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Carbohydrates

Copyright 1999-2008 by Joyce J. Diwan.

All rights reserved.

Molecular Biochemistry I

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Monosaccharides - simple sugars with multiple OH

groups. Based on number of carbons (3, 4, 5, 6), amonosaccharide is a triose, tetrose,pentose orhexose.

Disaccharides - 2 monosaccharides covalently linked.

Oligosaccharides - a few monosaccharides covalentlylinked.

Polysaccharides - polymers consisting of chains of

monosaccharide or disaccharide units.

I

(CH2O)n or H-C-OHI

Carbohydrates (glycans) have the followingbasic composition:

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Monosaccharides

Aldoses (e.g., glucose) have

an aldehyde group at one end.

Ketoses (e.g., fructose) have

a keto group, usually at C2.

C

C OHH

C HHO

C OHH

C OHH

CH2OH

D-glucose

OH

C HHO

C OHH

C OHH

CH2OH

CH2OH

C O

D-fructose

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D vs L Designation

D & L designations

are based on the

configuration aboutthe single asymmetric

C in glyceraldehyde.

The lower

representations are

Fischer Projections.

CHO

C

CH2OH

HO H

CHO

C

CH2OH

H OH

CHO

C

CH2OH

HO H

CHO

C

CH2OH

H OH

L-glyceraldehydeD-glyceraldehyde

L-glyceraldehydeD-glyceraldehyde

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Sugar Nomenclature

For sugars with more

than one chiral center,

D orLrefers to theasymmetric C farthest

from the aldehyde or

keto group.

Most naturally occurring

sugars are D isomers.

O H O H

C C

H C OH HO C H

HO C H H C OH

H C OH HO C H

H C OH HO C H

CH2OH CH2OH

D-glucose L-glucose

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D & L sugars are mirror

images of one another.

They have the samename, e.g., D-glucose

& L-glucose.

Other stereoisomers

have unique names,

e.g., glucose, mannose,

galactose, etc.

The number of stereoisomers is 2n, where n is thenumber of asymmetric centers.

The 6-C aldoses have 4 asymmetric centers. Thus there

are 16 stereoisomers (8 D-sugars and 8 L-sugars).

O H O H

C C

H C OH HO C HHO C H H C OH

H C OH HO C H

H C OH HO C H

CH2OH CH2OH

D-glucose L-glucose

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Hemiacetal & hemiketal formation

An aldehyde can

react with an

alcohol to form

a hemiacetal.

A ketone canreact with an

alcohol to form

a hemiketal.

O C

H

R

OH

O C

R

R'

OHC

R

R'

O

aldehyde alcohol hemiacetal

ketone alcohol hemiketal

C

H

R

O R'R' OH

"R OH "R

+

+

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Pentoses and

hexoses can cyclize

as the ketone or

aldehyde reacts

with a distal OH.

Glucose forms an

intra-molecularhemiacetal, as the

C1 aldehyde &

C5 OH react, to

form a 6-memberpyranose ring,

named after pyran.

These representations of the cyclic sugars are called

Haworth projections.

H O

OH

H

OHH

OH

CH2OH

H

OH

H HO

OH

H

OHH

OH

CH2OH

H

H

OH

-D-glucose -D-glucose

23

4

5

6

1 1

6

5

4

3 2

H

CHO

C OH

C HHO

C OHH

C OHH

CH2OH

1

5

2

3

4

6

D-glucose

(linear form)

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Fructose forms either

a 6-member pyranose ring, by reaction of the C2 keto

group with the OH on C6, or

a 5-member furanose ring, by reaction of the C2 keto

group with the OH on C5.

CH2OH

C O

C HHO

C OHH

C OHH

CH2OH

HOH2C

OH

CH2OH

H

OH H

H HO

O

1

6

5

4

3

2

6

5

4 3

2

1

D-fructose (linear) -D-fructofuranose

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Cyclization of glucose produces a new asymmetric center

at C1. The 2 stereoisomers are called anomers, & .

Haworth projections represent the cyclic sugars as havingessentially planar rings, with the OH at the anomeric C1:

(OHbelow the ring) (OH above the ring).

H O

OH

H

OHH

OH

CH2OH

H

-D-glucose

OH

H HO

OH

H

OHH

OH

CH2OH

H

H

OH

-D-glucose

23

4

5

6

11

6

5

4

3 2

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Because of the tetrahedral nature of carbon bonds,

pyranose sugars actually assume a "chair" or "boat"

configuration, depending on the sugar.

