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Chapter 4 The

Movement of

Substances into andout of cells

All cells are delimited by

a plasma membrane.

The size of cells is

limited by the surfacearea of plasma

membrane available for

transport of materials

into and out of the cell.

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Cells regulate internal

environment (homeostasis)

- regulate exchange ofmaterials with external

environment

- chemical concentration- pH

- electrical potential (voltage)

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Plasma Membrane

Fluid Mosaic Model composed of phospholipids, proteins,

steroids

- selectively (differentially) permeable boundary between the

cell and the external environment

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Substances

Vary in TheirRates of

Penetration

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Cells Maintain Gradients

Transmembrane gradient -

Concentration of a solute is

higher on one side of a

membrane than the other.

Ion electrochemical gradient -

Both an electrical gradient and

chemical gradient.

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The Functions Of Membranes1. Membranes define boundaries and serve as

permeability barrier

2. Membranes are sites

of organization and

localization

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The Functions Of Membranes

Passive Transport

a. Simple diffusion: H2O, O2,

CO2, and ethanol

high low

b. Facilitated diffusion with

the aid of transport proteins

high low

3. Membranes regulate the transport of solutes

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Active Transport

a.Pump proteins for

maintaining ion gradient

across the membrane: low->

high. (requires energy input)

b. Endocytosis and

exocytosis

3. Membranes regulate the transport of solutes

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4. Membranes detect and transmit electrical and

chemical signals signal transduction: detectsignals from outside of the cell and transmit them

into the cell interior.

5. Membranes mediatecell-to-cell

communication

intracellular

communication providedby gap junctions (animal

cells) and

plasmodesmata (plant

cells).


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Diffusion the movement of a substance from an area of

higher concentration to an area of lower

concentration

- results from kinetic energy of random motion of the

molecules

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Passive Transport

Passive transport does not require an input of energy

Substances move down or along concentration gradient

Two types:

Simple passive diffusion - Diffusion of a solute

through a membrane without transport protein

high low

Facilitated diffusion - Diffusion of a solute through a

membrane with the aid of a transport protein

high low

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Passive

Transport

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Osmosis

Water diffusion across a selectively permeableplasma membrane from an area with lowersolute concentration (higher waterconcentration) to an area with higher soluteconcentration (lower water concentration).

If the solutes cannot move, water movement canmake the cell shrink or swell as water leaves orenters the cell.

Osmotic pressure the pressure that wouldneed to be applied to stop the movement ofwater down a concentration gradient

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Water will move from

an are of higher water

concentration (lower

solute concentration)to an area of lower

water concentration

(higher solute

concentration).

- Net movement of

water

Campbell & Reece 2005

Where is the concentration of water higher or

where is the concentration of solute lower?

Osmosis

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Water Potential

-the stored energy of water that results from its

position (relates to pressure)

- water will move from an area of higher water

potential to an area of lower water potential

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Water will move from

an are of higher water

concentration (higher

water potential) to anarea of lower water

concentration (lower

water potential).

Campbell & Reece 2005

Where is the concentration of water higher?

Osmosis

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Solutions

Isotonic - Equal water and solute concentrations oneither side of the membrane.

Hypertonic - Solute concentration is higher (andwater concentration lower) on one side of themembrane.

Hypotonic - Solute concentration is lower (and waterconcentration higher) on one side of the membrane.

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Isotonic

Hypertonic

Hypotonic

Outside the cell Inside the cell

The solution andcell are isotonic

The solution is

hypertonic to the cell

The solution is

hypotonic to the cell

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Where is the concentration of solute lower?

or


Salt molecule

Elodea cell

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Where will the water move?

Salt molecule

Elodea cell

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Hypotonic

a solution in which the solute concentration is less

than that in the cell

- net movement of water into the cell

Cell is turgid water enters

central vacuole until the

vacuole is fully extended,

pushing cytoplasm up

against cell wall

In cells with no cell wall(animal cells) cell ruptures

(lysis)

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Elodea sp.

COASTAL FRESHWATER POND

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Salt molecule

Elodea cell

Where is the concentration of solute lower?

or


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Salt molecule

Elodea cell

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Hypertonic

a solution in which the solute concentration is

higher than that in the cell

- net movement of water out of the cell

Cell is plasmolyzed water

leaves the cell and the

protoplast shrinks away fromthe cell wall

Wilted leaf

Cell dies if plasma membrane

completely detaches from the

cell wall

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Animal Cells

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Isotonic

Hypertonic

Hypotonic

Outside the cell Inside the cell

The solution andcell are isotonic

The solution is

hypertonic to the cell

The solution is

hypotonic to the cell

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Plasma Membrane

Transport Proteins

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- transport polar molecules and ions across plasmamembrane

- highly selective (solute-specific)

Classes of Transport Proteins:

1. pump driven by energy from ATP or light

2. carrier3. channel

Transport Proteins

driven by energy fromelectrochemical gradients

Transport Proteins

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2. carrier 3. channel

- bind solute, undergo - form water-filled pores,conformational (shape) extend across membrane,change, carry solute across - open to allow dissolved

membrane solutes (ions) to cross

membrane- 500-10,000 molecules/sec - 10,000+++ molecules/sec

Transport Proteins- driven by energy from electrochemical gradients

high

low

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Channels

Form an open

passageway for the

direct diffusion of

ions or moleculesacross the

membrane and

down their gradient.

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Carriers Also known as

transporters

Conformational change

transports solute

Principal pathway forthe uptake of organic

molecules, such as

sugars, amino acids,

and nucleotides

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Passage of Solutes

channel, some carrier

carrier

Transport Proteins

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1. pump

- Plants and fungi proton pumps (H+-ATPase)

-

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Active Transport

Movement of a solute across a membrane againstits gradient from a region of low concentration tohigher concentration.

Energetically unfavorable and requires the input ofenergy (ATP).

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Couples conformational (shape) changes to

an energy source, such as ATP

Pump

P i d S d A i T

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Primary and Secondary Active Transport

Carrier - Cotransport, symportPump

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Vesicle-Mediated Transport

- Transport larger moleculessuch as proteins andpolysaccharides, and even verylarge particles.

Exocytosis - Material inside thecell, which is packaged intovesicles, is secreted into theextracellular medium.

i.e., polysaccharides(hemicellulose, pectin)

Endomembrane System

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Endomembrane System

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Vesicle-Mediated Transport

Transport larger molecules such as proteins andpolysaccharides, and even very large particles.

Endocytosis - Plasma membrane folds inward, to form

a vesicle that brings substances into the cell. Phagocytosis

Pinocytosis

Receptor-mediated endocytosis

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Phagocytosis-cell eating

- Ingestion of large, solid particles(bacteria, cellular debris) invesicles derived from plasmamembrane

- amoeba, cellular slime molds

- plants - root hair cells of legumesingest Rhizobium (nitrogen-fixing

bacterium)

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Pinocytosis-cell drinking

- cells take in liquids in vesiclesderived from plasma membrane

- Occurs in all eukaryotic cells

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Receptor-Mediated Endocytosis

coated pits

- specialized depression in plasmamembrane coated on thecytoplasmic surface with clathrin

- protein receptors on outer surfaceof plasma membrane bind moleculeto be transported into cell

- coated vesicle forms

-clathrin coat is shed

- vesicle fuses with endomembrane,

releases contents

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Cell-to-Cell Communication

Signal synthesized in one cell and transported toanother (chemical messengers)

Signal Transduction cell converts external signal to aresponse

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Cell-to-Cell Communication

-Cytoplasmic connectionbetween cells

Plasmodesmata plantcells

Gap-Junction animalcells

bio 109 chapter 4

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