AS CIE Cell membranes

D1: The Cell Membranes

Plan

1 Introduction:

1.1 Where are membranes found in the cell ?

1.2 Roles of the Cell Membranes

1.3 The fluid mosaic model

2 Membrane lipids:

2.1 Types of membrane lipids

2.2 Roles of membrane lipids

2.3 Examples

2

3

4

1

5

3 Membrane proteins:

3.1 Types of membrane proteins

3.2 Roles of membrane proteins

3.3 Examples

4 Membrane carbohydrates:

4.1 Types of membrane carbohydrates

4.2 Roles of membrane carbohydrates

4.3 Examples

5 Conclusions and Overviews:

Xavier DANIEL, Ph.D. AS

• Cell wall

• Plasma membrane

• Cytoplasm

• Free DNA

• Ribosomes

A Typical Prokaryotic Cell

0.1 – 10mm

Depending on the species

outside

inside

Where are membranes found in the cell ? 1


Rough ER Smooth ER

Centrosome

CYTOSKELETON

Microfilaments

Microtubules

Microvilli

Peroxisome

Lysosome

Golgi apparatus

Ribosomes

In animal cells but not plant cells:

Lysosomes

Centrioles

Flagella (in some plant sperm)

Nucleolus

Chromatin

NUCLEUS

Flagellum

Intermediate filaments

ENDOPLASMIC RETICULUM (ER)

Mitochondrion

Nuclear envelope

Plasma membrane

A Typical Animal Cell



4 4

Endomembrane System



In plant cells but not animal cells:

Chloroplasts

Central vacuole and tonoplast

Cell wall

Plasmodesmata

CYTOSKELETON

Ribosomes (small brown dots)

Central vacuole

Microfilaments

Intermediate

filaments

Microtubules

Rough

endoplasmic

reticulum Smooth

endoplasmic

reticulum

Chromatin

NUCLEUS

Nuclear envelope

Nucleolus

Chloroplast

Plasmodesmata Wall of adjacent cell

Cell wall

Golgi apparatus

Peroxisome

Tonoplast

Centrosome

Plasma membrane

Mitochondrion

A Typical Plant Cell




Plan

1 Introduction:




2 Membrane lipids:



2.3 Examples

2

3

4

1

5




3.3 Examples




4.3 Examples



Cell (plasma) membrane

1. Separates the components of a cell from its

environment

Surrounds the cell

The cell membrane is the boundary

IN food

- sugars

- proteins

- fats

salts

O2

H2O

OUT waste

- ammonia

- salts

- CO2

- H2O

products

- proteins

Cell needs materials in & products or waste out

Roles of the cell membranes 1

Controls what goes in

and what goes out of

the cell Xavier DANIEL, Ph.D. AS


Cell (plasma) membrane

1. Separates the components of a cell from its environment

Controls what goes in

and what goes out of

the cell

2. Keeps a stable environment inside the cell

Homeostasis of the cell


Organelle’s membranes

1’. Separate the components of an organelle from its environment

=

Cytoplasm

Surrounds the organelle

Organelle’s membrane is the boundary


What goes in ? What goes out ?

Depends on the organelle

Controls what goes in and what

goes out of each organelle



1’. Separate the components of an organelle from its environment


Controls what goes in and what

goes out of each organelle

2. Keeps a stable environment inside each organelle

Homeostasis of all organelles



2. Compartimentalize the molecules

What is needed in the nucleus may not be needed elsewhere

Genomic DNA

What is needed in the chloroplast may not be needed elsewhere

Chlorophyll

What is needed in the cytoplasm may not be needed elsewhere

Cytoskeleton

What is needed in the vacuole may not be needed elsewhere

Water and waste products

etc...




3. Compartimentalize the chemical reactions

Each organelle is specialized for some reactions only

More efficient ! No mutual inhibition !

Lysosome

Hydrolysis of molecules

Chloroplast

Photosynthesis

Mitochondrion

Last steps of Cellular respiration

Golgi

Modification of proteins

etc...




