Transcript Plasma Membrane

The Plasma Membrane and
Homeostasis
FLUID MOSAIC MODEL
Homeostasis – Maintaining
a Balance
 Cells must keep the proper
concentration of nutrients and
water and eliminate wastes.
 The plasma membrane is
selectively permeable – it will
allow some things to pass
through, while blocking other
things.
Structure of the Plasma
Membrane
 Lipid bilayer – two sheets of lipids
(phospholipids).
 Found around the cell, the nucleus,
vacuoles, mitochondria, and
chloroplasts.
 Embedded with proteins and
strengthened with cholesterol
molecules.
What’s a Phospholipid?
 It’s a pair of fatty acid chains and a
phosphate group attached to a glycerol
backbone.
 Polar (water-soluble) heads face
out and the nonpolar fatty acids
hang inside.
Membrane Proteins
Determine what particles can pass
through the membrane.
 2. Serve as enzymes (may speed
reactions).
 3. Act as markers that are
recognized by chemicals and
molecules from the inside and the
outside of the cell (the immune
system).
 1.
Cellular Transport
 Diffusion – movement of particles from
an area of high concentration to an area
of low concentration.
 Caused by Brownian motion (movement of
particles because of the movement of their
atoms).
 Continues until an equilibrium is reached (no
gradient).
 Dynamic equilibrium – particles move freely
and are evenly distributed.
Osmosis
Diffusion of water across a
selectively permeable
membrane.
Occurs until water is balanced
on both sides of the
membrane.
Cell Concentrations
1. Hypertonic solutions (external
environment) – more dissolved solute.
(less water)
2. Hypotonic solutions (external
environment)– less dissolved solute.
(more water)
3. Isotonic solutions (external environment)
– the same dissolved solute.
 QUESTION: What happens to the cell in
each situation?
ANSWER
1. Hypertonic solutions (external environment) –
water leaves the cell. If severe enough, the cell
shrivels and dies (plasmolysis)
2. Hypotonic solutions (external environment)–
water enters the cell. If severe enough, the cell
swells and bursts (cytolysis)
3. Isotonic solutions (external environment) –
water passes back and forth equally. There is
no adverse effect
Overcoming Osmosis
 Contractile vacuoles – expel
excess water from bacterial
cells that live in water.
 Turgor pressure – water
pressure in a plant cell. Loss
of turgor pressure causes
wilting (plasmolysis).
Cellular Transport
 Passive transport – no energy is needed to
move particles.
 Facilitated diffusion – embedded
proteins act as tunnels allowing
particles to “fall” through.
 Occurs when molecules are too
big to fit through the plasma
membrane
Ion channels
 Many ions are not soluble in lipids
 To enter the cell, they need to go through a protein
“tunnel” to get into the cell
 Examples: Na+, K+, Ca+2, Cl-
 These protein “tunnels” have “gates” that open or close
to allow ions into the cell or to leave the cell
 Again, this depends on the concentration gradient
 Stimuli in the cell determine when the gates open or
close
Cellular Transport
 Active transport – energy is needed to
move particles.
 Carrier proteins – embedded proteins change
shape to open and close passages across the
membrane.
 This system allows the cell to move
substances from a lower concentration to a
higher concentration
Example: Sodium-potassium pump
 The sodium-potassium pump is one of the active
transport mechanisms used in the conduction of a
nerve impulse.
 How it works: (open book to pg. 104) – FOLLOW
ALONG
Three Na+ ions (inside the cell) bind to a protein in the
cell membrane
2. You must use energy to move the Na+ ions out of the
cell so an ATP molecule is used (energy molecule) to
change the shape of the carrier protein
3. With a phosphate is bound to the carrier protein it has
“space” for two K+ to bind to the protein
1.
Sodium-potassium pump
4. When the two K+ bind to the carrier protein, the
protein again changes shape by releasing the
phosphate and allows the K+ to enter the cell

NOTE: Another driving force for the pump is an
attempt to maintain a balanced electric charge


You lose 3+ so it’s easier to add + into the cell
IN SHORT: You need ATP to change the shape of
proteins so that molecules can enter or leave the cell.
ENDOCYTOSIS VS EXOCYTOSIS
 There are two other ways to move substances into and
out of the cell:
 Endocytosis: the cell ingests external substances
(macromolecules, external fluid, other cells)
 The cell membrane engulfs the substance and forms a
vesicle
 The substance inside the vesicle is kept separate from
the rest of the cell by the phospholipid bilayer of the
vesicle
 These substances can be transported to the lysosome for
digestion or other membrane-bound organelles for
other functions
ENDOCYTOSIS – CONT.
 Types of endocytosis
 Pinocytosis: this creates a vesicle that is
transporting fluids
 Phagocytosis: creates a vesicle that transports
large particles or other cells
 Example: Your immune system creates a type
of phagocyte (cell that digests foreign
bacteria) called a macrophage that helps to
fight off bacterial infections
EXOCYTOSIS
 Exocytosis: when a substance is released from the cell by
binding a vesicle to the plasma membrane
 This process is basically the reverse of endocytosis
 This process is used for
 Elimination of large molecules from the cell (they are
large enough that they would damage the cell
membrane if allowed to leave through the plasma
membrane)
 Elimination of toxins that need to be kept separate
from cell interior
 Many endocrine cells use this method to release
hormones