Biology sem1- chap2

8/7/2019 Biology sem1- chap2

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LEARNING OUTCoMES

2.1 CELL MEMBRANEa) Describe structures and

function of cell wall

b) outlines of rolesof membrane within cells

*Description of Fluid Mosaic Model

*Roles of phospholipids and protein inmembrane

2.2 MOVEMENT OF MOLECULESa) Transport mechanisms

b) Explain passive transport

c) Explain endocytosis(pinocytosis& phagocytosis ) and exocytosis

*Included water potential concept

* Example : reabsoption of protein bythe proximal tubules


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Cell Wall


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Th e Structure of t h e Cell Wall

Differences, Plant and Animal CELLS -Presence of a CELL WALL IN PLANT

CELLS.Eg.Mushrooms /Yeast also have Cell Walls.Cell Walls of Fungi are made of CHITIN.

The cell wall is laid down during thedevelopment of cell and starts as a thinlayer of pectin, forms the middle lamella.


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Cellulose, secreted by the outer part of thecytoplasm, is laid down on the inside surface of the middle lamella.

This cellulose layer constitutes the primary wall.

Extra layers of cellulose are laid down on theinside surface of the primary wall, which form thesecondary wall-laid down after cell reached maxsize.



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Plasmodesmata and pits occur at intervals in the cellwall.

Plasmodesmata are fine cytoplasmic strands linking thecytoplasm of two neighbouring cells through fine pores inthe cell wall.

Pits arise in plases where the secondary wall is absent

altogether.

Where pits occur, two adjacent cells are separated bythe primary wall and middle lamella.



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F unctions of Cell Wall

Determines/maintains the shape of plantcells.

Prevent the cell membrane frombursting.Support, mechanical strength.


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P lasma Membrane


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Th e Structure of t h e Cell

MembraneThe cell membrane (or plasma membrane) surrounds allliving cells, and is the cell's most important organelle.

The membranes that surround the nucleus and other organelles are almost identical to the cell membrane.

The Cell Membrane CONTROLS the ease with whichsubstances pass into and out of the cell-some

substances easily cross the membrane, while otherscannot cross at all. For this reason, the Cell Membraneis said to be SELEC TIVELY P ERMEABLE .


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Membranes are composed of phospholipids, proteins and carbohydratesarranged in a fluid mosaic structure, asshown in this diagram.


Membrane


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Membrane


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P h osp h olipidsThe phospholipids form a thin, flexiblesheet, while the proteins "float" in thephospholipid sheet like icebergs, and thecarbohydrates extend out from theproteins.


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Th e p h osp h olipids are arranged in a bilayer, with their polar ,hy drop h ilic p h osp h ate h eads facing outwards ,

and their non-polar, hy drop h obic fatt y acid tails facing eac h ot h er in the middle of the bilayer.

Cell Membranes are made mostly of P HOS P HOLI P IDMOLECULES . (Phosphate + Lipid).

Lipid is a simple form of FAT.

Phospholipids are a kind of Lipid that consists of TWO FATTY ACIDS (TAILS), and PHOSPHATE GROUP (HEADS).

A Phospholipid Molecule has a POLAR "Head" and TwoNONPOLAR "Tails".


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POLAR - The two ends of the Phospholipid Molecule have differentproperties in Water.

The Phosphate Head is HYDRO P HILIC meaning "WA TERLOVING" . Because of its hydrophilic nature, the head of aPhospholipid will orient itself so that it is as close as possible towater molecules.

The Lipid Tails are HYDRO P HOBIC meaning "WA TER- F EARING" ,the Hydrophobic tails will tend to orient themselves away from water.

This hydrophobic layer acts as a barrier to all but the smallestmolecules, effectively isolating the two sides of the membrane.

Different kinds of membranes can contain phospholipids withdifferent fatty acids, affecting the strength and flexibility of themembrane, and animal cell membranes also contain cholesterollinking the fatty acids together and so stabilising and strengtheningthe membrane.


