Cell Boundaries
MIAMI BEACH SENIOR HIGH
MRS DIAZ
I. Cell (Plasma) Membrane
The cell membrane is composed of lipids
(because they can separate substances.)
Cell Membrane (cont’d)
The membrane is made of many phospholipids
which have two parts: hydrophilic (water loving)
heads and hydrophobic (water fearing) tails. The
heads are made of Glycerol (alcohol) and a
phosphate group and the tails are chains of fatty
acids.
Phospholipids
Phospholipids can move side to side and
allow water and other non-polar
molecules to pass into and out of the cell.
This is known as simple diffusion because
it does not require any energy and the
water or molecules are moving with/down
the concentration gradient. (high to low
concentration)
Cell Membrane (cont’d)
Notice also that the membrane is made of two
layers of lipids, thus we call it a phospholipid
bilayer.
Cell Membrane (cont’d)
Also found in the cell membrane are
certain proteins, which play an important
part in the cell by:
1) Coordinating cell-to-cell behaviors.
2) Adjusting the cells metabolism rates.
3) Initiating growth & division times.
Cell Membrane (cont’d)
Cholesterol
Another type of lipid in the cell
membrane is cholesterol which makes
the membrane stronger. . Embedded in
the phospholipid bilayer are
glycoproteins that
aid in identification
and in cell
recognition.
Proteins
Proteins called integral proteins go all the way
through the bilayer, while peripheral proteins
only stay on one side. Integral proteins are also
called a protein channel. Large molecules like
lipids or carbohydrates use protein channels to
move across membranes.
Carbohydrates
Some of the membrane proteins called
glycoproteins have carbohydrate chains
attached to help cells recognize each other
and certain molecules. Carbohydrate chains
attached directly to phospholipids are called
glycolipids
4 Functions of the cell
membrane
1. Cell to Cell recognition
2. Regulates what enters the
cell
3. Cell signaling
4. Separates the cell from the
environment
Passive Transport
Chapter 4 Section 1

Passive transport (no energy required) can occur in 4
different ways:
i) Simple Diffusion
ii) Osmosis
Passive Transport (cont’d)
 During passive transport substances always move
from high concentration to low concentration (this
is known as a concentration gradient).

After passive transport when the concentration of
molecules is the same we say equilibrium exists.
i) Diffusion
Diffusion is the movement of molecules
down a concentration gradient, from high
to low concentrations.
ii) Osmosis
Osmosis is the diffusion of water across a
semipermeable (cell) membrane.
Water will move down the concentration
gradient
b) Types of Solutions
1. Hypertonic: solution with a higher concentration
of dissolved particles than its surroundings.
2. Hypotonic: solution with a lower concentration
of dissolved particles than its surroundings.
3. Isotonic: solution with an equal concentration of
dissolved particles to its surroundings.
c) Water Balance in a Cell
1. Cells placed in a hypertonic environment (salt
water) will plasmolysis. – SHRINK!
2. Cells placed in a hypotonic
environment (fresh water) will
cytolysis. – BURST!
Cells placed in a isotonic
environment will be balanced
Water Movement Problems
explain what will happen in each example
Active Transport (cont’d)
Active transport is the movement of
molecules up (against) the concentration
gradient, from low to high.
This type of transport requires energy in
the form of ATP.
b) Phagocytosis
Process where a vacuole is formed around solid
particles before they are taken into the cell.
2) Exocytosis
Process by which large molecules are passed out
of the cell without going through the membrane.
Levels of Organization
Atoms
Molecules
Cells
Tissues
Organs
Organ Systems
Organism
Final Thought
“Knowledge is not simply another
commodity. On the contrary. Knowledge is
never used up. It increases by diffusion
and grows by dispersion.”
-Daniel J. Boorstin