Cells, Chapter 6, Campbell, 7E

Outline

• Cell Theory

• What is a cell?

• How do we study cells?

• Why are cells small?

• Prokaryotic cells

• Eukaryotic cells

– Organelles, cytoskeleton, etc.

– Plant cells

Cell Theory

• All organisms composed of one or more cells, and the life processes of metabolism and heredity occur in cells.

• Cells are the basic unit of structure and function.

• Cells arise only by division of previously existing cells.

• Cell structure is correlated to cellular function

What is a Cell?

• Must have

– plasma membrane

– genome

– cytoplasm

Figure 6.2  The size range of cells

To study cells, biologists use microscopes and the tools of biochemistry

Microscopy

• Microscopy – Light and electron

– Pass electromagnetic radiation through a specimen.

– Magnify cellular structures.

– Resolution is the distance two points can be apart and be distinguished as two separated points.

• 200 nanometers for light microscopes

• 0.2 nanometers for electron microscopes

Light Microscopy

• Passes visible light through a cell.

• Useful for resolving structures 200 nm or larger

• Necessary for viewing living cells

• Use different methods for enhancing visualization of cellular structures

Electron Microscopy (EM)

• Can visualize membranes, organelles, details up to 0.2 nm in diameter.

• Cells must be fixed and stained (i.e. dead)

• Transmission EM – electrons are transmitted through the specimen.

– Great for organelles, interior detail.

• Scanning EM – electrons are bounced off the surface of the object.

– Great for membranes, exterior detail.

Cell Fractionation

• Used by cell biologists to isolate organelles to study their functions.

– Cells are broken open using homogenization.

– Differential centrifugation:

Cell debris is centrifuged with increasing force. Different organelles can be isolated at each step.

Why are cells small?

• Surface to Volume ratio

– Allows for interaction with environment, obtains nutrients, disposes of wastes.

• The plasma membrane

– Functions as a selective barrier

– Allows sufficient passage of nutrients
and waste

Prokaryotes

• Simple cells, little internal structure:

– DNA in cytoplasm

– peptidoglycan cell walls

– ribosomes

Bacterium is analogous to a small business

• Reactions not compartmentalized, cell operates as a single unit.

• Can adapt to environment, multiply rapidly

• Plasma membrane carries functions performed by some organelles

– Partitions bacterial chromosome copies during cell division.

– Contains photosynthetic pigments

Eukaryotic Cells

• Eukaryotic cells

– Nucleus and Ribosomes

– The Endomembrane System

– Mitochondria, and Chloroplasts

– The Cytoskeleton

– Cell Exteriors

– Comparing and Contrasting Plant and Animal Cells

Eukaryotic Cells

• Analogous to a large corporation

• Compartmentalized functions

• Great diversity (genetic)

• Can form multicellular organisms

Features of Eukaryotic Cells

• DNA enclosed in a nucleus

• Membrane bound organelles

• Vesicles transport materials through the cells

• Multiple DNA molecules packaged into chromosome.

• Cytoskeleton

• Cell walls of plants and fungi very different from peptidoglycan cell walls of bacteria.

Figure 7.7  Overview of an animal cell

Figure 7.8  Overview of a plant cell

Compartmentalization

• Eukaryotic cells have membrane bound organelles and an extensive internal membranes.

• Internal membranes compartmentalize the functions of a eukaryotic cell.

– Highly organized

– Much more efficient

– Allows a larger cell

• Remember – cell size is limited by surface to volume ratio.

The Nucleus

• The nucleus contains a eukaryotic cell�fs DNA.

– Surrounded by a double membrane called the nuclear envelope.

– The nuclear envelope has pores that selectively allow macromolecules to enter and exit the nucleus.

– DNA is organized along with proteins to make a fibrous material called chromatin.

Figure 6.10  The nucleus and its envelope

Ribosomes

• Ribosomes are molecular machines that synthesize proteins.

– They are made of RNA and protein.

– Ribosomes are manufactured in the nucleus, but move to the cytoplasm where protein synthesis occurs

– The nucleolus is a densely staining structure that is the site of ribosomal RNA synthesis.

The Endomembrane System consists of:

• Nucleus

• Endoplasmic Reticulum

• Golgi Apparatus

• Vesicles

• Lysosomes

• Vacuoles

The Endoplasmic Reticulum (ER)

• Largest internal membrane system involved in biosynthesis.

• Rough ER exterior is studded with ribosomes

– Proteins are synthesized by the ribosomes, then drawn inside to be properly folded by chaperonins.

