Lecture 2A
Chapter 4: CELLS - Structure and Function

      A little philosophy: When learning, the student is usually taught the subject INITIALLY at a more superficial level, with lots of detail left out. Later, that knowledge is fine tuned and embellished . Thus most of the purple additions below are fine tunings and embellishments added to help you study for the final exam.

  1. Introduction.

    1. All organisms are composed of cells

    2. At a scale just below what humans can directly perceive, most cells are invisible, but with a microscope, we can plainly see that all living things contain cells. Cells that are visible to the unaided eye are, for example, unfertilized ostrich eggs, and just barely visible, many of the protozoans living in ponds. (Think back: are viruses alive?)

    3. Eukaryotic cells, by definition, have complex internal organization

      1. Outer surface structures face the environment (hooks, cilia, flagella, slime (glycocalyx, etc.) and waxy layers).

      2. The plasma membrane physically and chemically separates the outside environment from the cellıs internal environment.

      3. Cytoplasm includes the semifluid medium and, in the case of eukaryotic cells, organelles (recognizable, smaller structures) between the plasma membrane and the nucleus. (Prokaryotic creatures (bacteria) do not contain organelles.)

      4. The nucleus contains genes encoded in DNA.

  2. Introduction to the World of the Cell.

    1. Microscopes provide windows to the world of the cell (Module 4.1).

      1. Images formed by microscopes represent the object "under" the microscope.

      2. Magnification: the number of times larger the image appears than the object actually is.

      3. Resolution: clarity of the image.

      4. Images are formed in different ways by three types of microscopes that produced the images in the text. Each of these microscopes has advantages relative to the others, and a range of scales at which it functions best

      5. Light microscopes (LM) bend the light coming through an object. The bent light rays form larger images in the viewerıs eyes. Well-resolved LM images are limited to 1,000­2,000 times larger than life size. The LM is particularly good for looking at living cells and tissues (Figure 4.1A).

      6. Scanning electron microscopes (SEM) compose images on a TV screen, from electrons that bounce off the surfaces of the object. SEM images are usually about 10,000­20,000 times larger than life size. The SEM is particularly good for showing organismal and cellular surfaces under high magnification (Figure 4.1B).

      7. Transmission electron microscopes (TEM) compose images on camera film, from electrons that have traveled through very thin slices of the object and have been bent by magnetic lenses. TEM images are usually about 100,000­ 200,000 times larger than life size. The TEM is particularly useful for showing the internal structures of cells (Figure 4.1C).

    2. Eukaryotic cell sizes vary with their function (Module 4.2; Figure 4.2).

      1. Blood cells are very small, to allow them to flow through blood vessels, and to provide a large surface area for efficient gas exchange (Figure 4.3).

      2. Nerve cells can be very long, to communicate between different parts of an animalıs body.

      3. Bird eggs are very large, mostly composed of food reserves.

    3. Natural laws limit cell size (Module 4.3).

      1. Large cells have a smaller ratio of surface area to volume than do small cells (Figure 4.3).

      2. This fact imposes the upper limit on cell size (actually, cell volume) because materials have to flow across the surface to get to the inside. Larger cells require correspondingly greater surface area, which they do not have Unless they take the strategy of being very flat or filamentous. There is a type of amoeba which is nearly 3 ft across, and a fungal cell that is more than a acre in extent..

      3. The small size of cells is limited by the total size of all the molecules required for cellular activity (DNA, ribosomes, life-process-governing proteins, etc.).

    4. Prokaryotic cells are small and structurally simple (Module 4.4; Figure 4.4).

      1. The two groups (Kindoms?) of prokaryotic cells are the Bacteria and the Archaea. The latter are more closely related to eukaryotes than they are to the bacteria (Module 15.14).

      2. Usually relatively small (2­8 µm in length).

      3. Lack a nucleus: their "chromosomal" DNA is in direct contact with cytoplasm and is coiled into a nucleoid region. Their plasmid DNA's, if present, are like small versions of the super-coiled, circular chromosomal DNA, and are scattered throughout the cytoplasm, often in many copies.

      4. Cytoplasm includes 1 to 3 million ribosomes (protein factories) .

      5. Otherwise composed of a boundary plasma membrane, complex outer cell wall, which is a rigid monofilament net-like container, often with a sticky outer surface (Gram-positive bacteria such as those that make cheese and butter) or an exterior second membrane (Gram-negative bacteria such as E.coli). Other "optional" exterior equipment might be flagella and/or pili for attaching to things or for transmitting genes between cells.

      6. Mitochondria and chloroplasts in eukaryotic cells are probably derivatives of prokaryotic cells.

    5. Eukaryotic cells are partitioned into functional compartments (Module 4.5).

      1. Usually relatively larger (10­100 µm or more) in diameter.

      2. Internally complex, with organelles of two types: membranous and nonmembranous.

      3. Membranous organelles found in eukaryotic cells include the nucleus, endoplasmic reticulum, Golgi apparatus, mitochondria, chloroplasts (plants), lysosomes, and peroxisomes.

