Celula (Eng)

7/29/2019 Celula (Eng)

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Cytology, the study of the structure and

function of cells

The human body contains both somatic andsex cells

Histology is the science thatstudies the microscopic structure of

normal tissues.


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Cells are the building blocks of all plantsand animals

Cells are produced by the division of

preexisting cells Cells are the smallest units that perform all

vital physiological functions

Each cell maintains homeostasis at thecellular level

Homeostasis at higher levels reflects

combined, coordinated action of many cells

The cell theory states:


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Figure 3.1

The Diversity of Cells in the Human Body


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General Subdivisions of a CellA. Nucleus(regulatory

center of thecell)

B. CellMembrane(selectivelypermeableboundary

betweenthe cell andtheenviron-

ment)

C. Cytoplasm

(everything

between thecell membrane

and the

nuclear

com artment Organelles are individualcompartments in the cytoplasm


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Cell Membrane Cell membrane components

phospholipid bilayer

transmembrane (integral) and peripheral proteins

interior protein network elements of the cytoskeleton

cell surface markers glycocalyx (proteoglycans,

glycolipids, glycoproteins)


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Fluid-mosaic model of membrane

structure


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Plasma or Cell Membrane

OUTSIDE

INSIDE

LIPID BILAYER


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Cytoplasm

Membrane Proteins

Cholesterol

Outer

Surface

Fluid Mosaic Model

of Membrane Structure


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Functions of the cell membrane

Selectively isolates the cells contentsfrom the external environment andserves as a barrier

Regulate the exchange of substancesbetween the inside and outside of the

cell transport function

Receptor function


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The phospholipid bilayer is the fluid

portion of the membrane

Double layer

Polar head group: hydrophilic exterior

Non-polar hydrocarbon tails: hydrophobic interior


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Cholesterol molecules are part of

the lipid bilayer

Adds strengthAdds flexibility

Affects fluidity


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Classified by position:

Integral proteins

Peripheral proteins

Membrane proteins:

Classified by function:

Anchoring proteins

Recognition proteins Receptor proteins

Carrier proteins

Channels


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Membrane proteins


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Integral and peripheral proteins


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Types of membrane proteins

functional classification Transport proteins

For passage of materials through theplasma membrane Channel vs. carrier proteins

Receptor proteins Bind molecules and trigger cellular

responses Example: hormones

Recognition proteins Self vs. non-self (glycoprotein-based)

recognition

Markers during development


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2. Movement of substances across

membranes

Definitions

ConcentrationNumber of molecules in a given volume

GradientDifferences in concentration between two regions of

space. This causes molecules to move from one region to the

other (if no barrier to movement)

DiffusionNet movement of molecules from regions of high

concentration to regions of low concentration Considered as movement down its concentration gradient


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Diffusion of Dye in Water

Time 0Steep

Concentration

Gradient

Time 1Reduced

Concentration

Gradient

Dispersing

Time 2No

Concentration

Gradient

Random

Dispersal


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Passive and active transport

Passive transport

Movement of molecules down their

concentration gradients

Requires no net energy expenditure

The gradients themselves provide energy

Active transport

Movement of molecules against theirconcentration gradients

Requires energy!


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Passive transport

1. Simple diffusion

2. Facilitated diffusion

3. Osmosis4. Filtration

Remember that no energy is required, andmolecules move down their concentration

gradients


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Passive transport

1. Simple diffusion Molecules simply cross cell membrane on their own,

down their concentration gradients

Possible only for molecules that can cross the lipidbilayer on their own Lipid-soluble molecules

Examples: ethyl alcohol, vitamin A, steroid hormones

Very small molecules Examples: water, carbon dioxide

Rate depends upon Concentration gradient

Size

Lipid solubility


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Passive transport

2. Facilitated diffusion

Molecules move down their concentration

gradients (as for simple diffusion), but

Transport proteins assist these molecules in

crossing the membrane

No net energy expenditure! (This is a type

of diffusion)Example: transport of glucose


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Passive transport:Facilitated diffusion via a channel


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Passive transport:Facilitated diffusion via a carrier protein

Carrier proteinhas binding sitefor molecule

Molecule entersbinding site

Carrier protein changesshape, transportingmolecule across membrane

Carrier protein resumesoriginal shape

(Inside Cell)

