BIOL 235 – Chapter 3 Notes: The Cellular Level of Organization Cells – the living structural and functional units enclosed by a membrane Cell division – the process by which a new cell is formed by the division of one cell into two Cell biology – or cytology is the study of cellular structure and function the structure and function of a cell are intimately related The cell has three main parts 1. The plasma membrane - Forms the cell’s outer surface (separating internal environment from the external) - It is a selective barrier meaning that it regulates the flow of materials into and out of the cell helping to establish the appropriate environment for normal cellular activity - Plays a key role in communication among cells 2. The Cytoplasm - Consists of cellular contents between the plasma membrane and the nucleus - Has two components: cytosol (the fluid portion of the cytoplasm contains water, dissolved solutes, and suspended particles) and organelles (sit within the cytoplasm and have characteristic shapes and functions) 3. The Nucleus - Large organelle housing DNA - Within it each chromosome (single DNA molecule associated with several proteins) contains thousands of hereditary units called genes that control aspects of cellular structure and function The Plasma Membrane (in detail) - Flexible yet sturdy barrier - Fluid mosaic model – the molecular arrangement of the plasma resembles a continually moving sea of fluid lipids that contains a mosaic of different proteins (some floating freely – like icebergs, and others anchored at specific locations – like islands) - Lipids allow passage of several lipid-soluble molecules but act as barriers to any polar or charge substances that wish to enter or exit - Proteins allow the passage of some of these polar or charged substances into and out of the cell - Other proteins act as signal receptors or molecules that link the plasma to the intracellular or extracellular proteins The Lipid Bilayer of the Plasma Membrane - Two back to back layers made up of three types of lipid molecules (phospholipids, cholesterol and glycolipids ) – 75% are phospholipids - Arrangement is because lipids are amphipathic (meaning they are both polar and nonpolar) o In phospholipids the head is hydrophilic while the tail is hydrophobic, the phospholipids then orient themselves such that the heads are aligned and point outwards and the tails inwards – this way the heads meet a watery substance on both sides of the cells while the tails avoid the water o Cholesterol is weakly amphipathic (hydrocarbon tail is nonpolar while the hydroxyl group is the only polar part) o In glycolipids the carbohydrates form the hydrophilic heads while the fatty acid tails are nonpolar, the glycolipids face only towards the extracellular fluid which results in the bilayer being asymmetric Proteins in the Membrane - Integral (firmly embedded) or peripheral (free-floating) according to whether or not they are firmly embedded o Most integral proteins are transmembrane proteins
You've reached the end of your free preview.
Want to read all 10 pages?