TOPIC 2.1

Topic 2: Cells

By the end of this lesson, you should be able to:

• Outline the Cell Theory• Be able to compare relative sizes of

cells and cellular components• Be able to calculate linear

magnification of drawings• Understand gene selectivity• Understand the importance of stem

cells

What level of complexity is necessary for life?

• Aristotle (384 – 322BC)

History of the Cell

• Hooke, 1665– Looked through

compound microscope at cork samples and coined the term “cells”

History of the Cell

• Leeuwenhoek, 1675– Discovers

unicellular organisms in pond water

History of the Cell

• Matthias Schleiden & Theodor Schwann, 1838-1839– Suggested that plants and

animals; respectively, are composed of cells

– “The cell is the basic unit of living tissue”

• Rudolf Virchow (1858) noted that: “all cells come from pre-existing cells”

Cell Theory

Those early scientists did experiments on living things and developed CELL THEORY

Main Ideas of Cell Theory

All living things are made of one or more cells

1)

Cells are the basic units of structure & function of living things; “The smallest unit of life”

2)

All cells come from existing cells 3)

What is a theory?

Cell: “The smallest Unit of Life” Leeuwenhoek’s

unicellular organisms show all the signs of life Metabolism Response to stimuli Growth Reproduction Homeostasis Nutrition

2.1 Cell Theory2.1.3State that unicellular organisms carry out all

the functions of life. (1)MOVEMENT – Intracellular and/or extracellularRESPIRATION – Gas exchange. Not always O2 and CO2

NUTRITION – Need raw materials, i.e.- food, water, minerals

EXCRETION – Get rid of waste materialsREPRODUCTION – Ability to produce like organismsIRRATIBILITY – Respond to external stimuli GROWTH – Cells grow larger . . . and don’t forget . . .

What level of complexity is necessary for life?

• Xavier Bichat (1771-1802): An organ is composed of different tissues and several organs can be grouped together as an organ system (e.g. the digestive system)

• An idea of hierarchy of structure developed:

Organism

Organ-system

Organ

Tissue

Cell

2.1 Cell Theory

2.1.1Discuss the theory that living organisms are

composed of cells. (3)

Skeletal muscle and some fungal hyphae are not divided into cells but have a multinucleate cytoplasm. Some biologists consider unicellular organisms to be acellular.

2.1 Cell Theory

2.1.2 State that a virus is a

non-cellular structure consisting of DNA or RNA surrounded by a protein coat. (1)

2.1 Cell Theory

2.1 Cell Theory

2.1.3 State that all cells are formed from other cells.

(1)

x ref . 1.5- Mitosis, 8.1- Meiosis

2.1 Cell Theory

2.1.4Explain three advantages of using light microscopes.

(3)Advantages include:• colour images instead of monochrome, • a larger field of view,• easily prepared sample material, • the possibility of examining living material and

observing movement.

2.1 Cell Theory2.1.5Outline the advantages of using electron microscopes.(2)

Greater:

Resolution – the ability to distinguish between two points on an image. Like pixels in a digital camera.

Magnification – how much bigger a sample appears to be under the microscope than it is in real life.

3.1 Cell Theory

Transmission electron microscopes pass a beam of electrons through the specimen. The electrons that pass through the specimen are detected on a fluorescent screen on which the image is displayed.

Thin sections of specimen are needed for transmission electron microscopy as the electrons have to pass through the specimen for the image to be produced.

2.1 Cell Theory• Scanning electron microscopes pass a beam of electrons over

the surface of the specimen in the form of a ‘scanning’ beam.• Electrons are reflected off the surface of the specimen as it

has been previously coated in heavy metals.• It is these reflected electron beams that are focused on the

fluorescent screen in order to make up the image.• Larger, thicker structures can thus be seen under the scanning

electron microscope as the electrons do not have to pass through the sample in order to form the image.

• However the resolution of the scanning electron microscope is lower than that of the transmission electron microscope.

