DP Biology HL A2.2 Cell Structure

DP Biology HL A2.2 Cell Structure - Full notes

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DP Biology HL | A2.2 Cell Structure

DP BIOLOGY · THEME A: UNITY AND DIVERSITY

A2.2

Cell Structure

Full notes for HL · IB-style

Contents

1	Cells as the basic unit of life
2	Microscopy and magnification
3	Structures common to all cells
4	Prokaryotic cell structure
5	Eukaryotic cell structure
6	Plant, animal and fungal cells and atypical cells
7	Unicellular organisms and the functions of life
8	The origin of eukaryotic cells by endosymbiosis (HL only)
9	Cell differentiation and multicellularity (HL only)
10	Magnification calculations: practice
11	Light and electron microscopes compared
12	Advanced microscopy techniques
13	Cell theory and its exceptions
14	Prokaryotic cell structure in detail
15	Eukaryotic organelles: functions
16	Comparing animal, plant and fungal cells
17	Drawing and annotating cells
18	Surface area to volume ratio and cell size
19	Unicellular organisms: functions of life
20	Mixed review: worked problems
21	Additional exam-style practice
22	Practical: preparing slides
23	Cell structure and disease
24	Common misconceptions and links
25	Exam-style question with markscheme
26	Check your understanding
27	Summary and topic map

These are full teaching notes. Read each section in order, try the Think about it question before you read on, and do every Quick check before moving to the next section.

Before you start

Rate yourself on each statement now, and again after you've worked through the notes. Shade R (not yet), A (getting there) or G (confident).

I can…	R	A	G
outline the cell theory and its evidence
calculate magnification and actual size using scale bars
compare light and electron microscopy and advanced techniques
draw and label a prokaryotic cell
describe the structure and function of eukaryotic organelles
compare plant, animal and fungal cells and identify atypical cell structures
describe how unicellular organisms carry out the functions of life
explain the endosymbiotic theory and its evidence
explain cell differentiation and the evolution of multicellularity

Sections marked HL only (Sections 8 and 9) are Additional Higher Level content. Every other section is common to SL and HL.

Guiding questions

What are the features common to all cells and the features that differ? How has microscopy changed our understanding of cell structure?

How this topic is examined

A2.2 is one of the most heavily examined subtopics. Paper 1 tests organelle functions and prokaryote/eukaryote differences; Paper 2 asks you to calculate magnification, identify structures in micrographs and draw cells.

For drawings, use single clear lines, no shading, and label lines that end exactly on the structure. For calculations, convert units carefully and give the answer with units.

Command term	What the examiner wants
State / Identify	a brief answer with no explanation needed
Outline	a brief account or summary
Describe	a detailed account, including trends or features, without reasons
Explain	a detailed account including causes, reasons or mechanisms
Distinguish / Compare and contrast	give the differences (and similarities), point by point
Calculate	a numerical answer with working and units
Deduce / Suggest	reach a conclusion from data; propose a reasonable explanation
Evaluate / Discuss	weigh strengths and limitations and reach a judgement

How to use these notes

Each section teaches one part of the syllabus in full. You'll see these boxes throughout; each has one job:

Box	What it's for
Think about it	a question to get you thinking before a section; the answer comes in the section
Key point	the one idea you must not forget
Common mistake	an error examiners see every year, and how to avoid it
In the real world	where the biology shows up outside the classroom
Try this	something quick you can do yourself to test the idea
Worked example	full solutions with IB-style marks (M1 method, A1 answer, R1 reasoning)
Quick check	short questions to test yourself at the end of each section

1 Cells as the basic unit of life

All living things are made of cells.

Think about it

Viruses can reproduce, but only inside cells. Should a virus count as a living thing according to the cell theory?

The cell theory has three parts: all living organisms are composed of one or more cells; the cell is the smallest unit of life; and cells arise only from pre-existing cells.

The theory was developed in the nineteenth century as microscopes improved, building on observations by Robert Hooke (who coined the word 'cell' in 1665 after looking at cork) and Antonie van Leeuwenhoek (who first saw living single-celled organisms). Matthias Schleiden and Theodor Schwann proposed that plants and animals are made of cells, and Rudolf Virchow added that cells come only from other cells.

