DP Biology HL B2.3 Cell Specialization

DP Biology HL B2.3 Cell Specialization - Full notes

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DP Biology HL | B2.3 Cell Specialization

DP BIOLOGY · THEME B: FORM AND FUNCTION

B2.3

Cell Specialization

Full notes for HL · IB-style

Contents

1	From zygote to specialized cells
2	Stem cells
3	Cell size and the surface area to volume ratio
4	Adaptations to increase surface area to volume ratio
5	Specialized cells: examples
6	Specialized cells in the alveolus, muscle and gametes (HL only)
7	From zygote to specialized cells
8	Morphogens and positional information
9	Stem cells: types and potency
10	Stem cell niches
11	Therapeutic uses of stem cells
12	Ethics of stem cell research
13	Cell size and specialization
14	Surface area to volume ratio in specialized cells
15	Specialized cells in the alveolus
16	Specialized muscle cells
17	Gametes: structure and function
18	Plant cell specialization
19	Differentiation in plants
20	Mixed review: worked problems
21	Additional exam-style practice
22	Cancer: when specialization fails
23	Practical: observing specialized cells
24	Tissues, organs and systems
25	Common misconceptions and links
26	Exam-style question with markscheme
27	Check your understanding
28	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
explain how cells become specialized from a zygote
describe the properties of stem cells and distinguish totipotent, pluripotent and multipotent
explain the location and uses of stem cells
explain why cell size is limited by surface area to volume ratio
calculate surface area to volume ratios
explain adaptations that increase the surface area to volume ratio
relate the structure of specialized cells to their functions
explain the adaptations of type I and type II pneumocytes, cardiac and skeletal muscle, and sperm and egg cells

Sections marked HL only (Section 6) are Additional Higher Level content. Every other section is common to SL and HL.

Guiding questions

How do cells in a multicellular organism become different from each other? Why are cells so small, and how do some cells overcome the limits of size?

How this topic is examined

B2.3 is examined through surface area to volume calculations, stem cell definitions and ethical questions, and questions linking the structure of specialized cells to their function.

In calculations, show the surface area, the volume and the ratio separately, 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 From zygote to specialized cells

One cell becomes trillions of different cells.

Think about it

Every cell in your body, from neuron to skin cell, came from one fertilized egg and contains the same genes. How can they be so different?

A multicellular organism begins as a single cell, the zygote, formed by fertilization. The zygote divides by mitosis to form an embryo. At first the cells are all similar, but as the embryo develops, cells differentiate: they become specialized for particular functions.

All the cells have the same genome, but they express different genes. A red blood cell precursor expresses the genes for haemoglobin; a pancreatic beta cell expresses the insulin gene. Which genes are expressed depends partly on the position of the cell in the embryo, detected through chemical signals called morphogens. A morphogen diffuses from a source, forming a concentration gradient; cells respond differently depending on the concentration they receive, switching on different sets of genes.

Quick check

1. What is differentiation?

2. How do morphogens control differentiation?

Answers: 1. the process by which cells become specialized by expressing different genes 2. they form concentration gradients; cells respond to the concentration by expressing different genes

2 Stem cells

Unspecialized cells that can divide and differentiate.

Stem cells have two key properties: they can divide repeatedly (self-renewal) to produce more stem cells, and they can differentiate into specialized cell types. They are classified by how many cell types they can become.

Totipotent stem cells can differentiate into any cell type, including the cells of the placenta; the zygote and cells of the very early embryo are totipotent. Pluripotent stem cells can become almost any cell type of the body, but not the placenta; embryonic stem cells from the blastocyst are pluripotent. Multipotent stem cells can become a limited range of related cell types; bone marrow stem cells form the different types of blood cells.

Stem cell niches are locations in the adult body where stem cells are maintained: bone marrow (blood cells), the base of hair follicles, the lining of the gut, and the meristems of plants.

Uses of stem cells include bone marrow transplants for leukaemia (stem cells from a donor replace the patient's blood-forming cells), treatment of some forms of blindness, and research into replacing damaged tissue in conditions such as type 1 diabetes, Parkinson's disease and spinal cord injury. Scientists can also reprogram adult cells into induced pluripotent stem cells (iPS cells).

