transport

facilitated diffusion

Hydrophilic molecules and very large molecules cant pass through the cell membrane by simple diffusion. They need help to pass through the membrane. The help is provided by proteins called transport proteins. This process is known as facilitated diffusion. There are two types of transport proteins: channel proteins and carrier proteins. They work in different ways. You can see how they work in Figure 4.3. A channel protein forms a tiny hole called a pore in the cell membrane. This allows water or hydrophilic molecules to bypass the hydrophobic interior of the membrane. A carrier protein binds with a diffusing molecule. This causes the carrier protein to change shape. As it does, it carries the molecule across the membrane. This allows large molecules to pass through the cell membrane.

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simple diffusion

Simple diffusion occurs when a substance diffuses through a cell membrane without any help from other molecules. The substance simply passes through tiny spaces in the membrane. It moves from the side of the membrane where it is more concentrated to the side where it is less concentrated. You can see how this happens in Figure 4.2. Substances that cross cell membranes by simple diffusion must squeeze between the lipid molecules in the mem- brane. As a result, the diffusing molecules must be very small. Oxygen (O2 ) and carbon dioxide (CO2 ) are examples of molecules that can cross cell membranes this way. When you breathe in, oxygen is more concentrated in the air in your lungs than it is in your blood. So oxygen diffuses from your lungs to your blood. The reverse happens with carbon dioxide. Carbon dioxide is more concentrated in your blood than it is in the air in your lungs. So carbon dioxide diffuses out of your blood to your lungs.

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passive transport

Passive transport occurs when a substance passes through the cell membrane without needing any energy to pass through. This happens when a substance moves from an area where it is more concentrated to an area where it is less concentrated. Concentration is the number of particles of a substance in a given volume. Lets say you dissolve a teaspoon of salt in a cup of water. Then you dissolve two teaspoons of salt in another cup of water. The second solution will have a higher concentration of salt. Why does passive transport require no energy? A substance naturally moves from an area of higher to lower concentration. This is known as moving down the concentration gradient. The process is called diffusion. Its a little like a ball rolling down a hill. The ball naturally rolls from a higher to lower position without any added energy. You can see diffusion if you place a few drops of food coloring in a pan of water. Even without shaking or stirring, the food coloring gradually spreads throughout the water in the pan. Some substances can also diffuse through a cell membrane. This can occur in two ways: simple diffusion or facilitated diffusion.

the cell membrane

Youve probably blown soap bubbles like the child in Figure 4.1. In some ways, the thin film of soap molecules that forms a bubble resembles the cell membrane. Like the soap film, the cell membrane consists of a thin skin of molecules. You can see a model of the cell membrane in Figure below. The molecules that make up the cell membrane are mainly phospholipids. There are two layers of phospholipids. They are arranged so the lipid tails are on the inside of the membrane. They make the interior of the membrane hydrophobic, or "water fearing". The lipid heads point toward the outside of the membrane. The make the outer surfaces of the membrane hydrophilic, or "water loving". Different types of proteins are embedded in the lipid layers. The proteins are needed to help transport many substances across the membrane. The passage of a substance through a cell membrane is called transport. There are two basic ways that transport can occur: passive transport and active transport. For a good video introduction to passive and active transport, click on this link: . MEDIA Click image to the left or use the URL below. URL:

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sodiumpotassium pump

Sodium and potassium are two of the most important elements in living things. They are present mainly as positively charged ions dissolved in water. The sodium-potassium pump moves sodium ions (Na+ ) out of the cell and potassium ions (K+ ) into the cell. In both cases, the ions are moving from an area of lower to higher concentration. Energy in ATP is needed for this "uphill" process. Figure 4.4 shows how this pump works. Trace these steps from left to right in the figure: 1. Three sodium ions inside the cell bind with a carrier protein in the cell membrane. 2. The carrier protein receives a phosphate from ATP. This forms ADP (adenosine diphosphate) and releases energy. 3. The energy causes the carrier protein to change shape. As it does, it pumps the three sodium ions out of the cell. 4. Two potassium ions outside the cell next bind with the carrier protein. Then the process reverses, and the two potassium ions are pumped into the cell.

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active transport

Active transport occurs when a substance passes through the cell membrane with the help of extra energy. This happens when a substance moves from an area where it is less concentrated to an area where it is more concentrated. This is the opposite of diffusion. The substance moves up, instead of down, the concentration gradient. Like rolling a ball uphill, this requires an input of energy. The energy comes from the molecule named ATP (adenosine triphosphate). The energy allows special transport proteins called pumps to move substances to areas of higher concentration. An example is the sodium-potassium pump.

osmosis

Osmosis is the special case of the diffusion of water. Its an important means of transport in cells because the fluid inside and outside cells is mostly water. Water can pass through the cell membrane by simple diffusion, but it can happen more quickly with the help of channel proteins. Water moves in or out of a cell by osmosis until its concentration is the same on both sides of the cell membrane.

vesicle transport

Some substances are too big to be pumped across the cell membrane. They may enter or leave the cell by vesicle transport. This takes energy, so its another form of active transport. You can see how vesicle transport occurs in Figure 4.5. Vesicle transport out of the cell is called exocytosis. A vesicle containing the substance moves through the cytoplasm to the cell membrane. Then the vesicle fuses with the cell membrane and releases the substance outside the cell. You can watch this happening in this very short animation: MEDIA Click image to the left or use the URL below. URL: Vesicle transport into the cell is called endocytosis. The cell membrane engulfs the substance. Then a vesicle pinches off from the membrane and carries the substance into the cell.