Scientific theories are based on many observations; this is inductive reasoning. The cell theory is supported by observing cells in organisms of every kind. A few structures seem to challenge it, such as very long muscle fibres with many nuclei, but these are explained as modified cells (Section 6).

Quick check

1. State the three parts of the cell theory.

2. Who first observed living unicellular organisms?

Answers: 1. all organisms made of cells; cell is the smallest unit of life; cells come from pre-existing cells 2. Antonie van Leeuwenhoek

2 Microscopy and magnification

To study cells you must make them look bigger and see more detail.

2.1 Magnification and resolution

Magnification is how many times larger an image is than the actual object. Resolution is the ability to distinguish two points close together as separate: the higher the resolution, the more detail can be seen. Magnifying an image without improving resolution just makes a blurred image bigger.

Light microscopes use visible light and glass lenses. They can magnify up to about ×1 500 with a resolution of about 0.2 µm, enough to see cells and nuclei but not most organelles in detail. They can view living cells and show colour.

Electron microscopes use beams of electrons, which have a much shorter wavelength, so their resolution is far higher (about 0.1 to 1 nm). They reveal the ultrastructure of cells: membranes, ribosomes and organelle details. Transmission electron microscopes (TEM) pass electrons through a thin section; scanning electron microscopes (SEM) scan the surface to produce 3D images. Specimens must be dead and in a vacuum, and images are black and white (false colour may be added).

Feature	Light microscope	Electron microscope
Radiation	visible light	electrons
Maximum resolution	about 0.2 µm	about 0.1 to 1 nm
Maximum magnification	about ×1 500	over ×500 000
Specimens	living or dead	dead, in a vacuum
Image	colour	black and white

2.2 Calculating magnification and size

Magnification = image size ÷ actual size Actual size = image size ÷ magnification

Always convert to the same units first: 1 mm = 1 000 µm; 1 µm = 1 000 nm. A scale bar on a micrograph tells you the actual length represented by the bar.

WORKED EXAMPLE 1 Magnification calculations

(a) A mitochondrion is 3 µm long and its image is 45 mm long. Calculate the magnification. (b) A micrograph has a 10 µm scale bar that measures 25 mm. A cell in the image measures 60 mm. Calculate its actual length.

Solution

(a) Convert 45 mm to µm: 45 000 µmM1

Magnification = 45 000 ÷ 3 = ×15 000A1

(b) Magnification of scale bar = 25 000 ÷ 10 = ×2 500M1

Actual length = 60 000 ÷ 2 500 = 24 µmA1

The most common error is forgetting to convert mm to µm. Write the units at every step.

2.3 Developments in microscopy

Recent techniques have extended what microscopy can show. Fluorescent stains and immunofluorescence attach fluorescent molecules (often linked to antibodies) to specific proteins, so their location in a cell glows. Freeze-fracture splits frozen membranes to reveal proteins inside them. Cryogenic electron microscopy (cryo-EM) images molecules frozen in their natural state, allowing protein structures to be worked out at near-atomic resolution.

Quick check

1. Define resolution.

2. An image is 12 mm long at ×400. Find the actual size in µm.

3. Give one advantage of a light microscope over an electron microscope.

Answers: 1. ability to distinguish two close points as separate 2. 30 µm 3. can view living cells / colour / cheaper

3 Structures common to all cells

Every cell, from bacteria to neurons, shares four features.

All cells have: a plasma membrane that separates the cell from its surroundings and controls what enters and leaves; cytoplasm, a water-based fluid in which metabolic reactions take place; DNA as genetic material; and ribosomes, which synthesize proteins. These shared features reflect common ancestry (A2.1).

4 Prokaryotic cell structure

Small, simple, no nucleus.

Prokaryotes (bacteria and archaea) are cells without a nucleus or membrane-bound organelles. They are usually 1 to 10 µm long.

