Terms

 TERMS

Cell wall  -  The thick and rigid layer that covers the plasma membrane in plant cells. Composed of fat and sugar molecules in a matrix.

Chlorophyll  -  A pigment located within a chloroplast that absorbs light in plant cells, helping to convert light energy into biological energy in the process of photosynthesis.

Chloroplast  -  A double membrane-bound organelle found in plant cells that contains chlorophyll and is responsible for mediating photosynthesis.

Photosynthesis  -  A process in which plants convert sunlight into energy sources that can be used inside the cell to sustain life.

Vacuole  -  Membrane-bound, fluid-filled organelle found only in plant cells. Can compose up to 90% of a cell's volume and performs diverse functions in plant cells, including digestion of intracellular molecules.

Introduction of cell differences

CELL DIFFERENCES

INTRODUCTION

Structural Differences in Distinct Classes of Cells

Thus far we have been describing the structure and function of eukaryotic animal cells. As we have mentioned earlier, there are different classes of cells. In this short section, we will discuss the major cellular differences between prokaryotic and eukaryotic cells, and between animal and plant cells. The diversity among various cells is a result of the varying functions and complexity of those cells.

Because plants convert sunlight into energy rather than obtaining energy by eating food the way we humans do, plant cells require additional structures that can perform this function. While most structures between animal and plant cells are the same, there are a couple of additional structures that perform specific functions in plant cells that are unnecessary in animal cells.

In contrast to eukaryote cells, prokaryotic cells lack membrane-bound organelles. Such organization is necessary in eukaryotic cells because eukaryotes are often multi-cellular organisms whose cells communicate with the help of specific membrane-bound structures. Because prokaryotic cells do not form such complex networks, a single compartment containing all of the essential biomolecules for life is sufficient.

We will begin our discussion by contrasting plant and animal cells and then move on to the structural differences between prokaryotic and eukaryotic cells.

The Lipid Bilayer

THE LIPID BILAYER 

Lipid Bilayer Structure

The lipid bilayer is a universal component of all cell membranes. Its role is critical because its structural components provide the barrier that marks the boundaries of a cell. The structure is called a "lipid bilayer" because it is composed of two layers of fat cells organized in two sheets. The lipid bilayer is typically about five nanometers thick and surrounds all cells providing the cell membrane structure.

Lipids and Phospholipids

The structure of the lipid bilayer explains its function as a barrier. Lipids are fats, like oil, that are insoluble in water. There are two important regions of a lipid that provide the structure of the lipid bilayer. Each lipid molecule contains a hydrophilic region, also called a polar head region, and a hydrophobic, or nonpolar tail region.

Figure %: Basic Lipid Structure

The hydrophilic region is attracted to aqueous water conditions while the hydrophobic region is repelled from such conditions. Since a lipid molecule contains regions that are both polar and nonpolar, they are called amphipathic molecules.

The most abundant class of lipid molecule found in cell membranes is the phospholipid. The phospholipid molecule's polar head group contains a phosphate group. It also sports two nonpolar fatty acid chain groups as its tail.

Figure %: Phospholipid Structure

The fatty acid tail is composed of a string of carbons and hydrogens. It has a kink in one of the chains because of its double-bond structure.

The Bilayer

The phospholipids organize themselves in a bilayer to hide their hydrophobic tail regions and expose the hydrophilic regions to water. This organization is spontaneous, meaning it is a natural process and does not require energy. This structure forms the layer that is the wall between the inside and outside of the cell.

Figure %: Lipid Bilayer 

Properties of the Lipid Bilayer

As we have already mentioned, the most important property of the lipid bilayer is that it is a highly impermeable structure. Impermeable simply means that it does not allow molecules to freely pass across it. Only water and gases can easily pass through the bilayer. This property means that large molecules and small polar molecules cannot cross the bilayer, and thus the cell membrane, without the assistance of other structures.

Another important property of the lipid bilayer is its fluidity. The lipid bilayer contains lipid molecules, and, as we will discuss later, it also contains proteins. The bilayer's fluidity allows these structures mobility within the lipid bilayer. This fluidity is biologically important, influencing membrane transport. Fluidity is dependent on both the specific structure of the fatty acid chains and temperature (fluidity increases at lower temperatures).

Structurally, the lipid bilayer is asymmetrical: the lipid and protein composition in each of the two layers is different.