The representation above reflects the chair configuration

of the glucopyranose ring more accurately than the

Haworth projection.

O

H

HO

H

HO

H

OH

OHHH

OH

O

H

HO

H

HO

H

H

OHHOH

OH

-D-glucopyranose -D-glucopyranose

1

6

5

4

32

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Sugar derivatives

sugar alcohol - lacks an aldehyde or ketone; e.g., ribitol.

sugar acid - the aldehyde at C1, or OH at C6, is oxidized

to a carboxylic acid; e.g., gluconic acid, glucuronic acid.

CH2OH

C

C

C

CH2OH

H OH

H OH

H OH

D-ribitol

COOH

C

C

C

C

H OH

HO H

H OH

D-gluconic acid D-glucuronic acid

CH2OH

OHH

CHO

C

C

C

C

H OH

HO H

H OH

COOH

OHH

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Sugar derivatives

amino sugar- an amino group substitutes for a hydroxyl.An example is glucosamine.

The amino group may be acetylated, as in

N-acetylglucosamine.

H O

OH

H

OH

H

NH2H

OH

CH2OH

H

-D-glucosamine

H O

OH

H

OH

H

NH

OH

CH2OH

H

-D-N-acetylglucosamine

C CH3

O

H

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N-acetylneuraminate (N-acetylneuraminic acid, also

called sialic acid) is often found as a terminal residue

of oligosaccharide chains of glycoproteins.Sialic acid imparts negative charge to glycoproteins,

because its carboxyl group tends to dissociate a proton

at physiological pH, as shown here.

NH O

H

COO

OH

H

HOH

H

H

R

CH3C

O

HC

HC

CH2OH

OH

OH

N-acetylneuraminate (sialic acid)

R =

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Glycosidic Bonds

The anomeric hydroxyl and a hydroxyl of another sugar

or some other compound can join together, splitting outwater to form a glycosidic bond:

R-OH + HO-R' R-O-R' + H2OE.g., methanol reacts with the anomeric OH on glucoseto form methyl glucoside (methyl-glucopyranose).

O

H

HO

H

HO

H

OH

OHHH

OH

-D-glucopyranose

O

H

HO

H

HO

H

OCH3

OHHH

OH

methyl--D-glucopyranose

CH3-OH+

methanol

H2O

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Cellobiose, a product of cellulose breakdown, is the

otherwise equivalent anomer (O on C1 points up).The (14) glycosidic linkage is represented as a zig-zag,

but one glucose is actually flipped over relative to the other.

H O

OH

H

OHH

OH

CH2OH

H

O H

OH

H

OHH

OH

CH2OH

H

O

HH

1

23

5

4

6

1

23

4

5

6

maltose

H

O

OH

H

OHH

OH

CH2OH

H

O OH

H

H

OHH

OH

CH2OH

H

H

H

O1

23

4

5

6

1

23

4

5

6

cellobiose

Disaccharides:

Maltose, a cleavage

product of starch(e.g., amylose), is a

disaccharide with an

(14) glycosidic

link between C1 - C4OH of 2 glucoses.

It is the anomer(C1 O points down).

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Otherdisaccharides include:

Sucrose, common table sugar, has a glycosidic bondlinking the anomeric hydroxyls ofglucose & fructose.

Because the configuration at the anomeric C of glucose

is (O points down from ring), the linkage is (12).

The full name of sucrose is -D-glucopyranosyl-(12)-

-D-fructopyranose.)

Lactose, milk sugar, is composed ofgalactose &glucose, with (14) linkage from the anomeric OH of

galactose. Its full name is -D-galactopyranosyl-(14)-

-D-glucopyranose

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Polysaccharides:

Plants store glucose as amylose oramylopectin, glucose

polymers collectively called starch.

Glucose storage in polymeric form minimizes osmotic

effects.

Amylose is a glucose polymer with (14) linkages.The end of the polysaccharide with an anomeric C1 not

involved in a glycosidic bond is called the reducing end.