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P rotein A Variety of PROTEIN MOLECULES are EMBEDDED inthe Lipid Bilayer.

Some Proteins are Attached to the surface of the cellmembrane, these are called P ERI P HERAL P RO TEINS ,and are located on both the Internal and External

Surface.

The Proteins that are Embedded in the Lipid Bilayer arecalled INTEGRAL P RO TEINS .

Some Integral Proteins extend across the entire CellMembrane and are exposed to both the inside of the celland the exterior environment. Others extend only to theinside or only to the exterior surface.


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There are many kinds of Proteins in membranes; they

HELP to MOVE Material INTO and OUT of the Cell.

Some Integral Proteins form Ch annels or P ores throughwhich certain substances can pass.

Other Proteins (carrier protein) will have a specificbinding site for the substance it transports.

Integral Proteins exposed to the Cell's External

environment often have Carbohydrates attached to themserve as identification badges that allow cells torecognize each other and may act as Site where virusesor chemical messengers such as hormones can attach.


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Carbo hy dratesTh e carbo hy drates are attached to the membrane

proteins or sometimes to the phospholipids.Proteins with carbohydrates attached are calledglycoproteins,

While phospholipids with carbohydrates attached arecalled glycolipids.

The carbohydrates are short polysaccharides composedof a variety of different monosaccharides, and form a cell

coat or glycocalyx outside the cell membrane.The glycocalyx is involved in protection and cellrecognition, and antigens such as the ABO antigens onblood cells are usually cell-surface glycoproteins.


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Ch olesterolCell membranes also contain cholesterol,whose molecules are found in between thephospholipids molecules.

Important for increasing the fluidity of cell

membranes.


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F LUID MOSAIC MODEL O F CELL

MEMBRANESThe Lipids and Proteins of the Cell Membrane arealways in motion.

Phospholipids are able to drift across the membrane,changing places with their neighbor.

Proteins in and on the membrane Form PATTERNS, or MOSAICS.

Because the Membrane is FLUID with a MOSAIC of Proteins, scientists call the modern view of MembraneStructure THE F LUID MOSAIC MODEL.

The Pattern, or "Mosaic" of Lipids and Proteins in theCell Membrane is Constantly Changing.


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F unction of cell membranes

Forms boundary between the contents of thecell and its external environment.

Regulates the passage of substances into andout of the cell (certain substances).

Enables the cell to communicate with theexternal environment (receptor proteins thatreceive chemical messenger molecules e.ghormones from other cells).


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Helps cell to maintain structural

relationships with neighbouring cells.

Enable separate compartments to be

formed within a cell in which specificbiochemical pathways can occur.

Allow the attachment of enzymes.


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Organelles


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Cytoplasm and t h e c ytosol

Everything between the cell membrane and thenucleus is the cells cytoplasm.

Cytoplasm consists of two main components: Cytosol Organelles

Cytosol is a jellylike mixture that consists mostlyof water, along with protein, carbohydrates, salt,minerals and organic molecules.


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Cytoplasm and t h e c ytosol

Suspended in the cytosol are tinyorganelles.

Organelles are structures that work likeminiature organs, they carry out specificfunction in the cell.

The organelles plus the cytosol makes upthe cytoplasm.


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Nucleus


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Th e Structure of nucleusTHE NUCLEUS (plural, Nuclei) is often the most ProminentStructure within a Eukaryotic Cell.

It maintains its shape with the help of a Protein skeleton known asthe NUCLEAR MA TRIX.

The Nucleus is the CONTROL CENTER (BRAIN) of the Cell.

Most Cells have a Single Nucleus some cells have more than one.

The nucleus is surrounded by a Double Layer Membrane called theNUCLEAR ENVELO P E.


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The Nuclear Envelope is covered with many small pores throughwhich PROTEINS and CHEMICAL MESSAGES from the Nucleuscan pass. Golf Ball like dimples (pores).

The Nucleus contains DNA, the HEREDITARY MATERIAL OF

CELLS.

