Cell Structure and Function

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Cell Structure

•

In 1655, the English scientist Robert Hooke coined the

term “cellulae” for the small box-like structures he saw

while examining a thin slice of cork under a

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4

Basic Cell Structure

• All cells have the following basic structure:

All cells have the following basic structure:

• A thin, flexible

A thin, flexible

plasma membrane

surrounds the entire cell.

• The interior is filled with a semi-fluid

The interior is filled with a semi-fluid

material called the

cytoplasm

_cytoplasm

.

_.

• Also inside are specialized structures

Also inside are specialized structures

called organelles and the cell’s

genetic

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Prokaryotic Cells

•

Simplest organisms

–

Cytoplasm

is surrounded by plasma membrane and

encased in a rigid cell wall composed of peptidoglycan.

–

No distinct interior compartments

–

Some use flagellum for locomotion, threadlike structures

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8

Eukaryotic Cells

•

Characterized by compartmentalization by

an endomembrane system, and the

presence of membrane-bound organelles.

–

central vacuole

–

vesicles

–

chromosomes

–

cytoskeleton

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10 Membrane Function

Membrane Function

•

_{All cells are surrounded}

by a plasma membrane.

•

_{Cell membranes are}

composed of a lipid

bilayer with globular

proteins embedded in the

bilayer.

•

_{On the external surface,}

carbohydrate groups join

with lipids to form

glycolipids, and with

proteins to form

glycoproteins. These

function as cell identity

markers.

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11 Fluid Mosaic Model

Fluid Mosaic Model

•

_{In 1972, S. Singer and G. Nicolson proposed the Fluid}

Mosaic Model of membrane structure

Extracellular fluid

Carbohydrate Glycolipid

Transmembrane proteins Glycoprotein

Peripheral protein

Cholesterol

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12 Phospholipids

Phospholipids

• _Glycerol

_Glycerol

• _{Two fatty acids}

_{Two fatty acids}

• _{Phosphate group}

_{Phosphate group}

Hydrophilic heads

Hydrophobic tails

ECF WATER

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13 Phospholipid Bilayer

Phospholipid Bilayer

•

_{Mainly 2 layers of phospholipids; the non-polar tails}

point inward and the polar heads are on the surface.

•

_{Contains cholesterol in animal cells.}

•

_{Is fluid, allowing proteins to move around within the}

bilayer.

Polar

hydro-philic heads

Nonpolar hydro-phobic tails

Polar

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14 Steroid Cholesterol

Steroid Cholesterol

• _{Effects on membrane fluidity within}

_{Effects on membrane fluidity within}

the animal cell membrane

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15

Glycoprotein

Carbohydrate

Microfilaments

of cytoskeleton Cholesterol Peripheral

protein Integral_protein Glycolipid

Membrane Proteins

• _{A membrane is a collage of different proteins}

_{A membrane is a collage of different proteins}

embedded in the fluid matrix of the lipid bilayer

• _{Peripheral proteins are appendages loosely}

_{Peripheral proteins are appendages loosely}

bound to the surface of the membrane

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16 Integral proteins

Integral proteins

• _{Penetrate the hydrophobic core of the}

_{Penetrate the hydrophobic core of the}

lipid bilayer

• _{Are often transmembrane proteins,}

_{Are often transmembrane proteins,}

completely spanning the membrane

EXTRACELLULAR SIDE

N-terminus

C-terminus

 Helix

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17 Functions of Cell Membranes

Functions of Cell Membranes

• Regulate the passage of substance

_{Regulate the passage of substance}

into and out of cells and between cell

organelles and cytosol

• _{Detect chemical messengers arriving}

_{Detect chemical messengers arriving}

at the surface

• _{Link adjacent cells together by}

_{Link adjacent cells together by}

membrane junctions

• _{Anchor cells to the extracellular}

_{Anchor cells to the extracellular}

matrix

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18 6 Major Functions Of Membrane

6 Major Functions Of Membrane

Proteins

1. Transport. (left) A protein that spans the membrane may provide a hydrophilic channel across the

membrane that is selective for a particular solute.

