Dr Fatahiya Kashif The Cell. Dr Fatahiya Kashif The Cell.

Dr Fatahiya Kashif The Cell

Transport across the cell membrane


Objectives

• To appreciate how solutes and other materials move across the cell membrane

• Become familiar with different types of transport proteins

• An understanding of the importance of membrane transport to living cells


TYPES OF TRANSPORT

Passive transport Active transport


Passive transport

• Simple diffusion • Facilitated diffusion


Simple diffusion

• Downhill


SIMPLE DIFFUSION


Simple diffusion

• Fick law

J = P X A(C1-C2)

• J = rate of diffusion mmol/s• P = permeability coefficient • A = membrane surface area cm²• C = concentration of the substance mmol/L


Factors affecting diffusion rate

• Permeability coefficient – Diffusion coefficient• Temperature• Molecular radius • Viscosity

– Partition coefficient• Solubility in oil vs water

– Thickness of the membrane/distance • Lungs • Placenta



• Surface area– Lungs – Intestinal lining– Capillaries



• Concentration gradient – Effect on the rate?– What is equilibrium state? – How to achieve a constant rate of diffusion?– Gas exchange in lungs


FACILITATED DIFFUSION


Facilitated diffusion

• Carrier proteins – Glucose transporters• GLUT family • GLUT 1 all cells• GLUT 4 insulin dependent

– Muscle – Adipose


Simple diffusion vs carrier mediated/facilitated diffusion

Carrier mediated

Simple diffusion

Tran

spor

t rat

e

Concentration

Transport maximum (Tm)


Transport of charged ions

• What happens to charged molecules?– Permeability of the membrane• Carriers

– Electrical gradient– Electrochemical equilibrium– How to maintain an electrochemical gradient


ACTIVE TRANSPORT


Active transport

• Primary active transport • Secondary active transport


PRIMARY ACTIVE TRANSPORT Uphill


Pumps

• ATPases• Na⁺-K⁺ pump• Ca⁺⁺ ATPase• H⁺-K⁺ ATPase


Na⁺-K⁺ ATPase

• All cells • Electrical signaling in neurons• Secondary active transport


SODIUM

POTASSIUM


ATPADP+PiMg++

SODIUM

POTASSIUM


SODIUM

POTASSIUM


Ca⁺⁺ ATPase

• Intracellular Ca⁺⁺ concentration• Sarcoplasmic reticulum


Primary active transport of calcium


CALCIUM

Primary active transport of Calcium


CALCIUM


ATP


CALCIUM


ADP+Pi


H⁺-K⁺ ATPase

• Stomach • Kidney


H2CO3

HCO3-Cl-

Primary active transport of Hydrogen ions

Gastric parietal cellPlasma Lumen

ATPaseH+


H2CO3

H+

HCO3- Cl-



ATPase


H2CO3

H+HCO3- Cl-HCl




SECONDARY ACTIVE TRANSPORTUphill


Secondary active transport

• Cotransport/symport– Na⁺-glucose– Na⁺-aminoacid– Na⁺-Cl⁻ cotransporter– K⁺- Cl⁻ cotransporter– Na⁺- K⁺-2 Cl⁻ cotransporter

• Countertransport – Na⁺- Ca⁺⁺ exchanger– Na⁺- H⁺ exchanger– Cl⁻-HCO3⁻ exchanger


Glucose

Sodium

Co-transport of glucose with sodium ions

Dr Fatahiya Kashif The CellDr Fatahiya Kashif

Cotransport of ions

300

600

1200

Med

ull

aC

ort

ex

K+Na+2Cl-


Red blood cells

H2OCO2


Red blood cells

H2CO3

HCO3- H+


Red blood cells

H2CO3

H+HCO3-Cl-


RENAL TUBULAR CELL

TUBULAR LUMEN

INTERSTITIAL FLUID

BLOOD

3Na+

2K+

H2CO3

HCO3- H+

Renal regulation


RENAL TUBULAR CELL

TUBULAR LUMEN

INTERSTITIAL FLUID

BLOOD

H2CO3

HCO3- H+

Na+


Calcium

Sodium

Sodium counter-transport of Calcium ions


GLUT 5

GLUT 2

3Na+

2K+

O

Na+

O

Na+

Na+

Na+

Na+

Na+

O

SGLT-1

O

ONa+

O


GLUT 5

GLUT 2

O

Na+

O

Na+

Na+

Na+

Na+

Na+

O

SGLT-1O

Na+

O

GLUT 2O


Na+3Na+

2K+


Na+

Dr Fatahiya Kashif The Cell. Dr Fatahiya Kashif The Cell.

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