Transcript Day 2 Summary

BIO201 – Anatomy and
Physiology I
Biological Macromolecules
Kamal Gandhi
Lecture 2
Molecules
• Very few elements are functional in the body
in their inert, unchanged form
• Most elements, instead, are found as ions or
as parts of molecules
• A molecule is the result of two or more atoms
being bound together
• Atoms form bonds in order to complete their
valence shell of electrons
SPONCH
• The 6 SPONCH elements are vital for the
formation of biological macromolecules because
of their chemical bonding abilities
• S
• P
• O
• N
• C
• H
Table 2-1
Biological Marcomolecules
• The SPONCH elements make up the building
blocks of cells – the 4 biological macromolecules
–
–
–
–
Carbohydrates: short term energy storage
Lipids: long term energy storage, membranes
Proteins: cellular workhorse (functional part of a cell)
Nucleic acids: genetic information (blueprint of a cell)
• These macromolecules are long chains (polymers)
built from small parts (monomers)
Monomers and Polymers
• Individual subunits are combined with each
other to form large macromolecules
• Water is directly involved in these reactions
• Dehydration synthesis: a bond is formed by
the removal of water
• Hydrolysis: a bond is broken by the addition of
water
Fig. 5-2
HO
1
2
3
H
Short polymer
HO
Unlinked monomer
Dehydration removes a water
molecule, forming a new bond
HO
2
1
H
3
H2O
4
H
Longer polymer
(a) Dehydration reaction in the synthesis of a polymer
HO
1
2
3
4
Hydrolysis adds a water
molecule, breaking a bond
HO
1
2
(b) Hydrolysis of a polymer
3
H
H
H2O
HO
H
Carbohydrates
• The primary molecule used by cells to make energy is
carbohydrates
• Contains a [C(H2O)]n motif
• They can be used immediately to make ATP, the energy
molecule of a cell
• They can also be stored for “medium-term” in long
chains or polymers
• A few carbohydrates are more stable and are used as
structural molecules
• Carbohydrates typically contain carbonyl groups
Fig. 5-3
Trioses (C3H6O3)
Pentoses (C5H10O5)
Hexoses (C6H12O6)
Glyceraldehyde
Ribose
Glucose
Galactose
Dihydroxyacetone
Ribulose
Fructose
Carbohydrates
• The scientific name of carbohydrates are
“saccharides”
• A single unit of a saccharide is a monosaccharide
• There are three common monosaccharides that
are a part of your diet: glucose, fructose, and
galactose
• In a water environment (like a cell), these
molecules will circularize into a ring structure at
the carbonyl group
Fig. 5-4a
(a) Linear and ring forms
Dissaccharides
• In nature, the three monosaccharides
combine into disaccharides that are common
parts of your diet
– Maltose: glucose + glucose, a common part of
starchy foods
– Lactose: galactose + glucose, a common part of
dairy
– Sucrose: glucose + fructose, aka table sugar
Fig. 5-5
1–4
glycosidic
linkage
Glucose
Glucose
Maltose
(a) Dehydration reaction in the synthesis of maltose
1–2
glycosidic
linkage
Glucose
Fructose
(b) Dehydration reaction in the synthesis of sucrose
Sucrose
Polysaccharides
• Glucose is the primary sugar that almost all living
organisms use for energy
• When cells/organisms have extra glucose, they can store it
for short/medium term
• They do this by forming long chains of glucose –
polysaccharides
• In plants, longs chains of glucose are called starch
• While starch is made by the plant to store glucose, starchy
foods provide a large energy source in our diet
• In humans/animals, long chains of glucose are called
glycogen, and can be stored in the liver/muscles
Fig. 5-6
Chloroplast
Mitochondria
Starch
Glycogen granules
0.5 µm
1 µm
Glycogen
Amylose
Amylopectin
(a) Starch: a plant polysaccharide
(b) Glycogen: an animal polysaccharide
Structural polysaccharides
• In a few cases, chains of glucose form more
stable molecules that do not break down very
easily
• This is done by using an alternate form of
glucose
• The plant cell wall is made up of cellulose, a
chain of β-glucose
α vs β glucose
• When glucose forms it’s ring structure, the bond at C1
can form in two orientations (“up” vs “down”)
