bio201 – anatomy and physiology i biological macromolecules kamal gandhi lecture 2

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