2.1 Cell TheoryLight Light ElectronElectronCheap to purchase (£100 Cheap to purchase (£100 – 500)– 500)

Expensive to buy (over Expensive to buy (over £1,000,000)£1,000,000)

Cheap to operateCheap to operate Expensive to produce Expensive to produce electron beamselectron beams

Small and portableSmall and portable Large and requires Large and requires special roomsspecial rooms

Simple and easy Simple and easy preparationspreparations

Lengthy and complex Lengthy and complex preparationspreparations

Material rarely distorted Material rarely distorted by preparationby preparation

Preparation distorts Preparation distorts materialmaterial

Vacuum is not requiredVacuum is not required Vacuum is requiredVacuum is required

Natural color maintainedNatural color maintained All images in black and All images in black and whitewhite

Magnifies objects only up Magnifies objects only up to 2000 timesto 2000 times

Magnifies over 500,000 Magnifies over 500,000 timestimes

2.1 Cell Theory

2.1.4Define organelle. (1)• Literally ‘little organ’• An organelle is a discrete structure within a

cell, and has a specific function.i.e. – nucleus, cell membrane, mitochondria

2.1 Cell Theory2.1.4 Compare the relative sizes of molecules, cell membrane

thickness, viruses, bacteria, organelles and cells, using appropriate SI units Appreciation of relative size is required (2)

• molecules (1 nm), • thickness of membranes (10 nm), xref. 1.4• viruses (100 nm), • bacteria (1 µm), xref. 1.33• organelles (up to 10 µm), xref. 6.4.2, 7.1.3, 7.2.1• most cells (up to 100 µm). Don’t forget: all of these structures are in 3D space

2.1 Cell Theory

2.1 Cell Theory

2.1.5Calculate linear magnification of drawings. (2)Drawings should show cells and cell ultra-

structure with scale barsMagnification could also be stated, eg x250.

How Big are Cells?

Eukaryotic Cell Up to 100 μm

Organelles Up to 10 μm

Bacteria (prokaryote)

1 μm

Large Virus (HIV) 100 nm

Cell Membrane 10 nm

Molecules 1 nm

Weem et al., 2007

Adenovirus

E. coli

•1 micrometer (μm) is 1 millionth of a meter (10-6)

•1 nanometer (nm) is 1billionth (1 thousand milllionth) of a meter (10-9)

Why so small?

• If cell was larger….– Diffusion distance becomes too far to be

energy efficient– Surface to volume ratio becomes too

small to allow the necessary exchange

Why so small?

• The production rate of cellular heat/waste and consumption of resources is directly proportional to its volume

• Since everything goes via the cell membrane, the rate of uptake/removal is proportional to the cell surface area

2.1 Cell Theory

2.1.5Explain the importance of the surface area to volume

ratio as a factor limiting cell size. (3)The rate of metabolism of a cell is a function of its

mass:volume ratio, Whereas the rate of exchange of materials and energy

(heat) is a function of its surface area. Simple mathematical models involving cubes and the

changes in the ratio that occur as the sides increase by one unit could be compared.

Volume and surface to area ratios

1

1

10

Length of side = 1 cmSurface area = 6 cm2 (6 * 1 * 1)Volume = 1 cm3

Surface area:Volume ratio = 6:1

This means every 1 cm3 of cell has 6 cm2 of surface area

Length of side = 10 cmSurface area = 600 cm2 (6 * 10 * 10)Volume = 1000 cm3

Surface area:Volume ratio = 0.6:1

This means that every 1 cm3 of cell has 0.6 cm2 of surface area; 10 times less

10

Ratio of V:S.A.Ratio of V:S.A.

CubeCube Side Side LengthLength

Volume Volume (x(x33))

S.A. (6xS.A. (6x22)) Ratio Ratio (S.A./V)(S.A./V)

11 1 cm1 cm

22 10 cm10 cm

33 100 cm100 cm

1 cm3

1 000 cm3

1 000 000 cm3

6 cm2

600 cm2

60 000 cm2

6

0.6

0.06

Ways cells adapt to help this problem

• Protrusions• Flattening cell

Multicellular organisms have same problem

• Lungs– Alveoli help to increase surface area to

allow greater diffusion

• Intestines– Villi help to increase surface area to

allow greater absorption

• Circulatory system– Reduces diffusion distance

2.1 Cell Theory

2.1.7Define: (1)Tissue – A group of cells working together to

perform a common functionOrgan – A group of tissues working together to

perform a common functionOrgan System – A group of organs working

together to perform a common function

2.1.8 More is different!

• As a multicellular organism grows and develops it follows a structured plan

• The cells specialize (differentiate)• A developing multicellular organism

shows emergent properties• The whole is more than the sum

of its parts

2.1.7 Emergent properties

• Cells interact to acquire properties that, alone, they do not possess

• Example: The human brain– Individual neurons not capable of

thought but the cooperation and communication among the individuals enables us to think

Specialized cells

• In multicellular organisms, cells differentiate to become specialized for their function

• Only a small portion of the genes are necessary– Each human cell has 40,000 potential

genes– To avoid waste, cell activates only those

genes necessary to carry out its function

Stem Cells

• Unspecialized, “immortal” cells• Have not silenced their genes yet, therefore, can

potentially become any cell• Totipotent – Can become any type of cell

– Embryonic cells

• Pluripotent – Partly differentiated, are restricted to certain cell types (ex. Blood cells)– Most of your stem cells in your bone marrow are

pluripotent– Zygotic cells

• Multipotent – More differentiated but can still differentiate into a limited number of cell types

What can stem cells do for you?

