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BIO201 – Anatomy and Physiology I
Biological Macromolecules
Kamal GandhiLecture 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
Short polymer
HO 1 2 3 H HO H
Unlinked monomer
Dehydration removes a watermolecule, forming a new bond
HO
H2O
H1 2 3 4
Longer polymer
(a) Dehydration reaction in the synthesis of a polymer
HO 1 2 3 4 H
H2OHydrolysis adds a watermolecule, breaking a bond
HO HH HO1 2 3
(b) Hydrolysis of a polymer
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
Dihydroxyacetone
Ribulose
Keto
ses
Aldo
ses
Fructose
Glyceraldehyde
Ribose
Glucose Galactose
Hexoses (C6H12O6)Pentoses (C5H10O5)Trioses (C3H6O3)
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
(b) Dehydration reaction in the synthesis of sucrose
Glucose Fructose Sucrose
MaltoseGlucoseGlucose
(a) Dehydration reaction in the synthesis of maltose
1–4glycosidic
linkage
1–2glycosidic
linkage
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
(b) Glycogen: an animal polysaccharide
Starch
GlycogenAmylose
Chloroplast
(a) Starch: a plant polysaccharide
Amylopectin
Mitochondria Glycogen granules
0.5 µm
1 µm
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
(a) and glucose ring structures
Glucose Glucose
Fig. 5-7bc
(b) Starch: 1–4 linkage of glucose monomers
(c) Cellulose: 1–4 linkage of glucose monomers
Fig. 5-8
b Glucosemonomer
Cellulosemolecules
Microfibril
Cellulosemicrofibrilsin a plantcell wall
0.5 µm
10 µm
Cell walls
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)
(a) Dehydration reaction in the synthesis of a fatGlycerol
Fig. 5-11b
(b) Fat molecule (triacylglycerol)
Ester linkage
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
(a) Saturated fat
Structuralformula of asaturated fatmolecule
Stearic acid, asaturated fattyacid
Fig. 5-12b
(b) Unsaturated fat
Structural formulaof an unsaturatedfat molecule
Oleic acid, anunsaturatedfatty acid
cis doublebond causesbending
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)
Fig. 5-13ab
(b) Space-filling model(a) Structural formula
Fatty acids
Choline
Phosphate
Glycerol
Hyd
roph
obic
tails
Hyd
roph
ilic
head
Fig. 5-14
Hydrophilichead
Hydrophobictail 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
Aminogroup
Carboxylgroup
carbon
Fig. 5-17Nonpolar
Glycine(Gly or G)
Alanine(Ala or A)
Valine(Val or V)
Leucine(Leu or L)
Isoleucine(Ile or I)
Methionine(Met or M)
Phenylalanine(Phe or F)
Trypotphan(Trp or W)
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)
Electricallycharged
Acidic Basic
Aspartic acid(Asp or D)
Glutamic acid(Glu or E)
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)
Peptidebond
Fig. 5-18
Amino end(N-terminus)
Peptidebond
Side chains
Backbone
Carboxyl end(C-terminus)
(a)
(b)
Polypeptides
• As amino acids grow longer, they will start to fold into a 3-dimensional 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
Amino acidsubunits
+H3N Amino end
25
20
15
10
5
1
Primary Structure
Fig. 5-21c
Secondary Structure
pleated sheet
Examples ofamino acidsubunits
helix
Fig. 5-21f
Polypeptidebackbone
Hydrophobicinteractions andvan der Waalsinteractions
Disulfide bridge
Ionic bond
Hydrogenbond
Fig. 5-21e
Tertiary Structure Quaternary Structure
Fig. 5-21g
Polypeptidechain
Chains
HemeIron
Chains
CollagenHemoglobin
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
A ribbon model of lysozyme(a) (b) A space-filling model of lysozyme
GrooveGroove
Fig. 5-22
Primarystructure
Secondaryand tertiarystructures
Quaternarystructure
Normalhemoglobin(top view)
Primarystructure
Secondaryand tertiarystructures
Quaternarystructure
Function Function
subunit
Molecules donot associatewith oneanother; eachcarries oxygen.
Red bloodcell shape
Normal red bloodcells are full ofindividualhemoglobinmoledules, eachcarrying oxygen.
10 µm
Normal hemoglobin
1 2 3 4 5 6 7Val His Leu Thr Pro Glu Glu
Red bloodcell shape
subunit
Exposedhydrophobicregion
Sickle-cellhemoglobin
Moleculesinteract withone another andcrystallize intoa fiber; capacityto carry oxygenis greatly reduced.
Fibers of abnormalhemoglobin deformred blood cell intosickle shape.
