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Learning Outline

Introduction to CellsPre-A&P

The generalized cell

Human cells are eukaryotic cells (that is, complex cells with a nucleus)
Plasma membrane
Cytoplasm – cell stuff slide_icon

  • Cytosol – cell solutions
  • Organelles – cell structures
    • Membranous and nonmembranous


Basic parts of a cells
The “main” parts of a typical cell.  slide_icon

Cellular membranes book-icon-lionden-fw
Membranes of the cell
Plasma membrane – outer boundary of cell
Organelle membranes form boundaries around and within many organelles
All these membranes are cell membranes

Fluid mosaic model

  • Phospholipid bilayer with imbedded proteins and hybrid molecules picture_icon
  • Cholesterol (among phospholipid tails) stabilizes membrane
  • Rafts
    • Linked groups of membrane molecules that travel together like a raft within the fluid bilayer picture_icon

Membrane functions
See table in textbook Chapter 3 tv_icon

Cell membrane functions are cell functions (that is, many functions of cells that we will discuss are in reality jobs performed by the membranes of cells)


A “typical” cell model.
Click here for a larger image.
Campare to another cell model.

Other cell structures book-icon-lionden-fw
cell structures
Most cell structures are called organelles
We’ll review only the main types of cell structures (there are MANY others, with more being discovered all the time)

Nucleus book-icon-lionden-fw
Nuclear envelope

  • Nuclear pores
    • Nuclear pore complex (NPC) is the specific structure at each opening in the nuclear envelope
  • Chromation
    • DNA plus protein
    • Chromosome = condensed chromatin
    • Primary genetic code of the cell
    • Click here tv_icon to see a slick view of DNA that will help you understand where it is and what it is
  • Nucleolus – forms ribosome parts (rRNA)


Nucleus and ER

1. nuclear envelope 5. chromatin (DNA) strands
2. ribosomes 6. nucleus
3. nuclear pores 7. Rough ER
4. nucleolus 8. nucleoplasm

Plural is mitochondria
Double membrane – inner membrane folded into cristae

  • Interior is called matrix

Involved in transfer of energy from fuel molecules to ATP
Called the cell’s “power plant” or “battery charger”
Serial Endosymbiosis Theory – SET (Lynn Margulis) slide_icon
liontrack Please also review the Mini Lesson: SET and Organelles

Click image to enlarge it

Assembled as subunits of rRNA/protein in nucleolus
Attach to mRNA strands (containing a gene) to guide assembly of amino acids into a polypeptide or protein
Amino acids are brought to the ribosome by tRNA

Selected examples of important nucleic acids
rRNA,ribosomal RNA Forms ribosomes
messenger RNA
Unfolded strand contains gene (code for one polypeptide)
transfer RNA
Brings specific amino acids to ribosome and places them according to code on mRNA
nuclear DNA “Master” genetic code in the nucleus
mDNA or mtDNA
mitochondrial DNA
Additional “master” genetic code in the mitochondrion

Endoplasmic reticulum (ER)
Network of membranous canals and sacs

  • ER extends outward from the outer boundary of the nucleus

Rough ER (RER) has temporarily-attached ribosomes

  • Receives and processes polypeptides/proteins dropped off by ribosomes
  • Also called granular ER

Smooth ER (SER) has no ribosomes

  • Also processes proteins and is site of enzyme action, including manufacture of membrane components (thus, it makes “new” membrane for the cell)
  • Transports calcium ions (Ca++) into ER sacs, removing it from the cytosol (discussed later in course)
  • Also called agranular or nongranular ER

Golgi apparatus
Also called Golgi body or Golgi complex (named for Camillo Golgi)
Also called dictyosome (this is the official name in human anatomy)
Stack of separate, flattened sacs

  • Sacs made of membrane are often called cisternae (sing. cisterna)

Processes, sorts, packages proteins sent by ER tv_icon picture_icon

Golgi apparatus
click image to enlarge and see labels

Vesicle literally means “little vessels”

  • Fluid-filled “bubbles” of membrane
  • There are MANY types of vesicles in a cell


  • Transport vesicles (such as ER or Golgi vesicles)
  • Secretory vesicles
  • Lysosomes contain lysing (digesting) enzymes
  • Peroxisomes (bud from the ER) have enzymes that process H2O2 as they digest fats and detoxify poisons

Formation of secretory vesicles
This diagram illustrates the process by which . . .
A. Proteins made by ribosomes enter the ER and are processed, then moved in ER vesicles to the Golgi apparatus.

B. Golgi vesicles then shuttle the chemicals from one sac (cisterna) to the next, as the Golgi apparatus further processes the chemicals.

C. Finished chemicals are packaged in vesicles that then may be secreted from the cell.
Some ER and Golgi vesicles instead remain in the cell and the chemicals inside perform intracellular functions.