• Cell therapy– The totipotent potential of stem cells

permits their use to replenish damaged/missing cells of our own body

Cell Therapy

• Leukemia (cancer of the blood)– A bone marrow transplant can replenish blood cells lost

to leukemia/chemotherapy

• Skin grafts– Stem cells can re-grow skin damaged by

burns/accidents

• Corneal replacement– Re-grow cells of the eye for vision restoration

• Parkinson’s/Alzheimer’s– Stem cells possess the ability to re-grow brain cells

• Diabetes– Pancreas cells responsible for insulin production could

be re-grown for type I diabetes

Where do stem cells come from?

• Umbilical cord• Embryos

– Unused from in-vitro fertilization– Aborted

• Your own bone marrow – Although not totipotent, investigation is

determining whether they can be converted to be totipotent

• Requires embryonic investigation

TOPIC 2.2 and

2.3

2.2

Damon, et al Standard Level Biology, 2009

Escherichia coli

Electron micrograph of E.coli

2.2.32.2.3 Prokaryotic Cells Prokaryotic Cells

• CytoplasmCytoplasm– contains enzymes that catalyse the chemical contains enzymes that catalyse the chemical

reactions of meabolism and DNA in a region call reactions of meabolism and DNA in a region call the nucleoidthe nucleoid

• Pili (not in all cells)Pili (not in all cells)– Hairlike growth on the outside of cell membraneHairlike growth on the outside of cell membrane– Used for attachmentUsed for attachment– Main function is to join bacterial cells in Main function is to join bacterial cells in

preparation for DNA exchangepreparation for DNA exchange• Flagella (not in all cells)Flagella (not in all cells)

– longer than pililonger than pili– Used for motilityUsed for motility

Size 5-10µm

• ribosomes 70sribosomes 70s– synthesize proteins by translating messenger synthesize proteins by translating messenger

RNA. Some proteins stay in the cell and RNA. Some proteins stay in the cell and others are secretedothers are secreted

• Nucleoid (naked DNA)Nucleoid (naked DNA)– stores the genetic information that controls stores the genetic information that controls

the cell and is passed on to daughter cellsthe cell and is passed on to daughter cells– Single, long, continuous (circular)Single, long, continuous (circular)– Bacteria may contain plasmidsBacteria may contain plasmids

• small, circular DNA fragments that replicate small, circular DNA fragments that replicate independently of the bacterial chromosomeindependently of the bacterial chromosome

2.2.4 Bacterial Replication

• Reproduce by binary fission

• No exchange of genetic material

http://www.youtube.com/watch?v=vTzH1P3aQjghttp://www.youtube.com/watch?v=DY9DNWcqxI4&feature=related

Takes 20 min in good conditions

Eukaryotic CellsEukaryotic Cells

• Contains a nucleus and distinct Contains a nucleus and distinct organellesorganelles

• DNA is enclosed in a nuclear DNA is enclosed in a nuclear envelopeenvelope

• Cell division by mitosisCell division by mitosis

Size 50-150 µm

Weem et al., 2007

Eukaryotic CellsEukaryotic Cells

Ribosomes 80s – protein synthesisRibosomes 80s – protein synthesisRough endoplasmic reticulum (rER) – Rough endoplasmic reticulum (rER) –

synthesis of proteins to be secretedsynthesis of proteins to be secretedLysosome – holds digestive enzymesLysosome – holds digestive enzymesGolgi apparatus – for processing of proteins Golgi apparatus – for processing of proteins Mitochondrion – for aerobic respirationMitochondrion – for aerobic respirationNucleus – holds the chromosomesNucleus – holds the chromosomes

COMPARE PROKARYOTIC

AND EUKARYOTIC

CELLS

2.3.5 State three differencesbetween plant and animal cells.

Carbohydrates Carbohydrates stored as stored as starch.starch.

Carbohydrates Carbohydrates stored as stored as glycogen.glycogen.

Stores large amounts of liquid (juice). Larger size of cell.

Central Central VacuoleVacuole

XDoes not store large amounts of liquid. Smaller size of cell.

