1. Cell Biology Notes

8/18/2019 1. Cell Biology Notes

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Cell Biology

1. 1 Introduction to Cells

Cell Theory

>> According to the cell theory, living organisms are composed of cells.

The cell theory states that cells are basic unit structure and function of

every living thing. The three main ideas include:

1. Cells are the building blocks of structure in living things2. They are the smallest unit of life3. Cells are formed from one another, pre-eisting cells by division!. Cells also store all information they re"uire for gro#th, development

and behavior$. Cells are the location for all the chemical reactions needed for life,

metabolism.

Common Cell %eatures

• &urrounding cell membrane, that separates the contents of the cell

from its eternal environment.

• 'enetic material #hich store(s instructions for the cell(s activities.

• )ctivities are chemical reactions, catalysed by en*ymes made in the

cells

• Cell(s activities are po#ered by their o#n energy release system.

+ceptions to the Cell Theory

>> Looking for trends and discrepancies: although most organismsconform to the cell theory, there are exceptions

There are some tissues and organisms that are not made of typical cells:

Skeletal Muscle-

• t is made up of muscle bres.

• ike cells these bres are enclosed inside a membrane,

•

They are much larger than most cells /300 or more mm long• Contain hundreds of nuclei.


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Giant Algae-

• &uch as )cetabularia

• Can gro# to a length of as much

as 100mm

• e can epect them to consist of

many small cells• They only contain a single nucleus

so are not multicellular.

Aseptate Fungi

• Consists of thread- like

structures called hyphae.

• These hyphae are not divided

into sub-units containing singlenucleus.

• They are long undivided

sections of hypha, #hichcontain many nuclei.

Unicellular Organisms

>> Organisms consisting of only one cell carry out all functions of life in

that cell.

&tructure of unicellular organisms is more comple than most cells, in

multicellular organisms. nicellular organisms carry out seven functions of

life

• 4utrition - 5btaining food for energy and material, needed for

gro#th

• 6etabolism - Chemical reactions inside the cell to release energy

• 'ro#th - rreversible increase in si*e

• 7esponse - )bility to react to changes in the environment

• +cretion - To remove #aste products of metabolism

• 8omeostasis - 9eeping organism(s internal environment constant

• 7eproduction - roducing o;spring either seually or aseually

6any unicellular organisms have a method of movement, but some

remain in a ed position or merely drift in #ater or air currents.


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Sie o! Cells

1nm < a molecule10nm < cell membrane

100nm < virus

1000nm < =acterium /1>m

10000nm < 5rganelles /10>m

100000nm < Cells

/100>m

Actual Sie " Sie o! image # $agni%cation

>> Surface area to volume ratio is important in the limitation of cell size.

)s the si*e of a structure increases, the surface area to volume ratiodecreases. %or eample, using a cube:ength &urface )rea ?olume &):?

• )s organisms get bigger, their volume and surface area both get

bigger, but not by the same amount.

• )s a result, larger organisms have a slo#er rate of echange

/di;usion@radiation #ith their outside surroundings.

•

This is true for organelles, cells, tissues, organs and organisms.• )ll organisms need to echange substances such as food, #aste,

gases and heat #ith their surroundings.

• The rate of echange of substances depends on the surface area of

the organism #hich is in contact #ith its surroundings.

• %or this reason, cells are very small so that they are able to

echange substances eAciently

>> ulticellular organisms have properties that emerge from the

interaction of their cellular components.

• &ome unicellular organisms live together in colonies.

• +.g. ?olvo aureus, a variety of algae.

1cm 10B-2m centimeter

1mm 10B-3m milimeter

1>m 10B-m micromet

er1nm 10B-Dm nanomete

r


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• 5rganisms consisting of a single mass of cells, fused together, are

multicellular.

• ndividual cells in a group can organi*e themselves and interact #ith

each other to form a living organism #ith distinct overall properties.

• The characteristics of the #hole organism, including the fact that it

is alive, kno#n as emergent properties.

