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LIPIDS, MEMBRANE AND

CELLULAR TRANSPORT

CHAPTER VII

LIPID

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• Lipid is not a polymer

• But they still high tendency to associate through non

covalent forces

• They characterized by their structure

a-hydrophilic head

b-hydrophobic tail

b

a

LIPID

• Membrane or barrier of cell membrane that

control materials in and out of the cell

• Energy storage (triacylglyceride)

• Cushioning (adipose tissue)

• Transmission(signal transduction)

• Communication between cell (steroid and

hormone)

GENERAL FUNCTION OF

LIPIDS

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• Natural foods (unprocessed foods) contain two main types of fatty acids - saturated and unsaturated

– Saturated fatty acids - which come from animal fats (meat, lard, dairy products) and tropical oils such as coconut and palm oils - raise the levels of LDL cholesterol

– Unsaturated fats - which come from vegetable oils - in general do not increase cholesterol levels, and may reduce them

TRANS FATTY ACIDS

• Trans fatty acids (trans fats) are a third form of fatty

acids

• While trans fats do occur in tiny amounts in some

foods (particularly foods from animals), almost all

the trans fats now in our diets come from an

industrial process that partially hydrogenates (adds

hydrogen to) unsaturated fatty acids

• Trans fats, then, are a form of processed vegetable

oils

TRANS FATTY ACIDS

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Unsaturated Fatty Acids

Saturated Fatty Acids

• Propionic Acid

CH3CH2COOH

• Butyric Acid

CH3(CH2)2COOH

• Valeric Acid

CH3(CH2)3COOH

• Caproic Acid

CH3(CH2)4COOH

• Myristoleic acid

CH3(CH2)3CH=CH(CH2)7COOH

• Palmitoleic acid

CH3(CH2)5CH=CH(CH2)7COOH

• Sapienic acid

CH3(CH2)8CH=CH(CH2)4COOH

• Oleic acid

CH3(CH2)7CH=CH(CH2)7COOH

FATTY ACIDS:

THE COMMON NAMES

The difference between

cis and trans fatty acid’s

structure

Example of

trans fatty

acids:

Burger and fries

What Is Unhealthy About Trans Fats?

• Trans fats increase total cholesterol levels and

LDL cholesterol levels; worse (and in contrast

to saturated fats), they reduce HDL

cholesterol levels

• Trans fats also appear to interfere with the

body's usage of omega-3 fatty acids, which

are important for heart health

TRANS FATTY ACIDS

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1 Fatty Acid + 3 Glycerol = Triacylglycerol

• Remove water (condensation)

• Linked by ester bond

TRIACYLGLYCEROL

• Oil

– Unsaturated fatty acid

– Liquid at room temperature

– Can undergo hydrogenation process

• Fat

– Mainly saturated fatty acid

– Solid/semisolid at room temperature

TRIACYLGLYCEROL

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• Glycerophospholipids or Phosphoglycerides are glycerol

based phospholipids

• They are the main component of biological membranes such

as phospholid bilayer

Structures

• It contains a glycerol core with fatty acids

• They can be the same or different subunits of fatty acids

• Carbon 1 (tail, non-polar) contains a fatty acid,

typically saturated.

• Carbon 2 (tail, non-polar) contains a fatty acid,

typically unsaturated and in the cis conformation, thus

appearing "bent“

• Carbon 3 (head, polar) contains a phosphate group or an

alcohol attached to a phosphate group

GLYCEROPHOSPHOLIPIDS GLYCEROPHOSPHOLIPIDS

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Uses in membranes:

• Each glycerophospholipid molecule consists of a

small polar head group and two long hydrophobic chains

• In the cell membrane, the two layers of phospholipids are

arranged as follows: – The hydrophobic tails point to each other and form a fatty, hydrophobic center

– The ionic head groups are placed at the inner and outer surfaces of the cell

membrane

• This is a stable structure because the ionic hydrophilic head

groups interact with the aqueous media inside and outside

the cell, whereas the hydrophobic tails maximize hydrophobic

interactions with each other and are kept away from the

aqueous environments

GLYCEROPHOSPHOLIPIDS • Sphingolipids are a class of lipids containing a

backbone of sphingoid bases, a set of aliphatic amino alcohols that includes sphingosine

• Play important roles in signal transmission and cell recognition

• Sphingolipidoses, or disorders of sphingolipid metabolism, have particular impact on neural tissue

• A sphingolipid with an R group consisting of a hydrogen atom only is a ceramide

• Other common R groups include phosphocholine, yielding a sphingomyelin, and various sugar monomers or dimers, yielding cerebrosides and globosides, respectively

• Cerebrosides and globosides are collectively known as glycosphingolipids

SPHINGOLIPIDS

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SPHINGOLIPIDS

• Simple Sphingolipids

– Ceramides

• Complex Sphingolipids:

