2.3 Cell Transport

Selective Permeability

1. Permeable - small, uncharged mlc (CO2, O2)

2. Somewhat Perm. - small, polar mlc (H2O)

3. Not Perm. - large (proteins), charged (K+, Na+)

Gradients

1. Concentration Gradient

2. Electrochemical Gradient

Concentration Gradient

• differential concentrations of a substance across a space or membrane

• substance want to move from an area of high conc. to low conc.

• substance A will generally ignore conc. grad. of substance B

Electrochemical Gradient

concentration gradient + charge gradientCha

Charge Gradient

differential charges of a substance across a space or a membrane

BPQ

why does each substance maintain a concentration gradient in this system

Some mlc. are charged and others are too large and cannot easily pass through cell membrane → conc. grad. is maintained

Transport Across the Cell Membrane

1. Passive Transport

2. Active Transport

3. Endocytosis/Exocytosis

Passive Transport

• with the conc. grad.

• no energy needed

1. Simple Diffusion

2. Facilitated Diffusion

Active Transport

• with or against conc. grad.

• requires energy

1. Pumps

2. Secondary Active Transport

3. Endocytosis/Exocytosis

Endocytosis/Exocytosis

• moves large items or items in bulk

• uses vesicles

• also a form of active transport

Endocytosis forms

1. Phagocytosis

2. Pinocytosis

3. Receptor-Mediated Endocytosis

Simple Diffusion

• movement of a subst. from high to low conc. thru a plasma membrane

  • small nonpolar, small polar mlc. only

Factors that Affect Speed of (Simple) Diffusion

1. Conc. Grad. - higher conc. = diffuses faster

2. Size of mlc. - smaller = faster

3. Temperature - higher temp = faster

4. Solvent Density - lower density = faster

5. Solubility - more soluble = faster

   • hydrophobic vs hydrophilic

6. SA & Thickness of Plasma Membrane - more SA & thinner = faster

7. Distance Traveled - shorter dist. = faster

Facilitated Diffusion

• moves subst. across a plasma membrane from high to low conc. with use of integral proteins

  • small polar mlc., charged particles/ions

Channel Proteins

• Open to both sides of the plasma membrane

• No binding sites

• Diffuse faster

(Facilitated Diffusion ; integral protein)

Carrier Protein

• open to only one side of the plasma membrane at a time

• Binding sites

  • very selective

• Diffuse slower

(Facilitated Diffusion ; integral protein)

Aquaporin

• channel protein for water

(Facilitated Diffusion ; integral protein)

BPQ

why does water need a channel protein?

H2O can do simple diffusion, but Aquaporin helps move water (small, polar mlc) easily past the hydrophobic region of the cell membrane → faster

Pumps

• Carrier Proteins that move subst. against the conc. grad. using ATP

Types of Pumps

1. Uniporter

2. Symporter

3. Antiporter

   1. Sodium Potassium Pump

Uniporter

carries one mlc or ion (in one direction)

Symporter

carries two mlc in the same direction

Antiporter

carries two mlc in opposite directions

Sodium Potassium Pump

• Antiporter

  • 3 Na+ out of cell

  • 2 K+ into cell

Sodium Potassium Pump Function

• create cell resting potential

  • cell is slightly more negative

  • opposite of an action potential in nerve cells

• regulate osmolarity

• transport of other substances

Secondary Active Transport

Moving substances using the conc. grad. created from primary active transport

BPQ

why is so much energy spent on Sodium Potassium Pumps?

Sodium Potassium Pumps create ATP that helps facilitate the transport of other mlc.

• ex. Na helps glucose move against glucose’s conc. grad. via secondary active transport

Endocytosis

• plasma membrane of the cell invaginates, forming a pocket around the target particle

1. Phagocytosis

2. Pinocytosis

3. Receptor-Mediated Endocytosis

Phagocytosis

• intake of food or a pathogen to be broken down in a lysosome

Phago = eat

Pinocytosis

• intake of small mlc that dont need to be broken down in a lysosome → usually solutes

pina colada

Receptor-Mediated Endocytosis

• Receptor proteins are used to capture a specific target mlc

• viruses use these to enter the cell

Exocytosis

• The reverse of endocytosis

• used to expel material from a cell

  • waste products

  • secreted proteins