The DNA is in the form of a long Strand called CHROMA TIN.

During Cell Division, Chromatin strands COIL and CONDENSESinto thick structures called CHROMOSOMES .

Th e Structure of nucleus


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The Chromosomes in the nucleus contain coded"BLUE P RINTS" that control all cellular activity.

Most Nuclei contain at least ONE NUCLEOLUS (plural,Nucleoli).

The NUCLEOLUS MAKES (synthesizes) RIBOSOMES,WHICH IN TURN, BUILD PROTEINS.

When a Cell prepares to Reproduce, the NUCLEOLUSDISAPPEARS.

Th e Structure of nucleus


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F unctions of nucleus

The nucleus contains the hereditarymaterial (chromosomes) of an organism.

The nucleus is essential for cell division.

Controls the activities of a cell byregulating protein synthesis.


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Mitoc h ondriaTh e Structure of mitoc h ondrion

MITOCHONDRIA (MET-oh-KAHN-dree-uh) are found scatteredthroughout the Cytosol, and are relatively Large Organelles.

Mitochondria are the sites of Chemical Reactions that transfer Energy from Organic Compounds to ATP. Energy contain in food isreleased. Converted to ATP. ATP is the molecule that most Cellsuse as their main Energy Currency.

THE " P OWERHOUSE" O F THE CELL.

Mitochondria are Usually more numerous in Cells that have a HighEnergy Requirement - Your muscle cells contain a large number of mitochondria.


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Mitochondria is surrounded by TWO Membranes.

The smooth outer membrane serves as a boundarybetween the mitochondria and the cytosol.

The inner membrane has many long folds, known asCRIS TAE (KRIS-tee). The Cristae greatly increases thesurface area of the inner membrane, providing morespace for the Chemical Reactions to occur.

Mitochondria have their own DNA, and new mitochondriaarise only when existing ones Grow and divide.


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Th e Structure of mitoc h ondrion


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F unctions of mitoc h ondrion

Involved in cellular respiration, which is aseries of biochemical reactions that resultin the formation of ATP.


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Th e Structure of Endoplasmicreticulum

ENDO P LASMIC RE TICULUM (ER) is a system of membranous tubules and sacs.

The ER functions Primarily as an Intracellular Highway, apath along which molecules move from one part of thecell to another.

The amount of ER inside a cell fluctuates, depending on

the Cell's Activity.

Poisons, waste, and other toxic chemicals are madeharmless.


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Th e Structure of Endoplasmicreticulum

ER is an extensive network of membranesthat connect the Nuclear Envelope to theCell Membrane.

ransports materials through the cell.

Can be ROUGH OR SMOO TH.


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F unctions of Endoplasmicreticulum

ROUGH ER is studded with RIBOSOMES and processes

PROTEINS to be exported from the cell.

SMOO TH ER IS NO T Covered with RIBOSOMES and processes

LIPIDS and CARBOHYDRATES. The Smooth ER is

involved in the synthesis of steroids in gland cells,the regulation of calcium levels in muscle cells, andthe breakdown of toxic substances by liver cells.


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Golgi bod y


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Th e Structure of Golgi bod yGOLGI BODY (GOHL-jee) is the Processing, Packagingand Secreting Organelle of the Cell.

The Golgi Apparatus is a system of membranes.

Made of Flattened SAC like Structures calledCIS TERNAE .

Cisternae are continually being formed at one end of theGolgi body (the forming face) and continually beingbroken down into vesicles at the other end (the maturingface).


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F unctions of Golgi bod y

It works Closely with the ER, the Golgi Apparatus modifies proteins for export bythe cell.

Golgi apparatus is also the site of producing vesicles called Lysosomes.


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Ribosomes


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Th e Structure of ribosomes

Unlike most other organelles, Ribosomes AreNot Surrounded by a membrane.

They are Most Numerous Organelles in almostall cells.

Some are free in the Cytoplasm;

others line the membranes of ROUGHENDOPLASMIC RETICULUM.


