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Chapter 3
The Cellular Level of
Organization
Lecture slides prepared by Curtis DeFriez, Weber State University
The Cell
• A cell is the basic unit of all living things.
– Prokaryotic cells are simple cells that are Pro
(“before”) karyon (“nucleus”) – they have no nucleus.
• Most are unicellular bacteria.
– Eukaryotic cells are complex cells with a nucleus and
subcellular structures (organelles).
• All fungi, plants, and animals are eukaryotes.
A Generalized Cell
• All eukaryotic cells are composed of three main parts:
1. Plasma membrane or “plasmalemma”
2. Cytoplasm - a gelatin-like substance, plus structural fibers
and organelles (but
not the nucleus)
3. Nucleus - contains the
genetic library of the cell
A Generalized Cell
1. The plasma membrane forms the cell’s outer
boundary and separates the cell’s internal
environment from the outside environment.
– It is a selectively permeable barrier,
allowing the passage of some
things and not others.
– It plays a role in
cellular communication.
A Generalized Cell
2. The cytoplasm contains all the cellular
contents between the plasma membrane and the
nucleus.
– The cytosol is the fluid portion (mostly water).
– Organelles are subcellular
structures embedded
in the cytosol, having
characteristic shapes
and specific functions.
A Generalized Cell
3. The nucleus is a large organelle that contains
DNA in molecules called chromosomes.
– Each chromosome consists of
a single molecule of DNA and
associated packaging proteins.
– A chromosome contains
thousands of hereditary
units called genes.
A Generalized Cell

Fig. 3.1 shows a generalized body cell labeled with the
plasma membrane, cytoplasm (and organelles) and nucleus.
The Plasma Membrane
• The plasma membrane is much more than just a
“fence” – it is a flexible yet sturdy, “intelligent”
semipermeable regulator that:
– Covers and protects the cell
– Controls what goes in and comes out
– Links to other cells
– Flies certain “flags” to tell
other cells “who” it is
The Plasma Membrane
• The Fluid Mosaic Model describes the
arrangement of molecules within the membrane:
They resemble a sea of phospholipids with
protein “icebergs” floating in it.
– The lipids act as a barrier to certain polar substances.
– The proteins act as
“gatekeepers”, allowing
passage of specific
molecules and ions.
The Plasma Membrane
Interactions Animation
• Membrane Functions
You must be connected to the internet to run this
animation.
The Plasma Membrane
• The structure of the membrane
– Phospholipids form a lipid bilayer - cholesterol and
glycolipids (sugar-lipids) also contribute.
– Integral proteins - extend into or through the
bilayer.
• Transmembrane proteins (most integral proteins) span
the entire lipid bilayer.
• Peripheral proteins attach to the inner or outer surface
but do not extend through the membrane.
The Plasma Membrane
The Plasma Membrane
• Glycoproteins are membrane proteins with a
carbohydrate group attached that protrude into
the extracellular fluid.
• The Glycocalyx is the entire
“sugary coating” surrounding
the membrane (made up of
the carbohydrate portions
of the glycolipids and
glycoproteins).
The Plasma Membrane
• The Functions of the membrane
– Some integral proteins are ion channels.
– Transporters - selectively move substances through
the membrane.
– Receptors - for cellular recognition; a ligand is a
molecule that binds with a receptor.
– Enzymes - catalyze chemical reactions
– Others act as cell-identity markers.
The Plasma Membrane
Examples of different membrane proteins include
 Ion channels
 Carriers
 Receptors
The Plasma Membrane
Examples of different membrane proteins include
 Enzymes
 Linkers
 Cell identity markers
The Plasma Membrane
• Because of the distribution of lipids and the
proteins embedded in it, the membrane allows
some substances across but not others; this is
called Selective permeability
– Rule of thumb: small, neutrally-charged, lipidsoluble substances can
freely pass. Water is a
special case - it is highly polar,
yet still freely
permeable.
courtesy of Dr. Jim Hutchins
Membrane Permeability
• For those substances that are needed by the cell
but for which the membrane is impenetrable
(impermeable), transmembrane proteins act as
channels and transporters.
