Topic 2 : Cells

2.1 Cell TheoryLearning Objectives

1. Outline cell theory

2. Discuss the evidence for cell theory

3. State unicellular organisms carry out all the functions for life

Cell Theory

• All organisms are made of one or more cells

• Cells are the basic living units withinorganisms; all chemical reactions of life

take place within cells• All cells come from pre-existing cells via

mitosis or meiosis

TASK: Evidence for Cell Theory

1. State the 5 summary statements of Cell Theory (use your text books)

2. Use your text book pages 3 and 4 to summarize the evidence we have to support each statement of cell theory.

• (HINT: you should read these pages FIRST then summarize the key pieces of evidence

Mix and Match: 5 minutes

1 Movement A Producing offspring

2 Respiration B Getting rid of waste products

3 Sensitivity C Being able to move their parts

4 Growth D Turning food and oxygen into energy

5 Reproduction E Getting to full size, repairing old cells

6 Excretion F Responding to the outside world

7 Nutrition G Getting food where it’s needed

Functions of Life

• MRS GREN is the pneumonic to help us remember the functions of life.

• Unicellular organisms carry out all the functions of life. This is what defines a “living thing”.

1.Identify the functions of life Viruses carry out.2.Do they qualify as a “living” thing? Why? Why not?

3.Is FIRE alive? Use MRS GREN to justify your answer

2.1 Learning Objectives continued

4. Compare the relative sizes of molecules, cell membrane thickness, viruses, bacteria, organelles and cells

5. Calculate the linear magnification of drawings and / or the actual size of specimens in images of known magnification

Measurements & equivalents

1 millimetre (mm) 10-3 metre (m) 1/1 000 m

1 micrometre (µm) 10-6 metre (m) 1/1 000 000 m

1 nanometre (nm) 10-9 metre (m) 1/ 1 000 000 000 m

Measuring Cells

• To accurately measure cellular structures we need a suitable scale

Estimating cell size

Graticule = eypiece micrometer – a fine scale that fits inside an eyepiece lens

Stage micrometer = slide with a fine scale of known dimension etched onto it

Comparing relative sizes of molecules

Magnification• The size of an image of an object compared to its

actual size. • Calculated using the formula M = I/A

I = size of imageA = actual size of objectM = magnification

BUT you must remember to convert values to the same unit FIRST

M A

I

Calculations

M A

I

M = I/A I = M x A

A = I/MX

÷ ÷

Eg. Calculating Magnification

It is essential that the same unit is used for the size of the image and the size of the object:

• Eg. If an image measures 50mm (as printed on paper) and the object actually measures 5µm;

The size of the image should be converted to µm:Size of image = 50mm = 50 000µm

Then you can use this to calculate magnification:Therefore, magnification = 50 000/5 = 10,000

Practice Question: Calculate the linear magnification1. If a red blood cell has a diameter of 8 µm and

a student shows it with a diameter of 40 mm in a drawing, what is the magnification of the drawing?

Another Practice Question;

2. An image of a liver cell has a real scale bar next to it recording 10um, but you measure the scale bar and find it is 20mm. What is the magnification used?

ANSWER:• 20mm = 20,000 um

• Magnification = 20,000 \ 10 = 2000 X

Resolution

• Ability of a microscope to distinguish two objects as separate from one other

• The smaller & closer together the objects that can be distinguished as separate, the higher the resolution (resolving power)

• The resolving power of the light microscope is limited by the wavelength of light. It cannot resolve detail finer than 0.2µm

The importance of resolution

Light Microscope, low resolution

Electron Microscope, high resolution

The light microscope

The Electron Microscope

ChromosomesLight Microscope

Scanning Electron Microscope

Light Microscope: Anaphase

Animal Cell

Plant Cell

TEM – Spindle Fibres

Light vs. Electron microscopesFeature Light microscope Electron microscope

Radiation used Light rays Electron beams

Magnification x 2000 x 500 000

Resolving power 200 nm 0.2nm

Focused by Glass lenses Electromagnets

Biological material Living or dead Dead

Size Small & portable Very large & static

Preparation of material

Quick & simple Time-consuming & complex

Cost Relatively cheap VERY expensive

Learning Objective

Topic 2.1 Cell Theory continued

2.1.6. Explain the significance of surface area to volume ratio as a factor limiting cell size

Investigating the importance of Surface Area for Cells

• Raw materials enter a cell by diffusion via the cell membrane. The number of molecules which diffuse into a cell depends on its surface area. To function efficiently a cell needs a large surface area relative to its volume

• Diffusion is the passive movement of molecules from an area where they exist in higher concentration to an area of lower concentration

