This document provides an overview of cell membranes and transport mechanisms. It discusses how phospholipid bilayers naturally form membranes, allowing the separation of intracellular and extracellular environments. It describes passive transport mechanisms like diffusion and osmosis that move molecules down concentration gradients, as well as active transport processes like pumps that move molecules against gradients by expending cellular energy in ATP. The roles of these transport mechanisms in maintaining cell volume and movement of important ions and molecules in and out of cells are also summarized.

1. Inside the Cell
AP Biology - Summer 2013
WEEK 2

2. Biochem QUIZ
Write a quick sentence to describe the
difference between each pair of
concepts:
1. Organic molecules vs. inorganic
molecules
2. Hydrogen bonds vs. covalent bonds
3. Monomers vs. polymers
4. Carbohydrates vs. proteins
5. Substrate vs. catalyst

3. Biochem QUIZ
1. Organic molecules vs. inorganic molecules
Organic molecules contain covalently bonded carbon. Inorganic
molecules may not contain carbon, or the carbon is not covalently
bonded together.
2. Hydrogen bonds vs. covalent bonds
Hydrogen bonds are weak connections between different molecules.
Covalent bonds are strong bonds between atoms in the same
molecule.
3. Monomers vs. polymers
Monomers are single, small molecules. Polymers are long molecules
formed by connecting monomers.
4. Carbohydrates vs. proteins
Carbohydrates are made of carbon, hydrogen, and oxygen. Proteins
also include nitrogen, sulfur, and other atoms.
5. Substrate vs. catalyst
The substrate is the reactant, which changes during a chemical
reaction. The catalyst or enzyme helps the reaction happen, but it will
not change during the reaction.

4. The First Cells
Last week, we discussed the primordial soup,
a theory to describe the creation of the
macromolecules we need to form cells. But
why do we need macromolecules?
The first thing a cell needs
is a membrane that
separates the inside from
the outside.
And this membrane needs
to organize itself, because
there are no cells around
yet to help it get together.

5. INTRODUCING…THE PHOSPHOLIPID!
(the early Earth’s first self-organizing membrane
molecule)
 Featuring…
◦ A phosphate ―head‖
◦ Two lipid ―tails‖
◦ A glycerol
―backbone‖
 The fatty acid tails are
non-polar: they share
electrons equally and
have no charge.
 The phosphate head is
polar; it shares
electrons unequally
like water.

6.  The phosphate heads are hydrophilic—
because they are polar, they like to mix
with water.
 The lipid tails are hydrophobic—because
they are nonpolar, they hate water.
So if you put phospholipids in water, they naturally form
protective bubbles called micelles with the tails
pointing in.
But these micelles aren’t big enough to hold anything!

7. The Phospholipid Bilayer
 By instead having a
double layer, or
bilayer, cells can
have a watery inside
and outside, while the
fatty acid tails never
touch water.
 This membrane is semipermeable—
some things, like water and dissolved
gases, can pass through easily.
 Large molecules and charged ions
cannot go through without help.

8. The Fluid Mosaic Model
 It’s not all phospholipids … there are also
embedded membrane proteins
 Proteins can help with transport or
signalling  Cholesterol is an
important part of
the membrane. It
also has hydrophilic
and hydrophobic
parts. It stabilizes
and seals the
membrane.
 All the molecules
on the membrane
are fluid and

9. Why Are Cells Small?
 The plasma membrane is where all the action
happens in a cell—so it’s important to have a
high surface to volume ratio.
 The bigger the cell, the smaller this ratio,
meaning there is less plasma membrane to
go around.

10. DIFFUSION
the movement of any
particle from an area
of high concentration
to an area of low
concentration
Diffusion is caused
by ―Brownian motion‖
– the random motion
of particles in any
fluid (gas or liquid).
Concentration of red
is high on the left side
Everything is even—this is
dynamic equilibrium
(low on the
right side)
TIME

11. OSMOSIS
a special name for
diffusion when the
diffusing particle is
water, and it’s going
across a membrane
If the membrane is
selectively permeable
(like the cell
membrane!) then water
can get through when
other molecules can’t.
This can lead to uneven water distribution
on the different sides of a membrane.

12. Cells in Solution
 Isotonic solutions have the same concentration of
water inside and outside. Water passes in and
out, but equally in either direction.
 Hypertonic
solutions have
less water on the
outside. Water
rushes out of the
cell into the salty
or sugary
outside.
 Hypotonic
solutions have
more water
outside. Water
rushes into the
cell.

13.  Plant cells and animal cells react differently to
water concentrations. Animal cells in hypotonic
solution will lyse or burst, and cannot survive in
pure water.
 Plant cells, protected by their cell walls, simply

14. Passive Transport
 Osmosis happens automatically in the cell—
the membrane cannot stop it. It takes no
effort!
 Transport across the membrane with no
energy expended is called passive
transport.
 Larger molecules can
also move passively, but
they must be passed
through transport
proteins that form large
channels in the cell
membrane. This is called
facilitated diffusion.
 Different ions and
molecule types have
different specific

15. Active Transport
 For a molecule to move in the wrong
direction (up the concentration
gradient), the cell must apply energy
(ATP) in a process called active
transport.  Each type of ion
(K+, H+, Cl-,
Na+) has its
own specific
protein ―pump‖
for transport.

16. The Phospholipid Bouncer
 Passes through the membrane freely:
◦ Water (H2O)
◦ Dissolved gases (N2, O2, CO2 )
◦ Small uncharged polar molecules (urea,
alcohol)
 Transported through proteins:
◦ Ions (K+, Na+, Cl-)
◦ Small molecules (neurotransmitters)
 Endocytosis and exocytosis:
◦ Very large polymers (proteins)
◦ Bacteria and viruses

17. The Structure of ATP
 As we saw, active
transport requires the
cell to spend some
energy. The ―energy
currency‖ in a living
cell for interactions like
these is ATP,
adenosine tri-
phosphate.
 ATP is composed of
(a) an adenosine
nucleotide, (b) a ribose
sugar, and (c) three
phosphate groups

18. The ATP Energy Cycle
 When the cell needs some
energy from ATP, it gets it by
breaking a chemical bond—
specifically, the bond between
the second and third
phosphate group.
 Now there are only two
phosphate groups attached, so
it’s no longer tri-phosphate.
Instead, this low energy
molecule is called ADP,
adenosine di-phosphate.
 Luckily, ADP is reusable, and
can go to the mitochondrion in

25. If red blood cells cultured in an isotonic
medium are placed in distilled water,
they will most likely
(A) Remain unchanged
(B) Shrivel
(C) Swell and lyse
(D) Divide
(E) Become dormant

26. If red blood cells cultured in an isotonic
medium are placed in distilled water,
they will most likely
(A) Remain unchanged
(B) Shrivel
(C) Swell and lyse
(D) Divide
(E) Become dormant

27. Some videos
 Diffusion through a membrane:
http://www.youtube.com/watch?v=2Th
0PuORsWY&feature=related
 Diffusion and osmosis discussion
http://www.youtube.com/watch?v=W0
Dm06BsYBI