(right) Other transport proteins shuttle a substance from one side to the other by changing shape. Some of these proteins hydrolyze ATP as an energy ssource to actively pump substances across the membrane

2. Enzymatic activity. A protein built into the membrane may be an enzyme with its active site exposed to

substances in the adjacent solution. In some cases, several enzymes in a membrane are organized as a team that carries out sequential steps of a metabolic pathway.

3. Signal transduction. A membrane protein may have a binding site with a specific shape that fits the shape of a chemical messenger, such as a hormone. The external messenger (signal) may cause a conformational change in the protein (receptor) that relays the message to the inside of the cell.

ATP

Enzymes

Signal

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Cell-cell recognition. Some glyco-proteins serve as identification tags that are specifically recognized by other cells.

Intercellular joining. Membrane proteins of adjacent cells may hook together in various kinds of junctions, such as gap junctions or tight junctions

Attachment to the cytoskeleton and extracellular matrix (ECM). Microfilaments or other elements of the

cytoskeleton may be bonded to membrane proteins, a function that helps maintain cell shape and stabilizes the location of certain membrane proteins. Proteins that adhere to the ECM can coordinate extracellular and intracellular changes

4.

5.

6.

Glyco-protein

6 Major Functions Of Membrane Proteins

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Outside

Plasma membrane

Inside

Transporter Cell surface

receptor Enzyme

Cell surface identity

marker Cell adhesion Attachment to thecytoskeleton

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21 Membrane Transport

Membrane Transport

• _{The plasma membrane is the boundary that}

_{The plasma membrane is the boundary that}

separates the living cell from its nonliving

surroundings

• _{In order to survive, A cell must exchange}

_{In order to survive, A cell must exchange}

materials with its surroundings, a process

controlled by the plasma membrane

• _{Materials must enter and leave the cell through}

_{Materials must enter and leave the cell through}

the plasma membrane.

• _{Membrane structure results in selective}

_{Membrane structure results in selective}

permeability, it allows some substances to cross

it more easily than others

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22 Membrane Transport

Membrane Transport

• _{The plasma membrane exhibits}

_{The plasma membrane exhibits}

selective permeability - It allows some

substances to cross it more easily

than others

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23 Passive Transport

Passive Transport

• _{Passive transport is diffusion of a}

_{Passive transport is diffusion of a}

substance across a membrane with

no energy investment

• _{4 types}

_{4 types}

• _{Simple diffusion}

_{Simple diffusion}

• _Dialysis

_Dialysis

• _Osmosis

_Osmosis

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24 Solutions and Transport

Solutions and Transport

• Solution – homogeneous mixture of

_{Solution – homogeneous mixture of}

two or more components

• _{Solvent – dissolving medium}

_{Solvent – dissolving medium}

• _{Solutes – components in smaller quantities}

_{Solutes – components in smaller quantities}

within a solution

• Intracellular fluid – nucleoplasm and

_{Intracellular fluid – nucleoplasm and}

cytosol

• Extracellular fluid

_{Extracellular fluid}

• _{Interstitial fluid – fluid on the exterior of the cell}

_{Interstitial fluid – fluid on the exterior of the cell}

within tissues

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25 Diffusion

Diffusion

• _{The net movement of a substance from an area of higher}_{The net movement of a substance from an area of higher}

concentration to an area of lower concentration - down a

concentration gradient

• _{Caused by the constant random motion of all atoms and molecules}_{Caused by the constant random motion of all atoms and molecules} • _{Movement of individual atoms & molecules is random, but each}_{Movement of individual atoms & molecules is random, but each}

substance moves down its own concentration gradient.

Lump of sugar

No net movement at equilibrium

Random movement leads to net movement down a

concentration gradient

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26 Diffusion Across a Membrane

Diffusion Across a Membrane

• _{The membrane has pores large enough for the molecules to pass}_{The membrane has pores large enough for the molecules to pass}

through.

