Transcript Movement through cell membranes

Movement through cell
membranes
Diffusion
• Example: exchange of
oxygen and carbon
dioxide in lungs
• Molecules or ions moving
from areas of higher
concentration to lower
concentration
• Difference in
concentration is the
concentration gradient
• Concentrations are equal
= equilibrium
Diffusion in & out of a cell
1. the cell membrane is permeable
to that substance
2. concentration gradient exists such
that the substance is at a higher
concentration either outside or
inside the cell.
Diffusion in & out of a cell
example
• Intracellular Oxygen is always low b/c it is
constantly used during metabolism; extracellular
oxygen is high due to homeostatic mechanisms.
Concentration gradient always favors oxygen
diffusing into the cell.
• CO2 is a waste product of metabolism, and thus
is high inside cells; homeostasis maintains lower
levels of CO2 outside the cell. Concentration
gradient always favors CO2 to diffuse out of the
cell.
• Thus – never an equilibrium between CO2 and
O2
Facilitated
Diffusion
• Substances not able
to pass through the
lipid bilayer need
help from
membrane proteins
• Examples: glucose
and amino acids
• Glucose needs
insulin to help it
diffuse through
membranes of
certain cells.
• Example: distilled water
entering the cell
• Water molecules diffuse
from higher water
concentrations to lower
water concentrations
• In solutions: higher
concentration of solute,
lower concentration of
water; lower concentration
of solute, higher
concentration of water.
• Greater the
concentration of solute
(proteins), lower water
concentration, greater
osmotic pressure.
• Lower the
concentration of solute
(proteins), higher water
concentration, lower
osmotic pressure
Osmotic Pressure
• When intracellular and extracellular fluids
and pressure are the same = isotonic
• When extracellular fluid has greater
pressure than intracellular – water leaves
cell = hypertonic
• When intracellular fluid has greater
pressure than extracellular – water enters
cell = hypotonic
Filtration
• Forced movement of
molecules through
membranes
• Separate solids from water
• Tissue fluid forms when
water and small dissolved
substances are forced out
through thin porous walls of
blood capillaries, but larger
particles are left
• Force of this movement
comes from blood pressure,
generated by a higher
pressure inside the heart,
less pressure outside the
heart.
Active Transport
• Similar to facilitated
diffusion
• Differs in that particles
are moving from areas of
low concentration to high
concentration
• Carrier proteins also
called pumps
• Examples: sugars,
amino acids; sodium,
potassium, calcium, and
hydrogen ions
• Also absorb nutrients
into cells of the intestinal
walls
Endocytosis/Exocytosis
Endocytosis
• Molecules or other particles that are too
large to enter a cell by diffusion, facilitated
diffusion or active transport.
• Three types
– Phagocytosis: cell eating, takes in solids, ie.
Bacteria
– Pinocytosis: cell drinking, takes in tiny droplets,
ie. Water
– Receptor-mediated endocytosis: moves specific
particles into the cell, particle binds to a
receptor site protein.
Exocytosis
• Reverse process of endocytosis
• Cell secretes a substance stored in vesicle
• ie. Neurons – release neurotransmitters that
signal nerves, muscles, or glands