CHAPTER 7A TOUR OF THE CELL

Section A: How We Study Cells1. Microscopes provide windows to the world of the cell2. Cell biologists can isolate organelles to study their function

Objectives

• Distinguish between prokaryotic and eukaryotic cells

• Explain why there are upper and lower limits to cell size

• Explain the function of compartmentalization in eukaryotic cells

• Describe the structure and function of the nucleus

• Describe the structure and function of the eukaryotic ribosome

• List the components of the endomembrane system, describe their structures and functions and summarize the relationships among them

• Describe the types of vacuoles and how their functions differ

• Explain the role of peroxisomes in eukaryotic cells

• Describe the structure of a typical mitochondrion, detail its function and explain how compartmentaliztion in the mitochondrion is important to this function

• Explain the structure and functioning of the chloroplast

• Describe the functions of the cytoskeleton and distinguish among microtubules, microfilaments and intermediate filaments

• Describe the structure of flagella and cilia and briefly summarize the relationship between this structure and their functioning

1. Microscopes provide windows to the world of the cell

• The discovery and early study of cells progressed with the invention and improvement of microscopes in the 17th century.

• In a light microscope (LMs) visible light passes through the specimen and then through glass lenses.– The lenses refract light such that the image is

magnified into the eye or a video screen.– A light microscope can be used to resolve individual

cells

• Microscopes vary in magnification and resolving power.

• Magnification is the ratio of an object’s image to its real size.

• Resolving power is a measure of image clarity.– It is the minimum distance two points can be

separated and still viewed as two separate points.

– Resolution is limited by the shortest wavelength of the source, in this case light.

• The minimum resolution of a light microscope is about 2 microns, the size of a small bacterium

• Light microscopes can magnify effectively to about 1,000 times the size of the actual specimen.– At higher

magnifications, the image blurs.

Fig. 7.1

• A light microscope can resolve individual cells but it cannot resolve much of the internal anatomy, especially the organelles.

• To resolve smaller structures we use an electron microscope (EM), which focuses a beam of electrons through the specimen or onto its surface.– the practical limit of a modern EM is about

about 2 nm (the size of a single rhinovirus).

• Transmission electron microscopes (TEM) are used mainly to study the internal ultrastructure of cells.– A TEM aims an electron beam through a thin

section of the specimen.– The image is focused

and magnified by electromagnets.

– To enhance contrast, the thin sections are stained with atoms of heavy metals.

Fig. 7.2a

• Scanning electron microscopes (SEM) are useful for studying surface structures.– The sample surface is covered with a thin film

of gold.– The beam excites electrons on the surface.– These secondary electrons are collected and

focused on a screen.

• The SEM has great depth of field, resulting in an image that seems three-dimensional.

Fig. 7.2b

• Electron microscopes reveal organelles, but they can only be used on dead cells and they may introduce some artifacts.

• Light microscopes do not have as high a resolution, but they can be used to study live cells.

• Microscopes are a major tool in cytology, the study of cell structures.

• Cytology + biochemistry = modern cell biology.

Cell Theory1. All known living things are made up of cells.2. The cell is structural & functional unit of all living things.3. All cells come from pre-existing cells by division.

(Spontaneous Generation does not occur).

1838: Schleiden and Schwann proposed cell theory

These first three are the very basic foundations of Cell Theory. All six

of these components make up modern Cell Theory

4. Cells contains hereditary information which is passed from cell to cell during cell division.

5. All cells are basically the same in chemical composition.6. All energy flow (metabolism & biochemistry) of life occurs

within cells.    

2. Cell biologists can isolate organelles to study their functions

• The goal of cell fractionation is to separate the major organelles of the cells so that their individual functions can be studied.

• Uses an ultracentrifuge, a machine that can spin at up to 130,000 revolutions per minute and apply forces more than 1 million times gravity (1,000,000 g).

• Fractionation begins with homogenization, gently disrupting the cell.

