Learning Outline

Introduction to Cells

BIO 095

This module reviews what you hopefully already learned about cells in your previous courses.

There may be a few new concepts that you missed before . . . so be careful in your review.

 

The generalized cell

Human cells are eukaryotic cells (that is, complex cells with a nucleus)

Plasma membrane

Cytoplasm - cell stuff

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

Nucleus

The "main" parts of a typical cell.

 

Cellular membranes

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
  
  
• Cholesterol (among phospholipid tails) stabilizes membrane
  
  
• Rafts
  
  
  • Linked groups of membrane molecules that travel together like a raft within the fluid bilayer
    

Membrane functions

See table in textbook Chapter 3

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

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

Nuclear envelope

• Nuclear pores
  
  
  • Nuclear pore complex (NPC) is the specific structure at each opening in the nuclear envelope
    

Nucleoplasm

• Chromatin
  
  
  • DNA plus protein
    
    
  • Chromosome = condensed chromatin
    
    
  • Primary genetic code of the cell
    
    
  • Click here 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 2. ribosomes 3. nuclear pores 4. nucleolus

5. chromatin (DNA) strands 6. nucleus 7. Rough ER 8. nucleoplasm

Mitochondrion

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)

Please also review the Mini Lesson: SET and Organelles

Ribosome

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
mRNA messenger RNA	Unfolded strand contains gene (code for one polypeptide)
tRNA 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

Golgi apparatus
click image to enlarge and see labels

Vesicles

Vesicle literally means "little vessels"

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

Examples

• 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.
Nuclear membrane Nuclear pore Rough endoplasmic reticulum (RER) Smooth endoplasmic reticulum Ribosome attached to REM Macromolecules	Transport vesicles Golgi apparatus Cis (inner) face of Golgi apparatus Trans (outer) face of Golgi apparatus Cisternae of Golgi apparatus	Secretory vesicle Cell membrane Secretory vesicle fused to plasma membrane & releasing contents from cell Cell cytoplasm Extracellular environment

Proteasome

Hollow, drumlike cylinder made up of protein subunits

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

Proteasome
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)

Cytoskeleton

Made up of microfilaments, intermediate filaments, microtubules

• Adsorption of water on proteins and cross-linking of proteins gives cytoplasm a gel consistency
  

Functions include support, movement of cell, movement within cell, cell shape, connections with other cells

Centrosome = microtubule-organizing center (MTOC)

• 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)
  
  
  • Increase surface area for absorption
    
    
• Cilia (sing. cilium)
  
  
  • Groups of moving cilia move fluids along the surface of a sheet of cells
    
    
  • 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
    

Extracellular matrix (ECM)

Material outside of cells

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

Desmosome

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

Spot desmosome.
click to enlarge

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

Tight junction

• 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

Gap junction

• 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
    

Phases

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
  

Mitosis

• 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

Prophase

• Metaphase (positioning phase) - chromosomes align at equator of cell and attach to spindle fibers

Metaphase

• Anaphase (separation phase) - spindle fibers pull toward poles, separating the chromatids to form individual chromosomes moving away from their sisters

Anaphase

• 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

Telophase

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

Cytoknesis

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.