Cell StructureCell Structure

Units of LifeUnits of Life

Every living thing from the smallest of bacteria to the

largest redwood tree is made of cells. The discovery and study of cells was made possible by

the discovery of the microscope.

In 1665 Robert Hooke studied a thin slice of cork and saw many little boxes that reminded him of the tiny rooms that

monks lived in. He called these cavities cells.

What Hooke observed was actually the remains of cells. Anton van Leeuwenhoek was first to

observe living cells in 1673.

Cells vary widely in size. The nerve cells in a giraffe’s leg may be about 6 ½ feet long while some bacterial

cells are only .000008 in. across. Most cells are visible only with a microscope. We as scientists use the metric system to make exact cell measurements.

The SI system has seven base units.

• Length meter m

• Mass kilogram kg

• Time second s

• Temperature Kelvin K

• Amount mole mol

• Current ampere A

• Light candela cd

Metric PrefixesTera …………TrillionsGiga……..……BillionsMega………….MillionsKilo………..Thousands

Base UnitDeci…………….One tenthCenti………One hundrethMilli…..…One thousandthMicro….……One millionthNano………..One billionthPico….……..One trillionth

The microscope is probably the most widely used

instrument of the biologist.

Microscopes perform two main functions:

• Magnification shows an enlarged image.

• Resolution shows fine details.

One of the most popular microscopes in biology is the light

microscope.

The are other microscopes that offer higher magnification and

resolution than the light microscope. Let’s compare

them.

TYPE:Light Microscope

OPERATION:Light transmitted through a thin specimen forms an image which is enlarged by

convex lenses.

ADVANTAGES:Simple, easy, and quick to use in most environments.

Relatively inexpensive. Retains natural color

DISADVANTAGES:Relatively low magnification.

MAGNIFICATION: 2000X

TYPE:Scanning Electron Microscope (SEM)

OPERATION:A magnetic lense focuses a high voltage stream of electrons on to a sample. As the stream of

electrons are deflected by the sample, they are detected and a three dimensional image is assembled.

ADVANTAGES:High Magnification. Emits an X-ray spectrum.

Nondestructive

DISADVANTAGES:Sample must be a small, metallic solid.

MAGNIFICATION: 100,000X

TYPE:Transmission Electron Microscope (TEM)

OPERATION:A high energy beam of electrons passes through a thinly sliced sample and produces a

magnified image on a detector.

ADVANTAGES:High Magnification. Studies a wide range of samples.

DISADVANTAGES:Destructive to sample and not commonly available

MAGNIFICATION:200,000X

TYPE:Scanning Tunneling Microscope (STM)

OPERATION:An electron stylus traces the surface of a sample and produces a 3-D image.

ADVANTAGES:High magnification. Can even image atoms.

DISADVANTAGES:Not easily available

MAGNIFICATION:100,000,000X

Section TwoSection TwoCell FeaturesCell Features

About 150 years after the discovery of the cell scientists began to organize the works of the early microscopists into the cell theory.

The cell theory states: • All living things are made up of one or

more cells.

• Cells are the basic unit of structure and function in an organism.

• Cells come only from the reproduction of preexisting cells.

Matthias Schleiden, Theodor Schwann, and Rudolf Virchow are given credit for

organizing the cell theory. Schleiden and Schwann arrived at the first two premises

of the theory while working independently and Virchow established that cells must come from preexisting

cells.

The size of cells is limited by the ratio of their surface area to their volume. As cells grow

larger, this ratio moves toward 1:1 which limits the ability of nutrients, oxygen, and waste

products to be transported in and out of the cell.

Cells share some common structural features:

• The cell is surrounded by thin cell membrane which controls what goes in and out of the cell. The area between the cell membrane and the nucleus is a special protoplasm called the cytoplasm

• Cells contain a variety of internal structures called organelles that are suspended in a system of fibers called the cytoskeleton. An organelle is a cell component that performs a specific function in the cell.

• Most cells contain structures called ribosomes which make up proteins.

• Most cells have a definite membrane bound nucleus and distinct membrane bound organelles. These cells are called eukaryotes. Some unicellular organisms lack membrane bound organelles. They even lack a distinct nucleus. These cells are called prokaryotes.

ProkaryotesProkaryotes

Prokaryotes are the smallest, simplest cells.

Lack membrane bound organelles including a nucleus.

Oldest of cells Cannot carry out complicated functions Familiar prokaryotes include bacteria Prokaryotes can live in a wide range of

conditions Prokayotes carry out life functions in their cell

membrane and their DNA exists as a single loop

Prokaryotes cont.

