DP Biology SL B3.2 Transport

DP Biology SL B3.2 Transport - Full notes

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DP Biology SL | B3.2 Transport

DP BIOLOGY · THEME B: FORM AND FUNCTION

B3.2

Transport

Full notes for SL · IB-style

Contents

1	Why organisms need transport systems
2	Blood vessels: arteries, capillaries and veins
3	Pulse and coronary heart disease
4	Transport of water in plants: xylem and transpiration
5	Root uptake of water and minerals
6	Plant stems and roots in section
7	Arteries, veins and capillaries compared
8	Identifying vessels in micrographs
9	Measuring pulse rate
10	Coronary heart disease
11	Tissue fluid formation and return
12	The structure of the heart (introduction)
13	Water transport in plants: xylem
14	Factors affecting transpiration
15	Root pressure and water uptake
16	Phloem and translocation (introduction)
17	Comparing xylem and phloem
18	Mixed review: worked problems
19	Additional exam-style practice
20	Blood pressure measurement
21	Transport in other animals
22	Adaptations of plants to water stress
23	Common misconceptions and links
24	Exam-style question with markscheme
25	Check your understanding
26	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 why multicellular organisms need transport systems
relate the structures of arteries, capillaries and veins to their functions
explain how pulse rate is measured and the causes and consequences of coronary heart disease
explain the transport of water in xylem by cohesion-tension
explain how roots absorb water and minerals
identify tissues in sections of stems and roots

This booklet covers the SL content of this subtopic. Additional Higher Level material is not included.

Guiding questions

What adaptations facilitate transport of fluids around organisms? How do animals and plants move substances over long distances?

How this topic is examined

B3.2 is examined through diagrams and micrographs of blood vessels, stems and roots, data on blood pressure or transpiration, and explanations of xylem transport.

Answers should relate each structural feature (wall thickness, lumen size, valves, lignin) to its function.

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 Why organisms need transport systems

Diffusion is too slow over long distances.

Think about it

Oxygen takes about 2 seconds to diffuse 0.1 mm, but would take years to diffuse 1 m. How do large organisms get oxygen to every cell?

Single-celled and very small organisms can rely on diffusion to move substances. Large multicellular organisms cannot: their cells are far from the surface, their surface area to volume ratio is small, and their metabolic demands are high. They need mass transport systems that move fluids in bulk over long distances: the blood circulatory system in animals and xylem and phloem in plants.

In humans, blood carries oxygen, carbon dioxide, nutrients, wastes, hormones, antibodies and heat. In plants, xylem carries water and mineral ions from roots to leaves; phloem carries sugars and other organic compounds from where they are made or stored to where they are used.

2 Blood vessels: arteries, capillaries and veins

Each vessel is built for the pressure it carries.

Arteries carry blood away from the heart at high pressure. They have a thick wall with a thick layer of smooth muscle and elastic fibres, and a relatively narrow lumen. Elastic fibres stretch as blood surges in with each heartbeat and recoil between beats, maintaining pressure and smoothing flow. The thick wall withstands high pressure. The inner lining (endothelium) is smooth to reduce friction.

Capillaries are where exchange happens between blood and tissues. Their wall is just one cell thick (endothelium), giving a very short diffusion distance; their lumen is just wide enough for red blood cells (about 5 to 10 µm); they form dense networks giving a large surface area; and many have pores (fenestrations) allowing fluid and small molecules out. Blood flows slowly through them, allowing time for exchange.

Veins carry blood back to the heart at low pressure. They have a thin wall with little muscle and elastic tissue, a wide lumen (less resistance to flow) and valves that prevent backflow. Blood is pushed along by the contraction of surrounding skeletal muscles squeezing the veins.

Feature	Artery	Capillary	Vein
Wall	thick: muscle and elastic fibres	one cell thick	thin; little muscle
Lumen	narrow	very narrow (about 5–10 µm)	wide
Pressure	high, pulsatile	falling	low
Valves	no (except at heart exit)	no	yes
Function	carry blood from heart	exchange with tissues	return blood to heart

Common mistake

Saying arteries carry oxygenated blood and veins deoxygenated. Arteries carry blood away from the heart; the pulmonary artery carries deoxygenated blood.

