chapter 3: cell structure and...

In this chapter, you will learn about how cell structures have critical

roles to play in the health of an organism.

UNIT A Chapter 3: Cell Structure and Function

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Chapter 3: Cell Structure and Function

What other cellular organelles

have a similar function to the

lysosome?

Why doesn’t the cell “clean

up” the faulty lysosomes?

3.5 The Permeability of the Plasma Membrane

The plasma membrane is selectively permeable, allowing

passage of only certain molecules.

UNIT A Chapter 3: Cell Structure and Function Section 3.5

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Figure 3.17 How molecules cross

the plasma membrane. Molecules

that can diffuse across the plasma

membrane are shown with long

back-and-forth arrows. Substances

that cannot diffuse across the

membrane are indicated by the

curved arrows.

Passage of Molecules Across the Membrane


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• Some substances freely cross the membrane. They move

“down” their concentration gradient (from high

concentration to low concentration).

• Some substances are unable to freely cross and are

transported by proteins or vesicles. They may go “up,” or

against, their concentration gradient.

Diffusion

Diffusion is the movement of molecules down their

concentration gradient. It does not require energy. The rate of

diffusion is affected by factors such as temperature, pressure,

and molecule size.

• A solution contains a solute in a solvent. Diffusion occurs

until there is an equal distribution of solute and solvent.

Figure 3.18

Process of

Diffusion.


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Diffusion of Oxygen

Only a few types of molecules

can diffuse across the plasma

membrane.

• Gases can diffuse across the

bilayer

• Oxygen enters cells and

carbon dioxide leaves

• In lungs, oxygen moves

from the alveoli to blood

in the capillaries

Figure 3.19 Gas exchange in lungs. Oxygen (O2)

diffuses into the capillaries of the lungs because

there is a higher concentration of oxygen in the

alveoli (air sacs) than in the capillaries.


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Osmosis

Osmosis is the diffusion of water molecules across a

selectively permeable membrane due to a difference in

concentration.

• There is a net movement of water and changes in solute

concentration on both sides of the membrane

Figure 3.20 Osmosis demonstration.


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Isotonic solutions have the same

concentration of solute and solvent as the

solution inside the cell, and water will not

enter or leave the cell.

Hypotonic solutions have a lower

concentration of solute than solution inside

the cell, and water will enter the cell.

Hypertonic solutions have a higher

concentration of solute than solution inside

the cell, and water will leave the cell.

Isotonic, Hypotonic, and Hypertonic Solutions

Prefixes:

iso: the same as

hypo: less than

hyper: more than

_____________

tonicity: refers to

osmotic pressure


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Figure 3.21 Osmosis in animal and plant cells.

Transport by Carrier Proteins

The plasma membrane stops the passage of most molecules

into and out of the cell. However, biologically important

molecules do pass. They do so because of carrier proteins that

exist in the plasma membrane.

• Carrier proteins are specific and each binds to specific

molecules

• Carrier proteins are required for both facilitated transport

and active transport of substances across the plasma

membrane


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Figure 3.22 Facilitated transport.


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• Assists in transport of molecules across the membrane

by binding to those molecules

• Occurs down a concentration gradient and does not

require ATP

Facilitated Transport

Active Transport

• Assists transport of substances across the membrane by

binding to them

• Occurs against a concentration gradient and requires

energy, usually in the form of ATP

Proteins involved in active transport are often called pumps

because they use energy to pump substances against their

concentration gradient.

• One important carrier protein pump is the sodium-

potassium pump. It moves sodium ions to the outside of

the cell and potassium ions to the inside of the cell.


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Figure 3.23 The sodium-

potassium pump. The

same carrier protein

transports sodium ions

(Na+) to the outside of the

cell and potassium ions

(K+) to the inside of the cell

because it undergoes an

ATP-dependent change in

shape. Three sodium ions

are carried outward for

every two potassium ions

carried inward. Therefore,

the inside of the cell is

negatively charged

compared to the outside.


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Bulk Transport

Macromolecules are transported into and out of the cell by

vesicle formation, called membrane-assisted transport in

energy-dependent processes.

• Exocytosis is a way substances can exit a cell

• Endocytosis is way substances can enter a cell


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Exocytosis

During exocytosis, a vesicle fuses with the membrane and the

substance it is carrying is secreted outside of the cell.

• Neurotransmitters, hormones, and digestive enzymes are

examples of substances secreted in this way


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Figure 3.24 Exocytosis. Exocytosis

deposits substances on the outside

of the cell and allows secretion to

occur.

Endocytosis

During endocytosis, cells take in substances by vesicle

formation.

• The plasma membrane folds in on itself and then pinches

off to form an intracellular vesicle

Endocytosis occurs in one of three ways.

• Phagocytosis

• Pinocytosis

• Receptor-mediated endocytosis


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From Figure 3.25 Three methods of endocytosis. a. Phagocytosis occurs when the

substance to be transported into the cell is large. Amoebas ingest by phagocytosis.

Digestion occurs when the resulting vacuole fuses with a lysosome.

During phagocytosis, the material being taken into the cell is

large, such as a food particle or another cell.

• Common in unicellular organisms and occurs in certain

types of human white blood cells

Phagocytosis


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From Figure 3.25 Three methods of endocytosis. b. Pinocytosis occurs when a

macromolecule such as a polypeptide is transported into the cell. The result is a vesicle

(small vacuole).

During pinocytosis, vesicles form around liquid or very

small particles.

• Common in blood cells, intestinal cells, and plant root

cells

Pinocytosis


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