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  All cells have a cell membrane, an outer boundary that encloses the cell and separates the cell interior, called the cytoplasm, from it’s surroundings.

Cell Features  The cell membrane also regulates what enters and leaves the cell including gases, nutrients, and wastes

Cell Features Within the cytoplasm are many structures, often suspended in a system of microscopic fibers called the cytoskeleton

Cell Features  Most cells have ribosomes as well  Ribosomes are the cellular structures on which proteins 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 cells These 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 fibers Long slender microfilaments made of the protein actin Hollow tubes called microtubules made of the protein tubulin Thick ropes of protein called intermediate fibers

Cytoskeleton The 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

Cytoskeleton Microtubules 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

Cytoskeleton Intermediate 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 Proteins Cell 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