Cell Structure (AS Bio)

7/24/2019 Cell Structure (AS Bio)

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By: Dr.Mufaddal Page 1

ell structure

The basic unit of life, the cell, can be seen clearly only with the aid ofmicroscopes.

The light microscope uses light as a source of radiation, whereas the electronmicroscope uses electrons.

The electron microscope has greater resolution (allows more detail to beseen) than the light microscope, because electrons have a shorter

wavelength than light.

With a light microscope, cells may be measured using an eyepiece graticule

and a stage micrometer.

Using the formula A= I/M, the actual size of an object (A) or itsmagnification (M) can be found if its observed (image) size (I) is measuredand A or M, as appropriate, is known.

All cells are surrounded by a partially permeable cell surface membrane thatcontrols exchange between the cell and its environment.

All cells contain genetic material in the form of DNA, and ribosomes forprotein synthesis.

The simplest cells are prokaryotic cells, which are thought to have evolvedbefore, and given rise to the much more complex and much larger

eukaryotic cells.

Prokaryotic cells lack a true nucleus and have smaller ribosomes thaneukaryotic cells. They also lack membrane-bound organelles. Their DNA iscircular and lies naked in the cytoplasm.


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All eukaryotic cells possess a nucleus containing one or more nucleoli andDNA. The DNA is linear and bound to proteins to form chromatin.

The cytoplasm contains many membrane-bound organelles providing separate

compartments for specialized activities (division of labor). Organelles include

endoplasmic reticulum (ER), 80S ribosomes, mitochondria, Golgi apparatus andlysosomes. Animal cells also contain centrioles. Plant cells also containchloroplasts, often have a large, permanent, central vacuole and have a cell wall

containing cellulose. In eukaryotes, cells may be further organized into tissues,organs and systems.

MCQs:

1 Which type of membrane would be present in the largest quantity in aprokaryotic cell?

A cell surface membraneB mitochondrial cristaeC nuclear envelopeD smooth endoplasmic reticulum

2 Which type of cell would contain the greatest relative numbers ofmitochondria?

A bacterial cellB mesophyll cellC muscle cell

D parenchyma cell

3 In a cell that is specialized for secreting protein, which of thefollowing would be present in relatively large amounts?

A cell surface membraneB Golgi vesiclesC lysosomesD smooth endoplasmic reticulum

4 Which structure could be described as a microtubule-organizing

center?

A centriole

B Golgi apparatusC nucleus

D spindle


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5 What are microtubules made of?

A celluloseB DNAC lipid

D protein

6 Which structure could be found in a plant cell but not in a prokaryoticcell?

A 20 nm ribosomesB cell surface membraneC circular DNAD thylakoid

7 Which organelle makes lysosomes?

A Golgi apparatusB nucleusC ribosomeD smooth endoplasmic reticulum

8 A protein that is to be secreted from a cell would pass through asequence of cell organelles in the following order:

A Golgi apparatus rough endoplasmic reticulum secretory vesicle

B Golgi apparatus secretory vesiclerough endoplasmic reticulumC rough endoplasmic reticulumGolgi apparatus secretory vesicle

D secretory vesicleGolgi apparatus rough endoplasmic reticulum

9 A scientist calibrating an eyepiece graticule would notice what changewhen switching from a low-power lens to a high-power lens?

A The eyepiece units would appear closer together.B The eyepiece units would appear further apart.C The stage micrometer units would appear closer together.D The stage micrometer units would appear further apart.


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10 What explains the fact that an increase in the voltage used in a

transmission electron microscope results in an increase in the

resolution obtained?

A The electromagnetic lenses function more efficiently.

B Increasing the voltage increases the magnification.

C The electron beam can penetrate the specimen more easily.

D The wavelength of the electrons is shortened.


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Plan diagrams of tissue and organ, prokaryotic and

eukaryotic cells

An organusually contains many different types of cells. These are arranged ina particular pattern characteristic of the organ, with cells of a similar type foundtogether, forming distinctive tissues.A plan diagram shows the distributionof tissuesin an organ, not individual cells.