H O

OH

H

OHH

OH

CH2OH

H

O H

H

OHH

OH

CH2OH

H

O

HH H O

O

H

OHH

OH

CH2OH

HH H

O

H

OHH

OH

CH2OH

H

OH

HH O

O

H

OHH

OH

CH2OH

H

O

H

1

6

5

4

3

1

2

amylose

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Amylopectin is a glucose polymer with mainly (14)

linkages, but it also has branches formed by (16)

linkages. Branches are generally longer than shown above.

The branches produce a compact structure & provide

multiple chain ends at which enzymatic cleavage can occur.

H O

OH

H

OHH

OH

CH2OH

H

O H

H

OHH

OH

CH2OH

H

O

HH H O

O

H

OHH

OH

CH2

HH H O

H

OHH

OH

CH2OH

H

OH

HH O

O

H

OHH

OH

CH2OH

H

O

H

O

1 4

6

H O

H

OHH

OH

CH2OH

HH H O

H

OHH

OH

CH2OH

H

H

O

1

OH

3

4

5

2

amylopectin

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Glycogen, the glucose storage polymer in animals, is

similar in structure to amylopectin.

But glycogen has more(16) branches.

The highly branched structure permits rapid glucose releasefrom glycogen stores, e.g., in muscle during exercise.

The ability to rapidly mobilize glucose is more essential to

animals than to plants.

H O

OH

H

OHH

OH

CH2OH

H

O H

H

OHH

OH

CH2OH

H

O

HH H O

O

H

OHH

OH

CH2

HH H O

H

OHH

OH

CH2OH

H

OH

HH O

OH

OHH

OH

CH2OH

H

O

H

O

1 4

6

H O

H

OHH

OH

CH2OH

HH H O

H

OHH

OH

CH2OH

H

H

O

1

OH

3

4

5

2

glycogen

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Cellulose, a major constituent ofplant cell walls, consists

of long linear chains of glucose with (14) linkages.

Every other glucose is flipped over, due to linkages.This promotes intra-chain and inter-chain H-bonds and

cellulose

H O

OH

H

OHH

OH

CH2OH

H

O

H

OHH

OH

CH2OH

HO

H H O

O H

OHH

OH

CH2OH

H

H O

H

OHH

OH

CH2OH

H

H

OHH O

O H

OHH

OH

CH2OH

HO

H H H H

1

6

5

4

3

1

2

van der Waals interactions,

that cause cellulose chains to

be straight & rigid, and packwith a crystalline arrangement

in thick bundles - microfibrils.

See: Botany online website;

website at Georgia Tech.

Schematic of arrangement of

cellulose chains in a microfibril.

http://web.chemistry.gatech.edu/~williams/bCourse_Information/6521/carbo/glu/cellulose_int_2.jpghttp://www.biologie.uni-hamburg.de/b-online/e26/26a.htmhttp://web.chemistry.gatech.edu/~williams/bCourse_Information/6521/carbo/glu/cellulose_int_2.jpghttp://www.biologie.uni-hamburg.de/b-online/e26/26a.htmhttp://www.biologie.uni-hamburg.de/b-online/e26/26a.htmhttp://www.biologie.uni-hamburg.de/b-online/e26/26a.htmhttp://web.chemistry.gatech.edu/~williams/bCourse_Information/6521/carbo/glu/cellulose_int_2.jpghttp://web.chemistry.gatech.edu/~williams/bCourse_Information/6521/carbo/glu/cellulose_int_2.jpghttp://www.biologie.uni-hamburg.de/b-online/e26/26a.htmhttp://www.biologie.uni-hamburg.de/b-online/e26/26a.htmhttp://www.biologie.uni-hamburg.de/b-online/e26/26a.htmhttp://www.biologie.uni-hamburg.de/b-online/e26/26a.htmhttp://www.biologie.uni-hamburg.de/b-online/e26/26a.htmhttp://www.biologie.uni-hamburg.de/b-online/e26/26a.htmhttp://www.biologie.uni-hamburg.de/b-online/e26/26a.htmhttp://www.biologie.uni-hamburg.de/b-online/e26/26a.htmhttp://www.biologie.uni-hamburg.de/b-online/e26/26a.htmhttp://www.biologie.uni-hamburg.de/b-online/e26/26a.htmhttp://www.biologie.uni-hamburg.de/b-online/e26/26a.htmhttp://www.biologie.uni-hamburg.de/b-online/e26/26a.htmhttp://www.biologie.uni-hamburg.de/b-online/e26/26a.htmhttp://web.chemistry.gatech.edu/~williams/bCourse_Information/6521/carbo/glu/cellulose_int_2.jpghttp://web.chemistry.gatech.edu/~williams/bCourse_Information/6521/carbo/glu/cellulose_int_2.jpghttp://www.biologie.uni-hamburg.de/b-online/e26/26a.htmhttp://www.biologie.uni-hamburg.de/b-online/e26/26a.htmhttp://web.chemistry.gatech.edu/~williams/bCourse_Information/6521/carbo/glu/cellulose_int_2.jpghttp://web.chemistry.gatech.edu/~williams/bCourse_Information/6521/carbo/glu/cellulose_int_2.jpghttp://www.biologie.uni-hamburg.de/b-online/e26/26a.htmhttp://www.biologie.uni-hamburg.de/b-online/e26/26a.htmhttp://www.biologie.uni-hamburg.de/b-online/e26/26a.htmhttp://web.chemistry.gatech.edu/~williams/bCourse_Information/6521/carbo/glu/cellulose_int_2.jpghttp://web.chemistry.gatech.edu/~williams/bCourse_Information/6521/carbo/glu/cellulose_int_2.jpghttp://web.chemistry.gatech.edu/~williams/bCourse_Information/6521/carbo/glu/cellulose_int_2.jpghttp://www.biologie.uni-hamburg.de/b-online/e26/26a.htmhttp://www.biologie.uni-hamburg.de/b-online/e26/26a.htmhttp://www.biologie.uni-hamburg.de/b-online/e26/26a.htmhttp://www.biologie.uni-hamburg.de/b-online/e26/26a.htm

