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CELL MEMBRANE,
CYTOSKELETON &
CELL-CELL INTERACTIONS
Chapter 4
A. Cell Membrane Structure
Cell membrane is a phospholipid bilayer
embedded with mobile proteins.
Proteins
Phosphate head of
phospholipid is
hydrophilic.
Fatty acid tails are
hydrophobic.
Types of membrane proteins:
 Transport proteins - move
substances across membrane
 Cell surface proteins - establish
“self”
 Cellular adhesion molecules (CAMs) enable cells to stick to each other
 Receptor proteins - receive &
transmit messages into a cell
Additional molecules may be associated with
proteins & phospholipids:
 cholesterol
 sugar molecules
 glycoproteins
 glycolipids
B. Movement Across Membranes
Cell membranes are selectively
permeable.
1. Simple diffusion (passive)
Substance moves across phospholipids
from an area of high to an area of low
concentration, without using energy.
Substance moves down its
concentration gradient
Ex. O2, CO2
Simple Diffusion
Cell membrane
Transport protein
Diffusion continues until dynamic
equilibrium is reached.
Movement of water across membranes by
simple diffusion is called osmosis.
Water is driven to move because the
membrane is impermeable to solute(s).
Tonicity
Refers to differences in solute
concentration on either side of a
semipermeable membrane.
 Isotonic - both solutions have the same
solute concentrations.
 Hypotonic - solution with the lower
solute concentration.
 Hypertonic - solution with the higher
solute concentration.
What is effect of immersing an animal
cell in a hypertonic or hypotonic
solution?
What is effect of immersing a plant cell
in a hypertonic or hypotonic solution?
Cell immersed in
hypertonic solution
Cell immersed in
hypotonic solution
2. Facilitated Diffusion (passive)
Substance moves through a transport
protein from an area of high to an
area of low concentration, without
using energy.
Substance moves down its
concentration gradient
Ex. glucose
Facilitated Diffusion
Cell membrane
Transport protein
Cell:
1% sucrose
1% glucose
1% fructose
97% water
Environment:
3% sucrose
2% glucose
1% fructose
94% water
Assume cell membrane is permeable to
water, glucose & fructose, but
impermeable to sucrose.
In which direction will sucrose, glucose,
fructose & water move?
3. Active Transport (active)
Substance moves through a transport
protein from an area of low to an
area of high concentration; requires
energy.
Substance moves against its
concentration gradient
Ex. ions (Na+, K+, Cl-)
Active transport
Cell membrane
Transport protein
ATP
Active transport of Na+ & K+ through
the sodium-potassium pump (transport
protein).
4. Cotransport
The active transport of one
substance is coupled to the passive
transport of another.
Ex. sucrose (plant cells)
5. Exocytosis, Endocytosis &
Transcytosis
Movement of large particles across
membranes with the help of vesicles.
Exocytosis - vesicles move particles
out of the cell.
Ex. release of enzymes from head of
sperm; neurotransmitter release
Endocytosis - vesicles move particles
into the cell.
Three types of endocytosis:
ReceptorPinocytosis Phagocytosis
mediated
endocytosis
Transcytosis - combines endocytosis &
exocytosis.
Vesicles rapidly transport particles
through cells.
Ex. transport of nutrient monomers
through cells lining digestive tract &
into the bloodstream
C. Cytoskeleton
The structural framework of a cell.
1. Microtubules - hollow, thick elements
made of the protein tubulin.
Functions:
 move chromosomes
apart during cell
division
 form cilia & flagella
2. Microfilaments - long, thin elements
made of the protein actin.
Functions:
 connect cells to
each other
 move vesicles &
organelles within
cytoplasm
 help cells move
3. Intermediate filaments - elements
with diameters in between that of
microtubules & microfilaments.
Made of various proteins (ie. keratin)
Functions:
 maintain cell
shape
 connect cells to
each other & to
underlying tissue
(skin cells)
D. Intercellular Junctions
Structures that connect cells of
multicellular organisms to form tissues.
1. Animal cell Connections
Tight Junctions - cell
membranes of adjacent
cells are fused,
creating a tight seal.
Ex. cells lining small
intestine; cells lining
capillaries in brain
Desmosomes - intermediate
filaments weld cell
membranes of adjacent
cells together in isolated
spots.
Ex. skin cells
Gap Junctions - channels
that link the cytoplasm
of adjacent cells, allowing
exchange of materials.
Ex. heart muscle cells
2. Plant Cell Connections
Plasmodesmata - channels that link the
cytoplasm of adjacent plant cells,
allowing the exchange of cytoplasm &
organelles.
Ex. cells conducting water & nutrients
E. Cell-Cell Interactions
1. Cell Adhesion
Process that uses membrane proteins
called cellular adhesion molecules
(CAMs) to direct the migration of cells.
Various CAMs function in sequence to:
 guide WBCs to injury sites
 guide embryonic cells to help form
placenta
 establish nerve connections involved in
learning & memory
CAMs directing
WBCs to injury
sites.
2. Signal Transduction
Process by which cells receive, amplify,
& respond to outside stimuli.