UNIT A: Cell Biology

Chapter 2: The Molecules of Cells

Chapter 3: Cell Structure and Function: Section 3.5

Chapter 4: DNA Structure and Gene Expression

Chapter 5: Metabolism: Energy and Enzymes

Chapter 6: Cellular Respiration

Chapter 7: Photosynthesis

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 MembraneThe 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.

UNIT A Chapter 3: Cell Structure and Function Section 3.5

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UNIT A Chapter 3: Cell Structure and Function Section 3.5

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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

UNIT A Chapter 3: Cell Structure and Function Section 3.5

TO PREVIOUS SLIDE 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. Exocytosisdeposits 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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UNIT A Chapter 3: Cell Structure and Function Section 3.5

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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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From Figure 3.25 Three methods of endocytosis. c. Receptor-mediated endocytosis is a form of pinocytosis.

Receptor-mediated endocytosis is a type of pinocytosis. It involves receptor proteins that only bind to certain molecules.•The receptors are in coated pits. Once vesicles form, they become uncoated and fuse with lysosomes. Empty vesicles fuse with the plasma membrane and receptors return to their previous locations.

Receptor-Mediated Endocytosis

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Check Your Progress

1. Contrast diffusion with facilitated transport.

2. Explain the movement of water between hypotonic and hypertonic environments.

3. Describe the differences between facilitated and active transport.

4. Discuss the potential benefits of receptor-mediated endocytosis.

UNIT A Chapter 3: Cell Structure and Function Section 3.5

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UNIT A Chapter 3: Cell Structure and Function Section 3.5

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UNIT A Chapter 3: Cell Structure and Function Section 3.5

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