Type	Potency	Example source
Totipotent	any cell type, including placenta	zygote, early embryo (up to about 8 cells)
Pluripotent	almost any body cell type	embryonic stem cells; iPS cells
Multipotent	a limited range of cells	bone marrow, umbilical cord blood

2.1 Ethical issues

Obtaining embryonic stem cells usually destroys an early embryo, which raises ethical objections for people who believe the embryo has the moral status of a person. Supporters argue that the embryos used are often surplus from IVF and would otherwise be discarded, and that the potential medical benefits are very large. Adult and iPS stem cells avoid this problem, though they may have more limited potency or risks such as tumour formation.

Quick check

1. State two properties of stem cells.

2. Distinguish pluripotent and multipotent.

3. Give an ethical argument against using embryonic stem cells.

Answers: 1. self-renewal and ability to differentiate 2. pluripotent: almost any cell; multipotent: limited range 3. embryo is destroyed; moral status of embryo

3 Cell size and the surface area to volume ratio

As cells grow, their volume outpaces their surface.

A cell must exchange materials (oxygen, nutrients, waste) and heat with its surroundings across its surface, the plasma membrane. The rate of exchange depends on the surface area. The demand for exchange depends on the volume of the cell, because metabolism happens throughout it.

As a cell grows, its volume increases faster than its surface area. For a cube of side length L, surface area = 6L² and volume = L³, so the surface area to volume ratio = 6/L: doubling the side halves the ratio. A large cell would not be able to exchange materials fast enough to supply its whole volume. This is a key reason why cells are small and why large organisms are made of many cells.

WORKED EXAMPLE 1 Surface area to volume ratio

Calculate the surface area to volume ratio for cubes of side 1 µm, 2 µm and 4 µm.

Solution

1 µm: SA = 6 µm², V = 1 µm³, ratio 6 : 1A1

2 µm: SA = 24 µm², V = 8 µm³, ratio 3 : 1A1

4 µm: SA = 96 µm², V = 64 µm³, ratio 1.5 : 1A1

As size increases, the ratio decreases, so each unit of volume has less surface for exchangeR1

Figure 1. The surface area to volume ratio falls as cell size increases.

Quick check

1. Why do cells need a large surface area to volume ratio?

2. Calculate the SA:V ratio of a 3 µm cube.

Answers: 1. exchange across the surface must meet the needs of the whole volume 2. 54 : 27 = 2 : 1

4 Adaptations to increase surface area to volume ratio

Flatten, fold or extend.

Cells and tissues that exchange a lot of material are adapted to increase their surface area relative to volume. Flattening: red blood cells are biconcave discs, so no part of the cell is far from the surface, speeding oxygen uptake and release. Microvilli: epithelial cells of the small intestine and kidney tubules have thousands of finger-like projections of the plasma membrane, increasing the surface for absorption. Elongation: root hair cells extend into the soil, increasing the area for water and mineral uptake.

The same principle applies at larger scales: alveoli in lungs (B3.1), villi in the intestine and gill filaments in fish all increase surface area for exchange.

Quick check

1. Give three adaptations that increase surface area.

2. Why are red blood cells biconcave?

Answers: 1. flattening, microvilli, elongation (root hairs) 2. larger surface and shorter diffusion distance for oxygen

5 Specialized cells: examples

Structure matches function.

Cell	Function	Adaptations
Red blood cell	carries oxygen	biconcave; no nucleus; full of haemoglobin
Root hair cell	absorbs water and minerals	long extension; thin wall; many mitochondria for active transport
Neuron	transmits impulses	long axon; myelin sheath; many synaptic terminals
Palisade mesophyll cell	photosynthesis	many chloroplasts; column-shaped near upper surface
Small intestine epithelial cell	absorbs nutrients	microvilli; many mitochondria; tight junctions
Sperm cell	fertilizes egg	flagellum; mitochondria; acrosome with enzymes

Key point

Specialized cells are adapted by their shape, organelles and proteins. In answers, link each feature to its function with 'so that'.

6 Specialized cells in the alveolus, muscle and gametes (HL only)

Detailed adaptations of selected cells.

HL only

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

Type I pneumocytes form most of the alveolar wall. They are extremely thin (about 0.15 µm), giving a very short diffusion distance for gas exchange, and joined by tight junctions to prevent leakage. Type II pneumocytes are cuboidal cells that secrete surfactant, a fluid that reduces surface tension, stopping alveoli from collapsing and sticking together. They have many secretory vesicles (lamellar bodies).