• The version that cells use for energy is the “down”
orientation – α glucose
• Some organisms are able to make the “up” orientation
as well – β glucose
• Since most organisms do not have the enzymes needed
to breakdown β glucose, it is used as a stable,
structural molecule in plants (cellulose)
• Because we cannot breakdown β-glucose, this version
passes through the body unchanged - fiber
Fig. 5-7a
 Glucose
(a)  and  glucose ring structures
 Glucose
Fig. 5-7bc
(b) Starch: 1–4 linkage of  glucose monomers
(c) Cellulose: 1–4 linkage of  glucose monomers
Fig. 5-8
Cell walls
Cellulose
microfibrils
in a plant
cell wall
Microfibril
10 µm
0.5 µm
Cellulose
molecules
 Glucose
monomer
Lipids
• One of the most stable macromolecules are fats
• Because they are so stable, fats (lipids) can be used for
long-term energy storage
• A second, more important function of lipids in a cell is
that they are used to make cellular membranes
• There are two alternate forms of lipids that are utilized
for these functions – triglycerides and phospholipids
• A third, minor lipid in nature, though a very important
one for cells, are steroids, which are used to stabilize
membranes and for hormones
Triglycerides
• The form of fat that we use for long-term energy
storage (and to provide cushioning to organs,
insulation to the body, etc) is a triglyceride
• The “glyceride” part refers to the central sugar
molecule, a 3-C molecule called glycerol
• The “tri” part refers to the 3 fatty acids that are
attached to the glycerol, one to each carbon
• These fatty acids are long hydrocarbon chains that are
non-polar, making fats hydrophobic so they don’t
dissolve in water
Fig. 5-11a
Fatty acid
(palmitic acid)
Glycerol
(a) Dehydration reaction in the synthesis of a fat
Fig. 5-11b
Ester linkage
(b) Fat molecule (triacylglycerol)
Fatty acids
• One end of the fatty acid contains a carboxyl group,
allowing it to bind to the glycerol
• The hydrocarbon tail of a fatty acid can be of varying
length, typically 14-, 16-, or 18-C long
• The fatty acid tail is only made up of C and H; but
occasionally some of the Cs form double bonds
• In a saturated fat, there are no double bonds, and the
fat is therefore saturated with the maximum Hs
• In an unsaturated fat, there is a double bond, and so
there are less than the maximum number of Hs
Fig. 5-12a
Structural
formula of a
saturated fat
molecule
Stearic acid, a
saturated fatty
acid
(a) Saturated fat
Fig. 5-12b
Structural formula
of an unsaturated
fat molecule
Oleic acid,
an
unsaturated
fatty acid
(b) Unsaturated fat
cis double
bond causes
bending
Fats
• A saturated fat will allow the fat molecules to align closer
together, making these fats solid (at room temp)
• An unsaturated fatty acid will have a kink in the tail; which
prevents close packing of these fats, and so they tend to
be liquid (at room temp)
• Unsaturated fats can be mono- (one double bond) or poly(multiple double bonds) unsaturated
• A hydrogenated fat (like margarine) is an unsaturated fat
to which H has been added, causing it to lose its double
bond (which can be bad for you if it happens incorrectly)
Phospholipids
• The second major class of fat molecules are phospho-lipids,
which are used for virtually all cell membranes
• In these molecules, one of the fatty acids is replaced with a
phosphate group (PO4), which has a negative charge and is
therefore hydrophilic
• The phospholipid therefore has a hydrophilic head region
(the glycerol and phosphate) and a hydrophobic tail region
(the 2 remaining fatty acids)
• Because it it amphipathic, phospholipds will form a bilayer
structure in water (discussed more next lecture)
Hydrophobic tails
Hydrophilic head
Fig. 5-13ab
(a) Structural formula
Choline
Phosphate
Glycerol
Fatty acids
(b) Space-filling model
Fig. 5-14
Hydrophilic
head
Hydrophobic
tail
WATER
WATER
Steroids
• The third type of lipid is a steroid molecule
• In cells, steroids (sterols/cholesterols) are
important for maintaining stability as
temperatures change