Quick check

1. Why do arteries have elastic fibres?

2. How are capillaries adapted for exchange?

3. Why do veins have valves?

Answers: 1. stretch and recoil to maintain pressure 2. one-cell wall, pores, narrow lumen, large network 3. prevent backflow at low pressure

3 Pulse and coronary heart disease

The pulse is the artery stretching with each heartbeat.

Each heartbeat sends a surge of blood into the arteries, which stretch and recoil. This can be felt as the pulse in arteries close to the skin, such as the radial artery at the wrist or the carotid artery in the neck. Pulse rate equals heart rate. It can be measured by counting beats for 30 seconds and multiplying by two, or with a digital monitor.

Coronary heart disease (CHD) occurs when the coronary arteries, which supply the heart muscle itself with blood, become narrowed or blocked. Fatty deposits (atheroma, or plaque) build up in the artery wall in a process called atherosclerosis. The lumen narrows, reducing blood flow to the heart muscle and causing chest pain (angina). If a plaque ruptures, a blood clot (thrombosis) can form and completely block the artery, causing a heart attack (myocardial infarction): part of the heart muscle dies from lack of oxygen.

Risk factors include high blood pressure, smoking, high levels of LDL cholesterol, obesity, diabetes, lack of exercise, high salt intake, increasing age, male sex and family history. Epidemiological studies show correlations between these factors and CHD; careful study design is needed to show that a factor actually causes the disease.

Quick check

1. What causes coronary heart disease?

2. Give four risk factors for CHD.

Answers: 1. atherosclerosis narrowing or blocking coronary arteries 2. smoking, high blood pressure, high LDL cholesterol, obesity, inactivity, diabetes

4 Transport of water in plants: xylem and transpiration

Water is pulled up by evaporation from leaves.

Xylem vessels are long, continuous tubes formed from dead cells joined end to end, with the end walls broken down. Their walls are strengthened with lignin, deposited in rings or spirals, which prevents collapse under tension and makes them waterproof. Pits in the walls allow water to move sideways between vessels.

Water moves up the xylem by the cohesion-tension mechanism. Transpiration, the evaporation of water from the leaves, draws water out of the xylem in the leaf. Because water molecules are cohesive (hydrogen bonds, A1.1), the column of water is pulled upward, creating tension (negative pressure) in the xylem all the way down to the roots. Adhesion of water to the hydrophilic cellulose walls also helps. This is a passive process: the energy comes from the Sun, which drives evaporation.

Evidence for tension: tree trunks shrink slightly in diameter during the day when transpiration is fastest; if a xylem vessel is cut, air is drawn in rather than water leaking out.

Figure 1. The cohesion-tension mechanism of water transport.

Quick check

1. Why do xylem walls contain lignin?

2. What provides the energy for water movement in xylem?

3. Explain the role of cohesion.

Answers: 1. strengthen against collapse under tension; waterproof 2. solar energy driving evaporation 3. hydrogen bonds hold molecules together so the column is pulled up

5 Root uptake of water and minerals

Roots actively load minerals; water follows by osmosis.

Roots absorb water and mineral ions from the soil, mainly through root hair cells, which greatly increase the surface area. Mineral ions such as nitrate, phosphate and potassium are usually at lower concentration in the soil than in the root, so they are taken up by active transport, using ATP from respiration in the root cells (many mitochondria).

The accumulation of minerals lowers the water potential of root cells, so water enters by osmosis (through aquaporins). Water then moves across the root cortex to the xylem, either through cell walls (apoplast) or through the cytoplasm (symplast). At the endodermis, a waterproof band (the Casparian strip) blocks the cell-wall route, so water and minerals must pass through the cytoplasm of endodermal cells, which allows the plant to control which minerals enter the xylem.

Quick check

1. How are mineral ions absorbed by roots?

2. Why does water enter root hair cells?

Answers: 1. active transport using ATP 2. osmosis: lower water potential in root cells due to absorbed minerals

6 Plant stems and roots in section

Recognize the tissues in micrographs.