      4. Nonmembranous organelles found in eukaryotic cells include ribosomes, microtubules, centrioles, flagella, and the cytoskeleton.

      5. Animal cells are bounded by the plasma membrane alone, often have flagella, and lack a cell wall (Figure 4.5A).

      6. Plant cells are bounded by both a plasma membrane and a rigid cell wall (Figure 4.5B). In addition, plant cells usually have a central vacuole and chloroplasts, lack centrioles, and usually lack lysosomes and flagella.

      7. Cells of eukaryotes in other kingdoms vary in structure and components (protists: Figure 4.1A; Figures 4.13B, 16.22A­16.22D, 16.25A, and 16.25B; fungi: Figure 17.16D

      8. Membranes play an important role in defining many cellular structures. Introduce the phospholipid bilayer and the protein mosaic model of membrane structure, reminding students that a thorough discussion of the structure and function of membranes will come in Chapter 5 (Figure 5.12).

  3. Organelles of the Endomembrane System

    1. The nucleus is the genetic control center of the cell (Module 4.6; Figure 4.6).

      1. The nuclear envelope is a double membrane, perforated with pores through which material can pass into and out of the nucleus, which separates this organelle from the cytoplasm.

      2. DNA, with its coating of proteinaceous histones, can be seen as strands of chromatin dispersed inside the nucleus.Each strand of chromatin constitutes a chromosome.

      3. During cell reproduction chromosomes coil up and become visible through a light microscope.

      4. The nucleolus, also within the nucleus, is composed of chromatin, RNA, and protein. The function of nucleoli is the manufacture of ribosomes, because its DNA contains upwards of 10,000 copies of genes coding for rRNA and tRNA.

    2. Overview: Many cell organelles are related through the endomembrane system (Module 4.7).

      1. An extensive system of membranous organelles work together in the synthesis, storage, and export of molecules (Figures 4.11B and 4.14).

      2. Each of these organelles is bounded by a single membrane. Some are in the form of flattened sacs, some are rounded sacs, and some are tube-shaped.

    3. Rough endoplasmic reticulum makes membrane and proteins (Module 4.8;Figure 4.8A).

      1. Rough ER is composed of flattened sacs that often extend throughout the entire cytoplasm. (It is called 'rough' because the attached ribosomes give it a bumpy, rough appearance under SEM or TEM.)

      2. Ribosomes on rough ER make proteins, some of which are incorporated into the membrane; other proteins are packaged in membranous sacs that bud off the rough ER (Figure 4.8B).

    4. Smooth endoplasmic reticulum has a variety of functions (Module 4.9; Figure 4.9).

      1. One job of smooth ER is to synthesize lipids.

      2. In other forms of smooth ER, enzymes help process materials as they are transported from one place to another. An example of this function is the detoxification of drugs by smooth ER in liver cells.

      3. Other functions of smooth ER include the storage of calcium ions which are required for muscle contraction.

    5. The Golgi apparatus finishes, sorts, and ships cell products (Module 4.10; Figure4.10).

      1. Transport vesicles from the ER fuse on one end of a Golgi stack to form flattened sacs.

      2. These sacs move through the stack like a pile of pancakes added at one end and eaten from the other. Molecular processing occurs in the sacs as they move through the Golgi.

      3. At the far end, modified molecules are released in transport vesicles.

    6. Lysosomes digest the cellıs food and wastes (Module 4.11; Figure 4.11B).

      1. Lysosomes are one kind of vesicle produced at the far end of the Golgi.

      2. Within these vesicles are hydrolytic enzymes that break down the contents of other vesicles with which they fuse (Figure 3.3B).

      3. Abnormal lysosomes can cause fatal diseases (Module 4.12).

        1. Lysosomal storage diseases result from an inherited lack of one or more hydrolytic enzymes from lysosomes.

        2. In Pompeıs disease, lysosomes lack glycogen-digesting enzyme. In Tay-Sachs disease, lysosomes lack lipid-digesting enzymes.

    7. Vacuoles function in the general maintenance of the cell (Module 4.13).

      1. Vacuole is the general term given to other membrane-bounded sacs.

      2. Plants have central vacuoles that function in storage, play roles in plant cell growth, and may function as large lysosomes (Figure 4.13A).

      3. Contractile vacuoles in cells of freshwater protists (both protozoans and algae) function in water balance (Figure 4.13B).

  4. Energy-Converting Organelles.

    1. Chloroplasts convert solar energy to chemical energy (Module 4.15; Figure 4.15).

      Can you label the various components of this diagram of a part of a chloroplast?

      1. Found in most cells of plants and in cells of photosynthetic protists (algae).

      2. Double-membrane-bounded.

      3. Site of photosynthesis involving the complex derivative of porphyrin called: chlorophyll. Can you name any other porphyrins?

      4. The structure of the organelle fits its function. As we will see, the capturing of light and electron energizing occur on the grana, and chemical reactions that form food-storage molecules occur in the stroma

      5. Preview: Photosynthesis is covered in detail in Chapter 7 and the prokaryotic origin of chloroplasts is discussed in Module 16.20.

