Topic Membranes IB Biology - Period 5 Trevor Kosmo

Key Vocabulary Integral Protein Peripheral Protein Glycoprotein Cholesterol Plasma Membrane Hydrophilic Hydrophobic Diffusion Osmosis Active Transport Vesicle Endocytosis Exocytosis

Draw and label a diagram to show the structure of membranes. See Campbell: p. 142, figure 8.6

Explain how the hydrophobic and hydrophilic properties of phospholipids help to maintain the structure of cell membranes. The head of the phospholipid is polar and hydrophilic (water- loving), and these heads make up the outside of the phospholipid bilayer. The tail of the phospholipid is non-polar and hydrophobic (water-fearing). Because one end of the phospholipid is hydrophobic and the other is hydrophilic, phospholipids naturally form bilayers in which the heads are facing outward (toward the water), and the tails are facing inward (away from the water). Therefore, the characteristics of phospholipids enable the phospholipids to form a stable structure. See Campbell: pg 139, figure 8.1

List the functions of membrane proteins. Hormone binding sites Immobilized enzymes Cell adhesion Cell-to-cell communication Channels for passive transport Pumps for active transport See Campbell: pg. 144, figure 8.9

Define diffusion and osmosis. Diffusion is the total movement of particles from a region of higher concentration to a region of lower concentration. The difference in concentration that drives diffusion is called a concentration gradient. Osmosis is the passive movement of water molecules, across a partially permeable membrane, from a region of lower solute concentration to a region of higher solute concentration. See Campbell: pg

Explain passive transport across membranes by simple diffusion and facilitated diffusion. Passive transport happens naturally (it requires no energy from the cell) if there is a concentration gradient between one side of the membrane and the other. This is called simple diffusion. Membranes are partially permeable in that they allow some substances to diffuse through but not others. To allow some substances to diffuse through, channel proteins are needed. This is called facilitated diffusion. See Campbell: pg

Explain the role of protein pumps and ATP in active transport across membranes. Active transport is the movement of substances across membranes using energy from ATP. Active transport can move substances against the concentration gradient- from a region of lower concentration to a region of higher concentration. Protein pumps in the membrane are used for active transport. Each pump only transports particular substances, so cells can control what is absorbed and what is expelled. See Campbell: pg

2.4.6 (continued) The sodium-potassium pump is an important cellular process, and is an example of active transport. ATP is used to pump sodium into the cell and potassium out of the cell. (NOTE: the syllabus does not require knowledge of the sodium potassium pump; here it is just used as an example.)

Explain how vesicles are used to transport materials within a cell between the rough endoplasmic reticulum, the Golgi apparatus and the plasma membrane. Proteins are synthesized by ribosomes and then enter the rough endoplasmic reticulum. Vesicles bud off from the rER and carry the proteins to the Golgi apparatus. The Golgi apparatus modifies the proteins. Vesicles bud off from the Golgi apparatus and carry the modified proteins to the plasma membrane. Once the vesicle has reached the plasma membrane, exocytosis occurs: –Vesicles fuse with the plasma membrane –The contents of the vesicle are expelled –The membrane then flattens out again

Explain how the fluidity of the membrane allows it to change shape, break and re-form during endocytosis and exocytosis. Endocytosis is the movement of material into a cell by a process in which the plasma membrane engulfs extracellular material, forming membrane-bound sacs that enter the cytoplasm. Exocytosis is the movement of material out of a cell by a process in which intracellular material is enclosed within a vesicle that moves to the plasma membrae and fuses with it, releasing the material outside the cell. The cell membrane is fluid in that it is constantly in motion. The movement of the phospholipids changes the membrane's shape, and allows for temporary holes in the membrane that let materials flow in and out of the cell. If the membrane were not fluid in nature, it would not be able to fuse with vesicles in endocytosis and exocytosis. See Campbell: pg 151

2.4.8 (continued) Visual representation of the processes of endocytosis and exocytosis.