The Human Cell
Section 3.1 - Objectives
• By the end of this section, you should be able to:
Make a diagram of a cell membrane, showing the two parts of the
phospholipid bilayer, the positions of membrane proteins, cholesterol, and
Discuss the permeability of the lipid bilayer.
List five functions of the membrane proteins.
List five major ways substances cross membranes.
Describe the mechanism of diffusion.
List four factors that affect the rate of movement of substances through
Describe facilitated diffusion. How does it differ from simple diffusion?
Describe active transport. How does it differ from facilitated diffusion?
Define osmosis and describe the factors that affect the movement of water
across membranes. Define osmotic pressure.
Define isotonic, hypotonic, and hypertonic. Describe the effect of such
solutions on biological cells.
Describe how chemical and electrical gradients affect the movement of
molecules across membranes.
Define a resting membrane potential and state its normal polarity and
Describe the forces acting on ions and define equilibrium potential.
State the equilibrium potentials for K+, Na+, and Cl– in a nerve
Describe two functions of the Sodium/Potassium Pump.
Section 3.2 – Introduction
• The cell is so small and so simple in appearance when viewed with the light
microscope that it is difficult to conceive that each cell is a living entity.
Section 3.3 – Basic Cell Organelles
• All cells in the body share similar features and organelles.
Adense body within the cell nucleus
Contains the specific DNAthat produces the RNAfound in ribosomes.
• Golgi apparatus:
Packaging proteins from the rough ER into membrane-bound vesicles.
Two types of vesicles are produced by the Golgi bodies: 1. Secretory vesicles – which transport proteins to the cell membrane
for release into the extra-cell environment.
2. Storage vesicles – such as lysosome, whose contents are stored for
use within the cell.
• Secretory Vesicles:
Produced by the Golgi apparatus and are used to transport various types of
proteins out of the cell for use in other parts of the body.
The process by which the cell releases proteins through the membrane into
the extracellular env’t is called secretion.
• Free Ribosomes:
Are dense granules of RNAand protein.
Manufacture proteins from amino acids under the control of the cell’s
Two types of ribosomes in the cell:
1. Fixed ribosomes:
attached to the ER
2. Free ribosomes:
Float in the cytoplasm.
Often form in groups of 10-20 known as polyribosomes.
One type of storage vesicle produced by the Golgi apparatus.
Act as the digestive system of the cell.
Contain several kinds of enzymes that are used by the cell to destroy
damaged organelles, kill bacteria, and break down other kinds of
Cylindrical bundles of microtubules that are responsible for directing the
movement of DNAstrands during the process of cell division.
• Endoplasmic Reticulum:
Acontinuation of the cell’s nuclear membrane
The site for synthesis, storage and transport of proteins and lipid
Two types of ER:
1. Rough ER or Granular ER:
covered with rows of ribosomes
the site for protein synthesis
2. Smooth orAgranular ER:
Responsible for the synthesis of lipids and fatty acids.
Proteins manufactured here are packaged into vesicles that
transport them to Golgi apparatus.
Most of the body’s adenosine triphosphate (ATP) is generated here. Often called the “powerhouse of the cell” sinceATP is the primary cellular
mechanism for energy storage.
The # of mitochondrion in a cell is determined by that particular cell’s
Replicated themselves even if the cell isn’t undergoing division.
This will occur, for example, when a cell has increased energy
demands over a period of time, such as muscle cells that are
Section 3.4 – The Cell Membrane
• The cell membrane separates the intracellular environment from the extracellular
• Proteins, nucleotides, and other large molecules needed for the structure and
function of the cell cannot penetrate this membrane.
• Other molecules and many ions can penetrate the membrane.
• This is why the cell membrane is selectively permeable – It provides two-way
traffic for nutrients and waste needed to sustain metabolism, while it prevents the
passage of other substances between the intracellular and extracellular
Section 3.5 – Cell Membrane Structure
• The Cell Membrane:
Made up of proteins that form channels and pores, carbohydrate molecules
for cell recognition, and cholesterol for stability.
The phospholipid molecules are the most abundant components of the cell
• Phospholipid Molecule: Hydrophilic Head
Composed of a phosphate head and a lipid tail.
