A Tour of the Cell (Chapter 6)

The Discovery of the Cell: Hooke In 1665, Robert Hooke used an early compound microscope to look at a thin slice of cork, which is a plant material. He noticed that cork looked like thousands of tiny, empty chambers which he called “cells” We now know cells are the basic units of life.

The Discovery of the Cell: Von Leeuwenhoek The existence of cells was unknown for many years, but this changed with the invention of the microscope. Anton von Leeuwenhoek used a single-lens microscope to observe pond water and other things. The microscope revealed a world of tiny living organisms.

Developing the Cell Theory In 1838, Matthias Schleiden concluded that all plants were made of cells. In 1839, Theodor Schwann stated that all animals were made of cells. In 1855, Rudolph Virchow concluded that new cells were created only from division of pre-existing cells. These discoveries led to the cell theory.

Compound Light Microscope Combination of lenses and light used to magnify small objects held on a slide Live specimen may be observed (ie, pond water) Max magnification = 1000x

Electron Microscopes Image produced on a computer screen using a beam of electrons rather than light More powerful (300,000x or more) than light microscopes, but specimen cannot be alive Transmission Study of inner structure of a specimen Samples are cut into thin slices for viewing Images are 2-D Scanning Allows study of specimen surface Images are 3-D

Images from Electron Microscopes

The Cell Theory All living things are composed of cells. Cells are the basic units of structure and function in living things. New cells are produced from pre-existing cells.

Self-Assessment What are the three key ideas of the cell theory? What scientists contributed to the development of the cell theory? How did they contribute? Identify the type of microscope most useful for viewing each of the following: A group of cells in a thin layer of onion skin The details of the surface of a human hair The detailed structure of a mitochondria inside a muscle cell

Two Types of Cells All cells are surrounded by a barrier called a cell membrane and contain DNA Eukaryotic cells contain a nucleus & membrane organelles. (plants, animals, fungi & protists.) Prokaryotic cells do NOT contain a nucleus (still have DNA) and most organelles (do have ribsomes) and are classified as bacteria.

Cell Boundaries All cells are surrounded by a thin, flexible barrier, the plasma or cell membrane, which acts as a gate-keeper, regulating what enters and leaves the cell, and also provides some protection and support Many cells also produce a strong supporting layer around the membrane known as a cell wall. The main function of the cell wall is support and protection for the cell.

Structure of the Cell Membrane The composition of nearly all cell membranes is a double-layered sheet called a lipid bilayer. The cell membrane regulates what enters and leaves the cell. Lipid bilayer

Functions of the Plasma Membrane Embedded enzyme proteins help carry out chemical reactions of the cell Receptor proteins allow to receive chemical messages Surface molecules allow recognition & communication between cells Transport proteins serve as channels or pumps to move materials in and out of cells Proteins from adjacent cells allow intracellular joining

Diffusion Through Cell Boundaries Particles in a solution tend to move from an area where they are more concentrated to an area where they are less concentrated. (visualize this as a downhill movement, [high] to [low]) This process is called diffusion When the concentration of the solute is the same throughout a system, the system has reached equilibrium.

Osmosis The diffusion of water through a selectively permeable membrane. Osmosis is the diffusion of water through a selectively permeable membrane. In the first beaker, water is more concentrated on the right side of the membrane. As a result, the water diffuses (as shown in the second beaker) to the area of lower concentration.

Osmotic Pressure & Tonicity If you compare two solutions, the more concentrated solution (less water) is hypertonic. The more dilute (more water) solution is hypotonic. When the concentration is equal in both solutions, the solution is described as isotonic.

Effect of Tonicity on Cells

Facilitated Diffusion Cell membranes have protein channels that act as channels (pores) or carrier molecules, making it easy for certain molecules to cross. The movement of specific molecules across cell membranes through protein channels is known as facilitated diffusion. This process usually involves solute molecules (rather than solvent molecules) and is a form of passive transport because it requires no energy, ([high] to [low])

Active Transport Active transport involves the use of energy (ATP) to move substances across a cell membrane Ex) Movement of materials in the opposite direction from which the materials would normally move - against a concentration difference This is achieved by membrane proteins that act as pumps Visualize this as UPHILL, [low] to [high]) Large materials also move by active transport regardless of concentration gradient

Active Transport of Large Materials (“Bulk Transport”) Endocytosis is the process of taking large materials into the cell using vesicles, or pockets, of the cell membrane. (endo=enter) Phagocytosis – cell “eating”; actively moving solids into the cell Pinocytosis – cell “drinking”; actively moving liquids into the cell Exocytosis, the membrane of the vesicle surrounding the material fuses with the cell membrane, forcing the contents out of the cell (exo=exit)

Summary Assessment What is the difference between active and passive transport? Name and describe three examples of each. Distinguish between hypotonic, hypertonic and isotonic solutions. Explain what will happen to an animal cell in each. Explain what will happen to a plant cell in each.