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Active Transport, Endocytosis, and Exocytosis

Section 3.5

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Objectives

• SWBAT describe active transport.• SWBAT distinguish among endocytosis,

exocytosis, and phagocytosis.

• Main Ideas– Proteins can transport materials against a

concentration gradient.– Endocytosis and exocytosis transport materials

across the cell membrane in vesicles.

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Vocabulary Section 3.5

• Endocytosis (endocitosis)• Exocytosis (exocitosis)• Phagocytosis (fagocitosis)• Active transport (transporte activo)• ATP (adenosine triphosphate)

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Starter

• Small lipid molecules are in high concentration outside a cell. They slowly cross the membrane into the cell. What term describes this action? Does it require energy?

• Diffusion and no.

Two Types of Transport• There are two ways of transporting materials

across a cell membrane. They are:– Passive Transport – we have already looked at this.– Active Transport – actively drives molecules

across the cell membrane from a region of lower concentration to a region of higher concentration. Requires energy input.

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Active Transport

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Active Transport

Active transport drives molecules across a membrane from region of lower concentration to a region of higher concentration (remember our bicycle example).

ATP

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Active Transport

There are transport proteins that allow diffusion but there are others, often called pumps, that move materials against the concentration gradient.

ATP

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Active Transport

Active transport requires energy input from a cell and enables a cell to move a substance against its concentration gradient.

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Active Transport

Cells use active transport to get needed molecules across their cell membranes regardless of the concentration gradient to maintain homeostasis.

Sodium/Potassium Pump

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Both facilitated diffusion and active transport are used in neurons (brain cells) to propagate action potentials (electrical impulses). The sodium and potassium channels allow sodium ions and potassium ions to move across the cell membrane by facilitated diffusion. The movement of the ions is what allows the action potential to propagate along the neurons axon.

To move the sodium and potassium ions back to the sides of the neuron from which the came (sodium on the inside and potassium on the outside), a sodium potassium pump (using ATP) is employed by the cell. The return of sodium and potassium to their original sides of the cell membrane “resets” the neuron – preparing it for the next action potential.

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Question

• Are the protein pumps, like the Na/K pump in neurons, active or passive transport?

• Explain how a protein pump works?• What kind of energy does it use?

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ATP (Adenosine Triphosphate)

ATP is chemical energy made in a cell’s mitochondria.

Besides in neurons, like we have already seen, ATP is needed to drive many other processes – including the making of ATP.

ATP is used in the mitochondrial proton pump, moving hydrogen ions (H+) across the inner mitochondrial membrane. This proton pump is essential for the creation of ATP.

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ATP

Like with sodium and potassium in the neuron, the hydrogen ions are “pumped,” using ATP, across the mitochondrial inner membrane (against the H+ gradient).

They then diffuse across the membrane through a protein channel (an enzyme called ATP Synthase).

The enzyme uses the movement of the H+ to create ATP from a precursor called ADP.

ATP spent in active transport

ATP created via diffusion through protein channel

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ATP Video

• http://highered.mcgraw-hill.com/sites/9834092339/student_view0/chapter38/proton_pump.html

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Transport Proteins• All transport proteins/enzymes (which are

proteins) span a membrane.• Most change shape when they bind to a target

molecule or molecules. • As we have seen, some transport proteins bind to

only one type of molecule. • Others bind to 2 different types.

– Those that bind to two types can move both types of molecules either one way or opposite directions (like the sodium/potassium pump we saw in the neuron.

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Question

• How do transport proteins that are pumps differ from those that are channels?

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Endocytosis

• Endocytosis – the process of taking liquids or fairly large molecules into a cell by engulfing them in a membrane. – The cell membrane makes a pocket around the

substance.

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Endocytosis

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Endocytosis

• The pocket breaks off inside the cell and forms a vesicle.– The vesicle then fuses with a lysosome.

Lysosomal enzymes break down the vesicle membrane and the vesicle’s contents are release into the cell.

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Phagocytosis

• Phagocytosis – the word literally means “cell eating.”

• It is a special type of endocytosis which plays a major role in your immune system.

• White blood cells find foreign materials, such as bacteria, engulf them and destroy them. – They are your body’s enforcers.

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Exocytosis

Exocytosis is the opposite of endocytosis. It is the release of substances out of a cell by the fusion of a vesicle with the cell membrane.

Note: the vesicle pinches off from the Golgi Apparatus. Remember that the Golgi Apparatus is the cell’s “storage, packing, and shipping center.”

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Exocytosis

1. The cell forms a vesicle around material that needs to be removed or secreted.

2. The vesicle is transported to the cell membrane.

3. The vesicle membrane fuses with the cell membrane and releases the contents.

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Illustration of a Cell

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Exocytosis in Neuron

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Exocytosis in Neuron

The neurotransmitter is stored in vesicles in the terminus of the axon of the neuron.

When the action potential arrives, the vesicles, via exocytosis, release their neurotransmitter (for example, acetylcholine) into the synaptic cleft.

Membrane receptors on the neuron on the other side of the synaptic cleft stimulate is stimulated by the neurotransmitter to “fire” – transferring the action potential from one neuron to the other.

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Exocytosis – Neuron to Muscle

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Questions

• How do endocytosis and exocytosis differ from one another?

• List one function that exocytosis carries out in the human body?

• List one function that endocytosis carries out?

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Questions

• What might happen if vesicles in your neurons were suddenly unable to fuse with the cell membrane?

• How do endocytosis and exocytosis differ from diffusion?