Introduction to Cell Structure II Flashcards by Joel Glotfelty

Forms the interface between the cell and its environment and also functions as a permeability barrier

Plasma Membrane

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Consists of a lipid bi-layer with embedded proteins

Plasma Membrane

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Have very highly defined RERs and show many features characteristic of cells

Plasma cells

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Amphipathic molecules with polar hydrophilic groups and aliphatic side chains that are hydrophobic

Lipids

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The hydrophobic groups of the lipids are positioned to the interior of the

Bilayer

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There are several classes of lipids and sterols in the plasma membrane, including

-often asymmetrically distributed

Phospholipids, glycolipids, and cholesterol

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The fine structure of the plasma membrane is visible only with the

Electron microscope

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The cell-cell boundaries seen at the light microscopic level consists of both plasma membranes of adjacent cells with the

Extracellular matrix between them

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There are also proteins in the plasma membrane, many of which are

Glycosylated

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What are the two classes of plasma membrane proteins?

) Integral membrane proteins

2. ) Peripheral membrane proteins

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Traverse the entire width of the cell membrane and have three domains: Cytoplasmic, membrane spanning, and external

Integral membrane proteins

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Attached to the cytoplasmic/extracellular regions of the lipid bilayer non-covalently

Peripheral membrane proteins

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Cell membranes are fluid structures. Many, but not all proteins and lipids can move through the membrane in which way?

Laterally

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Distribution of proteins in cell membranes can be studied by

Freeze-fracture electron microscopy

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A frozen piece of membrane is split mechanically between the outer and inner lipid layers

Freeze-fracture EM

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The outter face is called the

E (external face)

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The inner leaflet is called the

P (protoplasmic or cytoplasmic) face

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More integral membrane proteins are associated with the P-face because the P-face is stabilized by attachment to the

Cytoskeleton

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Plasma membrane proteins can be stabilized by

-Best studied in RBCs

Cytoskeletal proteins

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An inherited disorder characterized by defects in the membranes of erythrocytes. RBCs are more spherical, less deformable, and more susceptible to destruction by phagocytes in the spleen

Hereditary spherocytosis

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Hereditary Spherocytosis is caused by a mutation in the cytoskeletal protein

Spectrin

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Cytoskeletal proteins that associate with several membrane proteins to stabilize the biconcave shape of normal erythrocytes

Spectrins

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The study of hereditary spherocytosis shows that interactions between the cell membrane and intracellular cytoskeletal proteins are needed to maintain normal

RBC morphology

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Serve as linker/anchor proteins, transporters, receptors, and enzymes

Membrane proteins

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The tall columnar cells making up the epithelium of the small intestine that functions in the absorption of nutrients

Enterocytes

Enterocytes have modifications to their surface to increase the surface area available for absorption. These modifications of the outer (apical or lumenal) surface are called the

Brush border (when viewed in LM) or Microvilli (EM)

Many cells exhibit microvilli at their surface, but few cells have them organized as a dense

Brush border

EM reveals that the brush border of enterocytes consists of envaginations of the apical cell membrane that are called

Microvilli

Extend as minute finger-like projections from the top (apical surface) of the cells and increase the absorptive surface

Microvilli

A "fuzzy coat" seen at the tips of the microvilli

Glycocalyx

An internal cytoskeleton of actin filaments supports the

Membrane Microvilli

Extends upward at the apical surface and encloses each microvilli

Plasma membrane

Within the core of each microvillus are filamentous structures composed primarily of the structural protein

Actin

These actin filaments are only visible by EM and extend downward to terminate in a filamentous complex at the top of the cells called the

Terminal Web (TW)

Each microvilli has a core of actin filaments and a glycocalyx at the

Surface

In intestinal epithelial cells, enzymes, especially those involved in the terminal stages of protein and carbohydrate digestion, are part of the

Glycocalyx

The glycocalyx consists of

Glycolipids, glycoproteins, and prteoglycans

In the core of each microvillus are actin, actin cross-linking proteins that stabilize the structure, and a motor protein that moves material along the actin filaments. What is the motor protein called?

Myosin

The terminal web at the base of each microvillus contains

Actin and intermediate filaments

Cells interact via several modifications of their lateral borders that together form a

Junctional Complex (JC)

Also contribute to junctional complexes that mediate interactions between epithelial cells

Membrane proteins

In LM, the locations of junctional complexes are identified by

Terminal bars (reddish dots at the tips of a cell junction)

Under EM, we see that cells maintain contact with each other by specializations of the lateral surface. These include

1. ) Tight or occluding junctions (Zona occludens) 2. ) Belt desmosomes (Zona adherens) 3. ) Spot desmosomes (Macula adherens 4. ) Communicating (gap) junctions

The zone of a junctional complex closes to the surface of the epithelial cells is called the

Tight (occluding) Junction (Zona Occludens)

For most cells (except RBCs) there are what two major cellular compartments?

1. ) Nucleus | 2. ) Cytoplasm

The cytoplasm has which two main components?

