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Chapter 3

Cell AnatomyCell = Chamber/Compartment

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• Basic Structure of the Cell – Plasma membrane – Cytoplasm containing organelles (little organs)– Nucleus

• Functions of the Cell– Basic unit of life– Protection and support through production and secretion of

various kinds of molecules– Movement. Various kinds occur because of specialized

proteins produced in the cell (Flagella and Cilla) – Communication. Cells produce and receive electrical and

chemical signals– Cell metabolism and energy release– Inheritance. Each cell contains DNA. Some cells are

specialized to gametes for exchange during sexual intercourse

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Plasma Membrane (phospholipid bilayer)

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Plasma Membranes

• Fluid-mosaic model• theory explaining how cell membranes are

constructed– Molecules of the cell membrane are

arranged in a sheet– The mosaic of molecules is fluid; that is,

the molecules are able to float around slowly

• Chemical attraction is the force that holds membranes together

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Membrane Lipids

• Primary structure of a cell membrane is a double layer of phospholipid molecules

– Heads are hydrophilic (water-loving)

– Tails are hydrophobic (water-fearing)

– Molecules arrange themselves in bilayers in water

– Cholesterol molecules are scattered among the phospholipids to allow the membrane to function properly at body temperature

– Most of the bilayer is hydrophobic; therefore water or water-soluble molecules do not pass through easily

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Membrane Lipids (cont)

What is it composed of?• Phospholipids and cholesterol predominate

– Phospholipids: bilayer. Polar heads facing water in the interior and exterior of the cell (hydrophilic); nonpolar tails facing each other on the interior of the membrane (hydrophobic)

– Cholesterol: interspersed among phospholipids. Amount determines fluid nature of the membrane

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Membrane Proteins

TYPES of Membrane Proteins

• Integral / intrinsic – Extend deeply into

membrane, often extending from one surface to the other

– Can form channels through the membrane

• Peripheral / extrinsic– Attached to integral proteins

at either the inner or outer surfaces of the lipid bilayer

• Functioning depends on 3-D shape and chemical characteristics. Markers, attachment sites, channels, receptors, enzymes, or carriers.

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1) Marker Molecules: Glycoproteins

• Carbohydrates attach to proteins

• Allow cells to identify one another or other molecules – Immunity– Intercellular

communication

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2) Attachment Sites

• Attachment sites to other cells or to extra/intracellular molecules.

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3) Channel Proteins (Integral): hydrophilic region faces inward; charge determines molecules that can pass through • Nongated ion channels:

always open– Responsible for the

permeability of the plasma membrane to ions when the plasma membrane is at rest

• Gated ion channels can be open or closed– Ligand gated ion channel:

open in response to small molecules that bind to proteins or glycoproteins

– Voltage-gated ion channel: open when there is a change in charge across the plasma membrane

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4) Receptor Molecules

• Proteins in membranes with an exposed receptor site

• Can attach to specific ligand molecules and act as an intercellular communication system

• Ligand can attach only to cells with that specific receptor

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5) Enzymes and Carrier Protein

• Enzymes: some act to catalyze reactions at outer/inner surface of plasma membrane.

• Carrier proteins: integral proteins move ions from one side of membrane to the other– Have specific binding sites– Protein change shape to transport ions or molecules

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Cytoplasm

• Cytoplasm: gel-like internal substance of cells that includes many organelles suspended in watery intracellular fluid called cytosol

• Cellular material outside nucleus but inside plasma membrane• Two major groups of organelles (Table 3-3)

– Membranous organelles are sacs or canals made of cell membranes

– Nonmembranous organelles are made of microscopic filaments or other nonmembranous materials

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Endoplasmic Reticulum

– 2 Types

Smooth ER

Rough ER

Endoplasmic ReticulumSmooth (SER):

• No ribosomes attached

• Synthesizes lipids and carbohydrates:

– phospholipids and cholesterol (membranes)

– steroid hormones (reproductive system)

– glycerides (storage in liver and fat cells)

– Storage of Ca++

– Fat metabolism and drug detoxification (liver)