4. Different cellular compartments

can have different conditions


2. Compartimentalize the molecules

3. Compartimentalize the chemical reactions

+


pH 4.8

Contains digestive

enzymes, optimum pH

4.5 - 4.8

pH 7.2

lysosome

cytosol

Membrane = tonoplast

acts as

a barrier


4. Different cellular compartments

can have different conditions



Plan

1 Introduction:




2 Membrane lipids:



2.3 Examples

2

3

4

1

5




3.3 Examples




4.3 Examples



Phospholipids are

the major structural

component of

membranes

Phospholipid

The fluid mosaic model 1


aqueous solution

Hydrophilic head

Hydrophobic tail

Phospholipids form

micelles when submerged

in water

air

One-layer membrane ? Impossible !



Hydrophilic

head

Hydrophobic

tail

WATER

WATER

Phospholipid bilayer


Mixes with water

Mixes with water

Does not mix

with water

Does not mix

with water

Mix together

Two-layer membrane ? It works !



polar hydrophilic

heads

nonpolar hydrophobic

tails

polar hydrophilic

heads



All membranes are phospholipid bilayers

with embedded proteins


Transmembrane (intrinsic) proteins span the membrane

Integral (extrinsic) proteins are on either side of the membrane


All membranes are phospholipid bilayers

with embedded proteins



Oligosaccharides

Cytoplasm

Membrane Proteins

Cholesterol

Outer

Surface

Phospholipid

Bilayer


Mostly in animal membranes

Attached to proteins: Glycoproteins

Attached to lipids: Glycolipids (not shown here)


Membranes are not static

They have a fluid consistency

Most membrane lipids and proteins can drift about

laterally in the plane of the membrane

Cholesterol enhances membrane fluidity

(animal membranes mostly)



The membranes’ “fluid” is

the double layer of phospholipids



The membranes’ “mosaic” is

a collection of various proteins



Membrane is a collage of proteins & other molecules

embedded in the fluid matrix of the lipid bilayer

Extracellular fluid

Cholesterol

Cytoplasm

Glycolipid

Transmembrane proteins

Filaments of cytoskeleton

Peripheral protein

Glycoprotein

Phospholipids



The Fluidity of Membranes

Phospholipids in the membranes

Can move within the bilayer

Lateral movement

(~107 times per second) Flip-flop (~ once per month)

(a) Movement of phospholipids



Proteins in the membranes

Can drift within the bilayer

EXPERIMENT Researchers labeled the plasma membrane proteins of a mouse

cell and a human cell with two different markers and fused the cells.

Using a microscope, they observed the markers on the hybrid cell.

Membrane proteins

Mouse cell

Human cell

Hybrid cell

Mixed

proteins

after

1 hour

RESULTS

CONCLUSION The mixing of the mouse and human membrane proteins

indicates that at least some membrane proteins move sideways within the plane

of the plasma membrane.

