Chapter 3Wonder Drug

A chance discovery revolutionized medicine

• In 1928, biologist Alexander Fleming noticed that the fungus Penicillium notatum was capable of killing many kinds of bacteria.

A chance discovery revolutionized medicine

• An antibiotic is a chemical that can slow or stop the growth of bacteria. Penicillin is an antibiotic produced by a living organism – Penicillium notatum.

Antibiotics target bacterial cells

• Penicillin and other antibiotics preferentially kill bacteria without harming their human or animal host because they target what is unique about bacterial cells.

The cell theory• The cell theory states that all living things are made of cells,

and every new cell comes from the division of a pre-existing cell.

Cells• Not all cells are alike. Cells come in many shapes and

sizes and perform various functions, depending on where they are found.

4.1 Microscopes reveal the world of the cell

• A variety of microscopes have been developed for a clearer view of cells and cellular structure

• The most frequently used microscope is the light microscope (LM)—like the one used in biology laboratories– Light passes through a specimen then through glass

lenses into the viewer’s eye– Specimens can be magnified up to 1,000 times the

actual size of the specimen

Copyright © 2009 Pearson Education, Inc.

Enlarges imageformed by objectivelens

Magnifies specimen,forming primaryimage

Eyepiece

Focuses lightthrough specimen

Ocularlens

Specimen

Objective lens

Condenserlens

Lightsource

4.1 Microscopes reveal the world of the cell

• Microscopes have limitations– Both the human eye and the microscope have limits

of resolution—the ability to distinguish between small structures

– Therefore, the light microscope cannot provide the details of a small cell’s structure

Copyright © 2009 Pearson Education, Inc.

Cells• Cells fall into two fundamentally different categories:– Prokaryotic cells, which

lack internal membrane-bound organelles

– Eukaryotic cells, which contain membrane-bound organelles, including a central nucleus

The cell membrane• All cells are

surrounded by a cell membrane – a phospholipid bilayer with embedded proteins that forms the boundary of all cells.

The cell membrane• The cell membrane forms a boundary between the external

environment and the cell’s cytoplasm (the gelatinous, aqueous interior of all cells).

The cell membrane

• The cell membrane is made of partly hydrophobic and partly hydrophilic phospholipids with proteins embedded in the membrane to perform particular functions.

• The cell membrane forms a semi-permeable barrier to substances on either side of it.

The cell membrane• The hydrophilic heads face out to interact

with water on both sides.

The cell membrane• The hydrophobic tails gather in the middle of

the membrane.

Ribosomes and DNA• In addition to a cell membrane, all cells have ribosomes and DNA.• Ribosomes are complexes of RNA and protein that carry out protein

synthesis.

Prokaryotic and eukaryotic cells• In prokaryotic cells, the DNA floats freely

within the cell’s cytoplasm (nucleoid)

Prokaryotic and eukaryotic cells• In eukaryotic cells, the DNA is housed within an

organelle called the nucleus.

Antibiotics target bacterial cells• Unlike human or other animal cells, most bacteria are

surrounded by a cell wall – a rigid structure enclosing the cell membrane of some cells that helps the cell maintain its shape.

Antibiotics target bacterial cells

• Bacterial cell walls are rigid due to the molecule peptidoglycan, a polymer made of sugars and amino acids that link to form a chainlike sheath around the cell.

Antibiotics target bacterial cells

• Without a cell wall, bacterial cells would fill up with water and burst due to osmosis. Osmosis is the diffusion of water across a semipermeable membrane from an area of lower solute concentration to an area of higher solute concentration.

Antibiotics target bacterial cells

• Most of the bacterial world falls into one of two categories:– Gram-positive – bacteria with a cell wall that

includes a thick layer of peptidoglycan that retains the Gram stain

– Gram-negative – bacteria with a cell wall that includes a thin layer of peptidoglycan surrounded by an outer lipid membrane that does not retain the Gram stain

Some antibiotics inhibit prokaryotic ribosomes

• While both eukaryotic and prokaryotic cells have ribosomes, their ribosomes are different sizes and have different structures.

Some antibiotics inhibit prokaryotic ribosomes

• Antibiotics that interfere with prokaryotic ribosomes leave eukaryotic ribosomes unaffected.

