Concept 4.5: Extracellular Structures Provide Support and Protection for Cells and Tissues

In Chapter 5 we will look at the role of the cell membrane in cell communication. Although the cell membrane is the functional barrier between the inside and the outside of a cell, cells produce molecules and secrete them to the outside of the cell membrane. There these molecules form structures that play essential roles in protecting, supporting, or attaching cells to each other. Because they are outside the cell membrane, these structures are said to be “extracellular.” In eukaryotes, these structures are made up of two components:

• A fibrous macromolecule
• A gel-like medium in which the fibers are embedded

The plant cell wall is an extracellular structure

The plant cell wall is a semirigid structure outside the cell membrane (FIGURE 4.15). The fibrous component is the polysaccharide cellulose (see Figure 2.10), and the gel-like matrix contains extensively cross-linked polysaccharides and proteins. The wall has three major roles:

• It provides support for the cell and limits the volume of a mature cell by remaining rigid.
• It acts as a barrier to infection by fungi and other organisms that can cause plant diseases.
• It contributes to plant form by controlling the direction of cell expansion during growth and development.

Figure 4.15: The Plant Cell Wall The semirigid cell wall provides support for plant cells. It is composed of cellulose fibers embedded in a matrix of polysaccharides and proteins.

Because of their thick cell walls, plant cells viewed under a light microscope appear to be entirely isolated from one another. But electron microscopy reveals that this is not the case. The cytoplasms of adjacent plant cells are connected by numerous cell membrane–lined channels called plasmodesmata. These are about 20–40 nm in diameter and extend through the cell walls (see Figure 4.7). Plasmodesmata allow water, ions, small molecules, hormones, and even some RNA and protein molecules to move between connected cells. In this way, energy-rich molecules such as sugars can be shared among cells, and plant hormones can affect growth at sites far from where they were synthesized. This intercellular communication integrates a plant organ composed of thousands of cells.

The extracellular matrix supports tissue functions in animals

Animal cells lack the semirigid wall that is characteristic of plant cells, but many animal cells are surrounded by, or in contact with, an extracellular matrix (FIGURE 4.16). The fibrous component of the extracellular matrix is the protein collagen, and the gel-like medium consists of proteoglycans, which are glycoproteins with long carbohydrate side chains. A third group of proteins links the collagen and the proteoglycan matrix together.

Figure 4.16: An Extracellular Matrix Cells in the kidney secrete an extracellular matrix called the basal lamina that separates them from nearby blood vessels. The basal lamina filters materials that pass between the kidney and the blood.

The extracellular matrices of animal cells have several roles:

• They hold cells together in tissues.
• They contribute to the physical properties of cartilage, skin, and other tissues. For example, the mineral component of bone is laid down on an organized extracellular matrix.
• They help filter materials passing between different tissues. This is especially important in the kidney.
• They help orient cell movements during embryonic development and during tissue repair.

Proteins connect the cell’s cell membrane to the extracellular matrix. These proteins (for example, integrin) span the cell membrane and have two binding sites: one on the interior of the cell, usually to microfilaments in the cytoplasm just below the cell surface, and the other to collagen in the extracellular matrix. These binding sites are noncovalent and reversible. When a cell moves its location in an organism, the first step is for integrin to change its three-dimensional structure so that it detaches from the collagen (FIGURE 4.17).

Figure 4.17: Cell Membrane Proteins Interact with the Extracellular Matrix In this example, integrin mediates the attachment of animal cells to the extracellular matrix.

Cell junctions connect adjacent cells

In a multicellular animal, specialized structures protrude from adjacent cells to “glue” them together. These cell junctions are most evident in electron micrographs of epithelial tissues, which are layers of cells that line body cavities or cover body surfaces (examples are skin and the lining of the windpipe leading to the lungs). These surfaces are often exposed to environmental factors that might disrupt the integrity of the tissues, so it is particularly important that their cells stick together tightly. There are three types of junctions (FIGURE 4.18):

• Tight junctions prevent substances from moving through spaces between cells. For example, the epithelium of the urinary bladder contains tight junctions to prevent urine from leaking out into the body.
• Desmosomes hold adjacent cells together with stable protein connections, but materials can still move around in the extracellular matrix. This provides mechanical stability for tissues such as skin that receive physical stress.
• Gap junctions are like plant plasmodesmata: they are channels that run between membrane pores in adjacent cells, allowing substances to pass between cells. In the heart, for example, gap junctions allow the rapid spread of electric current mediated by ions so the heart muscle cells can beat in unison.

Figure 4.18: Junctions Link Animal Cells Although all three types of junctions are shown in the cell at right, they don’t necessarily all occur in the same cell.

ANSWER Ideas about how the first cells may have formed focus on two questions: how and when. As to how cells could arise from a chemical-rich environment, most biologists assume that a cell membrane formed first and was necessary to provide a compartment for the chemical transformations of life to occur, separated from the environment (Concept 4.1). Biologists also assume that the first cells were relatively simple prokaryotes (Concept 4.2), without the organelles that define eukaryotic cells (Concept 4.3).

Jack Szostak, a Nobel laureate at Harvard University, builds synthetic cell models that give insights into the origin of cells. He and his colleagues make small membrane-lined droplets by putting fatty acids into water and then shaking the mixture. The lipids form water-filled droplets, each surrounded by a lipid bilayer “membrane” (see Figure 2.13). With water (and other molecules of the scientists’ choosing) trapped inside, these spheres have many properties characteristic of modern cells—so many that they have been called protocells (FIGURE 4.19). For example, the membrane barrier determines what goes in and out of a protocell, by excluding macromolecules like RNA but allowing smaller molecules such as nucleotides to pass through. Moreover, RNA inside the protocell can act as a catalyst, replicating itself from nucleotides that enter the protocell. The spheres are somewhat unstable, and under the microscope they can be seen to grow, elongate, and break, a possible precursor of more precise cell division.

Figure 4.19: A Protocell A protocell can be made in the lab and can carry out some functions of modern cells—in particular, it provides a compartment for biochemical reactions.

Is this really a cell, possibly like the one where life started? Certainly not: it cannot fully reproduce itself, and its capacities for metabolism are limited. But by providing a compartment for biochemical reactions with a boundary that separates it from the environment, the protocell is a model for the first cell.

When did the first cells on Earth appear? According to geologists, Earth is about 4.5 billion years old. Heat and atmospheric conditions precluded life for at least a half-billion years after Earth formed. The oldest fossils of multicellular organisms date from about 1.2 billion years ago.

In all probability, life began with single-celled organisms resembling modern bacteria. Unfortunately, such cells lack the structures that are typically preserved in fossils, and so they die without a trace. Recently, however, geochemist and paleontologist William Schopf at the University of California, Los Angeles used a new method of microscopy called confocal laser scanning microscopy, combined with chemical analyses, to identify fossil cells that are about 800 million years old. Some of these look like Szostak’s protocells. These were probably not the first cells, as there is chemical evidence in some rocks that life was present about 3.8 billion years ago. But so far, Schopf’s fossilized cells are the oldest cells that anyone has been able to find.