Ch. 7: Transport Across the Cell Membrane

Selectively permeable: • property of biological membranes

which allows some substances to pass more easily than others

Protein: too big to pass through

Selectively Permeable Membrane

Water molecule: can pass through pore

Enlargement of Membrane

Transport proteins:

• membrane proteins that transport specific molecules or ions across biological membranes:– may provide hydrophilic tunnel

through membrane– may bind to a substance and physically

move it across the membrane– are specific for the substance they

move

GLUCOSE

Binding

TransportRecovery

Dissociation

Movement across the cell membrane can be:

1. Passive • cell does not have

to expend energy

2. Active• energy-requiring process during which a

transport protein pumps a molecule across a membrane, against its conc. gradient; is energetically “uphill”

Passive Transport:

DIFFUSION • net movement of a substance

down a concentration gradient – results from KE of molecules– results from random molecular

movement– continues until equilibrium is

reached (molecules continue to move but there is no net directional movement)

Passive Transport: OSMOSIS

• diffusion of water across a selectively permeable membrane; water moves down its concentration gradient – continues until equil. is reached– at equil. water molecules move in

both directions at same rate

INSIDE THE CELL

OUTSIDE THE CELL

– relationship between solute concentration and movement of water can be described as:

• HYPERTONIC: a solution w/a greater solute concentration than that inside a cell

• HYPOTONIC: a solution w/a lower solute concentration compared to that inside a cell

• ISOTONIC: a solution w/an equal solute concentration compared to that inside a cell

Osmotic Environment

• Osmotic potential: A measure of the potential of water to move between regions of differing concentrations across a water-permeable membrane – osmotic potential of pure water = 0– more solute = negative osmotic

potentialHI LO

EX: Ψ= -0.40 (sol’n)

Ψ= -0.23 (cell)Water out

Glass tube

Sugar solution

Osmotic pressure

In cells with cell walls: • in a hypertonic environment,

PLASMOLYSIS occurs; cells shrivel and usually die

• in a hypotonic environment, water moves into cell, causing it to swell; cell becomes more TURGID.

Passive Transport: FACILITATED DIFFUSION

• diffusion of solutes across a membrane, with the help of transport proteins; passive transport because it is movement down a concentration gradient

ACTIVE TRANSPORT: • protein pumps use energy from

ATP

Examples of Active Transport protein

“pumps”:1. Sodium-Potassium Pump:

– actively pumps Na+ ions out / K+ ions in

– in every pump cycle, 3 Na+ leave and 2 K+ enter cell

– Na+ and K+ are moved against their gradients (both concentration and electric potential!)

OUTSIDE

INSIDE

• Membrane Potential: voltage across membrane; cell’s inside is negatively charged w/respect to outside – favors diffusion of cations into

cell and anions out of cell

• Electrochemical Gradient: diffusion gradient resulting from the combined effects of membrane potential and conc. gradient

**The Na+-K+ pump maintains the membrane potential…HOW?**

2. Proton Pump: pumps protons (H+ ions) out of the cell, creating a proton gradient (protons are more concentrated outside the membrane than inside); – protons then diffuse back into cell – the force of the proton pushing back

through the membrane is used to power the production of ATP

AND…

3. Cotransport / Coupled Channels: process where a single ATP-powered pump actively transports one solute and indirectly drives the transport of other solutes against their conc. gradients.

– Example: plants use a proton pump coupled with sucrose-H+ transport to load sucrose into specialized cells

ACTIVE TRANSPORT: EXOCYTOSIS & ENDOCYTOSIS

• transport of large molecules (e.g. proteins and polysaccharides) across cell membrane

Exocytosis Endocytosis

* exporting macromolecules by fusion of vesicles w/the plasma membrane* vesicle buds from ER or Golgi and migrates to plasma membrane* used by secretory cells to export products (e.g. insulin in pancreas)

* importing macromolecules by forming vesicles derived from plasma membrane* vesicle forms in localized region of plasma membrane* used by cells to incorporate extracellular substances

Three types of Endocytosis:

1) Phagocytosis: solid particles (“cell eating”)

2) Pinocytosis: fluid droplets (“cell drinking”)

3) Receptor-Mediated Endocytosis:

importing of specific macromolecules by

inward budding of vesicles formed from

COATED PITS

Phagocytosis

Pinocytosis

Receptor-Mediated Endocytosis

The cell in this beaker is _______ to the solution.

A. Hypotonic

B. Hypertonic

C. Isotonic

What type of transport does this illustrate?

A. Diffusion

B. Facilitated diffusion

C. Active transport

D. Osmosis

Which of the following diagrams does NOT represent an electrochemical gradient?

A BC

What process does this diagram represent?A. Proton pump

B. Exocytosis

C. Endocytosis

D. Cotransport