Cell membrane characteristics and transport mechanisms

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

The functions of the plasma membrane include:

Isolation

Regulation of exchange with the environment

Sensitivity to the environment

Structural support

The plasma membrane includes proteins

The proteins in the plasma membrane includes:

Anchoring proteins

Recognition proteins (immune system)

Receptors

Carrier proteins

channels

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

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

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 Depends on a concentration gradient. (What is a

concentration? A concentration gradient?)

The driving force is kinetic energy and it is influenced by:

Molecule size – the smaller the faster

Temperature – the warmer the faster

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

Membrane permeability

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

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 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

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)

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

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 energyMovement of glucose into cells

Osmosis Kinetic energyMovement of H2O in & out of cells

FiltrationHydrostatic 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