CELL MEMBRANE

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GAYATHIRIANSARI

LEARNING OBJECTIVE

Define cell

Identify the three major cell regions

(nucleus, cytoplasm, and plasma

membrane ) of a typical cell

Describe the structure of the plasma

membrane, and explain how the

various transport processes account

for the directional movements of

specific substances across the

plasma membrane.

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LEARNING OBJECTIVE

Define selective permeability,

passive transport, diffusion,

filtration (including facilitated

diffusion and osmosis ) active

transport, hypertonic, hypotonic,

and isotonic.

INTRODUCTION

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WHAT IS CELL?

Cells are the building

blocks of all living things

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THE MAJOR CELL REGIONS

Nucleus

Cytoplasm

Plasma membrane

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Nucleus

Cytoplasm

Plasma

Membrane

Rough ER

Pores

Nucleolus

Chromatin

Nuclear

Envelope

Nucleus

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MEMBRANE TRANSPORT

Intracellular fluid – A solution containing small

amounts of gases, nutrients, and salts, dissolved in

water

Interstitial fluid – A rich, nutritious, and unusual

“Soup” that continuously bathes the exterior of

cells

THE CELL MEMBRANE

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Called PLASMA/CYTOPLASMIC MEMBRANE

A fragile, transparent barrier that separates

interior and exterior environment

Consists of two lipid layers

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Nucleus

Cytoplasm

Plasma

Membrane

Polar heads of

Phospholipid

molecules

Bimolecular lipid

layer containing

proteins

Non polar tails of

Phospho lipid

molecules

PROPERTIES OF

CELL MEMBRANE

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SELECTIVE PERMEABILITY

A barrier allows some substances to

pass through it, while excluding others

Most of the lipid portion is

Phospholipids

Substantial amount of

cholesterol

The polar heads are

HYDROPHILIC (water loving)

Nonpolar tails are

HYDROPHOBIC(water hating)

Maintain homeostasis

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Embedded in the bilayer are proteins

Most of the membrane’s functions are

accomplished by the embedded

proteins.

Signal transduction (receptor) proteins bind

hormones and other substances on the

outside of the cell.

Binding triggers a change inside the cell.

Called signal transduction

Example: The binding of insulin to insulin receptors

causes the cell to put glucose transport proteins into the

membrane.

Fig. 5-1c

Messenger molecule

Activatedmolecule

Receptor

DIFFUSION

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DEFINITION

The movement of molecules from a area

in which they are highly concentrated

to a area in which they are less

concentrated (kinetic energy).

Constant movements of particles in

random directions is called Brownian

movement)

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FACTORS INFLUENCING

DIFFUSION:

Distance: diffusion is rapid over short distance

Resistance: particles diffusing within gas will move a greater distance than diffusing in liquids

Size and weight: the greater the weight of a particle the lower the diffusion coefficient

Shape: a large globular protein can diffuse faster than a long small thread like molecule e.g collagen

The greater the difference in concentration across a membrane the faster the rate of diffusion

Temperature

Higher temperature, faster diffusion

IMPORTANCE OF DIFFUSION

Respiratory exchange of gases.

Intestinal absorption of minerals, water soluble vitamins and renal absorption of urea.

Water, ions and small molecules pass largely by diffusion through plasma membranes.

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TRANSPORT

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TRANSPORT PROTEINS Passive Transport Proteins

allow water soluble substances (small polar

molecules and ions) to pass through the

membrane without any energy cost

Active Transport Proteins

The cell expends energy to transport water

soluble substances against their concentration

gradient

PASSIVE TRANSPORT

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Simple Diffusion

Nonpolar, hydrophobic molecules diffuse directly through the lipid bilayer

Simple diffusion does not require the use of transport proteins.

Examples: O2, CO2, steroids

Polar, hydrophilic substances cannot pass directly through the lipid bilayer

Examples: water, ions, carbohydrates

Simple Diffusion

small, nonpolar molecules(ex. O2, CO2)

Polar molecules

(ex. Glucose, water)

ions

(ex. H+, Na+, K+)

LIPID-SOLUBLE WATER-SOLUBLE

LIPID-SOLUBLE

Facilitated Diffusion

In facilitated diffusion small polar molecules

and ions diffuse through passive transport

proteins.

No energy needed

Most passive transport proteins are solute

specific

Example: glucose enter/leaves cells

through facilitated diffusion

Facilitated Diffusion

PROTEIN CHANNEL- to fit

Passive transport

proteinLower

concentration

Higher concentration of

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CARRIER PROTEING- specific shape

Concentration

Gradient

Transport

Protein

Lipid insoluble

solute

OSMOSIS Osmosis – diffusion of water across a

selectively permeable membrane

Water moves from an area of high water

concentration to an area of low water conc.

Is energy required ?

Water travels in/out of the cell through

aquaporins

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OSMOTIC PRESSURE

Pressure required to oppose the net

movement of water by osmosis

The greater the difference in water

concentration the greater is the osmotic

pressure required to prevent water molecules

moving via osmosis

Osmosis Terms

Consider two solutions separated

by a plasma membrane.

Hypertonic

solution with a relatively high concentration of solute

Hypotonic

solution with a relatively low concentration of solute

Isotonic

solutions with the same solute concentration

Isotonic solution Hypotonic solution Hypertonic solution

H2O H2O

(1) Normal (2) Lysed

H2O

H2O H2O H2O

Animal

cell

Plant

cell

(4) Flaccid (5) Turgid (6) Shriveled

(plasmolyzed)

(3) Shriveled

Plasma

membrane

H2O

H2O

See page 83

Osmosis and Animal Cells

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Isotonic solution

Osmotic pressure in fluid and cells is equal

What is the net movement of water?