• hen the component parts of a comple structure interact,

emergent properties arise. The #hole is greater than the sum of its

parts.

>> Specialized tissues can develop !y cell di"erentiation in multicellular

organisms.

• The human body has many speciali*ed cells. +ach cell has a uni"ue

and specic role.

e.g the function of the red blood cell is to carry oygen and thefunction of the rod cell in the retina is to absorb light and transmit

impulses to the brain.

• 6ulticellular organisms form tissues. They(re a group of cells

combine #hich together to speciali*e and perform the same

function.

- This allo#s them to be more eAcient

- They(re able to develop the ideal structure, #ith the en*ymes

needed to carry out the chemical reactions

• This ability of cells to develop di;erent #ays of carrying out

specic@speciali*ed functions is called di"erentiation.


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>> #i"erentiation involves the expression of some genes and not others

in a cell$s genome.

6ulticellular organisms are large and need to have speciali*ed parts to

their structure so that all the necessary functions of life can be performed.

&i'erentiation - The cells can become speciali*ed to perform their

function. These cells s#itch on, or epress, particular genes that correlate

#ith these specic functions. The epression of these genes #ill inEuence

the shapes, functions and adaptations #ith that cell.

%or eample, a muscle cell #ill only epress muscle genes, but not nerve

cell genes.

Specialiation in multicellular organisms is more eAcient for organisms

competing for a specic resource. 6ovement of nutrients, #ater, etc, can

happen faster and more e;ectively than passing bet#een cells throughdi;usion.

>> %he capacity of stem cells to divide and di"erentiate along di"erent

path&ays is necessary in em!ryonic development. 't also makes stem

cells suita!le for therapeutic uses.

• )t early stages in embryonic development, the cells are capable of

dividing many times to produce large amounts of tissue.

• They are also etremely versatile and can di;erentiate along

di;erent path#ays into any of the cell types found in a ne# animal.

• The name stem cells #as given to the *ygote and the cells of the

early embryo in the 1Dth century, because all adult tissue stems

from them.

• &tem cells have t#o key properties:

- &tem cells can divide again and again to produce high "uantities

of ne# cells. They are useful for tissue gro#th or replacement of lost

or damaged cells.

&tem cells are not fully di;erentiated, so they can di;erentiate in di;erent

#ays to produce di;erent cell types.

1. ( Ultrastructure o! Cells

)ro*aryotic Cell Structure

>> (rokaryotes have a simple cell structure &ithout compartmentalization

• rokaryotes #ere the rst organisms to evolve on this earth and

have the simplest cell structure

%unctions of rokaryotic Cells/ =acteria Cells


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- )lasma $em+rane: regulates

movements of materials into and

out of the cell

- Cell ,all: strengthens and

maintains the structure of the cell

- )ili: assists in attaching to othersurfaces, #hich is important for

genetic F4) transfer

- i+osomes/0S: protein

builders of the cell, #here protein

synthesis occurs

- Cytoplasm: Contains F4),

ribosomes, and organic

compounds needed to carry out

life processes.

- 2ucleoid3 &torage of geneticinformation, the site of F4)

replication.

- Flagella: helps the cell move

around, allo#ing mobility

4u*aryotic Cell Structure

>> )ukaryotes have compartmentalized cell structure

%unctions of +ukaryotic Cells

- 2ucleus: Contains thechromosomes consisting

of F4). ncoiled

chromosomes

/chromatin are spread

throughout. This is

#here F4) is replicated

and copied out to form

m74).- r4 /7ough

+ndoplasmic7eticulum: Contains

ribosomes. &ynthesi*es

proteins for the

secretion from the cell- Golgi apparatus3

rocesses proteins

brought in vesicles from

the r+7 for transport #ithin the cell- 5ysosomes: Contains digestive en*ymes, #hich can be

used to break do#n ingested food or break do#n organelles.


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t(s important for cell defence, digestion of harmful

organisms@chemicals- $itochondria: roduces energy in the form of )T for cell by

aerobic respiration. )lso digests fat if being used by the cell

for energy.

