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Chapter 3

Chapter 3 . Cells Structure & Function. Cell Theory. Cells are the building blocks of the human body 4 concepts of the cell theory: Cells are the building blocks of all plants and animals Cells are the smallest functioning units of life

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Chapter 3

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  1. Chapter 3 Cells Structure & Function

  2. Cell Theory • Cells are the building blocks of the human body • 4 concepts of the cell theory: • Cells are the building blocks of all plants and animals • Cells are the smallest functioning units of life • Cells are produced through the division of preexisting cells • Each cells maintain homeostasis

  3. The Study of Cells and Their Anatomy • Cytology – the study of the structure and function of cells • The 2 most common methods used to study cell and tissue structure are light microscopy and electron microscopy • Anatomy: • Extracellular fluid – watery medium surrounding cells • In tissues it’s called interstitial fluid • Plasma (Cell) membrane – separates the cell contents (cytoplasm) from extracellular fluid • Nucleus – control center for cellular operations • Cytoplasm subdivides into: • Cytosol – liquid in cell • Organelles – intracellular structures

  4. Nuclear envelope Chromatin Nucleolus Nucleus Smooth endoplasmic reticulum Plasma membrane Mitochondrion Cytosol Lysosome Centrioles Centrosome matrix Rough endoplasmic reticulum Ribosomes Golgi apparatus Secretion being released from cell by exocytosis Cytoskeletal elements • Microtubule • Intermediate filaments Peroxisome Figure 3.2

  5. Plasma Membrane • General functions include: • Physical isolation • A physical barrier that separates the inside of the cell from the extracellular fluid • Regulation of exchange with the environment • Controls the entry of ions and nutrients, the elimination of wastes, and the release of secretions • Sensitivity to the environment • Contains a variety of receptors that enable the cell to recognize and respond to specific molecules in the environment • Structural support • For tissues

  6. Extracellular fluid (watery environment) Hydrophilic head of phospholipid molecule Cholesterol Glycolipid Glycoprotein Carbohydrate of glycocalyx Outward-facing layer ofphospholipids Integral proteins Filament of cytoskeleton Peripheral proteins Bimolecular lipid layer containing proteins Inward-facing layer of phospholipids Hydrophobic tail of phospholipid molecule Cytoplasm (watery environment) Figure 3.3

  7. Plasma Membrane • Membrane Lipids • The phospholipids in a plasma membrane lie in 2 distinct layers: (phospholipidbilayer) • Phosphate heads: polar and hydrophilic • Fatty acid tails: nonpolar and hydrophobic • The hydrophobic tails won’t associate with water or charged molecules, allowing the plasma membrane to act as a physical barrier • Lipid-soluble molecules, oxygen, CO2, etc. are able to cross • Ions and water-soluble compounds cannot • Membrane Proteins • The most common of these membrane proteins span the width of the membrane 1 or more times and are known as transmembrane proteins • May function as receptors, channels, carriers, enzymes, anchors, and identifiers • Membrane Carbohydrates • Carbs form complex molecules with proteins and lipids on the outer surface of the membrane • Function as cell lubricants and adhesives, receptors for extracellular compounds, and form part of a recognition system from attacking the body’s cells and tissues

  8. Diffusion and Filtration • The permeability of the plasma membrane is the property that determines precisely which substances can enter or leave the cytoplasm. • If nothing can cross a membrane, it’s impermeable • If any substance can cross without difficulty, it’s freely permeable • Plasma membranes are selectively permeable • Based on size, charge, shape, solubility • Movement is either passive or active • Passive – move ions or molecules across the plasma membrane without any energy expenditure by the cell • Diffusion • Osmosis • Filtration • Active – require that the cell expend energy, usually in the form of ATP

  9. Diffusion • The net movement of molecules from an area of relatively high concentration to an area of relatively low concentration • The different areas create a concentration gradient • Movement happens until the gradient no longer exists • Diffusion Across Plasma Membranes • An ion or molecule can independently diffuse across a plasma membrane by either: • Moving across the lipid portion of the membrane • Passing through the channel protein in the membrane • Diffusion ability depends on lipid solubility and size

  10. Osmosis • The diffusion of water across a membrane • Because dissolved solute molecules occupy space that would otherwise be taken up by water molecules, the higher the solute concentration, the lower the water concentration • As a result, water molecules tend to flow across a membrane toward the solution containing the higher solute concentration, because this movement is down the concentration gradient for water molecules • Three characteristics of osmosis that are important to remember: • Osmosis is the diffusion of water molecules across a membrane • Osmosis occurs across a selectively permeable membrane that is freely permeable to water but is not freely permeable to solutes • In osmosis, water flows across a membrane toward the solution that has the higher concentration of solutes, because that is where the concentration of water is lower

