CELL AND FUNCTION

Cell Structure and Function

Organelles and Their Functions

In this lab you will look at the eukaryotic cells of plants and animals. Eukaryotic cells are distinguished from the more primitive prokaryotic cells by the presence of 1) cytoplasmic membranous organelles, 2) a nuclear membrane (i.e. a true nucleus), and 3) chromosomal proteins.  In this lab we will focus primarily on organelles, their functions within the cell and how they differ between plant and animal cells.

Think of the cell as a microscopic city. Like a real city it requires many services to keep it clean and running smoothly. Think of some of the services a real city needs: traffic control, waste disposal, and authority figure just to name a few. Like our imagined city a cell needs the same services. Organelles are the “workers” that provide these services. The following is a list describing the various functions of some common organelles.

 The NUCLEUS (“mayor of city hall”)

The nucleus houses the majority of genetic material of a cell. The nucleus is the “brain” of the cell and controls all activity within the cell. Using DNA as a blueprint
(like the blueprints of a city) the nucleus directs the production of proteins.  You will learn about this process in the DNA Transcription and Translation lab.
                                    
A nucleus with the DNA coiled into chromatin.                Electron microscope picture of a nucleus

RIBOSOMES (“lumber or brick yard”)

The ribosomes carry out manual labor in the form of protein synthesis for the nucleus. They bring together all the raw ingredients such as RNA (copies of the original DNA blueprints) and amino acids to assemble proteins. The proteins created are essential to cell and organismal function.  Think of proteins as machinery for cell functions much like electricity and plumbing are essential in a real city.  For example, enzymes are a type of protein without which life could not exist.
 
The large and small subunits of ribosomal RNA translating an mRNA strand into a polypeptide chain.
Refer to DNA Transcription and Translation for further reading.

The ENDOPLASMIC RETICULUM (“highways and road systems”)

There are two types of endoplasmic reticulum (ER) – Smooth ER and Rough ER. This extensive network makes up approximately one half of all membranous tissue of the cell and is the site of membrane and protein synthesis. The ER system is much like a road system along which industry can be found. Goods are manufactured and shipped to needed areas via the road system. Rough ER is named for the presence of ribosomes along its membrane and is the source of proteins. Smooth ER lacks ribosomes and is responsible for lipid synthesis and processes a variety of metabolic processes such as drug detoxification.

Can you tell the difference between the smooth and rough ER?

CELL MEMBRANE (“City Border”) and CELL WALL (“City Wall”)

Cell membranes are found in animal cells whereas cell walls are found in plant cells. Cell walls and membranes have similar functions. Like a city perimeter, cell membranes surround the cell and have the ability to regulate entrance and exit of substances, thereby maintaining internal balance. These membranes also protect the inner cell from outside forces. Cell walls, as the city analogy implies, are much stronger than cell membranes and protect cells from lysing (exploding) in extremely hypotonic (diluted) solutions. You will learn more about these concepts in the Biological Membranes lab.

        
Artist rendition of an animal cell membrane.                   Artist rendition of a plant cell wall.             

CYTOSKELETON (“steel girders”)

The cytoskeleton makes up the internal framework, like the steel girders that are the framework for buildings in a city that gives each cell its distinctive shape and high level of organization. It is important for cell movement and cell division (mitosis).

Picture of a cell’s cytoskeleton- a complex network of tubules and filaments.

CYTOPLASM (“lawns and parks”)

Cytoplasm is a semi-fluid substance (think gelatin) found inside the cell. The cytoplasm encases, cushions and protects the internal organelles. It is the cell landscape found in any space where organelles are not and therefore is much like the lawns and parks of our city.

The cytoplasm is the substance surrounding the visible vacuoles in this cell.

GOLGI APPARATUS (“post office”)

Like a post office, the golgi apparatus is used for shipping those goods created by the ER and ribosomes to the rest of cell.

            
EM picture of a golgi apparatus                    Artist rendition of the Golgi Complex

CHLOROPLASTS (“solar energy plant”)

Chloroplasts are organelles found only in plant cells. Like a solar energy plant they use sunlight to create energy for the city. Chloroplasts are the site of photosynthesis a process in which the plant uses carbon dioxide, water and sunlight to create energy in the form of glucose for the plant cell as well as heterotrophs that consume the plant.

Artist rendition of a chloroplast- site of photosynthesis in plant cells.