What will happen to cells when placed in such solution?

Most intravenous solutions are isotonic

ringer’s lactate, 5% dextrose, 50 g/L glucose, 9 g/L sodium chloride

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Isotonic

This solution will cause no shrinking or

swelling of normal body cells

Two solutions have the same concentration

of dissolved substances

Osmotic pressure in the fluid cells is equal

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Hypertonic Solution

This is a solution that develops a greater

osmotic pressure than cells

Hypertonic solutions have low water

concentration than cells resulting normal cells

losing water and shrinking

Example: ____________?

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Hypertonic solution

Solutions with greater osmotic pressure than

cells

What is the net movement of water?

What will happen to cells when placed in

such solution?

18 g/L NaCL

Given in edema

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Hypotonic

This solution cause normal cells to swell

These solution exert low osmotic pressure

then cells, water flows from bathing solution to

cell

Application: It is used to rehydrate the tissues

of extremely dehydrated patients

Example_____________?

Question time??? (contd..) Match the solutions in column A with their characteristics in

column B:

Solutions Characteristics

Isotonic A. Used to rehydrate the tissues of extremely

dehydrated patients.

Hypotonic B. Draws water out of the tissue spaces into the

blood steam.

Hypertonic C. No net movement of water occurs between

the fluid and the cell

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ACTIVE TRANSPORT

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ACTIVE TRANSPORT

Examples- solute pumping and bulk transport

Solute pumping is similar to facilitated diffusion

Both processes require protein carriers

Uses ATP to energize its protein carriers- SOLUTE

PUMPS

Sodium ions out and potassium ions into the cell is

absolutely necessary for normal transmission of

impulses by nerve cell

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(1) Binding of cytoplasmic

Na+ to the pump protein

stimulates phosphorylation by

ATP, which causes the pump

protein to change its shape.

(2) The shape change expels

Na+ to the outside.

Extracellular K+ binds,

causing release of the

phosphate group.

(3) Loss of phosphate

restores the original

conformation of the pump

protein. K+ is released and

Na+ sites are ready to bind

Na+ again; the cycle repeats.

Cytoplasm

Fig. 5-8-1

Transportprotein

Solute

Solute binding1

Fig. 5-8-2

Transportprotein

Solute

Solute binding1 Phosphorylation2

Fig. 5-8-3

Transportprotein

Solute

Solute binding1 Phosphorylation2 Transport3

Proteinchanges shape

Fig. 5-8-4

Transportprotein

Solute

Solute binding1 Phosphorylation2 Transport3

Proteinchanges shape

Protein reversion4

Phosphatedetaches

BULK FLOW Vesicles are used to transport large particles

across the PM.

Requires energy

Types:

Exocytosis

Endocytosis

Phagocytosis, pinocytosis, receptor-mediated

Vesicle

Fluid outside cell

Protein

Cytoplasm

Exocytosis

Bulk Flow

Exocytosis

Cytoplasmic vesicle merges with the PM

and releases its contents

Example:

Golgi body vesicles merge with the PM an

release their contents

How nerve cells release neurotransmittors

Vesicle forming

Endocytosis

Endocytosis can occur in three ways

• Phagocytosis ("cell eating")

• Pinocytosis ("cell drinking")

• Receptor-mediated endocytosis

Endocytosis

Endocytosis

PM sinks inward, pinches off and forms a vesicle

Vesicle often merges with Golgi for processing

and sorting of its contents

Endocytosis - terms

Phagocytosis – cell eating

Membrane sinks in and captures solid particles for

transport into the cell

Examples:

Solid particles often include: bacteria, cell

debris, or food

Pinocytosis – cell drinking

Cell brings in a liquid

Endocytosis - comments

Phagocytosis and pinocytosis are not

selective

Membrane sinks inward and captures whatever

particles/fluid present.

Vesicle forms and merges with the Golgi body…

Receptor Mediated Endocytosis

Receptor Mediated Endocytosis is a highly

specific form of endocytosis.

Receptor proteins on the outside of the cell bind

specific substances and bring them into the cell

by endocytosis

Receptor Mediated Endocytosis

1. Receptor proteins on PM bind specific

substances (vitamins, hormones..)

2. Membrane sinks in and forms a pit

– Called a coated pit

3. Pit pinches closed to form a vesicle around

bound substances

Cytoskeleton aids in pulling in the membrane and

vesicle formation

Fig. 5-9c

Coatedvesicle

Coatedpit

Specificmolecule

Receptor-mediated endocytosis

Coat proteinReceptor

Coatedpit

Material boundto receptor proteins

Plasma membrane

Fig. 5-9Phagocytosis

EXTRACELLULARFLUID

Pseudopodium

CYTOPLASM

Foodvacuole

“Food” orother particle

Pinocytosis

Plasmamembrane

Vesicle

Coatedvesicle

Coatedpit

Specificmolecule

Receptor-mediated endocytosis

Coat proteinReceptor

Coatedpit

Material boundto receptor proteins

Plasma membrane

Foodbeingingested

FILTRATION

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FILTRATION

The process by which water and solutes are

forced through a membrane by fluid or

hydrostatic pressure

The gradient is a pressure gradient that

actually pushes solute-containing fluid from

the higher pressure area to the lower

pressure area