• Furthermore, in our body, steroids serve as a
major class of hormone
Fig. 5-15
Fig. 7-5c
Cholesterol
(c) Cholesterol within the animal cell membrane
Proteins
• The protein is the most important part of a cell, because
it provides that cell with all of its functional ability
• Proteins can be described as our cellular workhorse
• It carries out all of the functions of a cell, including
structure, movement, support, signaling, and enzymes
• Proteins are chains of amino acids, linked together by
peptide bonds
• The function of an individual protein is based on its
structure, and the structure is based on the sequence of
these amino acids
Table 5-1
Amino acids
• There are 20 naturally occurring amino acids in
nature
• All amino acids share the same overall structure,
with a central Carbon bound to an amino group, a
carboxyl group, and a Hydrogen
• The 4th bond of the central carbon is to a variable
side group, called the R group
• The chemical characteristics of the R group gives
individual amino acids their different
characteristics
Fig. 5-UN1
 carbon
Amino
group
Carboxyl
group
Fig. 5-17
Nonpolar
Glycine
(Gly or G)
Valine
(Val or V)
Alanine
(Ala or A)
Methionine
(Met or M)
Leucine
(Leu or L)
Trypotphan
(Trp or W)
Phenylalanine
(Phe or F)
Isoleucine
(Ile or I)
Proline
(Pro or P)
Polar
Serine
(Ser or S)
Threonine
(Thr or T)
Cysteine
(Cys or C)
Tyrosine
(Tyr or Y)
Asparagine
(Asn or N)
Glutamine
(Gln or Q)
Electrically
charged
Acidic
Aspartic acid
(Asp or D)
Glutamic acid
(Glu or E)
Basic
Lysine
(Lys or K)
Arginine
(Arg or R)
Histidine
(His or H)
Peptide bonds
• Amino acids are linked together by peptide bonds into
long chains to make functional proteins
• A peptide bond is a repeatable bond formed between
the carboxyl group of one amino acid and the amino
group of the next amino acid
• Because this leave another free carboxyl group,
another amino acid can be added downstream
• As this process continues, it creates a direction to
proteins; the N-terminus (front end) and C-terminus
(back end)
Fig. 5-18
Peptide
bond
(a)
Side chains
Peptide
bond
Backbone
(b)
Amino end
(N-terminus)
Carboxyl end
(C-terminus)
Polypeptides
• As amino acids grow longer, they will start to fold into a 3dimensional structure
• This structure determines the function of the protein
• We typically define 4 different levels of protein structure
– Primary: the sequence of amino acids
– Secondary: folding into α-helices and β-pleated sheets,
caused by H-bonding of the backbone
– Tertiary: folding of the polypeptide caused by interactions
between side groups (disulfide bridges between cysteine,
H bonds, ionic bonds, van der Waals interactions)
– Quarternary: interactions between multiple polypeptides
Fig. 5-21a
Primary Structure
1
5
H3N
Amino end
+
10
Amino acid
subunits
15
20
25
Fig. 5-21c
Secondary Structure
 pleated sheet
Examples of
amino acid
subunits
 helix
Fig. 5-21f
Hydrophobic
interactions and
van der Waals
interactions
Polypeptide
backbone
Hydrogen
bond
Disulfide bridge
Ionic bond
Fig. 5-21e
Tertiary Structure
Quaternary Structure
Fig. 5-21g
Polypeptide
chain
 Chains
Iron
Heme
 Chains
Hemoglobin
Collagen
Structure determines function
• The 3D structure of a protein is vital to determining its
function
• Typically because the structure affects the interactions
of the protein with other molecules
• Protein structure can be altered by changing the
chemical environment (pH) or the physical
environment (temperature), causing proteins to
denature (unfold)
• Sometimes, changing just one amino acid can cause
the protein to misfold, creating the wrong structure
and a partially or non-functional protein
Fig. 5-19
Groove
Groove
(a)
A ribbon model of lysozyme
(b)
A space-filling model of lysozyme
Fig. 5-22
Normal hemoglobin
Primary
structure
Val His Leu Thr Pro Glu Glu
1
2
3
4
5
6
7
Secondary
and tertiary
structures
 subunit
Function
Normal
hemoglobin
(top view)
Secondary
and tertiary
structures
1
2
Normal red blood
cells are full of
individual
hemoglobin
moledules, each
carrying oxygen.
