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The cell membrane
• Separates the cell from its surrounding environment
• Controls movement of materials into and out of the cell
• Makes it possible for cell contents to be chemically different from environment
• Maintains homeostasis
Structure of cell membrane
• Two layers bilayer made of lipid (fat) molecules
• Hydrophilic = water loving
• Hydrophobic = water hating (fear of)
Cell Membrane
A phosopholipid is the major lipid found in the cell membrane. A. Each molecule has a polar head and
two nonpolar tails.1. Because of its hydrophilic nature,
the head will orient (point)
toward water.2. Because of it’s hydrophobic
nature,
the tails will orient away from
water.
CELL MEMBRANE
B. Because a cell is bathed inside and out in a watery solution, the phospholipid molecules form two layers, a lipid bilayer.
C. Cholesterol, a steroid, fits between the tails of the phosopholipid to provide support. CHOLESTEROL
B. Integral proteins are embedded in the bilayer.
1. Some I.P. extend across the entire cell membrane.
2. Some I.P. protrude only on one side or the other.
3. Some I.P. have carbohydrates attached which serve to:
a. hold adjoining cells together. b. act as a site where viruses can
attach. c. site where hormones can
attach.
4. Some I.P. have channels or pores through which substances can pass.5. Some I.P. bind to a substance on one side of the membrane and carry it to the other side.
Integral proteins
• Transport or move substances through membrane
• Receptors are sites that bind special messenger molecules that tell cell to start or stop some chemical activity
• enzymes
• Bind membrane to neighbor or other cell structures
CELL MEMBRANE Scientists use the term fluid mosaic model to
describe the cell membrane.Because of this fluidity, membrane
proteins and lipids can move laterally (side to
side) within the bilayer.
DIFFUSIONDIFFUSION THE MOVEMENT OF MOLECULES
FROM AN AREA OF:
HIGHER CONCENTRATION
TO AN AREA OF
LOWER CONCENTRATION.
WHAT DRIVES THE WHAT DRIVES THE DIFFUSION PROCESS?DIFFUSION PROCESS?
THE KINETIC ENERGY OF THE MOLECULES!
DIFFUSION AND THE CELL MEMBRANE!
SOME SUBSTANCES
EASILY CROSS THE CELL
MEMBRANE,
OTHERS CANNOT CROSS
AT ALL.
THE CELL MEMBRANE IS:
SELECTIVELY PERMEABLE!
Selectively Permeable:Some substances can pass through freely
Some can pass through only to a slight extent or at certain times
Some cannot go through at all
THE DIFFERENCE IN CONCENTRATION OF MOLECULES ACROSS A SPACE IS CALLED:
CONCENTRATION GRADIENT
WHAT CAUSES THIS TO TAKE PLACE?
PASSIVE TRANSPORT SOME SUBSTANCES CAN CROSS THE
CELL MEMBRANE WITHOUT ANY IMPUT OF ENERGY BY THE CELL.
THE SIMPLIST TYPE OF PASSIVE TRANSPORT IS:
EQUILIBRIUM WHEN THE
CONCENTRATION OF THE MOLECULES OF A
SUBSTANCE IS THE SAME THROUGHOUT!
WILL MOVEMENT STILL
CONTINUE?
IN WHICH DIRECTION?
OSMOSIS
THE PROCESS BY WHICH WATER MOLECULES DIFFUSE ACROSS A CELL MEMBRANE FROM AN AREA OF HIGHER
CONSENTRATION TO AN AREA OF LOWER CONSENTRATION!!!
PASSIVE TRANSPORT
DIRECTION OF OSMOSIS
DEPENDS ON THE CONCENTRATION OF THE SOLUTES ON THE TWO SIDES OF THE
MEMBRANE.
HYPOTONICHYPOTONIC HYPERTONICHYPERTONIC
ANIMAL PLANT
If the solution is isotonic relative to the cell, then the solute concentrations are the same on both sides of the membrane and water moves equally in both directions
A hypertonic solution has increased solute, and a net movement of water outside causing the cell to shrink.
A hypotonic solution has decreased solute concentration, and a net movement of water inside the cell, causing swelling or breakage.