WJEC Biology for AS: Student Bk

▪ ▪ Within the stroma are many closed, flattened sacs called thylakoids . A stack of thylakoids is a granum . Each granum comprises between two and a hundred parallel sacs. The photosynthetic pigments, such as chlorophyll, are found in the thylakoids. This arrangement produces a large surface area, efficient for trapping light energy. Basic structure of a chloroplast The endosymbiotic theory Endosymbiosis is a theory that describes the origin of chloroplasts and mitochondria. As far back as 1883, the division of chloroplasts was seen to closely resemble that of free- living cyanobacteria. In the 1920s, the idea that mitochondria were once independent bacteria was also suggested. Mitochondria and chloroplasts have 70S ribosomes and circular DNA. It is suggested that at least 1.5 × 10 9 years ago, some ancient bacteria with very fluid membranes engulfed others and maintained a symbiotic relationship with them. Some of those engulfed were very good at turning glucose and oxygen into ATP, evolving eventually into mitochondria. Some could turn carbon dioxide and water into glucose, evolving eventually into chloroplasts. In 1967, Lynn Margulis published a paper titled ‘The origin of mitosing cells’ which substantiated the idea of endosymbiosis. Now, biologists have plenty of evidence that both chloroplasts and mitochondria have their origins in free-living cells. Endoplasmic reticulum (ER) The endoplasmic reticulum is an elaborate system of parallel double membranes forming flattened sacs with interconnected, fluid-filled spaces between them, called cisternae . These are pink in the electron micrograph. The ER is connected with the nuclear envelope. This system allows the transport of materials through the cell. There are two types of ER: ▪ ▪ Rough ER (RER) has ribosomes on the outer surface and transports the proteins made there. RER is present in large amounts in cells that make a lot of protein, such as cells making amylase in the salivary glands. ▪ ▪ Smooth ER comprises membranes that lack ribosomes. It is associated with the synthesis and transport of lipids. Cells that store large quantities of carbohydrates, proteins and fats, including liver and secretory cells, have extensive ER. Electron micrograph of chloroplast Endoplasmic reticulum ⎧ ⎨ ⎩ stroma double membrane starch grain granum 70S ribosomes circular DNA inter-granal lamella / thylakoid thylakoid thylakoid membrane cisterna WJEC AS Level Biology: Chapter 2 34