The primary structure of the cell membrane is a double layer of
The hydrophilic heads of the phospholipid molecules that make up the cell
membrane face out into the water base solutions inside and outside of the
• Phospholipid Molecule: Hydrophilic Tail
The tails are oriented away from the aqueous and extracellular and
intracellular solutions into the cell membrane
• Cholesterol Molecule:
Found inserted into the non-polar lipid layer of the membrane.
Helps make the membrane impermeable to some water soluble molecules.
Helps to keep the membrane flexible over a wide temp range.
• Associated Protein: Enzyme Associated proteins can be attached to either the intracellular or
extracellular surface of the membrane.
Enzymes are a form of associated protein which acts as catalysts for
certain reactions immediately inside or outside the membrane.
• Carbohydrate Molecule:
Associated with extracellular membrane proteins or lipids.
Form a protective layer called the glycocalyx which plays a key role in the
immune response of the cell and in recognition of other cells in the body.
• Membrane Spanning Protein:
Embedded in the phospholipid bilayer
Span the entire width of the membrane
Act as gates or channels that control the movement of certain substances
into and out of the cell.
• Associated Protein: Structural
Attached to the inside surface of the cell membrane.
Support and strengthen the membrane while others may anchor some cell
organs to the intracellular side of the membrane.
Section 3.6 – Phospholipids
Made up of a phosphate "head" and fatty acid (or lipid) "tails."
The fatty acid tails of a phospholipid molecule are hydrophobic (they do
not like water)
The phosphate heads are hydrophilic (they like water).
When many phospholipids are thrown into water, they will align
themselves into a lipid bilayer so that the head groups all face out toward
the water and the tails away from the water - This is why they are arranged
in this way in the cell membrane.
Since the fatty acid tails are hydrophobic, they are the major barrier to
water and water-soluble substances (anything that dissolves in water) such
as ions, glucose, urea, and most of the other molecules found in living
Fat-soluble substances like oxygen, carbon dioxide, and steroid hormones
can penetrate this portion of the membrane with ease since they can
"dissolve" through the lipid region of the membrane. Section 3.7 – Membrane Proteins
• The other important components of the cell's membrane are the proteins.
Membrane proteins have many different functions, including the following:
For the attachment of chemical hormones and
That help with chemical reactions or breakdown molecules
3. Ion channels or pores:
That allow water-soluble substances, like ions, into the cell
4. Membrane-transport carriers:
That transport molecules across the membrane (this may
include gated channels)
5. Cell-identity markers:
Like antigens or glycoproteins.Antigens are foreign
particles that can stimulate the immune system. Section 3.8 – Membrane Proteins (cont.)
• One of the most important functions of the proteins is to transport substances
across the membrane. Let's have a look at the different ways substances cross the
membrane—some that require proteins and some that do not. Membrane-
transport mechanisms include the following:
1. Endocytosis/exocytosis (pinocytosis for small molecules)
2. Diffusion through the lipid bilayer (in the case of fat-soluble
3. Diffusion through protein channels (in the case of water and water-
4. Facilitated diffusion
5. Active transport
• Video: this is a cell that secretes large molecules that are unable to pass through
the lipid bilayer of the plasma membrane. They leave the cell through Exocytosis.
After production in a system of membranes called the ER the molecules are
packaged in a small sack of membrane called a vesicle. The vesicle moves to a
stack of membranes called the GA. Their membranes merge and the vesicle
releases its contents for modification.As the molecules leave the GAthey are
again packaged in a vesicle which moves to the plasma membrane.As membranes
merge again the contents leave the cell without actually crossing the plasma
membrane. This process can also occur in reverse allowing large molecules to
enter a cell – this is called Endocytosis.
Section 3.9 – Diffusion
• Diffusion: Diffusion is the movement of molecules from an area of high
concentration to low concentration due to the molecules' random thermal
• Consider the following example:
When a drop of dye is added to a glass of water, the dye molecules will be
localized to an area of high concentration.
These molecules of dye are constantly moving in a random manner,
bumping into each other and the water molecules.
The dye will slowly spread out from the area of high concentration to the
area of lower concentration down what is called the dye's chemical
This concentration gradient is much like a ski hill; a skier would move
down the hill from high elevation to low elevation down the elevation
The dye will continue to move until its concentration is uniform
throughout the glass of water, at which point there is no more
concentration gradient and the net movement is zero.