1. ) Cytosol | 2. ) Organelles

The cytoplasm outside of the organelles -contains many enzymes and reactions of intermediate metabolism

Cytosol

Free ribosomes/polysomes are located in the

Cytosol

Temporary constituents without membranes including glycogen granules and lipid droplets, which are located in the cytosol

Inclusions

The cytosol also contains a structure made up of actin, intermediate filaments, and microtubules, known as the

Cytoskeleton

Localized in bundles of microfilaments that extend throughout the cell and are visualized through IHC -also located in muscle cells and microvilli

Actin

Small cylindrical structures that are widely distributed through the cell and are composed of the protein tubulin -component of the cytoskeleton

Microtubules

Microtubules not only extend through the cytoplasm, but also contribute to

Centrioles, Cilia, and Mitotic Spindles

Complex membrane-bound structures within cells -includes the ER, Golgi, mitochondria, lysosomes, and peroxisomes

Organelles

An extensive cytoplasmic membrane system containing sites of protein and lipid synthesis and calcium storage

Endoplasmic reticulum

Consists of an extensive array of membrane tubules and their enclosed cisterns or spaces -Does not have ribosomes

Smooth ER

Enzymes in the smooth ER function in

1. ) Lipid and steroid synthesis 2. ) Detoxification 3. ) Drug metabolism 4. ) Calcium ion storage

A cell with an extensive SER and a lot of lipid droplets is likely making

Steroid hormones

The outer nuclear membrane contains ribosomes and is continuous with the

Rough ER

Basically a ribosome studded membrane-bounded space

Rough ER

The rRNA of ribosomes contributes to the basophilic staining properties of -Readily observed via LM

Secretory Cells (basophilia)

The Rough ER is abundant in cells synthesizing

Secretory proteins (i.e. plasma cells)

A few other proteins synthesized by the RER are not secreted by rather are retained in the cell, including

Lysosomal proteins and membrane proteins

Function in the synthesis, packaging, and secretion of digestive enzymes, and have linear arrays of rough ER that function in secretory protein synthesis

Pancreatic Acinar cells

Proteins synthesized in the rough ER are then transported to the

Golgi Complex

Contains several individual subdivisions composed of flattened sacs of membrane -made up of cis and trans faces

Golgi Apparatus

Closest to the rough ER

cis-Golgi network (forming face)

Furthest from the rough ER

trans-Golgi network (maturing face)

What is between the cis- and trans- golgi networks?

cis-, medial, and trans-cisternae

Used to transport material between the Golgi cisternae

Vesicles

Bud from the trans Golgi network

Secretory Vesicles

Proteins enter the Golgi at the

cis-face

Associated with the cis-Golgi is a series of tubules (the cis-Golgi network), which transport vesicles carrying newly synthesized protein from the

Rough ER

The trans-Golgi is the maturing face, directed toward the secretory granules and the apical or peripheral cytoplasm. The trans-Golgi is where proteins

Leave for sorting

In some cells, proteins will accumulate in the Golgi and be sorted prior to secretion. This occurs in

Secretory Granules

What are the two mechanisms by which release of granule contents from the cell (exocytosis) is regulated?

Constitutive secretion or Regulated secretion

Proteins are packaged into secretory granules for export from the cell in the

trans-Golgi

The continual secretion of proteins that are not stored

Constitutive secretion

What are two examples of cells that perform constitutive secretion?

Plasma cells and fibroblasts

Continually secrete immunoglobins for the entire two weeks of their lives

Plasma cells

Continually secrete ECM components

Fibroblasts

Few, if any, secretory granules accumulate in

Constitutive Secretion

A large number of granules accumulate in many secretory cells that undergo

Regulated Secretion

During a meal, the exocrine pancreas receives a signal from cholecystokinin (CCK) and the acinar cells secrete digestive enzymes into the duct system for transport to the duodenum. This is an example of

Regulated Secretion

In addition to exocytosis (secretion), cells can also take in material in a process called

Endocytosis

Receptors in the cell membrane (usually integral membrane proteins) can bind to signaling molecules or nutrients. The receptor-ligand complexes can then enter the cell via vesicle formation that is initiated when coat proteins associate with the

Cytoplasmic lipid layer

The ligand-receptor complexes and the coat proteins then form a -rapidly detaches from the surface to form a coated vesicle

Coated pit

This process of endocytosis is termed

Receptor-mediated endocytosis

The internalized vesicles then fuse with the

Early Endosomes

Distinct organelles consisting of vesicles containing material that has entered the cell

Endosomes

Receptors and ligands are separated, and the receptors are often recycled, in

Early endosomes

Route nutrients such as cholesterol to the lysosome where nutrients can be made available to the cell

Late endosomes

A cell that is undergoing regulatory cell secretion is characterized by well defined

Rough ER and Secretory Granules

A cell that is undergoing constitutive cell secretion is characterized by well defined

Roguh ER but NO secretory Granules

Mitochondria can move through the cytoplasm with the aid of

Cytoskeletal proteins

On the cristae of the inner mitochondrial membrane are spherical protein particles which couple oxidation to phosphorylation and

ATP synthesis

Mitochondria also participate actively in

Apoptosis

The numbers of mitochondrial cristae vary in different cells depending on

Energy needs (more energy = more cristae)

The liver is contains dark clumps in EM called

Glycogen granules

Mitochondria in adrenal cells are different because their cristae are

Tubular

Mitochondrial morphology can vary between

Cell types

Mitochondrial DNA does encode a few peptides, however, most mitochondrial proteins are encoded by -enter from the cytosol, not from the rough ER and Golgi pathway

Nuclear DNA

Membrane bound packets of hydrolytic enzymes that become active at acidic pH and are the sites of intracellular digestion

Lysosomes

An ATP-driven proton pump in the lysosomal membrane maintains the

Lysosomes process molecules that enter the cell from the -Ex: cholesterol, antigens

External environment

Also function in the removal of worn-out intracellular organelles (mitochondria, RER) as well as entire cells (erythrocytes and bacteria) following phagocytosis

Lysosomes

Which dye will allow you to see lysosomes?

Trypan Blue

Lysosomes that are in eosinophils can be detected in EM because they have a

Dark line down the center

Membrane bound organelles involved in hydrogen peroxide metabolism and multiple other important metabolic processes

Peroxisomes

Because of high enzyme concentrations, peroxisomes usually have a dense material in their matrix that forms a

Crystalline core