• _{Random movement of the molecules will cause some to pass}_{Random movement of the molecules will cause some to pass}

through the pores; this will happen more often on the side with more

molecules. The dye diffuses from where it is more concentrated to

where it is less concentrated

• _{This leads to a dynamic equilibrium: The solute molecules continue}_{This leads to a dynamic equilibrium: The solute molecules continue}

to cross the membrane, but at equal rates in both directions.

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27 Diffusion Across a Membrane

Diffusion Across a Membrane

• _{Two different solutes are separated by a membrane that is}_{Two different solutes are separated by a membrane that is}

permeable to both

• _{Each solute diffuses down its own concentration gradient.}_{Each solute diffuses down its own concentration gradient.}

• _{There will be a net diffusion of the purple molecules toward the left,}_{There will be a net diffusion of the purple molecules toward the left,}

even though the total solute concentration was initially greater on

the left side

Net diffusion

Net diffusion Equilibrium

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28 The Permeability of the Lipid Bilayer

The Permeability of the Lipid Bilayer

• _{Permeability Factors}

_{Permeability Factors}

•

_{Lipid solubility}

•

_Size

•

_Charge

•

_{Presence of channels and transporters}

• _{Hydrophobic molecules are lipid soluble and can}

_{Hydrophobic molecules are lipid soluble and can}

pass through the membrane rapidly

• _{Polar molecules do not cross the membrane}

_{Polar molecules do not cross the membrane}

rapidly

• _{Transport proteins allow passage of hydrophilic}

_{Transport proteins allow passage of hydrophilic}

substances across the membrane

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29 Passive Transport Processes

Passive Transport Processes

• _{3 special types of diffusion}_{3 special types of diffusion}

that involve movement of

materials across a

semipermeable membrane

• _{Dialysis/selective diffusion}_{Dialysis/selective diffusion}

of solutes

• _{Lipid-soluble materials}_{Lipid-soluble materials} • _{Small molecules that}_{Small molecules that}

can pass through

membrane pores

unassisted

• _{Facilitated diffusion -}_{Facilitated diffusion -}

substances require a

protein carrier for passive

transport

• _{Osmosis – simple diffusion}_{Osmosis – simple diffusion}

of water

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30 Osmosis

Osmosis

• _{Diffusion of the solvent across a}

_{Diffusion of the solvent across a}

semipermeable membrane.

• _{In living systems the solvent is}

_{In living systems the solvent is}

always water, so biologists

generally define osmosis as the

diffusion of water across a

semipermeable membrane:

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Lower

concentration of solute (sugar)

Higher brane: sugar mole-cules cannot pass through pores, but water molecules can

More free water molecules (higher concentration)

Water molecules cluster around sugar molecules

Fewer free water molecules (lower concentration)

Water moves from an area of higher free water concentration to an area of lower free water concentration



Osmosis

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32 Osmotic Pressure

Osmotic Pressure

• _{Osmotic pressure of a solution is the}

_{Osmotic pressure of a solution is the}

pressure needed to keep it in

equilibrium with pure H20.

• _{The higher the concentration of}

_{The higher the concentration of}

solutes in a solution, the higher its

osmotic pressure.

• _{Tonicity is the ability of a solution to}

_{Tonicity is the ability of a solution to}

cause a cell to gain or lose water –

based on the concentration of solutes

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33 Tonicity

Tonicity

•

_{If 2 solutions have equal [solutes], they are called}

isotonic

•

_{If one has a higher [solute], and lower [solvent], is}

hypertonic

•

_{The one with a lower [solute], and higher [solvent], is}

hypotonic

Hypotonic solution Isotonic solution Hypertonic solution

H₂O H2O H2O H2O

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34 Water Balance In Cells With Walls

Water Balance In Cells With Walls

Plant cell. Plant cells are turgid (firm) and generally healthiest in a hypotonic environ-ment, where the uptake of water is eventually balanced by the elastic wall pushing back on the cell.

(b)

H₂O H₂O

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35 My definition of Osmosis

My definition of Osmosis

• _{Osmosis is the diffusion of water}

_{Osmosis is the diffusion of water}

across a semi-permeable membrane

from a hypotonic solution to a

hypertonic solution

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36 Facilitated Diffusion

Facilitated Diffusion

• _{Diffusion of solutes through a semipermeable membrane with the}_{Diffusion of solutes through a semipermeable membrane with the}

help of special transport proteins i.e. large polar molecules and ions

that cannot pass through phospholipid bilayer.

