3.3 Transporting Molecules across the cell membrane

Extracellular (interstitial) fluid composition?

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• Has a higher concentration of sodium (Na⁺).

• Has a lower concentration of potassium (K⁺).

Intracellular fluid composition?

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• Has a higher concentration of potassium (K⁺).

• Has a lower concentration of sodium (Na⁺).

The plasma membrane is selective meaning what?

only certain substances can cross easily.

Passive transport is what?

movement that does NOT require ATP

Passive transport:

Substances move concentration gradient how?

____________→ ____________

High concentration → Low concentration

In passive transport substances can?

follow chemical/ and or electrical gradient when charged particles are involved.

Passive Transport
Diffusion is?

movement of a solute from high → low concentration.

Diffusion is caused by?

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• Caused by random molecular movement (Brownian motion)

• At equilibrium, molecules are still moving, but there is no net movement.

Simple diffusion =

= solute moves directly through the phospholipid bilayer.

Characteristics of simple diffusion?

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• High → low concentration

• No ATP

• No transport protein needed

• Works best for small, nonpolar/hydrophobic molecules

Factors that affect diffusion

Diffusion is faster with:

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• Higher temperature

• Smaller solute

• Less viscous solution

• Shorter distance

• Thinner membrane

• Larger surface area

• Steeper concentration gradient

Facilitated diffusion =

= a solute moves high → low concentration using a transmembrane transport protein.

What is facilitated diffusion used for?

Used for substances that cannot easily cross the phospholipid bilayer.

What are two types of transport proteins:

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• Channel proteins

• Carrier proteins

Characteristics of Channel proteins?

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• Form a water-filled pore/tunnel through the membrane.

• Solutes pass through the opening.

• Can allow many particles to pass quickly.

Characteristics of Carrier Proteins?

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• Have a specific binding site for a solute.

• Solute binds → protein changes shape → solute is moved across.

• Can become saturated when all binding sites are occupied.

Ion Channels allow what?

allow ions to move across the membrane.

Types of ion channels include? (when open)

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• Leakage channels → generally open

• Ligand-gated channels → open when a chemical/ligand binds

• Voltage-gated channels → open when membrane voltage changes

• Mechanosensitive channels → open when the membrane is mechanically deformed

What is osmosis?

 Diffusion of water (solvent) across a semipermeable membrane.

Water moves:

____________→ ______________

Low osmolarity → High osmolarity

What is Aquaporins?

Aquaporins are channels that allow water to cross the cell membrane.

What is osmolarity?

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• the total concentration of solutes in a solution.

• both permeable and nonpermeable solutes.

• Measured in osmoles per liter (Osm/L).

What is tonicity?

concentration of nonpermeable solutes across the membrane.

What does Tonicity predict?

predicts where water will move and what happens to the cell.

What is isotonic?

Equal concentration of effective/nonpermeable solutes inside and outside.

What happens to RBC during isotonic?

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• Water moves in and out at equal rates.

• No net water movement

• RBC stays its normal biconcave shape.

what is hypertonic?

Higher concentration of nonpermeable solutes outside the cell.

What happens to RBC during hypertonic?

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• Water moves out of the RBC

• RBC loses water

• Cell becomes shriveled/crenated

What is hypotonic?

Lower concentration of nonpermeable solutes outside the cell.

What happens to RBC during hypotonic?

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• Water moves into the RBC

• RBC swells

• In extreme conditions, it can lyse (burst).

Active transport moves substances ___________ their concentration gradient:

_____________ → ____________

• Against

• Low concentration → High concentration

Does Active transport require energy?

yes

does Primary active transport directly use ATP or not?

yes

Na⁺/K⁺ ATPase Pump

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The pump:

• Moves 3 Na⁺ OUT

• Moves 2 K⁺ IN

• Uses 1 ATP

• Both ions are moved against their concentration gradients.

(3 Na⁺ OUT → 2 K⁺ IN → 1 ATP)

Does secondary active transport directly use ATP or no?

does not directly use ATP

what does secondary active transport use?

electrochemical gradient created by primary active transport.

What is Symporter?

Moves substances in the same direction.

What is Antiporter?

Moves substances in opposite directions.

What is specific carrier protein?

A membrane protein that binds a specific substance and changes shape to move it across the cell membrane.

what is Vesicular Transport?

Used to move large materials that cannot cross the membrane through diffusion or transport proteins.

Exocytosis =

out

Characteristics of Exocytosis?

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• Vesicle moves to the plasma membrane.

• Vesicle fuses with the membrane.

• Contents are released outside the cell.

• The vesicle membrane becomes part of the plasma membrane.

Endocytosis =

in

Characteristics of Endosome?

• Plasma membrane folds inward.

• It forms a vesicle called an endosome.

• Material from outside the cell is brought into the cell.

• The vesicle can fuse with lysosomes for breakdown/recycling.

Phagocytosis = ____?

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Phagocytosis = “cell eating”

• Takes in large solid particles.

• Example: cellular debris or bacteria.

Pinocytosis = _______?

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Pinocytosis = “cell drinking”

• Takes in fluid and small dissolved solutes.