Botany 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

nuclear membrane

absent

present

chromosomes

usually singular, ring-shaped, consisting only of DNA, without associated proteins, and lack centromeres

multiple, not ring-shaped, consisting of DNA together with attached proteins and have centromeres

organelles

membrane-bound organelles are absent

membrane-bound organelles are present in the cytoplasm

size

diameter seldom exceeds 2 μm

diameter typically 20 μm or more

capacity to differentiate

lacks the capacity to differentiate into specialized tissues in multi-cellular organisms

great capacity to differentiate in structure w/in multi-cellular bodies

organisms

occurs only as bacteria and cyanophytes (bluegreen algae)

makes up bodies of protists, fungi, plants, and animals

Table 3.1. Differences between prokaryotic and eukaryotic cells.

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, 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. HormoneFig. 3.1. The phospholipid bi-layer that makes up the cell membrane. triggered 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 16

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. The 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 membranes are separated by a space of 20-40 nm, except where they fuse to form pores in the envelope. These nuclear pores Fig. 3.2. The different organelles found in the cytoplasm 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). 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. 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 endoplasmic reticulum can be distinguished in electron micrographs. One region is called the Rough Endoplasmic Reticulum because the many ribosomes attached to it give it a rough appearance. In contrast, the other region is called the Smooth Endoplasmic Reticulum 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. 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 number of ribosomes varies among cell types and in different stages of cell development. They are especially abundant in dividing cells because these cells make large amounts of protein. 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. Fig. 3.3. Vesicles forming from the Golgi complex.

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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. Substances secreted by exocytosis include polysaccharides used to build cell walls, nectars that flowers secrete, and oils or other resinous chemicals secreted from the glands on leaves and stems of mints and other fragrant plants. 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 in plants are peroxisomes, which occur primarily in leaves, and glyoxysomes, which are common in germinating oil-bearing seeds and the young seedlings that grow from them. 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 oxygen. Fig. 3.4. The process of exocytosis.

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. Vacuoles. Vacuoles are membranous sacs that enclose a variety of substances, often for only temporary storage.

Organelles Found Only in Plants The Cell Wall. The most easily observed part of a plant cell is the cell wall. In some cells, such as the cork cells in bark, the cell wall is the only remnant of a formerly living cell. Cell walls are dynamic parts of cells that can grow and change their shape and composition. Their composition varies in different cell types and from one species to another. Up to 60% of a cell wall may be 18

Fig. 3.5. The mitochondria.

