Chapter 6 magnification : ability to make an image larger than A - - PDF document

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Chapter 6 A Tour of the Cell

How We Study Cells

Microscopes opened up the world of cells Characteristics of Microscopes

• magnification: ability to make an image larger than

actual size

• resolution: power to show details clearly while enlarged

(if poor, objects seem fuzzy)

• constrast: accentuates different parts of the sample

Types of Microscopes

I. compound light

• light passes through one or more lenses
• object must be sliced thinly enough to be

transparent

• upper limitation is 2000X or 0.5 microns

(um) in diameter

• resolution limitation: 0.2 microns
• II. Electron Microscopes (1950’s)
• limited by physical characteristics of light
• can magnify an image up to 200,000 X
• beams of electrons produces enlarged

image

• resolution limitation: 0.002 nm across object

Types of Electron Microscopes

• 1. transmission electron microscope (TEM)
• used to study internal structures
• transmits a beam of electrons

through very thinly sliced specimen stained with heavy metals

• change density of cellular structures and electron transmission
• dead specimens
• 200,000 X magnification
• black and white only

Plant Cell

2. scanning electron microscope (SEM)

• used to study surface structures
• surface covered with thin film of gold
• beam excites electrons on surface
• produces three dimensional images
• 100,000 X mag.
• dead specimens only

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Isolating organelles

1. Cell fractionation - take apart cells, separate major organelles 2. Ultracentrifuge - applies force 1 million times the force of gravity to separate further the cell

• rganelles with the most dense

at the bottom

CELL THEORY

• 1. all living things are composed of cells
• 2. cells are basic units of structure and

function

• 3. all cells come from pre- existing cells

Activities of Life

• Most everything you think of a whole
• rganism needing to do, must be done at

the cellular level…

– reproduction – growth & development – energy utilization – response to the environment – homeostasis

Common features of all cells

• plasma membrane (cell membrane)
• cytosol: semi-fluid substance, holds cellular structures
• cytoplasm: cystsol + organelles
• cytoskeleton: microscopic protein fibers that keep cells shape
• ribosomes: make proteins
• DNA: controls all cell activities

Prokaryote vs Eukaryote

DNA in nucleoid region, without a membrane separating it from rest of cell Cell Wall: Made of Peptidoglycans Ribosome: Smaller, Free in cytoplasm, Simultaneous Transcription/Translation chromosomes in nucleus, membrane-enclosed organelle Cell Wall: made of Cellulose or Chitin Ribosomes: Larger, Free and Attached, Non-simultaneous

Cell Size

• most cells are 5-50 microns

surface area ratio (limits size of cells) inside of cell grows faster: cubed (V = L x W x H)

• utside of cell grows slower: squared

(A = L x W)

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Relationship of Surface Area to Volume

LENGTH OF SIDE (CM) TOTAL SURFACE AREA (CM2) TOTAL VOLUME (CM3) SURFACE AREA TO VOLUME RATIO 1 (1X1X6)= 6 (1X1X1)=1 6:1 2 (2X2X6)=24 (2X2X2)=8 24:8 3:1 3 (3X3X6)= 54 (3X3X3)=27 54:27 2:1

Surface Area Example

Cells must be small to maintain a large surface area to volume ratio Large S.A. allows  rates of chemical exchange between cell and environment Small Intestine: highly folded surface to increase absorption of nutrients Villi: finger-like projections on small intestine wall Microvilli: projections on each cell

Surface Area Example

Root hairs increase surface area for water and mineral absorption

Limits to Cell Size

• Metabolic requirements set uper limit of size

– In large cell, cannot move material in and out fast enough to support life

How to get Bigger

• Become multi-cellular

Cell Shape

• most spherical or cuboidal
• different shapes reflect

function

dermal epidermal cells white blood cells goblet cell

red blood cells

nerve cell

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Eukaryotic Cells

Animal Plant

Cytoplasmic channels- Connnect adjacent Cell’s cytoplasm

Cell Structure

Main components of eukaryotic cells

• 1. cell membrane (outer boundary)
• 2. nucleus (control center)
• 3. cytoplasm (material between nucleus

and membrane)

Nucleus

• control center of cell: directs all cell activities
• contains DNA
• continuous with rough ER
• site of DNA and RNA synthesis
• located in center of most cells

Structure:

• nuclear matrix - protein skeleton helps

maintain nucleus shape

• nuclear envelope (double membrane)
• contains chromatin: combination of

strands of DNA and protein

• nuclear pores: control substance

movement

• nucleoplasm: dense, protein rich
• nucleolus: region that forms ribosomal

subunits

• Cytosol

(between membrane and nucleus)

• Cytoplasm = cytosol + organelles

– gel like material between – contains water, salts, organic molecules – in constant motion (cytoplasmic streaming) – holds organelles

animation amoeba animation

Organelles

Organelle: tiny structure that performs special functions in the cell to maintain life

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Mitochondria

• powerhouse of cell (cell respiration)
• provides energy for cell in form of ATP
• membrane bound
• double membrane:
• most numerous in cells which use a lot of energy (muscle)
• self replicating, contain their own DNA
• cristae: folds of inner membrane

greatly enlarge surface area of inner membrane (more area for chemical reactions of respiration)