– Sphingomyelins

– Glycosphingolipids

• Cerebrosides

• Sulfatides

• Gangliosides

• Inositol

SPHINGOLIPIDS

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• Sphingoid bases are the fundamental building

blocks of all sphingolipids

• The main mammalian sphingoid bases are

dihydrosphingosine and sphingosine, while

dihydrosphingosine and phytosphingosine are

the principle sphingoid bases in yeast

• Sphingosine, dihydrosphingosine, and

phytosphingosine may be phosphorylated

SPHINGOLIPIDS

FUNCTIONS:

• Protect the cell surface against harmful

environmental factors by forming a

mechanically stable and chemically

resistant outer leaflet of the plasma

membrane lipid bilayer

• Cell recognition and signalling

SPHINGOLIPIDS

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STEROID

4 fused-rings…..3 CYCLOHEXANE, 1 CYCLOPENTANE

FUNCTIONS:

Steroid hormones

Produce sex difference or support reproduction androgens, estrogens, and progestagens

Corticosteroids include glucocorticoids and mineralocorticoids

Glucocorticoids:

Regulate many aspects of metabolism and immune function, whereas mineralocorticoids

help maintain blood volume and control renal excretion of electrolytes

Most medical 'steroid' drugs are corticosteroids

Anabolic steroids:

A class of steroids that interact with androgen receptors to increase muscle and bone

synthesis

There are natural and synthetic anabolic steroids

In popular language, the word "steroids" usually refers to anabolic steroids

Cholesterol, which modulates the fluidity of cell membranes and is the principal

constituent of the plaques implicated in atherosclerosis

STEROIDS

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MEMBRANE STRUCTURE • Integral proteins

– Span lipid bilayer

– Transmembrane proteins

– Hydrophobic regions consist of one or more stretches of nonpolar amino acids

– Often coiled into alpha helices

MEMBRANE STRUCTURE

EXTRACELLULAR

SIDE N-terminus

C-terminus CYTOPLASMIC

SIDE a Helix

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MEMBRANE STRUCTURE

Enzymes Signal

Receptor ATP

Transport Enzymatic activity Signal transduction

Glyco- protein

Cell-cell recognition Intercellular joining Attachment to the

cytoskeleton and extra-

cellular matrix (ECM)

MEMBRANE STRUCTURE

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THE ROLE OF MEMBRANE CARBOHYDRATES

IN CELL-CELL RECOGNITION

• Cells recognize each other by binding to surface

molecules, often carbohydrates, on the plasma

membrane

• Carbohydrates covalently bonded to lipids (glycolipids) or more often to proteins (glycoproteins)

• Much variability of extracellular carbohydrates among species, individuals, cell types in an individual

Plasma membrane:

Cytoplasmic face

Extracellular face Transmembrane

glycoprotein

Plasma membrane:

Secreted

protein

Vesicle

Golgi

apparatus

Glycolipid

Secretory

protein

Transmembrane

glycoproteins

ER

Synthesis and Sidedness of Membranes:

• Membranes distinct inside and outside faces

• Plasma membrane is added to by vesicles from ER & Golgi

• Secreted and integral membrane proteins, lipids and associated carbohydrates transported to membrane by these vesicles

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TRANSPORT ACROSS CELLULAR MEMBRANES

• To exchange materials with surroundings in part to take in nutrients and give off waste

• Exchange(or transport) regulated: selective permeability

• Structure Dictates Membrane Permeability: – Hydrophobic (nonpolar) molecules cross membrane

rapidly – e.g., hydrocarbons, oxygen, CO2 can dissolve in the lipid bilayer and pass

through the membrane rapidly

– Polar molecules cross slowly – e.g. sugars, charged proteins, water

HOW DO HYDROPHILIC SUBSTANCES CROSS

MEMBRANES?

Transport proteins

– Some create hydrophilic channels across membranes for

polar molecules or ions to pass through

– Example: Aquaporin : water channel protein

Carrier proteins – binds solutes & change the shape of carrier

– help to facilitate passage across membrane

– highly specific for transported solutes

– Examples: glucose transporter is a carrier protein for glucose only

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Molecules of dye Membrane (cross section)

WATER

Net diffusion Net diffusion Equilibrium

Diffusion of one solute

PASSIVE TRANSPORT: DIFFUSION

• Substances diffuse down their concentration gradient from high to low

• Substances reach dynamic equilibrium

• No work (no added energy) required

Net diffusion Net diffusion Equilibrium

Diffusion of two solutes

Net diffusion Net diffusion Equilibrium

PASSIVE TRANSPORT: DIFFUSION

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EFFECTS OF OSMOSIS ON WATER BALANCE

• Osmosis

– diffusion of water across a selectively permeable membrane

• Diffuses across a membrane from the region of lower solute (such as an ion) concentration to the region of higher solute concentration