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F unctions of ribosomes

Ribosomes are the site of P RO TEINSYN THESIS (Production or Construction) in a cell.


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Lysosomes


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Th e Structure of L ysosomes

Lysosomes are small spherical organellesthat enclose hydrolytic (digestive)enzymes within a single membrane.

Lysosomes are vesicles that bud (breakoff) from the Golgi apparatus and thatcontain digestive enzymes.


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F unctions of L ysosomesLysosomes are the Site of Food Digestion in the Cell.They can break down large molecules such as proteins,nucleic acids, carbohydrates, and phospholipids.

In the liver, they break down glycogen to release glucoseinto the blood stream.

Some white blood cells use lysosomes to break down

bacteria.

Within a cell, lysosomes digest worn-out organelles in aprocess called Autop h ag y .


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F unctions of L ysosomesLysosomes are also responsible for breaking down cellswhen it is time for the cells to die - Apoptosis .

The digestion of damaged or extra cells by the enzymesof their own lysosomes is called Autol ysis .

Lysosomes play a very important in maintaining anorganism's health by destroying cells no longer

functioning properly.

Lysosomes are common in the Cells of Animals, Fungi,and Protists, But they are Rare in Plant Cells.


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Ch loroplasts


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Bounded by a double membrane.

The interior of the chloroplast is filled with a gel-likematrix called the stroma.

Each Chloroplast encloses a system of Flattened,Membranous Sacs called THYLAKOIDS .

It is in the Thylakoids that Photosynthesis occurs.

Thylakoids are stacked to form grana (sing: granum).

Stacks of grana are joined by intergranal lamellae .

Intergranal lamellae and grana contain photosyntheticpigments (chlorophyll).


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F unctions of c h loroplast

CHLORO P LAS T, an organelle that convertsSUNLIGHT, CARBON DIOXIDE, AND WATERINTO SUGARS.

This process is called P HO TOSYN THESIS .

Light-dependent reactions of photosynthesisoccur on the lamellae while the light-independent reactions occur in the stroma.


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VesiclesTh e Structure and F unctions of vesicles

VESICLES - membrane sacs

Cells contain several types of vesicles, which performvarious roles.

Vesicles are small, spherically shaped sacs that aresurrounded by a single membrane and that are classifiedby their contents.

Vesicles often migrate to and merge with the plasmamembrane to release their contents outside of the cell.


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P EROXISOMES

Peroxisomes are similar to lysosomes but

contain different enzymes and are not producedby the Golgi apparatus.

Peroxisomes are abundant in liver and kidney

cells, where they neutralize free radicals(oxygen ions that can damage cells) anddetoxify alcohol and other drugs.


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P EROXISOMES

They are named for the HydrogenP eroxide , H2O2, they produce whenbraking down alcohol and killing bacteria.

Peroxisomes also break down fatty acids,which the mitochondria can then use as anenergy source.


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CYTOSKELE TON


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Th e Structure of c ytoskeletonThe c ytoskeleton is made of various types of special proteins.

Microtubules are hollow tubes made of globular proteins. Most notably, they are found in cilia ,flagella , and centrioles .

The arrangement of microtubules in cilia andflagella consists of nine doublets around theedge and two single microtubules in the center,all running the length of the structure.


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Th e Structure of c ytoskeletonThis is referred to as the nine-plus-two formula.

In centrioles, microtubules are arranged in 9 sets of 3 each.

Animal cells typically have a pair of centrioleslocated just outside the nucleus and oriented atright angles to each other.

Microfilaments are also part of the cytoskeletonand are made of solid rods of globular proteins.


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MICRO TUBULES Microtubules are HALLOW TUBES like

plumbing pipes. They are the Largest Strandsof the Cytoskeleton.

Microtubules are made of a PROTEIN calledTUBULIN .

MICRO F ILAMEN TS MICROFILAMENTS are NOT HALLOW and

have a structure that resembles ROPE madeof TWO TWISTED CHAINS OF PROTEINcalled AC TIN.