Skeletal muscle fibres are very long, multinucleate cells formed by the fusion of many cells, containing myofibrils with contractile proteins (B3.3), many mitochondria and a specialized ER (sarcoplasmic reticulum) storing calcium. Because they are formed by fusion, they challenge the idea that cells have one nucleus. Cardiac muscle cells are branched, usually have one nucleus, and are joined by intercalated discs that allow impulses to pass quickly so the heart contracts as one unit.

Gametes show extreme specialization. The egg cell is very large, with a store of nutrients in the cytoplasm, a protective jelly coat (zona pellucida) and cortical granules that prevent more than one sperm entering. The sperm cell is small and streamlined, with a flagellum for swimming, a midpiece packed with mitochondria for ATP, and an acrosome containing enzymes to digest the egg's coat.

Cell	Key adaptations
Type I pneumocyte	very thin; short diffusion distance
Type II pneumocyte	secretes surfactant; prevents alveolar collapse
Skeletal muscle fibre	multinucleate, long, many mitochondria, sarcoplasmic reticulum
Cardiac muscle cell	branched; intercalated discs
Egg	large; nutrient store; zona pellucida; cortical granules
Sperm	flagellum; mitochondria; acrosome

Quick check

1. Why are type I pneumocytes very thin?

2. What do type II pneumocytes secrete?

3. What is the function of the acrosome?

Answers: 1. short diffusion distance for gases 2. surfactant 3. releases enzymes to digest the egg's coat

7 From zygote to specialized cells

Differentiation.

A zygote divides by mitosis to form an embryo. Cells become specialized (differentiate) by expressing different genes; all cells contain the same genome but use different parts of it. Differentiation is usually irreversible in animals.

WORKED EXAMPLE 2 Gene expression (standard)

Explain how cells with the same genes become different.

Solution

Different genes are switched on or off in different cellsR1

Different proteins produced, giving different structures and functionsR1

8 Morphogens and positional information

Gradients guide development.

In embryos, morphogens are signalling molecules that form concentration gradients. Cells detect their position by the morphogen concentration and activate particular genes, which patterns body structures such as limbs and segments.

WORKED EXAMPLE 3 Morphogen (standard)

What is a morphogen?

Solution

A signalling molecule whose concentration gradient determines cell fateA1

9 Stem cells: types and potency

Totipotent to unipotent.

Type	Potency	Example
totipotent	any cell type, including placenta	zygote, early embryo cells
pluripotent	almost any body cell type	embryonic stem cells
multipotent	a limited range of related cells	bone marrow (blood cells)
unipotent	one cell type	skin stem cells

WORKED EXAMPLE 4 Potency (standard)

Distinguish pluripotent and multipotent stem cells.

Solution

Pluripotent: can become almost any cell typeR1

Multipotent: only a limited range of related typesR1

10 Stem cell niches

Where adult stem cells live.

Stem cell niches are microenvironments that maintain stem cells and regulate their division and differentiation: bone marrow (blood-forming stem cells) and hair follicles (skin and hair stem cells).

Quick check

1. Name two stem cell niches in adults.

Answers: 1. bone marrow; hair follicles

11 Therapeutic uses of stem cells

Treating disease.

Stem cells are used to treat leukaemia (bone marrow transplants), and are being researched for Parkinson's disease, type 1 diabetes, macular degeneration and spinal injuries. Induced pluripotent stem cells (iPSCs) are made by reprogramming adult cells.

WORKED EXAMPLE 5 Bone marrow (exam level)

Explain how bone marrow transplants treat leukaemia.

Solution

Patient's cancerous marrow destroyed by chemotherapy/radiationR1

Donor stem cells produce healthy blood cellsR1

In the real world

In 2012, Shinya Yamanaka received the Nobel Prize for showing that adult cells can be reprogrammed into pluripotent stem cells using four transcription factors.

Quick check

1. Give one advantage of iPSCs over embryonic stem cells.

Answers: 1. no embryo destroyed; patient's own cells reduce rejection

12 Ethics of stem cell research

Weighing benefits and concerns.

Embryonic stem cell research may destroy embryos, raising ethical objections. Arguments for include potential cures and that early embryos lack a nervous system. Alternatives include adult stem cells, umbilical cord blood and iPSCs.

WORKED EXAMPLE 6 Discussion (exam level)

Discuss the ethics of using embryonic stem cells.

Solution

For: pluripotent; potential treatments; surplus IVF embryosR1

Against: embryos destroyed; potential for lifeR1

13 Cell size and specialization

Size matched to function.