Cell wall: surrounds the plasma membrane and protects the cell and prevents it bursting. In bacteria it is made of peptidoglycan. Plasma membrane: controls entry and exit. Cytoplasm: contains enzymes and dissolved substances. Nucleoid: the region where the single, circular, 'naked' DNA molecule (not associated with histones) is found. Plasmids: small extra circles of DNA, often carrying genes such as antibiotic resistance. 70S ribosomes: smaller than eukaryotic ribosomes. Some prokaryotes also have pili (for attachment and exchanging DNA), flagella (for movement) and a capsule or slime layer (protection).

Figure 1. A generalized prokaryotic cell.

Common mistake

Saying prokaryotes have no DNA or no ribosomes. They have both: DNA free in the nucleoid and smaller (70S) ribosomes.

Quick check

1. What is the cell wall of bacteria made of?

2. What are plasmids?

3. How do prokaryotic ribosomes differ from eukaryotic ones?

Answers: 1. peptidoglycan 2. small circles of extra DNA 3. 70S, smaller than 80S

5 Eukaryotic cell structure

Membrane-bound compartments divide up the work.

Eukaryotic cells (animals, plants, fungi, protists) are larger, typically 10 to 100 µm, and have a nucleus and membrane-bound organelles. Dividing the cell into compartments allows different conditions in each, keeps potentially harmful enzymes separate, and concentrates reactants (B2.2).

Organelle	Structure	Function
Nucleus	double membrane (nuclear envelope) with pores; contains chromosomes (DNA with histones)	stores genetic information; site of transcription
80S ribosomes	free in cytoplasm or attached to rough ER	protein synthesis
Rough endoplasmic reticulum	flattened membrane sacs with ribosomes	synthesis of proteins for secretion or membranes
Smooth endoplasmic reticulum	membrane tubules without ribosomes	lipid synthesis; detoxification
Golgi apparatus	stack of flattened sacs	modifies, sorts and packages proteins into vesicles
Vesicles	small membrane-bound sacs	transport substances; exocytosis
Lysosomes	vesicles containing digestive enzymes	digest worn-out organelles and engulfed material
Mitochondria	double membrane; inner folded into cristae	aerobic respiration; ATP production
Chloroplasts (plants)	double membrane; thylakoids in stacks (grana)	photosynthesis
Vacuole	membrane-bound sac; large and permanent in plants	storage; maintains turgor in plants
Cytoskeleton	microtubules and microfilaments	shape, movement, transport within the cell
Centrioles (animals)	two cylinders of microtubules	organize spindle in cell division

Proteins made for secretion follow a path: synthesized on the rough ER, moved in vesicles to the Golgi apparatus, modified and packaged, then carried in vesicles to the plasma membrane and released by exocytosis.

Feature	Prokaryotic	Eukaryotic
Nucleus	absent; DNA in nucleoid	present, with double membrane
DNA	circular, naked; plasmids	linear chromosomes with histones
Ribosomes	70S	80S (70S in mitochondria and chloroplasts)
Membrane-bound organelles	absent	present
Size	1 to 10 µm	10 to 100 µm
Cell wall	peptidoglycan (bacteria)	cellulose (plants), chitin (fungi), absent (animals)

Quick check

1. What is the function of the Golgi apparatus?

2. Why is the inner mitochondrial membrane folded?

3. Give three differences between prokaryotic and eukaryotic cells.

Answers: 1. modifies, sorts, packages proteins 2. larger surface area for ATP production 3. nucleus; organelles; ribosome size; DNA form

6 Plant, animal and fungal cells and atypical cells

Same basic plan, different specializations.

Figure 2. Animal, plant and fungal cells compared.

Feature	Animal	Plant	Fungus
Cell wall	absent	cellulose	chitin
Chloroplasts	absent	present in photosynthetic cells	absent
Vacuole	small, temporary	large, permanent	present
Centrioles	present	absent	absent in most
Carbohydrate store	glycogen	starch	glycogen
Shape	variable	fixed, often regular	hyphae

Some cells do not fit the typical pattern. Skeletal muscle fibres are very long and contain many nuclei, formed by the fusion of many cells. Aseptate fungal hyphae are long tubes with many nuclei and no cross walls. Red blood cells in mammals lose their nucleus to make room for haemoglobin. Phloem sieve tube elements lose their nucleus and most organelles and depend on companion cells. These atypical cells are explained as adaptations, and they do not overturn the cell theory.

