Chapter Three Objectives

1. Define the cell theory and the three major parts of all cells.

2. Describe the chemical composition and functions of the plasma membrane using the fluid mosaic model.

3. Define microvilli, tight junctions, desmosomes, gap junctions, passive transport, active transport, endocytosis, and exocytosis.

4. Describe the structure and function of all cellular organelles.

5. Describe the structure and function of the three parts of the cytoskeleton.

6. Describe cilia, flagella, and parts of the nucleus

CELLS: THE LIVING UNITS

All organisms are cellular in nature, be they one-celled or multicellular. The individual cells are the structural units of all living things. The human body has trillions of them. Robert Hooke was the first to observe plant cells with a crude microscope in the late 1600's. In the mid-1800's, Schleiden and Schwann proposed that all living things were composed of cells. Then Virchow extended this idea by suggesting that cells arise only from other cells. Since the 1800's, research has provided us with four concepts known as the cell theory:

1. A cell is the basic structural and functional unit of living organisms.
2. The activity of an organism is dependent on both the individual and collective activities of its cells.
3. Principle of Complementarity - biochemical activities of cells are determined and made possible by the specific subcellular structures of cells.
4. Continuity of life has a cellular basis.

Perhaps the most striking property of a cell is its complex organization. Cells are amazingly diverse in both size and shape. Cells do have many common structural and functional features. All cells have three major parts: nucleus, cytoplasm, plasma membrane

Plasma Membrane:

Defines the boundary of the cell and acts as a fragile barrier. The membrane is thin, elastic, and semi-permeable. It is composed chiefly of a double layer, or bilayer of phospholipid molecules with protein molecules dispersed in it. The fluid mosaic model of membrane structure depicts the plasma membrane as:

1. Sandwich composed of two layers of phospholipid molecules, with the polar head exposed to water inside and outside the cell and the nonpolar tails facing each other and buried in the internal portion of the membrane.

2. Globular proteins are embedded in the bilayer. The amount of protein varies with membrane function and averages 50% of the mass of the membrane. Proteins function as: structural supports, pumps, carriers, receptors, enzymes, ion channels

3. The membrane is a dynamic fluid structure of about the consistency of olive oil. The lipid molecules are free to move laterally. Some of proteins float freely, while others are more restricted.

4. About 10% of the externally facing lipid molecules have attached sugar groups. Protein molecules also have sugar groups attached.

5. The physical and chemical properties of the cell membrane enables it to play an important role in determining what substances can enter or leave and in what quantity at any given time. Permeability is determined by:
solubility in lipids
size - very small lipid-insoluble substances (water, some ions, urea) can pass
electrical charge - must have the opposite charge of the cell membrane
carrier system - modifies permeablility

Specializations of the Plasma Membrane - See Figure 1

Microvilli: minute, finger-like projections which increase the surface area of the membrane. Found on the surface of absorptive cells such as kidney tubules and intestinal cells.

Membrane Junctions: two types of junctions formed between cells that make up tissues. (1.) junctions that fasten cells together (2.) junctions that permit transfer of ions and other molecules between cells.

In tight junctions protein molecules in adjacent membranes fuse together preventing free passage of molecules through intercellular space between cells. In the digestive tract they form a barrier to enzymes and microorganisms preventing them from entering the blood.

Desmosomes are spotlike patches that act as mechanical couplings between adjoining cells. The membranes do not actually touch, but are held together by fine glycoprotein filaments stretched between button-like thickening of the inner membranes. They are abundant in tissues subject to mechanical stress (skin, heart, muscles).

Gap Junctions function to allow direct passage of chemical substances between adjacent cells. The cells are connected by hollow cylinders composed of transmembrane proteins. Allows ions, sugars, and other small molecules to pass through. In adults they are found in electrically excitable tissues such as in the heart and muscles.

Membrane Transport: The plasma membrane is a selectively, or differentially, permeable barrier that allows some substances to pass while excluding others.

1. Passive Processes: substances penetrate the membrane without any energy input from the cell. Depends on the physical process of diffusion. Molecules diffuse down the concentration gradient from an area of high concentration to an area of low concentration.

2. Active Processes: the cell provides metabolic energy (ATP) to drive the movement of substances across the membrane. There are two major mechanisms: active transport and bulk transport.