  11. Osmosis (cont.) • The osmotic pressure of a solution is an indication of the force of water movement into that solution as a result of solute concentration • Pushing against a fluid generates hydrostatic pressure • Can be either isotonic (no net movement), hypotonic (water flows in and swells the cell and lyse), or hypertonic (water flows out of the cells and causes shriveling called crenation)

  12. Filtration • In this passive process, hydrostatic pressure forces water across a membrane • If solute molecules are small enough to fit through membrane pores, they will be carried through with the water • Filtration across specialized blood vessels in the kidneys is an essential step in the production of urine

  13. Carrier-Mediated Transport • Requires the presence of specialized membrane proteins • It can be passive (no ATP required) or active (ATP dependent), depending on the substance being transport and the nature of the transport mechanism • In CMT , membrane proteins bind specific ions or organic substrates and carry them across the plasma membrane • Passive = high conc. to low conc. Active = low conc. to high conc. • Countertransport – when one substance is moved into a cell while the other is moved out • Cotransport – 2 substances are moved into or out of a cell at the same time

  14. Carrier-Mediated Transport (cont.) • Facilitated Diffusion • Used when essential nutrients are insoluble in lipids and too large to fit through membrane channels. Passively transported across the membrane by carrier proteins • First, the molecule binds to a receptor site on the carrier protein. Then the shape of the protein changes, moving the molecule to the inside of the plasma membrane, where it is released into the cytoplasm • Active Transport • In this case, the high energy bond in ATP provides the energy needed to move ions or molecules across the membrane • Despite the energy cost, this transport has one great advantage : it isn’t concentration gradient dependent • All cells have ion pumps that actively transport Na+, K +, Ca2+, and Mg2+ across plasma membranes either in or out of the cell. If one kind of ion is moving in and another is moving out it is called an exchange pump • Exchange pumps mainly function in maintaining cell homeostasis • Sodium-potassium exchange pump

  15. Lipid-insoluble solutes (such as sugars or amino acids) Carrier-mediated facilitated diffusion via a proteincarrier specific for one chemical; binding of substrate causes shape change in transport protein

  16. Vesicular Transport • Involves the movement of materials within small membranous sacs called vesicles • Always an active process • 2 major categories: • Endocytosis – the packaging of extracellular materials in a vesicle at the cell surface for import into the cell • 3 types: receptor-mediated endocytosis (involves the formation of small vesicles at the membrane surface to import substances), pinocytosis (“cell drinking”, the formation of small vesicles filled with extracellular fluid), and phagocytosis (“cell eating”, produces vesicles containing solid objects that may be as large as the cell itself) • Exocytosis – the functional reverse of endocytosis. In exocytosis, a vesicle created inside the cell fuses with the plasma membrane and discharges its contents into the extracellular environment • Ejected material may be a secretion or waste product

  17. The Cytosol • Cytoplasm is a general term for material inside the cell including the cytosol and the organelles • Cytosol is the intracellular fluid , which contains dissolved nutrients, ions, soluble and insoluble proteins, and waste products • It differs from extracellular fluid in that: • Contains a higher concentration of potassium ions and a lower concentration of sodium ions • Has a high concentration of dissolved proteins • Usually contains small quantities of carbohydrates and large reserves of amino acids and lipids

  18. The Organelles • Internal structures that perform specific functions essential to normal cell structure, maintenance, and metabolism • Cytoskeleton – internal protein framework of various threadlike filaments and hollow tubules that gives the cytoplasm strength and flexibility • Microfilaments, intermediate filaments, and microtubules (anchors major organelles) • Microvilli – small, finger shaped projections of the plasma membrane on the exposed surfaces of many cells. Increase surface area for absorption • Centrioles, Cilia, and Flagella • Centrioles – cylindrical structure composed of short microtubules • Cilia – relatively long, slender extensions of the plasma membrane. Undergo active movements that require ATP • Flagella – move a cell through surrounding fluid, rather than moving the fluid past a stationary cell

  19. The Organelles (cont.) • Ribosomes – manufacture proteins using information provided by DNA. Can be either free or fixed • Proteasomes – remove and recycle damaged or denatured proteins and for breaking down abnormal proteins • The Endoplasmic Reticulum (ER) – a network of intracellular membranes connected to the membranous nuclear envelope surrounding the nucleus. 4 major functions: snythesis of proteins, carbs and lipids, storage of synthesized molecules/materials, transport of materials, and detoxification • Smooth ER and Rough ER: ratio depends on cell function • Golgi Apparatus – consists of a set of 5 or 6 flattened membranous discs. Main functions: modification and packaging of secretions, renewal or modification of the PM, and packaging of special exzymes. Creates lysosomes, secretory vesicles, and membrane renewal vesicles

