BIOLOGY
1111
Chapter
5
CELL
MEMBRANE
Northland
Community & Technical College
Instructor
Terry Wiseth
CELL
MEMBRANE
cell
membrane made up of:
1)
Phospholipids
2)
Sterols
3)
Proteins
4)
Glycoproteins
PHOSPHOLIPIDS
cell
membrane - semi-permeable lipid bilayer
two
layers of Phospholipids
hydrophilic
head
hydrophobic
acid tails
lipid
bilayer minimizes the number of hydrophobic groups exposed to water
sealing
behavior exhibited because the puncture is energetically unfavorable
leaves
too many hydrophobic groups exposed to the surrounding water
FLUID
MOSAIC MODEL
membrane
bilayer shows fluid behavior
molecules
of the bilayer are in constant motion
membrane
is composite of molecules allowing a “mosaic description”
cell
survival depends on fluidity characteristics of the cell membrane
extracellular
fluid temperature decreases cause a stiffening of the cell membrane
disrupts
membrane protein function
yeast
and bacteria cells under decreased temperatures rapidly synthesize unsaturated
fatty acids
infusion
of kinky lipids keep the membrane from stiffening up
STEROLS
common
sterols of cell membranes
Cholesterol
(animal cells)
Phytosterols
(plant cells)
prevent
packing of lipid cells in the cell membrane
GLYCOPROTEINS
proteins
are embedded in the bilayer
glycoproteins
- sugar-protein combinations
sugars
extend out to the extracellular fluid
ROUTES
ACROSS CELL MEMBRANES
1)
Diffusion / Osmosis
2)
Protein Mediated Diffusion
3)
Endocytosis and Exocytosis
DIFFUSION
diffusion
process is a key factor in movement of substance across cell membranes and
through the cytoplasm of the cell
movement
of molecules from areas of high concentrations to areas of lower concentrations
net
movement of molecules occurs down the concentration gradient
oxygen,
carbon dioxide and small molecules with no charge
diffuse
without assistance or consumption of energy
rate
of diffusion is dependent on:
1)
steepness of the concentration gradient
2)
temperature
3)
molecular size
4)
electrical gradients
5)
pressure gradients
OSMOSIS
movement
of water across a semi-permeable membrane in response to solute
concentration gradients
some
small molecules (like water) pass readily through the cell membrane
glucose
must be pumped across the membrane
water
concentration gradient is influenced by the number of molecules of solutes
that are present on both sides of the membrane
the
direction in which water moves across the membrane is influenced by tonicity
TONICITY
relative
concentrations of solutes in two fluids
water
tends to move where solute concentrations are greatest
1)
Isotonic=solute concentrations are equal
2)
Hypotonic=least solute concentrations
3)
Hypertonic=greatest solute concentrations
OSMOSIS
water
molecules move from hypotonic solutions to hypertonic solutions
cells
require built in mechanisms for adjusting to differences in tonicity between
cytoplasm and their surroundings
without
them cells will:
1)
shrivel (crenation)
2)
burst (lyse)
TURGOR
PRESSURE
water
moves into plant cells creating an internal pressure called turgor pressure
maintains
soft parts of plant in an erect posture
soil
must be hypotonic
plants
will wilt if soil has too many solutes (hypertonic)
ex: adding salty water
PROTEIN
MEDIATED DIFFUSION
glucose,
large water soluble molecules and charged ions
cannot
diffuse freely across the cell membrane
A)
Passive Transport
B)
Active Transport
PASSIVE
TRANSPORT
molecules
diffuse through non-selective channel proteins
selective
carrier proteins can move molecules of a solute across the cell membrane
by binding and changing shape
passive
transport occurs only down the concentration gradient
ACTIVE
TRANSPORT
transport
occurs when carrier proteins receive an energy boost from ATP
pump
solutes across the cell membrane against the concentration gradient
ex: calcium pump, sodium-potassium pump
cytoplasm
concentrations of K+ increase and Na+ decrease
TYPES
OF MEMBRANE PROTEINS
1)
Channel Proteins
2)
Active Transport Proteins
3)
Receptor Proteins
4)
Recognition Proteins
5)
Adhesion Proteins
CHANNEL
PROTEINS
Passive
and do not require any energy investment by the cell
most
remain open at all times
acts
as a pore for water-soluble ions
may
have molecular gates
ex: nerve cells during nerve impulses
1)
molecules only move down the concentration gradient
2)
does not require any energy input from the cell
FACILITATED
DIFFUSION
Does
not require energy
Highly
selective
Polar
compounds such as sugars and amino acids
Diffusion
is down the concentration gradient
ACTIVE
TRANSPORT PROTEINS
require
energy to transport molecules
1)
molecules are pumped against their concentration gradient
2)
requires an energy input from the cell
CARRIER
PROTEINS
a
change in shape allows bound substances to pass through the membrane
some
may require an energy input to actively pump substances across the membrane
CONTRACTILE
VACUOLES
paramecium
live in freshwater
cytoplasm
is hypertonic to the water the organism lives in
paramecium
expel excess water influx by utilizing a contractile vacuole
RECEPTOR
PROTEINS
have
binding sites for hormones
ex: hormone somatotropin
binds to receptors which triggers enzymes to activate cell growth and division
RECOGNITION
PROTEINS
act
as “molecular fingerprints” on the cell surface
self
recognition proteins identify cell type to other cells
white
blood cells are able to recognize “self” from “non-self”
glycoproteins
are extended into the extracellular fluid for easy access for recognition
ADHESION
PROTEINS
connect
cells together in a given tissue
glycoproteins
connect neighboring cells
some
may become cell junctions
ROUTES
ACROSS CELL MEMBRANES
1)
Diffusion / Osmosis
2)
Protein Mediated Diffusion
3)
Endocytosis and Exocytosis
EXOCYTOSIS
AND ENDOCYTOSIS
move
materials in bulk across cell membranes
EXOCYTOSIS
cytoplasmic
vesicles fuse to the cell membrane and contents are released to the outside of
the cell
ENDOCYTOSIS
1)
Phagocytosis
2)
Pinocytosis
region
of the cell membrane sinks inward and balloons around substances, pinching
itself off into a sealed vesicle which transports or stores the contents
PHAGOCYTOSIS
1)
Phagocytosis “cell eater”
ex: amoeba, white blood cells (macrophage)
lobes
of cytoplasm wrap around the trapped item and seal together
newly
formed vesicles fuse with lysosomes for digestion
PINOCYTOSIS
2)
Pinocytosis “cell drinking”
ex: intestinal cell taking up liquid droplets
END
CHAPTER
5