    2. Mitochondria harvest chemical energy from food (Module 4.16; Figure 4.16). Again, probable prokaryotic origins.

      In this simple diagram, name the parts that are NOT shown.

      1. Found in all cells of eukaryotes, except a few anaerobic protozoans.

      2. Double-membrane-bounded.

      3. Site of cellular respiration.

      4. The structure of the organelle fits its function. As we will see, the ATP-generating electron transport system is embedded in the inner membrane (cristae), and chemical reactions occur in compartments between membranes.

      5. Preview: Cellular respiration is covered in detail in Chapter 6 and the origin of mitochondria is discussed in Module 16.20.

  5. The Cytoskeleton and Related Structures.

    1. The cellıs internal skeleton helps organize its structure and activities (Module4.17; Figure 4.17A, Figure 4.17B).

      1. The organelles discussed up to this point, particularly the endomembrane system, provide cells with some support.

      2. The cytoskeleton adds to this support, plays a role in cell movement, and may have a role in cell signaling.

      3. The cytoskeleton is a three-dimensional meshwork of fibers: microfilaments, intermediate filaments, and microtubules.

      4. Microfilaments are solid rods composed of globular proteins. They play a role in cell movement, including contraction.

      5. Intermediate filaments are ropelike strands of fibrous proteins. These structures are tension bearing and anchor some organelles. NOTE: The cytoskeletal fibers of anchoring junctions are intermediate filaments (Module 4.19).

      6. Microtubules are hollow tubes composed of globular proteins. They guide the movement of organelles through the cell and are the basis of ciliary and flagellar movement.

    2. Cilia and flagella move when microtubules bend (Module 4.18).

      1. Although the terms cilium and flagellum refer to similar structures, the structures were named when their internal similarities were not appreciated. Cilia are short, numerous, and usually complexly organized. Flagella are longer, fewer, and less complexly organized.

      2. In both cases, these nonmembranous organelles are minute, tubular extensions of the plasma membrane that surround a complex arrangement of microtubules (Figure 4.18A).

      3. Cilia and eukaryotic flagella function to move whole cells or to move materials across or into cells. Prokaryotic flagella are not only constructed very differently that those of eukaryotes, but operate by truly rotating - nature's only example of the wheel!

      4. The underlying structure consists of nine microtubule doublets arranged in a cylinder around a central pair of microtubules. At the base within the cell body (basal body), the structure is slightly different (Figure 4.18A). A prokaryotic flagellum consists of a rigid helical inner protein that is surrounded by a loose membranous sheath. When the rotary basal organ turns the inner "rod", the flagellum churns.

      5. Various types of whipping movements of a whole flagellum or cilium occur when the microtubule doublets move relative to neighboring doublets. The connecting dynein arms apply the force (Figure 4.18B).

      6. Preview: Basal bodies are in the cytoplasm below these external extensions. They are identical in cross section to centrioles, which function in cell division (Figure 4.5A, Modules 8.7 and 8.8, Figure 8.7).

  6. Eukaryotic Cell Surfaces and Junctions.

    1. Cell surfaces protect, support, and join cells (Module 4.19).

      1. Prokaryotic cells and eukaryotic cells of many protists function independently of one another and relate directly to the outside environment.

      2. In multicellular plants, cell walls protect and support individual cells and join neighboring cells into interconnected and coordinated groups (tissues) (Figure

      3. Plant cell walls are multilayered and are composed of various mixtures of polysaccharides and proteins. The dominant polysaccharide in plant cells is cellulose.

      4. Plasmodesmata are channels through the cell walls connecting the cytoplasm of adjacent plant cells

      5. In multicellular animals, cells secrete and are embedded in sticky layers of glycoproteins, the extracellular matrix, which can protect and support the cell as well as regulate cell activity (Figure 4.19B).

      6. In animal tissues, cells are joined by several types of junctions.

        1. Tight junctions provide leak-proof barriers;

        2. Anchoring junctions join cells to each other or to the extracellular matrix, but allow passage of materials along the spaces between cells or attach cells to an extracellular matrix;

        3. Communicating junctions provide channels between cells for the movement of small molecules.

  7. Functional Categories of Organelles.

    1. Eukaryotic organelles comprise four functional categories (Module 4.20; Table 4.20).

      1. Manufacture: synthesis of macromolecules and transport within the cell.

      2. Breakdown: elimination and recycling of cellular materials.

      3. Energy processing: conversion of energy from one form to another.

      4. Support, movement, and communication: relationships with extracellular environments.

      5. Within each of the four categories there are structural similarities that underlie their functions.

      6. All four categories work together as an integrated team, producing the emergent properties at the cellular level.

    2. All life forms share fundamental features (Module 4.21).

      1. Review: The concept of the fundamental similarity of life is first discussed in Module 1.5.

      2. Cells are highly structured units.

      3. Cell structure and function are related at the cellular and subcellular (and supracellular) levels.

      4. Cells are set off from their external environment by membranes.

      5. Each cell has DNA as the genetic material.

      6. Each cell carries out metabolism.

      7. These features are likely to be characteristic of other life forms that may have evolved in our universe, although the materials and structures involved might be modified from the pattern seen on Earth.