Chap 3: Cells

• Cells Overview:

  • All cells of an individual originate from a single fertilized cell
  • Cell division and specialization give rise to different cell types like nerve, muscle, bone, and blood cells
  • Homeostasis is maintained to keep cells functioning normally
• Cell Function:

  1. Cell metabolism and energy use involve all chemical reactions, usually energy transfer and breaking down.

  2. Synthesis of molecules such as proteins, lipids, and nucleic acids.

  3. Communication between cells through chemical and electrical signals, like nerve cells to muscles for contraction.

  4. Reproduction and inheritance, including gametes and the transmission of genes
• Common Characteristics of Cells:

  • Plasma Membrane/cell membrane
  • Nucleus
  • Cytoplasm
  • Organelles, which are small organs performing specific functions
• Plasma Membrane:

  • Composed of a phospholipid bilayer with proteins attached
  • Acts as the outer boundary separating cytoplasmic and extracellular substances
  • Controls the entry and exit of substances
  • Contains receptor proteins for intercellular communication and marker molecules for cell identity
• Nucleus:

  • Enclosed by a nuclear envelope with nuclear pores
  • Contains chromatin, nucleoli, and ribosomal RNA (rRNA) and proteins
  • Acts as the control center housing genetic information for regulating protein synthesis and cell chemical reactions
• Cytoplasm:

  • Gel-like substance where organelles are located
• Cytoplasmic Organelles:

  • Ribosome: Site for protein synthesis
  • Rough Endoplasmic Reticulum: Synthesizes proteins usually transported to the Golgi
  • Smooth Endoplasmic Reticulum: Synthesizes lipids and detoxifies harmful chemicals
  • Golgi Apparatus: Modifies, packages, and secretes proteins and lipids
  • Lysosome: Contains digestive enzymes for breaking down materials
  • Peroxisome: Breaks down hydrogen peroxide and lipids
  • Proteasomes: Break down and recycle other proteins
  • Mitochondria: Sites of ATP synthesis
  • Centrioles: Involved in microtubule formation and cell division
• Extensions of Plasma Membrane:

  • Cilia: Movement requires ATP and is usually found in the respiratory and female reproductive tracts
  • Flagellum: Longer than cilia and used for locomotion
  • Microvilli: Increase surface area for absorption of nutrients
• How do we see cells? - Microscopes:

  • Light Microscope: Uses light for viewing
  • Electron Microscope: Uses a beam of electrons for higher magnification
  • Atomic Force Microscope: Scans samples using a tiny mechanical probe
• Plasma Membrane Functions:

  • Acts as a boundary separating intracellular and extracellular substances
  • Encloses and supports cell contents
  • Attaches cells to the external environment or other cells
  • Facilitates cell recognition and communication
  • Determines what moves in and out of the cell
• The fluid nature of the lipid bilayer in the plasma membrane:

  • Distributes molecules within the membrane
  • Allows for easy repair of damage
  • Enables membranes to fuse with each other
• Cholesterols present in the plasma membrane:

  • Influence the fluid nature of the membrane
  • Limit movements of phospholipids
  • Provide stability to the membrane
• Membrane proteins:

  • Selectively permeable, allowing only certain substances to enter
  • Function is determined by chemical characteristics and 3D shape
  • Classified into integral and peripheral membrane proteins
• Marker molecules:

  • Allow cells to identify one another or other molecules
  • Mostly glycolipids and glycoproteins
  • Examples include sperm cells recognizing oocytes and immune cells recognizing foreign cells
• Attachment proteins:

  • Allow cells to attach to other cells or extracellular molecules
  • Cadherins facilitate cell-to-cell attachment
  • Integrins facilitate cell-to-extracellular molecule attachment
• Transport proteins:

  • Move ions or molecules from one side to the other
  • Exhibit specificity, competition, and saturation
  • Include channel proteins and carrier proteins
• Channel proteins:

  • Form passageways through the plasma membrane for specific ions or molecules
  • Include leak ion channels and gated ion channels
  • Types of gated ion channels include ligand-gated and voltage-gated channels
• Carrier proteins (transporters):

  • Move ions or molecules across the plasma membrane
  • Change shape upon binding chemicals to allow entry
  • Classified based on the movement of ions/molecules into uniport, symport, and antiport
• ATP-powered pumps:

  • Require energy (ATP) to function
  • Transport substances against their concentration gradient
• Receptor proteins:

  • Have binding sites for chemical signals in the extracellular fluid
  • Trigger cellular responses upon binding
  • Linked to channel proteins or coupled to G protein complexes
• Enzymes:

  • Catalyze chemical reactions inside or outside the cell
  • Some are always active, while others are activated by membrane-bound receptors or G protein complexes
• Transport mechanisms:

  • Lipid-soluble molecules pass through the plasma membrane by dissolving in the lipid bilayer
  • Large non-lipid soluble molecules are transported by transport proteins or vesicles
• Passive transport mechanisms:

  • No use of ATP, movement from higher to lower concentration
  • Include diffusion, osmosis, and facilitated diffusion
• Diffusion:

  • Net movement from areas of higher to lower concentration
  • Rate influenced by concentration gradient and viscosity
  • Examples include oxygen, CO2, and lipids
• Osmosis:

  • Water diffusion across a selectively permeable membrane
  • Involves aquaporins and osmotic pressure
  • Examples include water movement in the intestines
• Facilitated diffusion:

  • Carrier proteins move substances without ATP
  • Specificity, saturation, and competition are involved
  • Mediates the transport of large, water-soluble, or electrically charged ions
• Active transport mechanisms:

  • Require ATP, transport from low to high concentration
  • Include active transport and secondary active transport
• Active transport:

  • ATP-powered pumps move substances across the plasma membrane
  • Transport substances against their concentration gradient
• Secondary active transport:

  • Indirectly uses energy from active transport mechanisms
  • Utilizes ion concentration gradients created by active transport
• Vesicular transport:

  • Requires energy (ATP) for bulk transport
  • Includes endocytosis and exocytosis for large-scale transport
• Endocytosis:

  • Process of taking substances into the cell
  • Includes receptor-mediated endocytosis and phagocytosis
• Endocytosis is the process of substances entering the cell, with receptor-mediated endocytosis allowing bacteria to be phagocytized but not healthy cells
• Phagocytosis involves taking in cells and solid particles (cell-eating), while pinocytosis takes in molecules dissolved in liquid (cell-drinking)
• Immune system cells ingest bacteria and cellular debris
• Exocytosis is the process of substances exiting the cell through secretory vesicles that fuse with the plasma membrane
• Transcytosis is a combination of endocytosis and exocytosis, where substances enter on one side of the cell and exit on the opposite side
• Proteins and other water-soluble molecules are transported out of cells through exocytosis
• The cytoplasm is composed of half cytosol and half organelles
• Cytosol is the fluid portion of the cytoplasm, containing dissolved ions, molecules, and suspended molecules and proteins
• The cytoskeleton supports the cell, holds the nucleus and other organelles, and is responsible for changes in cell shape and movement of organelles