+



Overview

Cell membranes separate living cell from non-living

surroundings and separate different organelles

thin barrier = 8nm thick

Controls traffic in & out of the cell/organelle

Made of phospholipids, proteins & other macromolecules

Is a fluid mosaic



Overview




Plan

1 Introduction:




2 Membrane lipids:



2.3 Examples

2

3

4

1

5




3.3 Examples




4.3 Examples



Types of membrane lipids 2

Two types of lipids in the membranes

Phospholipids Cholesterol

Mostly in animal cels


Phospholipids

Triglycerides that contain a Phosphate Group

One of the 3 fatty acids is replaced by a phosphate group

Variable

2 Types of membrane lipids


Phospholipids

Hydrophobic tail Hydrophilic head

AMPHIPATIC



Phospholipids




Most common Phospholipids in membranes




Outer side

of membranes

Phosphatidylcholine = Lecithin

Proeminent phospholipid in membranes




Inner side

of membranes

Phosphatidylserine




Inner side

of membranes

Phosphatidylethanolamine = Cephalin




Inner and Outer sides

of membranes

Phosphatidylinositol




Phosphatidyl

inositol

Phosphatidyl

ethanolamine

Phosphatidyl

serine

Phosphatidyl

choline



The lipid bilayer is asymmetric


Roles of Phospholipids 2

The kind of phospholipids influences

the movement of molecules in the membrane


Fatty Acids Saturated Unsaturated

Chain of

Carbon + Hydrogen

End in

COOH

Contain double bonds



The type of hydrocarbon tails in phospholipids

affects the fluidity and viscosity of the membranes

Fluid Viscous

Unsaturated hydrocarbon tails with kinks

Saturated hydro-

Carbon tails

2

1. Fluid membrane

Not tight

Molecules can drift

1. Viscous membrane

Tight

Molecules cannot drift

Roles of Phospholipids


1. Fluid membrane

Not tight

Molecules can drift

2



1. Viscous membrane

Tight

Molecules cannot drift

2



2

Aqueous

Aqueous

“Organic”

Can water go through ?

?

Phospholipids regulate the passage of molecules through membranes

YES

Can hydrophilic molecules

go through ?

? NO

Membranes are selectively permeable

Hydrophilic molecules

do not mix with lipids



2

Phospholipids regulate the passage of molecules through membranes

Membranes are selectively permeable

Phospholipids restrict the passage of

hydrophilic molecules

through membranes

Phospholipids make membranes become

Barriers between

1. Cells and their environment

2. Organelles and the cytoplasm

Aqueous

Aqueous

“Organic”



Types of membrane lipids 2

Two types of lipids in the membranes

Phospholipids Cholesterol

Only in animal cels


Lipids

Steroids Basic form = CHOLESTEROL

In animal membranes

Mostly in the plasma membrane




Cholesterol is amphipathic

Hydrophobic

Hydrophilic


2 Roles of cholesterol

1. Cholesterol maintains the integrity of the Cell Membrane

Part of the steroid ring

(the four hydrocarbon rings in between the hydroxyl group and the hydrocarbon "tail")

is closely attracted to part of the fatty acid chain on the nearest phospholipid

1. This helps slightly immobilize the surface of the membrane


2

2. Cholesterol regulates the fluidity of the membranes




1. This helps slightly immobilize the surface of the membrane

More cholesterol

Membrane is more ………

Less cholesterol

Membrane is more …….

Roles of cholesterol


2

3. Cholesterol reduces membranes’ permeability

to hydrophilic molecules




2. Makes the membrane less soluble to very small water-soluble molecules

that could otherwise pass through more easily

Very small hydrophilic molecules

Roles of cholesterol



Membranes contain many important proteins

Sometimes the “normal” phospholipid bilayer is not suitable

“islands” of Cholesterol + Sphingolipids make up Lipid Rafts



Sphingolipid

=

Shingosine + fatty acid (+ phosphate)

Shingosine



Sphingolipid

=

Shingosine + fatty acid + X

Shingosine Fatty acid

can vary

X Varies: carbohydrate, phosphate...

If X is a carbohydrate

Sphingolipid = Glycosphingolipid



Sphingolipid

=

Shingosine + fatty acid + R



Sphingolipid

=

Shingosine + fatty acid + X



Lipid rafts

=

Cholesterol + Sphingolipids

a) Shingolipid

b) Cholesterol



Lipid rafts



Plan

1 Introduction:




2 Membrane lipids:



2.3 Examples

2

3

4

1

5




3.3 Examples




4.3 Examples



Types of membrane proteins

Two types of proteins in the membranes

Integral

3

Span the membrane:

