Chapter 3: Cells – The Living
Units
Cellular Basis of Life
• Robert Hooke – late 1600s – observed plant
cells
• 1830s Schleiden & Schwann – concluded all
living things are composed of cells
• Rudolf Virchow – found – all cells come from
preexisting cells
1800s- Cell Theory
1. Cell is the basic structural and functional unit of
living organisms
2. Activity of organism – depends on individual and
collective activities of cells
3. Principal of complementary structure and
function – biochemical activities of cells are
dictated by relative numbers of subcellular
structures
4. Continuity of life from one generation to
another has a cellular basis
Human Body
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Trillions of cells
200 different cell types
Size ranges from 2 µm to 1 m
Structural unit of all living things
All cells composed of C, N, O, and other trace
elements
• All cells have same basic parts (generalized or
composite cell)
Human Cells
• All have 3 main parts
1. Plasma membrane – outer boundary
2. Cytoplasm – intracellular fluid packed with
organelles
3. Nucleus – controls cellular activates
Plasma Membrane
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Also called the cell membrane
Flexible
Defines the extent of cell
Separates intracellular fluid from extracellular
fluid
Fluid Mosaic Model
• Thin (7-10 nm) structure
• Composed of double layer – bilayer of lipid
molecules with protein molecules dispersed in
it
Membrane Lipids
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Phospholipids –
Polar head – hydrophilic – “water loving”
Non-polar tail – hydrophobic – “water fearing”
Polar heads are attracted to water, lie on inner and out
surfaces
• Sandwich like structure – heads on outside, tails on inside
• Majority of tails – unsaturated – have kinks and increase
membrane fluidity
Membrane Lipids
• Glycoproteins - lipids with sugars attached
– Only on outer membrane surface
– 5 % of total membrane
• Cholesterol – 20 % of total membrane
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Polar region hydroxyl group
Nonpolar region – fused ring
Stabilizes membrane
Increases mobility of phospholipids
• Lipid Rafts – 20 % of outer membrane
– Assemblies of saturated phospholipids
– Packed tightly together
– Platforms for receptor molecules and cell signaling
Membrane Proteins
• Make up approximately 50 % of membrane
• Responsible for specialized function
• 2 kinds of proteins
– Integral proteins
– Peripheral proteins
Membrane Proteins – Integral Protiens
• Inserted into membrane
• Some stick out only on one
side
• Others span entire membrane
– transmembrane proteins
– Hydrophobic and hydrophilic
regions
– Most involved in transport
– Form pores or channels
• Others – carriers – bind
substances and move them
into membrane
• Also receptors – relay
messages to interior of cell
Membrane Proteins – Peripheral
Proteins
• Not embedded in membrane
• Attached loosely to integral proteins
• Include filaments that support
membrane
• Some are enzymes
• Others – motor proteins – change
cell shape during division and cell
contraction
• Glycocaylx – “sugar coating” sugars
attached to proteins
• Highly specific biological markers –
allow cells to recognize each other
Membrane Junctions
• 3 factors that bind cells together
1. Glycoproteins in glycocaylx acts as adhesive
2. Wavy contours of membrane of adjacent cells fit together –
tongue and groove
3. Specialized membrane junctions
a. tight junction – integral proteins of adjacent cells fuse
together, prevent molecules from passing
b. desmosomes – anchoring junctions, mechanical couplings
c. gap junctions – nexus cells connected by hallow cylinders –
connections (transmembrane proteins), present in electrically
excitable tissue
Membrane Transport
• Extracellular fluid – interstitial fluid
• Rich, nutritious “soup”
• Amino acids, sugars, fatty acids, vitamins,
hormones, salts, wastes, etc.
