TRANSPORT OF MATERIAL ACROSS

THE CELL

PROCESS OF TRANSPORT ACROSS THE PLASMA MEMBRANE

•Motion of substances in and out of the cell

• Cell membranes are Selectively permeable

• Two Types of Transport Mechanisms:

1. Passive Transport 2. Active Transport

Membrane transport

•Passive transport is movement of molecules through the membrane in which no energy is required from the cell

•Active transport requires energy expenditure by the cell

1. Passive Transport

•Passive transport is movement of molecules through the membrane in which no energy is required from the cell.

• Molecules move in response to a concentration gradient.

- A concentration gradient is a difference between the concentration on one side of the membrane and that on the other side.

• Passive transport mechanisms only movement substances along the concentration gradient.

1. Passive Transport

•Passive transport mechanisms only movement substances along the concentration gradient:

- Substances move from an area of high concentration to an area of low concentration

1. Passive Transport

•Mechanisms of Passive Transport:

1. Diffusion

- movement of solute molecules from high solute concentration to low solute concentration

2. Osmosis

- movement of solvent water from high solvent concentration to low solvent concentration

Diffusion

• Diffusion is movement of solute molecules from high concentration to low concentration.

Diffusion

•There are two types of diffusion

1. Simple Diffusion

2. Facilitated Diffusion

1.Simple Diffusion

•Substances pass directly through the cell membrane

• The cell membrane has limited permeability to small polar molecules, water, and ions

• The motion of water across the membrane is known as osmosis

•The rate (molecules/s) of simplediffusion depends on the degree ofconcentration gradient

•As the gradient reaches equilibrium,diffusion slows

•At equilibrium, substances pass in andout of the membrane at equal rates

2.Facilitated Diffusion

•Substances must pass through transported proteins to get through the cell membrane.• The cell membrane is selectively permeable.•Carrier proteins bind to the molecule that they transport across the membrane.

2.Facilitated Diffusion

•Selective permeability: integral membrane proteins allow the cell to be selective about what passes through the membrane.

- Channel proteins have a polar interior allowing polar molecules to pass through.

- Carrier proteins bind to a specific molecule to facilitate its passage.

ion channels

Carrier Proteins

•Carrier proteins bind to a specific molecule to facilitate its passage.

Osmosis

•Osmotic concentration is determined by the concentration of all solutes in solution.

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2. Active Transport

Active transport

• Requires energy – ATP is used directly or indirectly to fuel active transport

• Able to moves substances against the concentration gradient - from low to high concentration

- allows cells to store concentrated substances

• Requires the use of carrier proteins

Active transport

Fig. 5.15-2

Fig. 5.15-3

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Bulk Transport

• Bulk transport of substances is accomplished by

1. Endocytosis – movement of substances into the cell

2. Exocytosis – movement of materials out of the cell

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Bulk Transport• Endocytosis occurs when the plasma membrane envelops food particles and liquids.

1. phagocytosis – the cell takes in particulate matter

2. pinocytosis – the cell takes in only fluid

3. receptor-mediated endocytosis – specific molecules are taken in after they bind to a receptor

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