cellulose; other components include hemicelluloses, pectins, lignins, and proteins. Almost all plant cells have cellulose-containing cell walls. Young cells and cells in actively growing areas have primary cell walls that are relatively thin and flexible. Examples of such cells include the dividing cells at tips of roots and shoots. The primary cell wall is usually 25% cellulose, the remainder being hemicelluloses, pectins, and glycoproteins. Certain kinds of cells stop growing when they reach maturity. When this occurs, these cells form a secondary cell wall inside the primary cell wall. The secondary cell wall is more rigid than the primary cell wall and therefore functions as a strong support structure. Although cellulose is one of their main components, the secondary walls of cells in wood are up to 25% lignin, which adds hardness and resists decay. Because of its lignin content, wood is one of the strongest materials known. Unlike primary cell walls, secondary cell walls are rigid and lack glycoproteins. Most types of cells that have secondary cell walls die when they reach maturity. Some cell walls, such as those of cork cells also contain suberin. Suberized tissues inhibit water loss through bark, which is why cork from the cork oak is useful in making stoppers for wine bottles. Cells that adjoin one another are probably held together by pectins. The pectic layer between cells is called the middle lamella. Primary cell walls have thin areas where many tiny connections, called plasmodesmata (sing. plasmodesma) occur between adjacent cells. Plasmodesmata are lined by the plasma membrane, thereby forming an uninterrupted channel for the movement of materials from one cell to another. This means that all cells in a plant are interconnected and have the potential to exchange substances through the plasmodesmata. The structure of plasmodesmata and the frequency of their occurrence in conducting and glandular cells suggest that these connections function in transport between cells. Cells probably do not exchange all materials freely; neighboring cells can differentiate into different cell types and maintain different internal concentrations of various chemicals. Water-conducting tissue is an important exception to the general occurrence of plasmodesmata. Cells of this tissue die as they mature, so they have no living material to share between them. Instead, they function as inanimate “straws” formed by many cells. Central Vacuoles. Vesicles from the ER and Golgi apparatus often fuse together to form a central (water) vacuoles. Immature cells of plants and animals may contain several small vacuoles, but in most plant cells these small vacuoles fuse into larger ones as the cell matures. A mature plant cell typically has one large vacuole that can occupy up to 95% of the cell’s volume. The membrane of the central vacuole has its own name, the tonoplast. As plant cells grow, most of the enlargement results from the absorption of water by the vacuoles. This absorption of water by the vacuole expands and pushes the rest of the cell’s contents into a thin layer against the cell wall. Vacuoles that are filled with water create a force, called turgor pressure, on the cell walls, which contributes to the structural rigidity of the cell. In addition to water, vacuoles contain enzymes and other proteins, water-soluble pigments, growth hormones, and ions. Vacuolar enzymes digest storage materials and components from other organelles for recycling into the cytosol. Pigments impart bright colors to flowers, fruits, and other plant parts. Some plants harbor toxic alkaloids or other secondary products in their vacuoles. Ions such as potassium and chloride are stored in vacuoles for easy retrieval to the cytosol when needed for cellular metabolism.

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Plastids. Plastids are classified according to the kinds of pigments or storage products they contain. Amyloplasts store starch and elaioplasts store oil. Colored, non-green plastids are chromoplasts and are usually red, orange, or yellow. Chloroplasts are another type of plastid. Pigments in chloroplasts include the chlorophylls, which create the green color of leaves and other green organs, and the carotenoids, which are the yellow, orange, or red colors of autumn leaves, tomatoes, and carrots. In addition to pigments, chloroplasts often contain starch and oil.

Comparison between Chloroplasts and Mitochondria Two kinds of organelles, chloroplasts and mitochondria, produce most of the ATP needed for cellular metabolism. These organelles are similar in several respects. Both are bounded by two membranes, and much of their internal membranes is folded and stacked to form complex compartments. Their internal membranes contain the enzyme ATPase, which uses the electrochemical energy of protons to phosphorylate ADP to ATP. They both contain DNA that controls synthesis of some of the enzymes necessary for their respective metabolic pathways. Finally, both are semiautonomous, meaning they grow and divide in the cell on their own. The differences between chloroplasts and mitochondria include their respective sources of energy for making ATP, their appearance, and their composition. Chloroplasts use the energy of light to make ATP, whereas mitochondria use the energy of chemical bonds. Chloroplasts contain chlorophyll, which makes them green, while mitochondria are colorless. Photosynthesis occurs in chloroplasts, and most respiration occurs in mitochondria. Each process requires a different set of enzymes. Chloroplasts have many shapes and sizes, but they are generally larger than mitochondria, which are often cigar-shaped.

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. chromosomes

nucleolus nuclear membrane centrioles and spindle fibers cellular membrane

prophase DNA complex coils (chromatids attached to one another by centromeres) and becomes easily stained disappears during late prophase disappears during late prophase migrates to opposite poles, forms spindle fibers intact

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

anaphase centromeres divide, chromatids move toward opposite poles

telophase chromosomes reach the general location of the centrioles

absent

reappears

absent

absent

spindle fibers attached to centromeres of chromatids intact

spindle fibers shorten pulling chromatids

reforms around each group of chromosomes spindle fibers disappear

intact

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

Table 3.2. Comparison between stages of mitosis.

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 and the nucleolus disappear. In 20

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. 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 3.3. 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. 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 divide and 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.

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