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instructional diagrams

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questions

Types of passive transport include

a. sodium-potassium pump

-->  b. facilitated diffusion

c. vesicle transport

d. endocytosis

_passage of a substance through a cell membrane from an area of higher to lower concentration that

a. facilitated diffusion

b. vesicle transport

c. channel protein

d. phospholipid

-->  e. simple diffusion

f. carrier protein

g. active transport

_transport molecule that forms a tiny pore in a cell membrane so another substance can pass through

a. facilitated diffusion

b. vesicle transport

-->  c. channel protein

d. phospholipid

e. simple diffusion

f. carrier protein

g. active transport

The energy needed for active transport comes from

a. channel proteins

-->  b. ATP molecules

c. carrier proteins

d. sodium ions

Which statement about exocytosis is true?

a. It is a type of passive transport

b. It is an example of facilitated diffusion

c. It moves a substances into a cell

-->  d. none of the above

_type of molecule that is the main component of a cell membrane

a. facilitated diffusion

b. vesicle transport

c. channel protein

-->  d. phospholipid

e. simple diffusion

f. carrier protein

g. active transport

_passage of a substance through a cell membrane from an area of higher to lower concentration that

-->  a. facilitated diffusion

b. vesicle transport

c. channel protein

d. phospholipid

e. simple diffusion

f. carrier protein

g. active transport

Which substance can cross a cell membrane only with added energy?

a. carbon dioxide molecule

b. oxygen molecule

c. water molecule

-->  d. potassium ion

_any type of transport through a cell membrane that requires energy

a. facilitated diffusion

b. vesicle transport

c. channel protein

d. phospholipid

e. simple diffusion

f. carrier protein

-->  g. active transport

In simple diffusion, a substances crosses a cell membrane by

-->  a. passing through tiny spaces in the membrane

b. moving through a channel created by a protein

c. first binding with a carrier protein

d. moving up a concentration gradient

_general term for the passage of a substance through a cell membrane by endocytosis or exocytosis

a. facilitated diffusion

-->  b. vesicle transport

c. channel protein

d. phospholipid

e. simple diffusion

f. carrier protein

g. active transport

_transport molecule that binds with a diffusing substance to carry it across a cell membrane

a. facilitated diffusion

b. vesicle transport

c. channel protein

d. phospholipid

e. simple diffusion

-->  f. carrier protein

g. active transport

When substances cross cell membranes by simple diffusion, they

a. squeeze between phospholipid molecules.

b. have help from channel proteins.

c. move from a higher to lower concentration.

-->  d. two of the above

Molecules that pass through cell membranes by facilitated diffusion include

a. large molecules.

b. hydrophobic molecules.

c. oxygen molecules.

-->  d. two of the above

The sodium-potassium pump is an example of

a. simple diffusion.

b. passive transport.

c. facilitated diffusion.

-->  d. none of the above

Which statement about endocytosis is false?

a. It is a form of active transport.

b. It requires the formation of a vesicle.

-->  c. It moves a substance out of a cell.

d. It requires energy.

The main difference between active and passive transport is the need for transport proteins.

a. true

-->  b. false

The surface of a cell membrane that faces the interior of the cell is hydrophobic.

a. true

-->  b. false

Examples of molecules that can cross cell membranes by simple diffusion include

a. water.

b. oxygen.

c. carbon dioxide.

-->  d. all of the above

Carbon dioxide passes from your blood to the air in your lungs by simple diffusion.

-->  a. true

b. false

The sodium-potassium pump involves

-->  a. carrier proteins.

b. channel proteins.

c. vesicles.

d. all of the above

Types of active transport include

-->  a. exocytosis.

b. facilitated diffusion.

c. osmosis.

d. all of the above

Small molecules can pass through a cell membrane by simple diffusion if they are hydrophobic.

-->  a. true

b. false

Water can pass more quickly through a cell membrane with the help of a carrier protein.

a. true

-->  b. false

_A substance naturally moves from an area of lower to higher concentration.

a. true

-->  b. false

_The two basic ways transport can occur are passive transport and diffusion.

a. true

-->  b. false

_Only very small molecules can move through a cell membrane by simple diffusion.

-->  a. true

b. false

_The interior of a cell membrane is hydrophobic.

-->  a. true

b. false

_Facilitated diffusion moves molecules across a cell membrane from an area of lower to higher con-

a. true

-->  b. false

_Active transport occurs when a substance moves up the concentration gradient to cross a cell mem-

-->  a. true

b. false

_Exocytosis releases substances outside the cell.

-->  a. true

b. false

diagram questions

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