• Osmosis is a specialized type of diffusion describing the movement of water molecules specifically

Surface Area: Volume ratio

Object A Object BVolume (cm3)How many blocks are there?Surface Area (cm2)How many 1cm2 faces are facing outwards?Surface Area: Volume Ratio(surface area/volume)

Agar Cells Experiment

• You will be given an experiment sheet• Follow the instructions carefully!• Remember to wear safety googles.• You have 20 minutes to complete the experiment

& have it cleaned up

• When you have finished you will have 15 minutes to complete the worksheet using the data you collect

Task: Hypothetical cells

A B C ED1cm 1.5 cm 2 cm 2.5 cm3 cm

Cell Length Surface Area Volume SA:Vol Ratio

A 1cm 6x1x1=6cm2 1x1x1=1cm3 6:1

B

C

D

E

QUESTIONS on the importance of Surface Area

1. Which increase faster as cell size increases: volume or surface area?

2. Describe what happens to the ratio of SA / V as cell size increases

3. Predict which cell would have the most efficient rate of diffusion—justify your answer

4. Is the shape of the cell important? I.e does it matter if a cell is round and fat compared to long and thin as long as the volume remains the same? Why?

Answers

1. Volume increases faster than surface area2. The ratio falls as cell size increases ie. Less SA compared with vol3. Cell A – the smallest cell4. Yes, shape is important the long cell would be more efficiently

supplied (faster rate of diffusion) with materials via diffusion as it has a greater surface area compared to its volume

Cell Length Surface Area Volume SA:Vol Ratio

A 1cm 6x1x1=6cm2 1x1x1=1cm3 6:1

B 1.5cm 13.5cm2 3.375cm3 4:1

C 2cm 24cm2 8cm3 3:1

D 2.5cm 37.5cm2 15.625cm3 2.4:1

E 3cm 54cm2 27cm3 2:1

HOMEWORK TASK

You are to complete the discussion questions (on the back of the Investigation Sheet) about how surface area to volume ratio acts as a limiting factor to cell size

2.1 Cell TheoryLearning Objectives

7. State that multicellular organisms show emergent properties

8. State the multicellular organisms differentiate to carry out specialized functions by expressing some of their genes but not others

9. State stem cells have the capacity to divide and have the ability to differentiate along different pathways

10.Outline at least one therapeutic use of stem cells

Emergent Properties

• Emergent properties are those where the whole is more than the sum of their parts. Good examples of emergence is a termite hill, an ant hill or the human brain.

• In the case of the human brain the individual neurons are not capable of thought but the communication and cooperation between the neurons makes it possible for the brain to think.

Multicellular Organisms show Emergent Properties

• This means that the organism can achieve MORE than the sum of what each cell could achieve individually.

• This is caused by the fact that cells interact, allowing them to perform tasks together that they could not achieve, even in part, if they were alone.

How Cells Differentiate• All multicellular organisms begin their lives as a single cell

that divides rapidly, and as it does so the cells, although identical genetically, differentiate.

• Cells are able to differentiate and carry out specialized functions by expressing some of their genes but not others.

E.g. Cells in your toe’s, fingers and heart all have the information (genes) to make the protein pigment for the color of your eyes but do not use it.

• The genes that are not expressed by the cell are stillpresent in the nucleus, but are packed away tightly so they cannot be accessed.

Stem Cells—Unspecialized cells•The have the capacity to divide and differentiate along a number of different pathways.•They are different to other cells in two ways1.They are undifferentiated which means they have not specialized into a certain type of cell. This means all their genes can still be expressed2.They are self-sustaining—they can divide and replicate for long periods of a time

Sources and Types of Stem Cells• A zygote is a source of stem cells because the cells can become

any type of cell and are therefore said to be ‘totipotent’.• After the zygote cells divide many times it becomes a ball of cells

(blastocyst). These cells can become almost any kind of cell and are considered ‘pluripotent’.

• Another source of stem cells is the umbilical cord of a new baby. Cells from the blood of the cord are considered ‘multipotent’ because they can become a limited number of particular cell types.

• Adults have stem cells in their bone marrow, brain and a limited number in muscles—these are all only multipotent

Therapeutic Uses of Stem Cells

The Promiseof Stem Cells

Stem cells are able to differentiate into a particular cell type when given a specific signal.

Theoretically this means you could signal a stem cell to specialize into a liver cell, then divide until you grow a whole liver!

Stem Cells are found in adults, but the Most promising types of Stem Cells for Therapy are

Embryonic Stem Cells

Embryonic Stem Cells

The embryo is destroyed by separating it into individual cells for the collection of ICM cells.

