Anatomy and Physiology

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UNIT 1 Chapter 5:

Cell structure

FUNCTIONAL ANATOMY OF CELLS

  • The typical cell (Figure 3-1)
  • Also called composite cell
  • Vary in size; all are microscopic (Table 3-1)
  • Vary in structure and function (Table 3-2)

FUNCTIONAL ANATOMY OF CELLS (cont.)

  • Cell structures
  • Plasma membrane: separates the cell from its surrounding environment
  • Cytoplasm: thick, gel-like substance inside the cell composed of numerous organelles suspended in watery cytosol; each type of organelle perform particular functions (Figure 3-2)
  • Nucleus: large membranous structure near the center of the cell

CELL MEMBRANES

  • Each cell contains a variety of membranes
  • Plasma membrane: outer boundary of cell (Fig 3-3)
  • Membranous organelles: sacs and canals made of the same material as the plasma membrane.

CELL MEMBRANES (cont.)

  • Groupings of membrane molecules form rafts that float as a unit in the membrane (Figure 3-4)
  • Rafts may pinch inward to bring material into the cell or organelle.

  • Primary structure of a cell membrane is a double layer of phospholipids molecules.

  • Heads are hydrophilic (“water loving”)

  • Tails are hydrophobic (“water fearing”)

  • Most of the bilayer is hydrophobic; therefore water and water-soluble molecules do not pass through easily

CELL MEMBRANES (cont.)

  • Membrane proteins (Table 3-4)
  • A cell controls what moves through the membrane by membrane proteins embedded in the phospholipid bilayer
  • Some membrane proteins have carbohydrates attached to them and, as a result, form glycoproteins that act as identification markers
  • Some membrane proteins are receptors that react to specific chemicals.

CYTOPLASM AND ORGANELLES

  • Cytoplasm: gel-like internal substance of cells that includes many organelles suspended in watery intracellular fluid called cytosol
  • Two major groups of organelles (Table 3-3)
  • Membranous organelles are sacs or canals made of cell membranes
  • Nonmembranous organelles are made of microscopic filaments or other nonmembranous materials

CYTOPLASM AND ORGANELLES (cont.)

  • Endoplasmic reticulum (Figure 3-5)
  • Made of membranous, walled canals and flat, curving sacs arranged in parallel rows throughout the cytoplasm; extend from the plasma membrane to the nucleus.
  • Proteins move through the canals.

CYTOPLASM AND ORGANELLES (cont.)

  • Two types of endoplasmic reticulum
  • Rough endoplasmic reticulum
  • Ribosomes dot the outer surface of the membranous walls
  • Ribosomes synthesize proteins, which move toward the Golgi apparatus and then eventually leave the cell
  • Function in protein synthesis and intracellular transportation
  • Smooth endoplasmic reticulum
  • No ribosomes border the membranous wall
  • Functions are less well established and probably more varied than those of the rough endoplasmic reticulum
  • Synthesizes certain lipids and carbohydrates .
  • Removes and stores calcium ions from the cell’s interior

CYTOPLASM AND ORGANELLES (cont.)

  • Ribosomes (Figure 3-6)
  • Many are attached to the rough endoplasmic reticulum and many lie free.

  • Each ribosome is a nonmembranous structure made of two pieces, a large subunit and a small subunit; each subunit is composed of rRNA and protein

  • Ribosomes in the endoplasmic reticulum make proteins for “export,” or to be embedded in the plasma membrane; free ribosomes make proteins for the cell’s domestic use.

CYTOPLASM AND ORGANELLES (cont.)

  • Golgi apparatus
  • Membranous organelle consisting of cisternae stacked on one another and located near the nucleus (Figure 3-7)
  • Processes protein molecules from the endoplasmic reticulum (Figure 3-8)
  • Processed proteins leave the final cisterna in a vesicle; contents may then be secreted to outside the cell

CYTOPLASM AND ORGANELLES (cont.)

  • Lysosomes (Figure 3-9)
  • Made of microscopic membranous sacs .

  • The cell’s own digestive system; enzymes in lysosomes digest the protein structures of defective cell parts, including plasma membrane proteins, and particles that have become trapped in the cell

CYTOPLASM AND ORGANELLES (cont.)

  • Proteasomes (Figure 3-10)
  • Hollow protein cylinders found throughout the cytoplasm
  • Break down abnormal or misfolded proteins and normal proteins no longer needed by the cell (and that may cause disease)

CYTOPLASM AND ORGANELLES (cont.)