• _{Two types of transport proteins can help ions and large polar}_{Two types of transport proteins can help ions and large polar}

molecules diffuse through cell membranes:

• _{Channel proteins – provide a narrow channel for the substance to pass}_{Channel proteins – provide a narrow channel for the substance to pass}

through.

• _{Carrier proteins – physically bind to the substance on one side of}_{Carrier proteins – physically bind to the substance on one side of}

membrane and release it on the other.

EXTRACELLULAR FLUID

Channel protein Solute CYTOPLASM

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37 Facilitated Diffusion

Facilitated Diffusion

• _Specific

_Specific

_{– each channel or carrier}

transports certain ions or molecules

only

• _Passive

_Passive

_{– direction of net movement}

is always down the concentration

gradient

• _Saturates

_Saturates

_{– once all transport}

proteins are in use, rate of diffusion

cannot be increased further

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38 Active Transport

Active Transport

• _{Uses energy (from ATP) to move a}

_{Uses energy (from ATP) to move a}

substance against its natural tendency

e.g. up a concentration gradient.

• _{Requires the use of carrier proteins}

_{Requires the use of carrier proteins}

(transport proteins that physically bind to

the substance being transported).

• _{2 types:}

_{2 types:}

• _{Membrane pump (protein-mediated active}

_{Membrane pump (protein-mediated active}

transport)

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39 Membrane Pump

Membrane Pump

• _{A carrier protein uses energy from}

_{A carrier protein uses energy from}

ATP to move a substance across a

membrane, up its concentration

gradient:

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•

_{One type of active transport system}

The Sodium-potassium Pump

2. Na+ binding stimulates phosphorylation by ATP. 1. Cytoplasmic Na+_binds

to the sodium-potassium pump.

6. K+_{is released and Na}+

sites are receptive again; the cycle repeats.

3. Phosphorylation causes the protein to change its conformation, expelling Na+_{to the outside.}

4. Extracellular K+_{binds to the}

protein, triggering release of the Phosphate group.

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41 Coupled transport

Coupled transport

•

_{2 stages:}

• _{Carrier protein uses ATP to move a substance across the}_{Carrier protein uses ATP to move a substance across the}

membrane against its concentration gradient. Storing energy.

• _{Coupled transport protein allows the substance to move down its}_{Coupled transport protein allows the substance to move down its}

concentration gradient using the stored energy to move a

second substance up its concentration gradient:

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42 Review: Passive And Active Transport Compared

Review: Passive And Active Transport Compared

Passive transport. Substances diffuse spontaneously down their concentration gradients, crossing a

membrane with no expenditure of energy by the cell. The rate of diffusion can be greatly increased by transport proteins in the membrane.

Active transport. Some transport proteins act as pumps, moving substances across a membrane against their concentration gradients. Energy for this work is usually supplied by ATP.

Diffusion. Hydrophobic molecules and (at a slow rate) very small uncharged polar molecules can diffuse through the lipid bilayer.

Facilitated diffusion. Many hydrophilic substances diffuse through membranes with the assistance of transport proteins, either channel or carrier proteins.

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43 Bulk Transport

Bulk Transport

• _{Allows small particles, or groups of}

_{Allows small particles, or groups of}

molecules to enter or leave a cell

without actually passing through the

membrane.

• _{2 mechanisms of bulk transport:}

_{2 mechanisms of bulk transport:}

endocytosis and exocytosis.

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44 Endocytosis

Endocytosis

• _{The plasma membrane envelops}

_{The plasma membrane envelops}

small particles or fluid, then seals on

itself to form a vesicle or vacuole

which enters the cell:

• _Phagocytosis

_Phagocytosis

• _Pinocytosis

_Pinocytosis

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Three Types Of Endocytosis

EXTRACELLULAR

An amoeba engulfing a bacterium via phagocytosis (TEM).

