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Cell. By Dr OJ Tsotetsi. CELL. Cells are the basic unit of life made from macromolecules . Cells serves as the structural building block to form tissues and organs         Each cell is functionally independent- it can live on its own under the right conditions.

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Cell

By

Dr OJ Tsotetsi


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CELL

Cells are the basic unit of life made from macromolecules.

Cells serves as the structural building block to form tissues and organs         Each cell is functionally independent- it can live on its own under the right conditions.

          1. it can define its boundaries and protect itself from external changes causing internal changes           2. it can use sugars to derive energy for different processes which keep it alive           3. it contains all the information required for replicating itself and interacting with other cells in order to produce a multicellular organism           4. It is even possible to reproduce the entire plant from almost any single cell of the plant


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  • Membrane Functions

  • A. Form selectively permeable barriersB. Transport phenomena

  • Passive diffusion

  • Active transport

  • C. Cell communication and signalingD. Cell-cell adhesion and cellular attachmentE. Cell identity and antigenicityF. Conductivity


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Types of cells

  • Prokaryotic cells- eg. bacteria             1. very simple-there are no organelles and most everything functions in the cytoplasm

  • Eukaryotic cells             1. all contain the organelles that subcompartmentalize the cell             2. includes unicellular algae and protists (e.g. ameba) that live alone or in colonies             3. includes multicellular organisms - animals, plants, fungi - where cells work together                 a. plant cells are unlike animal cells in that plant cells have chloroplasts and cell walls.                 Animal cells have neither of these. Plant cells also have relatively large vacuoles.


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Parts of the eukaryotic cell

A. Outside (boundary) of the cell             1. cell wall                 a. protects and supports cell                 b. made from carbohydrates- cellulose and pectin- polysaccharides                 c. strong but leaky- lets water and chemicals pass through- analogous to a cardboard box

Cell membrane made up of lipids, protein and to a less extend carbohydrates.

Major function is protection and support of the structure


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Fluid Mosaic Model of Membrane Structure

A. Mosaic: an object comprised of bits and pieces embedded in a supporting structure

1. membrane lipids form the supporting structure2. membrane proteins provide the bits and pieces3. both lipids and proteins may be mobile or 'fluid'

B. Membrane lipids: the supporting structure

1. phospholipids2. glycolipids3. cholesterol

C. Membrane proteins: the bits and pieces

1. integral (intrinsic) proteins2. peripheral (extrinsic) proteins


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The Membrane Lipids

A. phospholipids

1. most abundant of the lipids in membranes: form a lipid bilayer2. phospholipid composition

a. glycerol backbone covalently linked to:b. two long, non-polar fatty acid hydrocarbon chainsc. variable phosphate-containing polar group

3. phospholipids are amphiphilic (amphipathic) molecules:

a. hydrophobic ('water fearing') end: fatty acid chains

orient toward the interior of the membrane

b. hydrophilic ('water 'loving') end: phosphate group end

orients towards the extracellular space or cytoplasm


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some common membrane phospholipids

a. choline containing phospholipids

(1) phosphatidycholine(2) sphingomyelin

b. non-choline containing phospholipids

(1) phosphatidylserine(2) phosphatidylethanolamine(3) phosphatidylinositol

5. synthesis occurs in the membranes of the endoplasmic reticulum

All of the phospholipids are initially synthesized on the cytoplasmic side of the lipid bilayer. The phospholipids containing choline tend to get flipped to the opposite face of the lipid bilayer (which is topologically equivalent to the extracellular space) by enzymes known as 'flippases'. Once 'flipped', further flip-flopping is rare.


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Glycolipids

1. least common of the membrane lipids (~2%)2. always found in outer leaflet of plasma membrane*

3. general structure of a glycolipid is a variation on the phospholipid theme

a. two long hydrocarbon chains

(1) hydrophobic, non-polar part of molecule

b. carbohydrate component: one or more sugars

(2) hydrophilic, polar part of molecule

4. synthesis of glycolipids

a. starts in membranes of endoplasmic reticulumb. carbohydrates added in Golgi apparatus


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Cholesterol

1. steroid; lipid soluble; found in both leaflets of lipid bilayer2. amphiphilic: -OH group forms the polar end of the molecule3. synthesized in membranes of endoplasmic reticulum


