Cells and Tissues

• 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:

  1. Carbon (11%)

  2. Hydrogen (59%)

  3. Oxygen (24%)

  4. Nitrogen (2.5%)

  • CHON is equivalent to 96.5% of the organism’s weight
  • Cells are about 60% water
• Anatomy of a Generalized Cell:

  • Parts:

  1. Nucleus

  2. Cytoplasm

  3. Plasma Membrane
• The Nucleus:

  • Control center of the cell
  • Contains genetic material known as deoxyribonucleic acid, or DNA
  • Three regions:

  1. Nuclear envelope (membrane)

  2. Nucleolus

  3. Chromatin
• Nuclear Envelope (Membrane):

  • Consists of a double membrane that bounds the nucleus
  • Contains nuclear pores that allow for exchange of material with the rest of the cell
  • Encloses the jellylike fluid called the nucleoplasm
• Nucleolus:

  • Contains one or more dark-staining nucleoli
  • Sites of ribosome assembly
  • Ribosomes migrate into the cytoplasm through nuclear pores to serve as the site of protein synthesis
• Chromatin:

  • Composed of DNA wound around histones (proteins)
  • Scattered throughout the nucleus and present when the cell is not dividing
  • Condenses to form dense, rod-like bodies called chromosomes when the cell divides
• The Plasma Membrane:

  • Transparent barrier for cell contents
  • Contains cell contents
  • Separates cell contents from surrounding environment
  • Fluid mosaic model is constructed of:
  • Two layers of phospholipids arranged “tail to tail”
  • Cholesterol and proteins scattered among the phospholipids
  • Sugar groups may be attached to the phospholipids, forming glycolipids
• Phospholipid Arrangement in the Plasma Membrane:

  • Hydrophilic (“water loving”) polar “heads” are oriented on the inner and outer surfaces of the membrane
  • Hydrophobic (“water fearing”) nonpolar “tails” form the center (interior) of the membrane
  • This interior makes the plasma membrane relatively impermeable to most water-soluble molecules
• Role of Proteins:

  • Responsible for specialized membrane functions:
  • Enzymes
  • Receptors for hormones or other chemical messengers
  • Transport as channels or carriers
• Role of Sugars:

  Glycoproteins:

  • Branched sugars attached to proteins that abut the extracellular space

  Glycocalyx:

  • Fuzzy, sticky, sugar-rich area on the cell’s surface
• Cell Membrane Junctions:

  • Cells are bound together in three ways:

  1. Glycoproteins in the glycocalyx act as an adhesive or cellular glue

  2. Wavy contours of the membranes of adjacent cells fit together in a tongue-and-groove fashion

  3. Special cell membrane junctions are formed, which vary structurally depending on their roles
• • Molecules can travel directly from one cell to the next through these channels
• Main Types of Cell Junctions:

  1. Tight Junctions:

  • Impermeable junctions
  • Bind cells together into leakproof sheets
  • Plasma membranes fuse like a zipper to prevent substances from passing through extracellular space between cells

  2. Desmosomes:

  • Anchoring junctions, like rivets, that prevent cells from being pulled apart as a result of mechanical stress
  • Created by buttonlike thickenings of adjacent plasma membranes

  3. Gap Junctions (Communicating Junctions):

  • Allow communication between cells
  • Hollow cylinders of proteins (connexons) span the width of the abutting membranes
• The Cytoplasm:

  • The cellular material outside the nucleus and inside the plasma membrane
  • Site of most cellular activities
  • Includes cytosol, inclusions, and organelles
• Three Major Components of the Cytoplasm:

  Cytosol:

  • Fluid that suspends other elements and contains nutrients and electrolytes

  Inclusions:

  • Chemical substances, such as stored nutrients or cell products, that float in the cytosol

  Organelles:

  • Metabolic machinery of the cell that perform functions for the cell
  • Many are membrane-bound, allowing for compartmentalization of their functions
• Cell Extensions:

  • Surface extensions found in some cells

  Cillia:

  • Slender, microscopic, hair-like structures
  • Move materials across cell surface (e.g., mucus)
  • Location: Respiratory System

  Flagella:

  • Whip-like
  • Propel the cell
  • Location: Testes; sperm

  Microvilli:

  • Tiny, fingerlike extensions of the plasma membrane
  • Increase surface area for absorption
  • Location: Small intestinal cells
• Cell Diversity:

  • The human body houses over 200 different cell types
  • Cells vary in size, shape, and function
  • Cells vary in length from 1/12,000 of an inch to over 1 yard (nerve cells)
  • Cell shape reflects its specialized function
• Cells That Connect Body Parts:

  Fibroblasts:

  • Secretes cable-like fibers

  Erythrocytes (Red Blood Cell):

  • Carries oxygen in the bloodstream
• Cells That Cover and Line Body Organs:

  Epithelial Cell:

  • Packs together in sheets
  • Intermediate fibers resist tearing during rubbing or pulling
• Cells That Move Organs and Body Parts:

  Skeletal Muscle Cells:

  • Contractile filaments allow cells to shorten forcefully

  Smooth Muscle Cells
• Cells That Store Nutrients:

  Fat Cells:

  • Lipid droplets stored in the cytoplasm
• Cell That Fights Disease:

  White Blood Cells:

  • i.e., macrophage (phagocytic cell)
  • Digests infectious microorganisms
• Cell That Gathers Information and Controls Body Functions:

  Nerve Cell (neuron):

  • Receives and transmits messages to other body structures
• 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 Physiology:

  • Cells have the ability to:
  • Metabolize
  • Digest food
  • Dispose of wastes
  • Reproduce
  • Grow
  • Move
  • Respond to a stimulus
• Membrane Transport:

  Solution:

  • Homogeneous mixture of two or more components
  • Solvent: dissolving medium present in the larger quantity
  • Solutes: components in smaller quantities within a solution

  Intracellular Fluid:

  • Nucleoplasm and cytosol
  • Solution containing gases, nutrients, and salts dissolved in water

  Extracellular Fluid (Interstitial Fluid):

  • Fluid on the exterior of the cell
  • Contains thousands of ingredients, such as nutrients, hormones, neurotransmitters, salts, waste products
• The Plasma Membrane is a Selectively Permeable Barrier:

  • Some materials can pass through, while others are excluded
  • For example:
  • Nutrients can enter the cell
  • Undesirable substances are kept out
• Two Basic Methods of Transport:

  [1] Passive Processes:

  • Substances are transported across the membrane without any input from the cell

  [2] Active Processes:

  • The cell provides the metabolic energy (ATP) to drive the transport process
  • Active processes are used when:
  • Substances are too large to travel through membrane channels
  • The membrane may lack special protein carriers for the transport of certain substances
  • Substances may not be lipid-soluble
  • Substances may have to move against a concentration gradient
• Types of Passive Processes:

  1. Diffusion:

  • Molecule movement is from high concentration
• Passive processes:

  • Diffusion: movement of molecules from high to low concentration down a concentration gradient
  • Factors affecting diffusion: size of the molecule and temperature
  • Types of diffusion: Simple Diffusion, Osmosis, Facilitated Diffusion
  • Simple Diffusion: unassisted process for lipid-soluble or small solutes
  • Osmosis: diffusion of water across a selectively permeable membrane
  • Facilitated Diffusion: transports lipid-insoluble and large substances, like glucose
• Filtration:

  • Process where water and solutes are forced through a membrane by pressure
  • Critical for kidney function
  • Requires a pressure gradient to push solute-containing fluid from high to low pressure areas
• Active processes:

  • Active Transport and Vesicular Transport
  • Active processes use ATP to move substances across a membrane
  • Used when substances are too large, not lipid-soluble, or need to move against a concentration gradient
• Active Transport:

  • Involves protein carriers like solute pumps
  • ATP energizes the movement of substances against concentration gradients
  • Example: Sodium-Potassium Pump for nerve impulses
• Vesicular Transport:

  • Moves substances across the membrane without crossing it
  • Types: Exocytosis and Endocytosis
  • Exocytosis: actively secretes hormones, mucus, and other products
  • Endocytosis: engulfs extracellular substances in a vesicle for digestion
• Cell Division:

  • Cell Life Cycle consists of Interphase and Cell Division
  • Interphase: cell growth and metabolic processes
  • Cell Division: reproduces the cell through Mitosis and Cytokinesis
• Mitosis:

  • Division of the nucleus resulting in two daughter nuclei
  • Events: Prophase, Metaphase, Anaphase, Telophase
  • Chromatin coils into chromosomes, aligns in the center, splits, and reforms into chromatin
• Cytokinesis:

  • Division of the cytoplasm into two daughter cells
  • Cleavage furrow forms to pinch cells apart
  • Mitosis and cytokinesis usually occur together