The cell membrane is semipermeable

• With its densely packed collection of hydrophobic phospholipid tails, the cell membrane prevents many large molecules, like glucose, and hydrophilic substances, like sodium ions, from wandering across the cell membrane.

The cell membrane is semipermeable• Only small, uncharged molecules like

oxygen cross the membrane easily by a process known as simple diffusion.

• Simple diffusion is the movement of small, hydrophobic molecules across a membrane from an area of higher concentration to an area of lower concentration.

The cell membrane is semipermeable

• Simple diffusion takes advantage of the natural tendency of dissolved substances to spread out from an area of higher concentration to one of lower concentration.

• Because the substance is moving from the side of the membrane with a higher concentration to the side with a lower concentration, no energy is required to move substances across the membrane.

The cell membrane is semipermeable• Large or hydrophilic

molecules needed to survive are moved across the membrane by transport proteins.

The cell membrane is semipermeable

• Transport proteins sit in the membrane bilayer with one of their ends outside the cell and the other inside. They act as a channel, carrier, or pump to provide a passageway for large or hydrophilic molecules to cross the membrane.

The cell membrane is semipermeable

• Transport proteins are very specific: a protein that transports glucose will not transport calcium ions, for example.

The cell membrane is semipermeable

• When a substance uses a transport protein to move down a concentration gradient, the process is called facilitated diffusion. This process requires no energy because the substance is moving from a higher to a lower concentration.

The cell membrane is semipermeable

• Active transport is an energy-requiring process by which solutes are pumped from an area of lower concentration to an area of higher concentration with the help of transport proteins.

Eukaryotic cells have organelles• Both plant and animal eukaryotic cells are

characterized by the presence of multiple, distinct membrane-bound organelles.

Eukaryotic cells have organelles

• Each organelle is separated from the cell’s cytoplasm by a membrane similar to the cell’s outer membrane, and each performs a distinct function.

Eukaryotic cells have organelles• The nucleus encloses

the cell’s DNA. Important reactions for interpreting the genetic instructions contained in DNA take place here.

Eukaryotic cells have organelles• The nucleus is

surrounded by the nuclear envelope, a double membrane made of two lipid bilayers.

Eukaryotic cells have organelles• Mitochondria are the cell’s power plants – they help extract

energy from food and convert that energy into a useful form.

Eukaryotic cells have organelles

• The endoplasmic reticulum (ER) is a vast network of membrane-covered “pipes” that serve as a transport system throughout the cell.

Eukaryotic cells have organelles

• The “rough ER” is studded with ribosomes that make proteins.

Eukaryotic cells have organelles

• The “smooth ER” is the site of lipid production.

Eukaryotic cells have organelles

• The Golgi apparatus processes and packages proteins produced in the rough ER. The processed molecules are packaged into membrane vesicles, then targeted and transported to their final destinations.

Eukaryotic cells have organelles

• The nucleus, ER, and Golgi apparatus work together to produce and transport proteins.

Eukaryotic cells have organelles

• The nucleus provides instructions for protein production.

Eukaryotic cells have organelles

• Proteins are made in the ER and packaged into vesicles for transport to the Golgi apparatus.

Eukaryotic cells have organelles

• Proteins receive final modifications in the Golgi apparatus and are packaged into vesicles for transport to their final destination.

Eukaryotic cells have organelles• Lysosomes are full of digestive enzymes to

break down worn-out cell parts or molecules so they can be used to build new cellular structures.

Eukaryotic cells have organelles

• The cytoskeleton is a network of protein fibers that carry out a variety of functions, including cell support, cell movement, and movement of structures within cells. Each type of cytoskeletal fiber has a specific structure and function.

Eukaryotic cells have organelles• Plant and algal cells

have chloroplasts, which have two membranes surrounding them, as well as an internal system of stacked membrane discs. Chloroplasts are the sites of photosynthesis, the reactions that plants use to capture the energy of sunlight in a usable form.

The evolution of eukaryotic cells

• Endosymbiosis is the theory that free-living prokaryotic cells engulfed other free-living prokaryotic cells billions of years ago, forming eukaryotic organelles such as mitochondria and chloroplasts.