Active transport (solute pumping) requires a carrier protein and ATP to move molecules against the concentration gradient from low concentration to high concentration. Combines specifically and reversibly with the substance. Many active transport systems are coupled systems - they move more than one substance at a time.

Bulk transport moves large particles and macromolecules through the plasma membrane. It requires ATP. There are two kinds of bulk transport: exocytosis and endocytosis

Exocytosis moves substances out of the cell Substances to be released are enclosed within a membrane sac which migrates to the plasma membrane, fuses, and then ruptures releasing the contents of the sac.

Endocytosis is a means for allowing large particles or macromolecules to enter the cell. Substances to be taken in are progressively enclosed by a portion of the cell membrane. Once formed, the sac pinches off and moves into the cytoplasm where the contents are digested. There are two types of endocytosis: phagocytosis (when the substance to be ingested is a large particle) and pinocytosis (when the substances to be ingested are dissolved in water).

Interaction with other Cells: Glycoproteins of the cell membrane serve as highly specific cell surface markers. They aid in cellular recognition and cell interaction: blood group antigens, binding sites for certain toxins (cholera, tetanus), recognition of the egg by sperm, determining cellular life span, immune response, embryonic development

Cytoplasm - Cellular material inside the plasma membrane and outside the nucleus. It is the site where most cellular activities occur. It is a major functional area. The cytoplasm consists of two major elements: cytosol and organelles.

Cytosol is a viscous, semitransparent fluid composed mostly of water with some soluble proteins, salts, sugars and other solutes. Organelles are the metabolic machinery of the cell. Each carries out a specific function for the cell as a whole. Some synthesize proteins, others package those proteins, etc.

Cytoplasmic Organelles - See Figure 2 Specialized cellular compartments, each performing its own job to maintain the life of the cell. Most organelles are bounded by selectively permeable membranes which enable them to maintain an internal environment differing from the surrounding cytosol. This compartmentalization is crucial to the functioning of the cell; without it enzymes would be randomly mixed and biochemical activity would be chaotic.

Mitochondria - Sausage-shaped structure. Powerhouse - provide most of the ATP supply. Quantity per cell reflects the energy requirements of the cell. Each mitochondrion is surrounded by two lipid bilayer membranes.Inner membrane contains more protein, has shelf-like inward folds called cristae which protrude into the gel-like matrix.Enzymes in the matrix together with those on the cristae membranes cooperate to breakdown glucose and other nutrients to water and carbon dioxide. Some of the energy released is captured in the form of ATP.

Ribosomes - Small granules composed of proteins and a type of RNA. Each ribosome has two globular submits that fit together with the small subunit sitting on top of the larger subunit. It is the site of protein synthesis, where amino acids linked together to form a polypeptide chain. Some ribosomes float freely in the cytoplasm; others are attached to membranes, forming a complex called the rough endoplasmic reticulum. Free ribosomes make proteins for use in the cytosol. Membrane-bound ribosomes make proteins for export or use in the membrane.

Endoplasmic Reticulum (E.R) - Extensive system of interconnected parallel membranes that coils and twists through the cytoplasm, enclosing fluid-filled cavities or cisternae. Continuous with the nuclear membrane and account for about half the cell's membranes. Two distinct varieties of E.R.; rough and smooth. External surface of rough E.R. is studded with ribosomes.

As amino acids are assembled on the ribosome, they thread their way into the cisternae. This prevents the proteins for export from mixing with those that are being retained in the cell. Proteins for export become enclosed in membranous sacs pinched off from the E.R. (transport vesicles).Transport vesicles migrate to the Golgi apparatus for further processing of the proteins. Rough E.R. is abundant in cells specialized to secrete proteins such as gland cells, liver cells, plasma cells, etc).

Smooth E.R. is a continuation of the rough E.R. and consists of tubules arranged in branching network. Smooth E.R. plays no role in protein synthesis. Smooth E.R. is involved in lipid metabolism, synthesis of cholesterol and the lipid portion of lipoproteins, synthesis of steroid based hormones.