A cross section of leaf, stem and root.


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Prokaryotes and Eukaryotes

There are only 2 basic types of cells, primitive prokaryotes and the more

complex eukaryotes.

Prokaryotic cells(Pro=before, karyon = nucleus) are evolutionarilyancient. They were here first and for billions of years were the only form oflife. Today most life is prokaryotic, and these cells are supremely successful.All bacteria and bacteria-like Archaeaare prokaryotic organisms.

Eukaryotes (Eu=true, karyon= nucleus) can be single celled or multi-cellular organisms. Eukaryotic cells are more complex, having evolved from aprokaryote-like predecessor. Most of the living things that we are typicallyfamiliar with are composed of eukaryotic cells: animals, plants, fungi andprotists.

Prokaryotic cells:

much smaller

no membrane-bound nucleuses other membrane-bound organelles. The only

membrane is the plasma membrane.

the genetic material is naked within the cytoplasm

ribosomes are the only type of organelle

Eukaryotic cells

The main structure: a double membrane-bound nucleus separates the genetic material from the

rest of the cell.

an endomembrane system composed of different membrane-boundorganelles that transport materials around the cell: the rough and smoothendoplasmic reticulum, Golgi apparatus and vesicles.

energy producing organelles: mitochondria and chloroplasts, involved inmetabolism and energy conversion.


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Comparison of prokaryotic, animal and plant cells


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Viruses

Viruses are non-cellular organisms, which made up of genetic material and

protein that can invade living cells. These microorganisms belong to the family

of viridae and Genus of virus.

A virus is non-cellular organisms made up of genetic material and protein that

can invade living cells. They are considered both a living and non-living things.

In the year 1897, a scientist named Beijerinck discovered and coined the term

virus. The term virus is derived from Latin word-virus means poison. Later in

the year 1935, a scientist named Wendell Stanley discovered that these viruses

are composed of nucleic acids, protein and lips. The study of viruses is called as

virology.

Viruses are very small and they measured in nanometers. They can only be

seen with an electron microscope. They are composed of a core of DNA or RNAsurrounded by a protein coat they can only reproduce by infecting living cells.

Their size ranges from 20 nanometers to 250 nanometers.


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Cell structure and function

Describe and interpret drawings and photographs oftypicalanimaland plantcells. Note that plant cells are always surrounded by a

cell wall made of cellulose, never found around animal cells.

Typical animal and plant cells as seen using an electron microscope:

1. Functions of membrane systems and organelles:

The plasma membrane(cell surface membrane) controls what entersand leaves the cell.

Many membranes within the cell help to make different compartments fordifferent chemical reactions to take place.


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The nucleusis surrounded by nuclear envelope (pair of membranes).

The nucleus contains chromosomes, with very long molecule of DNA (DNAdetermines the sequences of amino acids to form protein molecules).

A darker area in the nucleus (no membrane) is called nucleolus: here new

ribosomes are made, following a code on part of the DNA.

Ribosomes (made of RNA & protein) are found free in the cytoplasm +attached to rough endoplasmic reticulum(RER).

The RERis network of membranes in the cytoplasm. The membranesenclose small spaces called cisternae.


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1. Ribosomeson the RER produce proteinsby linking amino acids. Thegrowing chains of amino acids move into the cisternae.

2. The cisternae break off to form little vesiclesthat travel to the Golgiapparatus.

3. Golgi apparatusmodifies the proteins (by adding carbohydrate

groups...).4. Vesicles containing modified proteins break away from Golgi apparatus

and move to the cell surface membrane---> secreted from the cellby exocytosis, releasing proteins.

The transport of vesicles.

Smooth endoplasmic reticulum (SER)

- less extensive (than RER)

- no ribosomes attached- cisternae more flattened

- involved in the synthesis of steroid hormones and the breakdown oftoxins.