MEMRANE PROTEINS

In addition to the lipid bilayer, the cell membrane also contains a number of proteins. We have already mentioned the presence of certain proteins in the cell membrane. In this section we will discuss the different classes of proteins found there. While the lipid bilayer provides the structure for the cell membrane, membrane proteins allow for many of the interactions that occur between cells. As we discussed in the previous section, membrane proteins are free to move within the lipid bilayer as a result of its fluidity. Although this is true for most proteins, they can also be confined to certain areas of the bilayer with enzymes. Membrane proteins perform various functions, and this diversity is reflected in the significantly different types of proteins associated with the lipid bilayer.

Classifications of Membrane Proteins

Proteins are generally broken down into the smaller classifications of integral proteins, peripheral proteins, and lipid-bound proteins.

Integral Proteins

Integral proteins are embedded within the lipid bilayer. They cannot easily be removed from the cell membrane without the use of harsh detergents that destroy the lipid bilayer. Integral proteins float rather freely within the bilayer, much like oceans in the sea. In addition, integral proteins are usually transmembrane proteins, extending through the lipid bilayer so that one end contacts the interior of the cell and the other touches the exterior. The stretch of the integral protein within the hydrophobic interior of the bilayer is also hydrophobic, made up of non-polar amino acids. Like the lipid bilayer, the exposed ends of the integral protein are hydrophilic.

Figure %: Membrane Proteins

When a protein crosses the lipid bilayer it adopts an alpha-helical configuration. Transmembrane proteins can either cross the lipid bilayer one or multiple times. The former are referred to as single-pass proteins and the later as multi-pass proteins. As a result of their structure, transmembrane proteins are the only class of proteins that can perform functions both inside and outside of the cell.

Peripheral Proteins

Peripheral proteins are attached to the exterior of the lipid bilayer. They are easily separable from the lipid bilayer, able to be removed without harming the bilayer in any way. Peripheral proteins are less mobile within the lipid bilayer.

Lipid-Bound Proteins

Lipid-bound proteins are located entirely within the boundaries of the lipid bilayer.

The Cell Surface

The protein and lipid cell membrane is covered with a layer of carbohydrate chains on its outer surface. This layer is called a cell coat or glycocalyx. The exact composition and distribution of these chains is very diverse. The chains are thought to provide the cell with protection against damage. Glycocalyx are only found on the surface of the cells of higher organism's.

Figure %: A detailed view of a Cell Membrane (phospholipid bilayer and associated proteins)

STRUCTURES RESPONSIBLE FOR MEMBRANE TRANSPORT

We have discussed how the lipid bilayer acts as an efficient barrier by only allowing a very small number of non-polar molecules to freely enter or exit a cell. While for the most part this selectivity is a valuable function and allows the cell to maintain its integrity, cells do need to move certain large, polar molecules such as amino acids, sugars, and nucleotides across their membranes. As a result, cell membranes require specific structures that allow for the transport of certain molecules.

Membrane Transport

There are a number of different ways that molecules can pass from one side of a cell membrane to the other. Some such means, like diffusion and osmosis, are natural processes that require no expenditure of energy from the cell and are called passive transport. Other methods of transport do require cellular energy and are called active transport. In addition to these two forms of transport, there exist other forms of transport such as endocytosis and exocytosis, which will be discuss later and do not require the same set of membrane proteins for their function.

Passive Transport

Diffusion is the natural phenomenon in which nonpolar molecules naturally flow from an area of higher concentration to an area of lower concentration. Osmosis is a similar process, but refers specifically to water molecules. Both of these classes of molecules we have already discussed as capable of crossing the lipid bilayer. As seen in , neither diffusion nor osmosis require the expenditure of energy.

Active Transport

Active transport occurs when a cell actively pumps a molecule across its membrane, against the natural direction dictated by diffusion, osmosis, or polarity. As seen in , such transport requires energy.

Figure %: Active and Passive Transport Proteins

Transport Proteins

Both of passive and active transport are mediated with the help of transmembrane proteins that act as transporters. shows the two main classes of transport proteins: carrier proteins and channel proteins. For the most part, carrier proteins mediate active transport while channel proteins mediate passive transport. Carrier proteins create an opening in the lipid bilayer by undergoing a conformational change upon the binding of the molecule. Channel proteins form hydrophilic pores across the lipid bilayer. When open, these pores allow specific molecules to pass through. There is one other class of transport proteins called ionophores. These are small, hydrophobic proteins that increase bilayer permeability for specific ions.