Quick check

1. What is a fungal cell wall made of?

2. Why do mammalian red blood cells lack a nucleus?

3. Why are skeletal muscle fibres multinucleate?

Answers: 1. chitin 2. more space for haemoglobin 3. formed by fusion of cells

7 Unicellular organisms and the functions of life

One cell must do everything.

A unicellular organism must carry out all the functions of life in a single cell: metabolism, growth, reproduction, response to stimuli, excretion of waste, homeostasis, and nutrition. Two examples are Paramecium (a heterotroph that ingests food) and Chlamydomonas (a photosynthetic autotroph).

Figure 3. Paramecium: one cell carries out every function of life.

Function	Paramecium	Chlamydomonas
Nutrition	cilia sweep food into the oral groove; food vacuoles	photosynthesis in a chloroplast
Movement / response	cilia; moves away from harmful stimuli	flagella; eyespot detects light
Excretion / homeostasis	contractile vacuoles expel excess water	contractile vacuoles
Reproduction	binary fission (asexual); conjugation	mitosis; sexual reproduction

8 The origin of eukaryotic cells by endosymbiosis (HL only)

Mitochondria and chloroplasts were once free-living bacteria.

HL only

This section is Additional Higher Level content. SL students do not need it.

The endosymbiotic theory, championed by Lynn Margulis, proposes that mitochondria and chloroplasts evolved from free-living prokaryotes that were engulfed by a larger host cell but not digested. The engulfed aerobic bacterium provided ATP; the host provided protection and nutrients. Over time it became the mitochondrion. Later, a photosynthetic cyanobacterium was engulfed in the same way, becoming the chloroplast.

Evidence: mitochondria and chloroplasts have a double membrane (the inner one from the bacterium, the outer from the engulfing vesicle); they contain their own circular, naked DNA like bacteria; they have 70S ribosomes; they divide by a process similar to binary fission; and their DNA sequences are most similar to those of particular groups of bacteria.

Quick check

1. Name two organelles thought to have originated by endosymbiosis.

2. Give three pieces of evidence for the theory.

Answers: 1. mitochondria and chloroplasts 2. double membrane; circular DNA; 70S ribosomes; binary fission

9 Cell differentiation and multicellularity (HL only)

One genome, many cell types.

HL only

This section is Additional Higher Level content. SL students do not need it.

In a multicellular organism, all cells contain the same genes, but different cells express different genes. Cell differentiation is the process by which cells become specialized for particular functions by switching on some genes and switching off others, often in response to signals from neighbouring cells or position in the embryo.

Multicellularity has evolved independently many times: in animals, plants, fungi and several groups of algae. Its advantages include larger size (escaping predators, reaching light), longer life (cells can be replaced), and specialization, which allows division of labour: different cells carry out different functions more efficiently than a single cell could.

Quick check

1. What is cell differentiation?

2. Give two advantages of multicellularity.

Answers: 1. specialization of cells by expressing different genes 2. larger size; longer life; specialization

10 Magnification calculations: practice

I = AM.

Magnification = image size ÷ actual size. Convert units first: 1 mm = 1000 μm; 1 μm = 1000 nm. Use a scale bar to find magnification: measure the bar and divide by the length it represents.

WORKED EXAMPLE 2 Actual size (standard)

A mitochondrion is 30 mm long in a micrograph at × 12 000. Find its actual length.

Solution

30 mm = 30 000 μmM1

30 000/12 000 = 2.5 μmA1

WORKED EXAMPLE 3 Scale bar (standard)

A 20 mm scale bar represents 5 μm. Find the magnification.

Solution

20 000 μm/5 μmM1

= × 4000A1

Quick check

1. A cell 40 μm wide appears 8 mm wide. Find the magnification.

2. A bacterium is 2 μm long. How long is its image at × 5000?

Answers: 1. × 200 2. 10 mm

11 Light and electron microscopes compared

Resolution matters.

Resolution is the ability to distinguish two close points as separate. Electron microscopes have much higher resolution because electrons have a much shorter wavelength than light.

