Cell Membrane &
Homeostasis
DEFINITIONS:



Diffusion: movement of molecules from region
of high concentration to low concentration
Diffusion Gradient: the concentration
spectrum (difference) of solute molecules from
high concentration to low concentration.
Osmosis: diffusion of water molecules across
a membrane from high water amounts (low
solute) to low water amounts (high solute).
DEFINITIONS:

Cell membranes are completely permeable to
water.


The environment the cell is exposed to can have a
dramatic effect on the cell.
Solute: a dissolved molecule in water.

Eg. Sodium chloride dissolved in water makes a
saline solution. The sodium chloride is the solute.
The water is the solvent.
DEFINITIONS:


Common cell solutes include salts, sugars, some
minerals (iron ions and calcium ions) and protons
(electrons from acids).
CONCENTRATION: amount of solute per unit
volume of solution. Concentration can be expressed in
mass/volume (g/100ml - percentage), ppm (parts per
million), and moles/volume (molarity). The greater the
mass or moles per unit volume, the more concentrated
the solution.
3 Osmotic Solution Terms
Isotonic

Same concentration of solute surrounding a
cell as inside the cell.
When a cell is placed in an isotonic solution, the
water diffuses into and out of the cell at the same
rate.
 The fluid that surrounds the body cells is said to be
“isotonic”.

Hypertonic

The surrounding solution contains a higher
concentration of solute relative to the cell.
When a cell is placed in a hypertonic solution, the
water diffuses out of the cell attempting to match
the solute concentration outside of it, causing the
cell to shrivel.
 The fluid surrounding the body cell is said to be
“hypertonic”.

Hypotonic

The surrounding solution contains a lower
concentration of solute relative to the cell (e.g.
the cell's cytoplasm).
When a cell is placed in a hypotonic solution, the
water diffuses into the cell in an attempt to dilute the
solutes inside the cell, causing the cell to swell and
possibly explode in animal cells.
 Plant cells have a strong cell wall that prevents
explosions.
 Plant cell central vacuoles will fill to maximum and
push against the cell wall – this is called high turgor
pressure.

CELL MEMBRANE FUNCTION
and STRUCTURE

The CELL MEMBRANE is chiefly responsible
for maintaining homeostasis inside a living cell
using different methods to transport molecules
in and out of the cell.
Too much water can burst the cell
 Too many wastes can poison the cell
 The cell cannot tolerate any great variations in ion
conditions.
osmosis animation

Jobs of the cell membrane
1.
2.
3.
4.
Isolate the cytoplasm from the external
environment
Regulate the exchange of substances (gases
and ions)
Communicate with other cells
Identification (proteins and carbohydrates on
its surface)
http://www.goldiesroom.org/AP%20Biology/AP%20Lecture%20Notes%20pdf/LN014--Ch05--Cell%20Transport.pdf
http://www.goldiesroom.org/AP%20Biology/AP%20Lecture%20Notes%20pdf/LN014--Ch05--Cell%20Transport.pdf
DESCRIPTION



The fluid mosaic model (S.J Singer)
selectively-permeable: allows some substances
in all the time, some only when needed, excludes
others, allows one-way flow of some.
Fluid portion is a double layer of phospholipids,
called the phospholipid bilayer.
large transport proteins, oligoproteins and
oligosaccharides aid in transport
energy is required from the cell
 aid in communication as well as identification

Phospholipid bilayer




Phospholipids contain a hydrophilic head and
a non-polar hydrophobic tail
Hydrogen bonds form between the
phospholipid “bilayer” and the watery
environment inside and outside of the cell.
Hydrophobic (water fearing) interactions force
the “tails" to face inward.
Phospholipids are not bonded to each other,
which makes the double layer fluid.

Cholesterol embedded in the membrane makes it
stronger and less fluid.
http://www.goldiesroom.org/AP%20Biology/AP%20Lecture%20Notes%20pdf/LN014--Ch05--Cell%20Transport.pdf

The different components of a plasma
membrane are integral proteins, peripheral
proteins, glycoproteins, phospholipids,
glycolipids, and in some cases cholesterol, and
lipoproteins.

Construction of the Cell Membrane - Learning
Activity
detailed cell membrane animation

Proteins Embedded in Membrane
Serve Different Functions

Transport Proteins


Channel Proteins


regulate movement of substance
form small openings for molecules to diffuse
through like water
Carrier Proteins

binding site on protein surface "grabs" certain
molecules and pulls them into the cell animation

Gated Channels
 similar to carrier proteins, not always
"open"—eg. Bind and pull in calcium ions
when needed. This requires cell energy—
active transport.

Receptor Proteins
 molecular triggers that set off cell responses
(such as release of hormones or opening of
channel proteins)
e.g. The junction
between nerve cells
requires the
transmission of
neurotransmitters
between synaptic
gaps—these chemicals
bind onto receptor
proteins.

Recognition Proteins - ID tags, to identify
cells to the body's immune system (called
antigens)
TRANSPORT MECHANISMS
1.
2.
PASSIVE TRANSPORT
ACTIVE TRANSPORT
1.
2.
ENDOCYTOSIS
EXOCYTOSIS
Types of Cellular Transport

Passive Transport
cell doesn’t use energy
1.
2.
3.