Rigid, cannot easily change shape.

Cell WallCell WallXFlexible, can easily change shape.

Can produce its own food.

ChloroplastChloroplastXCannot produce its own food

Plant CellsPlant CellsStructureStructureAnimal CellsAnimal Cells

2.3.62.3.6 Extracellular Extracellular componentscomponents

• The plant cell wall The plant cell wall maintains cell shape, prevents excessive water uptake, and holds the whole plant up against the force of gravity.

• Animal cells secrete glycoproteins that form the extracellular matrix. This functions in support, adhesion and movement

..

TOPIC 2.4

2.4 Membranes Go to the boardwork!!!!

2.4.1Draw a diagram to show the fluid mosaic model of a biological

membrane. • The diagram should show the phospholipid bilayer,

cholesterol, glycoproteins and integral and peripheral proteins.

• Integral proteins are embedded in the phospholipid of the membrane whereas peripheral proteins are attached to its surface.

2.4 Membranes

2.4.2Explain how the hydrophobic and hydrophilic

properties of phospholipids help to maintain the structure of cell membranes. (3)

Hydrophobic – ‘afraid of water’Hydrophilic – ‘loves water’

2.4 Membranes

2.4 Membranes

2.4 Membranes

2.4 Membranes

2.4 Membranes

2.4.3List the functions of membrane proteins including • hormone binding sites• enzymes • electron carriers• channels for passive transport • pumps for active transport.

MEMBRANE TRANSPORT MECHANISM

S

Passive TransportDiffusion

Simple diffusion

Facillitated diffusion

Osmosis

Requires no energyMoves from down the concentration gradientSome molecules pass through the membraneSome molecules use channels for facilitated diffusion

Diffusion2.4.4Define diffusion Diffusion is the passive movement of particles from a region of

high concentration to a region of low concentration (down a concentration gradient), until there is an equal distribution.

Define osmosis Osmosis is the passive movement of water molecules, across a

partially permeable membrane, from a region of lower solute concentration (high water concentration) to a region of higher solute concentration (low water concentration).

High Concentration

Low Concentration

Diffusion moves down the concentration gradient just like a ball rolling down a hill. It cannot roll uphill without energy.

Do not mix diffusion with osmosis!!!

Simple diffusion

Osmosis

v

Facillitated diffusion

channel carrier

EXAMPLES

Active TransportTransporters (proteins)

Directly use of energy

Indirectly use of energy

Requires energy (ATP) or adenosine triphosphateMovement of molecules or ions against concentration gradientPumps fit specific moleculesThe pump changes shape when ATP activates it, this moves the molecule across the membrane

Use of the energy already stored in the gradient of a directly-pumped ion

 Bind ATP directly and use the energy of its hydrolysis to drive active transport

Transport through vesicles

Endocytosis

Exocytosis

Chemiosmosis:Electron transport provides energy for the synthesis of ATP, but only indirectly. When electron transport chains pump H+ across the membrane, the protons become more concentrated on one side of the membrane than on the other. Such a concentration gradient stores potential energy. ATP is generated by a molecule called ATP synthase.

ATP synthase is a combination of proteins that act as both an ion channel and an enzyme. As an ion channel in the membrane of the mitochondria or thylakoids, it allows H+ to diffuse through it (facilitated diffusion). This actionspins a component of the ATP synthase. This rotation activates the active sites in the enzyme that attach phosphate groups to ADP molecules to generate ATP.

EXAMPLES

Indirectly use of energy ATP Synthase

Directly use of energy Na+/

K+ ATPase

It uses the energy from the hydrolysis of ATP toactively transport 3 Na+ ions out of the cellfor each 2 K+ ions pumped into the cell.

The picture is in Cell Resp ppt!!

Active TransportEx: Sodium – Potassium pump

2.4.7Explain how vesicles are used to transport materials

within a cell between the rough endoplasmic reticulum, Golgi apparatus and plasma membrane.

2.4.8 Describe how the fluidity of the membrane allows it to change shape, break and reform during endocytosis and exocytosis. THIS ITEM IS IN MEMBRANE´S BOARDWORK!!!!!

Transport through vesicles

2.4.7

2.4 Membranes

Endocytosis – the mass movement INTO the cell by the membrane ‘pinching’ into a vacuole

Exocytosis – the mass movement OUT of the cell by the fusion of a vacuole and the membrane

This is possible because the of the fluid properties of the membrane (able to break and reform easily, phospholipids not attached just attracted)

2.4.8

Exocytosis

Endocytosis