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Rough (RER)

-Has attached ribosomes

- Proteins produced and modified here

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Ribosomes - Tiny round bodies in the cytoplasm

made of a large and small subunit

- Build polypeptides in protein synthesis

– Two types:

• Free ribosomes in cytoplasm:

– manufacture proteins for cell

• Fixed ribosomes attached to ER membranes:

– manufacture proteins for secretion

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Golgi Apparatus

• Flattened membrane sacs stacked on each other (cisternae)

• Modification, packaging, distribution of proteins and lipids for secretion or internal use

Function of Golgi Apparatus

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Proteins made in the ER are packaged into tiny vesicles, which pinch off and move towards the golgi apparatus.Vesicles fuse with the golgi membrane and release the proteins.Enzymes within the golgi modify the proteinsThe processed molecules eventually pinch off and move towards the plasma membrane where they are secreted.

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Function of Golgi Apparatus

Lysosomes• Powerful enzyme containing vesicles

• Functions of Lysosomes

– Clean up inside cells:• Break down large molecules• Attack bacteria• Recycle damaged organelles• Eject wastes by exocytosis

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Proteasomes

– Hollow protein cylinders found throughout the cytoplasm

– Break down abnormal or misfolded proteins and normal proteins no longer needed by the cell (and that may cause disease)

– Break down protein molecules one at a time by tagging each one with a chain of ubiquitin molecules, unfolding the protein as it enters the proteasome, and then breaking apart peptide bonds

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Proteasomes

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Peroxisomes

• Peroxisomes– Smaller than lysosomes– Detoxify harmful substances in cell– Contain enzymes to break down fatty acids

and amino acids– Contain peroxidase and catalase– Hydrogen peroxide is a by-product of

breakdown

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Mitochondria• Major site of ATP

synthesis• Membranes

– Cristae: Infoldings of inner membrane

– Matrix: Substance located in space formed by inner membrane

• Mitochondria increase in number when cell energy requirements increase.

• Mitochondria contain DNA that codes for some of the proteins needed for mitochondria production.

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Nucleus- The cell’s control center

– Contains genetic material (DNA)

– Three distinct regions:

1. Nuclear Membrane

2. Nucleoli

3. Chromatin

Nuclear Structure and Content

• Nuclear Membrane

- double membrane connected by nuclear pores

-encloses nucleoplasm• Nucleoli

- site of ribosome formation• Chromatin

-carries cells DNA

-condenses into Chromosomes during cell division

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Cytoskeleton• Supports the cell but has to allow for

movements like changes in cell shape and movements of cilia

• Cell fibers:– Microfilaments: “cellular mm”

• Made of thin, twisted strands of protein molecules that lie parallel to the long axis of the cell

• Can slide past each other and cause shortening of the cell

• Actin and myosin are subcategories

– Microtubules: hollow, made of tubulin.

• Internal scaffold, transport, cell division, move things around cell

– Intermediate filaments: mechanical strength

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Cell Fibers

Centrosome

An area of the cytoplasm near the nucleus that coordinates the building and breaking apart of microtubules in the cell

Nonmembranous structure also called the microtubule organizing center

Center of microtubule formation

Before cell division, centrioles divide, move to ends of cell and organize spindle fibers– General location of the centrosome is

identified by the centrioles

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Cell ExtensionsCilia• Appendages projecting from cell

surfaces• Capable of movement• Moves materials over the cell surface• Shorter and more numerous than flagella• Coordinated oar like movements• Respiratory tract

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Flagella

• Similar to cilia but longer

• Usually only one per cell

• Move the cell itself in wave-like fashion

• Human Example: sperm cell

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Microvilli

• Extension of plasma membrane• Line intestines• Increase the cell surface area for absorption• Normally many on each cell• One tenth to one twentieth size of cilia• Do not move

Cell Connections

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Membrane junctions 1. Tight junctions – Adjacent plasma

membranes fused together tightly

2. Desmosomes – Anchoring junctions that prevent cells subjected to mechanical stress from being pulled apart

3. Gap junctions - Allow communication between cells