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Multisubunit Cellulose Synthase complexes in the plasma

membrane spin out from the cell surface microfibrilsconsisting of 36 parallel, interacting cellulose chains.

These microfibrils are very strong.

The role of cellulose is to impart strength and rigidity to

plant cell walls, which can withstand high hydrostatic

pressure gradients. Osmotic swelling is prevented.

Explore and compare structures of amylose & cellulose

using Chime.

cellulose

H O

OH

H

OHH

OH

CH2OH

H

O

H

OHH

OH

CH2OH

HO

H H O

O H

OHH

OH

CH2OH

H

H O

H

OHH

OH

CH2OH

H

H

OHH O

O H

OHH

OH

CH2OH

HO

H H H H

1

6

5

4

3

1

2

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Glycosaminoglycans (mucopolysaccharides) are linear

polymers ofrepeating disaccharides.

The constituent monosaccharides tend to be modified,

with acidic groups, amino groups, sulfated hydroxyl and

amino groups, etc.

Glycosaminoglycans tend to be negatively charged,

because of the prevalence of acidic groups.

H O

H

H

OHH

OH

COO

H

H O

OHH

H

NHCOCH3H

CH2OH

H

OO

D-glucuronate

O

1

23

4

5

61

23

4

5

6

N-acetyl-D-glucosamine

hyaluronate

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Hyaluronate(hyaluronan) is a glycosaminoglycan with a

repeating disaccharide consisting of 2 glucose derivatives,glucuronate (glucuronic acid) & N-acetyl-glucosamine.

The glycosidic linkages are (13) & (14).

H O

H

H

OHH

OH

COO

H

H O

OH H

H

NHCOCH3H

CH2OH

H

OO

D-glucuronate

O

1

23

4

5

6

1

23

4

5

6

N

-acetyl-D-glucosaminehyaluronate

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Proteoglycans are glycosaminoglycans that arecovalently linked to serine residues of specific

core proteins.

The glycosaminoglycan chain is synthesized by

sequential addition of sugar residues to the core protein.

heparan sulfateglycosaminoglycan

cytosol

core

protein

transmembrane-helix

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Some proteoglycans of the extracellular matrix bind

non-covalently to hyaluronate via protein domains calledlink modules. E.g.:

Multiple copies of the aggrecan proteoglycan associatewith hyaluronate in cartilage to form large complexes.

Versican, another proteoglycan, binds hyaluronate in

the extracellular matrix of loose connective tissues.