• Plasma Membrane – selectively permeable
• Allows some substances to pass, but NOT others
• Passive Processes – substances cross without
energy input
• Active Process – substances need energy input to
cross
Transport - Passive
• Diffusion – movement of molecules from an area of
high concentration to an area of low concentration
• Down the concentration gradient
• Kinetic energy of molecules moves them
• Speed of diffusion – influenced by: size – smaller the
molecules are the faster they move
• Molecules move until equilibrium is reached (no net
movement)
• Plasma membrane – physical barrier – but molecule will
diffuse if it is–
1. Lipid soluble
2. Small enough to pass through channels
3. Assisted by carrier
Transport - Passive
• Simple Diffusion – nonpolar and lipid soluble
substances diffuse directly though membrane
• Oxygen, carbon dioxide, fat-soluble vitamins,
etc,
Transport - Passive
• Facilitated Diffusion – molecules that can not pass
through the membrane by themselves
• Transported with the help of a protein
• Substance can binds the carrier protein in membrane
• Substance may also move through water filled
channels
• Carriers – integral proteins – allow substances to pass
through membrane
• Channels – transport proteins
• Transport water or ions through aqueous channels
Transport - Passive
• Osmosis – diffusion of solvent (water) through
a selectively permeable membrane
• Aquaporins (APQs) water specific channels in
the membrane
• Moves down the concentration gradient
• Also depends on the concentration of solutes
• Osmolarity – total concentration of all solute
particles in a solution
Transport Passive
• Osmosis cont –
• Water diffuses until hydrostatic pressure (back
pressure exerted by water against the
membrane) with in the cell is equal to its
osmotic pressure (tendency of water to move
into cell by osmosis)
• Tonicity – change in shape or tone of cells by
altering internal water volume
Solutions
• Isotonic – cells with the same concentration
of solutes on the inside and outside
• Hypertonic – solution has a higher
concentration than the inside of the cell
– Water moves out, cells shrinks
• Hypotonic – solution is more dilute than
inside of cell
– Water moves in, cell swells
Active Transport Processes
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Active Transport – requires protein
Combines specifically and reversibly with transported substances
Solute pumps – move solutes against the concentration gradient
Requires the input of energy
Symbort System – 2 substances transported the same way (both
inside or both outside)
• Antiport System – 2 substances transported opposite ways (one
inside and the other outside)
Primary Active Transport
• Hydrolysis of ATP  phosphorylation of
transport protein
• Protein changes shape – pumps solute across
membrane
• Ex. Na+-K+ pump – Na+K+ ATPase – drives
sodium out of cell and potassium in
Secondary Active Transport
• Single ATP powered pump indirectly drives
secondary active transport
• Na moves back into cell, another substance is
cotransported with it
• Ex. Sugar, amino acids, ions, etc
Vesicular Transport
• Fluids containing large particles and
macromolecules are transported in
membranous sacs – vesicles
• Exocytosis – process that ejects substances
from interior of cell
Vesicular Transport
• Endocytosis – process that moves substances into the cell
• Substances moved in by the infolding of the membrane – coated pit –
clathrin – protein coating, then vesicle detaches
• Phagocytosis – cells engulfs large solid material
– Bacteria, debris, etc
– Endocytic vesicle – phagosome
– Amoeboid motion – flowing of cytoplasm into temporary pseudopods
• Pinocytosis – fluid phase endocytosis
– Plasma membrane surrounds small volume of fluid containing dissolved
material
• Receptor mediated endocytosis – plasma membrane binds only certain
substances
Vesicular Transport
• Other protein coats –
• Caleolae – tubular or flask shaped
inpocketings of plasma membrane
• Coatomer (COP1 & COP2) proteins – vesicular
trafficking
• Transport substances between organelles
Plasma Membrane – Membrane
Potential
• Membrane Potential – voltage – electrical
potential energy resulting from separation of
oppositely charged particles
Plasma Membrane – Membrane
Potential
• Resting State – resting membrane potential – range -50
- -100mV
• Cell said to be polarized
• (-) indicates inside is negative compared to the outside
