Chapter 5

Homeostasis and Transport

Types of Transport

Passive transport does not require energy to complete.

Cells move materials across the membrane without energy.

This includes: diffusion, osmosis, facilitated diffusion, and diffusion through ion channels.

Diffusion Diffusion is the movement of materials across a

membrane using a concentration gradient. A concentration gradient is the difference

between the number of molecules on each side of the membrane.

Diffusion happens because the molecules wish to be in equilibrium or be equal on both sides of the membrane.

Even once equilibrium is achieved, molecules still move back and forth across the membrane randomly.

Osmosis

Osmosis is the diffusion of water across the cell membrane.

Only water is moved in osmosis.The direction of osmosis depends on the

concentration of molecules outside and inside of the cell.

HypotonicIf the concentration of molecules outside

the cell is lower than the concentration inside the cell’s cytoplasm, then the solution is called hypotonic.

Water will move into the cell to create equilibrium.

Hypertonic

When the solution outside the cell has more molecules than inside the cell, the solution is hypertonic.

Water will move out of the cell to create equilibrium.

Isotonic

Both the solution and the inside of the cell are equal.

Water will move both ways randomly to maintain equilibrium.

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Cells at Work: Diffusion and Osmosis

How Cells Deal with Osmosis

Some animals are only single celled organisms like the paramecium.

The paramecium is a freshwater pond dweller. These paramecia are functioning in a hypotonic

environment all the time so water is constantly diffusing into the organism.

Paramecia have contractile vacuoles that remove excess water from their bodies.

Sometimes when too much water is added, cells burst and this is called cytolysis.

How Plants Deal with Osmosis

Since plants have a cell wall, osmosis affects them differently.

When osmosis moves water into plant cells, the cell membrane swells but stops against the strong cell wall.

This is called turgor pressure. When plants don’t receive enough water,

plasmolysis occurs which makes the plants wilt. The cell membrane shrinks away from the cell

wall.

Facilitated Diffusion

This process is used for molecules that can’t move quickly through the cell membrane.

Carrier proteins help move the molecules.The molecules still move down the

concentration gradient so no energy is required.

Transport of glucose is facilitated diffusion.

Diffusion through Ion Channels

Ions such as sodium, potassium and calcium need a channel protein because they cannot pass through the membrane on their own.

Each type of channel is specific to an ion.Some channels are always open and

some have gates that open and close depending on the need for the ion.

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worksheet 5-1

Active Transport

This type of transport goes up or against the concentration gradient.

Since that is working uphill, energy is required.

This includes: sodium-potassium pump, endocytosis, and exocytosis

Sodium-Potassium Pump

This involves a carrier protein. To function normally, animal cells must have a

high concentration of sodium outside their cells and a high concentration of potassium inside their cells.

The sodium-potassium pump works to maintain these differences.

As sodium comes into the carrier protein and moves outside the cell, potassium comes into the protein and moves inside.

They switch places.

Endocytosis

Some substances such as food particles are too large to move through the cell membrane.

In endocytosis, the cell encloses external materials in a pouch made from the cell membrane and moves them into the cell.

The pouch is called a vesicle. There are two types of endocytosis: pincocytosis

involves transport of fluid and phagocytosis involves transport of solids or whole cells.

Many animals use phagocytosis to ingest bacteria or viruses that invade the body.

Exocytosis

Exocytosis is the reverse. In exocytosis, molecules or cells are transported

to the outside of the cell from the inside. Vesicles are used. Cells used exocytosis to release large molecules

like proteins into the body from the cell. Usually the ER and the golgi are involved in this

process.