• Then, the mixture is spun in a centrifuge to separate heavier pieces into the pellet while lighter particles remain in the solution. – As the process is repeated at higher speeds and

longer durations, smaller and smaller organelles can be collected in subsequent pellets.

• Cell fractionation prepares quantities of specific cell components.

• The functions of these organelles to be isolated, especially by the reactions or processes catalyzed by their proteins.– For example, one cellular fraction is enriched in

enzymes that function in cellular respiration.– Electron microscopy reveals that this fraction is rich in

the organelles called mitochondria.

• Cytology and biochemistry complement each other in connecting cellular structure and function.

Section B: A Panoramic View of the Cell

1.Prokaryotic and eukaryotic cells differ in size and complexity

2.Internal membranes compartmentalize the functions of a eukaryotic cell

1. Prokaryotic and eukaryotic cells differ in size and complexity

• All cells are surrounded by a plasma membrane. (What is this made of?)

• The “liquid” inside the membrane is the cytosol, which contains the organelles.

• All cells contain chromosomes which have genes in the form of DNA.

• All cells also have ribosomes, organelles that make proteins using the instructions contained in genes.

• A major difference between prokaryotic and eukaryotic cells is the location of chromosomes.

• In an eukaryotic cell, chromosomes are contained in a membrane-enclosed organelle, the nucleus.

• In a prokaryotic cell, the DNA is concentrated in the nucleoid without a membrane separating it from the rest of the cell.

Fig. 7.4 The prokaryotic cell is much simpler in structure, lacking a nucleus and the other membrane-enclosed organelles of the eukaryotic cell.

CD-Rom Activity 7.1

• This activity will help you to review and gain an understanding of the structures and functions of prokaryotic cells.

• In eukaryote cells, the chromosomes are contained within a membranous nuclear envelope.

• The region between the nucleus and the plasma membrane is the cytoplasm. (Sarah, this is for you.)

– All the material within the plasma membrane of a prokaryotic cell is cytoplasm. (This includes the organelles.)

• Within the cytoplasm of a eukaryotic cell is a variety of membrane-bounded organelles of specialized form and function.– These membrane-bounded organelles are absent

in prokaryotes.

• Eukaryotic cells are generally much bigger than prokaryotic cells.

• The logistics of carrying out metabolism set limits on cell size.– At the lower limit, the smallest bacteria,

mycoplasmas, are between 0.1 to 1.0 micron.– Most bacteria are 1-10 microns in diameter.– Eukaryotic cells are typically 10-100 microns

in diameter.

Fig. 7.5

• Metabolic requirements also set an upper limit to the size of a single cell.

• As a cell increases in size its volume increases faster than its surface area.– Smaller objects have a greater

ratio of surface area to volume.

• The plasma membrane functions as a selective barrier that allows passage of oxygen, nutrients, and wastes for the whole volume of the cell.

Fig. 7.6

• The volume of cytoplasm determines the need for this exchange.

• Rates of chemical exchange may be inadequate to maintain a cell with a very large cytoplasm.

• The need for a large surface to accommodate the volume explains the microscopic size of most cells.

• Larger organisms do not generally have larger cells than smaller organisms - simply more cells.

2. Internal membranes compartmentalize the functions of a eukaryotic cell

• A eukaryotic cell has extensive and elaborate internal membranes, which partition the cell into compartments.

• These membranes also participate in metabolism as many enzymes are built into membranes.

• The barriers created by membranes provide different local environments that facilitate specific metabolic functions.

• The general structure of a biological membrane is a double layer of phospholipids with other lipids and diverse proteins.

• Each type of membrane has a unique combination of lipids and proteins for its specific functions.– For example, those in the membranes of

mitochondria function in cellular respiration.

Fig. 7.7

CD-Rom Activity 7.2

• This activity will help you to review and gain an understanding of the structures and functions of animal cells.

Fig. 7.8

CD-Rom Activity 7.3

• This activity will help you to review and gain an understanding of the structures and functions of plant cells.