Chapter 4

Functional Anatomy of Prokaryotic and Eukaryotic Cells

Part 2

Structures Internal to the Cell Wall(The Prokaryotic Cell)

• Plasma (Cell or Cytoplasmic) Membrane• Cytoplasm• Nuclear Area• Ribosomes• Inclusions• Endospores

Figure 4.14a

Plasma Membrane• A thin structure lying inside the cell wall and

enclosing the cytoplasm of the cell• Consists primarily of phospholipids and

proteins (except Mycoplasma, contains sterols)

Plasma Membrane

Figure 4.14b

Plasma (Cytoplasmic) Membrane

• Phospholipid bilayer– polar head composed of a phosphate group and

glycerol; hydrophilic (water-loving); on the surfaces of the bilayer

– nonpolar tails composed of fatty acids; hydrophobic (water-fearing); interior of the bilayer

Plasma (Cytoplasmic) Membrane

• Peripheral proteins– at the inner or outer surface of the membrane– function:

• enzymes to catalyze chemical reaction• a “scaffold” for support• mediators of changes in membrane shape during

movement

Plasma (Cytoplasmic) Membrane

• Integral proteins (some are believed to span across the entire phospholipid bilayer = transmembrane proteins)– some are channels that have a pore or a hole to

allow substances to enter and exit the cell

• The dynamic arrangement of phospholipids and proteins comprising the plasma membrane– phospholipids and proteins move quite freely

within the membrane surface

Fluid Mosaic Model

Figure 4.14b

Fluid Mosaic Model

• Membrane is as viscous as olive oil to allow membrane proteins to move freely

• Proteins move to function without destroying the structure of the membrane

• Phospholipids rotate and move laterally

• Selective permeability (semipermeability) allows passage of some molecules– permeability depends on size, nature of

substances (easily dissolve in lipids), and transporter molecule

• Enzymes to break down nutrients and produce energy (ATP)

• Photosynthetic pigments (chromatophores or thylakoids) found in infoldings (Fig. 4.15)

Plasma Membrane

• Mesosomes are artifacts, not true cell structures

• Damage to the membrane by alcohols, quaternary ammonium (disinfectants) and polymyxin antibiotics causes leakage of cell contents.

Plasma Membrane

Movement of Material Across Membranes

• Materials move across plasma membranes by passive and active processes– Passive process substances move across the

membrane with the concentration gradient, or difference; no expenditure of energy (ATP)

– active process substances move across the membrane against the concentration gradient; require expenditure of energy (ATP)

• Passive processes (simple diffusion, facilitative diffusion, & Osmosis) stops when the molecules or ions are evenly distributed (equilibrium) – Simple diffusion: Movement of a solute from

an area of high concentration to an area of low concentration.

• used to transport small molecules (e.g. O & CO2)

Movement Across Membranes

Movement Across Membranes

– Facilitative diffusion: solute combines with a transporter protein in the membrane

• the transporter undergoes a change in shape to transport the substance

• Large molecules that cannot be transported into the cells are degraded (broken down) by extracellular enzymes produced by bacteria and released into the surrounding medium subunits move into the cell with the help of transporters

Movement Across Membranes

Figure 4.17

– Osmosis: movement of water across a selectively permeable membrane from an area of high water concentration to an area of lower water.

– Osmotic pressure: the pressure needed to stop the movement of water across the membrane.

Movement Across Membranes

Figure 4.18a

Fig. 4.18a & b

Figure 4.18c-e

Osmotic Solutions

• Bacterial cell may be subjected to any of three kinds of osmotic solutions (isotonic, hypotonic, & hypertonic)

• Active processes (active transport & group translocation) are used when a bacterial cell is in an nutrient low environment to accumulate the needed substances – Active transport of substances requires a

transporter protein and ATP.• move substance from outside to inside of a cell

although the concentration might be much higher inside the cell

Movement Across Membranes

Movement Across Membranes– Group translocation of substances requires a

transporter protein and PEP (phosphoenol-pyruvic acid, high-energy phosphate compounds).

• occurs only in prokaryotes• substance is chemically altered during transport across

the membrane (membrane is impermeable to the altered substance)

• allows a cell to accumulate various substances although they may be in low concentrations outside the cell

• Cytoplasm is the substance inside the plasma membrane– thick, aqueous, semitransparent, and elastic

Cytoplasm

Figure 4.6a, b

Cytoplasm

Cytoplasm

– about 80% water and contains primarily proteins (enzymes), carbohydrates, lipids, inorganic ions, and many low-molecular-weight compounds

• Major structure are a nuclear area (containing DNA), ribosomes, and inclusions (reserve deposits)

• Nuclear area (nucleoid) contains a single long, continuous, circularly arranged thread of double-stranded DNA (bacterial chromosome)

Nuclear Area

Figure 4.6a, b

Nuclear area (nucleoid) containing DNA

Plasmid

Nuclear Area– No nuclear envelope and do not include histones– can be spherical , elongated, or dumbbell-shaped– chromosomes attached to the plasma membrane

• Plasmids: small circular, double-stranded DNA– extrachromosomal genetic elements, replicate

independently of chromosomal DNA– can be transferred from one bacterium to another– used for gene manipulation in biotechnology