5
6
7
 subunit

Sickle-cell
hemoglobin

Function

Molecules
interact with
one another and
crystallize into
a fiber; capacity
to carry oxygen
is greatly reduced.
10 µm
Red blood
cell shape
4
Exposed
hydrophobic
region

Molecules do
not associate
with one
another; each
carries oxygen.
3

Quaternary
structure

Val His Leu Thr Pro Val Glu


Quaternary
structure
Sickle-cell hemoglobin
Primary
structure
10 µm
Red blood
cell shape
Fibers of abnormal
hemoglobin deform
red blood cell into
sickle shape.
Enzymes
• Perhaps the most important function of
proteins in a cell is to serve as a biological
catalyst (enzymes)
• A catalyst is a molecule that speeds up
chemical reactions without being changed by
the reaction
• It speeds up the reaction by requiring less
energy
• All chemical reactions that take place in a cell
require enzymes in order to occur under
Fig. 5-16
Substrate
(sucrose)
Glucose
Enzyme
(sucrase)
OH
Fructose
H O
H2O
Nucleic acids
• Nucleic acids serve as genetic information for a
cell
• This genetic information comes in two forms,
DNA (permanent copy) and RNA (temporary
copy)
• They provide the information necessary to
maintain and reproduce a cell
• They are also passed from the mother cell to the
two daughter cells during cell division; or from
parent to offspring during reproduction
Nucleic acids
• The permanent blueprint stored by a cell is
DNA
• The sequence of DNA is called the genome,
and it contains the information to make all of
the proteins the cell/organism might ever
need
• The code for one individual protein is called a
gene
• That gene gets transcribed into RNA, a
temporary copy of the blueprint for one
Fig. 5-26-3
DNA
1 Synthesis of
mRNA in the
nucleus
mRNA
NUCLEUS
CYTOPLASM
mRNA
2 Movement of
mRNA into cytoplasm
via nuclear pore
Ribosome
3 Synthesis
of protein
Polypeptide
Amino
acids
DNA
• DNA is a double helix of anti-parallel strands
held together by H-bonds between base pairs
• Each strand is a polymer of nucleotides
• A nucleotide consists of a sugar, a phosphate,
and a Nitrogenous base
• The sugar and phosphate make up the
backbone of each DNA strand
• The N-base sticks inside the backbone and
makes up the “rungs of the ladder”
Fig. 16-7a
5 end
Hydrogen bond
3 end
1 nm
3.4 nm
3 end
0.34 nm
(a) Key features of DNA structure (b) Partial chemical structure
5 end
Fig. 5-27
5 end
Nitrogenous bases
Pyrimidines
5C
3C
Nucleoside
Nitrogenous
base
Cytosine (C)
Thymine (T, in DNA)
Uracil (U, in RNA)
Purines
Phosphate
group
5C
Sugar
(pentose)
Adenine (A)
Guanine (G)
(b) Nucleotide
3C
Sugars
3 end
(a) Polynucleotide, or nucleic acid
Deoxyribose (in DNA)
(c) Nucleoside components: sugars
Ribose (in RNA)
DNA vs RNA
• DNA is double stranded, whereas RNA is single
stranded
• DNA uses deoxyribose as the central sugar,
