CHAPTER 3 Cell Structure and Function

Life has a cellular basis.

All organisms, including humans, are composed of cells.

There is no smaller unit of life that is able to: reproduce, respond, grow & develop, and take in

and use materials from the environment.

The work of 17th century scientists who had invented early microscopes and those that used them led to one of the fundamental principles of modern biology, the Cell Theory.

Cells

Cells

1. Cells are the basic working unit of life.2. All living things are made up of one or

more cells.3. New cells arise only from pre-existing

cells.

The Cell Theory States:

Cell Size

The ratio of surface-area-to-volume explains the small size of cells.

Nutrients enter a cell and wastes exit a cell at its surface.

The ability to get more materials in and out depends on a greater amount of surface area.

Volume represents cell needs.

Cell Size

Proportionally as a cell grows it becomes larger in volume and surface area decreases.

There is a limit to how large an actively metabolizing cell can become.

Cell division restores the amount of surface area needed for adequate exchange of materials.

Volume Increases Faster Than Surface Area!

MicroscopesLight microscopes

– Use glass lenses & light rays to obtain images

– Better suited for living specimens

– Lower magnification

Electron microscopes

– Use electrons to obtain images

– Obtain higher magnification and resolution

Microscope use and its parts will be studied in lab.

How Cells are Organized

Cells can be classified as either prokaryotic or eukaryotic.

Prokaryotic cells contain no membrane-bound organelles and lack a nucleus. They have a plasma membrane and contain cytoplasm; have ribosomes, and a ring of DNA. Most prokaryotes are bacteria.

How Cells are Organized

You and nearly all other life forms that you experience with your unaided eyes are eukaryotes.

The vast majority of eukaryotes that we knowingly interact with each day, mainly land plants and animals are large organisms, usually consisting of trillions of cells.

How Cells are Organized

Almost all eukaryotic cells have 3 main areas

1. An outer membrane called the plasma membrane.2. A semifluid medium inside called the cytoplasm.

The substance contains various organelles (small specialized structures).

3. A large central membrane-enclosed nucleus, where DNA is found.

Nucleus Central sphere of the cell

Plasma Membrane Outer boundary between the cell and the environment

Cytoplasm Everything between the plasma membrane and the nucleus Organelles

Specialized structures in the cytoplasm

Cellular Organization

Generalized Animal Cell

The plasma membrane is the boundary between the outside and inside of the cell.

The plasma membrane is a phospholipid bilayer with attached or embedded proteins.

At body temperature the bilayer is a liquid.

This allows the proteins to move about.

This structure of being liquid and having shifting proteins is described as the fluid-mosaic model.

Plasma Membrane - Structure

Short chains of sugars attach to the proteins and lipids and are called glycoproteins and glycolipids. They mark cells as belonging to a particular individual.

A common example of this can be seen in the different blood types of people.

Plasma Membrane - Structure

The plasma membrane keeps the cell intact.

The plasma membrane allows only certain molecules to enter and exit freely.

Therefore, the plasma membrane is said to be selectively permeable.

Plasma Membrane - Functions

Passive Transport

Passive transport is a way for molecules to enter or exit a cell without using cellular energy.

Diffusion is the random movement of molecules from an area of higher concentration to an area of lower concentration until they are equally distributed.

OsmosisOsmosis is the diffusion of water across a

semi-permeable membrane.

Tonicity

Tonicity is the term used to describe the solute concentration in a solution surrounding a cell. Example: 2% salt solution (98% water)

Solution Types - Identified by Tonicity

Isotonic – Solution outside of the cell is equal in concentration of water and solute.This causes cells to: Neither gain nor lose water. Normal cell size and shape maintained.

Hypotonic – Solution outside of the cell with a higher concentration of water and lower solute. This causes cells to: Gain water.Cells increase in size or burst.

Solution Types - Identified by Tonicity

Hypertonic – Solution outside of the cell has a lower concentration of water and higher solute.This causes cells to: Lose water.Cells shrink or shrivel in size.

Osmotic pressure controls water movement in our bodies.Examples: Absorption in the small and large intestine and in the kidneys.

Solution Types

Facilitated Transport

The transport of solutes from high to low concentration by means of protein carriers within the membrane.

This transport is also without using cell energy.

Active TransportOccurs when a molecule is moving from a lower to higher concentration.

Movement requires a protein carrier and the use of cellular energy.

Protein carriers act like a pump to move molecules against their concentration gradient (low to high).

EndocytosisEndocytosis occurs when a portion of the plasma membrane forms a pouch to surround a substance and fluid. The membrane pinches off and forms a vesicle.

Types of EndocytosisWhen cells like white blood cells do endocytosis and the membrane pushes out to engulf a particle like a bacterium, the process is called Phagocytosis.

Pinocytosis is taking in small molecules and fluid. The membrane invaginates forming a vesicle.

Exocytosis

The process is called Exocytosis when a vesicle fuses with the plasma membrane to move substances out of a cell.

