Cell Structure

Chapter 4

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What is Cell Theory?

Cells were discovered in 1665 by Robert Hooke.

Schleiden (studied plants) and Schwann (studied animals) proposed the Cell Theory.

Cell Theory

1. All organisms are composed of cells.

2. Cells are the smallest living things.

3. Cells arise only from pre-existing cells.

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Why is Cell size limited?

Cell size is limited:

-As cell size increases, it takes longer for material to diffuse from the cell membrane to the interior of the cell.

Surface area-to-volume ratio: as a cell increases in size, the volume increases 10x faster than the surface area

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Hydrogen atom

Amino acid

Logarithmic scale

Protein

Ribosome

Large virus (HIV)

Human red blood cellProkaryote

Human egg

Paramecium

Chicken egg

Adult human

Frog egg

ChloroplastMitochondrion

100 m

10 m

1 m

10 cm

1 cm

1 mm

100 mm

10 mm

1 mm

100 nm

10 nm

1 nm

0.1 nm (1 Å)

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Different kinds of microscopes are used to view different kinds of cells or components in a cell (with help of various staining procedures)

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All cells have certain structures in common

• Prokaryotes• DNA is circular and present in

nucleoid.• Cytoplasm is a semifluid matrix

without any membrane-bound organelles

• Two kinds: Bacteria and Archaea

• Eukaryotes• DNA is present in nucleus

(nuclear envelope)• Cytoplasm contains

membrane-bound organelles for support and cellular structure

• More complex than prokaryotes

Ribosome and plasma membrane are present in both

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Prokaryotic Cells

• Cell walls protect cell and maintain its shape

• Bacterial cell walls

- may be composed of peptidoglycan

- may be Gram positive or Gram negative

• Archaea cell walls lack peptidoglycan.

• Flagella is used to provide locomotion

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Eukaryotic Cells

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Eukaryotic Cells

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Nucleus in Eukaryotes

• Nucleus stores the genetic material of the cell in the form of multiple, linear chromosomes

• Surrounded by a nuclear envelope composed of two phospholipid bilayers

• Chromosomes – DNA is organized with proteins to form chromatin

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Ribosome in Eukaryotes

• Ribosomes

• Site of protein synthesis in the cell

• Composed of ribosomal RNA and proteins

• Found freely within the cytosol or attached to internal membranes

• Universal machines

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Endomembrane System in Eukaryotes

• Endomembrane system• A series of membranes throughout the cytoplasm• Divides cell into compartments where different

cellular functions occur• Channels passage of molecules through interior of

cell

1. endoplasmic reticulum

2. Golgi apparatus

3. lysosomes

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Endomembrane System in Eukaryotes

• Rough endoplasmic reticulum (RER)

• Membranes that create a network of channels throughout the cytoplasm

• Attachment of ribosomes to the membrane gives a rough appearance

• Synthesized proteins secreted are sent to either lysosomes or vacuoles or embedded in plasma membrane

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Smooth endoplasmic reticulum (SER)

• Relatively few ribosomes attached

• Functions:

-synthesis of membrane lipids

-calcium storage

-detoxification of foreign substances

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Golgi Apparatus

• Golgi apparatus• Flattened stacks of

interconnected membranes

• Collecting, packaging and distribution of materials to different parts of the cell

• Synthesis of cell wall components

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Lysosomes

• Lysosomes• Membrane bound

vesicles containing digestive enzymes to break down macromolecules

• Destroy cells or foreign matter that the cell has engulfed by phagocytosis

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Microbodies

Microbodies

-membrane bound vesicles

-contain enzymes

-not part of the endomembrane system

-Glyoxysomes in plants contain enzymes for converting fats to carbohydrates

-Peroxisomes contain oxidative enzymes and catalase that break H2O2 to water and oxygen

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Vacuole

• Vacuoles• Membrane-bound structures (tonoplast) with

various functions depending on the cell type

There are different types of vacuoles:-central vacuole in plant cells-contractile vacuole of some protists-vacuoles for storage

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Mitochondria

• Mitochondria is surrounded by 2 membranes-smooth outer membrane and folded inner membrane with layers called cristae

-Matrix is within the inner membrane

-Intermembrane space is located between the two membranes

-Contain their own DNA

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Mitochondria

Mitochondria

-organelles present in all types of eukaryotic cells

-contain oxidative metabolism enzymes for transferring the energy within macromolecules to ATP

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Chloroplasts

Chloroplasts-organelles present in cells of plants and some other eukaryotes-contain chlorophyll for photosynthesis-surrounded by 2 membranes-thylakoids are membranous sacs within the inner membrane-grana are stacks of thylakoids

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Mitochondria & Chloroplasts

• Endosymbiosis

• Proposal that eukaryotic organelles evolved through a symbiotic relationship

• One cell engulfed a second cell and a symbiotic relationship developed

• Mitochondria and chloroplasts are thought to have evolved this way

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Mitochondria & Chloroplasts

Much evidence supports this endosymbiosis theory.

Mitochondria and chloroplasts:-have 2 membranes-possess DNA and ribosomes-are about the size of a prokaryotic cell-divide by a process similar to bacteria

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Cytoskeleton

Cytoskeleton

-network of protein fibers found in all eukaryotic cells

-supports the shape of the cell

-keeps organelles in fixed locations

-helps move materials within the cell

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Cytoskeleton

• Cytoskeleton fibers include

• Actin filaments – responsible for cellular contractions, crawling, “pinching”

• Microtubules – provide organization to the cell and move materials within the cell

• Intermediate filaments – provide structural stability

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Movement within cells

Molecular motors

• Involve motor proteins

such as kinesin and

dynein

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Extracellular Structures - Cell Movement

Cell movement takes different forms.

-Crawling is accomplished via actin filaments and the protein myosin.

-Flagella undulate to move a cell.

-Cilia can be arranged in rows on the surface of a eukaryotic cell to propel a cell forward.

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Extracellular Structures - Cell Movement

The cilia and flagella of eukaryotic cells have a similar structure:-9-2 structure: 9 pairs of microtubules surrounded by a 2 central microtubules-Cilia are usually more numerous than flagella on a cell.

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Extracellular Structures

Cell walls

-present surrounding the cells of plants, fungi, and some protists

-the carbohydrates present in the cell wall vary depending on the cell type:

-plant and protist cell walls - cellulose

-fungal cell walls - chitin

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Extracellular Matrix

Extracellular matrix (ECM)

-surrounds animal cells

-composed of glycoproteins and fibrous proteins such as collagen

-may be connected to the cytoplasm via integrin proteins present in the plasma membrane

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