  20. The Organelles (cont.) • Lysosomes – filled with digestive enzymes. Perform cleanup and recycling functions within the cell • Peroxisomes – smaller than lysosomes and carry a different group of enzymes. Absorb and break down fatty acids and other organic compounds • Mitochondria – small organelles that provide energy via ATP bonds for the cell. Have a double membrane • Nucleus – control center for cellular operations. Stores all the information needed to control the synthesis of more than 100,000 different proteins. Determines both the structure of the cell and the and the functions it can perform by controlling which proteins synthesized, under what circumstances, and in what amounts

  21. Transcription and Translation • Transcription – the production of mRNA from a single strand of DNA • Takes place in the nucleus • Steps: • RNA polymerase binds to the promoter of a gene • Promotes the synthesis of an mRNA strand using complementary nucleotides • A sequence of 3 nitrogenous bases along the new mRNA strand represents a codon that corresponds to a triplet on the gene • At the DNA “stop” signal the mRNA detaches • Translation – the assembling of a protein by ribosomes, using the information carried by the RNA molecule • Takes place in the cytoplasm • Steps: • Begins at the “start” codon of mRNA (AUG) with the attachment of the first tRNA carrying an amino acid • The small and large ribosomal subunits join together to enclose the mRNA • A second tRNA arrives, carrying a different amino acid, and binds to the next codon • Ribosomal enzymes remove AA1 from its tRNA and attach it to AA2 with a peptide bond • This continues until it reaches the “stop” codon, where the strand detaches

  22. Nuclear envelope DNA Transcription RNA Processing Pre-mRNA mRNA Nuclear pores Ribosome Translation Polypeptide

  23. Nucleus Energized by ATP, the correct amino acid is attached to each species of tRNA by aminoacyl-tRNA synthetase enzyme. RNA polymerase mRNA Leu Template strand of DNA Amino acid 1 After mRNA synthesis in the nucleus, mRNA leaves the nucleus and attaches to a ribosome. Nuclear pore tRNA Nuclear membrane A G A 2 Translation begins as incoming aminoacyl-tRNA recognizes the complementary codon calling for it at the A site on the ribosome. It hydrogen-bonds to the codon via its anticodon. Released mRNA Aminoacyl-tRNA synthetase Leu 3 As the ribosome moves along the mRNA, and each codon is read in sequence, a new amino acid is added to the growing protein chain and the tRNA in the A site is translocated to the P site. Ile tRNA “head” bearing anticodon G A A Pro 4 Once its amino acid is released from the P site, tRNA is ratcheted to the E site and then released to reenter the cytoplasmic pool, ready to be recharged with a new amino acid. The polypeptide is released when the stop codon is read. U A U P site Large ribosomal subunit E site A site G C G A U U U A C C G C Small ribosomal subunit Codon 15 Codon 16 Codon 17 Direction of ribosome advance Portion of mRNA already translated

  24. SECOND BASE U C A G U UUU UCU UAU UGU Tyr Cys Phe C UUC UCC UAC UGC U Ser A UUA UCA UAA Stop UGA Stop Leu G UUG UCG UAG Stop UGG Trp U CUU CCU CAU CGU His C CUC CCC CAC CGC C Leu Pro Arg A CUA CCA CAA CGA Gln G CUG CCG CAG CGG U AUU ACU AAU AGU Asn Ser C Ile AUC ACC AAC AGC A Thr A AUA ACA AAA AGA Lys Arg Met or G AUG ACG AAG AGG Start U GUU GCU GAU GGU Asp C GUC GCC GAC GGC G Val Ala Gly A GUA GCA GAA GGA Glu G GUG GCG GAG GGG

  25. Cell Life Cycle • The duplication of a cell’s genetic material is called DNA replication, and nuclear division is called mitosis (only in somatic cells) • Sex cells = meiosis • Stages: • Interphase – where cells spend most of their lives, an interval of time in which they perform normal functions • Four subphases: • G1 (gap 1)—vigorous growth and metabolism • G0—gap phase in cells that permanently cease dividing • S (synthetic)—DNA replication • G2 (gap 2)—preparation for division • Stage 1 – prophase – begins when the chromosomes become visible, each with two chromatids joined at a centromere.Centrosomes separate and migrate toward opposite poles. Mitotic spindles and asters form • Stage 2 – metaphase – chromatids move to a narrow central plate called the metaphase plate • Stage 3 – anaphase – shortest phase, chromatids separate and begin moving to the poles of the cell • Stage 4 – telophase – the cell prepares to return to interphase. Nuclear membrane forms and chromosomes uncoil • Cytokinesis – the cytoplasmic division that forms 2 daughter cells. Marks the end of cell division

  26. G1 checkpoint (restriction point) S Growth and DNA synthesis G2 Growth and final preparations for division G1 Growth M G2 checkpoint

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