At this point, the dye reaches chemical equilibrium and net diffusion is
zero, although the dye and water molecules are still randomly moving
Section 3.10 – Diffusion (cont.)
• Electrically charged molecules, including ions like sodium ions (Na ), tend to
move toward areas of opposite charge; that is, positively charged ions move
toward negatively charged areas (and vice versa) down their electrical gradient.
• Therefore, charged ions can move down both their chemical concentration
gradient and electrical gradient.
• If the chemical and electrical gradients are in opposite directions, the movement
of the ion will depend on the balance of the two gradients and will stop moving
when the molecules reach electrochemical equilibrium (when the electrical force
is equal to and in opposite direction to the chemical force).
• Like charges repel; opposite charges attract
Section 3.11 – Diffusion of Lipid-Soluble Substances
• Substances that are lipid soluble can pass right through the cell membrane, while
those that are water soluble have a tougher time.
• Lipid-soluble (or fat-soluble) substances include oxygen, carbon dioxide, fatty
acids, and some steroid hormones.
• These molecules can diffuse right through the membrane's lipid bilayer and are
not stopped by the hydrophobic fatty acid chains.
Section 3.12 – Diffusion of Water-Soluble Substances • Substances that are water soluble cannot diffuse directly through the fatty acid
region of the cell membrane but may still cross the membrane.
• Some of these subst+nces, like water and many ions, for example, Na and
potassium ions (K ), appear to cross cell membranes through special protein
channels or pores.
• Each pore or channel is quite specific and will generally allow only one type of
Section 3.13 – Diffusion Factors
• The rate of movement of molecules through protein channels is limited by several
1. The size of protein channels, which is apprx 0.8nm, will limit the
size of the molecule. Sugar molecules, for example, are too large to
diffuse through protein pores.
2. The charge on the molecule (for example, Na+) will affect the
rate of movement through channels because the proteins that make
up the channels also have charges on them. Therefore, a + ion will
not got through a channel that has a + charge.
3. The greater the electrochemical gradient of a molecule, the
greater its rate of movement through the channels. Substances
move down both their [ ] and electrical gradients (recall: opposite
4. The number of channels in the membrane affects the rate. Even
if there is a large [ ] gradient for an ion, that ion will not move
across the membrane unless there are channels for it. The more
channels that exist, the more ions that will diffuse across the
Section 3.14 – Facilitated Diffusion
• Other water-soluble substances (such as sugars) that cannot diffuse through the
lipid bilayer and are too large to pass through protein channels still cross the
membrane at a relatively fast rate.
• These molecules attach to specific protein carriers on the membrane and cause a
change in the protein's shape.
• The result is either an opening of the protein channel through which the molecule
passes, or the protein rotates the molecule to the inner surface of the membrane
where it is released.
Section 3.15 – Facilitated Diffusion (cont.) • The process of facilitated diffusion is similar to simple diffusion in that it does not
require energy and it is powered by the concentration gradient of the molecule.
• It differs from simple diffusion because the rate of transport is limited by the
number of available proteins.
• Once the carriers are all occupied, the system becomes saturated and cannot
operate any faster.
• The speed at which the carrier can change shape or configuration is also limited;
once all the carriers are working and occupied, they are said to be saturated.
• Facilitated diffusion shows chemical specificity (a given carrier protein will
interact only with a specifically shaped molecule) and may be competitively
inhibited by molecules that are very similar in shape.
• The figure below shows another type of facilitated diffusion:
Section 3.16 –Active Transport
• Like facilitated diffusion, active transport requires protein carriers that span the
• This transport mechanism, like facilitated diffusion, can be saturated, shows
chemical specificity, and shows competitive inhibition.
• Unlike facilitated diffusion, however, active transport involves the use of energy.
This is because active transport moves molecules up their concentration gradients
from low concentration to high concentration.
• An example of active transport is the sodium-potassium pump: The energy comes from splitting adenosine triphosphate (ATP) to
adenosine diphosphate (ADP) and inorganic phosphate (PE). The
consequent release of energy powers the carrier movement.
• Video: The substance may