In a dicotyledonous stem cross-section, the vascular bundles are arranged in a ring near the outside. Each bundle has xylem towards the centre and phloem towards the outside, with cambium between them. The outer epidermis is covered with cuticle; inside is the cortex, and the centre is pith. The ring arrangement resists bending.

In a dicotyledonous root, the vascular tissue is in the centre, with xylem in a star shape (often X-shaped) and phloem between the arms. This central arrangement resists the pulling forces roots experience. The root is surrounded by the epidermis with root hairs, a wide cortex and the endodermis.

Feature	Dicot stem	Dicot root
Vascular tissue	bundles in a ring	central cylinder
Xylem	inner side of each bundle	star / X shape
Phloem	outer side of each bundle	between xylem arms
Mechanical role	resists bending	resists pulling

7 Arteries, veins and capillaries compared

Structure fits function.

Feature	Artery	Vein	Capillary
wall	thick, muscular, elastic	thin, little muscle	one cell thick
lumen	narrow	wide	very narrow (about 8 μm)
valves	none (except heart exits)	present	none
pressure	high, pulsatile	low	low, falling
function	carry blood away from heart	return blood to heart	exchange with tissues

WORKED EXAMPLE 1 Arteries (exam level)

Explain how the structure of arteries suits their function.

Solution

Thick muscle and elastic tissue withstand high pressureR1

Elastic recoil maintains pressure between beatsR1

8 Identifying vessels in micrographs

Using wall thickness.

In cross-sections, arteries have thick walls with a narrow, round lumen; veins have thinner walls with a wide, often irregular lumen; capillaries are tiny, with walls one cell thick, often containing a single red blood cell.

WORKED EXAMPLE 2 Ratio (standard)

An artery has wall thickness 1.2 mm and lumen diameter 2.0 mm; a vein has wall 0.5 mm and lumen 5.0 mm. Calculate wall-to-lumen ratios.

Solution

Artery 0.60; vein 0.10A1

Higher ratio identifies the arteryR1

9 Measuring pulse rate

Heart rate from arteries.

Each heartbeat sends a pressure wave through the arteries, felt as a pulse at the wrist (radial artery) or neck (carotid). Count beats for 15 s and multiply by 4, or use digital monitors. Exercise, temperature and stress increase pulse rate.

WORKED EXAMPLE 3 Calculation (basic)

21 beats are counted in 15 s. Calculate the heart rate.

Solution

21 × 4M1

= 84 beats per minuteA1

Try this

Measure your resting pulse rate three times, then after 2 minutes of step-ups. How long does it take to return to resting?

10 Coronary heart disease

Blocked coronary arteries.

Coronary arteries supply the heart muscle. Atherosclerosis (fatty plaques under the endothelium) narrows them, reducing blood flow (angina). A ruptured plaque can cause a clot (thrombosis) and a heart attack (myocardial infarction). Risk factors: high LDL cholesterol, smoking, high blood pressure, obesity, inactivity, genetics.

WORKED EXAMPLE 4 Risk factors (standard)

List three risk factors for coronary heart disease.

Solution

Smoking, high blood pressureA1

High saturated fat diet (LDL), obesity, lack of exerciseA1

Quick check

1. Explain how atherosclerosis can cause a heart attack.

Answers: 1. plaque narrows artery; rupture causes clot; heart muscle deprived of oxygen dies

11 Tissue fluid formation and return

Exchange at capillaries.

At the arterial end of a capillary, hydrostatic pressure forces fluid (water, glucose, ions, O2) out through gaps between cells, forming tissue fluid. At the venous end, lower hydrostatic pressure and the solute potential of plasma proteins draw some fluid back. Excess drains into lymph vessels and returns to the blood.

WORKED EXAMPLE 5 Pressure filtration (exam level)

Explain why proteins remain in the capillaries.

Solution

Proteins are too large to pass through the gaps between endothelial cellsR1

They lower the water potential of plasma, drawing water back at the venous endR1

Quick check

1. What happens if lymph vessels are blocked?

Answers: 1. tissue fluid accumulates (oedema)

12 The structure of the heart (introduction)

A double pump.

