Download Across the Membrane

Homeostasis and
Transport
Movement of cellular materials
in and out of the cell.
Cell membranes help cells maintain
homeostasis by controlling what
substances may enter or leave cells.
Cell membranes are selectivelypermeable in that they allow only
certain substances to pass.
I - Passive Transport
• No energy input from the cell.
.
No ATP energy molecules used.
Simplest type of Passive Transport:
(simple) Diffusion
It is the (random) movement of
molecules from an area of higher
concentration to an area of lower
concentration.
In other words…
• Materials move from where there’s more
to where there’s less.
This difference in concentration of
molecules (over a given area)is called
the concentration gradient
Diffusion movement occurs with, (or down) the concentration
gradient.
Diffusion is driven by the kinetic
energy of the molecules.
Kinetic Energy – Energy of Movement
from an
area of
low to
high
concentration
• Eventually the process of diffusion reaches
equilibrium.
• At equilibrium the concentration of
molecules are the same throughout the
space. (The concentration gradient
disappears)
Are the molecules still moving at
this point (equilibrium)?
• Yes. Molecules of a substance are always in
motion to some degree except at absolute zero.
The net movement is zero.
Osmosis
The process by which H2O molecules
diffuse across a cell membrane
(selectively permeable membrane) from
an area of HIGH concentration to LOW
concentration.
The net direction of water movement
depends on the relative amount of solutes
(dissolved substances) on the two sides of
the cell membrane.
THE THREE BASIC TYPES OF
SOLUTIONS:
• Hypotonic
• Hypertonic
• Isotonic
Hypotonic
• solute concentration outside the cell is
lower than inside the cell cytosol.
• water diffuses into the cell
Hypertonic
• solute concentration outside the cell is
higher than inside the cell cytosol.
• water diffuses out of the cell
Isotonic
• solute concentration outside and inside the
cell are equal thus, NO net movement of
water into or out of the cell
How do cells deal with osmosis?
• Isotonic – usually no difficulties
interesting chromosome
pattern makes this
cell have a happy
face
Hypotonic environment
• Unicellular freshwater organisms rid themselves
of excess water by an organelle called a
contractile vacuole. (Other cells cannot get rid
of the water and they may burst)
What happens to an
animal cell under
different conditions?
What happens to a
plant cell under
different
conditions?
Facilitated Diffusion
It is a type of passive transport in which
molecules pass through pores in the
cell membrane. Carrier proteins help
move molecules across the membrane.
II - ACTIVE TRANSPORT
Cell expend energy to move materials.
Active transport always needs ATP
energy molecules
Active Transport is usually against the
concentration gradient.
Materials move from low to high
concentration.
EXAMPLES OF ACTIVE TRANSPORT
• Cell Membrane Pumps (such as the
Sodium-Potassium pump) It involves
Carrier proteins
Sodium-Potassium Pump is important for nerve cells.
• Endocytosis - cells take in fluids, large
molecules, large particles and other cells.
Vesicles are formed to ingest materials.
Two major types of endocytosis:
• pinocytosis – movement of solutes or fluids
(Cell-drinking)
• phagocytosis – movement of large
particles or whole cells. (cell-eating)
Amoeba uses pseudopodia to engulf food.
Exocytosis (the reverse of endocytosis)
• Vesicles fuse with the cell membrane,
releasing their contents to the outside
environment.
• Unicellular organisms may get rid of
wastes through exocytosis.