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Understanding How the Cell Operates (organelles). Cell membrane. Key molecules Phospholipid bilayer Transport proteins Carbohydrates Cholesterol. Cell Membrane Animation. The membrane. Why is it Fluid? Why is it Mosaic? . Key Function of the Membrane. Helps to maintain homeostasis :

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cell membrane
Cell membrane
  • Key molecules
    • Phospholipidbilayer
    • Transport proteins
    • Carbohydrates
    • Cholesterol

Cell Membrane Animation

the membrane
The membrane
  • Why is it Fluid?
  • Why is it Mosaic?
key function of the membrane
Key Function of the Membrane
  • Helps to maintain homeostasis :
      • Controls substances that enter and leave the cell
      • Movement across the membrane can be
        • A. Passive ( No energy required
        • B Active (ATP required)
passive transport
Passive Transport
  • Moves molecules from high concentration to low concentration

Concentration Gradient (is a membrane required? )

passive diffusion how does it work
Passive Diffusion – How does it work
  • Driven by kinetic energy
  • The motion of molecules is
      • Constant
      • Random
      • Toward equilibrium (will continue to move “outward” unless it strikes something else)
  • Diffusion occurs naturally because of the kinetic energy
what is equilibrium
What is Equilibrium
  • Equal concentration of molecules throughout the medium
  • At equilibrium, do molecules stop moving?
  • Is equilibrium maintained over time?
  • Does diffusion require a membrane?
diffusion across membranes
Diffusion across membranes
    • Diffusion occurs only if the membrane is permeable to that molecule .
  • SIMPLE DIFFUSION: Diffusion across a membrane. Generally refers to solutes (sugar, salt) or gases
    • Size and type of molecule
    • Chemical nature of the membrane
    • Presence of pores in the membrane
  • Molecules that dissolve in lipids can pass directly through the lipid bilayer
      • CO2 and O2
      • Water
  • Small, not soluble in lipid may go through pores
      • Glucose
  • The passive transport of water
    • A solution is a solute in a solvent. Solutes can diffuse
    • It is also possible for solvents (like water) to diffuse


The movement of water from an area of high concentration (of water) to low concentration (of water).

how does osmosis work
  • The net direction ( remember there is always some movement, by chance, both ways) of osmosis depends on the relative concentration of solutes on either side of the membrane.
  • Solutes Low Solutes High
      • Water ---------------------
  • Solutes High Solutes Low
  • Water
direction of osmosis
Direction of Osmosis
  • In living things, consider the concentration of solute in the cytosol as the “reference”
  • When outside solute concentration is LESS than inside concentration, the environment is HYPOTONIC to the cell. WATER GOES INTO CELL
  • When the outside solute concentration is GREATER than the inside concentration the environment is HYPERTONIC to the cell. WATER GOES OUT OF CELL

Before osmosis After osmosis

Look carefully at what is happening which direction hasthe water moved after osmosis has occurred?

direction of osmosis14
Direction of Osmosis
  • When the outside concentration of solute equals the inside concentration of solute, the environment is ISOTONIC to the cell.
  • There IS movement back and forth across the membrane but the NET movement (In – Out) = zero.
  • IMPORTANT: hypo, hyper and iso refer to the RELATIVE SOLUTE concentration
how do cells deal with osmosis
How do cells deal with osmosis
  • When an isotonic external environment doesn’t exist, organisms MUST have mechanisms to “cope”
  • Freshwater example:
      • Contractile vacuole (paramecium) pumps out excess water gathered in the vacuole (active transport)
terms that deal with osmosis
Terms that deal with osmosis
  • Turgor pressure
    • Pressure exerted by water molecules against the cell wall (of plants)
    • Plasmolysis
      • Occurs in a hypertonic environment. Plasma membrane (plants) shrinks away from cell wall loss of turgor pressure  wilting
blood cells and non compensation
Blood Cells and “non-compensation
  • Crenation and Cytolysis

Hypertonic Isotonic Hypotonic

facilitated diffusion
Facilitated Diffusion
  • Movement of molecules across through carrier proteins
    • Occurs for molecules not soluble in lipids or too large to pass through phospholipid membrane
  • No energy required
    • High concentration to low concentration
    • EXAMPLE: Glucose
four steps of facilitated diffusion
Four steps of Facilitated diffusion
  • Glucose binds to a carrier protein (either side)
  • Carrier protein changes shape (no energy)
  • Molecule is released on other side
  • Carrier returns to its original shape.
properties of facilitated diffusion
Properties of Facilitated Diffusion
  • Can move things into OR out of a cell depending on the concentration gradient
  • Carrier proteins are specific for ONE type of molecule (ie glucose, or a specific amino acid)
diffusion through ion channels
Diffusion Through Ion Channels
  • Transport ions from high to low concentration
  • Necessary because ions are not soluble in lipids
    • Na+1
    • K+1
    • Cl-1
    • Ca2+

Each protein is specific for one type of ion

    • In some cells, “open ion channels”
    • Others have gates which respond to:
      • Stretching
      • Electrical signals
      • Chemicals

in the cytosol

or the environment

active transport protein mediated
Active Transport – Protein mediated
  • Moves materials from area of LOW concentration TO HIGH concentration
    • Move “up” the gradient
  • Requires energy
  • Use cell membrane “pumps”
    • ion channel and carrier proteins which require ATP
    • Example: Na-K pump – important in nerve impulse transmission
active transport vesicle mediated
Active Transport – Vesicle mediated
  • Used for movement of large molecules
  • Endocytosis
    • Brings materials IN to the cell
    • Forms a pouch (vesicle)
    • Vesicle can fuse with lysosome (digestion) or other membrane bound organelles
  • Pinocytosis (solutes/fluids)
  • Phagocytosis (large particles/whole cells – used to eat or for defense
active transport vesicle mediated24
Active Transport – Vesicle Mediated
  • Used for movement of large molecules
  • Exocytosis – substance is released from the cell through a vesicle which fuses to the membrane.
  • Used to release large molecules, waste, toxins.
  • Example:
    • Golgi releases proteins in vesicles that are released to the outside of the cell (nervous system and endocrine system release small molecules to control other cells this way.
  • Note use of endomembranesystem