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Summary


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Membrane Proteins

A. Integral (intrinsic) proteins

1. penetrate the bilayer or span the membrane entirely2. can only be removed from membranes by disrupting the phospholipid bilayer3. types:

covalently linked to membrane phospholipids or glycolipids


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a. transmembrane proteins

(1) single-pass(2) multiple-pass

Trans-membrane proteins have membrane spanning portions containing alpha helically arranged sequences of 20-25 hydrophobic amino acids. Short strings of hydrophilic amino acids separate the hydrophobic sequences from each other: These hydrophilic stretches tend to be found exposed to the more aqueous environments associated with the cytoplasm or the extracellular space.

b. covalently tethered integral membrane proteins

Tethered integral membrane proteins may be largely exposed to either the cytoplasm or aqueous extracellular space, but are


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4. many integral proteins are glycoproteins

a. covalently linked via asparagine, serine, or threonine to sugars

The sugars of glycoproteins are exclusively found on the extracellular side of the membrane or topological equivalent

5. synthesis of integral proteins:

a. occurs in the rough endoplasmic reticulumb. many integral proteins wind up as glycoproteins

(1) glycosylation begins in lumen of er(2) carbohydrates are modified in Golgi

6. integral proteins often form protein complexes having multiple subunits

7. functions

a. enzymaticb. receptorsc. transportd. communicatione. adhesion


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Peripheral (extrinsic) proteins

1. do not penetrate the phospholipid bilayer2. are not covalently linked to other membrane components3. form ionic links to membrane structures

a. can be dissociated from membranesb. dissociation does not disrupt membrane integrity

4. located on both extracellular and intracellular sides of the membrane

a. often link membrane to non-membrane structures

5. synthesis of peripheral proteins:

a. cytoplasmic (inner) side: made in cytoplasmb. extracellular (outer) side: made in er and exocytosed


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Membrane Dynamics

Lipid Asymmetry

1. phospholipids are asymmetrically distributed within the lipid bilayer

a. outer leaflet of plasma membrane

(1) phosphatidylcholine(2) sphingomyelin

b. inner leaflet of plasma membrane

(1) phosphatidylethanolamine(2) phosphatidylserine

flippase helps establish the phospholipid asymmetry

2. glycolipids exclusively found on outer half of membrane


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Lipid Mobility

mobility of membrane lipids:

a. rotational movementb. lateral movement

The lipids (especially the phospholipids) are mobile within their half of the lipid bilayer. Flip flop to the opposite side of the membrane is rare. Mobility of the phospholipids tends to increase as the number of double bonds ('kinks') between adjacent carbon atoms in the fatty acid chains increase.

2. cholesterol effects on membrane fluidity:

At high temperatures cholesterol tends to reduce membrane fluidity, probably by interacting with the hydrocarbon tails of the phospholipid and glycolipid molecules. At low temperatures cholesterol helps prevent membranes from freezing and thus tends to maintain membrane fluidity.


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Protein Asymmetry

1. many different kinds of proteins are in the cell membranes

a. each type has a unique conformation and orientationb. flip flop of proteins does not occurc. conformational changes of protein can occur

2. carbohydrates of glycoproteinsalways at outer surface

a. help form the 'glycocalyx' (along with glycolipids)


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Protein Mobility

rotational mobilitylateral diffusion

Protein mobility can vary greatly. Some proteins are free to move. Others may be tethered to structures in the cytoplasm or extracellular spaces, thus restricting their movement. Some types of cell junctions (e.g., tight junctions) can restrict protein movements to a specific membrane domain.


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Membrane Protein Conformational Changes

1. help explain many many membrane functions2. concept:

The binding of a membrane protein/glycoprotein to some other cellular or extracellular substance, molecule, ion, etc. can/will result in a 3-dimensional conformational change in that membrane protein.

That conformational change can/will, in turn, drive or inhibit some other cellular event.

Example 1: facilitated diffusion

Example 2: signaling


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Cytoplasm

1. cytosol - watery inside of cell composed of salts, proteins which act as enzymes  2. microtubules and microfilaments - cables made out of protein which stretch around the cell                 a. provide structure to the cell, like cables and posts on a suspension bridge                b. provide a structure for moving cell components around the cell -sort of like a moving conveyer belt.- 3. organelles - sub-compartments within the cell which provide different functions. Each organelle is surrounded by a membrane that makes it separate from the cytosol


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mitochondrion - Cell powerhouse. Converts compounds into energy through aerobic respiration. Also, plays a role in cellular response to toxic stimuli from the environment

golgi apparatus - membranous hollow sacs arranged in a stack          i. modifies proteins, lipids, and other substances from the ER         ii. packets of these materials move to the edge of the golgi where the golgi membrane is pinched off to make vesicle (package); this new vesicle moves to the plasma membrane where it leaves the cell, or it goes to other sites within the cell         iii. builds primary cell walls between newly divided nuclei

