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thebiotutor

AS Biology OCR

Unit F211: Cells, Exchange & Transport

Module 1.1 Cell Structure

Notes & Questions

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Explain the difference between magnification and resolution.

Magnification

o The number of times an image is greater in size than the object.

o Increases the ability to see details

Resolution

o The ability to distinguish two objects as separate from each other.

o Allows details to be seen with more clarity

o Determined by the wavelength of light or electron beam

State the resolution and magnification that can be achieved by a light microscope, a transmission electron microscope and a scanning electron microscope.

Light Microscope

o Uses light

o Light passes through thin specimen sample

o Sample

Living or non-living

o Low magnification = x1,500

o Low resolution = 200µm (half the λ of visible light)

o Stains - Dyes

Iodine

methyl blue

acetic orcein

o Easy and cheap to create samples

o Easy and cheap to setup

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Transmission Electron Microscope (TEM)

o Uses a beam of electrons which pass through the sample

o High magnification = x500,000

o High resolution = 0.1nm (The λ of electron beam).

o Sample

Non-living

o Creates a 2D image

o Contrast is created by structures of different densities.

Dense structures absorb more electrons and sop appear darker

o Stains

salts of heavy metals

o Difficult and expensive to create samples.

o Difficult and expensive to setup and use.

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Scanning Electron Microscope (SEM)

o Uses a beam of electrons which bounce off the surface of the specimen.

o High magnification = x100,000

o High resolution = 0.1nm (The λ of electron beam).

o Sample

Non-living

o Creates a 3D image of the surface

o Stains

salts of heavy metals

o Difficult and expensive to create samples.

o Difficult and expensive to setup and use.

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Explain the need for staining samples for use in light and electron microscopy.

It is necessary to give contrast between structures.

Stains are taken up differentially by objects with different densities.

o Light microscopy: e.g. iodine, methyl blue, acetic orcein o Electron microscopy: e.g. salts of heavy metals

Calculate the linear magnification of an image.

o Units of microscopy is always micrometers (µm) o You MUST always write you answer in µm. o Be careful that you convert the units of the image size into µm.

1 mm = 1,000 µm

1 cm = 10,000 µm

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A = actual size I = image size

M = magnification

Using a graticule to measure specimens (practical exercise)

o Place micrometer on the microscope stage. This is a tiny ruler, 1mm long and divided into 100 divisions

o Replace eyepiece with graticule eyepiece. This is marked in arbitrary units and needs to be calibrated.

o Calibrate the eyepiece by lining it up against the stage micrometer o You can now exchange the stage micrometer with the slide of the specimen

to be measured and use the calibrated eyepiece graticule to measure it. o REMEMBER. You need to recalibrate the eyepiece for each different

objective lens magnification.

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1 To convert millimetres to micrometres: A multiply by 1000 B divide by 100 C divide by 1000 D multiply by 100 2 The resolving power of a microscope is its ability to: A produce an enlarged image B separate two nearby points C show a large field of view D reduce the depth of view

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3 Actual size, apparent size and magnification are related by the formula: A actual size = image size x magnification B image size = actual size divided by magnification C magnification = image size x D image size = actual size x magnification 4 In comparison with optical microscopes, transmission electron microscopes have: A higher resolution but lower magnification B lower resolution but higher magnification C lower resolution and lower magnification D higher resolution and higher magnification 5 In the scanning electron microscope, the image is created from: A electrons that pass through the specimen B electrons that are reflected from the specimen C both A and B D neither A nor B

Describe and interpret drawings and photographs of eukaryotic cells as seen under an electron microscope and be able to recognise the following structures:

Animal Cell

Answers: 1.A 2.B 3.D 4.D 5.B

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E

D

C

B

A

Plant Cell

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Nucleus & Nucleolus

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Rough and smooth endoplasmic reticulum

Golgi apparatus

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Mitochondria

Lysosomes

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Chloroplasts

Centrioles

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Undulipodia (Flagella) and cilia

Outline the functions of the structures listed above.

