The plasma membrane functions

The functions of the plasma membrane include:

Isolation

Regulation of exchange with the environment

Sensitivity to the environment

Structural support

Plasma Membrane Physical barrier - separates intracellular fluids from

extracellular fluids Helps in maintaining homeostasis Plays a dynamic role in cellular activity – selectively

permeable

Fluid Mosaic Model

Double bilayer of phospholipids

Phospholipids have hydrophobic tails and hydrophilic heads

CH2

CH2

CH2

CH2

CH2

CH2

CH2

CH2

CH2

CH2

CH2

CH2

CH2

CH2

CH2

CH3

CH2

CH2

CH2

CH2

CH2

CH2

CH2

CH2

CHCH

CH2

CH2

CH2

CH2

CH2

CH2

CH2

CH3

CH2

CH2

CH3

CH3

CH3N+

OO

O–

P

OCH2

CH

CH2

C O C O

O O

Phosphategroup

Hydrophilic head

Hydrophobic tails

The plasma membrane includes proteins

Integral proteins

Within the membrane

Peripheral proteins

Bound to inner or outer surface of the

membrane

The plasma membrane includes proteins Anchoring proteins (stabilizers)

Attach to inside or outside structures Recognition proteins (identifiers)

Label cells as normal or abnormal Enzymes

Catalyze reactions Receptor proteins

Bind and respond to ligands (ions, hormones) Carrier proteins

Transport specific solutes through membrane Channels

Regulate water flow and solutes through membrane

Structures on the plasma membrane surfaces

Microvilli, Cilia,Stereocilia

Specialized junctions

Features of Apical Surface of Epithelium - Microvilli

Projections that increase surface area

Folding of the plasma membrane

http://cellbio.utmb.edu/microanatomy/epithelia/epith_lec.htm

Features of Apical Surface of Epithelium - Cilia

These structures are designed for motility.

Epithelia that need to move substances across their surface (like mucous in the air passages) have cilia.

Each cilium or flagellum has a basal body located at its base.

Basal bodies anchor the cilia or flagella and are thought to be responsible for their formation.

They look like centrioles and are believed to be derived from them

Flagella: (ex) spermatoza

Extra long cilia Moves cell

http://www.lbl.gov/Science-Articles/Archive/sabl/2006/Jul/02.html

Cell junctions – 3 groups

Tight junction

designed to restrict the movement of material between the cells they link

Gap junction

create cytoplasmatic communication bridges between cells

Anchoring junction

attach cells to one another or to extracellular matrix

http://www.phschool.com/science/biology_place/biocoach/biomembrane2/junctions.html

Membrane Junctions

Tight junction

Anchoring junction

Gap junction

Tight Junctions An intercellular junction between

cells in which the outer layers of the cell membranes fuse,

reducing the ability of larger molecules and water to pass between the cells.

Tight junctions prevent the free movement of molecules between cells in the intestine and allow the intestinal cell to control absorption

Gap junctions

Example – intercalated discs in the heart, electrical synapses

Cell transport mechanisms - How things enter and leave the cell

2 groups of movement

Passive transport – no energy is needed

Diffusion

Carrier-mediated

Active transport – requires ATTP

Pumps

Vesicular transport

Characteristics of selectivelypermeable membranes EXTRACELLULAR

FLUID

CYTOPLASM

Materials may crossthe plasma membrane

through active orpassive mechanisms.

Passive mechanismsdo not require ATP.

Active mechanismsrequire ATP.

Diffusion ismovement drivenby concentration

differences.

Carrier-mediatedtransport involves

carrier proteins, andthe movement maybe passive or active.

Vesicular transportinvolves theformation ofintracellular

vesicles; this is anactive process.

Plasmamembrane

Passive transport All molecules in the body are in constant motion

regardless of the presence of a membrane (kinetic energy)

Motion stops only at absolute zero

By international agreement, it is defined as 0K on the Kelvin scale, −273.15°C on the Celsius scale and −459.67°F on the Fahrenheit scale

When a membrane is present the movement in a certain direction can be limited or changed

A molecule will move in a certain direction until collide with another molecule. When this happens, the direction of the movement will change

Diffusion Diffusion is the tendency for molecules to spread out

evenly into the available space

The driving force is kinetic energy

Slow in air and water but important over small distances

Although each molecule moves randomly, diffusion of a population of molecules may exhibit a net movement in one direction

Depends on a concentration gradient. (What is a concentration? A concentration gradient?)

At dynamic equilibrium, as many molecules cross one way as cross in the other direction

Factors Affecting Diffusion Distance (inversely related) Molecule size (inversely related) Temperature (directly related) Gradient size (directly related)

Electrical forces Attraction of opposite charges (+,–) Repulsion of like charges (+,+ or –,–)

Diffusion

The movement of molecules will happen in ALL directions

What is usually important is the net rate of diffusion in a certain direction

The net movement will be from high to low concentration until equilibrium is reached

At equilibrium, the net movement is equal in all directions

When a membrane is present

Membrane can be:

Freely permeable (this does not apply to plasma membrane) – allows passage of all substances

Selectively permeable – permits passage of some materials and prevents passage of others

Impermeable – cells can be impermeable to specific substances, but no living cell has a completely impermeable membrane

Permeability characteristics of membranes

Freely permeable membranes Selectively permeable membranes Impermeable membranes

Freely permeable membranesallow any substance to pass withoutdifficulty.

Selectively permeable membranes,such as plasma membranes, permit thepassage of some materials and preventthe passage of others.

