2.1 A Background to Cell Structure

Cell Theory

• 1. All known living things are made up of cells.

• 2. The cell is structural & functional unit of all living things.

• 3. All cells come from pre-existing cells by division.

Modern cell theory adds the following:

• Cells contains hereditary information which is passed from cell to cell during cell division

• All cells are basically the same in chemical composition.

• All energy flow (metabolism & biochemistry) of life occurs within cells.    

Cell Theory

• The discovery of cells and their structure is linked to the development of the magnifying lenses, particularly the microscope.

Robert Hooke

• The first description of the cell is generally attributed to Robert Hooke (1635−1702)

• He described the microscopic units that made up the structure of a slice of cork and coined the term "cells" to refer to these units.

Antoni van Leeuwenhoek (1632−1723).

• This Dutch microscopist made over 500 microscopes.

• He observed moving particles in his microscopes and described the microorganisms that he saw in great detail.

• He is considered the father of microbiology.

Rudolf Virchow

• In 1855, Rudolf Virchow proposed an important extension of cell theory.

• "All living cells arise from pre-existing cells".

• This statement has become what is known as the "Biogenic law".

How do new cells arise?• Until the 18th century, many scientists believed in

spontaneous generation. This was the idea that non-living objects can give rise to living organisms.

• It was common “knowledge” that simple organisms like worms, beetles, frogs, amd salamanders could come from dust, mud, etc., and food left out, quickly “swarmed” with life.

• Jan Baptista van Helmont’s recipe for mice: – Place a dirty shirt or some rags in an open pot or

barrel containing a few grains of wheat or some wheat bran, and in 21 days, mice will appear. There will be adult males and females present, and they will be capable of mating and reproducing more mice.

Franscesco Redi

• In 1668, he did an experiment with flies and wide-mouth jars containing meat.

• This was a true scientific experiment

• In the control group of jars, flies were seen entering the jars. Later, maggots, then more flies were seen on the meat.

• In the gauze-covered jars, no flies were seen in the jars, but were observed around and on the gauze, and later a few maggots were seen on the meat.

• In the sealed jars, no maggots or flies were ever seen on the meat.

Conclusion(s): Only flies can make more flies.• In the uncovered jars, flies entered and laid

eggs on the meat. Maggots hatched from these eggs and grew into more adult flies.

• Adult flies laid eggs on the gauze on the gauze-covered jars. These eggs or the maggots from them dropped through the gauze onto the meat.

• In the sealed jars, no flies, maggots, nor eggs could enter, thus none were seen in those jars. Maggots arose only where flies were able to lay eggs.

This experiment disproved the idea of spontaneous generation for larger organisms

Louis Pasteur-1861

Pasteur’s conclusion

• There is no life force in air, and organisms do not arise by spontaneous generation in this manner.

• “Life is a germ, and a germ is Life. Never will the doctrine of spontaneous generation recover from the mortal blow of this simple experiment.”

Cell Structures

Compare prokaryotic cells and eukaryotic cells:

How are they similar?

How are they different?

Cell structure and functionANIMAL CELL

Plant Cell

Cell membraneStructure: Phospholipid bilayer with embedded

proteins

Function: • Acts as a barrier

between the inside of the cell and the outside environment.

• Import and export of substances from the cell

• Cell recognition: self vs non-self.

• Communications/ connections with other cells

Cytoplasm

CYTOPLASM• Consists of all the contents between the nucleus and the cell

membrane. • Made up of the cytosol and organelles such as mitochondria

and ribosomes.• The fluid part of the cytoplasm is the Cytosol. It is clear in

color and has a gel-like appearance. It is composed mainly of water and also contains enzymes, salts, and various organic molecules.

• Also located within the cytoplasm is the cytoskeleton, a network of fibers that help the cell maintain its shape and give it support.

• Function: The cytoplasm helps to move materials around the cell and also dissolves cellular waste.

Nucleus• The nucleus is the largest organelle in a cell.

It directs all activity in the cell.• It also controls the growth and reproduction of

the cell. • It is the location for most of the nucleic acids

(DNA and RNA) a cell makes.• The nucleus controls the shape and features

of the cell. • Double membrane with pores

• The nucleolus is an area of the nucleus where ribosomes are constructed.

• It is not membrane bound

• It is rich is proteins and nucleic acids, so it appears darker.

• There may be more than one nucleolus

• Nuclear pores: allow for passage of materials between cytoplasm and nucleoplasm

Cytoplasmic Organelles

Mitochondria• Site of cellular respiration:

provide the energy a cell needs to carry out its activities

• Membrane-bound organelles, and like the nucleus have a double membrane.

• They are about the size of bacteria but may have different shapes depending on the cell type.

• The outer membrane is fairly smooth.

• The inner membrane is highly convoluted, forming folds called cristae.

• The cristae greatly increase the inner membrane's surface area.

• It is on these cristae that food (sugar) is combined with oxygen to produce ATP - the primary energy source for the cell.

Endoplasmic Reticulum

• Comes in two forms: Rough ER and Smooth ER

• Rough ER: - sheets of unit membrane with ribosomes on the outside                - forms a tubular network throughout the cell

• Function - transports chemicals between cells and within cells                - provides a large surface area for protein synthesis

• Smooth ER: functions in the synthesis and transport of lipids (fats)

Ribosomes• Ribosomes are molecular machines that make all

the proteins found in every living thing on the planet.

• They are made in the nucleolus• They are found in the cytoplasm and attached to

the ER

Golgi Apparatus• AKA Golgi Body, Golgi

Complex• The Golgi is principally

responsible for directing molecular traffic in the cell

• Molecules are modified and packaged into vesicles for transport

Vacuoles

• Compartments which are filled with water and various molecules in solution. (dissolved salts, minerals, ions, proteins)

• They may contain solids which have been engulfed.

• In plants: usually one large vacuole

• In animals: many small vacuoles (look like vesicles)

Centrioles

• Found only in animal cells

• Paired organelles usually located together near the nucleus

• The centrioles are positioned so that they are at right angles to each other

• Function: form the spindle fibers in mitosis and meiosis- cell reproduction.

Lysosomes

• Double membrane sacs that contain chemicals to break down large food particles into smaller ones

• Break down old cell parts and release the substances to be used again.  

• Keeps the harsh chemicals from escaping and breaking down the rest of the cell

Cell wall• Plant cell walls are

made of cellulose. It is a polysaccharide that forms long straight chains and can be woven into fibers.

• The cell wall is structurally strong, but it is porous.

Chloropasts

• Found only in plant cells• They are the site of photosynthesis• Carbon dioxide + water glucose +

oxygen

• 6 CO2 + 6 H2O C6H12O6 + 6 O2

• This is the most important chemical reaction on the planet…it takes the energy from the sun and changes it to chemical energy that can be used by living things.

Endosymbiosis Theory