The Plasma Membrane

• Plasma Membrane

  • consists of a phospholipid bilayer
  • described using the Fluid-Mosaic Model
  • a selectively permeable membrane, which permits the movement of only certain molecules both in and out of the cell.
  • separates the fluid from outside the cell from the fluid inside the cell.
• The Phospholipid Bilayer

  • Made from 2 layers of phospholipids
  • each phospholipid has a
  • 1 polar, hydrophilic, lipophobic, phosphate-containing head
  • 2 non-polar (hydrogen atoms equally situated around carbons), hydrophobic, lipophilic, fatty acid tails.
  • Sponantaesouly forms in an aqueous environment
  • polar things and nonpolar things repel each other.
  • polar heads face away, fatty acid tails face towards each other.

  A) heads face away

  
• Fluid Mosaic Model


  The molecules inside the plasma membrane are not bound, but rather, are free-flowing, the fatty chains of phospholipids akin to a thick, oily, fluid. This allows it to bend and be flexible, like a fluid. It is a mosaic due to the various proteins and components that make up the phospholipid bilayer.
• Glycoproteins and Glycolipids


  both stabilise the environment and serve to detect what's coming in and out of the cell

  • glycoproteins serve as receptors for chemical signals
  • glycolipids facilitate cellular recognition
• Cholesterol

  • a packing molecule and important in regulating membrane fluidity
  • when the temperature rises, cholesterol pulls the phospholipids together
  • when the temperature drops, cholesterol prevents phospholipids from tightly packing together
• Other molecules in the phospholipid bilayer

  • Glycoproteins
  • proteins with attached carbohydrates,
  • Glycolipids
  • lipids with attached carbohydrates
  • Cholesterol
  • Proteins
  • Integral proteins
  • Peripheral proteins
  • Transmembrane proteins
  • Protein Channels
  • Carrier Proteins

  
• Transport Proteins /transmembrane proteins

  Protein Channels

  • Allows larger molecules, such as glucose, to diffuse through the plasma membrane by creating pores
  • facilitated diffusion
  • does not use ATP, since it allows molecules to be transported down their concentration gradient
  • e.g potassium channels, and aquaporins

  Carrier Proteins

  • Allows molecules to pass the plasma membrane by binding to them on one side, and releasing them on the other
  • Can be either up or down the concentration gradient
  • If it's up the conc gradient, it's active, and requires ATP
  • e.g sodium potassium pump,
• Transport across the membrane

  • Passive (no ATP required)
  • Diffusion
  • Osmosis
  • Active (ATP required)
  • Active transport
  • Bulk transport
• Passive Transport - Diffusion

  • Simple
  • molecules can move directly through the membrane without any assistance
  • small, nonpolar, uncharged or hydrophobic molecules
  • e.g oxygen diffusing in and carbon dioxide diffusing out
  • Facilitated
  • molecules that are too large or too charged to freely cross the plasma membrane can use a membrane protein, such as a protein channel, to move down their concentration gradient into or out of the cell.
  • can be faster than simple diffusion, and is why water - despite being able to diffuse, also has protein channels
  • e.g glucose
• Passive Transport - Osmosis

  • net movement of water from areas of low solute concentration (high water) to areas of high solute concentration (low water), across a partially permeable membrane
  • despite water being able to diffuse through the membrane, it can be aided with aquaporins. this increases the rate of diffusion
  • Tonicity
  • Hypertonic
  • Hypotonic
  • Isotonic
• Active Transport - Active transport

  • the movement of molecules (or ions) from regions of low concentration to regions of high concentration across a plasma membrane (against its concentration gradient)
  • ATP, performed by specific carrier proteins
  • e.g sodium-potassium pumps
• Active Transport - Bulk transport


  cytosis - a form of active transport involving the formation of membrane-bound vesicles or vacuoles

  • Endocytosis
  • Phagocytosis
  • Pinocytosis
  • Exocytosis
  • releases substances from the inside of the cell to the outside of the cell
• Endocytosis

  • the incorporation of substances from outside the cell to inside the cell as a membrane-bound vesicle or vacuole
  • the plasma membrane invaginates (folds in) around the molecules to be transported into the cell
  • Steps
  • Invagination of plasma membrane
  • Vesicle buds off from the plasma membrane
  • 3 Vesicle carries molecules into the cell, contents can then be digested either stored in lysosomes or digested by vacuole.
  • Phagocytosis
  • the engulfment/transport of solid particles through phagocytes
  • Pinocytosis
  • engulfment of liquids

  
• Exocytosis

  • occurs by the fusion of a vesicle membrane with the plasma membrane. the vesicle contents are then released to the outside of the cell
  • Steps
  • Vesicle carrying molecules for export moves to the perimeter of the cell
  • Vesicle fuses with the plasma membrane
  • The contents of the vesicle are expelled outside the cell

  
• Hypertonic Cell

  • When a cell is hypertonic relative to its surroundings, it gains water
  • cell is in a hypotonic solution
  • IN PLANTS
  • become turgid, but does not burst due to the cell wall
  • IN ANIMALS
  • the volume increases until they lyse (explode)
• Hypotonic Cell

  • When a cell is hypotonic relative to its surroundings, it loses water
  • cell is in a hypertonic solution
  • IN PLANTS
  • become plasmolysed, but retain shape due to the cell wall
  • IN ANIMALS
  • the volume decreases and they shrivel
• Isotonicity

  • cell and solution have equal solute concentrations, so there is no net movement of water.
  • It’s important to note that there is still movement of water into and out of a compartment, but the rate of water moving in is equal to the rate of water moving out so the net movement is zero.