1.Nuclear membrane 7.Transport vesicles 12.Secretory vesicle
2.Nuclear pore 8.Golgi apparatus 13.Cell membrane
3.Rough endoplasmic reticulum (RER) 9.Cis (inner) face of Golgi apparatus 14.Secretory vesicle fused to plasma membrane & releasing contents from cell
4.Smooth endoplasmic reticulum 10.Trans (outer) face of Golgi apparatus 15.Cell cytoplasm
5.Ribosome attached to REM 11.Cisternae of Golgi apparatus 16.Extracellular environment

Hollow, drumlike cylinder made up of protein subunits picture_icon
Found throughout cytoplasm
Breaks apart abnormal / misfolded proteins or proteins that are no longer needed

  • Small proteins called ubiquitins tag proteins for destruction by proteasomes
  • Breaks proteins into small segments, which are later broken apart to individual amino acids (which in turn are recycled)
  • Failure to dispose of misfolded proteins could result in buildup of harmful plaques

A protein (green) is shown moving from top to bottom through the hollow proteasome. Middle part is cut away to see where active enzymes cut the protein into small segments, which then move out ofthe bottom end.
(click image to enlarge)

Made up of picture_icon

  • Microfilaments
  • Intermediate filaments
  • Microtubules
  • Adsorption of water on proteins and cross-linking of proteins gives cytoplasm a gel consistency slide_icon

Functions include

  • Support
  • Movements of a cell
  • Movement within a cell
  • Forming/supporting a cell’s shape
  • Anchoring/forming connections with other cells (see below)


  • Also called microtubule-organizing center (MTOC) picture_icon
  • Guides formation and elongation of microtubules, as in mitosis Video of microtubule elongation
  • Includes two cylindrical centrioles

Molecular motors

  • Help to move materials or organelles within cell
  • Provide power to move the cytoskeleton (and thus move or change the shape of cells
  • Examples: dynein, myosin, kinesin

Cell extensions

  • Microvilli (sing. microvillus) picture_icon
    • Increase surface area for absorption
  • Cilia (sing. cilium) picture_icon
    • Groups of moving cilia move fluids along the surface of a sheet of cells picture_icon
    • Primary cilium
      • Most cells have one or more (blood cells do not)
      • Some have a sensory function (taste, smell, movement, etc)
      • Involved in replication of centrioles
  • Flagella (sing. flagellum)
    • Only the sperm cell has one
    • Molecular motors at base allow flagellem to move, thus enabling swimming of sperm picture_icon picture_icon

Extracellular matrix (ECM)
Material outside of cells picture_icon
The ECM is a complex arrangement of fibers and other molecules that interact with cells to perform body functions

See the discussion at the beginning of Chapter 5 of Anatomy & Physiology textbook

Cell connections
Cells must be held together in a multicellular organism, or the tissues would simply fall apart
In some tissues, cells are held together by fibrous “nets” that are not part of the cells themselves
In some tissues, cells form junctions with each other


  • Spot desmosomes: small patches of filaments from adjoining cells “tangle” together like Velcro patches, holding cells together (example: skin cells)


  • Belt desmosomes: connecting band (rather than small patch) encircling the cell and connecting it to nearby cells picture_icon

Tight junction slide_icon

  • Bands of protein units in adjoining cells “snap together” to form a tight seal all the way around one “end” of a cell, forming a sort of “collar” that sticks to the collars of nearby cells and thus forms a seal to prevent molecules from passing by a membrane made of cells held together by tight junctions (example: lining of intestines)


Tight junction
click to enlarge

Group junction picture_icon

  • Protein units form channels that link together to form “tunnels” that lead from one cell to the next
  • This arrangement not only joins cells structurally but also functionally, because molecules can move back and forth through gaps and the plasma membrane of each cell is now a continuous sheet—as if it’s now one giant cell (example: heart muscle cells)

Gap junction
click to enlarge

Cell Life Cycle
Life Cycles
All organisms have “life cycles” of development and reproduction—so do cells

  • Parent (mother) cell divides to produce two genetically identical daughter cells
    • Daughter cells may not be the same size or have exactly equal number of organelles

Cell cycle includes many phases of growth and development
Interphase (in-between phase)

  • Phase during which cell is not actively dividing
  • G1 phase (first growth [gap] phase) – new daughter cell is growing
  • S phase (synthesis phase) – cell prepares for eventual cell division by replicating nuclear DNA
    • So that there are now two identical sets (so that each daughter cell will receive one complete, identical set)
    • Each DNA molecule splits (unzips) and new, complementary nucleotides fill in the missing side
    • Each daughter DNA molecule (chromatid) is held together at the centromere
  • G2 phase (second growth [gap] phase) – cell continues to grow, often “stockpiling” extra cytoplasm for an eventual split
  • Centrioles (in the centrosome) are replicated during interphase


  • Coordinated division of the nuclear DNA and equal distribution to each daughter cell
    • Provides genetic integrity
    • Also called nuclear division or M phase
  • Phases of mitosis
    • Prophase (preliminary phase)
      • Nuclear envelope dissolves
      • Chromatin (DNA) strands shorten into compact chromosomes (each chromosome is made up of two chromatids held together by a centromere)
      • Each centriole moves toward an opposite pole of the cell, constructing a spindle of fibers (microtubules) across the cell
  • Metaphase (positioning phase) – chromosomes align at equator of cell and attach to spindle fibers
  • Anaphase (separation phase) – spindle fibers pull toward poles, separating the chromatids to form individual chromosomes moving away from their sisters
  • Telophase (end phase) – everything goes back to the way it “should be”—the chromosomes unwind to chromatin, the nuclear envelope reforms, the spindle is dismantled
  • Cytokinesis – the “pinching in” of the membrane and eventual separation of the two daughter cells; not as precise as nuclear division (mitosis); overlaps the end of mitosis


Daughter cells are now separate and in G1 of interphase

Chromosome numbers

Diploid number = 46 (for humans)

Haploid number = 23 (1/2 of diploid number)

Daughter cells should always have the diploid number of chromosomes—except egg cells and sperm cells, which should have the haploid number.

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Last updated: March 27, 2019 at 20:46 pm