Chapter 5- Cells PDF

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Summary

Jennifer Abruzzes class
Biology 1101
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Description

Transport across membranes  Small Materials- 5 forms o Passive Transport (3)  Simple Diffusion  Small lipid like molecules can cross cell membranes by simple diffusion  Small uncharged, small hydrophobic, neutral, lipid like -- all the same describers for the same thing.  Yes, it can pass: cholesterol-based steroids  No, it cannot pass: sugars and amino acids  Osmosis  Diffusion of solvent  Water is small but polar. It may cross the membrane but the presence of aquaporin's can significantly increase the rate of transport  Have proteins that give water a path to cross through membrane faster.  Cell membranes restrict the movement of many solutes so water molecules move down their gradient instead.  Movement of Water  Phrase: water follows the solutes  Hypertonic: higher concentration of materials.  Solution around cell has greater concentration of dissolved materials/environment  Dissolved material will NOT cross membrane  Water flows to the area with a higher concentration  Isotonic: same concentration of materials  Solution has the same concentration inside and outside the cell  NO GRADIENT  Hypotonic: low concentration of materials  Low concentration inside the cell  Water flows into the cell  Cells can burst.  Facilitated Diffusion  No energy is required to move the substance across membrane  Net movement is determined by gradient  Channels: gated  Ions or small molecules through a protein pore..  Ligand: chemical signal  Voltage: can sense change in charges  Has to have a protein present  Transporters/Carriers.  No energy  Binding, confrontational change and then release..  Determined by solute, number and direction.  Has a pocket for chemicals to sit in  2 forms: Inside or outside facing pockets that can change sides  Protein creates a path but still a form of diffusion  Uniporter: one solute

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 Symporter: two chemicals moving in the same direction  Antiporter: one chemical in and one out Active Transport  Requires cells to use energy  One substance will move up or against the gradient.  Mediated by proteins(pump and/or transporter)  Primary Transport  Direct hydrolysis of ATP  Protein will set in pocket until ATP interacts with the pocket..  Secondary transport  Uses pre-existing gradient to drive transport  Energy already used to build gradient.  Wont change shape until a secondary chemical interacts. Large Materials- 2 Forms Mediated by Membrane Vesicles  Endocytosis: material released to outside environment  Endocytosis: Material brought in to cell Phagocytosis: cellular eating pinocytosis : cellular drinking Receptor mediated endocytosic: a receptor is when you need a chemical bound and the cells bring in the receptor and chemical. Can unbind from chemical later. Movement of Water Phrase: water follows the solutes Hypertonic: higher concentration of materials. Isotonic: same concentration of materials Hypotonic: low concentration of materials

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5.1 MEMBRANE STRUCTURE 

Structure determines function

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Two primary components of membranes are phosoplipids and proteins 

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Carbohydrates are a third component Phosoplipid bilayer

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framework of a membrane

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2 layers of phospholipids

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amphipathic molecules

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membranes are a “mosaic” of lipid protein and carbohydrate molecules

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fluid-mosaic model

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proposed by S. Jonathan Singer and Garth Nicolson in 1972 membrane exhibits properties that resemble a fluid because lipids and proteins can move relative to each other within a membrane.

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Leaflet 

half of a phosoplipid bilayer

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each leaflet faces a different region

Transmembrane Proteins: has one or more regions that are physically inserted into the hydrophobic interior of the phospholipid bilayer. 

Regions are stretches of non-polar amino acids that span or travers the membrane from one leaflet to another Most transmembrane segments are folded into a -helix

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Non polar amino acids interact favorable with hydrophobic lipid tails

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Cannot be released from membrane unless membrane is dissolved.

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Peripheral Membrane proteins  

Called extrinsic proteins Do not interact with hydrophobic interior but noncovalently bound to regions of integral membrane proteins that project out from the membrane 

Or polar head grounds of phosoplipids

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Typically attached to the membrane by ionic or hydrogen bonds

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5.2 FLUIDITY OF MEMBRANES 

fluidity: means individual molecules remain in close association yet have the ability to readily move within a membrane

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semifluid: membranes are more accurately described 

in a fluid substance, molecules can move in three dimensions

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lipid composition affects membrane fluidity 

length of phosoplipid tails 

range from 14-24 carbon atoms with 16-18 being the most common

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shorter tails are less likely to interact meaning its more fluid

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double bonds in phospholipid tails

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when double bonds are present the lipid is unsaturated which causes a kink in the tail and makes it hard for them to interact and making it more fluid.

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Cholesterol 

Present in animal cells 

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Phytosterls are similar for plants Tends to stabilize membranes but depends on temperature

At higher temps it makes it less fluid but at lower temps makes it more fluid

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Membrane must be at ideal fluidity. If it’s to fluid, it can become leaky and to solid the membrane proteins will be inhibited.

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1970 Larry Frye and Michael Edidin conducted an experiment that verified the later movement of transmembrane proteins.

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5.3 SYNTHESIS OF MEMBRANE COMPONENTS IN EUKARYOTIC CELLS 

cellular membranes composed of lipids, proteins and carbohydrates.