MITOCHONDRIA (“energy plant”)

Mitochondria are found in both plant and animal cells and is the site of cellular respiration. Through this process that will be covered in the Photosynthesis and Respiration lab ATP is created which is used for energy by the cell.

Electron microscope picture of a mitochondria.

LYSOSOMES (“waste disposal and recycling”)

The lysosomes are digestive sacs that can break down macromolecules in the cell using the process of hydrolysis. The digestion is carried out with lysosomal enzymes found in the lysosome. Like waste disposal in a city, lysosomes help keep excessive or bulky macromolecules from building up in the cell.

Electron microscope picture of a lysosome.

VACUOLES and VESICLES (“warehouses, water towers or garbage dumps”)

Think of these membrane sacs that have a variety of functions as containment units for anything in excess in a city. They can hold many substances from organic molecules to simple excess water. Plant cells have a central vacuole that is important in maintaining plantturgidity. You can read more about this phenomenon in the Biological Membranes Lab.

Central vacuole of a plant cell.

Plant versus Animal Cells

Now that you know some important cell organelles let us identify those that distinguish plant cells from animal cells. From the descriptions above, we can identify three organelles unique to plant cells: 1) cell wall (versus a cell membrane in animal cells), 2) central vacuole (regular vacuoles are found in animal cells) and 3) chloroplasts (animals do not perform photosynthesis. This is what makes plants autotrophs and animals heterotrophs.)

CELL STRUCTURE	LOCATION	DESCRIPTION	FUNCTION
Cell Wall	Plant, Fungi, & Bacteria, but not animal cells	Outer layer Rigid & strong Made of cellulose	Support (grow tall) Protection allows H2O, O2, CO2 to diffuse in & out of cell
Cell Membrane	All cells	Plant - inside cell wall Animal - outer layer; cholesterol Double layer of phospholipids with proteins Selectively permeable	Support Protection Controls movement of materials in/out of cell Barrier between cell and its environment Maintains homeostasis
Nucleus	All cells except prokaryotes	Large, oval May contain 1 or more nucleoli Holds DNA	Controls cell activities Contains the hereditary material of the cell
Nuclear membrane	All cells except prokaryotes	Surrounds nucleus Double membrane Selectively permeable	Controls movement of materials in/out of nucleus
Cytoplasm	All cells	Clear, thick, jellylike material (cytosol) Organelles found inside cell membrane Contains the cytoskeleton fibers	Supports and protects cell organelles
Endoplasmic reticulum (ER)	All cells except prokaryotes	Network of tubes or membranes Smooth w/o ribosomes Rough with embedded ribosomes Connects to nuclear envelope & cell membrane	Carries materials through cell Aids in making proteins
Ribosome	All cells	Small bodies free or attached to ER Made of rRNA & protein	Synthesizes proteins
Mitochondrion	All cells except prokaryotes	Peanut shaped  Double membrane Outer membrane smooth Inner membrane folded into cristae	Breaks down sugar (glucose) molecules to release energy Site of aerobic cellular respiration
Vacuole	Plant cells have a single, large vacuole Animal cells have small vacuoles	Fluid-filled sacs Largest organelle in plant cells	Store food, water, metabolic & toxic wastes Store large amounts of food or sugars in plants
Lysosome	Plant - uncommon Animal - common	Small and round with a single membrane	Breaks down larger food molecules into smaller molecules Digests old cell parts
Chloroplast	Plants and algae	Green, oval  containing chlorophyll (green pigment) Double membrane with inner membrane modified into sacs called thylakoids Stacks of thylakoids called grana & interconnected Gel like innermost substance called stroma	Uses energy from sun to make food (glucose) for the plant Process called photosynthesis Release oxygen
nucleolus	All cells except prokaryotes	Found inside the cell's nucleus May have more than one Disappear during cell division	Make ribosomes
Golgi Apparatus	All cells except prokaryotes	Stacks of flattened sacs	Have a cis & trans face Modify proteins made by the cells Package & export proteins
Cilia	Animal cells, Protozoans	Have a 9-2 arrangement of microtubules Short, but numerous	Movement
Flagellum	Bacterial cells & Protozoans	Have a 9-2 arrangement of microtubules Long, but few in number	Movement
Centrioles	Animal cells	Paired structures near the nucleus Made of a cylinder of microtubule pairs	Separate  chromosome pairs during mitosis
Cytoskeleton	All cells	Made of microtubules 7 microfilaments	Strengthen cell & maintains the shape Moves organelles within the cell