•Chapter 7 •part 5

Methods of Transport across membranes

• Diffusion

• Osmosis

• Facilitated diffusion

• Active transport

definitions

• Diffusion is passive. No energy expended

• Osmosis is passive and is the movement of WATER

• Facilitated diffusion –passive. Use of proteins to carry polar molecules or ions across membrane

• Active transport- requires energy (ATP). Moving against a concentration gradient.

diffusion

• Movement of molecules from an area of high concentration to an area of low concentration

• Stops when equilibrium is reached

• Diffusion involved a net movement of particles down a concentration gradient

• The rate of diffusion is determined by the movement of the molecules, which is a function of size, shape and temperature

osmosis

• The diffusion of water across a selectively permeable membrane

• Net movement is from the region where the water molecules are more concentrated to the region where they are less concentrated

osmosis

osmosis

• A u-tube is divided into 2 sections by a selectively permeable membrane that allows water molecules to pass freely but excludes solute molecules. Net movement from the pure water side to the water/solute side

Tonicity

• Hypotonic solution- one solution has a lower concentration of solute than another

• Hypertonic solution- one solution has a higher concentration of solute than another

• isotonic solution- both solutions have same concentrations of solute

handout

• Refer to your hand out of red blood cells

• What happens to cells when placed in each of these types of solutions????

Facilitated diffusion

Spot the difference!

Key features

• It is passive – does not require energy but- moves against a gradient• It uses carrier proteins• Here the solute molecules “combine” with

carrier proteins in the membrane. These carrier molecules speed (or facilitate) the passage of the solute molecules across the membrane.

The molecules that use it:

• Large molecules such as glucose or charged ions.

Two types of protein are involved

• CARRIER PROTEINS – bind to a specific type of diffusing molecule. They have a highly specific hydrophilic region to which the solute molecule binds. This binding cause the protein to undergo a change in shape that moves the solute across the bilayer and release it on the other side

Carrier proteins

Carrier proteins

ION CHANNELS• These are formed by

proteins with a central pore that is lined with charged groups. They help the diffusion of charged particles such as Ca2+, Na+, K+, HCO3- and Cl ions.

• Some channels are gated and allow cells to regulate the flow of ions from one cell to another.

Ion channel

Comparison of diffusion and facilitated diffusion.

• What factors affect the diffusion rate?

Concentration difference

• In facilitated diffusion a point will be reached where increase in concentration differences are not accompanied by an increased rate. Why?

The system is saturated – there are only a limited number of carrier molecules per unit area of membrane.

The rate of movement reaches a maximum whenall the carrier molecules are fully loaded with

solute molecules

Water potentials

• Water moves from a place with a LESS NEGATIVE (higher) water potential to a place with a MORE NEGATIVE (lower) water potential.

• The water potential of pure water is 0 (zero).• Solutions have negative water potentials – the

more concentrated the solution, the more negative the water potential.

• Water diffuses from a solution with a less negative water potential (hypotonic) into one with a more negative water potential (hypertonic) until the solutions are the same concentration (isotonic).

Draw the diagrams

• Label which solution has the more negative water potential.

• Which solution is:Hypotonic

Hypertonic

Isotonic

Strong sugar

solution

Medium sugar

solution

Weak sugar

solution

Pota

to c

ells

More facts….

• The cell surface membrane is partially (and selectively) permeable.

• The membrane around the vacuole in plant cells is also partially permeable.

• Ions (and sugars) in the vacuole produce a negative water potential and water enters the vacuole by osmosis.

• The vacuole expands, pushes against the cell wall and makes the cell TURGID.

• Cell turgor provides support in plants.

Water potential formula:

Water potential= solute potential + pressure potential

Osmosis• The water potential of pure water is ZERO

• Water diffuses from a Less negative water potential

hypotonic

To a

More negative water potential

hypertonichypertonic

Until the solutions are ISOTONIC

Active Transport

Spot the difference

Definition

• Active transport is the transport of molecules or ions across a membrane by carrier proteins against a concentration gradient.

Key facts

• It requires energy from respiration• Factors that reduce respiration will also reduce

active transport: e.g. lower temperature; lack of oxygen; metabolic and respiratory inhibitors.

• Active transport involves carrier proteins in the membrane.

• The hydrolysis of ATP (process of decomposition where compound reacts with water to produce other compounds) releases the energy required for active transport.

• Cells involved in active transport

have a LARGE NUMBER of MITOCHONDRIA to provide the ATP required via AEROBIC RESPIRATION.

Example

• The sodium – potassium pump.

• Exists in most cell membranes.

• Actively removes sodium ions from the cell while actively accumulating potassium ions into them from their surroundings

•

Endocytosis and Exocytosis

• Endocytosis is the transport of large particles into the cell in vesicles formed by invagination of the cell surface membrane.

• Exocytosis is the reverse process and is used to secrete proteins, e.g digestive enzymes, out of the cells.

Endocytosis and Exocytosis

Endocytosis

Types of endocytosis

• Phagocytosis- cells ingest large, solid particles such as bacteria or food

• Pinocytosis- cell takes in dissolved materials. Tiny droplets

• Receptor mediated endocytosis- specific proteins or particles combine with receptor proteins embedded in the membrane

Exocytosis

Bozeman Biology Video

• http://www.youtube.com/watch?v=RPAZvs4hvGA