• matrix: fluid filled inner compartment

Ribosomes

• spherical structures which make proteins
• not surrounded by membrane
• composed of protein and rRNA
• site of protein synthesis
• free ribosomes: float in cytosol- make proteins used

within cell

• bound ribosomes: attached to rough ER- make proteins

for export from cell (secretion)

Endoplasmic reticulum: (ER)

intercellular highway complex membrane system of folded sacs and tunnels regulates protein traffic and performs metabolic functions Rough ER

• ribosomes stuck to membrane

surface

• package proteins for secretion and

inserted into ER

• can be stored or exported to

smooth ER

• prominent in cells that make a lot
• f protein

Smooth ER

• no ribosomes
• also stores and acts as an

intercellular highway for proteins and enzymes

• involved in:
• synthesis of steroids in gland
• cell regulation of Ca levels

in muscle

• cells break down toxic

substances in liver cells

Golgi Apparatus

• flattened system of membranes and sacs piles on each other

(like pancakes)

• very close to ER
• processes, packages, and secretes proteins for transport (o other

parts of cell (vesicles) and produce lysosomes

– Cis face: receives vesicles – Trans face: ships vesicles

animation

Steps of Protein Production and Transport

1. ribosomes make proteins on the rough ER- packaged into vesicles 2. vesicles transport the newly made proteins from the rough to the Golgi apparatus 3. in Golgi, proteins are processed and then packaged into NEW vesicles 4. vesicles move thru Golgi to cell membrane and release contents outside cell

animation 2

Lysosomes

• small round vesicles that contain digestive hydrolytic enzymes
• formed from Golgi Apparatus
• digest and remove waste from cell (old organelles, byproducts,

bact., viruses) animation

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Vacuoles

• Storage of materials (food, water, minerals,

pigments, poisons)

• Membrane bound
• Ex: food vacuoles, contractile vacuoles

Peroxisomes

• contain different oxidative enzymes than lysosomes
• break down toxic substances into H2O2

(remove H from substances and transfer them to O2)

then converts H2O2 to H2O

• detox alcohol and drugs
• break down fatty acids
• formed from proteins in cytosol, not Golgi

– Glyoxysomes (fat tissues of plant seeds) FA  sugar

Cytoskeleton (cell framework)

• maintains shape and size of cell
• composed of network of long protein strands

located in cytosol

• provides movement for organelles within

cytosol

• regulate biochemical activities

Cytoskeleton Structure

• A. Intermediate Filaments (medium size fibers)
• protein fibers coiled into cables
• maintain shape of cell
• permanent fixtures
• anchor nucleus and organelles

Cytoskeleton Structure

• B. Microtubules (largest fibers)
• long hollow coiled protein tubes (tubulin)
• maintain shape and support cells
• internal cell highways – move organelles thru cell
• form centrioles, spindles (cell division)
• motility (cilia and flagella)
• flagella: long whip-like structures used for movement (motility)
• cilia: short numerous hair like projections
• movement
• transport of substances across cell
• signal receiving antenna for

ex: ear drum: transmits sound waves respiratory tract: moves mucus etc. Motility requires interaction Basal body- anchors

• f cytoskeleton fibers with

cilia/flagella to cell motor proteins.

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Internal Organization

9 + 2 Arrangement

dynein animation

respiratory system animation

• Centrosomes: region near nucleus

– Microtubule organizing center – Contains centrioles – Used in cell division – Not found in plants or fungi – 9 + 0 arrangement

Cytoskeleton Structure

• C. Microfilaments (smallest fibers)
• two strands fine protein (actin) intertwined
• used in cytoplasmic streaming, muscle

contraction, ameboid movement

• smallest strands of cytoskeleton

cytoplasmic streaming

Extracellular Matrix (ECM)

• Outside plasma membrane
• Composed of glycoproteins (collagen/ proteoglycans)
• Strengthens tissues and transmits external signals to cell
• Fibronectins/integrins: attach cells to ECM

Intercellular Junctions (Animal Cells)

• Tight junctions: 2 cells

are fused to form watertight seal

• Desmosomes: “rivets”

that fasten cells into strong sheets

• Gap junctions: channels

through which ions, sugar, small molecules can pass

Intercellular Junctions (Plant Cells)

Plasmodesmata

• Channels in plant cell

walls that attach plasma nmembranes of bordering cells connect

• Water, solutes, some

proteins, RNA move through channels

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Plant Cells

Contain the same

• rganelles as

animal cells plus the following:

• 1. cell walls
• 2. vacuoles
• 3. plastids

Plant Cell wall

• rigid covering of plant cells, algae, and some bacteria
• composed of long chains of cellulose embedded in

hardened lignin and pectin

• very porous (O, H2O, CO2 easily

pass through)

• function: support & protection

Structure

• middle lamella
• laid first, formed from the cell plate

during cytokenesis

• pectin: gluey substance holds cells

together

• primary cell wall
• forms next, expanded inside the

middle lamella

• cellulose: structure and support
• secondary wall
• constructed between the plant cell

and primary wall after a maximum size has been reached and stops growing

• lignin: very stiff and hard, in woody

plants in bark structure and support

Plastids

Convert solar energy into chemical energy to be stored.

3 types (arise from proplastids)

• 1. chloroplasts- chlorophyll (green pigment)

used in photosynthesis 2. chromoplasts- synthesize and store red, orange, and yellow pigments (give plants unusual colors) 3. leucoplasts- store starches, proteins, and lipids colorless