Cell Membrane!Cell Membrane!Diffusion and Diffusion and

osmosisosmosis

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

Cell Membrane

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

Proteins of the cell membraneA. Peripheral proteins are located on the interior and exterior.

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

A cell cannot survive in isolation.

Nutrients and waste must pass through the cell membrane.

THE DIFFERENCE IN CONCENTRATION OF MOLECULES ACROSS A SPACE IS CALLED:

CONCENTRATION GRADIENT

WHAT CAUSES THIS TO TAKE PLACE?

Concentration is most at the top of the hill

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

Suppose you put a cell into a glass of salt water.

NaCl normal

NaCl above normal

Water moves out of the cell

Cell shrinks

NaCl below normal

Suppose you put a cell into a glass of less salt water (hypotonic)

NaCl normal

Water moves into cell

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.