- i+osomes60S: &ynthesi*e proteins, releasing it into thecytoplasm, in the form of en*ymes. Constructed in the

nucleolus.- Chloroplasts3 Contain the pigment, chlorophyll used in

photosynthesis. roduces glucose and other organic

compounds by photosynthesis.- 7esicles3 ) single membrane that contains Euid inside.

?esicles are small vacuoles that transport materials inside the

cell.- Centrioles3 Contains microtubules, #hich move

chromosomes during cell division. Centriole forms an anchor

point for microtubules during cell division.- Cilia#Fagella3 =oth used for locomotion.

Cell di8ision in pro*aryotes

>> (rokaryotes divide !y !inary *ssion.

rokaryotes, if given right condition, can multiply rapidly by +inary

%ssion. The cell #ill divide into t#o cells. The bacterial cells are

replicated, #hich gro# to full si*e and divide again. This is aseual

reproduction. n this process the cell is replicated to form t#o identical

daughter cells.

• The F4) is rst replicated, attaching itself to the plasma membrane

• The cell elongates to separate chromosomes

• The membrane folds in, pulling itself together in the middle

• The cell then splits into t#o daughter cells

The resolution o! electron microscopes

>> )lectron microscopes have a much higher resolution than light

microscopes.


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• 6aking separate parts of an obGect

distinguishable by eye is called

resolution.

• The maimum resolution of a light

microscope is about 0.2>m, #hich is

200 nanometers. The resolution

cannot be higher as it is limited by

the #avelength of light /!00-H00

nm.

• f the resolution is made any higher,

the image is blurry. Therefore, the

maimum magnication #ith a light

microscope is !00.

• =eams of electrons have a shorter

#avelength, therefore that electron

microscopes have a higher

resolution.

• The resolution of modern electron

microscopes is 0.001>m or 1nm.

+lectron microscopes therefore have

a 200 better resolution than light microscopes.

• hile light microscopes reveal the structure of cells, electron

microscopes reveal the ultrastructure.


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1.9 $em+rane Structure

Fluid $osaic $em+rane Structure

>> (hospholipids form !ilayers in &ater due to the amphipathic

properties of phospholipid molecules

• hospholipids are in the structure of every cellular membrane

o hospholipid molecules are amphipathic. There are t#o

parts to the molecule: one, #hich is attracted to #ater

/hydrophilic and the other part that isn(t attracted to #ater

/hydropho+ic.o The phosphate head is hydrophilic. The t#o fatty acid tails,

composed of hydrocarbon chains are hydrophobic.o hen phospholipids mi #ith #ater, they naturally arrange

into a bilayer position #ith the hydrophilic heads are facing

the #ater and the hydrophobic tails are facing in#ards a#ay

from the #ater.


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o The attraction of the hydrophobic tail in the center keeps the

membrane stable.

• The phospholipid bilayer consists of integral proteins, peripheral

proteins, glycoproteins and cholesterol.o Integral )roteins3 &pan from one side of the phospholipid

bilayer to the other. They are involved in transportingsubstances across the membrane

o )eripheral )roteins3 laced on the surface of the

membrane. 5ften slide and collide #ith each other, though

never Eip or change sides. They are involved in maintaining

the cell shape or motility. They can be en*ymes, #hich

cataly*e reactions in the cytoplasm.o Glycoproteins3 nvolved in cell recognition, #hich is part of

the immune system. Can also act as receptors in cell

signaling.

o Cholesterol3 =inds together lipid in the plasma membranereducing it(s Euidity

>> em!rane proteins are diverse in terms of structure, position in the

mem!rane and function

• Channel )rotein3 They span the membrane, allo#ing movement

of large molecules across. ithin these are passive and active

membrane pumps. They only allo# specic ions through

• eceptor )rotein3 These detect hormones arriving at cells to

signal changes in function. They are also involved in other cell

substance recognition as in the immune system.