Transport proteins are critical to cell life and cell interactions. They allow for the proper distribution of ions and molecules in multicellular organisms. Additionally, they can help to maintain proper intra- and extra-cellular pH levels, facilitate communication between cells, and are involved numerous other essential functions including protein sythesis.

Terms

TERMS

Active transport  -  The transport of molecules across a membrane and against their natural flow; mediated by carrier proteins and requiring outside energy.

Carrier protein  -  A protein responsible for mediating the active transport of molecules from one side of the lipid bilayer to the other. Transport is carried out by a conformational change that occurs within the protein that forms an opening for specific molecules to pass through.

Channel protein  -  A protein responsible for mediating the passive transport of molecules from one side of the lipid bilayer to the other. Transport is carried out by its membrane-spanning hydrophilic structure which, when open, allows molecules to pass through.

Diffusion  -  The transport process in which molecules naturally travel from an area of higher concentration to an area of lower concentration.

Glycocalyx  -  A layer of carbohydrates that coats the exterior of higher-ordered cells. Functions in protecting the cell from damage.

Hydrophilic  -  A polar molecule that selectively associates itself with water through hydrogen bonds.

Hydrophobic  -  A nonpolar molecule that does not readily associate with water through hydrogen bonds.

Integral protein  -  A membrane protein that cannot be easily removed from the lipid bilayer.

Ionophore  -  A class of membrane transport proteins. Small, hydrophobic molecules that increase membrane permeability to certain ions.

Lipid bilayer  -  A thin double layer of phospholipid molecules. Provides the structure of a cell membrane. Structure is a result of hydrophobic and hydrophilic forces.

Lipid-bound protein  -  Membrane proteins that are located entirely within the lipid bilayer, having no part touching either the inside or outside of the cell.

Multi-pass protein  -  Transmembrane proteins that cross the lipid bilayer more than one time.

Osmosis  -  The process by which water naturally travels from an area of high concentration to one of lower concentration.

Passive transport  -  Transport mediated by channel proteins. The movement of molecules across a membrane according to the natural flow.

Peripheral protein  -  A membrane protein that can be easily removed from the lipid bilayer.

Single-pass protein  -  A transmembrane protein that only crosses the lipid bilayer one time.

Transmembrane protein  -  A membrane protein that spans the lipid bilayer having portions in contact with both the inside and outside of the cell. Area within the lipid bilayer forms an alpha-helix.

Introduction to the cell membranes

CELL MEMBRANES

Introduction to the cell membranes

Cell membranes are very important structures to cells because they function as a barrier between the components of the cell and the outside environment. The cell membrane is not only responsible for creating a wall between inside and outside the cell, it must also act as a threshold through which select molecules can enter and exit the cell when necessary. The cell membrane is what defines the cell and keeps its components separate from outside cells or organisms.

The cell membrane is composed of a double layer of fat cells called a lipid bilayer in which membrane proteins are embedded. The structure of the lipid bilayer prevents the free passage of most molecules into and out of the cell. We will begin our discussion of the structure of the cell membrane by discussing the structure and properties of the lipid bilayer. We will then go on to discuss the role of membrane-bound proteins, and finally, will discuss membrane transport structures.

Introduction of the cell structure

INTRODUCTION OF THE CELL STRUCTURE

Introduction to the cell

The purpose of this guide is to provide an overview of the basic structural components of living cells. In reviewing these structures, we will also discuss their functions.

All living organisms are composed of cells. A cell is a small, membrane-bound compartment that contains all the chemicals and molecules that help support an organism's life. An understanding of the structure of cells is one of the first steps in comprehending the complex cellular interactions that direct and produce life.

Cells can be thought of as building blocks of organisms. Some organisms are composed of a single cell. Others, like ourselves, are composed of millions of cells that work together to perform the more complex functions that make us different from bacteria. It is difficult to imagine that humans are descendants of a single cell, but this is a common belief in the scientific world. Before we can understand how multiple cells can work together to create complex biological functions, it is necessary to understand what biological functions single cells are capable of performing on their own to sustain life.

There are different types of cells with individuated structures. Single-celled organisms have different cell structure than multi-celled organisms and plant cells have different structures from animal cells. These differences reflect differences in the functions that each of these classes of cells is required to perform. While the focus of this guide will be on the structures that compose complex multi-cellular organisms, we will begin our discussion of cell structure with a structure that is universal to all cells,membranes.

 By Nay