Cell Structure and Neuronal Signaling: Biology Lecture Notes, Lecture notes of Biology

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Lecture one 2024 .. - cell structure and the neuron structure

Biology (Gharbiya STEM High School)

Lecture one 2024 .. - cell structure and the neuron structure

Biology (Gharbiya STEM High School)

Biology Lecture Notes

(STEMer’s guide)

Lecture (one)

The Neuron

Prokaryotes

➢ Do not have a membrane bound nucleus. ➢ They are split into two domains called Bacteria and Archaea. ➢ Archaea have as much in common with eukaryotes as they do with bacteria. They are typically found in the extreme environments such as salty lakes and boiling hot springs. Unlike bacteria, the cell walls of archaea are not made from peptidoglycan. ➢ Most known prokaryotes are members of the domain Bacteria. ➢ The introduction of the two domains makes the kingdom Monera obsolete. ➢ The kingdom Monera was the kingdom containing all prokaryotes.

Structure of Prokaryotes ▪ Prokaryotes don't have a nucleus. Instead of a nucleus, prokaryotes usually have a single, circular double stranded molecule of DNA. This molecule is twisted into supercoils and is associated with histones in Archaea and with proteins that are different from histones in Bacteria. ▪ The DNA, RNA and protein complex in prokaryotes forms a structure visible under the light microscope called a nucleoid (also called the chromatin body, nuclear region, or nuclear body). The nucleoid is not enclosed by a membrane. ▪ There are two major shapes of bacteria: cocci (round) and bacilli (rod shaped). There are many other shapes, including helical. Helically shaped bacteria are called spirilla, if they are rigid. ▪ Otherwise, they are called spirochetes. Certain species of spirochetes may have given rise to eukaryotic flagella through a symbiotic relationship. ▪ Prokaryotes have no complex, membrane bound organelles. ▪ All living organisms contain both DNA and RNA, so prokaryotes have RNA.

▪ Since they translate proteins, prokaryotes have ribosomes. Prokaryotic ribosomes are smaller than eukaryotic ribosomes. They are made from a 50S and a 30S subunit to form a 70S ribosome.

▪ A prokaryote mayor may not contain a mesosome. Mesosomes are invaginations of the plasma membrane. They may be in the shape of tubules, lamellae, or vesicles. Under the light microscope mesosomes may appear as bubbles inside the bacterium. Their exact function is unknown, but they may be involved in cell wall formation during cellular division.

▪ Prokaryotes also have inclusion bodies. Inclusion bodies are granules of organic or inorganic matter that may be visible under a light microscope. Inclusion bodies may or may not be bow1d by a single layer membrane.

N.B.

Peptidoglycan o Porous, so it allows large molecules to pass through. Many antibiotics such as penicillin attack the amino acid crosslinks of peptidoglycan. o Lysozyme, an enzyme produced naturally by humans, attacks the disaccharide linkage in peptidoglycan. In both cases the cell wall is disrupted, and the cell lyses killing the bacterium.

One method of classification of bacteria is according to the type of

cell wall that they possess.

A staining technique, called gram staining , used to prepare bacteria for viewing under the

light microscope, stains two major cell wall types differently. o The first type is called gram-positive bacteria because its thick peptidoglycan cell wall prevents the gram stain from leaking out.

▪ These cells show up as purple when stained with this process.

▪ Gram-positive bacteria have a cell wall that is approximately four times thicker than the plasma membrane. ▪ The space between the plasma membrane and the cell wall is called the periplasmic space. The periplasmic space contains many proteins that help the bacteria acquire nutrition, such as hydrolytic enzymes.

o Gram-negative bacteria

▪ Appear pink when gram stained.

▪ Their thin peptidoglycan cell wall allows most of the gram stain to be washed off. ▪ The peptidoglycan of gram-negative bacteria is slightly different from that of gram- positive. Outside the cell wall, gram-negative bacteria have a phospholipid bilayer, this second membrane is more permeable than the first, even allowing molecules the size of glucose to pass right through. It is similar in structure to the plasma membrane, but also possesses lipopolysaccharides. The polysaccharide is a long chain of carbohydrates which protrudes outward from the cell. These polysaccharide chains can form a protective barrier from antibodies and many antibiotics. ▪ A lipoprotein in the outer membrane called Braun's lipoprotein points inward toward the cell wall and attaches covalently to the peptidoglycan. ▪ In gram-negative bacteria the periplasmic space is the space between the two membranes. (Different species of Archaea may stain positive or negative.). ▪ Many bacteria are wrapped in either a capsule or a slime layer. Both capsules and slime layers are usually made of polysaccharide. Slime layers are easily washed off, while capsules are not. A capsule can protect the bacterium from phagocytosis, desiccation, Some viruses, and some components of the immune response of an infected host. ▪ Some gram-negative bacteria possess fimbriae or pili (not to be confused with the sex pilus). Fimbriae are short tentacles, usually numbering in the thousands, that can attach a bacterium to a solid surface. They are not involved in cell motility.

