Cell Structure

I.         Prokaryotic and eukaryotic cells

            A.        A prokaryotic cell has no nucleus to separate the area of the nucleic acid from the rest of the cell.

            B.        A eukaryotic cell has a membrane bound nucleus in which the nucleic acid is held.

            C.        Cell size (Fig. 7.5)

                        1.         The lower size limit of cells is set by the physical requirement of needing to have enough space to house all the equipment necessary to perform metabolic reactions.

                        2.         The upper size limit is set by the need to be able to exchange materials with the environment and to distribute them throughout the cell. As a cell grows, it’s volume increases much faster than it’s surface area. Cells need a certain amount of membrane area to get materials into and out of the cell volume. This means that the more surface area for a given volume the better.

II.        Compartmentalization

            A.        Internal membranes divide the cell into separate compartments. This is important so that:

                        1.         Each reaction can occur in the ideal environment.

                        2.         Reactions part of a series can occur close to one another.

                        3.         Enzymes for a reaction can be built into the membrane of a compartment.

                        4.         Reactions that require different conditions can occur at the same time in the same cell.

III.       Organelles

            A.        Nucleus (Fig. 7.9)

                        1.         contains the genetic material in the form of DNA.

                        2.         The double layer of the nuclear membrane (or nuclear envelope) is perforated by pores through which materials can pass.

                        3.         Nuclear lamina is a network of filaments that gives structure to the nucleus.

            B.        Ribosomes

                        1.         Build protein for the cell.

                        2.         Free ribosomes are suspended in the cytosol and construct proteins that stay within the cytosol.

                        3.         Attached ribosomes are attached to the ER and construct proteins that will be part of other organelles, incorporated into membranes or exported from the cell.

            C.        Endomembrane system - many of the membranes in eukaryotic cells are part of a cell-wide system of connected membranes called the endomembrane system.

                        1.         Nuclear envelope - see (A) above

                        2.         Endoplasmic reticulum (ER) (Fig. 7.11)

                                    a.         Smooth ER - functions in the synthesis of lipds such as fatty acids, phospholipids, and steroids. Also, detoxify drugs, alcohol, and other poisons.

                                    b.         Rough ER

                                                (1)       As a polypeptide chain is synthesized by an attached ribosome, it is threaded through pores in the ER membrane into the space inside. The polypeptides are folded into their proper conformation and are packaged inside vesicles for export.

                                                (2)       Rough ER also functions in membrane production. As new membrane is synthesized, it can be transferred to parts of the endomembrane system as it is needed.

                        3.         Golgi (Fig. 7.12)

                                    a.         Vesicles from the ER go to the Golgi.

                                    b.         Proteins from the ER are modified and stored in the Golgi before being sent to their destination.

                                    c.         Cis face receives proteins while the trans face ships them.

                        4.         Lysosomes

                                    a.         A membrane-enclosed sac of hydrolytic enzymes.

                                    b.         These enzymes are used to digest proteins, polysaccharides, lipids, and nucleic acids.

                                    c.         The enzymes work best at a pH of about 5, which is maintained within the lysosome (an example of compartmentalization). Why have they evolved to work best at this pH?

                                    d.         The products of hydrolysis in the lysosome can be recycled by the cell into new macromolecules.

                                    e.         Lysosomes also function to digest particles ingested by phagocytosis.

                                    f.         Another function is the programmed destruction of certain cells during development.

                        5.         Vacuoles - membrane enclosed sacs

                                    a.         Food vacuoles (Fig. 7.14) - food particles ingested by phagocytosis are enclosed in food vacuoles before fusing with lysosomes for digestion.

                                    b.         Contractile vacuole - collects and pumps water out of the cells of freshwater protists.

                                    c.         Central vacuole (Fig. 7.15) - a large vacuole in plant cells that functions in the storage of a variety of materials such as proteins, ions, metabolic wastes, pigments, and water.

                        6.         Plasma membrane - surrounds the cell, regulating the exchange of materials with the extracellular environment. The plasma membrane will discussed in Chapter 8.

            D.        Peroxisomes - contain enzymes that transfer hydrogen from various substrates to oxygen, producing hydrogen peroxide. Another enzyme in the peroxisome converts the toxic hydrogen peroxide into water. They have several functions:

                        1.         Breaking fatty acids into smaller molecules that can be moved to mitochondria as fuel for cellular respiration.

                        2.         In the liver, detoxifying alcohol.

            E.        Mitochondria (Fig. 7.18)

                        1.         The sites of cellular respiration, generating energy (ATP) from sugars, fats and other fuels.

                        2.         Mitochondria contain their own ribosomes and DNA although most of the proteins in them are made in the cytosol under the direction of nuclear genes.

                        3.         Enclosed by a double membrane. Th inner membrane is highly folded to increase surface area. Many enzymes required for cellular respiration are built into the inner membrane.

            F.        Chloroplasts (Fig. 7.19)

                        1.         The sites of photosynthesis where solar energy is converted to chemical energy in the form of carbohydrate.

                        2.         Enclosed by a double membrane.

            G.        Cytoskeleton (Table 7.2)

                        1.         Provides structural support to the cell which helps maintain shape. By dismantling and rebuilding cytoskeleton, the cell can change shape.

                        2.         Movement (Fig. 7.21) - Pieces of cytoskeleton can move past one another and organelles can move along the cytoskeleton using motor molecules. In this way, organelles can move throughout the cytosol.

                        3.         Cilia and flagella (Fig. 7.24 and 7.25) - The figure shows the 9+2 arrangement of microtubules. The protein arms extending from some tubes to neighboring tubes change shape using energy from ATP which causes the tubes to slide past one another.

                        4.         Microfilaments (actin filaments) (Fig. 7.27)- actin filaments, interspersed with thicker myosin filaments, are arranged in parallel rows in muscle cells. As actin and mysosin filaments slide past one another, the muscle contracts.

            H.        Extracellular matrix (ECM) (Fig. 7.29) - the ECM provides support and anchorage for cells but also allows cells to respond to the environment:

                        1.         During development, cells can migrate to the appropriate location in the embryo by matching their ECM with that of the cells around them.

                        2.         Physical signals outside the cell can be transmitted inside the cell through parts of the ECM.

            I.         Intercellular junctions - although cells of plants and animals are individual, they often interact and communicate through special areas of direct, physical contact.

                        1.         Plant cells

                                    a.         Plasmodesmata (Fig. 7.28) - channels in the walls of plant cells through which materials, including cytoplasm, pass. The plasma membranes of adjacent cells are continuous through plasmodesmata.

                        2.         Animal cells

                                    a.         Tight junctions (Fig. 7.30) - sealing junctions

                                                (1)       The membranes of neighbouring cells are fused together to form a seal that prevents the leakage of extracellular fluid across a layer of cells.

                                    b.         Desmosomes (Fig. 7.30) - anchoring junctions

                                                (1)       Proteins fasten adjacent cells together like snaps or rivets.

                                    c.         Gap junctions (Fig. 7.30) - communication junctions

                                                (1)       Cytoplasmic channels between adjacent cells. Small molecules such as sugars, salts, and amino acids can pass through.