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INTERMEDIA TE F ILAMEN TS

Intermediate filaments are rods thatanchor the nucleus and some other organelles to their place in the cell.


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CILIA AND F LAGELLA


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Cilia and Flagella are Hairlike Organelles that extendfrom the surface of the cell, where they assist inmovement.

Microtubules are sometimes bundled into structurescalled CILIA AND F LAGELLA .

CILIA ARE SHORT HAIRLIKE PROJECTIONS.

F LAGELLA ARE LONG WHIPLIKE PROJECTIONS.

The Cilia and Flagella of all Eukaryotes consist of ONEPAIR OF MICROTUBULES SURROUNDED BY NINEMORE PAIRS.

CILIA ARE O F TEN NUMEROUS.

F LAGELLA ARE O F TEN SINGULAR.


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Functions of cytoskeletonJust as your body depends on your skeleton tomaintain its shape and size, so a Cell needs structuresto maintain its shape and size.

In Animal Cells, an internal framework calledCYTOSKELE TON maintains the Shape of the Cell.

THE CYTOSKELETON MAINTAINS THE THREE-DIMENSIONAL STRUCTURE OF THE CELL,PARTICIPATES IN THE MOVEMENT OFORGANELLES WITHIN THE CYTOSOL, AND HELPSTHE CELL MOVE.


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Microtubules have THREE FUNCTIONS:

To maintain the shape of the cell and hold organellesin place.

To serve as tracks for organelles and molecules tomove along within the cell.

When the Cell is about to divide, two short cylindersof Microtubules at right angles known as Centriolescan be found situated in the cytoplasm near thenuclear envelope.

Centrioles organize the micortubules of thecytoskeleton during Cell Division in animal cells, plantcells lack centrioles.


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Function of Microfilaments

MICROFILAMENTS can CONTRACT,causing movement.

Muscle Cells have many microfilaments.

Intermediate filaments:maintain theinternal shape of the nucleus.


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Function of Cilia and flagella

Unicellular organisms such asParamecium and Euglena use Cilia andFlagella to move through water.

Sperm use flagella to swim to the egg.

In Humans, beating Cilia line parts of therespiratory system, moving dust particlesand bacteria away from the lungs.


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Movement of molecules / substances inand out of cell

y Cell membranes are a barrier to mostsubstances, and this property allowsmaterials to be concentrated inside cells,excluded from cells, or simply separatedfrom the outside environment.


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Obviously materials need to be able to

enter and leave cells, and there arevarious methods by which substances canmove across a cell membrane:

Passive Transport (simple diffusion, facilitateddiffusion and osmosis)

Active Transport

Osmosis Bulk transport (exocytosis and endocytosis)


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P assive transport

Passive transport is the transport of substances across a membrane by atrans-membrane protein molecule.

No energy required

Move due to gradient


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Simple Diffusion (or LipidDiffusion)


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A few substances can diffuse directly through the lipidbilayer part of the membrane.

The only substances that can do this are lipid-solublemolecules such as steroids, or very small molecules,such as H2O, O2 and CO2.

For these molecules the membrane is no barrier at all.

Since simple diffusion is (obviously) a passive diffusionprocess, no energy is involved and substances can onlymove down their concentration gradient.

Simple diffusion cannot be controlled by the cell, in thesense of being switched on or off.


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Two major groups of integral membraneproteins are involved in facilitated diffusion:

By carrier proteinBy channel protein


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F acilitated diffusion

l d d ff


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F acilitated diffusion via carrier proteins

A carrier protein will have a specific bindingsite for the substance it transports.

The substance will bind on the side where it ata high concentration and be released where itis at a low concentration.

Once they bind, the protein changes shapeand the molecules come off the binding siteon the other side of the membrane.

F l d d ff h l


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F acilitated diffusion via c h annelproteins / ion c h annels

Ion C h annels / c h annel proteins do not really bind thesolute, but are like hydrophilic pores through themembrane that open and allow certain types of solutes,usuall y inorganic ions, to pass t h roug h .