Nuclear Area– associated with plasma membrane proteins– usually contain from 5 - 100 genes that are

generally not crucial for the survival of the bacterium; maybe gained or lost without harming the cell

– Under certain condition, provide advantage to cells

• carry genes for antibiotic resistance, tolerance to toxic metals, production of toxins and synthesis of enzymes

Figure 4.6a

Ribosomes

Ribosomes

• Sites of protein synthesis– If actively growing, have high number

Figure 4.19

Ribosomes• Composed of two subunits (each consists of

protein and ribosomal RNA (rRNA)– 70S ribosomes (S = Svedberg units) has

subunits of a small 30S (one molecule of rRNA) and a larger 50S (two molecule of rRNA)

Streptomycin and Gentamicin

Erythromycin and Chloramphenicol

Inclusions

• Reserve deposits– may accumulate certain nutrients when they are

plentiful and use them when it is low or deficient in the environment

– macromolecules concentrated in inclusions does not cause the increase in osmotic pressure

– may serve as a basis for identification (some are limited to a small number of species)

Inclusions

• Matachromatic granules (volutin)– large inclusions; sometimes stain red with

certain blue dyes (methylene blue)– inorganic phosphate (polyphostate) reserve that

can be used to synthesize ATP– generally formed in cells grown in phosphate-

rich environments– characteristic of Corynebacterium diphtheriae

(cause diphtheria)– also found in algae, fungi and protozoa

Inclusions

• Polysaccharide granules– consist of glycogen and starch; glycogen appear

reddish brown and starch appear blue when stained with iodine

– energy reserve• Lipid inclusions

– stained by fat-soluble dyes (e.g. Sudan dyes)– energy reserve

Inclusions

– Poly--hydroxybutyric acid (polymer, common lipid-storage material unique to bacteria)

– Appear in various species of Mycobacterium Bacillus, Azotobacter, Spirillum, and other genera

• Sulfur granules– energy reserve deposited by certain bacteria

(e.g. sulfur bacteria, Thiobacillus)

Inclusions

• Carboxysomes– contain enzyme ribulose 1,5-diphosphate

carboxylase for CO2 fixation during photosynthesis (CO2 is used as the sole carbon source)

– found in nitrifying bacteria, cyanobacteria, and thiobacilli

Inclusions

• Gas vacuoles– hollow cavities found in many aquatic

prokaryotes (cyanobacteria, anoxygenic photosynthetic bacteria, and halobacteria)

– consists of rows of several individual gas vesicles

– maintain buoyancy

Inclusions• Magnetosomes (Fig. 4.20)

– inclusions of iron oxide formed by several gram-negative bacteria (Aquaspirillum magnetotacticum)

– act like magnets; allow bacteria to move downward until they reach a suitable attachment site

– In vitro, can decompose hydrogen peroxide (toxic by-product of respiration); protect cell against accumulation of hydrogen peroxide

• Specialized resting cells; unique to bacteria• Can remain dormant for thousands of years• Bacillus & Clostridium (both are gram-positive)• Dehydrated cells with thick walls and additional

layer– contains only DNA, small amounts of RNA,

ribosomes, enzymes, and a few important small molecules (e.g dipicolinic acid and calcium ions)

Endospores

Endospores

• Resistant to desiccation, heat, freezing, (toxic) chemicals, and radiation due to a thick spore coat– true endospores found in gram-positive bacteria– Coxiella burnetti (gram-negative, cause Q

fever) form endospore like structures• Sporulation (sporogenesis): process of

endospore formation – takes several hours within a vegetative (parent)

cell

Endospores

– triggered by adverse environmental condition (e.g. nutrient depletion)

– Endospore can be located terminally, subterminally, or centrally

• Germination: process of returning to vegetative state– Triggered by physical or chemical damage to

the spore coat

The Eukaryotic Cell

• Include algae, protozoa, fungi, higher plants, and animals

• Larger and structurally more complex• Contains membrane-enclosed organelles

Flagella and Cilia

• Projections used for Cellular locomotion or for moving substances along the cell surface

• Both flagella and cilia are anchored to the plasma membrane by a basal body

• Flagellum moves in a wavelike manner

Flagella and Cilia

Figure 4.23a, b

– projections are few and are long = flagella– Projections are numerous and short, resembles

hairs = cilia

• Microtubules– Long, hollow tubes made up of a protein (tubulin)

• 9 pairs + 2 arrangements of a flagellum or cilium

Figure 4.23c

• Cell wall (simpler than prokaryotic cell wall)– Plants, algae, fungi– Carbohydrates

• Cellulose, chitin, glucan, mannan• Glycocalyx

– Carbohydrates extending from animal plasma membrane

– Bonded to proteins and lipids in membrane

Cell Wall and Glycocalyx

• Phospholipid bilayer• Peripheral proteins• Integral proteins• Transmembrane proteins• Sterols (e.g. cholesterol)• Glycocalyx carbohydrates

Plasma Membrane

• Selective permeability allows passage of some molecules

• Simple diffusion• Facilitative diffusion• Osmosis• Active transport

Plasma Membrane

Plasma Membrane

• Endocytosis– Phagocytosis: Pseudopods extend and engulf

particles– Pinocytosis: Membrane folds inward bringing

in fluid and dissolved substances