Passive Transport Passive transport is the cellular process of moving molecules and other substances across membranes. Passive transport differs from active transport in that it does not involve any chemical energy.
Search term Chapter 15Transport across Cell Membranes Three-dimensional structure of a recombinant cardiac gap junction membrane channel determined by electron crystallography. These channels allow the direct exchange of ions and small molecules between adjacent cells.
Its permeability properties ensure that essential molecules such as glucoseamino acids, and lipids readily enter the cell, metabolic intermediates remain in the cell, and waste compounds leave the cell.
In short, the selective permeability of the plasma membrane allows the cell to maintain a constant internal environment. In several earlier chapters, we examined the components and structural organization of cell membranes see Figures and The phospholipid bilayer — the basic structural unit of biomembranes — is essentially impermeable to most water-soluble molecules, such as glucose and amino acids, and to ions.
Transport of such molecules and ions across all cellular membranes is mediated by transport proteins associated with the underlying bilayer. Because different cell types require different mixtures of low-molecular-weight compounds, the plasma membrane of each cell type contains a specific set of transport proteins that allow only certain ions or molecules to cross.
Similarly, organelles within the cell often have a different internal environment from that of the surrounding cytosoland organelle membranes contain specific transport proteins that maintain this difference.
In animals, sheets of epithelial cells line all the body cavities e. Epithelial cells frequently transport ions or small molecules from one side to the other.
Those lining the small intestine, for instance, transport products of digestion e. In order for epithelial cells to carry out these transport functions, their plasma membrane must be organized into at least two discrete regions, each with different sets of transport proteins.
In addition, specialized regions of the plasma membrane interconnect epithelial cells, imparting strength and rigidity to the sheet and preventing material on one side from moving between the cells to the other.
In the first two sections of this chapter, we discuss the protein -independent movement of small hydrophobic molecules across phospholipid bilayers and present an overview of the various types of transport proteins present in cell membranes.
We then describe each of the main types of transport proteins.
We also explain how specific combinations of transport proteins in different subcellular membranes enable cells to carry out essential physiological processes, including the maintenance of cytosolic pHthe transport of glucose across the absorptive intestinal epitheliumthe accumulation of sucrose and salts in plant-cell vacuoles, and the directed flow of water in both plants and animals.
Often the same type of transport protein is involved in quite different physiological processes.Because different cell types require different mixtures of low-molecular-weight compounds, the plasma membrane of each cell type contains a specific set of transport proteins that allow only certain ions or molecules to cross.
Feb 27, · Hank describes how cells regulate their contents and communicate with one another via mechanisms within the cell membrane.
Crash Course Biology is now available on DVD! Read and learn for free about the following article: Passive transport and active transport across a cell membrane article. Water and Solute Movement | The Cell Membrane | Cells and Diffusion | Active and Passive Transport | Carrier-assisted Transport.
Types of transport molecules | Vesicle-mediated transport | Learning Objectives | Terms | Links | References. Water and Solute Movement | Back to Top. Cell membranes act as barriers to most, but not all, molecules. Passive Transport Passive transport is the movement of molecules across the cell membrane and does not require energy.
It is dependent on the permeability of the cell membrane. There are three main kinds of passive transport - Diffusion, Osmosis and Facilitated Diffusion. Carrier mediated membrane transport.
Movement of materials from an area of high concentration to a lower one FASTER than expected by the laws of diffusion.
Uses a protein carrier molecule. Does NOT require expenditure of metabolic energy and is sodium independent.