The heart has four chambers: two atria and two ventricles. The right side pumps deoxygenated blood to the lungs; the left side pumps oxygenated blood to the body. The left ventricle wall is thicker because it pumps blood at higher pressure around the whole body. Valves prevent backflow.

WORKED EXAMPLE 6 Wall thickness (standard)

Explain why the left ventricle has a thicker wall than the right.

Solution

Pumps blood to the whole body, against greater resistanceR1

Generates higher pressureR1

13 Water transport in plants: xylem

Transpiration pull.

Water moves from roots to leaves through xylem vessels by transpiration pull. Evaporation from leaves lowers water potential, creating tension; cohesion holds the column together; adhesion to lignified walls supports it. Xylem vessels are dead, with no end walls, and are reinforced with lignin.

WORKED EXAMPLE 7 Lignin (standard)

Explain why xylem vessel walls need lignin.

Solution

Withstand the tension (negative pressure) without collapsingR1

14 Factors affecting transpiration

Measuring with a potometer.

Factor	Effect on transpiration	Reason
light intensity	increases	stomata open
temperature	increases	faster evaporation and diffusion
humidity	decreases	smaller water vapour gradient
wind	increases	removes humid air near stomata

WORKED EXAMPLE 8 Potometer (exam level)

A bubble in a potometer capillary (radius 0.5 mm) moves 30 mm in 5 minutes. Calculate the rate of water uptake.

Solution

Volume = π(0.5)2 × 30 = 23.6 mm3M1

Rate = 4.7 mm3 min−1A1

Quick check

1. Why does a potometer measure water uptake, not transpiration?

Answers: 1. some water is used in photosynthesis or retained in cells

15 Root pressure and water uptake

Entering the roots.

Root hair cells absorb water by osmosis because they actively take up mineral ions, lowering their water potential. Water moves across the root cortex to the xylem. Active pumping of ions into the xylem can generate root pressure, seen as guttation.

Quick check

1. Explain why root hairs need mitochondria.

Answers: 1. ATP for active uptake of mineral ions

16 Phloem and translocation (introduction)

Moving sugars.

Phloem transports sucrose and other organic compounds from sources (leaves) to sinks (roots, fruits, growing tips) in sieve tube elements, supported by companion cells. Unlike xylem, flow can be in either direction and requires energy.

WORKED EXAMPLE 9 Source and sink (standard)

Explain why a potato tuber is a sink in summer but a source in spring.

Solution

Summer: stores sugars as starchR1

Spring: starch broken down and sugars transported to new shootsR1

17 Comparing xylem and phloem

Two transport tissues.

Feature	Xylem	Phloem
cells	dead vessels	living sieve tubes and companion cells
transports	water and minerals	sucrose, amino acids
direction	upwards only	up or down
energy	passive	active loading
walls	lignified	cellulose; sieve plates

18 Mixed review: worked problems

Bringing the ideas together.

WORKED EXAMPLE 10 Mixed problem (exam level)

Explain how blood pressure and the structure of capillaries allow exchange of materials with tissues.

Solution

High hydrostatic pressure at arterial end forces fluid outR1

Thin walls with gaps; large surface areaR1

Fluid returns at venous end; excess via lymphR1

19 Additional exam-style practice

Paper 1 and Paper 2.

WORKED EXAMPLE 11 Paper 1 question (standard)

Which vessel has valves and a wide lumen? A. artery B. arteriole C. capillary D. vein

Solution

DA1

WORKED EXAMPLE 12 Paper 2 question (exam level)

Explain the cohesion–tension theory of water movement in xylem.

Solution

Transpiration creates tension in leavesR1

Cohesion between water molecules transmits the pullR1

Adhesion to xylem walls; continuous columnR1

20 Blood pressure measurement

Systolic and diastolic.

Blood pressure is given as systolic over diastolic in mmHg (e.g. 120/80). High blood pressure (hypertension) damages arteries and increases the risk of CHD and stroke.

Quick check

1. What does the 120 in 120/80 represent?

Answers: 1. systolic pressure (when the ventricles contract)

21 Transport in other animals

Open and closed systems.

