Section 1 Passive TransportChapter 5

Objectives

• Explain how an equilibrium is established as a result of diffusion.

• Distinguish between diffusion and osmosis.

• Explain how substances cross the cell membrane through facilitated diffusion.

• Explain how ion channels assist the diffusion of ions across the cell membrane.

Section 1 Passive TransportChapter 5

• Passive transport involves the movement of molecules across the cell membrane without an input of energy by the cell.

• Diffusion is the movement of molecules from an area of higher concentration to an area of lower concentration– As a result of diffusion, the

concentration of many types of substances eventually becomes balanced on both sides of a membrane

DiffusionConcentration gradient is the difference in

the concentration of molecules across a distance

• Equilibrium is a state that exists when the concentration of a substance is the same throughout a space (homeostasis) Big Bang Theory

High Conc. Water Low Conc. Water

Low Conc. Solute High Conc. Solute

DiffusionWhat does this look like on the microscopic

level?Animation

***Another example of diffusion would be the dispersal of ink in

a beaker of water

Diffusion cont.

Chapter 5Section 1 Passive Transport

Remember that diffusion is movement from high concentrations to low concentrations,

DiffusionDiffusion Across Membranes

Molecules can diffuse across a cell membrane by dissolving in the phospholipid bilayer or by passing through pores in the membrane Diffusion across a membrane is “simple diffusion”

(only certain molecules pass through) NO energy is expended (used up)

Section 1 Passive TransportChapter 5

Osmosis

• Osmosis is the diffusion (or passing) of water across a cell membrane. – Since water is moving from

a higher to lower concentration, osmosis does not require cells to expend energy. Therefore, osmosis is the passive transport of water

The Direction of Osmosis

• The NET direction of osmosis is determined by the relative concentrations of solute on either side of a membrane

• There are specific names for solutions depending on their solute concentrations:– Hypotonic– Hypertonic– Isotonic

Osmosis: Hypotonic Solutions

• Hypotonic (“Hypo-” is Greek for “under”)– When the concentration of solute molecules outside the cell is

lower than their concentration in the cytosol (cell), the solution outside is hypotonic to the cytosol (cell)

– What direction do you think water will diffuse?• Water will diffuse INTO the cell until equilibrium is

established

Osmosis: Hypertonic Solutions

• Hypertonic (“Hyper-” is Greek for “over”)

– When the concentration of solute molecules outside the cell is higher than their concentration in the cytosol(cell), the solution outside is hypertonic to the cytosol (cell).

– What direction do you think water will diffuse?• Water will diffuse OUT OF the cell until equilibrium is

established

Osmosis: Isotonic Solutions

• Isotonic (“Iso-” is Greek for “equal”)– When the concentration of solute molecules inside and

outside the cell are equal, the solution outside is isotonic to the cytosol(cell).

– What direction do you think water will diffuse?• Water will diffuse IN AND OUT AT EQUAL RATES so

there is no net movement of water (it’s in equilibrium!)

Chapter 5

Hypertonic, Hypotonic, Isotonic Solutions

Section 1 Passive Transport

Table 5-1, pg 99

Section 1 Passive TransportChapter 5

Osmosis, continued• The cell wall of plants are strong enough to resist the pressure

(turgor pressure) exerted by water in the central vacuole.

• The condition of turgor pressure decreasing is called plasmolysis, and is the reason that plants wilt if they don’t receive enough water

• The bursting of cells due to the increase of turgor pressure is called cytolysis

Section 1 Passive TransportChapter 5

Facilitated Diffusion

• Facilitated diffusion is the transport of substances through a cell membrane along a concentration gradient with the aid of carrier proteins

• Carrier protein is a protein that transports substances across a cell membrane

– In facilitated diffusion, a molecule binds to a carrier protein on one side of the cell membrane.

– The carrier protein then changes its shape and transports the molecule down its concentration gradient to the other side of the membrane.

Section 1 Passive TransportChapter 5

Facilitated Diffusion

• Facilitated diffusion does not require additional energy, so it is considered passive transport

– Can work in two directions, both into and out of cell depending on concentration levels

• A good example of facilitated diffusion is the transport of glucose (sugar).

– Pg. 101, figure 5-5

Chapter 5

Facilitated Diffusion

Section 1 Passive Transport

Section 1 Passive TransportChapter 5

Diffusion Through Ion Channels

• Ion channels are proteins, or groups of proteins, that provide small passageways across the cell membrane through which specific ions can diffuse.– Each type of ion channel is usually specific for one type of

ion– Ex. A calcium ion channel will allow only calcium ions to

pass through it

– Ions diffuse by moving through gates in cell membrane: these gates may open or close in response to three kinds of stimuli: stretching of the cell membrane, electrical signals, or chemicals in cytosol or external environment

Chapter 5

Ion Channels

Section 1 Passive Transport

Section 2 Active TransportChapter 5

Objectives

• Distinguish between passive transport and active transport.

• Explain how the sodium-potassium pump operates.

• Compare endocytosis and exocytosis.

Section 2 Active TransportChapter 5

• Active transport moves molecules across the cell membrane from an area of lower concentration to an area of higher concentration.

• Unlike passive transport, active transport requires cells to expend energy (ATP).

Active Transport

•Contractile vacuoles are found in paramecia and help them pump water into and out of their cell. These require energy are a type of ACTIVE transport.

Cell Membrane Pumps:

Sodium-Potassium Pump

• Some types of active transport are performed by carrier proteins called cell membrane pumps.

• The sodium-potassium pump move three Na+ ions out of the cell for every two K+ ions it moves into the cytosol

• ATP supplies the energy that drives the pump

Animation

Chapter 5

Sodium-Potassium Pump

Section 2 Active Transport

Section 2 Active TransportChapter 5

Movement in Vesicles

• Some substances, such as macromolecules and nutrients, are too large to pass through the cell membrane by diffusion or osmosis– In endocytosis, cells ingest external materials by

folding around them and forming a pouch– This is transferring molecules into the cell

– The pouch then pinches off and becomes a membrane-bound organelle called a vesicle.

Movement in Vesicles: Endocytosis

• There are two types of endocytosis:– Pinocytosis (“cell drinking”) – vesicle contains

solutes or fluids

– Phagocytosis (“cell eating”) – vesicle contains large particles or other cells

Section 2 Active TransportChapter 5

Movement in Vesicles, continued

• Exocytosis– In exocytosis, vesicles made by the cell fuse with

the cell membrane, releasing their contents into the external environment (out of the cell).

– Cells may use exocytosis to release large molecules such as proteins, waste products, or toxins that would damage the cell if they were released within the cytosol

– Exocytosis involves ridding the cell of material by discharging it from sacs at the cell surface

Chapter 5

Endocytosis and Exocytosis

Section 2 Active Transport

These two types of movement are exact opposites of each other, in and out