2: Exchange & Transport Systems for A-Level Biology (OCR B

What mathematical relationship compares the surface area available for exchange with the internal volume of a cell or organism?

Surface area to volume ratio

Often written as SA:V ratio; determines how efficiently substances can move in or out.

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As the size of a cell increases, how does the ratio of surface area to internal volume change?

Decreases

Volume increases faster than surface area as size increases.

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Why do very small organisms rely mainly on diffusion across their body surface for exchange?

Large surface area to volume ratio

A high ratio allows sufficient exchange of substances directly across the surface.

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Why is diffusion alone often insufficient for exchange in large multicellular organisms?

Small surface area to volume ratio

As organisms grow, their volume increases faster than surface area, limiting diffusion efficiency.

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What general type of biological feature develops in larger organisms to help maintain efficient exchange with the environment?

Exchange systems

Examples include lungs, gills, and circulatory systems.

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True or False:

As an organism grows larger, its volume increases at a faster rate than its surface area.

True

This mathematical relationship explains why SA:V decreases with increasing size.

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True or False:

Large organisms usually rely entirely on diffusion across their body surface for gas exchange.

False

Most large organisms require specialised exchange surfaces and transport systems.

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Fill in the blank:

As cells become larger, their ______ increases more rapidly than their surface area.

Volume

This leads to a reduced surface area to volume ratio.

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How does a higher surface area to volume ratio affect the rate of diffusion into or out of a cell?

Increases diffusion rate

More surface is available relative to the internal volume requiring exchange.

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What type of relationship exists between surface area to volume ratio and metabolic rate in organisms?

Positive relationship

Organisms with higher SA:V generally have higher metabolic rates.

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True or False:

Cells with a higher surface area to volume ratio typically exchange substances more efficiently with their environment.

True

A greater relative surface area allows faster diffusion.

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Fill in the blank:

In diffusion experiments, agar blocks containing indicator can be used to investigate how _______________________________ affects the rate at which a substance diffuses.

Surface area to volume ratio

Smaller agar cubes change colour more quickly due to higher SA:V.

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What mathematical skill is required when comparing exchange efficiency in cells of different shapes or sizes?

Calculating surface area to volume ratios

Students may calculate SA and volume from given dimensions of cells.

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What feature of gas exchange surfaces shortens the distance gases must diffuse between organisms and their environment?

Thin diffusion surface

Efficient gas exchange surfaces minimise the path length for diffusion.

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What process allows oxygen to enter and carbon dioxide to leave a single-celled organism across its surface membrane?

Diffusion

Movement occurs down a concentration gradient without energy input.

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In insects, what small external openings allow gases to enter the tracheal system?

Spiracles

Spiracles can open or close to regulate gas exchange and limit water loss.

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What are the larger air tubes in insects that carry air from spiracles into the body?

Tracheae

These tubes are strengthened by chitin to prevent collapse.

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What very fine tubes in insects deliver oxygen directly to respiring tissues?

Tracheoles

Tracheoles provide a large surface area for gas exchange with cells.

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In fish gills, what thin plates contain capillaries and provide a large surface for gas exchange?

Gill lamellae

These structures maximise surface area and minimise diffusion distance.

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What larger structures in fish gills support rows of lamellae?

Gill filaments

Water flows over filaments while blood flows through lamellae.

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What principle describes water flowing over fish gills in the opposite direction to blood flow?

Counter-current principle

Maintains a diffusion gradient along the entire length of the lamellae.

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True or False:

In the counter-current principle, blood and water flow in the same direction across the gill surface.

False

Opposite flow maintains a steep diffusion gradient for oxygen uptake.

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What tiny adjustable pores in dicot leaves allow gases to enter and leave the leaf?

Stomata

Guard cells control the opening and closing of stomata.

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What internal leaf tissue contains large air spaces that facilitate gas diffusion?

Mesophyll

The spongy mesophyll provides a large internal surface area.

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What **compromise** must terrestrial insects balance when regulating **spiracle opening**?

Gas exchange and water loss ## Footnote Opening spiracles allows oxygen entry but increases evaporation.

What type of **plant** shows adaptations to **reduce water loss** in dry environments?

Xerophytic plants ## Footnote Examples include cacti and marram grass with specialised leaf structures.

What small **air sacs** in the lungs provide the main **surface for gas exchange**?

Alveoli ## Footnote Humans have millions of alveoli creating a very large surface area.

What small **airways** branch from **bronchi** and lead to alveoli?

Bronchioles ## Footnote They regulate airflow within the lungs.

What two **tubes** carry air from the **trachea** into each lung?

Bronchi ## Footnote Each bronchus branches repeatedly to form bronchioles.