Some Thorny Ethical

Questions

Is it ethical to harvest embryonic stem cells from the

“extra” embryos created during in vitro fertilization?

Are these masses of cells a human?

TASKS

Use your text books to outline at least TWO therapeutic uses for stem cells.

You should describe WHY the use is needed AND how stem cells are used (where are they obtained from, how…)

Outline the ethical debate surrounding stem cell research—why are embryonic stem cells more favored that adult? What are some the issues people have with this research?

2.2 Prokaryotic cellsLearning Objectives

1. Draw and label a diagram of E.coli as an example of a Prokaryote

2. Annotate the diagram of E.coli to describe the function of each structure

3. Identify structures of E.coli in electron micrographs4. State that prokaryotic cells divide by binary fission

Prokaryotes = before the nucleus

• Primitive cells – do not have a nucleus or any membrane-bound organelles

• Probably the first living things to evolve on Earth a few billion years ago

Prokaryotes

• Eg. bacteria & blue-green algae• May have plasmids (satellite DNA( which can

replicate independent of the main chromosome

• Rigid cell wall containing murein• Mesosomes = inner extensions

of the cell membrane = siteof respiration

Prokaryotes

Prokaryotic Features

Binary Fission

Prokaryotes vs Eukaryotes

Prokaryotic vs Eukaryotic Cells

• “pro” = before• “eu” = true• “karyo” = nucleus• Therefore Prokaryotic cells lack a nucleus• Eukaryotic cells possess a nucleus• Several other differences

Eukaryotes

2.3 Eukaryotic CellsLearning Objectives

1. Draw and label the structures of a liver cell as an example of an animal cell

2. Annotate the labelled liver cell to describe the functions of each structure in an animal cell

3. Identify structures in electron micrographs of liver cells

Generalised Animal Cell

Plasma Membrane

Nucleus• Largest organelle in

cell• 10µm diameter• Surrounded by

nuclear membrane which has pores to allow materials to pass into/out of nucleus

• Connected to ER

Nucleus

Nuclear Envelope & Pores

Nucleolus

• Not an organelle• dark-staining region inside the

nucleus - cloud of ribosome parts in the process of being constructed.

• May be several nucleoli• Once the ribosome parts are

constructed, they are exported to the cytoplasm and incorporated into the rough endoplasmic reticulum.

Nucleolus

• Site of rRNA synthesis & ribosome assembly

Cells without nuclei

• Red blood cells – lose their nuclei and this enables them to carry more haemoglobin & so can pick up more oxygen

• Phloem sieve tubes provide the transport system for sucrose in plants – they have lost most of the cell organelles including nuclei to make it easier for materials to flow through the cell

Endoplasmic Reticulum

h

• Structure:– Complex system of double membranes– The fluid-filled spaces between the membranes

are called cisternae– The ER is continuous with the nuclear

membrane

Endoplasmic Reticulum (ER)

Chromatin

Nucleolus

Pore

NUCLEUS

Two membranesof nuclearenvelope

ROUGHENDOPLASMICRETICULUM

Ribosomes

Double membrane of RER

Cisternae

Endoplasmic Reticulum (ER)• Functions:

– Form an extensive transport system throughout cell– Production & packaging of proteins (RER)– Synthesis of lipids & steroids (SER)– Collection, storage & distribution of these materials

Rough Endoplasmic Reticulum

1 2

3

4Transport vesiclebuds off

Ribosome

Sugarchain

Glycoprotein

Secretory(glyco-) proteininside transportvesicle

ROUGH ER

Polypeptide

SMOOTH ER

ROUGH ER

Nuclearenvelope

Ribosomes

SMOOTH ER ROUGH ER

Smooth Endoplasmic Reticulum (SER)

• No ribosomes• Site of lipid

synthesis• Cells that make

lipids/steroids eg. liver cells & testis contain lots of SER

The Golgi Apparatus

• Structure – stacks of membranous sacks• Function – Receives and modifies molecules from

the ER that need to be secreted from the cell.• Directs proteins made in the ER to the correct

cellular compartment or to the plasma membrane for secretion.