Feature	Light microscope	Electron microscope
radiation	light	electron beam
maximum resolution	about 200 nm	about 0.2 nm (TEM)
maximum useful magnification	× 1500	× 500 000+
specimens	living or dead	dead (vacuum)
colour	natural or stained colour	black and white (false colour added)

WORKED EXAMPLE 4 Choosing (standard)

Which microscope would you use to observe ribosomes? Explain.

Solution

Electron microscopeA1

Ribosomes (about 20–30 nm) are below the resolution of light microscopesR1

12 Advanced microscopy techniques

Seeing more.

Fluorescent stains and immunofluorescence label specific molecules with fluorescent antibodies. Freeze-fracture reveals the interior of membranes, showing proteins embedded in the bilayer. Cryogenic electron microscopy determines 3D structures of proteins at near-atomic resolution.

WORKED EXAMPLE 5 Freeze-fracture (standard)

What did freeze-fracture images show about membranes?

Solution

Proteins embedded in and spanning the bilayerR1

Supported the fluid mosaic modelR1

13 Cell theory and its exceptions

Testing a generalization.

Cell theory: all living organisms are made of cells; the cell is the smallest unit of life; cells arise from pre-existing cells. Atypical examples test it: striated muscle fibres (multinucleate, very long), aseptate fungal hyphae (no cross walls), red blood cells (no nucleus), and phloem sieve tubes (no nucleus).

WORKED EXAMPLE 6 Atypical cells (exam level)

Explain why striated muscle fibres challenge cell theory.

Solution

Each fibre is very long and contains many nucleiR1

Formed by fusion of cells, so not a typical single-nucleus cellR1

Quick check

1. Why are mature red blood cells considered atypical?

Answers: 1. they have no nucleus

14 Prokaryotic cell structure in detail

Simple but effective.

Prokaryotes have: a cell wall (peptidoglycan), plasma membrane, cytoplasm, 70S ribosomes, a nucleoid with naked circular DNA, plasmids, and sometimes pili, flagella and a capsule. They lack membrane-bound organelles.

WORKED EXAMPLE 7 Structures (standard)

State the functions of plasmids and pili.

Solution

Plasmids: extra genes, e.g. antibiotic resistanceA1

Pili: attachment and transfer of DNA (conjugation)A1

Quick check

1. Where is DNA found in a prokaryote?

Answers: 1. in the nucleoid region (and plasmids)

15 Eukaryotic organelles: functions

Compartmentalized cells.

Organelle	Function
nucleus	contains chromosomes; site of transcription
rough ER	synthesizes and transports proteins
smooth ER	lipid synthesis; detoxification
Golgi apparatus	modifies and packages proteins for secretion
mitochondrion	aerobic respiration (ATP production)
chloroplast	photosynthesis
lysosome	digestion by hydrolytic enzymes
vacuole	storage; turgor in plants
80S ribosome	protein synthesis

WORKED EXAMPLE 8 Secretory cell (exam level)

Explain why pancreatic cells that secrete enzymes have many rough ER and Golgi.

Solution

Rough ER synthesizes large amounts of protein (enzymes)R1

Golgi modifies and packages them into vesicles for exocytosisR1

16 Comparing animal, plant and fungal cells

Shared and distinct features.

Feature	Animal	Plant	Fungus
cell wall	absent	cellulose	chitin
chloroplasts	absent	present	absent
vacuole	small, temporary	large, permanent	present
centrioles	present	absent	absent (in most)
energy storage	glycogen	starch	glycogen

WORKED EXAMPLE 9 Identifying (standard)

A cell has a chitin wall and no chloroplasts. Identify the kingdom.

Solution

FungiA1

17 Drawing and annotating cells

Scientific drawings.

Biological drawings use a sharp pencil, clear continuous lines, no shading, labels with ruled lines that do not cross, and a title with the magnification. Annotations add functions to labels.

WORKED EXAMPLE 10 Annotation (standard)

Annotate a mitochondrion in a drawing of a liver cell.

Solution

Label: mitochondrionA1

Annotation: site of aerobic respiration producing ATPA1

Common mistake

Shading or sketching structures. Drawings should show outlines with single clear lines and be proportional.

