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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.

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Cell structure l.jpg

Cell Structure

Chapter 3



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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 cell’s 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 cell’s 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 cell’s 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 Leeuwenhoek’s 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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