Principles of Biology

ByFrank H. Osborne, Ph. D.

Cells

THE CELL THEORY

• The word theory in science means "explanation." The cell theory is the accepted explanation about cells. It was developed after many observations of living things by biologists.

The Cell TheoryThe Cell Theory (Explanation)• All living things are made of cells.• All cells arise from pre-existing cellsExceptions• Viruses are considered to be non-living.• Mitochondria and chloroplasts can replicate

on their own.

Basic Types of CellsThere are two basic types of cells. The cells

that came first are prokaryotic (pro means "before" and karyo means "nucleus"). These cells came before there was a nucleus.

The cells with a nucleus are eukaryotic (eu means "true"). So, eukaryotic cells have a "true nucleus" meaning that the cellular nucleus has a membrane around it.

Biology of CellsThe word cell was used to describe these

objects because the first ones seen in cork reminded the observer of the cells in a monastery. Cells are too small to see with the unaided eye so we use microscopes.

Biology of CellsMicroscopes are used to see cells because they

are very small. There are two types of microscopes.

• Simple microscope--one lens like a magnifier.

• Compound microscope--two lenses working in series to make a magnified image of the specimen.

Biology of Cells

Simple microscope Compound microscope

Biology of CellsCellular structures include:• Cell membrane--made of two layers of

phospholipid and surrounds the cytoplasm.• Nucleus--control center of the cell surrounded

by a nuclear membrane.• Cytoplasmic structures--perform various

cellular functions: mitochondria, chloroplasts, and others.

Biology of CellsWhen viewed with the light microscope, only

the nucleus and cytoplasm are visible. In plant cells chloroplasts are also visible.

With the electron microscope, many more fine details are visible. These are referred to as ultrastructure.

GENERALIZED CELLSA generalized cell is a drawing that contains

all parts of the cell. In reality, there are many cells that lack one or more parts. A red blood cell, for example, contains cytoplasm and a cell membrane but the other parts are missing.

Cellular UltrastructureDiagram of Cellular Ultrastructure

Eukaryotic/Prokaryotic CellsEukaryotic cells are found in all higher

organisms. They contain a well-defined nucleus surrounded by a membrane.

The prokaryotic organisms, Bacteria and Archaea, do not have a membrane-bounded nucleus.

Cell MembraneThe cell membrane is a structure that surrounds

each cell.• All living cells have a cell membrane• It is the boundary between the cell

and its environment.• It has the same function in both

plant and animal cells.

Proteins of the Cell MembraneTransport proteins assist passage of large

molecules or ions into the cell.Integral proteins help maintain the structure of

the cell membrane. Some also serve as signal transmitters.

Peripheral proteins are associated with the integral proteins. They assist in transmitting messages into the cell.

Cell Membrane Structure

Cell Membrane StructureIn the electron microscope, a single

membrane, as illustrated, appears as two parallel lines 80 Angstrom units apart.

The "Fluid Mosaic" ModelThe proteins in the membrane can move

around inside the two layers of lipids and even protrude through the surface. This gives the surface of the membrane the appearance of a mosaic. So, the cell membrane is called a "fluid mosaic."

The "Fluid Mosaic" Model

The NucleusThe nucleus is the central organelle that

controls the cell. It is surrounded by a membrane known as the nuclear membrane. It contains chromosomes and a nucleolus.

Cellular UltrastructureThe nucleus controls the cell.

The Nucleus

Parts of the Nucleus• The nuclear membrane surrounds the

nucleus. It contains pores and channels that connect to the endoplasmic reticulum, a network of channels that is found in the cytoplasm .

• The chromosomes are structures that contain DNA and control the activities of the cell.

• The nucleolus is a large, round structure within the cell. It is responsible for the formation of ribosomes.

Cellular UltrastructureThe nuclear membrane surrounds the nucleus.

Cellular UltrastructureChromatin contains the DNA.

Cellular UltrastructureThe nucleolus helps form ribosomal parts.

The CytoplasmThe cytoplasm is the part of the cell that is

found outside the nucleus. It is bounded on the outside by the cell membrane. The cytoplasm contains many different organelles that perform various functions for the cell.

Cellular UltrastructureThe cytoplasmic membrane surrounds the cell.

MitochondriaMitochondria are the structures that produce

energy for the cell. They contain an internal membrane that is folded to form cristae. Each single crista is an individual fold of this membrane where cellular respiration occurs.

Mitochondria

Cellular UltrastructureMitochondria provide energy for the cell.

Endoplasmic ReticulumThe endoplasmic reticulum (ER) is a series of

channels in the cytoplasm. It provides rapid movement of materials within the cell.

Endoplasmic ReticulumRough ER has ribosomes attached to it. Its

appearance in the electron microscope is that of sandpaper, hence "rough."

Smooth ER does not have ribosomes attached to its membranes. It forms part of the internal membrane system of the cell.

Cellular UltrastructureEndoplasmic reticulum is for communications.

Cellular UltrastructureIt has ribosomes for protein synthesis.

Ribosomes• Ribosomes are the places where

proteins are made. Each ribosome has a small subunit and a large subunit.

• When a protein is being synthesized, you find ribosomes attached to a piece of messenger RNA. This structure is sometimes called a polysome.