18 Surface area to volume ratio and cell size

Why cells are small.

As a cell grows, its volume increases faster than its surface area. Exchange of materials depends on surface area, while demand depends on volume. A large SA:V ratio allows efficient exchange, which limits cell size.

WORKED EXAMPLE 11 Calculation (standard)

Calculate the SA:V ratio of cubes with sides 1 μm and 3 μm.

Solution

1 μm: 6/1 = 6 : 1A1

3 μm: 54/27 = 2 : 1A1

Quick check

1. Find the SA:V ratio of a cube with side 5 μm.

Answers: 1. 150/125 = 1.2 : 1

19 Unicellular organisms: functions of life

Doing everything in one cell.

Unicellular organisms carry out all functions of life: nutrition, metabolism, growth, response, excretion, homeostasis and reproduction. Paramecium (heterotroph) feeds by ingesting food into vacuoles; Chlamydomonas (autotroph) photosynthesizes and swims towards light using its eyespot and flagella.

WORKED EXAMPLE 12 Functions (exam level)

Outline how Paramecium carries out homeostasis and excretion.

Solution

Contractile vacuoles expel excess water (osmoregulation)R1

Waste diffuses out across the membraneR1

20 Mixed review: worked problems

Bringing the ideas together.

WORKED EXAMPLE 13 Mixed problem (exam level)

In an electron micrograph at × 25 000, a nucleus measures 125 mm. (a) Find its actual diameter. (b) Could it be seen with a light microscope?

Solution

(a) 125 000 μm/25 000 = 5 μmM1 A1

(b) yes: 5 μm is well above 200 nm resolutionR1

21 Additional exam-style practice

Paper 1 and Paper 2.

WORKED EXAMPLE 14 Paper 1 question (standard)

Which structure is found in both prokaryotic and eukaryotic cells? A. nucleus B. mitochondrion C. ribosome D. Golgi apparatus

Solution

CA1

WORKED EXAMPLE 15 Paper 2 question (exam level)

Compare the structure of prokaryotic and eukaryotic cells.

Solution

Both: membrane, cytoplasm, ribosomes, DNAR1

Prokaryotes: 70S ribosomes, naked circular DNA, no membrane-bound organellesR1

Eukaryotes: 80S ribosomes, nucleus, organelles, linear DNA with histonesR1

22 Practical: preparing slides

Temporary mounts and stains.

Temporary mounts: a thin specimen in a drop of water or stain on a slide, covered with a coverslip lowered at an angle to avoid air bubbles. Iodine stains starch and nuclei; methylene blue stains nuclei in animal cells.

WORKED EXAMPLE 16 Coverslip (standard)

Why is the coverslip lowered at an angle?

Solution

To avoid trapping air bubblesA1

Try this

Peel the inner epidermis from an onion scale, mount it in iodine solution, and observe the cell walls and nuclei at low and high power.

23 Cell structure and disease

When organelles fail.

Faulty lysosomal enzymes cause lysosomal storage diseases (e.g. Tay–Sachs); mitochondrial DNA mutations cause disorders affecting muscles and nerves; defects in cilia cause respiratory problems. Understanding organelles helps diagnose and treat disease.

Quick check

1. Why do mitochondrial disorders often affect muscles and the brain?

Answers: 1. these tissues have high ATP demands

24 Common misconceptions and links

Students often think…	Actually…
magnification and resolution are the same	resolution is the detail; magnification is the enlargement
prokaryotes have no DNA	their DNA is circular in the nucleoid
all plant cells have chloroplasts	only photosynthetic cells do; root cells do not
fungal walls are cellulose	fungal walls are chitin

How A2.2 connects to the rest of the course

Topic	Connection
A2.1 Origins of cells (HL)	the first prokaryotic cells
B2.2 Organelles	compartmentalization in detail
B2.3 Cell specialization	surface area to volume, stem cells
D2.1 Cell division	mitosis and binary fission
A2.3 Viruses (HL)	acellular structures

25 Exam-style question with markscheme

Try this under exam conditions (about 15 minutes) before reading the markscheme.

Text A. An electron micrograph of a pancreatic cell shows extensive rough ER, many Golgi stacks and numerous mitochondria. The scale bar shows 5 µm.