PINOCYTOSIS

Pinocytosis vesicles forming (arrows) in a cell lining a small blood vessel (TEM).

0.5 µm In pinocytosis, the cell

“gulps” droplets of

extracellular fluid into tiny vesicles. It is not the fluid itself that is needed by the cell, but the molecules dissolved in the droplet. Because any and all

included solutes are taken into the cell, pinocytosis is nonspecific in the substances it transports.

Plasma membrane

Vesicle In phagocytosis, a cell

engulfs a particle by Wrapping pseudopodia around it and packaging it within a membrane-enclosed sac large enough to be classified as a vacuole. The

particle is digested after the vacuole fuses with a lysosome containing hydrolytic enzymes.

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46 Process of Phagocytosis

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47

A coated pit and a coated vesicle

enables the cell to acquire bulk quantities of specific substances, even though those substances may not be very concentrated in the extracellular fluid. Embedded in the membrane are proteins with specific receptor sites exposed to the extracellular fluid. The receptor proteins are usually already clustered in regions of the membrane called coated pits, which are lined on their cytoplasmic side by a fuzzy layer of coat proteins.

Extracellular substances (ligands) bind to these receptors. When binding occurs, the coated pit forms a vesicle containing the ligand molecules. Notice that there are

relatively more bound molecules (purple) inside the vesicle, other molecules

(green) are also present. After this ingested material is liberated from the vesicle, the receptors are recycled to the plasma membrane by the same vesicle.

Receptor-mediated Endocytosis

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48 Exocytosis

Exocytosis

• _{The reverse of endocytosis}

_{The reverse of endocytosis}

• _{During this process, the membrane of a vesicle}

_{During this process, the membrane of a vesicle}

fuses with the plasma membrane and its

contents are released outside the cell:

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Cell Junctions

•

Long-lasting or permanent connections between

adjacent cells, 3 types of cell junctions:

Tight junctions prevent fluid from moving across a layer of cells

Tight junction

0.5 µm

1 µm

Space between

cells Plasma membranes_{of adjacent cells}

Extracellular

At tight junctions, the membranes of neighboring cells are very tightly pressed against each other, bound together by specific proteins (purple). Forming continu-ous seals around the cells, tight junctions prevent leakage of extracellular fluid across A layer of epithelial cells.

Desmosomes (also called anchoring junctions) function like rivets, fastening cells Together into strong sheets. Intermediate Filaments made of sturdy keratin proteins Anchor desmosomes in the cytoplasm.

Gap junctions (also called communicating junctions) provide cytoplasmic channels from one cell to an adjacent cell. Gap junctions consist of special membrane proteins that surround a pore through which ions, sugars, amino acids, and other small molecules may pass. Gap junctions are necessary for commu-nication between cells in many types of tissues, including heart muscle and animal embryos.

TIGHT JUNCTIONS

DESMOSOMES

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Surface of nuclear envelope.

Pore complexes (TEM). Nuclear lamina (TEM). Close-up of

The Nucleus And The Nuclear Envelope

• Repository for genetic material called chromatin - DNA and proteins

• Nucleolus: holds chromatin and ribosomal subunits - region of intensive

ribosomal RNA synthesis

• Nuclear envelope: Surface of nucleus bound by two phospholipid bilayer

membranes - Double membrane with pores

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Chromosomes

•

DNA of eukaryotes is divided into linear

chromosomes.

–

Exist as strands of chromatin, except

during cell division

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Ribosomes

•

Ribosomes are RNA-protein complexes composed of two

subunits that join and attach to messenger RNA.

–

Site of protein synthesis

–

Assembled in nucleoli

ER

Ribosomes _Cytosol

Free ribosomes

Bound ribosomes

Large subunit

Small subunit

TEM showing ER and ribosomes Diagram of a ribosome

0.5 µm

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Endomembrane System

•

Compartmentalizes cell, channeling passage

of molecules through cell’s interior.

–

Endoplasmic reticulum



Rough ER - studded with ribosomes

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54

Rough ER

• Rough ER is especially abundant in cells that secrete proteins.

