Biology 12

84 MHR • Unit 1 Metabolic Processes

Structure of Chloroplasts

In plant cells, photosynthesis occurs within

chloroplasts. Chloroplasts have a double membrane

and contain membrane pockets called thylakoids

(see Figure 3.20). Thylakoids occur in stacked,

parcel-like structures called grana (singular granum),

which are held together by support structures

called lamellae. The stroma, a thick, enzyme-rich

liquid, fills the interior of each chloroplast.

Mesophyll cells in the leaves of plants are

specialized for photosynthesis and contain

numerous chloroplasts. These cells provide the

chloroplasts with the two important ingredients

necessary for photosynthesis — carbon dioxide and

water. Gas exchange (oxygen and carbon dioxide)

occurs through pores on the underside of leaves,

and water is delivered via veins that extend to the

roots of the plant.

Within the grana, solar light energy is captured

by the thylakoids. This energy is used to form

ATP molecules, which fuel the production of

carbohydrates. These carbohydrate molecules are

then used to synthesize glucose — the molecules

used in cellular respiration. The thylakoid

membrane in the chloroplast is the site of ATP

production, using chemiosmosis and complex

structures functionally similar to those found in

mitochondria.

Stages of Photosynthesis

As the previous section suggests, there are two main

stages of photosynthesis: the photoand synthesis

stages. The first stage of photosynthesis converts

solar energy into chemical energy. The second stage

uses this energy to produce PGAL, which is then

used to form glucose (see Figure 3.21). The photo

reactions require light and are called light-

dependent reactions. The synthesisreactions do

not require light directly, and are called light-

independent reactions. However, light seems to be

important in activating enzymes in both the photo

and synthesisreactions.

Figure 3.21Two stages of photosynthesis. The first stage

(A) consists of reactions that require light energy. The

second stage (B) involves the synthesis of glucose molecules.

As light strikes the leaf of a plant, the energy is

captured by pigments in the chloroplasts. These

pigments, known as chlorophylls, absorb various

wavelengths of visible light (see Figure 3.22). The

two most important types of chlorophyll are

chlorophyll aand chlorophyll b. Photosynthesis is

most active at light wavelengths of about 400 nm

to 450 nm and 650 nm to 700 nm. The colour of

chlorophyll, green, is a result of the absorption of

mainly blue and red parts of the visible light

spectrum. In the following MiniLab, you will

extract chlorophyll from leaves and examine the

colour and properties of both types of chlorophyll.

sunlight

photo reactions

synthesis

reactions

organic

molecules

A B

leaf cross section

mesophyll cell

chloroplast

vacuole

nucleus

cell wall granum

thylakoid stroma

lamella

inner

membrane

outer

membrane

opening for C

to enter leaf

O 2

Figure 3.20Structure

of a chloroplast