whereas RNA uses ribose
• The 4 bases in DNA are A, C, G, and T
• The 4 bases in RNA are A, C, G, and U
Fig. 5-27ab
5' end
5'C
3'C
Nucleoside
Nitrogenous
base
5'C
Phosphate
group
5'C
3'C
(b) Nucleotide
3' end
(a) Polynucleotide, or nucleic acid
3'C
Sugar
(pentose)
Fig. 5-27c-2
Sugars
Deoxyribose (in DNA)
(c) Nucleoside components: sugars
Ribose (in RNA)
Fig. 5-27c-1
Nitrogenous bases
Pyrimidines
Cytosine (C)
Thymine (T, in DNA)
Uracil (U, in RNA)
Purines
Adenine (A)
Guanine (G)
(c) Nucleoside components: nitrogenous bases
Nucleic acids
• DNA and RNA serve as genetic information 
they are the blueprint to make proteins
• Protein function is based on structure, which
is based on the sequence of amino acids
• DNA serves as a blueprint for proteins through
the sequence of bases that make up an
individual gene
• Through the genetic code, the sequence of
bases gets translated into the sequence of
amino acids to make up different proteins
Third mRNA base (3 end of codon)
First mRNA base (5 end of codon)
Fig. 17-5
Second mRNA base
Chromosomes
• The human genome consists of 3 Gbp of DNA
• If unwound, this makes up 6 feet of DNA that
must fit into each and every cell of the body
• Therefore, DNA in a cell cannot be allowed to
completely unwind
• Instead, in a cell DNA is wrapped around
proteins called histones  chromosomes
• A human cell has 46 chromosomes; i.e. 46
segments of DNA wrapped around proteins
• These chromosomes come in homologous
pairs – one from mom and one from dad
Fig. 16-21a
Nucleosome
(10 nm in diameter)
DNA
double helix
(2 nm in diameter)
H1
Histones
DNA, the double helix
Histones
Histone tail
Nucleosomes, or “beads
on a string” (10-nm fiber)
Fig. 16-21b
Chromatid
(700 nm)
30-nm fiber
Loops
Scaffold
300-nm fiber
Replicated
chromosome
(1,400 nm)
30-nm fiber
Looped domains
(300-nm fiber)
Metaphase
chromosome
Sex Determination Is Directed By Our
Genome
• Humans have 23
pairs of
chromosomes
– 22 pairs of
autosomes
– X and Y = 1 pair of
sex chromosomes
Figure 26-1
Prokaryotes vs Eukaryotes
• No nucleus vs True nucleus
• Many similarities
– Common biological macromolecules
– Common genetic code
– Common metabolic pathways
– Common physical/cell structure
• Many differences
– Size
– Cellular complexity
– Metabolic diversity
Prokaryotic and Eukaryotic Cells: An Overview
• Prokaryotes
– Lack nucleus
– Lack various internal structures bound with
phospholipid membranes
– Are small (~1.0 µm in diameter)
– Have a simple structure
– Include bacteria and archaea
© 2012 Pearson Education Inc.
Figure 3.2 Typical prokaryotic cell
Inclusions
Ribosome
Cytoplasm
Flagellum
Nucleoid
Glycocalyx
Cell wall
Cytoplasmic membrane
Prokaryotic and Eukaryotic Cells: An Overview
• Eukaryotes
–
–
–
–
–
© 2012 Pearson Education Inc.