• 4nymes3 ntegral proteins in the membrane may be en*ymes /i.e

)T maltase

• 4lectron Carriers3 These are a chain of peripheral and integral

proteins that allo# electrons to pass across the membrane. )ctive

pumps use )T to move specic substances across the membrane.


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>> +holesterol is a component of animal cell mem!ranes

Cholesterol is a component of animal cell membranes. 6ost of cholesterol

molecule is hydrophobicI one end is hydrophilicI so cholesterol ts

bet#een phospholipids in the membrane.

• Cholesterol restricts movement of phospholipid molecules

• 7educes Euidity of the membrane

• 7educes permeability of the membrane to hydrophilic particles such

as 82 ions.

This is important as animal cells need to maintain concentration

di;erences of these ions across membranesI hence di;usion through the

membrane must be restricted.

1.: $em+rane Transport

>> (articles move across mem!ranes !y simple di"usion, facilitated

di"usion, osmosis and active transport

&i'usion is the passive movement of particles from a region of high

concentration to a region of lo# concentration, as a result of the random

motion of particles.

Fi;usion occurs because more particles move from the area of higher

concentration to an area of lo#er concentration.

Fi;usion through a cell membrane #ill occur if the membrane is fully

permeable to the solute. n the case of the phospholipid bilayer, it is

permeable to non-polar substances, such glycerol, as #ell as oygen and

carbon dioide. They #ill di;use "uickly via this route.

&imple Fi;usion: The molecules are smallI hence they can simply pass

through the phospholipid molecules of the membrane, as it o;ers little

resistance. +amples include 5J and C5J, as #ell as lipid molecules

%acilitated di;usion: %or larger molecules, there are channel proteins to

take the through the membrane. These have comple shapes, #hich

provide a channel through the protein, or the pore. t acts as a shield

against the non-charged regions of the membrane for the molecule.

These channels only allo# a specic type of substance through, butthere is no control over the direction of movement.


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o +.g chloride channels allo# only chlorine ions to pass through

assive movement means that no energy /)T is used for the movement

of molecules from one side of the membrane to the other. &imple and

facilitated di;usion are passive movements that re"uire no energy.

Osmosis is the passive movement of #ater molecules from a region oflo# #ater concentration to a region of high #ater concentration, across apartially permeable membrane.

)ttraction bet#een solute particles and #ater molecules are the reason for#ater moving to regions #ith ahigher solute concentration

Acti8e Transport is the movement of substances across membranes

using energy from )T. )ctive transport can move substances against

concentration gradient < from a region of lo#er to a region of higher

concentration

rotein pumps in the membrane are used for active transport. +ach pump

only transports particular substances, so cells can control #hat is

absorbed and #hat is epelled. umps #ork in a specic direction < the


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substance can only enter the pump on one side and can only eit on the

other.

o They are called active because they need energy to function

o This energy causes the shape of the protein, allo#ing it to move the

molecule across the membrane

&odium otassium pump creates an electro-chemical gradient across the

membrane of all cells. The inside of the cell has a negative charge

compared to outside. n nerve cells, the pump is modied to create

electrochemical phenomena.

KK The Euidity of membranes allo#s materials to be taken into cells by

endocytosis or released by eocytosis. ?esicles move materials #ithin

cells.


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The Euidity of membranes allo#s them to move and change shape. &mall

pieces of the membrane can be pinched o; the plasma membrane to

create a 8esicle containing some material from outside the cell. This is

endocytosis.

4ndocytosis3 ) vesicle is formed #hen the plasma membrane in

folds, then breaks o;. art of the membrane is pulled in#ards and a

droplet of Euid is enclosed #hen it(s pinched o;. They can then move the

contents through the cytoplasm. The continuity of the plasma membrane

is not disrupted.

4;ocytosis3 The vesicle membrane fuses #ith the plasma

membrane, and its contents are secreted. The vesicle fuses #ith plasma

membrane, and its contents are epelled.

?esicles move materials #ithin the cell, for eample vesicles move

proteins from the rough +7 to the 'olgi apparatus.

1.< Cell &i8ision

The ole o! $itosis

6itosis is division of the nucleus into t#o genetically identical daughternuclei.