Bacterial flagella are long, hollow, rigid, helical cylinders made from a globular

protein, called flagellin; these should not be confused with eukaryotic flagella, which composed of microtubule'. They rotate counterclockwise (from the point of view of looking at the cell from the outside) to propel the bacterium in a single direction. When they are rotated clockwise, the bacterium tumbles. The tumbling acts to change the orientation of the bacterium allowing it to move forward in a new direction. The flagellum is propelled using the energy from a proton gradient rather than by ATP. Some bacteria can move via a gliding motion that has not yet been explained. Spirochetes, the flexible, helical shaped bacteria, can move through viscous fluids by flexing and spinning.

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Which of the following structures are found in prokaryotes? I. A cell wall containing peptidoglycan. II. A membrane lacking cholesterol. III. Ribosomes A. I only B. II only C. I and II only D. I, II, and 1l

o As well as phospholipids, the plasma membrane contains other types of lipids such as glycolipids. Unlike eukaryotic membranes, prokaryotic plasma membranes usually do not contain steroids such as cholesterol. Instead, some bacterial membranes contain steroid- like molecules called hopanoids. o Different lipid types are arranged asymmetrically between the leaflets. For instance, glycolipids are found on the outer leaflet only. o Also embedded within the plasma membrane arc proteins. Most of the functional aspects of membranes are due to their proteins. o Membrane proteins act as transporters, receptors, attachment sites, and enzymes. Amphipathic proteins that traverse the membrane from the inside of the cell to the outside are called integral or intrinsic proteins. Peripheral or extrinsic proteins are situated entirely on the surfaces of the membrane. They are ionically bonded to integral proteins or the polar group of a lipid. o Both integral and peripheral proteins may contain carbohydrate chains making them glycoproteins. The carbohydrate portion of membrane glycoproteins always protrudes toward the outside of the cell.

o Lipoproteins (sometimes called lipid anchored proteins) also exist in some plasma membranes with their lipid portions embedded in the membrane and their protein portions at the surfaces. o Membrane proteins are distributed asymmetrically throughout the membrane and between the leaflets.

o Neither proteins nor lipids flip easily from one leaflet to the other. Since the forces holding the entire membrane together are intermolecular, the membrane is fluid; its parts can move laterally but cannot separate. The model of the membrane as just described is known as the fluid mosaic model. A mosaic is a picture made by placing many small, colored pieces side by side. The mosaic aspect of the membrane is reflected in the asymmetrical layout of its proteins. In eukaryotic membranes, cholesterol moderates membrane fluidity. In the prokaryotic plasma membrane, hopanoids probably reduce the fluidity of the membrane.

Fungi

▪ Fungi represent a distinct kingdom of organisms with tremendous diversity. ▪ Three divisions exist within this kingdom: Zygomycota, Ascomycota, and Basidiomycota ▪ Like plants, fungi are separated into divisions not phyla. ▪ All fungi are eukaryotic heterotrophs that obtain their food by absorption rather than by ingestion, they secrete their digestive enzymes outside their bodies and then absorb the products of digestion. ▪ Although most fungi are considered saprophytic, many fungi do not distinguish between living and dead matter, and thus can be potent pathogens (disease causing). (Saprophytic means to live off dead organic matter.) ▪ Most fungi possess cell walls, called septa , made of the polysaccharide, chitin. ▪ Chitin is more resistant to microbial attack than is cellulose. It is s the same substance of which the exoskeleton of arthropods is made. (Arthropods are insects and crustaceans.) ▪ Septa are usually perforated to allow exchange of cytoplasm between cells, called cytoplasmic streaming. Cytoplasmic streaming allows for very rapid growth.

