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• The plasma membrane is composed of a phospholipid bilayer which is a lipid where one of the fatty acids has been replaced by a phosphate group.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