• The nucleus of a eukaryotic cell can divide from t#o genetically

identical nuclei by a process called mitosis.• 6itosis allo#s the cell to divide itself into t#o daughter cells, each

#ith one of the nuclei and therefore genetically identical to theother.

• =efore mitosis can occur all of the F4) nucleus must be replicated.

• This happens during nterphase the period before mitosis.

• +ach chromosome is converted from a single F4) molecule into t#o

identical F4) molecules called Chromatids.

• Furing mitosis one of these chromatids passes to each daughter

nucleus.


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6itosis is involved #henever cells #ith genetically identical nuclei arere"uired in eukaryotes: during embryonic development, gro#th, tissuerepair and aseual reproduction.)lthough mitosis is a continuous process cytologists have divided theevents into four phases

•

rophase• 6etaphase

• )naphase

• Telophase

Interphase This is #hen the F4) is replicated. The cell #illreplicates its centrosome, #hich is important for the movement ofchromosomes.

t consists of 3 phases:


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• Anaphase The centromeres divide, the spindle bers

shorten and the chromatids are pulled by thecentromere to the opposite poles. 5nce separated,they are referred to as chromosomes.

Telophase The nuclear membrane reforms

around both groups of chromosomes at theopposite ends of the cell. The begin to

decondense and become chromatin again.The nucleolusreforms. nterphase then follo#s the division of cytoplasm.

Cyto*inesis The division of the cytoplasm to form t#o cells. t occursafter mitosis, and is di;erent in plant and animal cells.

• n plant cells a ne# cell is formed across the e"uator of the cell,

#ith plasma membrane on both sides. This divides the cell into t#o.

• n animal cells the plasma membrane at the e"uator is pulled

in#ards until it meets in the centre of the cell, dividing it in t#o.

Mitotic Index

s the ratio bet#een the number of cells in mitosis in a tissue and the totalnumber of observed cells. ) high inde indicates a fast gro#ing tumour.

5ne cell of each of the four stages of mitosis is identied right.

mitotic inde; " num+er o! cells in mitosis # total num+er o! cells

Cell Cycle in 4u*aryotes

The cell cycle is the se"uence of events bet#een one cell division and thenet. t has t#o phases: interphase and cell division.

Interphase3 very active in the life of a cell #hen many metabolicreactions occur, reactions like cell respiration also occur during cell

division, but F4) replication in the nucleus and protein synthesis in thecytoplasm happen during interphase.

• 6itochondria and cytoplasm increase as they gro# and divide

• n plant cells, the chloroplasts increase in the same #ay

Uncontrolla+le Cell &i8ision

o Tumors, or cancers, are cell mass formed as a result of uncontrolled

cell division. They can occur in any tissue.

o n a tumor, the normal repressed state of mitosis is disrupted bymutation to the proto- oncogene. )s a result, the cells begin to


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divide uncontrollably. The proto-oncogene mutates into theoncogene, resulting in the loss of control of cell division.

o The cells form an irregular mass of cellsI the tumor. &ome cells may

break a#ay and form a secondary tumor else#here. +ventually theytake over the surrounding, healthy cells, #hich can lead to

malfunction and death.o t is caused by damage to F4) chromosomes. The accumulation of

mistakes in F4) causes cancer, #hich is #hy it is more common inolder people. )nother cause is damage to the gene that codes forp$3, the protein that stops the copying of damaged F4).

o The damage to the F4) can result from ioni*ing radiation /L-rays,

gamma rays..., some chemicals /tar in tobacco smoke as #ell asvirus infections. &ome factors are also inherited.

o The development of cancer re"uires at least t#o mutationsI one of

the proto-oncogeneI t#o of the tumor suppressor.o Cancer eerts its deleterious e;ect on the body by destroying the

adGacent tissues /such as compressing nerves, eroding blood

vessels, replacing normal functioning cells /such as replacing bloodforming cells in the bone marro# or the heart muscles so that theheart fails.

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1. Cell Biology Notes