WJEC Biology for AS: Student Bk

Study point Not all plant cells contain chloroplasts. Root cells, for example, being underground, would have no use for them. Mitochondria Mitochondria are often cylindrical and 1–10 μm in length. They comprise: ▪ ▪ Two membranes, separated by a narrow, fluid-filled inter-membrane space . The inner membrane is folded inwards to form cristae . ▪ ▪ An organic matrix , which is a solution containing many compounds, including lipids and proteins. ▪ ▪ Small (70S) ribosomes and a small circle of DNA which enable mitochondria to make some of their own proteins and self-replicate. The function of mitochondria is to produce ATP in aerobic respiration. Some of the reactions occur in the matrix and others on the inner membrane. The cristae provide a large surface area for the attachment of enzymes involved in respiration. Metabolically active cells, such as muscle cells, need a plentiful supply of ATP. They contain many mitochondria, reflecting the high metabolic activity taking place. Electron micrograph of section through a mitochondrion Being cylindrical, mitochondria have a larger surface area than a sphere of the same volume, in other words, their surface area to volume ratio is bigger. Compared with a sphere, being a cylinder reduces the diffusion distance between the edge and the centre, making aerobic respiration more efficient. Chloroplasts Chloroplasts occur in the cells of photosynthesising tissue. In many plants the highest concentration is in the palisade mesophyll cells, just below the upper surface of the leaf. Each chloroplast is surrounded by two membranes , comprising the chloroplast envelope. ▪ ▪ The stroma is fluid-filled and contains some of the products of photosynthesis, including lipid droplets and starch grains, which can take up a large part of the stroma. ▪ ▪ Like mitochondria, they contain 70S ribosomes and circular DNA which enable them to make some of their own proteins and self-replicate. Diagram of section through a mitochondrion For a cylinder of length, l = 1.0 µm and diameter, d = 0.5 µm: d = 2 × radius radius, r = 0.25 µm Volume, V = πr 2 l = π × 0.25 2 × 1.0 = 0.20 µm 3 (2 dp) For a sphere of volume 0.20 µm 3 V = 0.20 = 3 4 πr 3 ∴ r 3 = 4 π 0.20 × 3 ∴ r = 0.36 µm (2 dp) So if a sphere and a cylinder have the same volume, the cylinder has a smaller radius. outer membrane inner membrane inter-membrane space cristae 70S ribosomes matrix DNA Cell structure and organisation 33