Golgi BodiesGolgi bodies serve as packaging organelles for

materials to be exported outside of the cell. They start out as flat membranes that fill up as the material enters them. They become spherical and migrate toward the outside.

Cellular UltrastructureThe Golgi apparatus is for external secretions.

Lysosomes• Lysosomes are structures that are

membrane-bounded and contain digestive enzymes.

• Lysosomes digest large particles that are brought into the cell.

• The process of digestion is the reverse of dehydration synthesis.

Vacuoles• Vacuoles are areas in the cytoplasm that have a

membrane surrounding them.• They are filled with cytoplasmic liquids or

storage materials of various kinds.

The Cytoskeleton• The cell has a network of flexible fibers within

the cytoplasm. This network of fibers is called the cytoskeleton.

• The fibers are made of polymers of proteins known as microtubules. These fibers have elastic properties that provide flexibility to the cell. They act as little muscles inside the cell.

Plant Cell Structures• A generalized plant cell is shown below. The

cell wall and plastids are found only in plants.

Cell wall

Chloroplast

Cell Wall• The cell wall is made of cellulose.• It is considered non-living.• Cellulose fibers form primary (thin) and

secondary (thick) walls. Different types of plant cells have different thicknesses of cell wall.

• The cell membrane is pushed up against the inside of the cell wall by turgor pressure so it cannot be seen.

Cell Wall LayersThe middle lamella is produced when the plant

cell divides.Each cell produces a thin primary wall on its side

of the middle lamella. All plant cells generally have a primary wall.

The secondary wall is much thicker. Not all plant cells produce secondary walls.

PlastidsPlastids are structures found only in plant cells.

There are three kinds which are chloroplasts, chromoplasts and leukoplasts.

Diagram of chloroplast structure.Stroma

Chloroplast Structure

Grana

Stroma

Chloroplasts• Chloroplasts are plant organelles that make

food by the process of photosynthesis.• Photosynthesis is the synthesis of organic

materials from inorganic raw materials.• Using light energy the chloroplast converts

carbon dioxide and water to glucose.• The chloroplast contains layers of membranes

called grana that contain chlorophyll. Chlorophyll traps the light energy from the Sun.

ChromoplastsChromoplasts are plastids that are colored but do

not perform photosynthesis. Most contain carotenoid pigments that are related to carotene. These pigments are various shades of orange or yellow. The pigments can be separated from leaves by means of a technique known as chromatography.

Leukoplasts"Leuko" means lacking color or without color.

Leukoplasts are plastids that do not have any color. They are used for storage of materials. An example is the starch grain which is found within the cells of potatoes.

Cilia and FlagellaCilia and flagella are organelles of locomotion.

They contain contractile proteins. The proteins allow cilia and flagella to move. Flagella are much larger than cilia. Cilia are more numerous on a cell than flagella. A cell will have either cilia or flagella but not both.

The contractile proteins of cilia and flagella form a characteristic arrangement. They come in pairs.

Cilia and FlagellaThere are nine pairs around the outside with two

fibers in the middle. This structural configuration is known as the 9+2 arrangement.

Cilia and FlagellaFlagella are attached to the cell membrane.

Inside the cell membrane is a basal body, which is anchored in the cytoplasm by rootlets. The filament, is located outside the cell membrane.

CentriolesCentrioles are animal cell structures

made of microtubules. The centriole is located in the cytoplasm just outside of the nuclear membrane. It is used by animal cells during cell division.

Centrioles produce a network of fibers known as the spindle. Spindle fibers pull the chromosomes apart during the process of mitosis.

Summary Outline of Cell StructuresStructures common to all eukaryotic cells Cell membrane Ribosomes Nucleus Golgi bodies Mitochondria Lysosomes Endoplasmic reticulum Vacuoles

Summary Outline of Cell StructuresStructures found Structures foundonly in plant cells only in animal cells Cell wall Cilia and flagella Chloroplasts Centrioles Plastids

PRINCIPLES AND DEFINITIONSDiffusion• Diffusion is the net movement of the particles of

a particular substance from an area of higher concentration of that substance to an area of lower concentration of that substance, at constant temperature and pressure.

PRINCIPLES AND DEFINITIONSOsmosis• Osmosis is the diffusion of water through a

differentially (semi-)permeable membrane.• Generally water moves from the outside of the

cell to the inside of the cell because the concentration of water is higher outside the cell.

• Osmosis is one of the most misunderstood concepts in biology.

PRINCIPLES AND DEFINITIONSOsmosis• Osmosis must involve water passing through a

membrane. If not, it is not osmosis.

PRINCIPLES AND DEFINITIONSActive Transport• Active transport is the movement of a

substance across the cell membrane requiring the use of energy from the cell.

• Cellular energy is produced by the mitochondria and stored in the form of adenosine triphosphate (ATP).

PRINCIPLES AND DEFINITIONSEndocytosis and Phagocytosis• Endocytosis is the movement of substances into

the cell. If the substances are very large, such as bacteria, phagocytosis is used to allow them to enter the cell.

• "Phago" refers to eating, so phagocytosis is the process by which the cell eats.

PRINCIPLES AND DEFINITIONSPhagocytosis

The End

Principles of BiologyCells