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