Plasma Membrane

o Description

Creates the outer surface of animal cells and lies just on the inside of the cell wall in plants and prokaryotes

It is made of lipids and proteins.

o Function

Regulates the movement of substances into and out of cells

Also used in cell signaling

Cell Wall (Plant = cellulose and Fungi = chitin)

o Description

Rigid structure that surrounds plant cells and prokaryotes

Plant cell walls are made from a cellulose (carbohydrate) network

Prokaryotes cell walls are made from peptidoglycan

o Function

Supports cells

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Nucleus

o Description

Large organelle surrounded by a nuclear envelope (double membrane), which contains many pores.

Contains chromatin and often a structure called the nucleolus

o Function

Chromatin is made from proteins and DNA

Pores allow RNA to move between the nucleus and the cytoplasm

Nucleolus makes ribosomes

Lysosome

o Description

A round organelle surrounded by a membrane, with no clear internal structure

o Function

Contains digestive enzymes.

These are kept separate from the cytoplasm by the surrounding membrane, and can be used to digest invading cells or to break down worn out components of the cell.

Ribosome

o Description

A very small organelle that either floats free in the cytoplasm or is attached to the rough endoplasmic reticulum

o Function

Site where proteins are made.

Rough Endoplasmic Reticulum

o Description

A system of membranes enclosing a fluid-filled space.

The surface is covered in ribosomes giving the rough appearance.

o Function

Folds and processes proteins by adding sugar chains that have been synthesized at the ribosomes.

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Smooth Endoplasmic Reticulum

o Description

Similar to rough endoplasmic reticulum, except with no ribosomes.

o Function

Synthesises and processes lipids

Vesicle

o Description

A small fluid-filled sac in the cytoplasm, surrounded by a membrane.

o Function

Transports substances in and out of the cell (via the plasma membrane) and between organelles.

Some are formed by the golgi apparatus or the endoplasmic reticulum, while others are formed at the cell surface.

Golgi Apparatus

o Description

A group of fluid-filled flattened sacs.

Vesicles are often seen at the edges of sacs.

o Function

It processes (adding or trimming sugar chains) and packages new lipids and proteins.

It also makes liposomes. Very useful at transporting hydrophillic substances

Mitochondrion

o Description

They are usually oval-shaped.

They have a double membrane – the inner one is folded to form structures called cristae.

Inside is the matrix, which contains enzymes involved in respiration.

o Function

The site of aerobic respiration, where ATP is produced.

They’re found in large numbers in cells that are very active and require a lot of energy.

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Chloroplast (Plant Only)

o Description

A small, flattened structure found in plant cells.

Surrounded by a double membrane and also has membranes inside called thylakoids

Thylakoids are stacked up to form grana

Grana are linked together by lamellae – thin, flat pieces of thylakoid membrane.

o Function

The site where photosynthesis takes place

Centriole (not in plants)

o Description

Small hollow cylinders containing a ring of microtubules

o Function

Involved in the separation of chromosomes during cell division.

Cilia

o Description

Small, hair-like structures found in large numbers on the surface membrane of some animal cells.

In cross section, they have an outer membrane and a ring of 9 pairs of protein microtubules inside, with a single pair of microtubules in the middle.

o Function

Microtubules allow the cilia to move.

This movement (wafting) is used to move substances along the cell surface.

Undulipodia (Flagellum in prokaryotes)

o Description

Like cilia but longer and found in fewer numbers.

They protrude from the cell surface and are surrounded by the plasma membrane

o Function

The microtubules contract to move the undulipodia

They propel the cell forward (E.g. sperm).

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Outline the interrelationship between the organelles involved in the production and secretion of proteins.

1. Messenger RNA leaves the nucleus after transcription of the DNA code

2. Protein synthesis occurs at ribosomes either on the RER or free in the cytoplasm. The ribosomes translate the code into polypeptides

3. Ribosomes on the RER make proteins that are excreted from the cell or attached to the cell membrane.

They are folded and processed (E.g sugar chains are added) in the RER.

4. Free ribosomes make proteins that stay in the cell cytoplasm.

5. Proteins are transported in vesicles which ‘bud off’ from the RER and travel to the Golgi apparatus

6. The Golgi apparatus processes the proteins further by trimming the sugar chains or adding additional chains.

7. The proteins enter more vesicles to be transported around the cell

8. Proteins leave the cell by exocytosis. The vesicle membrane fuses with the plasma membrane. Requires ATP

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Explain the importance of the cytoskeleton in providing mechanical strength to cells, aiding transport within cells and enabling cell movement.