• Smooth ER

– Carbohydrate and lipid synthesis, detoxification

Figure 6.12  Endoplasmic reticulum (ER)

The Golgi Apparatus

• Delivery system of the cell.

• Flattened interconnected sacs called Golgi Bodies make up the apparatus.

– Stacked folds called cisternae.

– Receiving end is cis face

– Discharging end is trans face

• Function: collection, packaging and distribution of molecules.

Figure 6.13  The Golgi apparatus

Lysosomes

• Membrane bound digestive vesicles

• Bud from the endomembrane system

• Contain enzymes that degrade macromolecules, organelles, and cellular debris

• Very acidic.

– Lysosomal enzymes function in an acid environment.

Vacuoles

• Food Vacuoles store complex energy containing molecules.

• Contractile Vacuoles pump water out of the cell.

• Central Vacuole provides hydrostatic pressure and storage .

Figure 6.15  The plant cell vacuole

–

Mitochondria

• Powerhouse of the cell

– Center of oxidative metabolism

• Bounded by a double membrane with many in-folds called cristae.

• Two compartments:

– Matrix

– Intermembrane space

Chloroplasts

• Site of photosynthesis

• Convert light energy to chemical energy

• Larger and more complex than mitochondria

• Bounded by a double membrane

• Grana - Stacked membranes that form closed compartments

• Thylakoids –disk shaped membranes that form the grana.

• Stroma – the fluid in the matrix of the chloroplast.

Role of the Cytoskeleton

• Function in shape, transport, and movement.

• Fibers made of polymers of protein subunits.

• Three types of cytoskeletal fibers:

– Actin filaments

– Microtubules

– Intermediate filaments

Table 6.1 The Structure and Function of the Cytoskeleton

Actin Filament Structure

• Long fibers about 7 nanometers in diameter

• Composed of two protein chains loosely twined together

• Subunit – actin

• Actin filaments will spontaneously form

• Cellular proteins regulate actin filament formation

Actin Filament Function

• Maintenance of cell shape

• Cell movement: contraction, crawling, amoeboid motion, cytoplasmic streaming

• Pinching off of daughter cells during cell division

• Formation of cellular extensions, shape

Figure 6.26 A structural role of microfilaments

Figure 6.27  Microfilaments and motility

Microtubule Structure

• Hollow tubes about 25 nanometers in diameter

• Composed of a circle of filaments around a central core

• Subunit making up the filaments is tubulin.

• Usually form from a microtubule organizing center (MTOC)

Microtubule Function

• Maintenance of shape

• Cell motility

– Flagella

– Cilia

• Motor proteins propel organelles and materials around the cell along microtubule cables.

• Move chromosomes during mitosis/meiosis.

• Make up centrioles

Figure 6.23  A comparison of the beating of flagella and cilia

Figure 6.21 Motor proteins and the cytoskeleton

Figure 6.22 Centrosome containing a pair of centrioles

Centrosomes (Centrioles)

• Barrel shaped, composed of a protein called tubulin

• Some contain DNA

• Help assemble microtubules

• Found in animal cells, but NOT plant or fungi cells.

Intermediate Filaments

• 8-10 nm in diameter

• Very stable

• Many different kinds

– Subunits differ

• Many cell roles

• Example: Keratin in skin, hair and nails

–

Cell Exteriors

• Surfaces

– Plant cell walls

– Extracellular matrix found on animal cells

• Junctions

– Plasmodesmata

– Desmosomes

– Gap junctions

Plant Cell Walls

• Plant cells are encased by cell walls

• Made of cellulose

• Protects, maintains shape, prevents excess water uptake

• Plant cell walls are unique to the plant kingdom.

– Animal cells do not have cell walls.

– Fungi have cell walls made of chitin, and bacterial cell walls are made of peptidoglycan.

Extracellular Matrix

• Found in animals

• Surrounds cells

• Functions in support, adhesion, movement and regulation/communication.

• Made of collagen woven together by proteoglycans.

Intracellular Junctions

• Allow interaction and communication between neighboring cells.

• Plant cells have channels called plasmodesmata.

– Cytosol can pass from plant cell to plant cell.

• Animal cells

– Tight junctions – water tight seal

– Desmosomes anchor cells together

– Gap junctions provide cytoplasmic channels between cells.

Figure 6.31  Intercellular junctions in animals

Comparison of Plant and Animal Cells

• Plant cells contain several structures not found in animal cells

– Central vacuole

• Storage of sugars, ions

• Maintains turgor pressure

– Cell Wall

• Protection and support

• Composed of cellulose

– Chloroplasts

• Animal cells contain centrosomes, which are not found in plant or fungal cells.

The End.