Chapter 7- Membrane Structure and Function*

Transcription

1 Chapter 7- Membrane Structure and Function* *Lecture notes are to be used as a study guide only and do not represent the comprehensive information you will need to know for the exams. Life at the Edge The plasma membrane separates the living cell from its external environment. The plasma membrane determines the molecules that will enter and exit the cell by a process called selective permeability. Proteins associated with the plasma membrane help to determine the molecules that will move into and out of the cells. Concept 7.1 : Cellular membranes are fluid mosaics of lipids and proteins The primary components of membranes are lipids and proteins. The most common type of lipid is phospholipids. A phospholipid is an amphipathic molecule it has both hydrophilic and hydrophobic regions (Figure 7.2). Membranes are described using the fluid mosaic model (Figure 7.3). Mosaic because of the various proteins associated with the phospholipids. [See also Inquiry Do membrane proteins move? Figure 7.4.] The Fluidity of Membranes The phospholipids of membranes move very easy laterally, and not very easily by flip-flopping. Proteins associated with the membranes also move through the phospholipids (Figure 7.6). Membranes remain fluid until a certain temperature at which the phospholipids stop moving. The more phospholipids have unsaturated fatty acid tails, the more fluid the membrane will remain (Figure 7.5a). In animal cells, the steroid cholesterol has an effect on the fluidity of the membrane. At body temperature cholesterol makes the membrane less fluid by restraining phospholipid movement. At lower temperatures cholesterol prevents the phospholipids from packing close together (Figure 7.5b). Evolution of Differences in Membrane Lipid Composition The specific chemical structure of the hydrocarbon tails of phospholipids seems to have evolved such that different species can exist in different environments. Natural selection has apparently favored organisms whose mix of membrane lipids ensures an appropriate level of membrane fluidity for their environment. 1

2 Membrane Proteins and Their Functions The mosaic aspect of membranes comes from the fact that the membrane is a collage of different proteins clustered in groups, embedded in the fluid matrix of the lipid bilayer. Proteins determine most of the membrane s function. There are two main groups of membrane proteins: 1) integral proteins, proteins that span the width of the phospholipid bilayer. These proteins have a hydrophobic region in area of the fatty acid tails of phospholipids. The hydrophilic regions of these proteins are exposed on either side of the membrane. Another term associated with these proteins is transmembrane proteins (Figure 7.6); and 2) peripheral proteins are not embedded in the lipid bilayer, they are loosely associated to the surface of the membrane. Figure 7.7 shows the major functions of different membrane proteins. Learning about membrane proteins is important in the medical field as some microbes use membrane proteins to gain access to entry into the cell, as seen in HIV (Figure 7.8) The Role of Membrane Carbohydrates in Cell-Cell Recognition Cell-cell recognition is important for a cell s ability to recognize different cells. Carbohydrates are important in this recognition. Glycolipids are carbohydrates that are covalently bonded to lipids. Glycoproteins are carbohydrates that are covalently bonded to proteins. The glycolipids and glycoproteins are on the external side of the membrane and allow for distinguishing markers on cells. Synthesis and Sidedness of Membranes Each side of the membrane has a unique composition of lipids and proteins (Figure 7.9). Concept 7.2 : Membrane structure results in selective permeability The biological membrane is a supramolecular structure many molecules exist in an ordered structure. The membrane has the ability to regulate transport across cellular boundaries. Nutrients enter the cell and wastes are transported out of it. Cell membranes are selectively permeable. The Permeability of the Lipid Bilayer Certain hydrophobic molecules such as CO 2 and O 2 can easily cross the cell membrane. Polar molecules do not pass as easily across the hydrophobic region of the lipid bilayer. 2

