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Steroid hormones

Steroid hormones

Figure 3.2. Small uncharged molecules can pass through membranes by simple diffusion, but ions cannot.

proteins (e.g., clathrin adaptor protein, page 229). Membrane proteins are free to move laterally, within the plane of the membrane. Integral membrane proteins are often glycosylated— that is, have sugar residues attached—on the side facing the extracellular medium.

Straight Through the Membrane: Diffusion Through the Bilayer

Molecules of oxygen are uncharged. Although they dissolve readily enough in water, they are also able to dissolve in the hydrophobic interior of lipid bilayers. Oxygen molecules can therefore pass from the extracellular fluid into the interior of the plasma membrane, and from there pass on into the cytoplasm, in a simple diffusion process (Fig. 3.2). Three other small molecules with important roles in biology—carbon dioxide, nitric oxide, and water itself—also pass across the plasma membrane by simple diffusion, as do the uncharged hormones of the steroid family. In contrast, charged ions cannot dissolve in hydrophobic regions (page 21) and therefore cannot cross membranes by simple diffusion.

Example 3.1

Rapid Diffusion in the Lungs

Blood is composed of cells, red and white, and plasma, a solution of sodium chloride, other ions, various organic molecules and proteins. Red blood cells are very simple. They have no nucleus and their plasma membrane encloses a cytosol packed with the oxygen-carrying protein hemoglobin, bathed in a salt solution. The sodium concentration in the red blood cytosol is much lower than the sodium concentration in plasma. It is important that the sodium concentration in the cytosol remain low: if it increases, the red blood cells will swell and then burst as water rushes in. As red blood cells pass through the lungs, they quickly gain oxygen because oxygen molecules can pass rapidly across the plasma membrane by simple diffusion. The cells do not, however, gain sodium ions from the plasma as they pass around the body because the lipid bilayer is impermeable to ions. Red blood cells therefore remain the right size to pass though even the tiniest blood vessels in our bodies.

Beyond the Cell Membrane: The Extracellular Matrix

In connective tissue (page 13), the spaces between cells are filled by an extracellular matrix consisting of polysaccharides and proteins such as collagen together with combinations of the two called proteoglycans. The proteins provide tensile strength and elasticity while the polysaccharides form a hydrated gel that expands to fill the extracellular space.

The sugar-based extracellular matrix reaches its highest form of expression in the cell walls of plants (Fig. 3.3). The plant cell wall consists primarily of cellulose microfibrils linked together by other polysaccharide molecules such as hemicellulose and pectin. The thickness of the wall is determined largely by its pectin content. In healthy plant cells the plasma membrane is constantly pressing against the cell wall.

In plants the extracellular medium is a much more dilute solution than the intracel-lular solution. If no other forces operated, water would diffuse into plant cells down its concentration gradient, a process called osmosis. However, plant cells are prevented from expanding by the inextensible cellulose microfibrils in their cell walls. As water diffuses in, the hydrostatic pressure in the cells rises, making the combination of cell plus cell wall stiff. The same effect can be seen when a bicycle inner tube is blown up inside the tire, and is called turgor. If plants are starved of water, water will leave the cell interior, hydrostatic pressure drops, and the cells lose their turgor pressure, like a tire when the tube inside is deflated. As a result the nonwoody parts of the plant wilt.

Figure 3.3. The cell wall directs plant cell growth.

Newly generated plant cells first lay down a primary cell wall of cellulose microfibrils that are orientated in one direction around the plant cell (Fig. 3.3a). Hydrostatic pressure therefore tends to cause a growing plant cell to elongate in one direction, perpendicular to the axis of the microfibrils. Once the growth of the cell is complete, more layers of cellulose and/or other compounds are added, most notably the polyphenolic compound lignin, to form the secondary cell wall (Fig. 3.3b).