Diffusion
Facilitated Diffusion
Osmosis
Weeee!!!
high
Active Transport
cell does use energy
1.
2.
3.
Protein Pumps
Endocytosis
Exocytosis
low
This is
gonna be
hard
work!!
high
•Animations of Active &
Passive Transport
low
1. Passive Transport (p. 198)


Simple Diffusion - water, oxygen and other
molecules move from areas of high
concentration to areas of low concentration,
down a concentration gradient.
Note – Osmosis is the diffusion of water
Diffusion animation
Passive Transport Animation
1. Passive Transport (cont’d)

Facilitation Diffusion - diffusion that is
enabled by proteins (channel or carrier
proteins) which bind onto required molecules so
that they flow into the cell.
Animation: How Facilitated Diffusion Works

Contractiles Vacuoles are found in freshwater
microorganisms - they pump out excess water.
Recall:
 Turgor pressure occurs in plants cells as their
central water vacuoles fill with water.
Factors Affecting Rate of
Diffusion
1. Size


small molecules can slip through
phospholipids bilayer easier than large
molecules
very large molecules may not be able to diffuse
at all
2. Concentration

the greater the concentration gradient (bigger
range) the quicker a material diffuses (makes the
molecules want to move faster) – think of a
crowded room
3. Temperature

In general as temperature increases – molecules
move faster which translates into faster diffusion
4. Polarity of molecules

Water-soluble (polar) molecules will not easily
move through the membrane because they are
stopped by the middle water-insoluble
(nonpolar) layer
5. Surface Area

As a cell’s size increases its volume increases much
quicker than it’s surface area.


If cell size is doubled, it would require 8 times more
nutrients and have 8 times s much waste. SA only
increases by a factor of 4 – not enough surface area
through which nutrients and wastes could move.


If you double individual lengths (1 cm to 2 cm) the surface
areas increases 4 times, and the volume increases 8 times.
Cell would either starve or be poisoned (waste products)
Cells divide before they come too large to function.
2. Active Transport (p. 199)

Involves moving molecules "uphill" against the
concentration gradient, which requires energy.
Uses carrier protein molecules as receptors.
 One may transport calcium ions another glucose
molecules.
 There are hundreds of these types of protein
molecules.
*Each one changes shape to accommodate a specific
molecule.

2. Active Transport (cont’d)

Their activity can be stopped from transporting
molecules with inhibitors (unfortunately, these
are usually poisons) which:


either destroy the membrane protein
or just plug it up
(e.g. for your neurons – tetanus &
botulinum-B secrete a poison that
suppress the Na/K pump)

active transport animation
http://www.biology4kids.com/files/cell2_activetran.html
Sodium-Potassium Pump

Pumps out 3 sodium atoms for ever 2 potassium
atoms taken in against gradient in the cell.
ATP and the Na/K Pump
Animation: How the Sodium Potassium Pump Works
The H+/K+ ATPase


The parietal cells of your stomach (lining) use this
pump to secrete gastric juice.
These cells transport hydrogen ions (H+) from a
concentration of about 4 x 10-8 M within the cell to a
concentration of about 0.15 M in the gastric juice
(giving it a pH close to 2).


Recall: pH – power of the H+ ion
Small wonder that parietal cells are stuffed with
mitochondria and use huge amounts of energy as they
carry out this three-million fold concentration of
protons.
The H+/K+ ATPase
Ca2+ ATPases



In resting skeletal muscle, there is a much higher
concentration of calcium ions (Ca2+) in the
sarcoplasmic reticulum (__) than in the cytosol
(_________).
Activation of the _______ fiber allows some of this
Ca2+ to pass by fascilitated diffusion into the cytosol
where it triggers contraction. After contraction, this
Ca2+ is pumped back into the sarcoplasmic reticulum.
This is done by a Ca2+ ATPase that uses the energy
from each molecule of ATP to pump 2 Ca2+ ions.
Exocytosis


Moves large, complex molecules such as
proteins out of the cell membrane.
Large molecules, food, or fluid droplets are
packaged in membrane-bound sacs called
vesicles.
Endocytosis

Endocytosis moves large particles (huge
molecules or molecular conglomerates) into a
cell.

endo & exocystosis animations
Phagocytosis
Phagocytosis is another type of endocytosis used for
massive transport.
Cell membrane extends out forming pseudopods
(fingerlike projections) that surround the particle.
 Membrane pouch encloses the material & pinches off
inside the cell making a vesicle.
 Vesicle can fuse with lysosomes(digestive organelles)
or release their contents in the cytoplasm
Animation: Phagocytosis
HowStuffWorks "Phagocytosis"


Used by ameba to feed & white blood cells to
kill bacteria.
Known as “killer cells"

Pinocytosis is another type of endocytosis



Cell membrane surrounds fluid droplets
Fluids taken into membrane-bound vesicle
Known as “cell drinking”
•Exocytosis is used to remove
large products from the cell such
as wastes, mucus, & cell products
such as hormones and
antibodies.
•Exocytosis is the process used
by our memory cells (white
blood cells that produce
antibodies to fight infection).
•It is also used by our gland cells
to secrete hormones when
needed.
phagocytosis

animation
In Summary
Essential Biochemistry - Membrane Transport
Transport Flowchart
Transport of Materials
Across a Membrane
Active
Ion Endocytosis
Exocytosis
Pump
Pinocytosis Phagocytosis
Passive
Simple
Osmosis Facilitated
Diffusion
Diffusion