Golgi Apparatus - Consists of 4-8 flattened menbranous sacs stacked one upon the other. Located near the nucleus. Principal traffic director for cellular proteins. Major function is to modify, concentrate, and package proteins for export. Protein-containing vesicles pinch off from the Golgi and migrate to the plasma membrane and discharge their contents from the cell by exocytosis. Secretory cells have prominent Golgi apparatus.

Lysosomes - Membranous sacs containing hydrolytic enzymes. Provide sites where digestion can proceed safely within the cell. Large and abundant in phagocytes. Function to digest ingested particles, worn-out nonfunctional organelles, non-useful tissues. The breakdown of bone to release calcium ions also reflects lysosomal activity.

Cvtoskeletal Elements - An elaborate network of protein structures called microfilaments, microtubules, and intermediate filaments are located through out the cytoplasm. Cytoskeleton acts as the cells "bones and muscles" helping to support intercellular structures and generate various cell movements.

Microfilaments - Thin strands of the contractile protein actin. Nearly all cells have a dense cross-linked network of microfilaments attached to the cytoplasmic side of the plasma membrane. Braces and strengthens the cell surface. Most involved in cell motility or cell shape change. Form the core of microvilli. Provide for amoeboid movement and vesicle formation during endocytosis. Together with myosin, fonn the cleavage ring that pinches the cell in two during cell division.

Intermediate Filaments - Tough, insoluble protein fibers with high tensile strength. Act as internal guy wires to resist pulling forces on the cell.

Microtubules - Long, flexible hollow tubules. Have a variety of cellular roles; most importantly, they appear to be the overall organizers of the cytoskeleton. Help to position and suspend organelles at specific locations within the cell. Form the walls of more complex organelles called centrioles.

Cilia and Flagella – Hair-like, motile, cellular extensions. Occur in large numbers on the free surface of certain cells. Cilia and flagella arise from microtubule-organizing structures called basal bodies just beneath the cell surface.

Cilia produce a pushing motion in a single direction. Cilia do not act independently of each other, instead their actions are coordinated, creating a current at the cell surface. Important in moving substances in one direction across cell surfaces.

Flagella are essentially the same as cilia, somewhat longer and usually there are only 1-2 associated with a cell. Function to propel the cell. Only cells in humans with flagella are sperm cells.

Nucleus - Control center for the cell. Most cells have only one nucleus, although some (skeletal, bone destruction cells, some liver cells) are multinucleated. All cells within the human body are nucleated except red blood cells. The nucleus is the largest organelle in the cell. Spherical to oval in shape. Three distinct regions or structures: nuclear membrane, nucleoli, chromatin.

The nuclear membrane encloses a jelly like colloidal fluid called nucleoplasm. The nuclear membrane is a double membrane surrounding the nucleus. Each membrane is a phospholipid bilayer. The outer membrane is continuous with the E.R. and may be studded with ribosomes. At various points, the two layers fuse and nuclear pores penetrate through the fused regions.The nuclear membrane is selectively permeable. Because of the larger pores passage of substances is much freer. Protein molecules imported from the cytoplasm and RNA exported from the nucleus pass easily through the pores.

The nucleoli are dark staining spherical bodies found in the nucleus and are made of rRNA and proteins. Typically 1-2 per cell, but there may be more. They function as ribosome-producing machines. As RNA is produced within a nucleolus, they are combined there with proteins forming the two kinds of ribosomal subunits. Ribosomal subunits leave the nucleus, enter the cytoplasm and are assembled into functional units.

The chromatin appears as a fine, unevenly stained network. It is composed of approximately equal amounts of DNA and globular histone proteins. Nucleosomes are the fundamental units of chromatin. They are spherical clusters of 8 histone proteins connected like beads on a string by DNA molecules that wind around them. This provides a physical means for packing very long DNA molecules into a compact form. Histones are believed to influence the activity of the genes contained in the DNA. See Figure 3

Changes in the shape of histones exposes different DNA segments, or genes, so that they can "dictate" the specifications for protein synthesis. Active chromatin segments are referred to as extended chromatin or euchromatin and are not usually visible under the light microscope. Nonactive chromatin segments, called condensed chromatin, or heterochromatin, are darker staining and so are more easily detected. When a cell is preparing to divide, the chromatin threads coil and condense enormously to form the short, bar-like bodies called chromosomes.

Figure 1

Figure 2

Figure 3