Learning Objectives
- Describe the components of biological membranes.
- Explain the basic principles of the fluid mosaic model of biomembranes.
- Outline the properties of the lipid bilayers and associated proteins that compose biomembranes.
- List the functions of membrane proteins.
- Explain the different ways proteins can be associated with a membrane.
- Describe a transmembrane domain.
- Describe the factors involved in membrane fluidity.
- Describe the location of lipid synthesis in eukaryotic cells
- Explain the process of glycosylation and proved examples of its importance
- Compare simple diffusion and facilitated diffusion.
- Differentiate between channel proteins and transporter proteins.
- Explain the movement of water by osmosis.
- Differentiate between active transport and diffusion.
- Describe the function of the Na+/K+ pump.
- Explain the energetics of coupled transport.
- Distinguish between endocytosis and exocytosis and give examples of each in living cells.
Chapter Summary
Phospholipids are the foundation of all known biological membranes. The characteristic lipid bilayer forms as a result of the interactions among nonpolar phospholipid tails, polar phospholipid heads, and the surrounding water. The nonpolar tails face inward toward each other while the polar heads face outward toward the water. The arrangement of the lipid bilayer is stable, yet fluid. The membranes of living organisms are assembled from four components. The phospholipid bilayer provides an impermeable flexible matrix in which the other components are arranged. Transmembrane proteins that float within the bilayer are channels through which various molecules pass. A supporting protein network, anchored to the actin filament cytoskeleton, prevents these channels from moving. The glycocalyx consisting of sugars and membrane proteins provide a cell’s identity.
All of the cell’s activities are in one way or another tied to the membrane that separates its interior from the environment. Net diffusion occurs when the materials on one side of the membrane have a different concentration than the materials on the other side. Facilitated transport of materials is necessary to control the entrance and exit of particular molecules. Facilitated diffusion is a simple process that utilizes protein carriers that are specific to certain molecules. It is a passive process driven by the concentration of molecules on the inside and the outside of the membrane. Osmosis is a specialized form of diffusion associated specifically with the movement of water molecules. Many cells are isosmotic to the environment to avoid excessive inward or outward movement of water. Other cells must constantly export water from their interior to accommodate the natural inward movement. Most plant cells, on the other hand, are hyperosmotic with respect to their immediate environment. The resulting turgor pressure within the cell pushes the cytoplasm against the cell wall and makes a plant cell rigid.
Large molecules enter the cell by endocytosis, a nonselective process. Endocytosis of particulate material is called phagocytosis while endocytosis of liquid material is called pinocytosis. Exocytosis is the reverse mechanism and is used by plants to construct the cell wall and by animals to secrete various internally produced chemicals. Receptor-mediated endocytosis is a complicated mechanism that involves the transport of materials via coated vesicles. Some molecules are transported into or out of the cell independent of concentration. This process requires the expenditure of energy in the form of ATP and is called active transport. Such transport channels are coupled to a sodiumpotassium pump. The proton pump produces ATP through two special transmembrane protein channels through a process called chemiosmosis.
PowerPoint Presentation
Virtual Lectures
Section 5.1: Membrane Structure and Section 5.2: Synthesis of Membrane Components in Eukaryotic Cells
Section 5.3: Membrane Transport
a. Diffusion
b. Osmosis
c. Electrochemical Gradients and Active Transport
** make sure to press OK when the LMS implementation screen appears.
Animations
1. Bilayer Arrangement of Phospholipids
3. Function of Membrane Proteins
4. Semi-permeable Nature of Membranes
5. Overview of Membrane Transport
7. Osmosis
8. Tonicity
9. Active Transport: Calcium Pumps
10.Exocytosis via the Endomembrane System
11.Bulk Transport: Phagocytosis
Miscellaneous
This is a video from the Virtual Cell project funded by the National Science Foundation. For more information, see http://vcell.ndsu.nodak.edu/animations. All of these videos pertain to the production, modification, and transport of proteins throughout the cell.
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