Cell sizes vary with function: sperm cells (about 50 μm, small and motile), egg cells (about 120 μm in humans, large food store), red blood cells (8 μm, small to fit capillaries), motor neurons (up to 1 m long to transmit signals), striated muscle fibres (several cm long).

WORKED EXAMPLE 7 Size (standard)

Explain why the human egg is much larger than a sperm cell.

Solution

Contains nutrients and organelles for the early embryoR1

14 Surface area to volume ratio in specialized cells

Maximizing exchange.

Cells involved in exchange have large SA:V ratios: biconcave red blood cells, flattened type I pneumocytes, root hair cells with long projections, and microvilli on intestinal epithelial cells.

WORKED EXAMPLE 8 Red blood cells (standard)

Explain how the shape of red blood cells aids their function.

Solution

Biconcave disc: large surface area for oxygen diffusionR1

Thin centre: short diffusion distanceR1

15 Specialized cells in the alveolus

Type I and type II pneumocytes.

Type I pneumocytes are extremely thin for gas exchange. Type II pneumocytes secrete surfactant (reduces surface tension, preventing alveoli from collapsing) and have many secretory vesicles and microvilli.

WORKED EXAMPLE 9 Surfactant (standard)

Explain the role of surfactant.

Solution

Reduces surface tension of water lining alveoliR1

Prevents alveoli collapsing on exhalationR1

16 Specialized muscle cells

Striated, cardiac and smooth.

Striated muscle fibres are long, multinucleate (formed by fusion), with many mitochondria and myofibrils. Cardiac muscle cells are branched with intercalated discs. Smooth muscle cells are spindle-shaped with one nucleus.

Quick check

1. Why are striated muscle fibres multinucleate?

Answers: 1. they form by the fusion of many cells

17 Gametes: structure and function

Egg and sperm.

Feature	Sperm	Egg
size	small (about 50 μm)	large (about 120 μm)
motility	flagellum	non-motile
mitochondria	many in midpiece	many in cytoplasm
special features	acrosome with enzymes	zona pellucida, cortical granules
numbers	millions produced	one released per cycle

WORKED EXAMPLE 10 Acrosome (standard)

State the role of the acrosome.

Solution

Releases enzymes that digest the zona pellucidaA1

18 Plant cell specialization

Root hairs, guard cells, xylem, phloem.

Root hair cells have long extensions (large surface area) for water and mineral absorption; guard cells control stomatal opening; xylem vessels are dead, hollow and lignified for water transport; phloem sieve tubes have few organelles and sieve plates, supported by companion cells.

WORKED EXAMPLE 11 Xylem (standard)

Explain how xylem vessels are adapted for water transport.

Solution

No cell contents or end walls: continuous tubesR1

Lignin strengthens walls against tensionR1

19 Differentiation in plants

Meristems.

Plants retain stem cells in meristems (shoot and root tips, cambium), allowing growth throughout life. Many plant cells remain totipotent, which allows cloning by tissue culture.

Quick check

1. Why can whole plants be grown from small pieces of tissue?

Answers: 1. many plant cells are totipotent

20 Mixed review: worked problems

Bringing the ideas together.

WORKED EXAMPLE 12 Mixed problem (exam level)

Outline the properties of stem cells that make them useful in medicine.

Solution

Can divide repeatedly (self-renewal)R1

Can differentiate into specialized cell typesR1

Replace damaged tissuesR1

21 Additional exam-style practice

Paper 1 and Paper 2.

WORKED EXAMPLE 13 Paper 1 question (standard)

Which cells are totipotent? A. bone marrow stem cells B. cells of a 4-cell embryo C. skin cells D. neurons

Solution

BA1

WORKED EXAMPLE 14 Paper 2 question (exam level)

Explain how type I pneumocytes are adapted for gas exchange.

Solution

Very thin (flattened): short diffusion distanceR1

Large surface area covering most of the alveolar wallR1

22 Cancer: when specialization fails

Uncontrolled division.

Cancer cells divide uncontrollably and often lose their specialized features (dedifferentiate). They can invade tissues and spread (metastasize). Understanding the control of differentiation helps develop cancer treatments.

Quick check

1. Define metastasis.

Answers: 1. spread of cancer cells to other parts of the body to form secondary tumours

23 Practical: observing specialized cells

Prepared slides.

Prepared slides of blood smears, muscle tissue, root tips and leaf sections show specialized cells. Draw cells with scale bars and annotate adaptations.