(OutsideCell)

DiffusionChannelProtein

Diffu

sio

n

Gradient

Molecule inTransit


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Facilitated diffusion ispassive diffusion withthe help of transport

proteins


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Passive transport

3. Osmosis Movement of water from a high [water] to an

area of low [water] concentration across a semi-permeable membrane

Note here that water can pass through, but glucosecannot


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The effects of osmosis

Compare solute and water concentrations

outside vs. inside the cell (sketches)


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H2O

H2O


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Active Transport

1. Movement via active transport proteins

(sodium-potassium pump)

Remember that energy is required, and

molecules are moved against their

concentration gradients


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Bulk (vesicular) transport Exocytosis - movement of materials out of the cell by fusion of

vesicles with the plasma membrane Example - export or removal of wastes in single-celled organisms

Example cells exporting proteins

Endocytosis Infolding of the plasma membrane to bring largematerials into the cell Pinocytosis, "cell drinking" extra cellular fluid and materials

suspended in it (water and solutes) are enclosed in invaginating vesicle Used in digestive tract

Receptor-mediated endocytosis more specific with receptor bindingto molecules, bringing them in and concentrating into a coated pit The way insulin gets into your cells.

Phagocytosis "cell eating" brings large materials into a cell bywrapping extensions of the plasma membrane around the materials and

fusing the extension together. How the human immune system ingests whole bacteria or one-celled

creatures eat

pseudopodia false feet plasma membrane extensions


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Bulk (vesicular) transport1. Endocytosis

Three types of endocytosis

Pinocytosis

cell drinking

Extracellular fluid taken in

Receptor-mediated endocytosis

Specific for particular molecules

Molecules bind to receptors.

Receptor-molecule complex taken in

Phagocytosis

Large particles engulfed


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Mmm...yummy bacteria!!

Help! Im to be broken

down to mere macro-molecules!!


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Bulk (vesicular) transport2. Exocytosis

3. Transcytosis

Transcytosis in endothelial cells of the


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Transcytosis in endothelial cells of the

capillaryCan see this phenomena

in continuous capillariesMuscle, connective tissue,

exocrine glands and nervous

tissue

Transport

macromolecules in both

directions.

Pinocytotic vesicles can

cross cell in about

2-3 minutes.

lumen


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Tight Junctions

Seal tissues andprevent leaks

Link epithelial cellstogether

Prevent things frommoving through theintercellular space

Restrict migration of

proteins andphospholipids


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Tight junctions

Extracellular surfaces of twoadjacent plasma membranes are

joined together so there is no

extracellular space between them

Occurs in a band around the entirecell


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Belt desmosome

Zonula adherens

Another belt

around the cell

Below the tight

junctions An anchorage

junction

Associated withactin filaments

Space between

membranescanbe seen


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Desmosomes

Like spot welds!

Dense plaques with

fibers attached-Anchor cells

together from one

side to the other.

These cells

withstand lots of

abuse!


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Spot Desmosomes

A region between two cells where

membranes are separated by 20nm

Dense accumulation of protein at the

cytoplasmic surface of the membrane


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Desmosomes, contd

Keratin fibers extend from

the cytoplasmic surface

to other side of cell tonext desmosome

Holds adjacent cells

together in areas ofstretching

skin, cardiac muscle


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Hemidesmosome

Assymetrical structures

A plate anchors the basal part of cell to

the basal lamina

This plate contains IFs called keratins or

tonofilaments

Membrane plaque linkinghemidesmosome to basal lamina via

anchoring filaments

Contributes to overall stability of


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Hemidesmosomes


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Gap junctions

Protein channels link the cytosols of

cells

Passage of small molecules and ions(Na+, K+)

Excludes large molecules

Transmits electrical activity betweencardiac and smooth muscle cells

Allows chemical messengers to cross from

one cell to another

Coordinates activities between cells


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Gap junction connexons

A connexon is a cylinder with a central

open pore

One gap junction connexon is made upof six connexins

The pore is a hydrophilic channel

between two cytoplasms Plasma membranes come within 2-4nm

of each other


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Gap junctions


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Put Them All Together


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Cellular Junctions

Occluding jxns

zonula adherens

macula adherens

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