Lysosomes – proteins are synthesized by rough ER and transported to the golgi apparatus for where enzymes are packed into vesicles

Function: destruction of foreign substances


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chloroplast (50-100 per cell) - site of photosynthesis.           i. allows production of sugars from sunlight and carbon dioxide           ii. only found in plants and algae- other cells have to find sugar from outside the cell

vacuole or tonoplast- stores compounds that may interfere with other things in the cell.                 Dominates the inside of a plant cell.                    - sugars, salts, pigments (e.g. red pigment in beets and purple onions, acids (lemon acids)

Peroxisomes – are membrane – bound organelles containing enzymes that participate in oxidative reactions


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ribosomes - site of protein synthesis-

           i. many different proteins have to be made by the cell- the proteins that a cell makes directs the cell's function and identity              ii. ribosomes use the information coded in the DNA of the nucleus to produce proteins

endoplasmic reticulum (ER) - a network of folded membranes throughout the cytoplasm          i. rough ER has attached ribosomes, active in protein synthesis          ii. smooth ER lacks ribosomes and functions in the transport and packaging of proteins as well as the synthesis of lipids


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Inside the cell

Cell is made up of cytoplasm and the nucleus

The two are separated by the nuclear membrane


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NUCLEUS

Consist of chromosomes, chromatitin and nucleolus.

Surrounded by nuclear envelop which is porous and is a double membraned.

nucleus- contains the genetic information which tells the cell machinery which proteins, carbohydrates and lipids to make and how they are assembled.                i. this genetic information is coded in DNA (deoxyribonucleic acid)


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What is a cell made from?

  • Four groups of biologically important molecules:

  • lipids

  • carbohydrates

  • nucleic acids

  • proteins

  • Water


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Lipids

Composed of Carbon, Oxygen, Hydrogen atoms (COH) in building blocks of Fatty acids

Fats (solid) and oils (liquid at room temperature)            1. fats associated with animals - butter, lard             2. oils associated with plants - corn oil, olive oil

Main characteristic of lipids - won't dissolve in water and is repelled from water

Roles of lipids            1. food- high energy (many C-H bonds), has more energy than any other molecule             2. part of cell membranes             3. also- waxes (cutin, suberin), hormones (testosterone, estrogen), certain vitamins, certain pigments (chlorophyll)

Basic form for energy storage- monoglycerides, diglycerides, triglycerides


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CARBOHYDRATES

Composed of COH -makes building blocks of monosaccharides

Roles   1. energy storage (many C-H bonds) - sugar/starch energy source   2. structural (especially in plants- cellulose) / animals with proteins  3. carbon sources for making other building blocks (such as ribose and deoxyribose for nucleic acids, amino acids) 3 main types             1. monosaccharides (simple sugars)             2. disaccharides             3. polysaccharides (poly = many)                 a. polymers- composed of repeating subunits of monosaccharides -


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Nucleic acids

DNA (deoxyribonucleic acid; master information carrying molecule for the cell), RNA (ribonucleic acid; Copy of DNA molecule

Function- Contains the information for entire cell-expressed through protein synthesis

Polymers of nucleotides- composed of:             1. base- organic molecule with nitrogen- cytosine, guanine, thymine, adenine, and uracil   (uracil is in RNA only)             2. sugar- ribose, deoxyribose             3. phosphate Shape of DNA molecule- double helix (DNAOther important nucleotide compounds- example- ATP (universal in organisms; role- energy transfer or exchange)


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Proteins

Composed of COH and Nitrogen (four main elements) -building block is amino acids (20 different)

Roles             1. basic building blocks of cell - much of cell structure             2. part of cell membranes (help control entrance and exit of materials through membranes)             3. important in animal structure: hair, nails, connective tissue (tendons, cartilage), muscles             4. enzymes- facilitate chemical reactions

Composed of amino acids

1. repeating amino acids joined by the peptide bond forms a protein 2. 20 of them in proteins  3. 2 functional groups:-NH2 (amino group) & -COOH (acid group)


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order of amino acids is important- order determines the 3-dimensional shape of the molecule. This is significant because the function follows form: the biological activity of a protein depends largely on its 3-dimensional structure.


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Water

Water is the most abundant and important molecules in cells.

Human are about two third water

Water is a solvent, reactant or product.

It is an important determinant of biological structure as lipid bilayer, folded proteins and macromolecules are all stabilised by by the hydrophobic effect

Properties of water is very important


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