Cell Structure

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Cell Structure

• Chapter 3

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Examples of Cells
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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

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Cell Structure

• A plasma membrane surrounds the cell and controls
  which substances move in and out
• Plasma membrane
• A cells outermost membrane
• Lipid bilayer
• Structural foundation of cell membranes mainly
  phospholipids arranged tail-to-tail in a bilayer

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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 cells plasma
  membrane

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Organelles

• Cell metabolism occurs in cytoplasm and internal
  compartments, including organelles
• Organelle
• Structure that carries out a specialized
  metabolic function inside a cell

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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 cells DNA
• Nucleoid
• Region of cytoplasm where DNA is concentrated in
  a prokaryotic cell

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The Cell Theory

• Cell theory is the fundamental theory of biology
• Cell theory
• 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

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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

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Bacteria on the Tip of a Pin

• Bacteria are the smallest and simplest cells

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Animalcules and Beasties

• No one knew cells existed until microscopes were
  invented
• 1600s van Leeuwenhoeks microscope

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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

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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

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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
  two-dimensional fluid of mixed composition

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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

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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

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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

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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

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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 worn-out
  cell parts, and inactivates toxins

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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

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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

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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

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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

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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

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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

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Flagella

• Eukaryotic flagella are whiplike structures that
  propel cells such as sperm through fluid
• Different internal structure and motion than
  prokaryotic flagella

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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

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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

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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

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