– They assist the entrance of certain substances that
either can’t pass at all (glucose) or for which the cell
needs to hasten passage
(ions).
Transport Processes
• Passive processes involve substances moving
across the cell membranes without the input of
any energy - they are said to move “with” or
“down” their concentration gradient ([gradient] ,
where [ ] indicates “concentration”).
• Active processes involve the use of energy,
primarily from the breakdown of ATP, to move a
substance against its [gradient].
Transport Processes
• Passive processes
– Diffusion of solutes
– Diffusion of water (called osmosis)
– Facilitated diffusion (requires a specific channel or a
carrier molecule, but no energy is used)
• Active processes
– Various types of transporters are used, and energy is
required.
Passive Transport Processes
• Diffusion is the passive spread of particles through random
motion, from areas of high concentration to areas of low
concentration.
– It is affected by the amount of
substance and the steepness of
the concentration gradient.
– Temperature
– Surface area
– Diffusion distance
Passive Transport Processes
• Channel-Mediated Facilitated Diffusion
• Carrier-Mediated Facilitated Diffusion
Passive Transport Processes
• An example of Channel-Mediated Facilitated Diffusion
is the passage of potassium ions through a gated K+
Channel
• An example of Carrier-Mediated Facilitated Diffusion is
the passage of glucose across the cell membrane.
•
Passive
Transport
Processes
Osmosis is the net movement of water through a
selectively permeable membrane from an area of high
water concentration to one of lower water concentration.
• Water can pass through plasma membrane in 2 ways:
– through lipid bilayer by simple diffusion
– through aquaporins (integral membrane proteins)
Passive Transport Processes
In the third tube, the force generated by the movement of
water from the left to the right side is called osmotic
pressure.
Passive Transport Processes
In the body, tonicity refers to the concentration of salt
solutions in the blood and elsewhere. Since semipermeable
membranes separate these fluid compartments, osmosis of
water is free to occur between
any fluid space and another.
The effect of changing tonicity
is demonstrated in this graphic,
as water moves in and out
of red blood cells.
Active Transport Processes
Solutes can also be actively transported across a plasma
membrane against their concentration gradient ([low] to
[high]) by using energy (usually in the form of ATP).
The sodium-potassium pump is found in all cells.
Secondary Active Transport Mechanisms
• Antiporters carry two substances across the membrane in
opposite directions.
• Symporters carry two substances across the membrane in the
same direction.
Transport in Vesicles
 Vesicle - a small spherical sac formed by budding off from a
membrane
 Endocytosis - materials move into a cell in a vesicle formed from the
plasma membrane
three types: receptor-mediated endocytosis
phagocytosis
bulk-phase endocytosis (pinocytosis)
 Exocytosis - vesicles fuse with the plasma membrane, releasing their
contents into the extracellular fluid
 Transcytosis - a combination of endocytosis and exocytosis
ReceptorMediated
Endocytosis
Phagocytosis
Bulk-phase Endocytosis
Transport
• Transport Across the Plasma Membrane
Interactions Animation
You must be connected to the internet to run this animation.
Transport
Interactions Animations
• Transport Across the Plasma Membrane
You must be connected to the internet to run this
animation.
Cytoplasm - 2 Components
1. Cytosol - intracellular fluid, surrounding the organelles
- The site of many chemical reactions
- Energy is usually released by these reactions.
- Reactions provide the building blocks for cell maintenance,
structure, function and growth.
2. Organelles
- Specialized structures within the cell
The Cytoskeleton
• Network of protein filaments throughout the
cytosol
• Provides structural support for the cell
The Cytoskeleton
• Types
– Microfilaments
– Intermediate
filaments
– Microtubules
Organelles
Centrosome - located near the nucleus, consists
of two centrioles and pericentriolar material
Organelles
• Cilia - short, hairlike projections from
the cell surface,
move fluids along a
cell surface
• Flagella - longer
than cilia, move an
entire cell; only
example is the sperm
cell’s tail
Organelles