Chemistry, Third edition

PHOTOSYNTHESIS

Photosynthesis

What is photosynthesis?

Photosynthesis is a complicated sequence of chemical reactions, driven by absorbed

sunlight, in which carbohydrates are made. Photosynthesis is one of the most

important of all reactions which involve light, because plants (either directly or in-

directly) are the main source of our food.

The overall reaction involved in photosynthesis may be represented as

6CO 2 (g) + 6H 2 O(l) + h"→C 6 H 12 O 6 (s) + 6O 2 (g)

glucose

Key points about photosynthesis

●This reaction is highly endothermic with H—o +2800 kJ mol–1. The reaction

does not occur directly but in many steps, and in effect the energy for the conver-

sion is obtained from sunlight (h"). This energy is initially trapped (i.e. absorbed)

by chlorophyll, forming excited chlorophyllmolecules. The excited chlorophyll

molecules then pass on this energy to enable the sequence of reactions to occur.

●Cellulose (which provides plants with their bulk and mass) is manufactured

within the plant by bonding together a large number (typically 300–2500) of glu-

cose molecules in a straight chain, with the elimination of one water molecule

from each glucose unit. (Taking H 2 O from the formula for glucose, C 6 H 12 O 6 ,

gives C 6 H 10 O 5 , so that cellulose can be represented by the formula (C 6 H 10 O 5 )n.)

●Starch (which is used by plants as a ‘fuel’ to supply energy for respiration and

growth) is also made from glucose molecules linked together although, unlike cel-

lulose, both straight and branched chains are present.

●In photosynthesis, the carbon (cellulose-based) skeleton of plant tissue and its

stored carbohydrates are ‘chemically assembled’ from carbon dioxide gas. The

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Beer–Lambert plot

The absorbance of different concentrations of manganate(VII) ions at 526 nm in a 1.0 cm

glass cell was found by experiment:

[MnO 4 – (aq)]/mol dm–3 Absorbance at 526 nm

0.0 0.00

8.7 10 –5 0.25

12.2 10 –5 0.35

14.0 10 –5 0.40

17.5 10 –5 0.50

(i) Convert the concentrations to mol m–3.

(ii)Plot absorbance against concentration and hence calculate the molar absorption

coefficient of the MnO 4 – (aq) ion at 526 nm.

Exercise 20Q