Transcript Chapter 7 ppt

CHAPTER 8
Membrane Structure and Function
Membrane Structure

Plasma Membrane – selectively permeable
membrane that allows some substances to diffuse
across.
 Amphipathic
– condition where a molecule has
hydrophilic and hydrophobic regions.
 Phospholipids
allow for the hydrophilic head due to a polar
phosphate group and hydrophobic fatty acid tails (nonpolar
hydrocarbon chains).
Membrane Structure (Continued)

Fluid Mosaic Model – lipid bilayer involving the
lateral movement of lipids, proteins, and
carbohydrates (oligosaccharides).
Most lipids and proteins drift laterally within the membrane.
 Molecules rarely flip within the membrane due to negative
interactions of hydrophobic and hydrophilic regions.
 Unsaturated hydrocarbon tails enhance fluidity because of
the non-kinking of single carbon to carbon bonds.
 Cholesterol inhibits fluidity at warm temperatures and limits
packing of lipids at lower temperatures.

Membrane Structure (Continued)

Cell-Cell Recognition – the ability to recognize a
cell that another cell encounters by looking at
extracellular membrane carbohydrates such as:
 Oligosaccharides
 Glycolipids
 Glycoproteins
Transport Across Membranes

Permeability of the lipid bilayer
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
Nonpolar molecules- dissolve in membranes; move across with
ease (hydrocarbons, oxygen, carbon dioxide)
Polar molecules- small polar molecules pass through membrane
lipids with ease (water, ethanol) but large molecules such as
glucose will have difficulty passing
Transport Proteins- integral membrane proteins that
transport specific molecules or ions across biological
membranes

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
May be a channel protein
May physically move molecules using ATP
Are specific to molecules they translocate
Transport Across Membranes
(Continued)

Passive Transport (diffusion) – net movement of a
substance down a concentration gradient (graded
concentration change over a distance) over a biological
membrane


Continues until a dynamic equilibrium is reached
Osmosis – passive transport of water that results in
movement of water due to the total solute concentration.
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Hypertonic Solution – greater solute concentration
Hypotonic Solution – lower solute concentration
Isotonic Solution – equal solute concentration

Water is moving at the same rate in both directions.
Transport Across Membranes
(Continued)

Balance of water uptake and loss

Water balance of cells without walls (animal, protists)



Hypertonic Solution – cell will crenate (shrivel)
Hypotonic Solution – cell will lyse (burst)
Osmoregulation – organisms may pump water out of vacuoles
when present in a hypotonic solution or pump slats out when
present in a hypertonic solution.

Water balance of cells with walls

Hypotonic Solution – cell will become turgid (hard)
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Ideal state for plants
Isotonic Solution – cell will become flaccid (limp)
Hypertonic Solution – cell will plasmolyze (pulling away of the cell wall from
the plasma membrane)
Transport Across Membranes
(Continued)

Facilitated Diffusion – diffusion of solutes across
the membrane with the aid of transport proteins.
 Solute
is transported down its concentration gradient
 Transport proteins are specific for the solutes they
transport having binding sites analogous to an enzymes
active site
 Transport proteins change conformations to deposit
solute on the other side of the membrane
Transport Across Membranes
(Continued)
Active Transport – energy-requiring process during
which a transport protein pumps a molecule across a
membrane against its concentration gradient
 Example
Sodium-Potassium Pump


Protein oscillates between 2 conformations


High affinity for NA+ towards the cytoplasm
High affinity for K+ towards the cell’s exterior
ATP helps power the movement
 Translocates solutes across their gradient

Transportation of Large Molecules
Exocytosis
Endocytosis
Exports macromolecules from a Process of importing
cell by fusion of vesicles with the macromolecules into cell by
plasma membrane
forming vesicles derived from
the plasma membrane
Vesicle usually budded from ER Vesicle forms from a localized
or Golgi then migrates to PM
region of the PM; is pinched off
into the cytoplasm
Used by secretory cells to
Used by cell to incorporate
export products
extracellular substances
Three Types of Endocytosis
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Phagocytosis – cell eating; pinches off food
vacuole that will attach to a lysosyme containing
hydrolytic enzymes
Pinocytosis – cell drinking; droplets are formed
into vesicles and the solute in the droplet is
extracted
Receptor-Mediated Endocytosis – importing of
molecules from another cell