Have nucleus
Have internal membrane-bound organelles
Are larger (10–100 µm in diameter)
Have more complex structure
Include algae, protozoa, fungi, animals, and plants
Figure 3.3 Typical eukaryotic cell
Nuclear envelope
Nuclear pore
Nucleolus
Lysosome
Mitochondrion
Centriole
Secretory vesicle
Golgi body
Cilium
Transport vesicles
Ribosomes
Rough endoplasmic
reticulum
Smooth endoplasmic
reticulum
Cytoplasmic
membrane
Cytoskeleton
Cells
• A cell is the functional unit of biology
• All living things are made up of cells
• A cell must contain the information and ability necessary
to maintain itself and reproduce itself
• Therefore, all cells must contain 4 basic components
– Chromosomes: genetic information for the cell
– Cell/plasma membrane: semi-permeable boundary
– Ribosomes: protein factory of the cell
– Cytosol/cytoplasm: the internal liquid portion of the
cell
Eukaryotic cells
• Human cells are eukaryotic
• Eukaryotes are defined by having a nucleus
(and other internal membrane-bound
organelles)
• These organelles allow for
compartmentalization of individual functions
for the cell
Nucleus
• The defining feature of a eukaryotic cell
• It is a double-membraned organelle with the
primary role of storing and protecting DNA
• In order to fit inside the nucleus (or cell in
general), the DNA gets wrapped around
proteins  chromosome
• Within the nucleus is the nucleolus, the site of
ribosome production
• To move RNA and ribosomes out of the
nucleus, it must contain nuclear pores,
through which movement is regulated
Fig. 6-10
Nucleus
1 µm
Nucleolus
Chromatin
Nuclear envelope:
Inner membrane
Outer membrane
Nuclear pore
Pore
complex
Surface of
nuclear envelope
Rough ER
Ribosome
1 µm
0.25 µm
Close-up of nuclear
envelope
Pore complexes (TEM)
Nuclear lamina (TEM)
Ribosomes
• Ribosomes are “protein factories”
• They translate RNA into proteins in the cell
cytoplasm
• Ribosomes are found in two locations, freefloating in th cytoplasm or bound to the rough
ER
• Free-floating ribosomes tend to make proteins
that will function within the cytoplasm or
nucleus
• Bound ribosomes tend to make proteins that
Fig. 6-11
Cytosol
Endoplasmic reticulum (ER)
Free ribosomes
Bound ribosomes
Large
subunit
0.5 µm
TEM showing ER and ribosomes
Small
subunit
Diagram of a ribosome
Endoplasmic reticulum (ER)
• Organelle contiguous with the outer nuclear
membrane, whose job is typically production
• Two types: rough and smooth
• Rough ER: looks “rough” because of the
presence of ribosomes on the surface; makes
proteins
• Smooth ER: typically involved in lipid synthesis
and sugar storage/modification
Fig. 6-12
Smooth ER
Rough ER
ER lumen
Cisternae
Ribosomes
Transport vesicle
Smooth ER
Nuclear
envelope
Transitional ER
Rough ER
200 nm
Golgi apparatus (body)
• The storage and transport center of the cell
(FedEx)
• Products from the ER get delivered to the
Golgi, which packages them, modifies them as
needed, and directs them to the correct
location within or out of the cell
• Also, products brought into the cell often get
directed to the Golgi for proper sorting
• Consists of stacked membrane sacks
• Products get delivered by small transport
Fig. 6-13
cis face
(“receiving” side of Golgi
apparatus)
0.1 µm
Cisternae
trans face
(“shipping” side of Golgi
apparatus)
TEM of Golgi apparatus
Lysosome/Peroxisome
• Two organelles involved in breakdown
• As cellular portions get “old and worn-down,” or
as external products are engulfed and must get
broken down, they are sent to these organelles
• Peroxisome
– Oxidative breakdown
– Uses toxic oxygen species like peroxide & superoxides