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Most components are made at the ER

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Cytosol and endomembrane system work together to synthesize most lipids.

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Lipids are transported via vesicles to Golgi, lysosomes, vacuoles, or plasma membrane.

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Lipid exchange proteins: extracts a lipid into another membrane, diffuse through the cell and insert the lipid into another membrane. Can be done between any 2 membranes, even between endomembrane system and semiautonomous systems.

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Glycosylation: process of covalently attaching a carbohydrate to a lipid or protein. 

When a carbohydrate is attached to a lipid it’s called a glycolipid

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When a carbohydrate is attached to a protein it’s called a glycoprotein.

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Glycolipids and glycoproteins are used in call surface recognition.

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Two forms of glycosylation in eukaryotes 

N-linked- also in archaea, involves attachment of a carbohydrate to the amino acid asparagine in a polypeptide. Carbohydrate is attached to a nitrogen atom on asparagine.

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O-Linked- occurs in Golgi only involves addition of a string of sugars to the oxygen atom of serine or threonine side chains. 

Important in animals for production of proteoglycans

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Helps organize the extra cellular matrix around cells.

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Also component of mucus

5.4 OVERVIEW OF MEMBRANE TRANSPORT 

membrane transport: movement of ions and molecules across biological membranes selectively permeability: allowing passage of some ions and molecules but not others.  Simple  : when a substance moves from a region of high concentration to a region of low

concentration.  2. Facilitated diffusion: a transport protein provides a passageway for a substance

to move across membrane.  3. Active transport: moves a substance from an area of low concentration to an

area of high concentration.  

Requires energy

Phosoplipids are barriers to ions and hydrophilic molecules 

Called solutes when they are suspended in water

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Called solution when they are dissolved in water. High Permeability

Moderate Low

Very Low

Gases

Water

ions

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urea

Small uncharged molecules

Polar organic molecules

Charged polar molecules and macromolecules

Transmembrane gradient or concentration gradient: concentration of a solute is higher on one side of a membrane Electrochemical gradient: dual gradient with both electoral and chemical components. 

Occurs when solutes have a positive or negative change

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Isotonic: when solutes are the same concentration inside and outside the cell

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Hypertonic: a higher concentration of solute on one side or the other of the cell.

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Hypotonic: lower concentration of solute on one side of the cell.

osmosis: if solutes cannot be readily moved across membrane, water will move to the hypotonic side.    

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when a cell ruptures: osmotic lysis when a cell dehydrates: crenation cell walls prevent ruptures plasmolysis: when water exits cell and plasma membrane pulls away from cell wall. 5.5 TRANSPORT PROTEINS Transport proteins: transmembrane proteins that provide passageways for movement of ions and hydrophilic molecules across bilayer.  Two categories: cannels and transporters Channel: transmembrane protein that forms an open passageway for the facilitated diffusion of ions or molecules.  most channels are gated  open to allow the diffusion of solutes and close to prohibit diffusion. Transporters (carriers): bind solutes in a hydrophilic pocket and undergoes a conformational change that switches pocket from one side to the other.  Provide the principal pathway for the uptake of organic molecules such as sugars, amino acids and nucleotides.  In animals they also allow cells to take in hormones and neurotransmitters.  Also play a key role in export  Waste products of cellular metabolism must be released before they reach toxic levels.  Ion transport- regulates pH  Named according to number of solutes they bind and direction they transport those solutes.  Uniporters: bind a single ion and transport across membrane  Symporters: bind two or more ions and transport molecules in the same direction.  Antiporters: bind two or more ions and transport molecules in opposite direction. Active transport is energetically unfavorable and requires an input of energy.

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Primary active transport: involves the function of a pump – a type of transporter that directly uses energy to transport a solute against a concentration gradient.  Secondary active transport: involves the use of pre-existing gradient to drive the active transport of another solute. Active transport was discovered in the 1940s Electrogenic pump: generates an electrical gradient. Important functions of Ion Electrochemical Gradients – ATP is commonly the source of energy.  Transport of ions and molecules  Production of energy intermediates  Osmotic regulation  Neuronal signaling  Muscle contraction  Bacterial swimming 5.6 EXOCYTOSIS AND ENDOCYTOSIS Exocytosis: materials inside the cell is packaged into vesicles and excreted into the extracellular environment.  Vesicles are usually from Golgi Endocytosis: the plasma membrane folds inwards to form a vesicle and bring substances into cell.  Three types:  Receptor-mediated endocytosis: a receptor in plasma membrane is specific for a given cargo. Cargo molecules bind to specific receptors and stimulate it causing the plasma membranes to form a vesicle.  Pinocytosis: (Greek meaning cell-drinking) a vesicle is formed from plasma membrane as a well for cells to internalize the extracellular fluid.  Important in cells that are active in nutrient absorption.  Phagocytosis: (Greek meaning cell-eating) a vesicle is formed from an enormous membrane called phagosome and it engulfs a large particle.  Only certain cells can carry out this process, such as macrophages (immune system of animals)...