Compare the surface:volume ratio of a cube that is 1 cm X 1 cm X 1 cm with that of a cube that is 10 cm X 10 cm X 10 cm.

Smaller cube (1 cm X 1 cm X 1 cm)

The surface area of one side = 1 cm X 1 cm = 1 square cm (or 1 cm²).

There are 6 sides, so the total surface area = 6 X  cm² = 6 cm².

Volume = 1 cm X 1 cm X 1 cm = 1 cubic cm (or 1 cm³)

Surface:Volume = 6 cm²/1 cm³ = 6 cm²/cm³ (or 6 square cm of surface area for each cubic cm of volume)

Larger cube (10 cm X 10 cm X 10 cm)

The surface area of one side = 10 cm X 10 cm = 100 square cm (or 100 cm²).

There are 6 sides, so the total surface area = 600 X  cm² = 600 cm².

Volume = 10 cm X 10 cm X 10 cm = 1000 cubic cm (or 1000 cm³)

Surface:Volume = 600 cm²/1000 cm³ = 0.6 cm²/cm³ (or 0.6 square cm of surface area for each cubic cm of volume).

Comparison

Notice that the larger cube has more surface area and more volume but less surface area for each cubic centimeter of volume.   For any given geometric object (cubes, spheres, etc.), smaller objects have a greater surface to volume ratio (surface:volume) than larger objects of the same shape.

All organisms are composed of cells, and a cell is the smallest unit of living matter.

Cells come only from preexisting cells.

Procaryotic Cells

Bacteria are procaryotes. Their cells are very small and very simple. Click here to view the chapter on prokaryotes in the BIO 102 notes.

Eucaryotic cells

All other cells are eucaryotic cells. These include protists, fungi, plants, and animals. The remainder of this chapter is devoted to eucaryotic cells.

Cells contain structures called organelles. The structure and function of the major organelles found in eucaryotic cells are described below.

All cells are surrounded by a plasma membrane. It separates the contents of the cell from its environment and regulates the passage of molecules into and out of the cell. 

The membrane contains proteins that have a variety of functions. These functions will be discussed in more detail in the chapter on membranes.

Cytoplasm is the material enclosed by the plasma membrane, excluding the nucleus.

Some organisms have a cell wall that functions to support and protect the cell. Animal cells do not have a cell wall.

The nuclei can be seen in the photograph of human cheek cells below.

The nucleus contains DNA. Recall that DNA contains instructions needed to produce proteins that control  metabolism and other cell functions.

One nucleus can serve a limited amount of cytoplasm, so large cells are often multinucleate, that is, they contain several nuclei.

Chromatin is the grainy threadlike DNA. During cell division, it coils up to produce visible structures called chromosomes.

A double membrane (nuclear envelope) surrounds the nucleus. Nuclear pores allow materials to pass into and out of the nucleus.

Ribosomes read the code in mRNA and synthesize protein according to the instructions in the mRNA.

The symbols to the right are used in the drawings of protein synthesis below.

 

The ribosome attaches to the mRNA.

 

As ribosomes move along messenger RNA (mRNA), the amino acids are added to a growing chain to form a particular protein. In these drawings, the ribosome moves from left to right.

 

In this drawing, the protein is nearly complete. When the ribosome reaches the end of the genetic message, it will become detached from the mRNA.

Several ribosomes may be attached to a strand of mRNA forming a unit called a polysome.

Ribosomes are made of RNA and proteins.

Ribosomes are composed of 2 subunits. In eucaryotic cells, the subunits are made in the nucleolus and move into the cytoplasm to form the ribosomes.

Nucleolus

The nucleolus is a structure within the nucleus where the ribosomal subunits are produced.

It appears darker than the nucleus in cells that have been stained.

The endoplasmic reticulum is a membranous network that extends throughout the cell.

It is continuous with the nuclear envelope and the plasma membrane.

Rough Endoplasmic Reticulum

The rough appearance of rough endoplasmic reticulum is due to the presence of ribosomes on the membrane.

The rough ER functions in protein synthesis, especially proteins that are to be secreted to outside the cell (example: hormones). Proteins enter the lumen of the endoplasmic reticulum while still being synthesized.

In addition to protein synthesis, the rough endoplasmic reticulum also functions in the modification of newly formed proteins. For example, some enzymes may add carbohydrate chains forming glycoproteins. Other enzymes function to fold the newly-synthesized proteins into their proper shape.

Vesicles are small sacs that pinch off the endoplasmic reticulum or golgi apparatus (discussed below) and transport molecules to other parts of the cell.

Smooth Endoplasmic Reticulum

Smooth endoplasmic reticulum has no ribosomes attached to it. It is continuous with rough endoplasmic reticulum.

Vesicles pinch off the smooth endoplasmic reticulum and carry materials to other parts of the cell such as the plasma membrane or Golgi apparatus.

Smooth endoplasmic reticulum generally functions to produce lipid compounds such as phospholipids, steroids, and fatty acids.

Certain kinds of cells have smooth endoplasmic reticulum with a specialized function. The following are some examples:

Smooth endoplasmic reticulum is abundant in the adrenal cortex and the testes where it produces steroid hormones.

