THE TRANSPORT OF SUBSTANCES ACROSS A CELL MEMBRANE.

THE TRANSPORT OF SUBSTANCES ACROSS A CELL MEMBRANE

PASSIVE TRANSPORT

Processes that enable substances to move into and out of cells without an input of energy from the cell

Powered by a concentration gradient

Three types:

a) Diffusion

b) Osmosis

c) Facilitated Diffusion

PASSIVE TRANSPORT: DIFFUSION

Random movement of substances from regions of higher concentration to lower concentration

Continues until equilibrium (afterward no net change)

PASSIVE TRANSPORT: DIFFUSION

Factors affecting diffusion

- Relative concentrations

- How readily a molecule or ion crosses the membranea) Molecule size: faster for smaller moleculesb) Molecule polarity: faster for non-polar or small polar

moleculesc) Molecule/Ion charge: faster for uncharged molecules or

ions

- Temperature: faster at higher temperatures

PASSIVE TRANSPORT: OSMOSIS

The movement of WATER from higher concentration to lower concentration across a SEMI-PERMEABLE MEMBRANE

- Membrane is impermeable to the solute

- Water moves until concentrations are equal (afterward, no net change)


HYPERTONIC: Solution with higher concentration

HYPOTONIC: Solution with lower concentration

ISOTONIC: Solutions with equal concentrations

PASSIVE TRANSPORT: FACILITATED DIFFUSION

Diffusion across a membrane faciliated by a membrane protein

Two types of proteins exist

- Channel Proteins

- Carrier Proteins


CHANNEL PROTEINS

- Tubular shape composed of one or more helixes

- Can remain open or have gates that respond to a variety of signals (Ex: hormones, electric charge, presssure, light, etc)

- Generally permit ions or small polar molecules to pass


CHANNEL PROTEINS

- Exterior: amino acids with non-polar side chains

- Internal: highly specific for the molecules that can enter (Ex: sodium channels allow Na+ to enter)


CHANNEL PROTEINS

Cystic Fibrosis – defective chloride ion channel iwhich interrupts movement of water into and out of cell

- Results in thick mucus in the breathing passages and pancreas


CARRIER PROTEINS

- Bind a specific molecule and change shape to transport them across cell membrane

- Generally transport larger molecules (Ex: glucose, amino acids)

- Slower rate of diffusion than channel proteins


CARRIER PROTEINS

- Exterior: non-polar amino acids

- Interior: lined with amino acids that are specific to the molecule being carried


CARRIER PROTEINS

Cystinurea – inability for carrier proteins to remove cystine and other amino acids from urine

- Cystine crystallizes into painful stones that block urine flow

ACTIVE TRANSPORT

Active transport of substances across a cell membrane against their concentration gradient

Requires energy usually in the form of ATP

ACTIVE TRANSPORT

ATP – Adenosine triphosphate

- Derived from an adenosine nucleotide but with a triple phosphate group

- The hydrolysis of the end phosphate releases energy

ACTIVE TRANSPORT

ATP – Adenosine triphosphate

ADP – Adenosine diphosphate

AMP – Adenosine monophosphate

ACTIVE TRANSPORT

Two types of active transport:

a) Primary Active Transport

b) Secondary Active Transport

ACTIVE TRANSPORT- PRIMARY ACTIVE TRANSPORT

Cellular process using ATP directly to move molecules or ions across a cell membrane


SODIUM-POTASSIUM PUMP

- 3 Na+ transported out of the cell against its gradient

- 2 K+ transported into the cell against its gradient



- 3 Na+ bind the ion pump on the inside of the cell membrane

- ATP binds the ion pump and is hydrolysed to ADP + Pi

- ADP is released and Pi attaches to ion pump

- Ion pump changes shape

- Na+ released outside cell and 2 K+ bind

- Pi is released from the pump and the pump returns to its original shape releasing 2 K+ inside cell

ACTIVE TRANSPORT- SECONDARY ACTIVE TRANSPORT

Ion pumps result in:

- A difference in charge/electric potential

- A difference in concentration gradient

*** Electrochemical gradient ***


Use of an electrochemical gradient as a source of energy to transport molecules or ions across a cell membrane


HYDROGEN-SUCROSE PUMP

- H+ pumped out of the cell by a hydrogen ion pump using ATP

- Resulting electrochemical gradient powers movement of sucrose against its gradient from outside to inside of cell


HYDROGEN-SUCROSE PUMP

MEMBRANE-ASSISTED TRANSPORT

Used to transport large macromolecules

Requires energy from cell

Two types:

a) Endocytosis

b) Exocytosis

MEMBRANE-ASSISTED TRANSPORT - ENDOCYTOSIS

Cell engulfs material

Folds cell membrane around material and pinches off to form a vesicle inside the cell

Three types:

a) Phagocytosis

b) Pinocytosis

c) Receptor-Mediated Endocytosis


PHAGOCYTOSIS – cell eating

- Endocytosis with solid particles


PINOCYTOSIS – cell drinking

- Endocytosis with liquid particles


RECEPTOR-MEDIATED ENDOCYTOSIS

- Receptor proteins are found on a portion of the cell membrane (coated pit)

- Receptor proteins bind molecules and pit folds inwards to form a vesicle

- Contents are used by cell or digested by cell

- Receptor proteins are recycled into the cell membrane


RECEPTOR-MEDIATED ENDOCYTOSIS

MEMBRANE-ASSISTED TRANSPORT - EXOCYTOSIS

Vacuoles fuse with cell membrane to release contents outside the cell

Vesicle becomes part of cell membrane

MEMBRANE-ASSISTED TRANSPORT - EXOCYTOSIS

Ex: Plant cells – used to construct cell wall

Animal cells – releasing hormones, neurotransmitters, digestive enzymes, etc

THE TRANSPORT OF SUBSTANCES ACROSS A CELL MEMBRANE.

Documents

Transcript of THE TRANSPORT OF SUBSTANCES ACROSS A CELL MEMBRANE.