The Cell Theory

Robert Hooke (1635-1703
) first described cells in 1665.

Modern cell theory includes four principles:

All living organisms are composed of cells.

Nothing smaller than a cell is considered to be alive.

Life evolved only once, and all living organisms have descended from the earliest cells.

Cells arise only from preexisting cells.

Why Aren't Cells Larger? Cell size  (Fig 5.3)

surface area-to-volume relationships  (Fig 5.2)


The Plasma Membrane

polar ends of phospholipids (Fig. 5.4)

nonpolar fatty acid chains

bilayer of phospholipids

within the membrane

transmembrane proteins

cell surface proteins. (Fig. 5.6)

Membrane defects. EX: Cystic fibrosis

Prokaryotic Cells (before nucleus) = "bacteria" (Fig 5.9 and Fig. 5.8)

Prokaryotic cells evolved 2 billion years before eukaryotic cells.

Have a rigid cell wall around their plasma membranes, and ribosomes, but no internal membrane-bounded organelles. Some have one or more flagella.

Eukaryotic Cells (true nucleus) = cells of animals (Fig. 5.10), plants (Fig.5.11).

Cell Organelles

Eukaryotic cells are thought to have evolved as a result of two processes: infolding of the membrane system (Fig. 20.1) and  endosymbiosis (Fig 5.18) of several types of prokaryotic cells (Fig. 20.2). A more detailed image here and here. And a movie!

The Nucleus: The Cell's Control Center (Fig 5.12)

Nuclear Membrane


lipid bilayer forms the nuclear envelope


nuclear pores





The Endomembrane System - (Fig. 5.15) a tightly packed mass of membranes that divides the cell into compartments, channels the transport of materials, and provides surfaces on which enzymes act.

Endoplasmic Reticulum: The Transportation System - (Fig. 5.13)


Rough ER has ribosomes associated with it (makes proteins), smooth ER does not.


The Golgi Apparatus: The Delivery System - (Fig. 5.14)


- movie

Organelles that contain their own DNA, giving evidence for their ancient past as solitary bacteria.

Mitochondria: Powerhouses of the Cell  (Fig. 5.16)

(in plants and photsynthetic protists) (Fig. 5.17)

The Cytoskeleton: Internal Framework of the Cell

     determine the shape of animal cells

     provide a place for enzymes to attach so that metabolic reactions are more efficient

     move chromosomes during cell division Centrioles - (Fig. 5.20)

     cilia and flagella (9+2) arrangement (Fig. 5.23)
     anchor organelles.

Cell Movement


Cilia and flagella
(Fig. 5.23) exhibit a 9+2 arrangement of microtubules.. ciliates and choanoflagellates and sperm and paramecium.  Movie!

Special Things About Plant Cells

Vacuoles: (Fig. 5.21)

Cell Walls
: (Fig. 5.26)  



Transport Across Cell Membranes

Movement across the cell membrane occurs in three ways:
     1. Water leaks through membrane imperfections.
     2. Particles and liquids are engulfed by the membrane folding around them.
     3. Proteins in the membrane allow the passage of certain molecules.

Diffusion (Fig. 5.28) - Movie

Osmosis - (Fig. 5.29) Movie

Bulk Passage Into and Out of the Cell

endocytosis. (Fig. 5.30) Movie of endocytosis and exocytosis

(Fig. 5.31)



receptor-mediated endocytosis (Fig 5.32)

Selective permeability

channel proteins


carrier proteins

Passive transport
(Fig 5.33) Movie


Active transport (Fig 5.34) Movie