CELLS

• Cells are the structural units of all living things
• The human body has 50 to 100 trillion cells
• Cell Theory:

  • A cell is the basic structural and functional unit of living organisms
  • The activity of an organism depends on the collective activities of its cells
  • Biochemical activities of cells are dictated by their structure (anatomy) which determines their function (physiology)
  • Continuity of life has a cellular basis
  • Most cells are composed of four elements: Carbon, Hydrogen, Oxygen, Nitrogen
  • Cells are about 60% water
• Anatomy of a generalized cell:

  • Three main regions: Nucleus, Cytoplasm, Plasma membrane
• The Nucleus:

  • Control center of the cell
  • Contains genetic material known as DNA needed for building proteins and cell reproduction
  • Three regions: Nuclear envelope (membrane), Nucleolus, Chromatin
• The Plasma Membrane:

  • Transparent barrier for cell contents
  • Fluid mosaic model constructed of phospholipids, cholesterol, proteins, and sugar groups
  • Role of proteins: enzymes, receptors, transport channels or carriers
  • Role of sugars: glycoproteins, glycocalyx
  • Main types of cell junctions: Tight junctions, Desmosomes, Gap junctions
• The Cytoplasm:

  • Cellular material outside the nucleus and inside the plasma membrane
  • Site of most cellular activities
  • Includes cytosol, inclusions, and organelles like mitochondria, ribosomes, endoplasmic reticulum, Golgi apparatus, lysosomes, peroxisomes, centrioles
• Cell Extensions:

  • Cilia move materials across the cell surface
  • Flagella propel the cell
  • Microvilli increase surface area for absorption
• Cell Diversity:

  • The human body houses over 200 different cell types
  • Cells vary in size, shape, and function
  • Examples of different cell types and their functions
• Cell physiology includes the ability of cells to metabolize, digest food, dispose of wastes, reproduce, grow, move, respond to a stimulus
• Membrane transport involves passive processes like diffusion and filtration, where substances are transported across the membrane without any input from the cell
• Diffusion is a passive process where molecule movement is from high concentration to low concentration, down a concentration gradient
• Osmosis is the simple diffusion of water across a selectively permeable membrane, with isotonic, hypertonic, and hypotonic solutions affecting cell size
• Facilitated diffusion transports lipid-insoluble and large substances, like glucose, via protein membrane channels or carriers
• Endocytosis involves the cell engulfing large particles in phagocytosis, "gulping" droplets of extracellular fluid in pinocytosis, or taking up specific target molecules in receptor-mediated endocytosis
• Cell division involves the cell life cycle with two major periods: interphase (metabolic phase) and cell division, which includes preparations like DNA replication and events like mitosis and cytokinesis
• The nucleus is the control center of the cell, containing genetic material known as DNA, which is essential for building proteins and cell reproduction
• The nucleus has three regions:

  • Nuclear envelope (membrane): consists of a double membrane that bounds the nucleus, contains nuclear pores for material exchange, and encloses the nucleoplasm
  • Nucleolus: contains one or more dark-staining nucleoli, sites of ribosome assembly, where ribosomes migrate into the cytoplasm through nuclear pores for protein synthesis
  • Chromatin: composed of DNA wound around histones, scattered throughout the nucleus when the cell is not dividing, condenses into chromosomes during cell division
• Mitochondria ○ “Powerhouses” of the cell ○ Mitochondrial wall consists of a double membrane with cristae on the inner membrane ○ Carry out reactions in which oxygen is used to break down food into ATP molecules


  Ribosomes ○ Made of protein and ribosomal RNA ○ Sites of protein synthesis in the cell ○ Found at two locations: ■ Free in the cytoplasm ■ As part of the rough endoplasmic reticulum
• Endoplasmic reticulum (ER) ○ Fluid-filled tunnels (or canals) that carry substances within the cell ○ Continuous with the nuclear membrane ○ Two types: ■ Rough ER ● Rough endoplasmic reticulum ● Studded with ribosomes ● Synthesizes proteins ● Transport vesicles move proteins within cell ● Abundant in cells that make and export proteins ■ Smooth ER ● Smooth endoplasmic reticulum ● Lacks ribosomes ● Functions in lipid metabolism ● Detoxification of drugs and pesticides
• Golgi apparatus ○ Appears as a stack of flattened membranes associated with tiny vesicles ○ Modifies and packages proteins arriving from the rough ER via transport vesicles ○ Produces different types of packages ○ Secretory vesicles (pathway 1) ○ In-house proteins and lipids (pathway 2) ○ Lysosomes (pathway 3)
• Centrioles ○ Rod-shaped bodies made of nine triplets of microtubules ○ Generate microtubules ○ Direct the formation of mitotic spindle during cell division
• Peroxisomes ○ Membranous sacs of oxidase enzymes ○ Detoxify harmful substances such as alcohol and formaldehyde ○ Break down free radicals (highly reactive chemicals) ○ Free radicals are converted to hydrogen peroxide and then to water ○ Replicate by pinching in half or budding from the ER
• Lysosomes ○ Membranous “bags” that contain digestive enzymes ○ Enzymes can digest worn-out or nonusable cell structures ○ House phagocytes that dispose of bacteria and cell debris
• Cell Extensions ● Surface extensions found in some cells ● Cilia move materials across the cell surface ○ Located in the respiratory system to move mucus ● Flagella propel the cell ○ The only flagellated cell in the human body is sperm ● Microvilli are tiny, fingerlike extensions of the plasma membrane ○ Increase surface area for absorption
• Cells that connect body parts ● Fibroblast ○ Secretes cable-like fibers
• Cell that stores nutrients ○ Fat cells ○ Lipid droplets stored in cytoplasm
• Cells that move organs and body parts ○ Skeletal muscle and smooth muscle cells ■ Contractile filaments allow cells to shorten forcefully
• Cells that cover and line body organs ○ Epithelial cell ■ Packs together in sheets ■ Intermediate fibers resist tearing during rubbing or pulling
• Cells of reproduction ○ Oocyte (female) ■ Largest cell in the body ■ Divides to become an embryo upon fertilization ○ Sperm (male) ○ Built for swimming to the egg for fertilization ○ Flagellum acts as a motile whip
• Cell that gathers information and controls body functions ○ Nerve cell (neuron) ■ Receives and transmits messages to other body structures
• Cell that fights disease ○ White blood cells, such as the macrophage (a phagocytic cell) ■ Digests infectious microorganisms
• Vacuoles ○ Large vacuole present in plant cells ○ Contains water and dissolved nutrients ○ Helps maintain turgidity of plant cells ○ Small vacuoles found in animal cells ○ Store waste products and pigments