Transport Mechanisms

The four major categories of transport:filtrationdiffusion

mediated transportvesicular transport

Transport due to Pressure Gradients;Filtration across Membranes

• Material can move through the body due to pressure gradients.

• The heart generates hydrostatic pressure, causing blood to circulate. Solutes are carried with blood (bulk flow).

• In filtration, hydrostatic pressure causes water to cross membranes. Solutes may cross the membrane, depending upon the size of pores.

hydrostatic pressure

capillarymembranetissue space

Brownian Motion/Diffusion

• All molecules have random movement due to heat

• The random motion of molecules is called Brownian motion, and is the basis of diffusion.

Diffusion• Diffusion is defined as the movement of solute

from an area of high concentration to an area of low concentration.

• At equilibrium, the net movement of solute stops (although random motion continues)

• The difference in concentration between the area of high concentration versus low concentration is the concentration gradient.

Characteristics of Diffusion

•Diffusion does not require energy expenditure.•Factors affecting the rate of diffusion:

• faster at higher temperatures• faster with higher concentration gradient• faster for smaller solutes• slower in thicker solutions

•Diffusion can occur across membranes, if membrane is permeable to the solute.

Osmosis

• Osmosis is the movement of water across a membrane.

• Cell membranes are selectively permeable, allowing the solvent (but not all solutes) to cross the membrane.

• Water diffuses from areas where solute is less concentrated (more water) to where solute is more concentrated (less water).

Osmotic Pressure of Solutions

• The amount of force required to prevent the movement of water by osmosis is called the osmotic pressure.

• The greater the concentration of a solution, the greater the osmotic pressure will be.

mmHg

osmotic pressure

gravity

Osmotic Pressure of Solutions

• Terms to describe solutions based on their relative osmotic pressures:

- isosmotic: two solutions with the same osmotic pressures

- hyperosmotic: describes a solution with greater osmotic pressure than another

-hyposmotic: describes a solution with less osmotic pressure than another

Effects of Solutions of Cells You can describe solutions based on what happens when cells are added to them:- isotonic solution: cell doesn’t absorb or lose water- hypertonic solution: water moves out of cell, causing shrinkage (crenation) of the cell -hypotonic solution: water moves into the cell, causing swelling and rupture (lysis)Whether the solution is isotonic, hypertonic, or hypotonic depends upon the concentration of the solute, and whether the solute can cross the plasma membrane.

Effects of Solutions on Cells

Isotonic Hypertonic Hypotonic

Mediated Transport Mechanisms• Substances which cannot diffuse across the plasma

membrane can enter or leave cells by mediated transport.

• Characteristics of mediated transport:

- involves a carrier protein

--each carrier protein moves a specific type of solute (specificity)

--similar types of molecules can bind to the carrier molecule, and compete for transport (competition)

- - the rate of transport across the membrane is finite, with an upper limit at high solute concentrations (saturable)..

Specificity, Competition, Saturation

Specificity Competition

Saturation

Types of Mediated Transport

• There are two main types of mediated transport: facilitated diffusion and active transport.

• Facilitated diffusion:- carrier-mediated movement across membrane

- - follows concentration gradient

- - does not require energy

- - rate of diffusion dependent upon size of the concentration gradient

- - carrier is specific

- - can be bidirectional, depending on concentration gradient

- - example: glucose transport

Types of Mediated TransportActive Transport:

- carrier-mediated process that requires energy (ATP)

- maximum rate of transfer depends on the number of carrier molecules AND the energy supply

- substances can be moved AGAINST a concentration gradient (or with it)

- unidirectional transport

- materials can be concentrated in the cell

- active transport of two molecules can be linked

- example: Na+/K+ pump

Vesicular Transport

Materials can move in or out of cells through formation of vesicles

- a vesicle is a membranous sac

- large volumes of fluid and solutes can be transported (a form of bulk flow)- requires energy (ATP)- two forms: endocytosis and exocytosis

Endocytosis

• Endocytosis is the packaging of extracellular material for import into the cell.

• There are three forms of endocytosis:• receptor-mediated endocytosis• phagocytosis• pinocytosis

Receptor-Mediated Endocytosis

• Specific for a target molecule• binding of target to receptor triggers

internalization of receptor and target in a coated vesicle

• coated vesicle fuses with a lysosome, freeing target from the receptor

• target molecule enters the cytoplasm through diffusion or active transport

• receptors can be recycled• example: cholesterol uptake by cells

Phagocytosis (“cell eating”)

• the cell extends a pseudopod around a solid object

• the pseudopod engulfs the object, forming a vesicle

• the vesicle fuses with lysosomes to digest the object

• usually NOT for nutrition in human cells• some specificity involved• example: macrophages

Pinocytosis (“cell drinking”)

• similar to phagocytosis, except that the material ingested is liquid

• very nonspecific

• nutrients (lipids, sugars, amino acids) enter the cytoplasm via diffusion or active diffusion

• pinocytotic vesicle returns to the surface

• more common than phagocytosis

Exocytosis

Essentially, the reverse of endocytosis:

- a vesicle with the cell fuses with the plasma membrane and releases contents into the extracellular space.

- the released material may be a product of the cell (protein, hormone, etc.) or waste product from endocytotic vesicles or lysosomes.

Next Lecture....

Organic and Inorganic Molecules