• Diffusion – causes ionic imbalances that polarize the
membrane, active transport maintains membrane
polarization
• Ions – K+ and protein – inside cell
• Na+ and Cl- outside cell
• Membrane somewhat permeable to K+ - leaky
channels
• Protein anions cannot follow
Plasma Membrane – Membrane
Potential
• Membrane becomes negative = -90 mV
• Na+ also a factor – attracted to cell interior –
bring membrane to -70 mV
• Active transport – depends on diffusion
• More Na+ in, the more is pumped out
• Na+/K+ pump – 3 Na out for 2 K in
• Electrochemical gradient – electrical and
concentration (chemical) forces
Cell-Environment Interactions
• Cell Adhesion Molecules (CAMs)
• Key role in embryonic development, wound
repair, and immunity
• Sticky glycoproteins
Cell-Environment Interactions
• Functions –
1. Molecular “Velcro” cells use to anchor themselves to
molecules in extracellular space and to each other
2. The “arms: that migrating cells use to haul themselves
past one another
3. SOS signals – sticking out from blood vessels lining
that rally WBC to infected or damaged area
4. Mechanical sensors – respond to tension at cell
surface by stimulating synthesis or degradation of
adhesive membrane junction
5. Transmitters of intracellular signals that direct cell
migration, proliferation, and specialization
Roles
• Membrane receptors – integral proteins and
glycoproteins that serve as binding sites
1. Cell Signaling – coming together and
touching of cells
- Cells recognize each other
- Essential for normal development and
immunity
Roles
2. Chemical Signaling –
- Ligands – signaling chemicals, bind to specific plasma
membrane receptors
- Include – neurotransmitters, hormones, paracrines
- G-Protein – linked receptors – exert effects through Gprotein
- Second messengers are generated and connect plasma
membrane events to internal
- Ex. Cyclic AMP – Ca2+
- Activates a protein kinase cascade
- Nitric oxide (NO) – another messenger – important
signaling molecule
Cytoplasm
• “cell forming’ material
• Cell material between the plasma membrane
and the nucleus
• 3 major elements – cytosol, organelles, and
inclusions
Cytoplasm
1. Cytosol
• Viscous semitransparent fluid
• Colloid and solution properties
• Dissolved (in water) – protein, sugar, salts, and solutes
2. Organelles • Carry out specific functions – synthesize proteins,
package proteins, etc.
• Chemical substances not always present
3. Inclusions • Stored nutrients, lipid droplets, pigment, water
containing vacuoles, crystals of various types
Organelles
• “little organs”
• Nonmembranous – lack membranes
• Membranous – with membranes
Mitochondria
• Lozenge-shaped membranous organelles
• Power plants of cells
• 2 membranes –
– Outer – smooth and featureless
– Inner – folds inward forming cristae
• Gel-like substance inside
• Food particles – glucose – broken down into water and
carbon dioxide by enzymes
• Metabolites broken down and oxidized – energy
released and captured
• Aerobic cellular respiration
Mitochondria
• Contain their own DNA, RNA, and ribosomes
• Can reproduce themselves
• 37 genes – direct synthesis of 1% of proteins
needed
• Similar to bacteria (purple bacteria phylum)
• Believed to have arose from bacteria that
evaded plant and animal cells
Ribosomes
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Small dark staining granules
Protein and a variety of RNA – ribosomal RNA
Site of protein synthesis
Some float freely
Others attached to rough ER
Free ribosomes – make soluble proteins
Rough ER – make proteins destined for cell
membranes or export
Endoplasmic Reticulum (ER)
• “network” with in cytoplasm
• Intracellular connected tubes and parallel
membranes enclosed fluid filled cavities or
cisternae
• Continuous with the nuclear membrane
• Half of cells total membrane
• 2 types – smooth and rough
Rough Endoplasmic Reticulum (RER)
• Surface covered with ribosomes
• Proteins from ribosomes – enter and are
modified
• “Membrane factory”
Smooth Endoplasmic Reticulum (SER)
• Continuous with the rough ER
• No role in protein synthesis
• Involved in:
1. Lipid metabolism, cholesterol synthesis, synthesis of
lipid components – of lipoproteins
2. Synthesis of steroid hormones (sex hormones)
3. Absorption, synthesis, and transport of fats
4. Detox of drugs, pesticides, and carcinogens
5. Breakdown of stored glycogen into free glucose
- Muscle – also stores calcium
Golgi Apparatus