Insects have an open circulatory system: haemolymph bathes organs directly in a body cavity. Vertebrates have closed systems. Fish have a single circulation (heart → gills → body); mammals have a double circulation, maintaining high pressure to the body.

WORKED EXAMPLE 13 Double circulation (standard)

Explain the advantage of a double circulation.

Solution

Blood returns to the heart after the lungsR1

Pumped again at high pressure to the bodyR1

22 Adaptations of plants to water stress

Saving water.

Xerophytes (cacti, marram grass) reduce water loss with thick waxy cuticles, rolled leaves, sunken stomata, hairs and reduced leaf area; some use CAM photosynthesis, opening stomata at night. Halophytes tolerate salty soils.

Quick check

1. Why do rolled leaves reduce transpiration?

Answers: 1. trap humid air near stomata, reducing the diffusion gradient

23 Common misconceptions and links

Students often think…	Actually…
arteries always carry oxygenated blood	the pulmonary artery carries deoxygenated blood
xylem pumps water upward	water is pulled by tension from transpiration
mineral uptake is by diffusion	it is mainly active transport

How B3.2 connects to the rest of the course

Topic	Connection
A1.1 Water	cohesion and adhesion
B3.1 Gas exchange	transpiration and blood gases
B2.1 Membranes (HL)	active transport and osmosis
D2.3 Water potential	osmosis in roots
C3.1 Integration	heart rate control

24 Exam-style question with markscheme

Try this under exam conditions (about 18 minutes) before reading the markscheme.

Text A. A micrograph shows two blood vessels: vessel X has a thick wall and narrow round lumen; vessel Y has a thin wall and wide irregular lumen.

(a) Compare the structure of arteries and veins. [3]

(b) Explain how capillaries are adapted for exchange. [3]

(d) Outline the causes of coronary heart disease. [3]

Markscheme

Part	Answer	Marks
(a)	arteries thicker walls, more muscle and elastic tissue	A1
	arteries narrower lumen	A1
	veins have valves	A1
(b)	wall one cell thick	R1
	pores / fenestrations	R1
	large network; slow flow	R1
(c)	transpiration from leaves	R1
	cohesion between water molecules	R1
	tension pulls column up; adhesion to walls	R1
(d)	atheroma in coronary arteries	R1
	narrows lumen; reduces flow	R1
	clot blocks artery; heart muscle deprived of oxygen	R1

Examiner's comments. In (c), weaker answers said water is 'sucked up by the roots'. The driving force is transpiration from leaves.

25 Check your understanding

Section A: multiple choice

1. Which vessel has valves?

A. vein B. artery C. capillary D. aorta

2. Capillary walls are

A. muscular B. elastic C. one cell thick D. lignified

3. Elastic fibres in arteries

A. prevent backflow B. maintain pressure between beats C. allow exchange D. carry oxygen

4. Water moves up xylem mainly because of

A. active transport B. root pressure only C. gravity D. transpiration and cohesion

5. Mineral ions enter root hairs by

A. osmosis B. active transport C. simple diffusion only D. mass flow

6. Atherosclerosis is

A. fatty deposits narrowing arteries B. valve failure C. a blood clot in a vein D. a heart rhythm problem

Section B: structured

7. Relate the structure of arteries to their function. [4]

8. Explain the cohesion-tension theory. [4]

9. Distinguish between stems and roots in section. [3]

Answers

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

7. Thick muscular, elastic wall withstands and maintains pressure; narrow lumen; smooth endothelium.

8. Transpiration; cohesion; tension; adhesion; continuous column; passive.

9. Stem: bundles in ring; root: central star-shaped xylem.

26 Summary and topic map

Idea	Remember
Need	diffusion too slow; mass transport
Arteries	thick, muscular, elastic; high pressure
Capillaries	one cell thick; exchange
Veins	thin, wide lumen, valves
CHD	atherosclerosis; risk factors
Xylem	lignin; cohesion-tension; transpiration
Roots	active mineral uptake; osmosis

Before the exam, make sure you…

•relate each vessel feature to function
•explain xylem transport starting from transpiration
•distinguish correlation from causation for CHD risk factors
•identify stem and root sections

Transport 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 2. Topic map for Transport.

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