Mitochondria

- have an envelope: outer membrane + inner membrane (folded to

form cristae).- here aerobic respiration takes place ---> ATP.

the first stage(Krebs cycle) - in the matrix;

the final stage (oxidative phosphorylation) - on cristae's membranes.


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Mitochondria.

Lysosomes

- little packages of hydrolytic (digestive) enzymes with membrane- form by breaking off from Golgi apparatus

- to digest food taken into the cell- to digest bacteria/other cells taken into the cell by phagocytosis- to break down unwanted/damaged organelles within the cell.


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Centrioles

- found only in animal cells, not plant cells- 2 centrioles lie at right angles to each other- made of microtubules, arranged in a circular pattern

- Microtubules form the spindle during cell division in animal cells.

Chloroplasts

- found only in some plant cells- surrounded by an envelope of 2 membranes- the background material (stroma) contains many paired membranes

(thylakolds).- thylakolds form stacks called grana (contain chlorophyll---> absorbs energy

from sunlight).- may contain starch grains (from sugars produced in photosynthesis).


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In photosynthesis:

- first reactions (light dependent reactions and photophosphorylation) takeplace on the membranes.

- the final stages (Calvin cycle) take place in the stroma.

Longitudinal section through a chloroplast.


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Microscopy

Most cells are very small,and their structures can only be seen by usinga microscope.

1. Light microscopes:

light rays pass through the specimenon a slide

focused by an objective lensand an eyepiece lens.

---> magnified imageof the specimen on the retina ofyour eye/screen/camera.

2. Electron microscopes:

uses beams of electrons

specimen very thin, placed in a vacuumto allow electrons to pass

through it.

electrons are focused onto a screen/photographic film--->magnified

imageof the specimen.


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3. Magnification and Resolution

Amount of magnificationdepends on the resolutionof the microscope(ability to distinguish 2 objects as separate).

The smaller the objects that can be distinguished --> the higher theresolution.

Wavelength: beam of electrons > light microscope with electron microscope, we can see much more fine detail of a cell.

Units: millimeter, micrometer, nanometer

4. Magnification calculations:

Work out the real size of an object knowing the magnification:

a. This drawing of a mitochondrion has been magnified 100 000 times.

Use ruler to measure its length in mm (50 mm).

Convert this measurement to m by multiplying by 1 000.

50 x 1 000 = 50 000 g


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Substitute into the equation:

b. This is a the drawing of a chloroplast:

- The magnification for this drawing:

- The length of the chloroplast:

Measure the length of the image in mm (80 mm) and convert tom ---> 80000 m.

Calculate its real length:

5. Measuring cells using a graticule:

Eyepiece graticuleis a little scale bar placed in the eyepiece of lightmicroscope.

The graticule is marked off in 'graticule units'.

Turn the eyepiece so that the graticule scale lies over the object: the widthof one cell is 23 graticule units.


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Calibration: the conversion of graticule unitsinto real units(mm, m).

Use a special slide called a stage micrometerthat is marked off in a tinyscale. The smallest markings are often 0.01 mm (10 g) apart.

Take the specimen off the stage or the microscope and replace it with thestage micrometer. Use the same objective lens.

Line up the micrometer scale and the eyepiece graticule scale (by turningthe eyepiece and moving the micrometer on the stage). Make sure that 2large markings on each scale are lined up.


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The 50 mark (stage micrometer) is lined up with the 1.0 mark (eyepiecegraticule).

Work towards the right until you see another two lines lined up.

The 68 mark (stage micrometer) is lined up with the 9.0 mark (eyepiece

graticule). So you can say that:

80 small eyepiece graticule markings = 18 stage micrometer markings

= 18 x 0.01 mm= 0.18 mm

= 180 mSo 1 small eyepiece graticule marking = 180: 80

= 2.25 m

The plant cell was 23 eyepiece graticule units long --> its real width is:

23 x 2.25 = 51.75 m

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Answers to MCQs

1 A

2 C

3 B

4 A

5 D

6 D

7 A

8 C

9 D

10 D

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Cell Structure (AS Bio)