• The direction of osmosis is determined only by a difference in total solute concentration

Lower

concentration

of solute (sugar)

Higher

concentration

of sugar

Same concentration

of sugar

Selectively

permeable mem-

brane: sugar mole-

cules cannot pass

through pores, but

water molecules can

H2O

Osmosis

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WATER BALANCE OF CELLS WITHOUT WALLS

Tonicity

ability of a solution to cause a cell to gain or lose water

Isotonic solution

solute concentration is equal inside and outside the cell -->

no net water movement cell remains same size

Hypertonic solution

external solute concentration is greater than that inside the

cell-->cell loses water

Hypotonic solution

external solute concentration is less than that inside the cell--> cell

gains water

WATER BALANCE OF CELLS WITH

WALLS VS NO WALLS

• Cell walls help maintain water balance

• Plant cell in hypotonic solution swells -->turgid (firm)

• Plant cell and its surroundings isotonic--> no net water movement; the cell becomes flaccid (limp), and the plant may wilt

• In hypertonic environment, plant cells lose water--> membrane pulls away from the wall: plasmolysis

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EXTRACELLULAR

FLUID

Channel protein Solute

CYTOPLASM

Facilitated diffusion transport proteins speed movement of molecules

across the plasma membrane

PASSIVE TRANSPORT AIDED BY PROTEINS

CHANNEL PROTEIN

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Carrier protein Solute

PASSIVE TRANSPORT AIDED BY PROTEINS

CARRIER PROTEIN

Cytoplasmic Na+ bonds to

the sodium-potassium pump

CYTOPLASM Na+

[Na+] low

[K+] high

Na+

Na+

EXTRACELLULAR

FLUID

[Na+] high

[K+] low

Na+

Na+

Na+

ATP

ADP

P

Na+ binding stimulates

phosphorylation by ATP.

Na+ Na+

Na+

Phosphorylation causes

the protein to change its

conformation, expelling Na+

to the outside.

P

Extracellular K+ binds

to the protein, triggering

release of the phosphate

group.

P P

Loss of the phosphate

restores the protein’s

original conformation.

K+ is released and Na+

sites are receptive again;

the cycle repeats.

ACTIVE TRANSPORT

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Diffusion Facilitated diffusion

Passive transport

ATP

Active transport

H+

ATP

CYTOPLASM

EXTRACELLULAR

FLUID

Proton pump

H+

H+

H+

H+

H+

+

+

+

+

+

–

–

–

–

–

ELECTROGENIC PUMPS

• A transport protein that generates a voltage across a membrane--> opposite charges across membrane (membrane potential)

• Example: In animals, Na-K pump

• In plant fungi and bacteria, proton pump Requires ATP (active transport)

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H+

ATP

Proton pump

Sucrose-H+

cotransporter

Diffusion

of H+

Sucrose

H+

H+

H+

H+

H+ H+

+

+

+

+

+

+

–

–

–

–

–

–

COTRANSPORT

• Coupled Transport by a Membrane Protein • When active transport of one solute indirectly drives transport

of another

• Plants commonly use the proton gradient generated by proton pumps to drive transport of nutrients into the cell

HOW DO LARGE MOLECULES MOVE IN AND OUT OF CELLS?

• Small molecules and water enter or leave the cell through the lipid bilayer or by transport proteins

• Large molecules, such as polysaccharides and proteins, cross the membrane via vesicles

Plasma membrane:

Cytoplasmic face

Extracellular face Transmembrane

glycoprotein

Plasma membrane:

Secreted

protein

Vesicle

Golgi

apparatus

Glycolipid

Secretory

protein

Transmembrane

glycoproteins

ER

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• Exocytosis – Transport vesicles with cargo migrate to the membrane, fuse

with it, and are release contents

– Example:

– Many secretory cells use exocytosis to export their products

– Pancreatic cells (beta-cells) secrete insulin

• Endocytosis

– Cell takes in macromolecules by forming vesicles at the

plasma membrane

– Reversal of exocytosis, involving different proteins

HOW DO LARGE MOLECULES MOVE IN AND OUT OF CELLS?

• Three types of endocytosis:

– Phagocytosis (“cellular eating”): • Cell engulfs particle in a vacuole

– Pinocytosis (“cellular drinking”): • Cell creates vesicle around fluid

– Receptor-mediated endocytosis: • Binding of ligands to receptors triggers vesicle

formation

ENDOCYTOSIS

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Receptor

RECEPTOR-MEDIATED ENDOCYTOSIS

Ligand

Coated

pit

Coated

vesicle

Coat protein

Coat

protein

Plasma

membrane 0.25 µm

A coated pit

and a coated

vesicle formed

during

receptor-

mediated

endocytosis

(TEMs).

ENDOCYTOSIS