• Stacked and flattened membranous sacs
• Traffic director for cellular proteins
• Modify, concentrate and package proteins and
lipids
• Secretory vesicles or granules migrated to
plasma membrane and discharge contents
Lysosomes
• Inactive digestive enzymes
• Cells demolition crew
1. Digest particles taken in by endocytosis, particularly
bacteria, viruses, toxins
2. Degrade worn-out or nonfunctional organelles
3. Metabolic functions – glycogen breakdown and
release
4. Breakdown of nonuseful tissues – uterine lining
during menstruation
5. Breaking down of bone to release calcium into blood
- Autolysis – lysosome rupture and cell digests itself
Endomembrane System
• System of organelles that work together to:
1. Produce, store, and export biological
materials
2. Degrade potential harmful substances
• ER, Golgi, secretory vessels, and lysozomes,
also nuclear membrane
Peroxisomes
• “peroxide bodies”
• Membranous sacs with a variety of powerful
enzymes
• Oxidases and catalyzes
• Oxidase – use molecular oxygen to detoxify
harmful substances – alcohol/fermaldehyde
• Neutralize dangerous free radicals – highly
reactive chemicals with unpaired electrons
• Convert to hydrogen peroxide
• Numerous in liver and kidney
Cytoskeleton
• “cell skeleton”
• Elaborate network of
rods running through
cytosol
• 3 parts –
microfilaments,
intermediate filaments,
and microtubules
Cytoskeleton - Microtubules
• Elements with
largest diameter
• Hallow tubes made
of spherical protein
subunits- tubulins
• Organelles
attached
• Motor proteins –
move and
reposition
organelles
Cytoskeleton - Microfilaments
• Thinnest elements
of cytoskeleton
• Strands of protein –
actin
• Act together with
myosin to generate
contractile forces
Cytoskeleton – Intermediate Filaments
• Though insoluble protein fibers
• Woven ropes
• Internal wires to resist pulling forces acting on
cell
Centrosome and Centrioles
• Centrosome – cell center, microtubule
organizing center
• Centrioles – small barrel shaped organelles
oriented at right angles to each other
Cellular Extensions
• Cilia – whip like motile cellular extensions on cell
surface
• Move substances in one direction across cell
surface
• Flagella – long projection
• Ex. Sperm cell – commonly only a tail
Bases – basal bodies – 9 + 2 pattern of microtubules
Micorvilli – minute, fingerlike extensions of plasma
membrane, increase the SA of plasma membrane
Nucleus
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Control center
Genes
All body cells have nucleus
RBC eject their nucleus – anucleate
Average – 5 µm
Largest organelle
3 regions – nuclear envelope, nucleoli, and
chromatin
Nuclear Envelope
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Double membrane bilayer
Outer membrane continuous with ER
Inner membrane lined by nuclear lamina
Nuclear pores – protein (NPC) – aqueous
transport channel – entry and exist
• Selectively permeable membrane
• Nucleoplasm – jellylike fluid on inside
Nucleoli
• Dark-staining spherical bodies with in the
nucleus
• Typically 1 or 2 but maybe more
• Sites where ribosomal subunits are assembled
• Associated with the nucleolar organizer
regions – genetic information for synthesizing
rRNA
• rRNA – combined with proton = ribosome
Chromatin
• Fine, unevenly stained network
(microscope)
• Really bumpy threads
• Composed of 30: DNA, 60% histone
proteins, 10% RNA chains
• Fundamental unit – nucleosomes –
cluster of 8 histone proteins
connected by DNA
• Pack DNA
• Ready to divide – condense into
chromosomes
Cell Life Cycle
• Series of changes a cell goes through
• 2 major periods
1. Interphase
2. Cell Division
Interphase
• Period from cell formation to cell division
• Subphases:
• G1 (gap 1) phase –
– Cell metabolically active
– Synthesizes protein and grows
• S phase – DNA replicated
– New histones made and assembled into Chromatin
• G2 (gap 2) phase – brief
– Enzymes and proteins needed for division are
synthesized
Interphase – DNA Replication
1. DNA helix unwinds
2. Helicase enzyme untwists helix, replication
fork – Y-shaped separation
3. Each strand serves as a template
4. DNA Polymerase – attaches complementary
nucleotides, leading strand and lagging
strand
5. DNA ligase hooks segments of lagging strand
together, leading strand - continuous
Cell Division
• M (mitotic) phase
• 2 events – mitosis and
cytokinesis
• Mitosis – division of the
nucleus
• 4 phases
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4.