HO

H

H

OHH

OH

COO

H

H O

OH H

H

NHCOCH3H

CH2OH

H

OO

D-glucuronate

O

1

23

4

5

61

23

4

5

6

N-acetyl-D-glucosamine

hyaluronate

Websites on:

Aggrecan

Aggrecan &

versican.

http://www.glycoforum.gr.jp/science/word/proteoglycan/PGA03E.htmlhttp://www.glycoforum.gr.jp/science/word/proteoglycan/PGA03E.htmlhttp://www.glycoforum.gr.jp/science/word/proteoglycan/PGA03E.htmlhttp://www.glycoforum.gr.jp/science/word/proteoglycan/PGA03E.htmlhttp://www.glycoforum.gr.jp/science/word/proteoglycan/PGA03E.htmlhttp://www.glycoforum.gr.jp/science/word/proteoglycan/PGA03E.htmlhttp://www.glycoforum.gr.jp/science/word/proteoglycan/PGA03E.htmlhttp://www.glycoforum.gr.jp/science/word/proteoglycan/PGA03E.htmlhttp://www.glycoforum.gr.jp/science/word/proteoglycan/PGA03E.htmlhttp://www.glycoforum.gr.jp/science/word/proteoglycan/PGA03E.htmlhttp://www.glycoforum.gr.jp/science/word/proteoglycan/PGA03E.htmlhttp://www.glycoforum.gr.jp/science/word/proteoglycan/PGA03E.htmlhttp://www.glycoforum.gr.jp/science/word/proteoglycan/PGA03E.htmlhttp://www.glycoforum.gr.jp/science/word/proteoglycan/PGA03E.htmlhttp://www.glycoforum.gr.jp/science/word/proteoglycan/PGA03E.htmlhttp://www.cmb.lu.se/ctb/html/Aggrecan.htmhttp://www.glycoforum.gr.jp/science/word/proteoglycan/PGA03E.htmlhttp://www.glycoforum.gr.jp/science/word/proteoglycan/PGA03E.htmlhttp://www.glycoforum.gr.jp/science/word/proteoglycan/PGA03E.htmlhttp://www.glycoforum.gr.jp/science/word/proteoglycan/PGA03E.htmlhttp://www.cmb.lu.se/ctb/html/Aggrecan.htm

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Heparan sulfate is initially synthesized on a membrane-

embedded core protein as a polymer of alternating

N

-acetylglucosamine and glucuronate residues.Later, in segments of the polymer, glucuronate residues

may be converted to the sulfated sugariduronic acid,

while N-acetylglucosamine residues may be deacetylated

and/or sulfated.

H O

H

OSO3

H

OH

H

COO

O H

H

NHSO3

H

OH

CH2OSO3

H

H

H

O

O

heparin or heparan sulfate - examples of residues

iduronate-2-sulfate N-sulfo-glucosamine-6-sulfate

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Heparin, a soluble glycosaminoglycan

found in granules of mast cells, has a

structure similar to that of heparansulfates, but is more highly sulfated.

When released into the blood, it

inhibits clot formation by interacting

with the protein antithrombin.

Heparin has an extended helical

conformation.

heparin: (IDS-SGN)5

PDB 1RID

C O N S

Charge repulsion by the many negatively charged groupsmay contribute to this conformation.

Heparin shown has 10 residues, alternating IDS (iduronate-

2-sulfate) & SGN (N-sulfo-glucosamine-6-sulfate).

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The core protein of a syndecan heparan sulfate

proteoglycan includes a single transmembrane -helix,as in the simplified diagram above.

The core protein of a glypican heparan sulfate

proteoglycan is attached to the outer surface of the

plasma membrane via covalent linkage to a modified

phosphatidylinositol lipid.

heparan sulfateglycosaminoglycan

cytosol

coreprotein

transmembrane-helix

Some cell surface heparan

sulfate glycosaminoglycansremain covalently linked to

core proteins embedded in

the plasma membrane.

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Proteins involved in signaling & adhesion at the cell

surface recognize & bind heparan sulfate chains.

E.g., binding of some growth factors (small proteins) tocell surface receptors is enhanced by their binding also to

heparan sulfates.