10 µm
Sickle-cell hemoglobin
GluProThrLeuHisVal Val
1 2 3 4 5 6 7
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 biological time and energy constraints
Fig. 5-16
Enzyme(sucrase)
Substrate(sucrose)
Fructose
Glucose
OH
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
• * Since they provide the information to make a cell function, and the functional part of a cell are the proteins, nucleic acids are a blueprint to make proteins
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 protein
• The RNA is then translated into a protein
Fig. 5-26-3
mRNA
Synthesis ofmRNA in thenucleus
DNA
NUCLEUS
mRNA
CYTOPLASM
Movement ofmRNA into cytoplasmvia nuclear pore
Ribosome
AminoacidsPolypeptide
Synthesisof protein
1
2
3
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
Hydrogen bond 3 end
5 end
3.4 nm
0.34 nm
3 end
5 end
(b) Partial chemical structure(a) Key features of DNA structure
1 nm
Fig. 5-27
5 end
Nucleoside
Nitrogenousbase
Phosphategroup Sugar
(pentose)
(b) Nucleotide
(a) Polynucleotide, or nucleic acid
3 end
3C
3C
5C
5C
Nitrogenous bases
Pyrimidines
Cytosine (C) Thymine (T, in DNA) Uracil (U, in RNA)
Purines
Adenine (A) Guanine (G)
Sugars
Deoxyribose (in DNA) Ribose (in RNA)
(c) Nucleoside components: sugars
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. 5-27ab5' end
5'C
3'C
5'C
3'C
3' end
(a) Polynucleotide, or nucleic acid
(b) Nucleotide
Nucleoside
Nitrogenousbase
3'C
5'C
Phosphategroup Sugar
(pentose)
Fig. 5-27c-2
Ribose (in RNA)Deoxyribose (in DNA)
Sugars
(c) Nucleoside components: sugars
Fig. 5-27c-1
(c) Nucleoside components: nitrogenous bases
Purines
Guanine (G)Adenine (A)
Cytosine (C) Thymine (T, in DNA) Uracil (U, in RNA)
Nitrogenous bases
Pyrimidines
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
Fig. 17-5Second mRNA base
Firs
t mRN
A ba
se (5
en
d of
cod
on)
Third
mRN
A ba
se (3
en
d of
cod
on)
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
DNA double helix (2 nm in diameter)
Nucleosome(10 nm in diameter)
Histones Histone tailH1
DNA, the double helix Histones Nucleosomes, or “beads on a string” (10-nm fiber)
Fig. 16-21b
30-nm fiber
Chromatid (700 nm)
Loops Scaffold
300-nm fiber
Replicated chromosome (1,400 nm)
30-nm fiber Looped domains (300-nm fiber)
Metaphase chromosome
Figure 26-1
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
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
• 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.
Prokaryotic and Eukaryotic Cells: An Overview
Figure 3.2 Typical prokaryotic cell
Ribosome
Cytoplasm
Nucleoid
GlycocalyxCell wall Cytoplasmic membrane
Inclusions
Flagellum
• Eukaryotes– 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
© 2012 Pearson Education Inc.
Prokaryotic and Eukaryotic Cells: An Overview
Figure 3.3 Typical eukaryotic cell
Nucleolus
Cilium
Ribosomes
Nuclear envelope
Nuclear pore
Lysosome
Mitochondrion
Centriole
Secretory vesicle
Golgi body
Transport vesicles
Rough endoplasmicreticulum
Smooth endoplasmicreticulum
Cytoplasmicmembrane
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
NucleolusNucleus
Rough ER
Nuclear lamina (TEM)
Close-up of nuclear envelope
1 µm
1 µm
0.25 µm
Ribosome
Pore complex
Nuclear pore
Outer membraneInner membraneNuclear envelope:
Chromatin
Surface ofnuclear envelope
Pore complexes (TEM)
Ribosomes
• Ribosomes are “protein factories”• They translate RNA into proteins in the cell
cytoplasm• Ribosomes are found in two locations, free-
floating 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 will function within an organelle or will be secreted out of the cell
Fig. 6-11
Cytosol
Endoplasmic reticulum (ER)
Free ribosomes
Bound ribosomes
Large subunit
Small subunit
Diagram of a ribosomeTEM showing ER and ribosomes
0.5 µm
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. 6-12Smooth ER
Rough ER Nuclear envelope
Transitional ER
Rough ERSmooth ERTransport vesicle
RibosomesCisternaeER lumen
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
vesicles
Fig. 6-13
cis face(“receiving” side of Golgi apparatus) Cisternae
trans face(“shipping” side of Golgi apparatus)
TEM of Golgi apparatus
0.1 µm
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
Lysosome
Digestiveenzymes
Lysosome
Plasmamembrane
Food vacuole
(a) Phagocytosis
Digestion
(b) Autophagy
Peroxisome
Vesicle
Lysosome
Mitochondrion
Peroxisomefragment
Mitochondrionfragment
Vesicle containingtwo damaged organelles
1 µm
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
Central vacuole
Nucleus
Cell wall
Chloroplast
5 µm
Fig. 6-16-3
Smooth ER
Nucleus
Rough ER
Plasma membrane
cis Golgi
trans Golgi
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 chemical reactions
Fig. 6-17
Free ribosomesin the mitochondrial matrix
Intermembrane spaceOuter membrane
Inner membraneCristae
Matrix
0.1 µm
Fig. 9-UN3
becomes oxidized
becomes reduced
Fig. 8-12
P iADP +
Energy fromcatabolism (exergonic,energy-releasingprocesses)
Energy for cellularwork (endergonic,energy-consumingprocesses)
ATP + H2O
Fig. 9-6-3
Mitochondrion
Substrate-levelphosphorylation
ATP
Cytosol
Glucose Pyruvate
Glycolysis
Electronscarried
via NADH
Substrate-levelphosphorylation
ATP
Electrons carriedvia NADH and
FADH2
Oxidativephosphorylation
ATP
Citricacidcycle
Oxidativephosphorylation:electron transport
andchemiosmosis
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
Thylakoid
Stroma
Granum
Inner and outer membranes
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, but is also very important for movement of the cell and organelles
Table 6-1
10 µm 10 µm 10 µm
Column of tubulin dimers
Tubulin dimer
Actin subunit
25 nm
7 nm
Keratin proteins
Fibrous subunit (keratins coiled together)
8–12 nm
Fig. 6-23
5 µm
Direction of swimming
(a) Motion of flagella
Direction of organism’s movement
Power stroke Recovery stroke
(b) Motion of cilia15 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 (primarily in plants)
Fig. 6-32
Tight junction
0.5 µm
1 µmDesmosome
Gap junction
Extracellularmatrix
0.1 µm
Plasma membranesof adjacent cells
Spacebetweencells
Gapjunctions
Desmosome
Intermediatefilaments
Tight junction
Tight junctions preventfluid from movingacross a layer of cells