Zoology Notes: 006 Chapter 3

Chapter 3. Cells After atoms and molecules, the next higher level of complexity in living organisms includes cells and their components. All living things are made up of cells. Some cell components occur in all living cells, while others occur only in the cells of leaves, roots, or other parts of plants. Depending on their components, cells can divide, grow, transport substance, secrete substances, or harvest energy from organic molecules. Most types of cells also contain genetic material that controls the activities of the cell. This genetic material is inherited by new cells after cell division. The Cell Theory The modern version states that: - Cells are the morphological and physiological units of all living things. - The properties of a given organism depend on those of its individual cells. - Cells originate only from other cells, and continuity is maintained through the genetic material. Prokaryotic and Eukaryotic Cells All living species are composed of eukaryotic or prokaryotic cells. The differences between prokaryotic and eukaryotic cells are: Prokaryotic Cell Eukaryotic Cell absent present usually singular, ring-shaped, multiple, not ring-shaped, consisting only of DNA, without consisting of DNA together with associated proteins, and lack attached proteins and have centromeres centromeres organelles membrane-bound organelles are membrane-bound organelles are absent present in the cytoplasm size diameter seldom exceeds 2 μm diameter typically 20 μm or more capacity to lacks the capacity to differentiate great capacity to differentiate in differentiate into specialized tissues in multistructure w/in multi-cellular cellular organisms bodies organisms occurs only as bacteria and makes up bodies of protists, fungi, cyanophytes (blue-green algae) plants, and animals Fig. 3.1. Differences between prokaryotic and eukaryotic cells. nuclear membrane chromosomes

Structures Found in the Cell Looking through a light microscope, the only animal cell structures that can be seen are the nucleus, the cytoplasm, and the cell membrane. In plants cells, these structures can also be seen in addition to the cell wall. Other organelles can only be seen through an electron microscope. Organelles are usually membrane-bound structures inside the cytoplasm that have specific metabolic functions. These organelles float in the hyaloplasm. The hyaloplasm, or cytosol, is the clear, aqueous medium that bathes all cytoplasmic bodies and serves as a reservoir of solutes and water. Organelles that are common in plants and animals include the cell membrane, the nucleus, nucleoli, endoplasmic reticulum, ribosomes, golgi apparatus, mitochondria, and microbodies. Organelles that can only be seen in plants include the cell wall, central vacuole, and plastids. Substances inside the cytoplasm that do not have metabolic roles are called inclusion bodies. Inclusion bodies are passive, often very temporary materials such as pigments, 18

secretory granules, and aggregates of stored proteins, lipids, or carbohydrates, which can be utilized by the cell in its life processes. Cell Membrane. The cell membrane may also be called the plasma membrane, plasmalemma, or cytolemma. It is selectively permeable, depending on the lipid content of the membrane, allowing entry of certain molecules into the cytoplasm while disallowing others. The cell membrane also contains pumps which regulate the ion concentrations within the cell and its immediate vicinity. It contains a variety of enzymes and has specific receptor sites which mediate important cell functions such as endocytosis, phagocytosis, antigen recognition, and antibody production. Hormonetriggered cellular events also depend on specific surface receptors. The cell membrane is composed of phospholipids and proteins. Phospholipids form the basic structure of the membrane referred to as bi-layer, two parallel layers with their hydrophilic heads facing the aquaeous medium on the membrane surface and their hydrophobic tails facing the interior of the membrane. Proteins partially or completely penetrate the phospholipids bi-layer and are responsible for functional properties of the membrane. You may also find other structures on or near the cellular surface. Microvilli are finger-like projections of the plasma membrane that increase the surface area for absorption. Desmosomes are oval disks with anchoring fibrils that lie just within the plasma membranes of epithelial cells subject to being stretched. Gap junctions are hollow “pipes” formed by a ring of six dumbbell-shaped protein subunits that penetrate the plasma membrane of certain tissues and allow free flow of materials from cell to cell. Cilia and flagella are motile fibrils that protrude from the surface of certain types of cells, being covered by an extension off the plasma membrane.

The Fig. 3.1. The phospholipid bi-layer that makes up the cell membrane.