Prophase
Metaphase
Anaphase
Telophase
Cell Division
• Cytokinesis – division of the cytoplasm
• Begins in late anaphase
• Plasma membrane drawn inward (cleavage
furrow) by contractile ring
• 2 cells pinched apart
Control of Cell Cycle
• Signals for cell division
1. Ratio of SA to volume
- 64 fold increase in volume, only 16 fold
increase in SA
- Inadequate SA for nutrient and waste
exchange
2. Chemical Signals – growth factors, hormones,
etc.
Control of Cell Cycle
• Cyclins and Cdks (cyclin dependent kinases)
• Proteins and enzymes that signal the cell to
divide
• Also checkpoints for cell division – MPF – Mphase promoting factor – OK – signal to pass
G2 and enter M
Protein Synthesis
• DNA – blueprint for proteins
• Gene - sequence of DNA that carries instructions
for proteins
• DNA bases – A, C, T, G
• 3 bas segments – triplet – “word” that specifies
an amino acid
• Most genes have exons – which code for amino
acids separated by introns – noncoding segments
of DNA
• Introns – can range from 60100,000
nucleotides long
– “junk DNA” – must be cut out to make a protein
Protein Synthesis
• DNA can not leave the nucleus so need a
carrier
• RNA – (carrier)
• 3 types
1. Messenger RNA (mRNA) – long nucleotide
strand, “half DNA” codes for a protein
2. Ribosomal RNA (rRNA) – part of ribosome
3. Transfer RNA (tRNA) – small roughly L-shaped
molecules
Protein Synthesis
2 Parts
1. Transcription – DNA  mRNA
- initiation, elongation and
termination
- Promoter – start point
- RNA polymerase – initiates
transcription, pulls strands
apart, aligns RNA nucleotides
- Termination signal –
transcription ends – mRNA
pulls off
Protein Synthesis
• mRNA – must
then be
modified –
splicosomes –
cut introns
(junk) and
splice (glue
together)
remaining
exons
Protein Synthesis
2. Translation –
- base sequence (mRNA) 
converted to amino acids
(protein)
- Codon – 3 bas sequence
on mRNA – 64 codons for
20 amino acids
- Anticodon - 3 base
sequence on tRNA – brings
in amino acid
- Amino acids hook together
– form a chain = PROTEIN
DNA – other roles
• Antisense RNA – can intercept and bind to
protein coding mRNA, prevent it from being
translated into protein
• microRNAs – small RNAs that can interfere
and suppress mRNA
• Ribositches – folded mRNA, code for a protein
– Can turn on protein synthesis in response to
changes in the environment
Protein Degradation
• Proteins - no longer useful
• Ubiquitins – proteins that attach to old/bad
protein – mark it for destruction
• Proteasomes – digest the protein
OUTSIDE _ Extracellular Materials
• Body fluids – interstitial fluid, blood plasma,
cerebrospinal fluid
• Cellular secretions – substances that aid in
digestion and act as lubricants
• Extracellular Matrix – jelly-like, composed of
proteins and polysaccharides
• “cell glue” holds cells together
Development
• Embryo –cells – chemical signals that direct
pathways of development
• Cell differentiation – development of specific
and distinctive features of cells
• Apoptosis -programmed cell death
• Hyperplasia – accelerated growth
• Atrophy – decrease in size of organ or body
tissue
Development
• Aging –
• Telomere – string of nucleotides at the end of a
chromosome that protect it from fraying
• Get shorter with each cell division
• Aging – shorten, not protected
• Telomerase – enzyme that protects from
degradation
• Found in egg and sperm cells, NOT in normal
body cells
• “fountain of youth”