CHAPTER 4 A Tour of the Cell

By Dr. Par MohammadianOverview: •Microscopes•Cells

ProkaryotesEukaryotes

Animal CellsPlants Cells

•Cell Junctions

• Microscopes

INTRODUCTION TO THE WORLD OF THE CELL

• Using a microscope, Robert Hooke discovered cells in 1665

• Cells are the building blocks of all life

• Organisms are either

– Single-celled, such as most bacteria– Multicelled, such as plants, animals

Microscopes provide windows to the world of the cell

The light microscope (LM) enables us to see the overall shape and structure of a cell

• Micrograph: A photograph taken through a microscope.

• LM 109X: The image under the light microscope is 109 times the actual size of the organisms.

• Magnification: Increase in the apparent size of an object.

• Resolving power: Measure of the clarity of an image.

• Electron microscopes (EM) were invented in the 1950s

• They use a beam of electrons instead of light

• The greater resolving power of electron microscopes – allows greater magnification (0.2 nm vs. LM 0.2 µm)– reveals cellular details

• LM remains the major tool to study living organisms because in EM specimen must be held in vacuum.

• Scanning electron microscope (SEM) used to study the detailed architecture of the surface of a cell

• Transmission electron microscope (TEM) is useful for exploring the internal structure of a cell

• At minimum, a cell must be large enough to house the parts it needs to survive and reproduce

• The maximum size of a cell is limited by the amount of surface needed to obtain nutrients from the environment and dispose of wastes

Natural laws limit cell size

• CellsProkaryotic CellsEukaryotic Cells

Animal CellsPlant Cells

• The countless cells on earth fall into two categories

The Two Major Categories of Cells

– Prokaryotic cells– Eukaryotic cells

• Prokaryotic and eukaryotic cells differ in several respects

• Prokaryotic cells

– Are smaller than eukaryotic cells and are simple– Lack internal structures surrounded by membranes– Lack a nucleus

• A prokaryotic cell is enclosed by a plasma membrane (protects & shape of the cell) and is usually encased in a rigid cell wall (further protection)

– The cell wall may be covered by a sticky capsule (help glue prok. to surfaces)

– Inside the cell are its DNA and other parts

• All other life forms (such as animals, plants, protists, or fungi) are made up of one or more eukaryotic cells

• These are larger and more complex than prokaryotic cells

• Eukaryotes are distinguished by the presence of a true nucleus

Eukaryotic cells are partitioned into functional compartments

• The plasma membrane controls the cell’s contact with the environment

• The cytoplasm contains organelles

• Many organelles have membranes as boundaries– These compartmentalize the

interior of the cell– This allows the cell to carry

out a variety of activities simultaneously

• A plant cell has some structures that an animal cell lacks:

– Chloroplasts – A rigid cell wall– Central vacuole

• The largest organelle is usually the nucleus

• The nucleus is separated from the cytoplasm by the nuclear envelope

• The nucleus is the cellular control center– It contains the DNA that directs the cell’s activities

The nucleus is the cell’s genetic control center

– Genes in the nucleus store information necessary to produce proteins

• The plasma membrane separates the living cell from its nonliving surroundings

MEMBRANE STRUCTURE AND FUNCTION

• The membranes of cells are composed of

A Fluid Mosaic of Lipids and Proteins

– Lipids– Proteins

• The endomembrane system is a collection of membranous organelles– These organelles manufacture and distribute cell

products– The endomembrane system divides the cell into

compartments– Endoplasmic reticulum (ER) is part of the

endomembrane system

Cytoplasm

• The rough ER manufactures membranes• Ribosomes on its surface produce proteins

Rough endoplasmic reticulum makes membrane and proteins

1) Synthesized polypeptide is passed into ER

2) Sugars attach to polypeptides forming glycoprotein

3) ER packages it in tinny sacs called transport vesicles

4) It buds off from the ER & secretory vesicles travel to Golgi apparatus.

• The “roughness” of the rough ER is due to ribosomes that stud the outside of the ER membrane

• The functions of the rough ER include

– Producing proteins– Producing new membrane

• Smooth ER synthesizes lipids

• In some cells, it regulates carbohydrate metabolism and breaks down toxins and drugs

• Stores Ca ions

Smooth endoplasmic reticulum has a variety of functions

• The Golgi apparatus consists of stacks of membranous sacs – These receive and modify ER products, then send

them on to other organelles or to the cell membrane

The Golgi apparatus finishes, sorts, and ships cell products

• Lysosomes are sacs of digestive enzymes

Lysosomes digest the cell’s food and wastes

• Lysosomal enzymes

– digest food– destroy bacteria– recycle damaged organelles– function in embryonic

development in animals

• Vacuoles are membranous sacsVacuoles

– Two types are the contractile vacuoles of protists and the central vacuoles of plants

–The vacuole has lysosomal and storage functions

• Cells require a constant energy supply to do all the work of life

CHLOROPLASTS AND MITOCHONDRIA: ENERGY CONVERSION

• Chloroplasts are the sites of photosynthesis, the conversion of light energy to chemical energy

CHLOROPLASTS

• Mitochondria are the sites of cellular respiration, which involves the production of ATP from food molecules

Mitochondria

• A network of protein fibers makes up the cytoskeleton

The cell’s internal skeleton helps organize its structure and activities

THE CYTOSKELETON AND RELATED STRUCTURES

• One function of the cytoskeleton

Maintaining Cell Shape

– Provide mechanical support to the cell and maintain its shape

• The cytoskeleton can change the shape of a cell

– This allows cells like amoebae to move

• Microfilaments of actin enable cells to change shape and move

• Intermediate filaments reinforce the cell and anchor certain organelles

• Microtubules – give the cell rigidity– provide anchors for organelles– act as tracks for organelle movement

• Eukaryotic cilia and flagella are locomotor appendages that protrude from certain cells

• A cilia or flagellum is composed of a core of microtubules wrapped in an extension of the plasma membrane

Cilia and flagella move when microtubules bend

• Flagella propel the cell in a whiplike motion

• Cilia move in a coordinated back-and-forth motion

• Some cilia or flagella extend from nonmoving cells

– The human windpipe is lined with cilia

•Cell Junctions

• Cells interact with their environments and each other via their surfaces

• Plant cells are supported by rigid cell walls made largely of cellulose– They connect by plasmodesmata, channels that allow

them to share water, food, and chemical messages

Cell surfaces protect, support, and join cells

EUKARYOTIC CELL SURFACES AND JUNCTIONS

• Plant cells are encased by cell walls

Plant Cell Walls and Cell Junctions

– These provide support for the plant cells

• Animal cells lack cell walls

Animal Cell Surfaces and Cell Junctions

– They secrete a sticky covering called the extracellular matrix

– This layer helps hold cells together

• Animal cells connect by various types of junctions– Tight junctions– Adhering junctions– Communicating junctions

• Tight junctions can bind cells together into leakproof sheets

• Anchoring junctions link animal cells

• Communicating junctions allow substances to flow from cell to cell