In general, channels are quite specific for the type of solute they will transport and transport through channelsis quite a bit faster than by carrier proteins.

Channel Proteins form a water-filled pore or channel inthe membrane. This allows charged substances (usuallyions) to diffuse across membranes.

F ili d diff i i h l


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F acilitated diffusion via c h annelproteins / ion c h annels

Most channels can be gated (opened or closed),allowing the cell to control the entry and exit of ions.

When the gate is open, the channel transports,and when the gate is closed, the channel isclosed.

thus of particular importance in the physiology of excitable cells like neurons and muscle cells.


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Definitions of osmosis

Osmosis is the net movement of water molecules from a region of h ig h er water concentration to a region of lower water concentration through a partially permeablemembrane.

Osmosis is the net movement of water molecules from a region of h ig h er water potential to a region of lower water potential through a partially permeablemembrane.


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Concept of osmotic pressure


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Osmotic pressure can be measured in non-livingsystem using as osmometer, which contains an

artificial partially permeable membrane.

The bottom of a filter funnel is covered with amembrane permeable to water but not toglucose.

The filter funnel is filled with a glucose solutionand immersed in pure water to the level of asolution.

Later; glucose solution is risen in the filter funnel,level of water in the outer container has fallen.


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Water moved from container into the filter funnelby osmosis.

A piston is used to exert pressure on an identicalglucose solution.

The osmotic pressure is t h e pressure t h atmust be applied to prevent entr y of water into t h e glucose solution.

The opposite force, th e solute potential, is t h etendenc y of t h e glucose solution to gainwater from pure water across membrane.

C f l i l ( i


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Concept of solute potential (osmoticpotential) ( s solute potential)

Solute potential is the potential of a solution to gainwater.

It is always negative and has a magnitude equal to theosmotic pressure of a solution.

The more concentrate a solution, the greater its osmoticpressure and the lower (more negative) its solutepotential.

Water moves from a region of higher (less negative)solute potential to one of lower (more negative) solutepotential.


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Th e addition of a Solute Decreases Water P otential.

Solute P otential (Osmotic P otential)represents t h e effects of solutes on Water P otential.

Solutes lower t h e water potential b y diluting

th e water molecules.


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Osmotic pressure

Solute potential

Dilute solution

Concentrated solution

Concentrated solution h as h ig h

osmotic pressure but low solutepotential

Dilute solution h as lowosmotic pressure but h ig h solute potential

Relationship between osmotic pressure and solute potential

Concept of water potential


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Concept of water potential( Water potentials - )

``Osmosis can be quantified using water potential, so wecan calculate which way water will move, and how fast.

Water potential ( , the Greek letter psi, pronounced"sy") is simply the effective concentration of water. It ismeasured in units of pressure (Pa, or usually kPa), andthe rule is that water always "falls" from a high to a lowwater potential (in other words it's a bit like gravitypotential or electrical potential).

100% pure water has = = 0, which is the highestpossible water potential, so all solutions have < 0, andyou cannot get > 0.


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The water potential of a solution is defined asthe tendency for water molecules to leave the

solution by osmosis.

The more concentrate a solution is, the lower isits water potential and the smaller the tendencyfor water to leave the solution.

In concentrate solution, most water molecules

are associated with solute molecules and aretherefore not free to move about.

P ure Water h as t h e Hig h est Water P otential w h ic h is set atZERO


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ZERO.

Pressure

The term Hydrostatic Pressure is used to indicate the pressurecomponent of Water Potential.

Positive Pressures increase the Water Potential.

Positive Pressure would bring Water Molecules closer together.

Negative Pressures reduce Water Potential.

They pull water molecules further apart.

Th is lowers t h e potential energ y of water.

Water Potential = Solute (Osmotic) Potential + Pressure Potential


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Osmosis in animal and plant cells

The concentration of the solution thatsurrounds a cell will affect the state of thecell, due to osmosis.