• Prokaryotes maintain their shape with a cell wall of polysaccharide fibers connected by amino acids.

• Many prokaryotes are covered by a sticky capsule which allows them to attach to surfaces.

• Many prokaryotes have a whip-like flagella that allows them to move.

Eukaryotic CellsEukaryotic Cells

Eukaryotes have a definite membrane bound nucleus and

organelles. Membranes channel fluids through the cell

and pinch off to form vessicles. Eukaryotes can move using

cilia and flagella.

The cell cytoskeleton provides an interior framework for the cell. It might consist of 3 types

of fibers:

• Long, slender fibers of actin located just under the cell membrane pull and push to change the shape of the cell.

• Hollow microtubules provide channels for cell structures to move along.

• Larger intermediate fibers provide a framework to support the cell.

The Cell MembraneThe Cell Membrane

The cell cannot be totally separated from its environment. It must be able to take in food, oxygen, and

water and get rid of waste. The cell is able to do this through a

selectively permeable cell membrane.

The cell membrane is made up of a double layer of phospholipids

molecules that have their hydrophilic head turned outward and

their hydrophobic tails turned inward. This allows the cell to be

bathed in a watery environment yet

control what passes through.

Membrane proteins are located in the bilipid layer and provide many functions for the cell.

• Surface proteins attach to a carbohydrate on the cell’s surface to mark its idenity. And interact with other cells.

• Receptor proteins bind to specific particles.

• Transport proteins move substances into and out of the cell.

The cell membrane is not static. It is constantly changing to adjust to the

metabolism of the cell like the surface of a soap bubble. This constantly changing system is explained by the fluid mosaic

model in which the lipid bilayer is explained better as a fluid than as a solid.

Section ThreeSection Three

Cell OrganellesCell Organelles

The nucleus is the most obvious structure of the eukaryotic cell. It keeps

its shape with the help of a protein skeleton known as the nuclear matrix.

The nucleus is bound by a double nuclear membrane or nuclear

envelope that is not a barrier but contains a number of nuclear pores that

communicate with the cytosol. The nucleus holds DNA which makes up

chromosomes to be passed along in cell division.

The spherical nucleolus is found in the nucleus and builds

ribosomes.

Ribosomes and the Ribosomes and the Endoplasmic ReticulumEndoplasmic Reticulum

The endoplasmic reticulum is a system of tubules and sacs that connect all parts of the cell. It acts as a communication system the fluids, gases, enzymes and proteins can move along. There are

two basic types of ER, the smooth ER and the rough ER. The rough ER is located near the

nucleus and is thickly covered with ribosomes while the smooth ER is located in more distant parts of the cell and serves to form lipid covered vesicles

in some cells.

Ribosomes are the most numerous of the cell organelles. Some are found free

in the cytosol while many are found along another organelle called the

endoplasmic reticulum. The ribosomes are not membrane bound and are made

up of two substances, RNA and proteins. Ribosomes synthesize proteins for the

cell.

Lysosomes are small membrane bound organelles that serve in digestion in

animal, fungi, and protist cells. Lysosomes are rare in plant cells. Lysosomes can digest unneeded

proteins, carbohydrates, RNA, DNA, lipids, viruses, or bacteria. They digest body parts in shaping organisms during

development as in the absorption of a frog’s tail.

Scattered throughout the cytosol are comparatively large, bean shaped organelles called mitochondria.

Cells that have a high energy requirement such as muscle cells have thousands of mitochondria.

The mitochondria carry out respiration to build ATP from

nutrient molecules. The interior of the mitochondria is filled with

complicated folds called cristae.

Mitochondria have their own DNA and replicate only from other mitochondria.

Some number of mitochondria may change in cells with a high energy

demand as in muscles.

Plant CellsPlant Cells

Plant cells have a distinct cell wall to keep their shape and help them to

perform distinct functions. The cell wall

forms in layers. Fibers of cellulose, a polysaccharide, form the layers and are

held in place by proteins.

Vacuoles are a second characteristic of plant cells. They are large fluid-filled

vesicles that may occupy up to 90 percent of the cells volume. They may

store water, wastes, enzymes, pigments, and in some plants, poisons. Their

pressure helps the plant cell keep its shape.

A third characteristic of plant cells is chloroplasts. They are

surrounded by two membranes and store lipids and starch. They also contain pigments that give green

plants their color and the ability to organize food.