Cell Junctions

In multicellular organisms, and particularly in epithelia, it is often necessary for neighboring cells within a tissue to be connected together. This function is provided by cell junctions. In animal cells there are three types of junctions. Those that form a tight seal between adjacent cells are known as tight junctions; those that allow communication between cells are known as gap junctions. A third class of cell junction that anchors cells together, allowing the tissue to be stretched without tearing, are called anchoring junctions. Plant cells do not have tight junctions, gap junctions, or anchoring junctions but do contain a unique class of communicating junction known as plasmodesmata.

Tight junctions are found wherever flow of extracellular medium is to be restricted and are particularly common in epithelial cells such as those lining the small intestine. The plasma membranes of adjacent cells are pressed together so tightly that no intercellular space exists between them (Fig. 1.8 on page 14). Tight junctions between the epithelial cells of the intestine ensure that the only way that molecules can get from the lumen of the intestine to the blood supply that lies beneath is by passing through the cells, a route that can be selective.

Gap junctions are specialized structures that allow cell-to-cell communication in animals (Fig. 3.4). When two cells form a gap junction, ions and small molecules can pass directly from the cytosol of one cell to the cytosol of the other cell without going into the extracellular fluid. Since ions can move through the junction, changes in electrical voltage

gap junction channel plasma plasma membrane membrane

Figure 3.4. Gap junctions allow solute and electrical current to pass from the cytosol of one cell to the cytosol of its neighbor.

cytosol of left hand v_ cell gap junction channel plasma plasma membrane membrane

Figure 3.4. Gap junctions allow solute and electrical current to pass from the cytosol of one cell to the cytosol of its neighbor.

are also rapidly transmitted from cell to cell by this route. The structure that makes this possible is the gap junction channel. Channels, as we will see in Chapter 14, are water-filled holes through membranes. When two gap junction channels or connexons meet, they form a water-filled tube that runs all the way through the plasma membrane of the first cell, across the small gap between the cells, and through the plasma membrane of the second cell. In the middle of the channel is a continuous hole about 1.5 nm in diameter. This hole is large enough to allow small ions through (and therefore to pass electrical current) together with amino acids and nucleotides, but it is too small for proteins or nucleic acids. Gap junctions are especially important in the heart, where they allow an electrical signal to pass rapidly between all the cardiac muscle cells, ensuring that they all contract at the proper time. Each gap junction channel is composed of six protein subunits that can twist against each other to open and close the central channel in a process called gating (page 314) that allows the cell to control the degree to which it shares solute with its neighbor. The plasmodesmata that perforate the cell walls of many plant tissues (Fig. 1.7 on page 10) serve much the same purpose as the gap junctions of animal cells but are much bigger and cannot shut quickly. Some plant viruses use plasmodesmata to spread from cell to cell.

In the days leading up to ovulation, oocytes (cells that undergo meiosis to give rise to eggs) that are to be released from the follicle are kept in a state of suspended development by a chemical in their cytosol called cyclic AMP (page 350). The oocytes themselves do not make cAMP. Rather, the follicle cells that surround them in the ovary make cAMP, which then passes through gap junctions into the oocyte.

Only when the oocyte is released and begins its passage down the Fallopian tube does it prepare for fertilization. In particular, the process of meiosis (page 404) is completed so as to remove half the chromosomes, so that when the sperm adds its complement of chromosomes the fertilized egg will have the correct number.

Anchoring junctions bind cells tightly together and are found in tissues such as the skin and heart that are subjected to mechanical stress. These junctions are described later (page 396).

Three of the major cell organelles, the nucleus, mitochondrion, and, in plant cells, the chloro-plast, share two distinctive features. They are all enclosed within an envelope consisting of two parallel membranes and they all contain the genetic material DNA.

The Nucleus

The nucleus is often the most prominent cell organelle. It contains the genome, the cell's database, which is encoded in molecules of the nucleic acid, DNA. The nucleus is bounded by a nuclear envelope composed of two membranes separated by an intermembrane space (Fig. 3.5). The inner membrane of the nuclear envelope is lined by a meshwork of proteins

Gap Junctions Keep Eggs Ready But Waiting

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