Biology 3.2

Cell Features

The Cell Theory In 1838, the German botanist Mattias

Schleiden concluded that cells make up not only the stems of plants but every part of a plant.

A year later, German zoologist Theodore Schwann determined that cells animals are also made of cells

In 1858, Rudolph Virchow, a German physician, determined that cells came from other cells

Cell Theory The observations of Schleiden,

Schwann, and Virchow form the cell theory which has three parts

All living things are made of one or more cells

Cells are the basic units of structure and function in organisms

All cells arise from existing cells

Cell Size Small cells function more efficiently

than large cells.

Small cells can exchange substances more readily than larger cells because small objects have a higher surface area-to-volume ratio than larger objects

As a result, substances do not need to travel as far to reach the center of a smaller cell

Common Cell Features Al l ce l l s have a ce l l

membrane , an ou te r boundary tha t enc loses the ce l l and separa tes the ce l l i n te r io r, ca l led the cy top lasm, f rom i t ’s su r round ings .

Cell Features The cel l membrane

a lso regulates what enters and leaves the cel l

inc luding gases,

nut r ients,

and wastes

Cell Features

With in the cy top lasm are many s t ruc tu res , o f ten suspended in a sys tem o f m ic roscop ic f ibers ca l led the cy toske le ton

Cell Features Most cel ls have

r ibosomes as wel l

Ribosomes are the cel lu lar s t ructures on which prote ins are made

Cellular Structure

All cells also have DNA

Which provides Instructions for

1. Making proteins

2. Regulates cellular activities

3. Enables cells to reproduce

Features of Cells The smallest and simplest cells are prokaryotes

Prokaryotes are single-celled organisms that lack a nucleus and other internal compartments

Without separate compartments to isolate materials, prokaryotes cells cannot carry out many specialized functions

For nearly two billion years, prokaryotes were the only organisms on Earth

Characteristics of Prokaryotes

Prokaryotes can exist in a broad range of environmental conditions

The cytoplasm of a prokaryotes contains everything inside the cell membrane

A prokaryotes enzymes and ribosomes are free to move around in the cytoplasm because there are no internal structures that divide the cell into compartments

Prokaryotes In prokaryotes, the genetic material is a single circular

molecule of DNA

Prokaryotic cells have a cell wall surrounding the cell membrane that provides structure and support

Prokaryotes lack an

Internal support skeleton so

They rely on a strong cell

wall to give the cell shape

Prokaryotes Some prokaryotes cell walls are surrounded by a structure

called a capsule. The capsule enables the prokaryotes to cling to almost anything; teeth, skin, food, etc.

Many prokaryotes have flagella which are long, threadlike structures that protrude from the cell’s surface and enable movement

Prokaryotic flagella rotate

propelling the organism up

to 20 cell lengths per second

Eukaryotic cells

The first cells with compartments were primitive eukaryotic cells which evolved about 2.5 billion years ago

A eukaryote is an organism whose cells have a nucleus

The nucleus is an internal compartment that houses the cell’s DNA

Eukaryotic cells

Other internal compartments, or

organelles, enable eukaryotic

cells to function in ways

different from prokaryotes

An organelle is a structure that carries out specific activities in the cell

A complex system of internal membranes connects some organelles within the cytoplasm

Eukaryotic cellsThese membranes provide channels

that guide the distribution of substances within the cell

Many single celled eukaryotes use flagella for movement

Short hairlike structures called cilia protrude from the surface of many eukaryotic cells

Flagella or cilia propel some cells through their environment

In other cells, cilia or flagella move substances across the cell’s surface

Eukaryotic cells

A web of protein fibers makes up the cytoskeleton

The cytoskeleton holds the cell together and keeps cell membranes from collapsing

Cytoskeleton The cytoskeleton provides the

internal framework of an animal cell much like our skeletons provide our internal framework

The cytoskeleton is composed of an intricate network of protein fibers anchored to the inside of the plasma membrane

Cytoskeleton By linking one region to another,

they support the shape of the cell, much as steel beams anchor the sides of a building together

Other fibers attach the nucleus and other organelles to fixed locations in the cell

Because protein fibers are too small for a light microscope to reveal, biologists visualize the cytoskeleton by attaching flourescent dyes to antibodies.

Cytoskeleton There are three types of

cytoskeleton fibersLong slender

microfilaments made of the protein actin

Hollow tubes called microtubules made of the protein tubulin

Thick ropes of protein called intermediate fibers

CytoskeletonThe actin fibers of the

cytoskeleton form a network just beneath the cell surface that is anchored to membrane proteins embedded within the cell membrane

By contracting or expanding, actin plays a major role in determining the shape of animal cells by pulling the plasma membrane in some places and pushing it out in others

CytoskeletonMicrotubules within the

cytoskeleton act as a highway system for the transportation of information from the nucleus to different parts of the cell

RNA molecules are transported along microtubular “rails” that extend through the interior of the cell like train tracks

CytoskeletonIntermediate Fibers of the

cytoskeleton provide a frame on which ribosomes and enzymes can be confined to particular regions of the cell

The cell can organize complex metabolic activities efficiently by anchoring particular enzymes near one another

The Cell Membrane The cytoplasm of a cell is

contained by it’s membrane

Cell membranes are not rigid like an eggshell but fluid like a soap bubble

The fluidity of a cell membrane is caused by lipids

Lipids form a barrier that separates inside the cell from outside the cell

The Cell Membrane This barrier only allows certain

substances to pass through in or out

This ability to control what enters or leaves the cell is caused by the way phospholipids interact with water

A phospholipid is a lipid made of a phosphate group and two fatty acids

The Cell Membrane As shown at right, a

phospholipid has both a polar “head” and two nonpolar “tails”

The head of a phospholid is polar and is attracted to water.

In contrast, the two fatty acids, the “tails” are nonpolar and therefore they are repelled by water

The Cell Membrane In a cell membrane, the

phospholipids are arranged in a double layer called a lipid bilayer

The lipid bilayer allows lipids and substances that dissolve in lipids to pass through

Member Proteins Various proteins are

located in the lipid bilayer of a cell membrane

The motion and fluidity of phospholipids enables some membrane proteins to move around within the lipid bilayer

Member ProteinsCell membranes contain different types of

proteins, among them

marker proteins which attach to carbohydrates

Receptor proteins bind specific substances

Enzymes embedded in the cell membrane are involved in biochemical reactions within the cell

Transport proteins aid the movement of substances into and out of the cell