Instant Notes: Plant Biology

Plant cells produce a range of monosaccharides, from 3-carbon triose sugars

such as dihydroxyacetone and glyceraldehyde to 6-carbon hexoses such as

D-glucose, D-fructose, D-mannose and D-galactose (Fig. 4). Sucrose is a

disaccharide, made of glucose and fructose. Polysaccharides are large polymers

of these monosaccharides. By far the most important of these as a storage

product is starch, made up of α-(1–4) and α-(1–6) branched D-glucose residues

(Fig. 4). Cellulose, a major structural component of cell walls, is another poly-

saccharide, made up of β-Dglucose (Fig. 4).

Sucrose,
polysaccharides
and starch

J5 – Amino acid, lipid, polysaccharide and secondary product metabolism 157

RC

O

O

O O

H 2

H 2

C

C

C

H

O

O

C

C

R

R

Fig. 3. The structure of a triacylglycerol.

Triose sugars (3C)

Tetrose (4C)

Pentose (5C)

Hexose (6C)

Heptose (7C)

sugars

Important

hexoses (6C)

Ring structures

H

CO

CHOH

CH 2 OH

CH 2 OH

CH 2 OH

HO

C

H C

D-erythrose D-ribose D-glucose D-sedoheptulose

4C 5C 6C 7C

OH

HOHC

H 2 C

HO

C

HOHC

HOHC

HOHC

OH

H 2 COH

H 2 C

HO

C

HOHC

HO C H

HOHC

HOHC

OH

H 2 C

C

HOC H

O

H 2 C

H C

OH

H C

H C

OH

OH

OH

OH

HO

C

HOHC

HO C H

HOHC

H C OH

CH 2 OH

HO

C

HOHC

HO C

HO

H

C H

H C OH

CH 2 OH

HO

C

HO C H

HO C

H

H

C OH

OH

HOOH HOH

H

HHH

OH

O

H C

CH 2 OH

CH 2 OH

CO

HOC H

HOHC

H C OH

CH 2 OH

CH 2 OH

C O

Glyceraldehyde Dihydroxyacetone

D-glucose D-fructose

α-D-glucose β-D-glucose β-D-fructose

D-mannose D-galactose

HOOH HH

H

HOHH

OH

CH 2 OHO

HO H

CH 2 OH

HOH 2 COHO

HHHO

Fig. 4. Structures of major sugars. Note that for ring structures, αorβ depends on the posi-

tion of the hydroxyl (OH) groups adjacent to the oxygen which closes the ring.