CHAPTER 4

A TOUR OF THE CELL

Sect. 4.2 Cell Size

-Most cells are between 1 & 100um in diameter (Fig. 4.2A p. 54)

-Plasma membrane (PM): selective barrier that consists of proteins & lipids (phospholipid bilayer)

-Cell size is limited due to:

-a single nucleus controlling the entire cell

-enough surface area in relation to volume to obtain nutrients and dispose of wastes

Two Types of Cells Prokaryotic• No membrane

bound organelles, only nucleoid region

• Has ribosomes & DNA

• Cell wall, capsule & pili help attach, flagella

• Found in the Domains Bacteria & Archaea

Eukaryotic• Have cytoplasm –

region between the nucleus and the cell membrane (consists of a semifluid medium called cytosol)

• Contain organelles – structures w/special functions

• Found in protists, plants, fungi, and animals

Sect. 4.3

The Nucleus(genetic control center of a

eukaryotic cell)

Nuclear Envelope

-boundary around nucleus, separating it from the contents of the cytoplasm

-Double membrane – each with a phospholipid bilayer

Sect. 4.5

Contents of the Nucleus

- nearly all of the cell’s DNA is here (organized along w/proteins into chromosomes)

- unless the cells are dividing, chromosomes are seen as a tangled up mess called chromatin

- nucleolus functions in the synthesis of ribosomes

Ribosomes

Function

• assemble enzymes and the entire cell’s other proteins

• there are genetic instructions for this to occur

•not membrane bound

Sect. 4.4

Location

• exist in 2 places w/in cytoplasm

1) free ribosomes - suspended in cytosol (make proteins for cell use)

2) bound ribosomes – attached to endoplasmic reticulum (make proteins for export outside cell)

• ribosomes usually occur in clusters called polysomes

Endomembrane System

Endoplasmic Reticulum (ER)

• a network of interconnected compartments, continuous w/the outer membrane on the nuclear envelope p. 58

W/in the ER is a space known as the lumen

Sect. 4.6

• 2 distinct regions of the ER (interconnected flattened sacs)

Rough ER - has ribosomes on surface

- makes proteins destined to be secreted

- makes membranes

Sect. 4.8

Smooth ER - lack ribosomes (p. 59), membrane is continuous w/rough ER

- serves as a transition area for ER products

- makes steroids and sex hormones

- tolerance to drugs

- calcium for muscles

Sect. 4.7

Golgi Complex (p. 60)

• after leaving the ER, most transport vesicles travel to this organelle

•manufactures, warehouses, and ships (products of the ER are modified, stored, and shipped to other locations in transport vesicles)

Sect. 4.9

- consists of flattened sacs stacked up like pancakes (not connected)- molecules move from sac to sac

Lysosomes

• membrane-bound bag of hydrolytic (digestive) enzymes

- cell uses these enzymes to digest macromolecules

- these enzymes work best at pH 5

Sect. 4.10

• made by the rough ER, processed, and released by the Golgi

• function by using phagocytosis (engulfs particles) then digests them

• also function in recycling cell’s own organic material (recycling centers for damaged organelles)

• ex: webbing between fingers in embryonic development

• lysosomal storage diseases (missing one of the hydrolytic enzymes):

Pompe’s disease (liver can’t break down polysaccharide glycogen)

Tay-Sachs (overload of lipids on nervous system)

Sect. 4.11

Microbodies• bounded by a single membrane

• compartments specialized for specific metabolic pathways (each has a particular kind of enzymes)

2 kinds of microbodies:

Peroxisomes - have enzymes which transfer H from various substrates to O (produce H2O2 as a byproduct)

Glyoxysomes - contain enzymes to convert fats to sugar (in plants)

Vacuole

• a large membrane sac

• various functions:

food vacuole - formed by phagocytosis

contractile vacuole - found in freshwater protists (pumps out excess water from the cell)

Sect. 4.12

Central vacuole - found in mature plant cells - bounded by a tonoplast (membrane)

- stores organic compounds

Energy Transducers(have their own DNA)

Chloroplasts

• formed only in plants

• functions in photosynthesis

• chloroplast – contains a green pigment (chlorophyll)

Sect. 4.14

p. 63

• chloroplast’s elaborative structure:

-consists of flattened green sacs which are stacked up

-sacs called thylakoids

-stack of thylakoids called a granum (if more than one granum, it’s called grana)

-stroma: thick fluid surrounding thylakoids

Mitochondria (p. 63)

• site for cellular respiration

• found in plants and animals

Sect. 4.15

• these foldings divide inner region of mitochondria

- intermembrane space – region between inner and outer membranes

- has inner membrane that’s convoluted within foldings called cristae (this greatly increases surface area to enhance ATP production)

- mitochondrial matrix – is enclosed by the inner membrane (most of the respiration takes place here)

Cytoskeleton – supportive meshwork of fine fibers for

structural supportMicrofilaments

(thinnest type of fiber)

• solid rods of globular proteins called actin – linked in chains

• best known for role in muscle contraction

Sect. 4.16

• protein called myosin is imbedded in the actin molecule

• in plants, microfilaments are involved in cytoplasmic streaming (cytoplasm flows in a particular direction ex: moving chloroplasts)

Intermediate Filaments

- ropelike

- reinforces cell shape

- anchors certain organelles (nucleus is caged by these to keep in place)

Microtubules

• hollow rods of globular proteins called tubulins

• found in cytoplasm of all eukaryotic cells

• radiate out from cell centers

• give cell shape and reinforce it

• help w/animal cell division

p. 64

• also help other organelles navigate through cytoplasm

• w/in cell center are 2 centrioles – consist of 9 sets of triplet microtubules and must be 2 centrioles (a pair) in the center

9 sets

p. 65 Fig. 4.17C

Cilia and Flagella

Cilia – short fingerlike projections used for locomotion

Flagellum – a whip-like tail

• both have a core of microtubules and protrude from the cell

Sect. 4.17

• wrapped in plasma membrane

• anchored to the cell by a basal body – similar to a centriole

Ex: cilia on cells in wind pipe

flagellum on sperm

Cell Surface

Cell Wall

• plants only - thicker than PM

• there is a thin, flexible primary cell wall (between the 2 cell walls is the middle lamella - like glue) (pectin)

Sect. 4.18

• cell wall strengthens as the cell matures

• some plants add a secondary cell wall between the PM and the primary cell wall• plants use the plasmodesmata

- channels in the cell wall- strands of cytoplasm connect

one cell to another

Extracellular Matrix

• no cell wall in animals

• glycocalyx - fuzzy coat

• made of carbohydrates and helps cells stick together - strong surfaces

Intercellular Junctions

• provides a means by which many cells can be integrated into one functional organism

• cell to cell contact in animals provided by 3 junctions:

1. Tight junctions - binds cells together ex: digestive tract

2. Anchoring junctions - attach adjacent cells to each other to stretch ex: skin

3. Gap junctions - allow water and other small molecules to flow between neighboring cells (similar to plasmodesmata in plants)