WORKED EXAMPLE 15 Calibration (standard)

Why must an eyepiece graticule be calibrated?

Solution

Its divisions have no fixed size; their value depends on the objective lensR1

Try this

Examine a blood smear slide. Estimate the diameter of a red blood cell using an eyepiece graticule calibrated with a stage micrometer.

24 Tissues, organs and systems

Levels of organization.

Specialized cells form tissues (groups of similar cells with a common function), tissues form organs, and organs form organ systems. For example, epithelial, muscle and nervous tissues make up the stomach, part of the digestive system.

Quick check

1. Give the levels of organization in order.

Answers: 1. cell, tissue, organ, organ system, organism

25 Common misconceptions and links

Students often think…	Actually…
different cells have different genes	they have the same genes but express different ones
larger cells have a larger SA:V ratio	larger cells have a smaller ratio
all stem cells can become any cell	potency varies: totipotent, pluripotent, multipotent

How B2.3 connects to the rest of the course

Topic	Connection
A2.2 Cell structure	eukaryotic cells; differentiation (HL)
D2.2 Gene expression (HL)	control of which genes are switched on
B3.1 Gas exchange	surface area adaptations
D3.1 Reproduction	gametes
B3.3 Muscle (HL)	muscle fibre structure

26 Exam-style question with markscheme

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

Figure 2. Surface area to volume ratio of cubes.

(a) Calculate the surface area to volume ratio of a cube of side 5 µm. [2]

(b) Explain why cells remain small. [3]

(d) Outline one therapeutic use of stem cells. [2]

(e) Explain how type I pneumocytes are adapted for gas exchange. [2]

Markscheme

Part	Answer	Marks
(a)	SA 150 µm², V 125 µm³	M1
	ratio 1.2 : 1	A1
(b)	volume increases faster than surface area	R1
	exchange depends on surface; demand on volume	R1
	large cells could not exchange materials fast enough	R1
(c)	totipotent: any cell including placenta	A1
	pluripotent: almost any body cell, not placenta	A1
(d)	e.g. bone marrow transplant for leukaemia	A1
	replaces blood-forming cells	R1
(e)	very thin	R1
	short diffusion distance	R1

Examiner's comments. In (b), the best answers explained both the supply (surface) and the demand (volume) sides.

27 Check your understanding

Section A: multiple choice

1. Totipotent stem cells can form

A. any cell type including placenta B. only blood cells C. only nerve cells D. almost any body cell but not placenta

2. Bone marrow stem cells are

A. totipotent B. pluripotent C. multipotent D. unipotent only

3. The SA:V ratio of a 2 µm cube is

A. 6 : 1 B. 3 : 1 C. 1.5 : 1 D. 2 : 1

4. Microvilli increase

A. volume B. cell division C. protein synthesis D. surface area for absorption

5. Differentiation involves

A. loss of genes B. expression of different genes C. gaining genes D. changing DNA sequence

6. A morphogen is

A. a signal molecule that forms a concentration gradient B. a type of stem cell C. an enzyme D. a hormone in plants only

7. Surfactant is secreted by

A. type I pneumocytes B. red blood cells C. capillary cells D. type II pneumocytes

Section B: structured

8. Explain why the surface area to volume ratio limits cell size. [3]

9. Discuss the ethics of using embryonic stem cells. [4]

10. Explain how a root hair cell is adapted to its function. [3]

11. Compare cardiac and skeletal muscle cells. [3]

Answers

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

8. Volume grows faster than surface; exchange insufficient; diffusion distance too long.

9. Against: embryo destroyed; for: surplus IVF embryos, medical benefit; alternatives (adult, iPS).

10. Long extension increases surface; thin wall; mitochondria for active transport of minerals.

11. Skeletal: long, multinucleate, unbranched; cardiac: branched, single nucleus, intercalated discs.

28 Summary and topic map

Idea	Remember
Differentiation	same genes, different expression; morphogens
Stem cells	self-renewal and differentiation
Potency	totipotent, pluripotent, multipotent
Niches and uses	bone marrow; leukaemia treatment
SA:V ratio	falls as size increases; limits cell size
Adaptations	flattening, microvilli, elongation
HL	pneumocytes, muscle cells, gametes

Before the exam, make sure you…

•show SA, volume and ratio separately
•define stem cell potency precisely
•link each adaptation to its function
•present both sides of ethical debates

Cell Specialization 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 3. Topic map for Cell Specialization.

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