cells

• Cells are the basic, living structural and functional units of the body
• Cell biology is the study of cellular structure and function
• Principal Parts of the Cell:

  • Plasma membrane:
  • Flexible, selective barrier between cell and its environment
  • Key role in communication among cells
  • Cytoplasm:
  • Cytosol fluid between plasma membrane and nucleus, surrounding several types of organelles
  • Nucleus:
  • Large organelle housing cell’s chromosomes (DNA molecule associated with several proteins)
• Plasma membrane:

  • Fluid mosaic model
  • Fluid lipids containing mosaic of different proteins
  • Lipid bilayer:
  • Phospholipids
  • Cholesterol
  • Glycolipids
  • Cholesterol:
  • Steroid lipids with polar and nonpolar regions
  • Scattered throughout both lipid layers
  • Glycolipid:
  • Lipid with attached polar carbohydrate group
  • Only on extracellular side of cell membrane
• Membrane Protein Arrangement:

  • Integral protein:
  • Firmly embedded in lipid bilayer
  • Many transmembrane proteins extend into both cytosol and extracellular fluid
  • Some glycoproteins with carbohydrate group extending into extracellular fluid
  • Peripheral protein:
  • Attached to polar heads of phospholipid
  • Attached to integral proteins on inner or outer surface of membrane
• Membrane Protein Functions:

  • Ion channels
  • Carriers
  • Receptors
  • Enzymes
  • Linkers
  • Cell-identity markers
• Membrane Characteristics:

  • Membrane fluidity
  • Selective membrane permeability
  • Gradients across plasma membrane
• Transport Across Plasma Membrane:

  • Critical to life of cell
  • Passive process
  • Active process
• Diffusion:

  • Passive transport process with kinetic energy
  • Factors affecting rate
• Diffusion Through a Membrane:

  • Simple diffusion
  • Channel-mediated facilitated diffusion
  • Carrier-mediated facilitated diffusion
• Diffusion of Water Through a Membrane - Osmosis:

  • Movement of water through a selectively permeable membrane
  • Hydrostatic pressure
  • Osmotic pressure
  • Tonicity
• Active Transport Through a Membrane:

  • Primary active transport
  • Secondary active transport
  • Transport in vesicles
• Endocytosis:

  • Receptor-mediated endocytosis
  • Phagocytosis
  • Bulk-phase endocytosis
• Exocytosis and Transcytosis:

  • Exocytosis
  • Transcytosis
• Cytoplasm:

  • Cytosol
  • Cytoskeleton
  • Organelles
• Cytoskeleton:

  • Microfilaments
  • Intermediate fibers
  • Microtubules
• Organelle: Centrosome:

  • Two components located near the nucleus
  • Pair of centrioles
• Centrosome:

  • Nucleus
  • Pair of centrioles
  • Cylindrical arrangement of nine clusters of three microtubules (triplets)
  • Long axis of one is at right angles to the other in pair
  • Pericentriolar material
  • Tubulin protein around the two centrioles
  • Organizing center for microtubule formation and growth of mitotic spindle for cell division
• Cilia and Flagella:

  • Motile extensions of cell surface that contain microtubules surrounded by plasma membrane
  • Cylindrical arrangement of nine clusters of two microtubules (doublets) around central pair
  • Anchored to basal body just below plasma membrane
  • Cilia:
  • Numerous, short, hair-like
  • Oar-like movement of extracellular liquid
  • Flagella:
  • Single, long, tail-like
  • Only example in human propels sperm
• Ribosomes:

  • Sites of protein synthesis
  • Proteins for export from cell by exocytosis when attached to endoplasmic reticulum
  • Proteins for use inside cell when unattached in cytosol
  • Two structural sub-units
  • Contain ribosomal RNA
  • Large and small sub-units synthesized in nucleolus of nucleus
  • Combine in the cytoplasm to function
• Endoplasmic Reticulum:

  • Membranous network throughout the cytoplasm with connections to the nuclear envelope
  • Rough ER:
  • Continuous with nuclear envelope and studded with ribosomes, thus “rough” appearance
  • Synthesized proteins enter spaces of the ER for processing and sorting for insertion into other organelles or exocytosis (secretion)
  • Smooth ER:
  • Extends from rough ER lacking ribosomes
  • Synthesizes fatty acids and steroids, detoxifies drugs in liver cells, and stores calcium for contraction in muscle cells
• Golgi Complex:

  • Stack of cisternae - flattened membranous sacs
  • Located between rough ER and plasma membrane
  • Enzymes in cisternae modify, sort and package proteins in vesicles from ER
  • Processed proteins leave Golgi complex:
  • Secretory vesicles for exocytosis
  • Membrane vesicles to plasma membrane
  • Transport vesicles to other cell organelles
• Vesicular Organelles:

  • Membrane-enclosed vesicles in cytosol
  • Lysosomes:
  • Contain digestive enzymes in acidic pH
  • Break down worn-out organelles or cells
  • Can also destroy entire cell by autolysis or extracellular digestion
  • Peroxisomes:
  • Contain oxidases for metabolism of fatty acids, amino acids, and toxic substances
  • Proteasomes:
  • Contain proteases to degrade proteins to amino acids
• Mitochondria:

  • Double membrane with fluid-filled spaces
  • Outer membrane smooth
  • Inner membrane folded in cisternae increasing surface area for aerobic cellular respiration
  • Fluid inside cisternae is matrix with enzymes
  • Produce most of cell’s ATP during aerobic phase of cellular respiration
  • Have own DNA with genes for some protein synthesis and can self-replicate at times of increased energy demand or before cell division
• Nucleus:

  • Cell’s control center and houses hereditary units called genes
  • Spherical or oval structure, usually most prominent internal feature of a cell
  • Nuclear envelope - double membrane separates nucleus from cytoplasm
  • Nuclear pores - proteins form opening through envelope, controlling substance movement in and out of nucleus
  • Nucleolus - cluster of protein, DNA, and RNA, for synthesis of ribosomal sub-units
  • Chromosomes - contain genetic information
• Chromosomes:

  • Long DNA molecules coiled with proteins
  • Nucleosomes - unit of DNA wrapped around histone proteins
  • Linker DNA - holds adjacent nucleosomes together
  • Chromatin - complex of folded DNA and proteins in non-dividing cell
  • Pair of chromatids forms a chromosome:
  • Just before cell division, DNA replicates (duplicates)
  • DNA and protein condenses into linear structure
  • Pair of chromatids held together by a centromere
  • Gene is segment on chromosome
• Genetic Expression:

  • A gene’s DNA is the template for synthesis of specific proteins
  • Base triplet - three-nucleotide sequence of genetic information in DNA
  • Codon - complementary three-nucleotide sequence in RNA which “codes for” a particular amino acid
  • Two-step process of protein synthesis:
  • Transcription: DNA transcribed into RNA
  • Translation: RNA translated into amino acid
• Cell Division:

  • Somatic - body cells other than gametes
  • Mitosis - equal division of nuclear material
  • Cytokinesis - division of cytoplasm and other organelles
  • Cell cycle - sequence of events to produce two genetically identical body cells for growth and repair
  • Reproductive - form egg or sperm
  • Meiosis - special two-step division
  • Number of chromosomes in nucleus is reduced by half
• Cell Cycle:

  • Interphase - metabolically active cell
  • G1 phase: cell replicates organelles and centrosomes
  • S phase: DNA replicates - each molecule composed of an old and a newly synthesized strand in identical sequence of nucleotide base pairs
  • G2 phase: growth continues in preparation for mitosis
  • DNA replication is completed before mitosis
• Mitosis:

  • Prophase:
  • Chromatin fibers of already replicated DNA condense into paired chromatids; nucleolus and nuclear envelope disappear; each centrosome moves to opposite pole of cell
  • Metaphase:
  • Centromeres of chromatid pairs line up at metaphase plate
  • Anaphase:
  • Centromeres split; identical sets of chromosomes move to opposite poles of cell pulled by spindle microtubules
  • Telophase:
  • Nuclear envelope and nucleoli reappear; chromosomes resume chromatin form; mitotic spindle disappears
• Cell Cycle and Cancer:

  • One way in which cell division is prevented is that cells in which normal DNA repair after replication has not occurred are induced to die (apoptosis) during G2 phase.
  • In normal cells, somatic division is controlled or suppressed by tumour-suppressor genes:
  • The protein p53 arrests cells in the G1 phase preventing division.
  • If one or more of these are altered and malfunction it leads to tumour formation (masses of cells formed by rapid division) by cells still containing errors in their DNA
• Meiosis:

  • Two consecutive nuclear divisions resulting in 4 daughter cells genetically unlike each other and unlike starting cell that produced them
  • Haploid; each daughter cell contains a single set of 23 chromosomes
  • Crossing-over: exchange of parts of chromatids between chromosomes of a pair
  • Sexual reproduction produces an organism from union of two gametes produced by meiosis, one from each parent
• Practice Questions:

  1. Name and describe the three main parts of a cell.

  2. Describe the structure and functions of the plasma membrane.

  3. Describe the processes that transport substances across the plasma membrane.

  4. Describe the structure and functions of cytoplasm, cytosol, and organelles.

  5. Describe the structure and functions of the nucleus.

  6. Outline the sequence of events involved in protein synthesis.

  7. Discuss the stages, events, and significance of somatic cell division.

  8. Describe how cells differ in size and shape.

  9. Describe the cellular changes that occur with aging.
• 10. Describe the disorders that affect cells.