The Nucleus and theProduction of Proteins

The nucleus and several organelles are involved in the production and processing of proteins.

The nucleus is the largest structure in the cell, usually spherical, found near the center and stores genetic information.

Segments of DNA threads called genes contain information for the production of specific proteins.

The nucleus is thus called the control center of the cell.

Threads found inside are made of DNA and protein and are called chromatin.

These threads are immersed in a semifluid medium called nucleoplasm.

The nucleolus is a dark dense region where ribosomal RNA is produced and joins with protein to form subunits called ribosomes.

The nuclear envelope separates the nucleus from the cytoplasm and is perforated with passageways called nuclear pores.

Nuclear Envelope

Nuclear pores Nucleoplasm

Nucleous

Endoplasmicreticulum

RibosomesRibosomes are tiny granules composed proteins and rRNA. Ribosomes are where protein synthesis occurs.Ribosomes are often attached to membranes in the cytoplasm called the endoplasmic reticulum (ER).Ribosomes can also be found alone in the cytoplasm or in groups called polyribosomes.

The Endomembrane System

The endomembrane system consists of the:

Nuclear envelopeEndoplasmic reticulum

Golgi apparatusLysosomes

Vesicles

Figure 3.14 Endomembrane System

The Endoplasmic Reticulum

The endoplasmic reticulum (ER) is a system of membranes within the cytoplasm and continuous with the nuclear envelope.

Two types are:

Rough ER is studded with ribosomes on the cytoplasm side.

Proteins are synthesized here.

The Endoplasmic Reticulum

Smooth ER lacks ribosomes and is the site of synthesis of phospholipids and other substances.

The ER forms vesicles, which are membrane sacs used to transport substances.

Endoplasmic ReticulumSmooth ER

Rough ERRibosomes

The Golgi Apparatus

The Golgi apparatus consists of a stack of curved saccules.

The Golgi apparatus receives protein and lipid-filled vesicles from the ER.

Ends of the saccules expand forming vesicles, which leave the Golgi apparatus.

The Golgi apparatus processes, packages, secretes, and distributes substances within and out of the cell.

The Golgi Apparatus

Lysosomes

Lysosomes are vesicles produced by the Golgi apparatus.

Lysosomes contain hydrolytic enzymes.

All body cells have lysosomes, but certain white blood cells have many and use them to help protect us from disease-causing microbes.

LysosomesLysosomes can fuse with vesicles to digest their contents.

They engulf, digest, and release simpler substances into the cytoplasm for reuse.

The CytoskeletonThe cytoskeleton is a network of protein fibers that crisscross within the cytoplasm.

The cytoskeleton is made up of different types of fibers that help maintain a cell’s shape and can anchor the organelles or assist with their movement.

Cell Movement

Cilia, tiny hairlike structures on cell surfaces and Flagella, large single projections from cells are involved in movement.

Cilia and FlagellaCilia that line the respiratory tract sweep debris trapped in mucus out and cilia in the oviducts move eggs along.Sperm cells have flagella that move them to find and fertilize eggs.

Mitochondria Structure

Mitochondria are found in plant and animal cells.

Mitochondria are bounded by a double membrane.

The inner membrane is folded to form shelves called cristae.

The folds project into the matrix an inner space filled with a gel-like fluid that contains enzymes, which are used to break down glucose products.

Figure 3.17 Mitochondrion Structure - Page 59

Additional Mitochondrion Facts

Mitochondria were originally prokaryotes.

Mitochondria contain their own DNA and ribosomes.

Mitochondria are only produced by other mitochondria.

In humans and most multicellular organisms mitochondria are maternal inherited from the mothers egg.

Mitochondria FunctionMitochondria are often called the powerhouses of the cell.

Mitochondria convert the chemical energy of glucose into the chemical energy of ATP.

This cell process is called cellular respiration.

Summary of Cellular RespirationGlucose is combined with oxygen and breaks

down to carbon dioxide and water.

The Overall Chemical Reaction:

C6H12O6 + 6O2 6CO2 + 6H2O + Energy

Glucose + Oxygen Carbon Dioxide + Water + Energy

Cellular respiration!

Summary of Cellular Respiration

Cellular respiration provides the energy to run cell processes.

Three pathways called glycolysis, the citric acid cycle, and electron transport are involved.

They allow the chemical bond energy from glucose to be slowly released and used to produce ATP.

Altogether, 36 ATP molecules result from the breakdown of 1 glucose molecule.

Summary of Cellular Respiration

The first pathway (glycolysis) occurs outside mitochondria and breaks down six carbon glucose into two smaller three carbon molecules.

The second pathway (citric acid cycle) occurs within the inner matrix, which contains the enzymes that continue the breakdown of the three carbon molecules.

The third pathway (electron transport) occurs along the inner membrane folds called cristae where most of the ATP is produced.

Summary of Cellular Respiration

Figure 3.19 – Page 60