Nucleus. The nucleus is usually the most conspicuous organelle in a cell. It contains most of a cell’s DNA, which occurs with proteins in thread-like chromosomes. The nucleus is surrounded by two membranes, together called the nuclear envelope. The outer membrane is continuous with the endoplasmic reticulum. The inner and outer nuclear 19

membranes are separated by a space of 20-40 nm, except where they fuse to form pores in the envelope. These nuclear pores are small circular openings, 30-100 nm in diameter, bordered by proteins that probably influence the passage of molecules between the nucleus and the rest of the cell. Inside the nucleus is a smaller structure, the nucleolus, which serves as the site for the synthesis of ribosomal RNA (rRNA).

Fig. 3.2. The different organelles found in the cytoplasm

Microfilaments and Microtubules. Microfilaments are thread-like aggregates of protein molecules that serve to maintain cell shape, bring about changes in cell shape, and allow cells to contract. Microtubules are hollow tubules, much stouter than microfilaments, made of a unique protein, tubulin. They too, can maintain cell shape, and also serve as spindle fibers that separate the chromosomes during cell division. Centrioles. Centrioles occur as a single pair of tin can-shaped organelles in the cells of animals, fungi, and certain lower plants. During cell division the pair separate, move to opposite ends of the cell, and produce spindle fibers that separate the chromosomes. Ribosomes. Ribosomes are organelles that serve as the site for the biosynthesis of large varieties of proteins destined either for extra- or intra-cellular use. Ribosomes are either attached to membranes or move freely in the cytosol (the semi-fluid matrix between organelles). The number of ribosomes varies among cell types and in different 20

stages of cell development. They are especially abundant in dividing cells because these cells make large amounts of protein. Endoplasmic Reticulum. The endoplasmic reticulum is a network of channels or tubules which constitutes the bulk of the endo-membrane system. It is continuous with the nuclear membrane. Two regions of ER can be distinguished in electron micrographs. One region is called the rough ER because the many ribosomes attached to it give it a rough appearance. In contrast, the other region is called the smooth ER because it has no ribosomes attached to it. The smooth ER, in most cells, makes up the terminal portions of rough ER. It gives rise to transfer vesicles that carry substances synthesized within the rough ER to other location, especially the golgi complex. Functions of the smooth ER include: - Steroid hormone synthesis in the testicular interstitial cells, cells of the corpus luteum, and cells of the adrenal cortex - Synthesis of complex lipids and drug detoxification in hepatocytes - Lipid resynthesis in the intestinal absorptive cells - Release and capture of Ca++ ions in striated muscle cells - Concentration of Cl- ions in gastric parietal cells Golgi Complex. A Golgi complex (Golgi apparatus) is usually two-sided, with one side facing the smooth ER and one side facing the plasma membrane. They receive material from the smooth ER, either through direct connections or in vesicles released by the ER. These vesicles contain proteins, lipids, and other substances, which are often chemically modified in the golgi bodies and then sorted into separate packets. These packets eventually move to the edge of the golgi bodies near the outer face, where the golgi body membrane is pinched off into another vesicle. This vesicle moves to the plasma membrane or to other sites in the cell. Vesicles that move to the plasma membrane are secretory vesicles, because they fuse with plasma membrane and secrete their contents to the exterior of the cell. This type of secretion is called exocytosis. Endocytosis, the reverse process, involves taking substances into the cell. Pinocytosis is a type of endocytosis that involves taking up of liquids and diluted substances. Phagocytosis, another type of endocytosis, involves taking in of larger substances even bacteria. Fig. 3.3. The process of exocytosis.

Microbodies. The smallest membrane bound organelles in a cell are called microbodies. These tiny organelles are often associated with membranes of the ER, but they may also be closely associated with chloroplast and mitochondria. Different types of microbodies have specific enzymes for certain metabolic pathways. Two of the most important kinds of microbodies are lysosomes and peroxisomes.

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Lysosomes are involved in the hydrolysis of foreign (heterophagosomes) or intracellular sub-stances (autophagosomes) using hydrolytic enzymes. These enzymes also serve to digest aging organelles or sometimes liberate their enzymes en masse, causing “cell suicide” (autodigestion). They are not present in plants. Peroxisomes are the major sites of oxygen utilization within the cell and are particularly rich in catalase which converts toxic hydrogen peroxide (H2O2), formed during certain metabolic processes, into harmless water and Fig. 3.4. Vesicles forming from the Golgi complex. oxygen.

