A View of the cell Ch. 7

Before the microscope, many believed that diseases were caused by curses and supernatural spirits.

Microscopes enabled scientists to view and study cells, the basic units of living organisms.

The first microscope was used by van Leeuwenhoek in the 1600’s and it is considered a simple light microscope because it had only one lens.

Over the next 200 years, microscopes improved by grinding higher quality lenses. The compound microscope was also developed, which uses a series of lenses to magnify objects in steps.

Compound light microscopes can view objects up to about 1500 times their actual size.

Robert Hooke used a compound light microscope to study cork, the dead cells of Oak bark. He observed small geometric shapes like boxes which he called cells.

This and other discoveries gave rise to the Cell Theory, one of the fundamental ideas of modern biology.

The Cell Theory 1. All organisms are composed of one or more cells. 2. The cell is the basic unit of structure and organization of organisms. 3. All cells come from preexisting cells.

In the 1930’s and 1940’s the electron microscope was developed. It uses a beam of electrons instead of light and can magnify up to 500,000 times.

With better microscopes scientists observed that all cells contain small, specialized structures, called organelles.

Cells can be divided into two broad groups: those that contain membrane bound organelles and those that do not. These are called prokaryotes and eukaryotes

Prokaryotes are cells that do not contain membrane bound organelles. Ex. Unicellular organisms such as bacteria.

Eukaryotes are cells that contain membrane-bound organelles. Ex. Most multicellular organisms are made up of eukaryotic cells. Also amoeba and some yeast and algae.

Eukaryotes are larger and contain a true nucleus. They can carry out numerous metabolic chemical reactions at the same time.

The plasma membrane maintains a state of balance in the cell by controlling the flow of nutrients into the cell and a flow of waste out of the cell. It does this through a process of selective permeability.

The plasma membrane is composed of a phospholipid bilayer which is a lipid where one of the fatty acids has been replaced by a phosphate group.

The polar phosphate group allows the cell membrane to interact with its watery environment because water itself is polar. The fatty tails avoid water.

When the phospholipid molecules come together to form this “sandwich”, a barrier is created that is water-soluble at its outer surfaces and water insoluble in the middle.

Water-soluble molecules will not easily move through the membrane because they are stopped by the water- insoluble layer.

This model is called the fluid mosaic model. The proteins create a mosaic or pattern with the phospholipids like boats with their decks above water and hulls beneath it.

Cholesterol is found in the cell membrane where it helps stabilize the phospholipids by keeping them from sticking together.

Transport proteins move needed substances or waste materials through the plasma membrane. Other proteins play a role in attaching the cells inner structure to the membrane and help the cell identify chemical signals.

All plant cells have a cell wall which gives support and protection to the cell. It is a cellulose mesh which is highly permeable. Fungi, bacteria, and some protists also have a cell wall.

The master set of directions for making proteins is found in chromatin, which are strands of DNA found in the nucleus.

The nucleolus is found inside the cell nucleus and makes ribosomes. Ribosomes are the sites where the cell produces proteins according to the directions of DNA.

Cytoplasm is the clear, gelatinous fluid within a cell which holds all the organelles as well as nutrients and waste waiting for export.

The endoplasmic reticulum is the site of cellular chemical reactions. It is folded up like an accordion and thus fits into a small space.

The golgi apparatus is a flattened stack of tubular membranes that modify proteins made by the ribosomes, which sort and package proteins like mail at the post office.

Vacuoles are temporary storage compartments used to store food, enzymes, and other materials. Most animal cells don’t contain vacuoles.

Lysosomes are organelles that contain digestive enzymes. They digest excess or worn out organelles, food particles and engulfed viruses or bacteria.

The cell requires a large amount of energy to perform all its vital functions. Two organelles: chloroplasts and mitochondria provide this energy.

Chloroplasts are organelles in plant cells that contain chlorphyll and capture light energy and convert it to chemical energy. They contain a double membrane which contains the stroma and grana which help trap the sun’s energy.

Mitochondria are organelles in plant and animal cells that transform energy for the cell and store it in the bonds of other molecules.

Mitochondria can occur in varying numbers depending on the cells function. Liver cells can hold up to 2000 mitochondria.

The cytoskeleton forms a framework for the cell like the skeleton. Microtubules and microfilaments, that can be dismantled and reassembled elsewhere, make up the cytoskeleton.

Centrioles are organelles found in animal cells and aid in cell division. Cilia and flagella are organelles on the surface of the cell that are made of microtubules and aid in locomotion and feeding

Cilia are short numerous and look like hairs. They move like oars in a rowboat. Flagella are longer and move in a whip- like motion. Cells have only one or two flagella.