There are three possible concentrations of


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There are three possible concentrations of solution to consider:

Hypotonic Solutes in cell more than outside Outside solvent will flow into cell

Isotonic Solutes equal inside & out of cell

Hypertonic Solutes greater outside cell Fluid will flow out of cell


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Osmosis in animal cells


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Hypotonic solution

When a red blood cell is placed in a solution whosewater potential is higher than that of the cell(hy potonic solution ), there will be a net gain of water by endomosmosis , causing t h e cell swell .

If a large volume of water entres the cell, th e cellburst and the content are realeased.

Because animal cell lacks a cell wall to prevent thecell form bursting.


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Osmosis in plant cellsP lacing in a solution wit h a Hig h er water potential ( hy potonic)

Water will move through the plasma membrane andtonoplast, and into the vacuole by endosmosis.

The cell swells.

Does not burst because the cell wall is stretched anddevelops a tension, resisting further uptake of water intothe cell and therefore further expansion of the cell.


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Osmosis in plant cellsP lacing in a solution wit h a Hig h er water potential ( hy potonic)

When water enters a plant cell, the cell contents start to

push against the inside of the cell wall.

In turn, the cell wall pushes back on the expanding cell(pressure potential ) and it opposes the continueduptake of water into the cell by endosmosis.

The pressure potential reaches its maximum when thecell wall is stretched as much as it can be and the cellcannot take in any more water ( full turgor is achieved).


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Th e Sodium- P otassium P ump

The sodium-potassium pump uses activetransport to move 3 sodium ions to the outside of the cell for each 2 potassium ions that it movesin.

It is found in all human cells, especially nerveand muscle cells.

One third of the bodys energy expenditure isused to operate the sodium-potassium pump.


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Examples of Active Transport

Plants move minerals (inorganic ions) into their roots byactive transport.

The gills of marine fish have cells that can remove saltfrom the body by pumping it into the salt water.

The thyroid gland cells bring in iodine for use inproducing hormones.

Cells in the vertebrate kidney reabsorb sodium ions fromurine.


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Mec h anism of operation of t h e


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Three sodium ions enter the pump.

ATP bonds to the pump.

One phosphate bond in the ATP molecule breaks,releasing its energy to the pump protein. The pumpprotein changes shape, releasing the sodium ions tothe outside.

The two potassium binding sites are also exposed tothe outside, allowing two potassium ions to enter thepump.

Mec h anism of operation of t h eSodium- P otassium P ump


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B lk


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Bulk transport The processes described so far only apply to small

molecules.

Large molecules (such as proteins, polysaccharidesand nucleotides) and even whole cells are moved inand out of cells by using membrane vesicles.

Materials are released from cells by exocytosis or taken into cells by endocytosis. The processesdescribed so far only apply to small molecules.

Large molecules (such as proteins, polysaccharides andnucleotides) and even whole cells are moved in and out of cells by using membrane vesicles.Materials are released from cells by exocytosis or taken intocells by endocytosis.


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E d i


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Endoc ytosis is the transport of materials into acell.

Materials are enclosed by a fold of the cellmembrane, which then pinches shut to form aclosed vesicle.

There are two types of endocytosis: pinocytosis phagocytosis

Endoc ytosis

P i i


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P inoc ytosis

P i i


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When the materials and the vesicles aresmall (such as a protein molecule) theprocess is known as pinocytosis (celldrinking).

P inoc ytosis

P h t i


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P h agoc ytosis


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E t i


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Exoc ytosis

E t i


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Exoc ytosis is the transport of materialsout of a cell. It is the exact reverse of endocytosis.

Materials to be exported must first beenclosed in a membrane vesicle, usually

from the RER and Golgi Body.

Exoc ytosis

E t i


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Hormones and digestive enzymes aresecreted by exocytosis from the secretorycells of the intestine and endocrine glands.

Sometimes materials can pass straightthrough cells without ever making contactwith the cytoplasm by being taken in byendocytosis at one end of a cell andpassing out by exocytosis at the other end.

Exoc ytosis


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Biology sem1- chap2