– As a polypeptide is synthesized on a ribosome attached to rough ER, it is threaded into the

cisternal space through a pore formed by a protein complex in the ER membrane.

– As it enters the cisternal space, the new protein folds into its native conformation.

– Most secretory polypeptides are glycoproteins, proteins to which a carbohydrate is attached.

– Secretory proteins are packaged in transport vesicles that carry them to their next stage.

• Rough ER is also a membrane factory.

– Membrane-bound proteins are synthesized directly into the membrane.

– Enzymes in the rough ER also synthesize phospholipids from precursors in the cytosol.

– As the ER membrane expands, membrane can be transferred as transport vesicles to other

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Smooth ER

• The smooth ER is rich in enzymes and plays a role in a variety of metabolic processes. • Enzymes of smooth ER synthesize lipids, including oils, phospholipids, and steroids. • These include the sex hormones of vertebrates and adrenal steroids.

• In the smooth ER of the liver, enzymes help detoxify poisons and drugs such as

alcohol and barbiturates.

• Smooth ER stores calcium ions.

 Muscle cells have a specialized smooth ER that pumps calcium ions from the cytosol and stores them in its cisternal space.

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The Golgi apparatus

• The Golgi apparatus is the shipping and receiving center for cell

products.

– Many transport vesicles from the ER travel to the Golgi apparatus for

modification of their contents.

– The Golgi is a center of manufacturing, warehousing, sorting, and

shipping.

– The Golgi apparatus consists of flattened membranous sacs—cisternae

—looking like a stack of pita bread.

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57

Functions Of The Golgi Apparatus

TEM of Golgi apparatus

cis face

(“receiving” side of Golgi apparatus)

Vesicles move from ER to Golgi Vesicles also

transport certain proteins back to ER

Vesicles coalesce to form new cis Golgi cisternae

Cisternal leave Golgi, carrying specific proteins to other locations or to the plasma mem-brane for secretion Vesicles transport specific

proteins backward to newer Golgi cisternae

Cisternae

trans face

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58

Membrane Bound Organelles

•

Lysosomes – vesicle

containing digestive

enzymes that break down

food/foreign particles

•

Vacuoles – food storage

and water regulation

•

Peroxisomes - contain

enzymes that catalyze the

removal of electrons and

associated hydrogen

atoms

(a) Phagocytosis: lysosome digesting food

1 µm

Lysosome contains active hydrolytic enzymes

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59

Mitochondria

• Sites of cellular respiration, ATP synthesis

• Bound by a double membrane surrounding fluid-filled matrix. • The inner membranes of mitochondria are cristae

• The matrix contains enzymes that break down carbohydrates and

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60

Cytoskeleton

•

The eukaryotic cytoskeleton is a network of

filaments and tubules that extends from the

nucleus to the plasma membrane that support

cell shape and anchor organelles.

•

Protein fibers

–

Actin filaments



cell movement

–

Intermediate filaments

–

Microtubules

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61

Centrioles

•

Centrioles

are short

cylinders with a 9 + 0

pattern of microtubule

triplets.

•

Centrioles may be

involved in microtubule

formation and

disassembly during cell

division and in the

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62

Cilia and Flagella

•

Contain specialized arrangements of microtubules

•

Are locomotor appendages of some cells

•

Cilia and flagella share a common ultrastructure

(a) Outer doublets cross-linking

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63

Cilia and Flagella

•

Cilia (small and numerous) and flagella (large and single)

have a 9 + 2 pattern of microtubules and are involved in

cell movement.

•

Cilia and flagella move when the microtubule doublets

slide past one another.

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64

(a) Motion of flagella. A flagellum usually undulates, its snakelike motion driving a cell in the same direction as the axis of the

flagellum. Propulsion of a human sperm cell is an example of flagellatelocomotion (LM).

1 µm Direction of swimming

Cilia and Flagella

(b) Motion of cilia. Cilia have a and-forth motion that moves the cell in a direction perpendicular to the axis of the cilium. A dense nap of cilia, beating at a rate of about 40 to 60 strokes a second, covers this Colpidium, a

freshwater protozoan (SEM).