• ANY CELL that secretes STUFF has lots of these! (e.g. glands in the pancreas and intestinal wall, salivary glands)

Golgi Apparatus

Golgiapparatus

“Receiving” side ofGolgi apparatus

Transportvesiclefrom ER

Newvesicleforming

Transport vesiclefrom the Golgi

Golgi apparatus

“Shipping”side of Golgiapparatus

Golgi & Protein Trafficking

Golgi - Functions

• Assembling glycoproteins such as mucin by combining carbohydrate and protein

• Transporting and storing lipids• Formation of lysosomes• Production of digestive enzymes• Secretion of carbohydrates for the formation

of plant cell walls and insect cuticles

Golgi Apparatus

Lysosomes• Structure – small vacuoles

formed from Golgi; contain hydrolytic enzymes which can digest material

• Function – release enzymes to destroy worn-out organelles; phagocytosis; exocytosis; cause the cell to self-destruct (autolysis) by releasing enzymes

Lysosomes

• Phagocytosis:Digestion of material that has been taken into the cell eg. White blood cells engulf bacteria – lysosome fuses with vesicle to digest the bacterium

• Lysosomes release enzymes that destroy worn-out organelles in the cell

Lysosomes

Cell TransportRough ER

Transport vesicle(containing inactivehydrolytic enzymes)

Golgiapparatus

Plasmamembrane

LYSOSOMES“Food”

Engulfmentof particle

Foodvacuole

Digestion

Lysosomeengulfingdamagedorganelle

Cell TransportTransport vesiclefrom ER

Rough ER

Transport vesiclefrom Golgi

Plasmamembrane

Vacuole

LysosomeGolgiapparatusNuclear

envelope

Smooth ER

Nucleus

Mitochondria

• Structure - 2 membranes – outer membrane controls the entry & exit of materials; inner membrane forms many folds called cristae; filled with jelly-like matrix

• Function – site of aerobic respiration (creation of ATP)

Mitochondria

Mitochondria – False colour SEM

Mitochondria - TEM

Mitochondria: Cristae & Matrix

• Cristae – surface of crista covered in cytochromes which create ATP

• Matrix - contains ribosomes and loops of DNA which enables the mitochondria to replicate themselves when the cell divides

Ribosomes• Structure – small, dense organelles approx 20nm

diameter composed 1 large & 1 small subunit. Manufactured in nucleolus from rRNA and protein

• Function – site of protein synthesis in the cell

• Site of protein synthesis

Ribosomes

Many of the cell’s ribosomes are attached to the ER (Rough ER). However they also occur free in they cytoplasm

Ribosomes

Ribosomes & Protein Synthesis

• Many ribosomes can read the same mRNA strand to create many copies of the polypeptide at the same time

Generalised Plant Cell

Chloroplasts

Structure – Double membrane - chloroplast envelope; inside is the fluid stroma and granum (stacks of thylakoids)

Function - site of photosynthesis

Chloroplasts

Large surface area for light absorption

Chloroplasts

SEM TEMStarch grain

Thylakoid – contains chlorophyll

Centrioles

Centrioles

• Structure – two short bundles of microtubules positioned at right angles to each other; located just outside nucleus in clear area of cytoplasm called the centrosome; wall of each centriole is made up of nine triplets of microtubules

• Function – during cell division, they migrate to opposite poles of the cell where they produced the spindle to assist movement of chromosomes

Vacuole

• Structure - Plant cells have a large, permanent vacuole bounded by a membrane called the tonoplast

• Vacuole contains cell sap – a solution of sugars, amino acids, mineral salts & waste chemicals

Vacuoles

• Function – act as food stores; accumulate waste products; contain pigments which give colour to parts of plant eg. petals; help maintain turgidity

Electron micrograph of plant cell

TASKS

1. “Cell Apprentice” – make a 30-60 second elevator pitch to explain why your organelle shouldn’t be fired from the cell

2. Cut & Paste – Create the generalised animal cell

2.3 Eukaryotic CellsLearning Objectives

4. Compare prokaryotic and eukaryotic cells5. State 3 differences between plant and animal

cells6. Outline the roles of extracellular components

(including cell wall in plants and glycoprotein's in animals)

Prokaryotes vs Eukaryotes

Prokaryotes EukaryotesAverage diameter 0.5-5um Up to 40um diameter – often 1000x10

000 times volume of prokaryotic cells

DNA is circular & free in cytoplasm DNA is linear, within nucleus – nuclear “envelope” = 2 membranes

DNA is naked DNA associated with protein, forming chromosomes

Smaller ribosomes -18nm Larger ribosomes – 22nm

No ER present ER present, to which ribosomes may be attached

Very few organelles – none membrane-bound

Many cell organelles present – many single or double membranes

Cell wall present Cell wall sometimes present eg. In plants

Protein Synthesis

TASK:Use the diagram to compare & contrast protein synthesis in prokaryotes and eukaryotes

ProkaryotesProkaryotes EukaryotesEukaryotes

Where transcription

occurs

Cytoplasm – because no

nucleus

Nucleus

RNA Processing?

No - because no introns

Yes

Where translation

occurs

Cytoplasm – simultaneously

with transcription

Cytoplasm

How many genes

transcribed?

Usually several related genes =

operon

Usually only one