• Lysosome (not found in plants)
– Enzymatic breakdown
– Uses degradative enzymes to digest macromolecules
Fig. 6-14
Nucleus
1 µm
Vesicle containing
two damaged organelles
1 µm
Mitochondrion
fragment
Peroxisome
fragment
Lysosome
Lysosome
Digestive
enzymes
Plasma
membrane
Lysosome
Peroxisome
Digestion
Food vacuole
Vesicle
(a) Phagocytosis
(b) Autophagy
Mitochondrion
Digestion
Vacuoles
• Many cells need to store components
• For storage, vesicles will congregate into one
organelle called a storage vacuole
• Different types of cells have individual
vacuoles to store various different molecules
• Plant cells often contain a large Central
Vacuole, which stores primarily water and
provides rigidity to the cell
Fig. 6-15
Central vacuole
Cytosol
Nucleus
Central
vacuole
Cell wall
Chloroplast
5 µm
Fig. 6-16-3
Nucleus
Rough ER
Smooth ER
cis Golgi
trans Golgi
Plasma
membrane
Mitochondria
• Powerhouse of the cell
• Site of Cellular Respiration, where ATP is made
• ATP: adenosine triphosphate
– Adenine + ribose + 3 phosphates
– cellular battery used to charge chemical reactions
• All cellular ATP is charged in the mitochondria,
then gets delivered to other parts of the cell
where it is broken down into ADP
• Breaking the terminal phosphate bond releases
energy, which can be used to power other
Fig. 6-17
Intermembrane space
Outer
membrane
Free ribosomes
in the
mitochondrial
matrix
Inner
membrane
Cristae
Matrix
0.1 µm
Fig. 9-UN3
becomes oxidized
becomes reduced
Fig. 8-12
ATP + H2O
Energy from
catabolism (exergonic,
energy-releasing
processes)
ADP + P i
Energy for cellular
work (endergonic,
energy-consuming
processes)
Fig. 9-6-3
Electrons carried
via NADH and
FADH2
Electrons
carried
via NADH
Citric
acid
cycle
Glycolysis
Pyruvate
Glucose
Oxidative
phosphorylation:
electron transport
and
chemiosmosis
Mitochondrion
Cytosol
ATP
ATP
ATP
Substrate-level
phosphorylation
Substrate-level
phosphorylation
Oxidative
phosphorylation
Chloroplast
• Found only in plant cells
• Site of photosynthesis
• Photosynthesis: Using light energy to
synthesize glucose from CO2 in the air
Fig. 6-18
Ribosomes
Stroma
Inner and outer
membranes
Granum
Thylakoid
1 µm
Cytoskeleton
• Cells are not just free-floating bags of
organelles, but instead are full of internal
structure
• This internal structure comes from their
cytoskeleton
• There are 3 main classes of cytoskeletal
molecules
– Microfilaments: smallest type, made of actin
– Intermediate filaments: diverse array of proteins
– Microtubules: largest type, made of tubulin
• The cytoskeleton provides internal structure,
Table 6-1
10 µm
10 µm
10 µm
Column of tubulin dimers
Keratin proteins
Actin subunit
Fibrous subunit (keratins
coiled together)
25 nm
7 nm


Tubulin dimer
8–12 nm
Fig. 6-23
Direction of swimming
(a) Motion of flagella
5 µm
Direction of organism’s movement
Power stroke
Recovery stroke
(b) Motion of cilia
15 µm
Cellular connections
• For multicellular organisms, cells must be able
to communicate outside individual cells to
work together
• Many cells are connected to each other,
creating layers of tissues and organs
• These cells are often connected to an
extracellular matrix (ECM) or basement
membrane
• Many cells are interconnected through
communication sites called tight/gap junctions
(primarily in animals) or desmosomes
Fig. 6-32
Tight junction
Tight junctions prevent
fluid from moving
across a layer of cells
0.5 µm
Tight junction
Intermediate
filaments
Desmosome
Desmosome
Gap
junctions
Space
between
cells
Plasma membranes
of adjacent cells
Extracellular
matrix
1 µm
Gap junction
0.1 µm