Chapter 4.8

Specialized Plant Organelles

AP BiologyFall 2010

Objective

• Describe the structure and function of the chloroplast

• Understand the differences between chloroplasts, chromoplasts, and amyloplasts

• Describe the structure and function of the central vacuole

Chloroplasts and Plastids

• Plastids: organelles that function in photosynthesis or storage in plants

• Three types:– Chloroplasts– Chromoplasts – Amyloplasts

Chloroplasts

• Chloroplast: oval or disk shaped organelles, bounded by a double membrane, and specialized for photosynthesis

Chloroplasts

• Structure:– Two outer membranes enclose their semifluid

interior, the stroma – In the stroma, third membrane forms a single

compartment that is commonly folded in intricate ways

– Fold resemble stack of flattened disks, called granum

– Photosynthesis occurs in thylakoid membrane

Chlorplasts

• In simpler terms:– Stroma: semifluid

interior– Thylakoid: contains

chlorophyll and other pigments, individual units

– Granum: stacks of thylakoids

Chloroplasts

• In the innermost membrane, stacked disks (thylakoids), pigments, and enzymes trap sunlight energy to form ATP and NADPH

• Sugars and starches are formed in the fluid substance (stroma) surrounding the stacks

• Pigments such as chlorophyll (green) confer distinctive colours to the chloroplasts

Chloroplasts

• Chlorophyll reflect green light• Many kinds of accessory pigments assist

chlorophyll in capturing light• Energy drives reactions– ATP and NADPH form – ATP and NADPH used at sites in stroma where

sugars, starches assembled – Starch grains: new starch molecules that

accumulate in stroma

Chloroplasts

• Chloroplasts are like photosynthetic bacteria and may have evolved through endosymbiosis like mitochondria

Chromoplasts

• Chromoplasts: store red and brown pigments (carotenoids) that give colour to flowers, autumn leaves, fruits, and roots

• Have no chlorophyll • Attract animals

Amyloplasts

• Amyloplasts: colourless, lack pigments– Store starch grains, and are abundant in stem

cells, tubers, and seeds – Found in stems, potato tubers (underground

stems), and seeds

Comparison

Central Vacuoles

• Central Vacuoles: – Fluid filled vacuoles – In a mature plant, the central vacuole may occupy

50 to 90% of the cell interior – Store amino acids, sugars, ions, and wastes – Vacuole enlarges during growth and greatly

increases the cell’s outer surface area • Enlarged cell, with more surface area, has an

enhanced ability to absorb nutrients

Central Vacuole

Review

1. Describe differences between amyloplasts, chloroplasts, and chromoplasts

2. What are the main structural components of the chloroplasts?

Answers

1. Amyloplasts are pigment free, store starch grains. Chromoplasts have no chlorophyll, have abundance of carotenoids, colours attract animals. Chloroplasts are specialized organelles for photosynthesis, contain chlorophyll.

2. Clorplasts: – Stroma: semifluid interior– Thylakoid: contains chlorophyll and other pigments,

individual units – Granum: stacks of thylakoids