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Cell Structure
Chapter 3
Examples of Cells
3.2 What, Exactly, Is a Cell?
Cells are the fundamental units of all life
 All cells start life with a plasma membrane,
cytoplasm, and a region of DNA which, in
eukaryotic cells only, is enclosed by a nucleus
Cell Structure
 A plasma membrane surrounds the cell and
controls which substances move in and out
 Plasma membrane
• A cell’s outermost membrane
 Lipid bilayer
• Structural foundation of cell membranes; mainly
phospholipids arranged tail-to-tail in a bilayer
Cytoplasm
 An important part of homeostasis is maintaining
the composition of cytoplasm, which differs from
fluid outside the cell
 Cytoplasm
• Semifluid substance enclosed by a cell’s plasma
membrane
Organelles
 Cell metabolism occurs in cytoplasm and
internal compartments, including organelles
 Organelle
• Structure that carries out a specialized metabolic
function inside a cell
Prokaryotes and Eukaryotes
 Cells are classed as eukaryotes or prokaryotes
based on how DNA is housed in the cell
 Nucleus
• Organelle with two membranes that holds a
eukaryotic cell’s DNA
 Nucleoid
• Region of cytoplasm where DNA is concentrated
in a prokaryotic cell
The Cell Theory
 Cell theory is the fundamental theory of biology
 Cell theory
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All organisms consist of one or more cells
The cell is the smallest unit of life
Each new cell arises from another cell
A cell passes hereditary information to its
offspring
3.3 Measuring Cells
 Most cells are visible only with the help of
microscopes
 Different types of microscopes use light or
electrons to reveal different details of cells
Bacteria on the Tip of a Pin
 Bacteria are the smallest and simplest cells
“Animalcules and Beasties”
 No one knew cells existed until microscopes
were invented
 1600s: van Leeuwenhoek’s microscope
Hooke, Schleiden, and Schwann
 1600s: Robert Hooke improved the microscope
and coined the term “cell”
 1839: Matthias Schleiden and Theodore
Schwann realized cells were alive and proposed
the cell theory
3.4 The Structure of Cell Membranes
 The plasma membrane is basically a lipid bilayer
balloon filled with fluid
 The nonpolar tails of both layers are sandwiched
between the polar heads
The Fluid Mosaic Model
 A cell membrane is a mosaic of proteins and
lipids (mainly phospholipids) that functions as a
selectively permeable barrier that separates an
internal environment from an external one
 Fluid mosaic model
• A cell membrane can be considered a twodimensional fluid of mixed composition
Membrane Proteins
 Proteins associated with a membrane carry out
most membrane functions
• Transport proteins passively or actively assist
specific ions or molecules across a membrane
• Enzymes speed chemical processes
• Adhesion proteins help cells stick together
• Recognition proteins tag cells as “self”
• Receptor proteins bind to a particular substance
outside the cell
3.5 Introducing Prokaryotic Cells
 Domains Bacteria and Archaea make up the
prokaryotes
 Prokaryotes are single-celled organisms with no
nucleus, but many have a cell wall and one or
more flagella or pili
3.6 A Peek Inside a Eukaryotic Cell
 All eukaryotic cells start life with a nucleus,
ribosomes, organelles of the endomembrane
system (including endoplasmic reticulum,
vesicles, Golgi bodies), mitochondria, and other
organelles
The Nucleus
 Pores, receptors, and transport proteins in the
nuclear envelope control the movement of
molecules into and out of the nucleus
 Nuclear envelope
• A double membrane that constitutes the outer
boundary of the nucleus
The Endomembrane System
 The endomembrane system includes rough and
smooth endoplasmic reticulum, vesicles, and
Golgi bodies
 Endomembrane system
• Series of interacting organelles between the
nucleus and plasma membrane
• Makes and modifies lipids and proteins
• Recycles molecules and particles such as wornout cell parts, and inactivates toxins
The Endomembrane System
 Endoplasmic reticulum (ER)
• A continuous system of sacs and tubes that is an
extension of the nuclear envelope
• Rough ER is studded with ribosomes (for protein
production)
• Smooth ER has no ribosomes
The Endomembrane System
 Vesicle
• Small, membrane-enclosed, saclike organelle
• Stores, transports, or degrades its contents
 Peroxisome
• Enzyme-filled vesicle that breaks down amino
acids, fatty acids, and toxic substances
 Lysosome
• Vesicle with enzymes for intracellular digestion
The Endomembrane System
 Golgi body
• Organelle that modifies polypeptides and lipids
• Sorts and packages the finished products into
transport vesicles
 Vacuole
• A fluid-filled organelle that isolates or disposes of
wastes, debris, or toxic materials
Mitochondria and Chloroplasts
 Mitochondria and chloroplasts have their own
DNA – they resemble bacteria and may have
evolved by endosymbiosis
 Mitochondrion
• Double-membraned organelle that produces ATP
 Chloroplast
• Organelle of photosynthesis
The Cytoskeleton
 Cytoskeleton
• Dynamic network of protein filaments that
support, organize, and move eukaryotic cells and
their internal structures
 The cytoskeleton interacts with accessory
proteins, such as motor proteins
Cilia and False Feet
 Cilia
• Short, hairlike structures that project from the
plasma membrane of some eukaryotic cells
• Coordinated beating stirs fluid, propels motile cells
• Moved by organized arrays of microtubules
• Example: clears particles from airways
Flagella
 Eukaryotic flagella are whiplike structures that
propel cells such as sperm through fluid
• Different internal structure and motion than
prokaryotic flagella
False Feet
 Pseudopod (false foot)
• Extendable lobe of membrane-enclosed
cytoplasm for movement or to engulf prey
• Moved by motor proteins attached to
microfilaments that drag the plasma membrane
• Example: amoebas
3.7 Cell Surface Specializations
 Cell junctions
• Connect a cell structurally and functionally to
another cell or to extracellular matrix (ECM)
 Extracellular matrix (ECM)
• Complex mixture of substances secreted by cells
• Supports cells and tissues
• Functions in cell signaling
Types of Animal Cell Junctions
 Tight junction
• An array of fibrous proteins that joins epithelial cells
and prevents fluids from leaking between them
 Adhering junction
• Anchors cells to each other or to extracellular matrix
 Gap junction
• Forms a channel across plasma membranes of
adjoining animal cells