Organelles in cells are surrounded by the cytoskeleton.

The cytoskeleton is a network of protein threads running through the cytoplasm.

In eukaryotic cells the protein threads are arranged as microfilaments (small solid strands) and microtubules (tiny protein cylinders)

The Cytoskeleton has 4 main functions

o Support the cells organelles, keeping them in position.

o Strengthen the cell and maintain its shape

o Transport materials within the cell

o Cause the cell to move, either moving cilia or the whole cell as with undulipodia.

3

5

7

2

4

1

6

8

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Compare and contrast, with the aid of diagrams and electron micrographs, the structure of prokaryotic cells and eukaryotic cells

Prokaryote Eukaryote

Diameter of Cell 0.2 - 2.0µm 10 - 100µm

Nucleus No nuclear membrane True nucleus

Membrane bound organelles Absent Present

Cell wall Peptidoglycan Cellulose (plants) or chitin (fungi)

Plasma membrane Lacks carbohydrates and sterols Carbohydrates and sterols

present

Ribosomes 70s Smaller 80s larger

Chromosome Single circular chromosome

lacks histones Multiple chromosomes

with histones

Cell Division Binary fission Involves mitosis

Prokaryote Eukaryote

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Compare and contrast, with the aid of diagrams and electron micrographs, the structure and ultrastructure of plant cells and animal cells.

Eukaryote Prokaryote

Animal Cell Plant Cell Bacterial Cell

Plasma Membrane

Cell Wall Cellulose Peptidoglycan

Nucleus

Lysosome

Ribosome

Rough ER

Smooth ER

Vesicle

Golgi apparatus

Mitochondrion Mesomes

Chloroplast

Centriole

Cilia

Undulipodia Flagella

1. State the maximum magnification that can be achieved by a light microscope and a transmission electron microscope.

Select your answers from the list below.

10x 40x 100x 400x 1500x 25 000x 50 000x 500 000x

light microscope ................................... x

transmission electron microscope ................................... x

[Total 2 marks]

2. Describe what is meant by the term resolution.

..................................................................................................................................

..................................................................................................................................

..................................................................................................................................

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..................................................................................................................................

[Total 2 marks]

3. In the lungs, goblet cells secrete mucus. The mucus is then moved by cilia.

Name one cellular structure from the list below that is associated with each of the following functions. You must select a structure once only.

mitochondria ribosome Golgi vesicle centriole nucleus cytoskeleton

(i) release of energy .......................................................................................

(ii) movement of cilia .......................................................................................

(iii) secrete mucus ............................................................................................

[Total 3 marks]

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4. The figure below is a diagram of an animal cell as seen using a transmission electron microscope.

A

B

C

D

20 mµ

E

F

(i) Name the structures of the cell labelled A, B, C and D.

A ....................................................................

B ....................................................................

C ....................................................................

D ....................................................................

[4]

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(ii) Structures C and E are examples of the same organelle.

Suggest why E looks so different to C.

.........................................................................................................................

.........................................................................................................................

.........................................................................................................................

.........................................................................................................................

[2]

(iii) Calculate the actual length of structure C.

Show your working and give your answer in micrometres (µm).

Answer = .................................................. µm

[2]

[Total 8 marks]

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5. The table below compares features of typical eukaryotic and prokaryotic cells.

(i) Complete the table by placing one of the following, as appropriate, in each empty box of the table.

• a tick ( )

• a cross ( )

• the words ‘sometimes present’

Some of the boxes have been completed for you.

eukaryotic cell prokaryotic cell

cell wall sometimes present

nuclear envelope

Golgi apparatus

ribosomes

flagellum sometimes present

(ii) Outline the roles of the Golgi apparatus and the ribosomes.

Golgi apparatus ..............................................................................................

.........................................................................................................................

.........................................................................................................................

[4]

Ribosomes ......................................................................................................

.........................................................................................................................

[2]

[Total 6 marks]