Cell Membrane & Passive Transport
Notes:

The cell membrane is composed of a phospholipid bilayer, which allows it to be
selectively permeable.
o Selective permeability means that only certain substances can pass
through.

Phospholipid—lipid made of a phosphate group and two fatty acids
o Has a polar head and 2 non polar tails
attracted to water
repelled by water

The phospholipids are arranged in a double layer called the phospholipid bilayer.

Membrane Proteins:
o There are multiple proteins located in the lipid bilayer of a cell membrane.
o Protein types:

Marker proteins—identify cell type

Receptor proteins—binds to substances outside the cell

Transport proteins—aid in the movement of substances in and out
of the cell

Passive Transport:
o Passive transport—movement across the cell membrane that does not
require energy
o Concentration gradient—a difference in concentration of a substance
across a space

Substances will move from an area of higher concentration to an
area of lower concentration until equilibrium is reached (same
concentration).
o Movement of Substances:

Diffusion—the movement of a substance from an area of high
concentration to an area of low concentration caused by the
random movement of particles.

For the cell to reach equilibrium, substances must pass through the
cell membrane.

Due to selective permeability and the polarity of the lipid
bilayer, many substances are repelled and therefore cannot
diffuse across the membrane.

Small, non-polar substances can diffuse across without
difficulty or assistance.

Osmosis—the diffusion of water through a selectively permeable
membrane.

Also involves the movement of a substance down its
concentration gradient (from high to low concentration).

Quick Review: What is a solution? (substance dissolved in another substance)

Many substances in and out of the cell are dissolved in water
to make a solution.

The direction of movement across the membrane is
determined by the concentration of solutions inside and
outside of the cells.

Handout hypotonic, hypertonic, isotonic page

Most ions and polar molecules cannot pass across the cell
membrane because they cannot pass through the non-polar interior
or the lipid bilayer.

Transport proteins, or channels, provide polar passageways
through which ions and polar molecules can move across
the cell membrane.
o Each channel allows only a specific substance to
pass through.
o Ex: Ion channels

An ion channel is a transport protein with a
polar pore through which ions can pass.

The pores of some ion channels are always
open, while others can be closed by channel
gates.

Facilitated diffusion—the transport of substances through a cell
membrane down the concentration gradient (high to low), with the
aid of carrier proteins.

Facilitated diffusion uses carrier proteins, which carry a
specific substance down their concentration gradient to the
other side of the membrane.

Step 1: A molecule outside the cell binds to a carrier protein
on the cell membrane.

Step 2: The carrier protein transports the molecule across
the cell membrane.

Step 3: The molecule is released from the carrier protein
into the inside of the cell.
Homework: Passive transport worksheet