plasma membrane pulls from the cell wall
• Hypotonic solutions


  Gain water. The concentration of nonpenetrating solutes is lower outside of cell. Animal cells will swell and lyse. Plant cells will work optimally
• Water potential

  Physical property that predicts the direction water will flow. Includes the effects of solute concentration and physical pressure. Water will flow: high water potential to low water potential. Low solute to high solute concentration, high pressure to low pressure.
• Why are membrane carbohydrates important?

  They help in cell-cell recognition. Ex: When the immune system rejects foreign bodies. OR the sorting of different cells into tissues of organs in an animal embryo
• Glycolipid

  Maintaining cell membrane stability/aid in cellular recognition
• Glycoprotein

  Involved with the immune system and cellular interactions
• Integral protein

  Channeling/transporting molecules across the membrane
• Peripheral protein

  Provide support, help in communication, and molecular transportation
• Diffusion

  When particles move to occupy all of the available space. They move randomly, but populations can be directional.
• Concentration gradient

  The area in which the density of chemical substances changes