3 Transport Proteins Hydrophilic substances can pass through the lipid bilayer by passing through transport proteins that span the membrane. Some transport proteins, called channel proteins, have a hydrophilic center that allows charged to pass across the lipid portion. An example of a channel protein is an aquaporin that allows for the passage of water across the membrane. Some transport proteins are called carrier proteins that escort their passengers across the membrane. The selective permeability of a membrane depends on both the discriminating barrier of the lipid bilayer and the specific proteins built into the membrane. There is also direction across the membrane. Concept 7.3 : Passive transport is diffusion of a substance across a membrane with no energy investment Diffusion is the movement of molecules that spread out in the available space (Figure 7.10a). Molecules will diffuse from an area where they are more concentrated to an area where they are less concentrated. Substances will diffuse down their concentration gradient. Energy is not needed for diffusion to occur (Figure 7.10b). The diffusion of a substance across a biological membrane is called passive transport because the cell does not have to use energy to make it happen. Effects of Osmosis on Water Balance The diffusion of water across a selectively permeable membrane in response to a solute concentration is called osmosis. Water moves from areas of low solute concentration to areas of higher solute concentration (Figure 7.11). Water Balance of Cells Without Walls Tonicity is the ability of a surrounding solution to cause a cell to gain or lose water. The tonicity of a solution depends on its concentration of solutes that can not cross the membrane relative to that inside the cell. An isotonic solution will cause no net movement of water across the plasma membrane. A hypertonic solution will cause the cell to lose water, and shrivel. A hypotonic solution will cause the cell to swell because excess water enters the cell (Figure 7.12a). Cells that lack rigid cell walls have other adaptation to survive called osmoregulation. Paramecium caudatum has a contractile vacuole to expel excess water (Figure 7.13). 3

4 Water Balance of Cells with Walls Organisms like plants have a cell wall. When excess water enters the cell, the cell wall opposes further water uptake by a process called turgor pressure. At this point the cell is turgid. If a plant cell is in an isotonic environment, then it becomes flaccid. If the plant cell is in a hypertonic solution then it can undergo plasmolysis, which causes plant cell death (Figure 7.12b). Facilitated Diffusion: Passive Transport Aided by Proteins Facilitated diffusion is the transport of molecules across the lipid bilayer by a transport protein that spans the membrane. Channel proteins allow molecules to easily pass from one side of the membrane to the other (Figure 7.14a). Channel proteins that transport ions are called ion gated channels that open or close in response to a stimulus (figure 7.14b). No energy input is required, thus it is passive transport. Concept 7.4 : Active transport uses energy to move solutes against their gradients Some transport proteins can move solutes against their concentration gradient across the plasma membrane. The Need for Energy in Active Transport Active transport uses energy to move solutes across the membrane against its concentration gradient. This type of transport protein is a carrier protein. Active transport enables a cell to maintain internal concentrations of small solutes that differ from concentrations in its environment. ATP usually supplies the energy for active transport. The sodium-potassium pump is an example of active transport (Figure 7.15). The difference between active and passive transport is reviewed in Figure How Ion Pumps Maintain Membrane Potential All cells have electrical potential energy across their membrane referred to as voltage. The cytoplasmic side of the membrane is negatively charged. The voltage across the membrane is called membrane potential. The membrane potential is like a battery. Due to the charge difference between inside the cell and outside, the membrane potential favors the passage of cations inside the cell, and anions outside the cell. The electrochemical gradient describes two forces that that affect ion movement across the membrane: a chemical force and an electrical force. A transport protein that generates voltage across a membrane is called an electrogenic pump. The main electrogenic pump of plants is a proton pump (Figure 7.17). 4

5 Cotransport: Coupled Transport by a Membrane Protein Cotransport systems couple (pair) the movement of molecules across the membrane in opposite directions. As one transport protein pumps a molecule out of the cell, its cotransporter pumps a molecule inside the cell. Energy is used in this process so it is an active transport system (Figure 7.18). Concept 7.5 : Bulk transport across the plasma membrane occurs by exocytosis and endocytosis Large molecules such as proteins can cross the membrane by bulk transport in vesicles. This process requires energy. Exocytosis Exocytosis is the bulk transport process of moving molecules outside the cell. Vesicles bud from the Golgi apparatus and fuse with the plasma membrane. Endocytosis Endocytosis is the bulk transport of molecules moving inside the cell. Types of endocytosis in animal cells are: 1) phagocytosis, 2) pinocytosis, and 3) receptor-mediated endocytosis (Figure 7.19). 5