Nothing can pass through impermeablemembranes. Cells may be impermeableto specific substances, but no living cellhas an impermeable membrane.

Protein Protein Protein

Lipids Lipids Lipids

Ions Ions Ions

— — —Water Water Water

Carbohydrates Carbohydrates Carbohydrates

Selectively permeable membranes Selective based on:

1. Characteristics of material to pass Size Electrical charge Molecular shape Lipid solubility

2. Characteristics of membrane What lipids and proteins present How components are arranged

Diffusion through cell membrane Diffusion is divided into 2 types:

1. Simple diffusion – the movement of particles through the membrane with no assistance

Nonpolar / lipid-soluble substances that diffuse directly through the lipid bilayer

Gases readily diffuse through lipid bilayer. (Ex. movement of oxygen inside cells and CO2 outside)

Diffusion of water and other lipid-insoluble molecules happens via protein channels

The channels are highly selective as a result of the diameter, shape, charge and chemical bonds

Diffusion of lipid-soluble materials

Diffusion of lipid-insoluble materials

Diffusion through cell membrane 2. facilitated diffusion - Assisted by carrier protein

Materials are bound to specific proteins and move through water-filled protein channels (big polar molecules; ex. – glucose)

The facilitated diffusion rate depends on the rate in which the carrier protein molecule can undergo changes that allow passage

Carrier Proteins Are integral transmembrane proteins Show specificity for certain polar molecules Their number will influence the amount that can be

transferred through the membrane

Osmosis Osmosis is a simple diffusion of water.

It occurs through a selectively permeable membrane Occurs when the concentration of a water is different on

opposite sides of a membrane

Membrane must be freely permeable to water, selectively permeable to solutes

Osmosis – osmolality, osmolarity and osmotic pressure

Osmolality (molecular weight) - One osmole is 1 gram molecular weight

Osmolarity (concentration) - One osmole in one liter Osmotic pressure – defined by the concentration of

solute particles in a solution Is defined by the number of particles, not their size

or nature Each particle in a solution, regardless of its

mass, exerts the same pressure against the membrane

Effects of Solutions of Varying Tonicity

Tonicity – description of how the solution affects a cell

Isotonic – solutions with the same solute concentration as that of the cytosol

Hypertonic – solutions having greater solute concentration than that of the cytosol

Hypotonic – solutions having lesser solute concentration than that of the cytosol

Passive Membrane Transport: Filtration

The passage of water and solutes through a membrane by hydrostatic pressure

Pressure gradient pushes solute-containing fluid from a higher-pressure area to a lower-pressure area

Depending on the size of the membrane pores

only solutes of a certain size may pass through it.

Transport that uses ATP

A movement that can be against concentration gradient

Uses ATP to move solutes across a membrane

Two types:

Active transport - use of carrier proteins

Vesicular transport

Types of Active Transport

2 types according to the source of energy used for the transport

Primary active transport

The energy for the transport derived directly from a high energy molecule – ATP

The hydrolysis of ATP causes phosphorylation of a transport protein that in turn changes its shape.

That change “promotes” the passage of materials (ex. Sodium-potassium pump)

2

EXTRACELLULAR

FLUID [Na+] high [K+] low

[Na+] low

[K+] high

Na+

Na+

Na+

Na+

Na+

Na+

CYTOPLASM ATP

ADP P

Na+ Na+

Na+

P 3

K+

K+ 6

K+

K+

5 4

K+

K+

P P

1

Fig. 7-16-7

Types of Active Transport

Secondary active transports – one ATP-powered pump can drive secondary transport of other solutes.

The energy is derived from the energy stored in creating the concentration gradient

This concentration difference was created by the primary active transport that used ATP

Secondary transport, like the primary, depends on carrier proteins, but without the need of energy

Fig. 7-19

Proton pump

–

–

–

–

–

–

+

+

+

+

+

+

ATP

H+

H+

H+

H+

H+

H+

H+

H+

Diffusionof H+

Sucrose-H+

cotransporter

Sucrose

Sucrose

Active transport

Symport system – two substances are moved across a membrane in the same direction

Antiport system – two substances are moved across a membrane in opposite directions (Na/K)

Vesicular Transport Transport of large particles and macromolecules across plasma

membrane using vesicles and ATP

Endocytosis – enables large particles and macromolecules to enter the cell. Few types:

Receptor-mediated endocytosis – selective process that depends on the binding of extracellular material to a specific receptor

This binding initiates the endocytosis

Phagocytosis – “cell eating”; endocytosis of solid objects

pseudopods engulf solids and bring them into the cell’s interior

Happens in specialized cells

Pinocytosis – “cell drinking”; endocytosis of liquids.

This is not a selective process and does not involve receptor

Vesicular Transport Exocytosis – moves

substance from the cell interior to the extracellular space

Transcytosis – moving substances into, across, and then out of a cell

Vesicular trafficking – moving substances from one area in the cell to another

Passive Membrane Transport – Review

Process Energy Source Example

Simple diffusion Kinetic energyMovement of O2 through membrane

Facilitated diffusion Kinetic energy Movement of glucose into

cells

Osmosis Kinetic energyMovement of H2O in & out of cells

Filtration Hydrostatic pressure Formation of kidney filtrate

Active Membrane Transport – Review

Process Energy Source Example

Active transport of solutes ATPMovement of ions across membranes

Exocytosis ATP Neurotransmitter secretion

Endocytosis ATPWhite blood cell phagocytosis