Regulated cell surface Sulfenzymes may remove sulfate

groups at particular locations on heparan sulfate chains toalter affinity

for signal

proteins, e.g.,

growth factors. H O

H

OSO3

H

OH

H

COO

O H

H

NHSO3

H

OH

CH2OSO3

H

H

H

O

O

heparin or heparan sulfate - examples of residues

iduronate-2-sulfate N-sulfo-glucosamine-6-sulfate

Diagramby Kirkpatrick

& Selleck.

http://dx.doi.org/doi:10.1242/jcs.03432http://dx.doi.org/doi:10.1242/jcs.03432http://dx.doi.org/doi:10.1242/jcs.03432http://dx.doi.org/doi:10.1242/jcs.03432http://dx.doi.org/doi:10.1242/jcs.03432

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O-linked oligosaccharide chains of glycoproteins vary

in complexity.

They link to a protein via a glycosidic bond between asugar residue & a serine or threonineOH.

O-linked oligosaccharides have roles in recognition,

interaction, and enzyme regulation.

H O

OH

O

H

HNH

OH

CH2OH

H

C CH3

O

-D-N

-acetylglucosamine

CH2 CH

C

NH

O

H

serineresidue

Oligosaccharides

that are covalentlyattached to proteins

or to membrane

lipids may be linear

or branched chains.

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H O

OH

O

H

HNH

OH

CH2OH

H

C CH3

O

-D-N-acetylglucosamine

CH2 CH

C

NH

O

H

serine

residue

N-acetylglucosamine (GlcNAc) is a common O-linkedglycosylation of protein serine or threonine residues.

Many cellular proteins, including enzymes & transcription

factors, are regulated by reversible GlcNAc attachment.

Often attachment of GlcNAc to a protein OH alternates

with phosphorylation, with these 2 modifications having

opposite regulatory effects (stimulation or inhibition).

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N-linkedoligosaccharides of glycoproteins tend to becomplex and branched.

First N-acetylglucosamine is linked to a protein via the

side-chain N of an asparagine residue in a particular

3-amino acid sequence.

H O

OH

HN

H

H

HNH

OH

CH2OH

H

C CH3

O

C CH2 CH

O HN

C

HN

O

HC

C

HN

HC

R

O

C

R

O

Asn

X

Ser or ThrN-acetylglucosamine

Initial sugar in N-linked

glycoprotein oligosaccharide

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Additional monosaccharides are added, and the N-linked

oligosaccharide chain is modified by removal and addition

of residues, to yield a characteristic branched structure.

NAN

Gal

NAG

Man

NAG

Gal

NAN

Man

Man

NAG

Gal

NAN

NAG

NAG

Asn

Fuc

N-linked oligosaccharide

Key:

NAN = N-acetylneuraminate

Gal = galactose

NAG= N-acetylglucosamine

Man= mannose

Fuc = fucose

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Many proteins secreted by cells have attached N-linked

oligosaccharide chains.

Genetic diseases have been attributed to deficiency of

particular enzymes involved in synthesizing or modifying

oligosaccharide chains of these glycoproteins.

Such diseases, and gene knockout studies in mice, havebeen used to define pathways of modification of

oligosaccharide chains of glycoproteins and glycolipids.

Carbohydrate chains of plasma membrane glycoproteinsand glycolipids usually face the outside of the cell.

They have roles in cell-cell interaction and signaling, and

in forming a protective layer on the surface of some cells.

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Lectins are glycoproteins that recognize and bind to

specific oligosaccharides.

Concanavalin A & wheat germ agglutinin are plantlectins that have been useful research tools.

The C-type lectin-like domain is a Ca++-binding

carbohydrate recognition domain in many animal lectins.

Recognition/binding of CHO moieties of glycoproteins,

glycolipids & proteoglycans by animal lectins is a factor in:

cell-cell recognition

adhesion of cells to the extracellular matrix

interaction of cells with chemokines and growth factors

recognition of disease-causing microorganisms

initiation and control of inflammation.

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Examples of animal lectins:

Mannan-binding lectin (MBL) is a glycoprotein foundin blood plasma.

It binds cell surface carbohydrates ofdisease-causing

microorganisms & promotes phagocytosis of theseorganisms as part of the immune response.

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A cleavage site just outside the transmembrane -helix

provides a mechanism for regulated release of some lectinsfrom the cell surface.

A cytosolic domain participates in regulated interaction

ith th ti t k l t

transmembrane

-helix

lectin domainselectin

cytoskeletonbinding domain

cytosol

outside

Selectins are integral proteins

of mammalian cell plasma

membranes with roles incell-cell recognition & binding.

The C-type lectin-like domain

is at the end of a multi-domain

extracellular segment extending

out from the cell surface.