(a) Calculate the magnification of the micrograph if the 5 µm scale bar measures 20 mm. [2]

(b) The mitochondrion in the micrograph measures 12 mm. Calculate its actual length. [2]

(d) Outline the path of a secreted protein through a cell. [3]

(e) Explain why electron microscopes reveal more detail than light microscopes. [2]

(f) State two features of mitochondria that support the endosymbiotic theory. [2]

Markscheme

Part	Answer	Marks
(a)	20 000 ÷ 5	M1
	= ×4 000	A1
(b)	12 000 ÷ 4 000	M1
	= 3 µm	A1
(c)	70S in prokaryotes, 80S in eukaryotes	A1
(d)	made on ribosomes of rough ER	R1
	vesicle to Golgi; modified and packaged	R1
	vesicle to plasma membrane; exocytosis	R1
(e)	electrons have shorter wavelength	R1
	higher resolution	R1
(f)	double membrane; circular DNA; 70S ribosomes; binary fission (any two)	A2

Examiner's comments. In (a) and (b), the most common error was not converting mm to µm. In (d), the best answers named each structure in order.

26 Check your understanding

Section A: multiple choice

1. Which structure is found in prokaryotes but not eukaryotes?

A. nucleoid B. nucleus C. mitochondrion D. Golgi apparatus

2. Resolution is

A. the enlargement of an image B. the size of the specimen C. the ability to distinguish two points as separate D. the brightness of the image

3. An image 30 mm long represents a 6 µm structure. The magnification is

A. ×5 B. ×5 000 C. ×500 D. ×50 000

4. The function of the Golgi apparatus is

A. ATP synthesis B. photosynthesis C. DNA replication D. modifying and packaging proteins

5. Fungal cell walls contain

A. cellulose B. chitin C. peptidoglycan D. starch

6. Which cell is multinucleate?

A. skeletal muscle fibre B. red blood cell C. neuron D. guard cell

7. Plasmids are

A. ribosomes B. organelles C. proteins D. small circular DNA molecules

8. Lysosomes contain

A. DNA B. chlorophyll C. digestive enzymes D. starch

9. Evidence for endosymbiosis includes

A. 80S ribosomes in chloroplasts B. a single membrane C. linear DNA with histones D. 70S ribosomes in mitochondria

Section B: structured

10. Compare prokaryotic and eukaryotic cells. [4]

11. Draw and label a prokaryotic cell. [4]

12. Compare plant and animal cells. [3]

13. A cell is 25 µm wide and its image 50 mm. Calculate the magnification. [2]

14. Explain the evidence for the endosymbiotic theory. [4]

Answers

1. A 2. C 3. B 4. D 5. B 6. A 7. D 8. C 9. D

10. Nucleus vs nucleoid; organelles; 80S vs 70S; linear vs circular DNA; size; both have membrane, cytoplasm, ribosomes, DNA.

11. Wall, membrane, cytoplasm, nucleoid, 70S ribosomes, plasmid, (flagellum, pili).

12. Plant: wall, chloroplasts, large vacuole, starch; animal: centrioles, glycogen, no wall.

13. 50 000 ÷ 25 = ×2 000.

14. Double membrane; circular naked DNA; 70S ribosomes; binary fission; DNA similar to bacteria.

27 Summary and topic map

Idea	Remember
Cell theory	cells make up organisms; smallest unit; from pre-existing cells
Magnification	image ÷ actual; convert units
Microscopes	light vs electron; resolution
Prokaryotes	nucleoid, plasmids, 70S, peptidoglycan wall
Eukaryotes	nucleus, organelles, 80S
Cell types	plant, animal, fungus; atypical cells
Unicellular	all functions of life in one cell
HL	endosymbiosis; differentiation; multicellularity

Before the exam, make sure you…

•convert to µm before calculating
•label organelles with precise functions
•use comparison tables for cell types
•draw with clear lines, no shading

Cell Structure on one page

Cover this page with a sheet of paper and try to rebuild it from memory. Whatever you can't recall is what to revise next.

Figure 4. Topic map for Cell Structure.

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