The Eukaryotic Cell

The Nucleus

• The major feature distinguishing eukaryotic cells from prokaryotic cells is the nucleus of the eukaryote.
• The nucleus contains all of the DNA in an animal cell (except for a small amount in the mitochondria).
• The aqueous 'soup' inside the nucleus is called the nucleoplasm.
• The nucleus is wrapped in a double phospholipid bilayer called the nuclear envelope or membrane. The nuclear envelope is perforated with large holes called nuclear pores. RNA can exit the nucleus through the nuclear pores, but DNA cannot.
• Within the nucleus is an area called the nucleolus where rRNA is transcribed and the subunits of the ribosomes are assembled. The nucleolus is not separated from the nucleus by a membrane.

N.B. Only eukaryotes have nuclei. If you remember that DNA cannot leave the nucleus, then you will remember that transcription must take place in the nucleus. RNA(S) and ribosomes leave the nucleus through nuclear pores, translation must take place in the cytoplasm.

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▪ Some proteins are activated within the secretory vesicles. For instance, proinsulin is cleaved to insulin only after the secretory vesicle buds off the Golgi. Lysosomes contain acid hydrolases (hydrolytic enzymes that function best in an acid environment) such as proteases, lipases, nucleases and glycosidases. Together, these enzymes are capable of breaking down every major type of macromolecule within the cell. ▪ Lysosomes generally have an interior pH of 5. They fuse with endocytotic vesicles (the vesicles formed by phagocytosis and pinocytosis), and digest their contents. ▪ Any material not degraded by the lysosome is ejected from the cell through exocytosis. ▪ Lysosomes also take up and degrade cytosolic proteins in an endocytotic process. ▪ Under certain conditions lysosomes (more than one) will rupture and release their contents into the cytosol killing the cell in a process called autolysis. Autolysis is useful in the formation of certain organs and tissues, like in the destruction of the tissue between the digits of a human fetus in order to form fingers. ▪ Endoplasmic reticulum, which lacks ribosomes, is called agranular or smooth endoplasmic reticulum. Rough ER tends to resemble flattened sacs, whereas smooth ER tends to be tubular. Smooth ER plays several important roles in the cell. ▪ Smooth ER contains glucose 6-phosphatase, the enzyme used in the liver, the intestinal epithelial cells, and renal tubule epithelial cells, to hydrolyze glucose 6-phosphate to glucose, an important step in the production of glucose from glycogen. ▪ Triglycerides are produced in the smooth ER and stored in fat droplets. Adipocytes are cells containing predominately fat droplets. Such cells are important in energy storage and body temperature regulation. ▪ The smooth ER and the cytosol share in the role of cholesterol formation and its subsequent conversion to various steroids. ▪ Most of the phospholipids in the cell membrane are originally synthesized in the smooth ER. ▪ The phospholipids are all synthesized on the cytosol side of the membrane and then some are flipped to the other side by proteins called phospholipid translocators located exclusively in the smooth ER. ▪ Finally, smooth ER oxidizes foreign substances, detoxifying drugs, pesticides, toxins, and pollutants.

Cellular Filaments

cytoskeleton The structure and motility of a cell is detennined by a network of filaments known as the cytoskeleton. ▪ The cytoskeleton anchors some membrane proteins and other cellular components, moves components within the cell, and moves the cell itself.

▪ Two major types of filaments in the cytoskeleton are microtubules and microfilaments. ▪ Microtubules are larger than microfilaments. They are rigid hollow tubes made from a protein called tubulin. Although tubulin is a globular protein, under certain cellular conditions it polymerizes into long straight filaments. Thirteen of these filaments lie alongside each other to form the tube. The spiral appearance is due to the two types of tubulin, α-tubulin and β-tubulin used in the synthesis. The mitotic spindle is made from microtubules.

Flagella and cilia are specialized structures also made from microtubules. ▪ The major portion of each flagellum and cilium, called the axoneme , contains nine pairs of microtubules forming a circle around two lone microtubules in an arrangement known as 9+2. Cross bridges made from a protein called dynein connect each of the outer pairs of microtubules to their neighbor. ▪ The cross bridges cause the microtubule pairs to slide along their neighbors creating a whip action in cilia causing fluid to move laterally, or a wiggle action in flagella causing fluid to move directly away from the cell.

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Human Anatomy & physiology

The main systems of the human body are: