O O

Ester linkage

Fatty acids -hydrocarbon chains

Hydrophilic region {

Hydrophobic region

Hydrophilic region {

Hydrophilic region {

Hydrophobic region

Hydrophilic region {

Surface protein

FIGURE 2.04 Phospholipid Molecule

Phospholipid molecules of the kind found in membranes have a hydrophilic head group attached via a phosphate group to glycerol. Two fatty acids are also attached to the glycerol via ester linkages.

FIGURE 2.03 A Biological Membrane

Membranes do not merely separate living tissue from the non-living exterior. They are also the site of many biosynthetic and energy yielding reactions.

A biological membrane is formed by phospholipid and protein. The phospholipid layers are oriented with their hydrophobic tails inward and their hydrophilic heads outward. Proteins may be within the membrane (integral) or lying on the membrane surfaces.

synthesize its own macromolecules. Each must have a genome, a set of genes carried on molecules of DNA. [Partial exceptions occur in the case of multicellular organisms, where responsibilities may be distributed among specialized cells and some cells may lack a complete genome.]

A cell must also have a surrounding membrane that separates the cell interior, the cytoplasm, from the outside world. The cell membrane, or cytoplasmic membrane, is made from a double layer of phospholipids together with proteins (Fig. 2.03). [Some single-celled protozoa, such as Paramecium, have multiple nuclei within each single cell. In addition, in certain tissues of some multi-cellular organisms several nuclei may share the same cytoplasm and be surrounded by only a single cytoplasmic membrane. Such an arrangement is known as a syncytium when it is derived from multiple fused cells.] Phospholipid molecules consist of a water-soluble head group, including phosphate, found at the surface of the membrane, and a lipid portion consisting of two hydrophobic chains that form the body of the membrane (Fig. 2.04).The phospholipids form a hydrophobic layer that greatly retards the entry and exit of water-soluble molecules. For the cell to grow, it must take up nutrients. For this, transport proteins, which penetrate through the membrane, are necessary. Many of the metabolic reactions involved in the breakdown of nutrients to release energy are catalyzed by soluble enzymes located in the cytoplasm. Other energy-yielding series of reactions, such as the respiratory chain or the photosynthetic system, are located in membranes. The proteins may be within or attached to the membrane surfaces (Fig. 2.03).

The cytoplasmic membrane is physically weak and flexible. Many cells therefore have a tough structural layer, the cell wall, outside the cell membrane. Most bacterial and plant cells have hard cell walls, though animal cells usually do not. Thus a cell wall is not an essential part of a living cell.

Soluble enzymes located in the cytoplasm catalyze biosynthesis of the low molecular weight precursors to protein and nucleic acids. However, assembly of proteins requires a special organelle, the ribosome (Fig. 2.05).This is a subcellular machine that consists of several molecules of RNA and around 50 proteins. It uses information that is carried from the genome into the cytoplasm by special RNA molecules, known as messenger RNA. The ribosome decodes the nucleic acid-encoded genetic information on the messenger RNA to make protein molecules.

cytoplasm The portion of a cell that is inside the cell membrane but outside the nucleus membrane A thin flexible structural layer made of protein and phospholipid that is found surrounding all living cells messenger RNA (mRNA) The class of RNA molecule that carries genetic information from the genes to the rest of the cell

The genome

information

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