  • Peroxisomes
  • Small membranous sacs containing enzymes that detoxify harmful substances that enter the cells.
  • Often seen in kidney and liver cells.

  • Mitochondria (Figure 3-11)
  • Composed of microscopic sacs; wall composed of inner and outer membranes separated by fluid.

  • The “power plants” of cells; mitochondrial enzymes catalyze series of reactions that provide most of a cell’s energy supply

  • Each mitochondrion has a DNA molecule, which allows it to produce its own enzymes and replicate copies of itself

CYTOPLASM AND ORGANELLES (cont.)

NUCLEUS

  • Definition: spherical body in center of cell enclosed by an envelope with many pores.
  • Structure (Figure 3-12)
  • Consists of a nuclear envelope (made of two membranes, each with essentially the same molecular structure as the plasma membrane) surrounding nucleoplasm
  • The nuclear envelope has holes called nuclear pores

NUCLEUS (cont.)

  • Structure (cont.)
  • Contains DNA (heredity molecules), which appear as:
  • Chromatin threads or granules in nondividing cells
  • Chromosomes in early stages of cell division
  • Functions of the nucleus are functions of DNA molecules; DNA determines the structure and function of cells as well as heredity


CYTOSKELETON

  • The cell’s internal supporting framework; made of rigid, rodlike pieces that provide support and allow movement and mechanisms that can move the cell or its parts (Figure 3-14)

CYTOSKELETON (cont.)

  • Cell fibers

  • Fibers appear to support the endoplasmic reticulum, mitochondria, and “free” ribosomes

  • Microfilaments: smallest cell fibers (Figure 3-15)
  • Made of thin, twisted strands of protein molecules that lie parallel to the long axis of the cell

CYTOSKELETON (cont.)

  • Intermediate filaments: twisted protein strands slightly thicker than microfilaments; form much of the supporting framework in many types of cells.
  • Microtubules: tiny, hollow tubes that are the thickest of the cell fibers.
  • Made of protein subunits arranged in a spiral fashion.
  • Function to move things around inside the cell.

CYTOSKELETON (cont.)

  • Centrosome (Figure 3-16)
  • An area of the cytoplasm near the nucleus that coordinates the building and breaking apart of microtubules in the cell.
  • Nonmembranous structure also called the microtubule organizing center.
  • Plays an important role during cell division.
  • General location of the centrosome is identified by the centrioles.

CYTOSKELETON (cont.)

  • Molecular motors
  • Motor proteins include dynein, myosin, and kinesin (Figure 3-17)
  • Molecular motors can pull larger structures along microtubules and microfilaments providing intracellular transport and movements of the entire cell.

CYTOSKELETON (cont.)

  • Cell extensions
  • Cytoskeleton forms projections that extend the plasma membrane outward to form tiny, fingerlike processes
  • Three types of these processes; each has specific functions (Figure 3-18)

CYTOSKELETON (cont.)

  • Microvilli

  • Found in epithelial cells that line the intestines and other areas where absorption is important.

  • Cilia and flagella:

cell processes that have cylinders made of microtubules and molecular motors (Figure 3-19)

  • Cilia are shorter and more numerous than flagella; cilia have coordinated oarlike movements that brush material past the cell’s surface
  • Flagella are found only on human sperm cells; flagella move with a tail-like movement that propels the sperm cell forward

CELL CONNECTIONS

  • Cells are held together by fibrous nets that surround groups of cells (e.g., muscle cells), or cells have direct connections to each other
  • Three types of direct cell connections (Figure 3-20)

CELL CONNECTIONS: DIRECT

  • Desmosome
  • Fibers on the outer surface of each desmosome interlock with each other; anchored internally by intermediate filaments of the cytoskeleton.
  • Spot desmosomes are like “spot welds” at various points connecting adjacent membranes.
  • Belt desmosomes encircle the entire cell .
  • Gap junctions: membrane channels of adjacent plasma membranes adhere to each other; have two effects.
  • Form gaps or “tunnels” that join the cytoplasm of two cells
  • Fuse two plasma membranes into a single structure.
  • Tight junctions
  • Occur in cells that are joined by “collars” of tightly fused material.
  • Molecules cannot permeate the cracks of tight junctions
  • Occur in the lining of the intestines and other parts of the body where controlling what gets through a sheet of cells is important.