The smooth endoplasmic reticulum of liver cells helps detoxify drugs in the blood.

Calcium ions needed for contraction are stored in the smooth endoplasmic reticulum of muscle cells.

Golgi Complex (also Golgi Apparatus or Golgi Body)

The Golgi complex is a stack of 3 to 20 flattened, slightly curved saccules which appear like a stack of pancakes.

It receives vesicles that contain molecules from the endoplasmic reticulum. Chemical reactions within the Golgi complex modify the molecules. Materials are passed from one saccule to the next through vesicles that form at the end of the saccule, pinch off, and fuse with the next. Processed molecules are pinched off in a vesicle.

Vesicles arriving at the Golgi complex from the endoplasmic reticulum are received into the forming face. The processed molecules leave at the maturing face.

Lysosomes are membrane-bound vesicles containing hydrolytic (digestive) enzymes produced by the Golgi complex.

They fuse with other vesicles formed around material that has entered the cell, allowing the digestion of the vesicle contents. For example, bacteria that are engulfed by white blood cells are destroyed by enzymes contained within the lysosomes.

Cells also use lysosomes to kill themselves. This important process occurs during the formation of fingers during embryonic development, the reduction in the size of a tadpole tail as it matures, and the abscission of tree leaves in the autumn.

Cellular Secretion

Ribosomes attached to the rough endoplasmic reticulum function to produce proteins. Various chemical reactions may occur within the rough endoplasmic reticulum which modify the proteins. Vesicles pinch off of the rough endoplasmic reticulum, carrying the protein molecules to the golgi apparatus for further modification. The completed molecules are then packaged into vesicles by the golgi apparatus and move to the plasma membrane where they fuse with the plasma membrane, emptying their contents. Some vesicles such as lysosomes remain within the cell.

Chloroplasts are not found in animal cells. They are included in this discussion so that students can understand the relationship between chloroplasts, mitochondria, and the flow of energy in an ecosystem.

The diagram below illustrates how energy from sunlight is used for the energy requirements of cells.

Photosynthesis is a process by which light energy is used to make sugar from CO₂ and H₂O. The equation that summarizes these reactions is:

Energy + 6CO₂ + 6H₂O ?  C₆H₁₂O₆ + 6O₂

In eucaryotes, photosynthesis occurs in chloroplasts. 

The photograph below is an Elodea leaf (X 400). The numerous green structures are chloroplasts.

Animal cells do not have chloroplasts and therefore cannot make their own food. Instead, they must eat food that has been synthesized by other organisms.

Cellular respiration refers to the chemical reactions that break down glucose to CO₂ and H₂O, releasing the energy stored within its bonds.

This process requires oxygen in aerobic organisms. Anaerobic organisms do not require oxygen, but produce much less ATP per glucose molecule.

Aerobic cellular respiration occurs in the mitochondria.

The drawing below shows the double-membrane structure of a mitochondrion.

Cilia and Flagella

Cilia and flagella are hairlike structures projecting from the cell that function to move the cell by their movements. They are outward projections of the interior of the cell, containing cytoplasm and enclosed by the plasma membrane.

Cilia are shorter than flagella but both are similar in construction.

Examples:

Sperm use flagella to move.

Cells lining the upper respiratory tract are ciliated (have cilia). The cilia move mucous and debris upward to the mouth where it is swallowed. The diagram below is a cross section of a human trachea (400 X). Note the cilia on the upper surface.

The cytoskeleton is a network of protein elements that extends through the cytoplasm in eucaryotic cells.

Microtubules

Microtubules are small cylindrical fibers that change in length by assembling and disassembling.

The fibers are lengthened and shortened as they assemble or disassemble from one or both ends.

The assembly of microtubules is controlled by an area near the nucleus called the centrosome.

The centrosome contains two barrel-shaped structures that are composed of microtubules. These structures, called centrioles, are oriented at right angles to each other.

Microtubules play a role in moving things within the cell and also in moving the cillia and flagella. For example, they are associated with movement of vesicles from the golgi complex to the plasma membrane. During cell division, they move the chromosomes into the newly-forming cells.

Actin Filaments (Microfilaments)

Actin filaments are long, thin protein fibers that generally assist in movement of the cell.

Because they can assemble and disassemble so quickly, the shape of a cell can change rapidly.

Microvilli and pseudopodia move by the action of actin filaments. 

Actin filaments are important in muscle contraction. 

During cell division a ring of actin filaments that surrounds the cell constricts, pinching the cell into two.

Intermediate Filaments

Intermediate filaments are intermediate in size. Actin filaments are smallest and microtubules are largest elements of the cytoskeleton.

Intermediate filaments are important in maintaining the cell?s shape, providing mechanical support; preventing excessive stretching, and supporting other organelles.

Draw a typical human cell. Use arrows to label each structure. Next to each label, write a brief description of the function of that structure. Include the following structures in your diagrams.

plasma membrane

nucleus

nucleolus

ribosomes

rough endoplasmic reticulum

smooth endoplasmic reticulum

golgi apparatus

lysosomes

mitochondria