Chapter Four: A Tour of the Cell

Questions:

1. What are the similarities and differences between the chloroplast and mitochondria?
2. What are three types of cell junctions found in animal tissues?  What are their functions?
3. What are some of the main differences and similarities between prokaryotic and eukaryotic cells?

Answers: 

1. The chloroplast is only found in the plant cell, while the mitochondria is found in both the animal and plant cell.  The chloroplast is the site of photosynthesis in the plant cell.  This means that it converses light energy from the sun into the chemical energy of sugar molecules.   The mitochondria carries out cellular respiration in the cell, using the energy from food to make ATP (adenosine triphosphate) that is essential for cellular work.  Both of these organelles have different structures that suit their particular function.  One similarity between these two organelles is the fact that they both evolved from being small prokaryotic cells that began living in other cells.     
2. Three types of cell junctions are tight junctions, anchoring junctions, and gap junctions.  Tight junctions are knit tightly together by proteins, preventing leakage of extracellular fluid across a layer of epithelial tissue.  Anchoring junctions are made up of filaments consisting of sturdy keratin proteins that keep these junctions glued to the cytoplasm.  They fasten the cells together into strong sheets.  Gap junctions work as channels that allow small molecules to flow through pores lined with proteins.  These pores allow for communication between neighboring cells. 
3. All cells have a plasma membrane, DNA, ribosomes and cytoplasm.  However, prokaryotic cells are much smaller than eukaryotic cells and are relatively simpler.  They don't even have a membrane bound nucleus.  The eukaryotic cell is much more complex.  Not only does it have a membrane bound nucleus, but it also only has a membrane.  The prokaryotic cell has a cell wall that protects it from its environment.  

Important Facts: 

1. Eukaryotic cells are separated into four different functional compartments.  They are manufacturing, breakdown of molecules, energy processing, and structural support, movement and communication.  Manufacturing involves the nucleus, ribosomes, the golgi apparatus, and the endoplasmic reticulum.  Breakdown of molecules involves peroxisomes, lysosomes, and vacuoles.  Energy processing involves the mitochondria in animal cells, and the chloroplast in plant cells.  Structural support, movement and communication involve the cytoskeleton, plasma membrane, and cell wall.   
2. The endoplasmic reticulum (ER) is a network of membranes and sacs.  It makes up much of the cell.  There are two types of ER: the smooth ER and the rough ER.  The smooth ER has three main functions.  These include the synthesis of lipids, metabolism of carbohydrates, and the detoxification of drugs and poisons.  The rough ER is called rough because it has ribosomes on its structure.  These ribosomes synthesize proteins.  Then the polypeptide chains travel across the ER membrane.
3. Endosymbiosis suggests that the mitochondria and chloroplast were small prokaryotes before they began growing in larger cells.  This hypothesis was proposed because both organelles contain DNA and ribosomes.  The DNA has a similar structure to the DNA found in prokaryotes.  Also the ribosomes are more similar to those found in prokaryotes rather than the ones found in eukaryotes.   
4. The extracellular matrix helps hold cells together and protects the plasma membrane.  It is mainly made up of glycoproteins that form strong collagen fibers outside the cell.  These fibers are connected to a network of other types of glycoproteins.  These glycoproteins connect to a polysaccharide molecule.  The ECM attaches to the cell through other glycoproteins that bind to integrins (membrane proteins).  Integrins transmit information between the ECM and the cytoskeleton.  
5. The light microscope can magnify living and dead cells up to 1,000 times.  The ultrastructure of cells can be revealed by scanning and transmission electron microscopes which have greater resolution and magnification.  

Diagram:  This diagram shows the structure of the plasma membrane as well as where it lies in relation to the entire cell.  It shows what the plasma membrane is composed of in great detail. 

Key Terms: 

Endomembrane System- The system of membranes within a cell that includes the nuclear envelope, endoplasmic reticulum, Golgi apparatus, lysosomes, vacuoles, and the plasma membrane.

Lysosomes- Sacs of hydrolytic enzymes that can digest large molecules (proteins, polysaccharides, fats, nucleic acids)  and recycle them into the cell. 
Plasma Membrane- Forms the boundary of a cell and selectively permits the passage of materials into and out of the cell.  Its structure is made up of phospholipids, proteins, and carbohydrates.
Nucleus- Contains most of DNA essential for the cell and makes mRNA that will later produce proteins.  It is the control center of the cell.
Ribosomes- Sites of protein synthesis in the cell.
Golgi Apparatus- The postal system of the cell.  Here, proteins are modified, stored, and shipped.
Cytoskeleton- Network of protein fibers that run throughout the cytoplasm.  It is responsible for support, motility, and regulating biochemical activities.
Central Vacuoles- Store and breakdown some waste products.  Take up 80% of the plant cell.
Cell Wall- Plant cells only.  Helps maintain the shape of the cell.  Mainly made of cellulose.
Plasmodesmata- Channels that penetrate adjacent plant cell walls and allow the passage of some molecules from cell to cell.
Peroxisomes- Single membrane bound structures in the cell.  They are responsible for various metabolic functions that involve the production of hydrogen peroxide and break down fatty acids that are then sent to the mitochondria for fuel.

Summary: This chapter focuses on the differences between prokaryotic cells and eukaryotic cells.  It also deals with the different organelles and where they lie in the four key functional groups of a cell: manufacturing, breakdown, energy processing, and communication between cells.  The chapter goes into detail describing the structure and function of each organelle and why they are important to the cell.

Relevant Video: 
http://www.youtube.com/watch?v=Hmwvj9X4GNY&feature=related