transmembrane

On either side

of the membrane

Peripheral

Bound to protein Bound to lipids


Within membrane

nonpolar amino acids

hydrophobic

anchors protein into membrane

On outer surfaces of

membrane

polar amino acids

hydrophilic

extend into extracellular

fluid & into cytosol

Polar areas of protein

Nonpolar areas of protein

Types of membrane proteins 3

Transmembrane proteins


Functions of membrane proteins

Six main functions for proteins in the membranes

3

Transportation Enzymes Receptor Cell adhesion Attachment

to cytoskeleton

/ ECM

Recognition

outside outside outside inside / outside inside


Channel proteins have pores that allow passage of

ions and small water-soluble molecules

Passive transport

Carrier proteins bind to molecules and change

shape for delivery across membrane

Active transport

3

1. Transport proteins

Regulate movement of ions and small hydrophilic molecules across the membranes



3

2. Receptor proteins = Signal transduction proteins

Usually after the cell received a signal from outside the cell

Signal = hormone and other substances

Pass on a signal from outside to inside the cell

The cell will respond to the signal that was passed on

Messenger molecule

Activated molecule

Receptor



3

3. Enzymes = Effector proteins

Catalyze reactions inside the cell

One or more than one different enzymes grouped together



3

3. Signal transduction

Receptor + Regulator + Enzyme



3

4. Recognition proteins “Identification tags” “Cell signature”

Very important in Immunity

Usually glycoproteins

To differentiate between “self” and “non-self”

Glyco-protein

Cell A

Cell B



3

5. Cell adhesion proteins “intercellular compatibility”

Important in Gap junctions and Tight junctions

Cell A

Cell B



3

6. Attachment to Cytoskeleton / ECM

ECM

Microfilaments

Maintain cell shape

and

Stabilizes the location of

some membrane proteins

coordinate extracellular and

intracellular changes

Integrins



Outside

Plasma membrane

Inside

Transporter Cell surface receptor

Enzyme

activity

Cell surface identity marker

Attachment to the cytoskeleton

Cell adhesion

Outside

Plasma membrane

Inside

Functions of membrane proteins 3



Plan

1 Introduction:




2 Membrane lipids:



2.3 Examples

2

3

4

1

5




3.3 Examples




4.3 Examples



Usually branched molecules of 15 or less sugar units

Some are bonded to lipids: Glycolipids

Most are bonded to proteins: Glycoproteins

Glycolipids + Glycoproteins = Glycocalyx

Function: Cell-cell recognition

Membrane Carbohydrates 4



Plan

1 Introduction:




2 Membrane lipids:



2.3 Examples

2

3

4

1

5




3.3 Examples




4.3 Examples



Membrane proteins and lipids

Are synthesized in the ER and Golgi apparatus

ER

Transmembrane

glycoproteins

Secretory

protein

Glycolipid

Golgi

apparatus

Vesicle

Transmembrane

glycoprotein

Membrane glycolipid

Plasma membrane: Cytoplasmic face

Extracellular face

Secreted

protein

4

1

2

3

Conclusion and Overviews 5


Summary The unit membrane consists of a phospholipid bilayer

Phospholipids consist of a polar, hydrophilic phosphate head and a non-polar, hydrophobic tail consisting of fatty acid chains.

Proteins also occur in the membrane and float freely throughout it.

The model for membrane structure is known as the fluid mosaic model.

Peripheral proteins occur on the inner or outer face of the membrane and integral proteins extend through both lipid layers.

Membrane bound enzymes occur allowing structured metabolic pathways.

5 Conclusion and overviews


Summary

Glycoproteins form the glycocalyx and allow cell to cell recognition.

Receptor proteins can act as binding sites for hormones and other substances and can transmit the information to the interior of the cell.

A variety of carrier proteins allow for the controlled movement of substance through the membrane using both passive diffusion or active transport.

Non-polar, lipid soluble molecules diffuse through the phospholipid bilayer.

Ionic, polar molecules require carrier proteins to enable them to pass through the membrane.

Membrane structure loses integrity with high temperature or presence of organic solvents such as alcohol, thereby increasing permeability.



Macromolecules found in Membranes

Lipids


Cholesterol Sphingolipids

Proteins

Transport Receptor

Enzymes Recognition

Cell adhesion Attachment to Cytoskelton / ECM

Carbohydrates

Oligosaccharides on glycoproteins

Oligosaccharides on glycolipids



AS CIE Cell membranes

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