Download Cell Membranes

CELL MEMBRANE
STRUCTURE AND FUNCTION
FLUID MOSAICS OF LIPIDS AND PROTEINS
COMPOSITION

Phospholipids
 Give
membrane hydrophilic and hydrophobic
properties
 Phosphate
– hydrophilic, face aqueous exterior or interior
 Fatty acid – hydrophobic, face each other inside the
membrane
Proteins
 Cholesterol, Glycolipids, and Glycoproteins

COMPOSITION
Phospholipids
 Proteins: many different roles

 Hydrophilic
 Have
charged and polar side groups
 Hydrophobic
 Have

nonpolar side groups
Cholesterol, Glycolipids, and Glycoproteins
COMPOSITION
Phospholipids
 Proteins
 Cholesterol, Glycolipids, and Glycoproteins

 Cholesterol:
resists changes in membrane fluidity
caused by changes in temperature
 Glycolipids & Glycoproteins: have a carbohydrate
attached; cell-cell recognition
SELECTIVE PERMEABILITY
THE PURPOSE: TO SEPARATE THE INTERNAL
FROM THE EXTERNAL ENVIRONMENT

Can Pass Through:
 Small,
Uncharged polar
 Small Nonpolar

Can NOT Pass Through (without assistance):
 Hydrophilic
 Large
 Polar
 Ions
 Water
substances
Note: Plant Cell Walls
are made of cellulose
and are external to the
cell membrane. They are
also found in
Prokaryotes and Fungi.
PASSIVE AND ACTIVE TRANSPORT
PASSIVE TRANSPORT
 Does
not require the input of energy
 Net movement of molecules from high to
low concentration
 Used
to import resources and export wastes
 Water moves across the membrane through
proteins called “aquaporins.”
 Osmosis: the diffusion of water across a
selectively permeable membrane.
 Tonicity: the ability of a solution to cause a
cell to gain or lose water
PASSIVE TRANSPORT
Solutions
Isotonic
outside a cell may be…
– same concentration of solutes
as the interior of the cell; water moves
across membrane at the same rate in both
directions
KEY: term – exterior solution; effect – direction of water flow
PASSIVE TRANSPORT
Solutions
outside a cell may be…
Hypotonic
– lower concentration of solute
than the interior of the cell; water moves
across membrane into the cell faster than
it flows out, causing it to swell and lyse
(burst)
KEY: term – exterior solution; effect – direction of water flow
PASSIVE TRANSPORT
Solutions
outside a cell may be…
Hypertonic
- higher concentration of solute
than the interior of the cell; water moves
across membrane out of the cell faster
than it flows in, causing it to shrivel and
possibly die
KEY: term – exterior solution; effect – direction of water flow
ACTIVE TRANSPORT
Requires energy to move molecules from low to
high concentration
 Free energy often from ATP
 Uses membrane proteins

 Example:
Sodium-potassium pump
EXOCYTOSIS AND ENDOCYTOSIS
MOVING LARGE MOLECULES BETWEEN THE
INTERNAL AND EXTERNAL ENVIRONMENT
Exocytosis – internal vesicles fuse with the
plasma membrane to secrete macromolecules
out of the cell
 Endocytosis – the cell takes in macromolecules
and other particles by forming vesicles from the
plasma membrane
