Download Plasma Membrane

Cell Membrane &
Cellular Transport
HOMEOSTASIS AND TRANSPORT
Cell membranes help organisms maintain homeostasis
by controlling what substances may enter or leave
cells
Some substances can cross the cell membrane
without any input of energy by the cell
The movement of such substances across the
membrane is known as passive transport
To stay alive, a cell must exchange materials such as
food, water, & wastes with its environment
These materials must cross the cell or plasma
membrane
Plasma Membrane
Boundary that separates the living cell from
it’s non-living surroundings.
Phospholipid bilayer (2 parts)
hydrophilic heads
hydrophobic tails
Phospholipid
Phospholipid Bilayer
Plasma Membrane - cont.
Controls traffic into and out of the cell with
phospholipids and transport proteins.
Selectively permeable
(allows some molecules to move
across cell membrane)
Transport protein
Small molecules like water,
oxygen, & carbon dioxide
can move in and out freely
Large molecules like
proteins & carbohydrates
cannot move easily across
the plasma membrane
The Cell Membrane is semi
permeable or selectively
permeable only allowing
certain molecules to pass
through
Diffusion
The net movement of a substance
(molecules) down a concentration gradient
from an area of high concentration to an
area of low concentration.
When the molecules are even throughout a space- it
is called EQUILIBRIUM
Passive transport: NO energy is required.
Facilitated diffusion: Type of passive transport
which uses transport proteins.
Diffusion
Diffusion is the movement of molecules from an area of higher
concentration to an area of lower concentration
Small molecules can pass through the cell membrane by a process
called diffusion
Diffusion across a membrane is a type of passive transport because it
does not require energy
Diffusion continues until the concentration of the molecules is the
same on both sides of a membrane
Diffusion is driven by the
kinetic energy of the
molecules
Kinetic energy keeps
molecules in constant motion
causing the molecules to
move randomly away from
each other in a liquid or a gas
The rate of diffusion depends
on temperature, size of the
molecules, & type of
molecules diffusing
Molecules diffuse faster at
higher temperatures than at
lower temperatures
Smaller molecules diffuse
faster than larger molecules
Osmosis
The diffusion of water across a semi~permeable membrane is
called osmosis
The water moves from a high concentration to low
concentration.
3 kinds of Osmosis in cells:
Hypotonic, Hypertonic, & Isotonic
OSMOSIS
This diagram shows water
molecules moving across a
selectively permeable
membrane. Water
molecules are the small
blue shapes, and the solute
is the green.
What's happening? The
solute (green blobs) is
more concentrated on the
right side, which pulls the
water molecules toward
that side. The green blobs
would move to the left to
spread out evenly, but the
membrane won't let those
pass
Solutions
Solutions have two parts --- the solute
which is being dissolved in the solvent
Water serves as the main solvent in
living things
Hypertonic Solution
Solute concentration outside the cell is higher (less water)
Water diffuses out of the cell until equilibrium is reached
Cells will shrink & die if too much water is lost
Plant cells become flaccid (wilt); called plasmolysis
Hypotonic Solution
Solute concentration is more
inside the cell.
Water moves into the cell until
equilibrium is reached
Animal cells swell & burst
(lysis) if they take in too much
water
Cytolysis is the bursting of cells
Plant cells become turgid due to
water pressing outward against
cell wall
. Turgor pressure in plant cells
helps them keep their shape
Isotonic Solutions
Concentration of solutes same inside & outside the
cell
Water moves into & out of cell at an equal rate so
there is no net movement of water
Animal Cells
Animal cells placed into a hypotonic
solution will HEMOLYSIS (EXPLODE).
Animal cells placed into a hypertonic
solution will CRENATE (SHRIVEL).
Hemolysis
Crenation
Red
Blood
Cells
Plant Cells
Firmness or tension (vacuole full) that is
found in plant cells (cell wall) that are in a
hypotonic environment is called TURGID.
This process is called TURGOR
PRESSURE.
Water
Water
Central
Vacuole
Cell
Wall
Water
Plant Cells
When the plasma membrane pulls away from
the cell wall (vacuole empty) in a hypertonic
environment (loss of water) is called
PLASMOLYSIS.
Water
Water
plasma membrane
Cell
Wall
Water
Effect of Solutions on Cells
Facilitated
Diffusion
Faster than simple diffusion
Considered passive
transport because extra
energy not used
Occurs down a
concentration gradient
Involves carrier proteins
embedded in a cell’s
membrane to help move
across certain solutes such
as glucose
Carrier molecules change
shape when solute attaches
to them
Change in carrier protein
shape helps move solute
across the membrane
Active Transport
The movement of molecules (small or large) across
the plasma membrane in which energy (ATP) is
required.
Moves materials against their concentration gradient
from an area of lower to higher concentration
Sodium-Potassium Pump
The mechanism that uses energy (active transport)
released from splitting ATP to transport Sodium
(Na+) out of and Potassium (K+) into cells.
extracellular
fluid
intracellular
fluid
K+
K+
Na+
Na+
Bulk Transport
Moves large, complex
molecules such as proteins
across the cell membrane
Large molecules, food, or
fluid droplets are packaged
in membrane-bound sacs
called vesicles
Endocytosis moves large
particles into a cell
There are two forms of
endocytosis:
a) Pinocytosis - a form of endocytosis
that transports liquids into the cell.
b) Phagocytosis - a form of endocytosis
that transports solids into the cell
Exocytosis is used to remove large
products from the cell such as
wastes, mucus, & cell products
 Exocytosis is the movement of a
substance out of a cell via
vesicles. Exocytosis is the reverse of
endocytosis
Animation of endocytosis
& exocytosis