Mitochondria. Many of the reactions of aerobic respiration are catalyzed by enzymes bound to mitochondrial membranes. The chief function of the mitochondria is to supply energy to the cell through cellular respiration, thus earning the distinction of being the “powerhouse of the cell”. A cell may contain several hundred mitochondria, usually depending on the energy requirement of a cell. Dividing cells and cells that are metabolically active need large amounts of energy and usually have the largest numbers of mitochondria.

Fig. 3.5. The mitochondria.

Vacuoles. Vacuoles are membranous sacs that enclose a variety of substances, often for only temporary storage. Cell Division There are two types of cell division that occur in living things depending on the type of cell: mitosis and meiosis. Mitosis occurs in body cells (soma cells) while meiosis occurs only in sex cells (egg cells and sperm cells). Mitosis. Mitosis is the type of cell division resulting in equal number of chromosomes. This ensures genetic equality of the daughter cells. It occurs in embryonic development, growth, repair of injury, and in replacement of body covering at molting. Four phases comprise the mitotic division: prophase, metaphase, anaphase, and telophase. In prophase, genetic material becomes evident as distinct chromosomes that shorten, thicken, and stain deeply. Towards the end of prophase the nuclear membrane 22 Fig. 3.6. The different stages of mitosis.

and the nucleolus disappear. In metaphase, chromosomes lie radially in an equatorial plate and separate. In anaphase, halved chromosomes move toward their respective poles. Telophase is marked by the end of polar movement, formation of nuclear membrane and the formation of cell membrane across the former plane of the equatorial plate. prophase DNA complex coils (chromatids attached to one another by centromeres) and becomes easily stained disappears during late prophase disappears during late prophase

metaphase arranged in a line along the median plane, centromeres attached to spindle fibers

anaphase centromeres divide, chromatids move toward opposite poles

telophase chromosomes reach the general location of the centrioles

absent

absent

reappears

absent

absent

centrioles and spindle fibers

migrates to opposite poles, forms spindle fibers

spindle fibers shorten pulling chromatids

cellular membrane

intact

spindle fibers attached to centromeres of chromatids intact

reforms around each group of chromosomes spindle fibers disappear

chromosomes

nucleolus nuclear membrane

intact

indents at the point of the equatorial plane dividing the cytolasm into two

Table 3.1. Comparison between stages of mitosis.

The period between cell divisions wherein the cell builds up genetic material to start another cycle is called interphase. It is divided into three phases. Phase Gap1 (G1)

usually lasts 8 hrs or longer depending on the type of cell and level of nutrition; characterized by growth of daughter cells by undergoing internal chemical changes in preparation for DNA replication Synthesis (S) typically lasts about 8 hrs; period of DNA replication or synthesis Gap2 (G2) usually lasts 5 hrs; beginning of active mitosis, replication of organelles Table 3.2. Description of the phases of interphase.

Meiosis. In meiosis, cell division results in the reduction of chromosomal number to haploid (half the normal number of chromosomes) set. Daughter cells (egg and sperm cells) unite during fertilization carrying genes from both parents to provide the correct number of chromosomes. Although both types of cell division involves the same phase (prophase, metaphase, anaphase, and telophase), meiotic cell division consists of two successive cell division named meiosis I and meiosis II. Meiosis I. In prophase I, the members of each chromosome pair come together (synapsis). This is essential for the orderly separation of the two members of each chromosome pair in the ensuing anaphase. Crossing-over may occur at this phase. Crossing over is the exchange in position of one part of one strand of chromosomes with the equivalent part of the other strand. During the metaphase I, the centromeres do not divide so during anaphase, the two members of each homologous chromosomes pair are separated. Meiosis I is often called the “reductional phase” because at its end each daughter cell contains only one member of each chromosome pair, although each chromosome still consists of two DNA molecules, or chromatids, held together by the undivided centromere. 23

Meiosis II. Depending on the species, meiosis II may begin at once or be delayed. In either case, DNA replication does not occur. When meiosis II starts, the chromosomes move to the midline of the new spindle. The centromeres finally divideand one of the two chromatids of each chromosome passes to each daughter cell. The result is four haploid cells with each chromosome now consisting of only one DNA molecule.

Fig. 3.6. Gametogenesis (spermatogenesis and oogenesis) and Meiosis.

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