What structure connects the **bronchi** to the **throat** and is supported by rings of cartilage?

Trachea ## Footnote The cartilage prevents the airway from collapsing during breathing.

What **muscular sheet** separates the **thoracic cavity** from the abdominal cavity and plays a key role in breathing?

Diaphragm ## Footnote Contraction flattens the diaphragm and increases thoracic volume.

During **inhalation**, what happens to the **diaphragm**?

Contracts and flattens ## Footnote This increases thoracic volume and lowers pressure in the lungs.

# True or False: **External intercostal muscles** contract during inhalation to **raise the rib cage**.

True ## Footnote This increases thoracic volume and helps draw air into the lungs.

What **equation** is used to calculate **pulmonary ventilation rate** from breathing measurements?

PVR = tidal volume × breathing rate

# Fill in the blank: **Pulmonary ventilation rate** equals **tidal volume** multiplied by \_\_\_\_\_\_.

Breathing rate ## Footnote Expressed as PVR = tidal volume × breathing rate.

What feature of the **alveolar epithelium** allows **rapid gas diffusion** between air and blood?

Single cell thick ## Footnote The epithelium and capillary walls form a very thin diffusion barrier.

# True or False: **Smoking** and **air pollution** can increase the incidence of lung disease by damaging **gas exchange surfaces**.

True ## Footnote Damage can reduce efficiency of ventilation and gas exchange.

What **protein** in **red blood cells** is responsible for transporting **oxygen** in the blood?

Haemoglobin ## Footnote A globular protein with a quaternary structure that binds oxygen reversibly.

Which **cells in the blood** are specialised for carrying **oxygen** around the body?

Red blood cells ## Footnote Also called erythrocytes; they contain haemoglobin and lack a nucleus in mammals.

What is the name of the **curve** that shows the **percentage saturation of haemoglobin** with oxygen at different **partial pressures of oxygen**?

Oxyhaemoglobin dissociation curve ## Footnote Demonstrates how readily haemoglobin loads and unloads oxygen under different conditions.

What property of **haemoglobin** means that the binding of the **first oxygen molecule** increases the affinity for additional oxygen molecules?

Cooperative binding ## Footnote Caused by a conformational change in haemoglobin after the first oxygen binds.

How does increasing **carbon dioxide concentration** affect the **affinity of haemoglobin for oxygen**?

Decreases ## Footnote This shift promotes oxygen release in respiring tissues.

What term describes the effect of **carbon dioxide concentration** on the **dissociation of oxyhaemoglobin**?

Bohr effect ## Footnote Higher CO₂ lowers haemoglobin affinity for oxygen, aiding oxygen delivery to tissues.

# True or False: The **oxyhaemoglobin dissociation curve** is linear because each **oxygen molecule binds independently**.

False ## Footnote The curve is sigmoidal due to cooperative binding between haemoglobin subunits.

Why do animals living in **low-oxygen environments** often possess different types of **haemoglobin**?

They have different oxygen affinity. ## Footnote Adapted haemoglobins bind oxygen more readily when oxygen availability is low.

Which **blood vessels** supply **oxygenated blood** directly to the **heart muscle**?

Coronary arteries ## Footnote These branch from the aorta and provide the heart’s own oxygen supply.

Which **blood vessel** carries **deoxygenated blood** from the body into the **right atrium**?

Vena cava ## Footnote Includes the superior and inferior vena cava.

Which **artery** carries **deoxygenated blood** from the heart to the **lungs**?

Pulmonary artery ## Footnote The only artery that normally carries deoxygenated blood.

Which **vein** carries **oxygenated blood** from the **lungs to the heart**?

Pulmonary vein ## Footnote Returns oxygenated blood to the left atrium.

What term describes the **volume of blood pumped** by one **ventricle of the heart per beat**?

Stroke volume ## Footnote Usually measured in cm³ per beat.

What term describes the **volume of blood pumped** by the **heart per minute**?

Cardiac output ## Footnote Calculated as stroke volume × heart rate.

# Fill in the blank: Cardiac output equals **stroke volume** multiplied by \_\_\_\_\_\_.

heart rate ## Footnote CO = stroke volume × heart rate.

What **structure in the heart** prevents **backflow of blood** during the **cardiac cycle**?

Valves ## Footnote Include atrioventricular valves and semilunar valves.

# True or False: During **ventricular systole**, the **semilunar valves** open to allow blood to **leave the heart**.

True ## Footnote Blood is ejected into the aorta and pulmonary artery.

Which type of **blood vessel** has **thick muscular walls** and **elastic fibres** to withstand high pressure?

Arteries ## Footnote Their elastic tissue allows stretching and recoil during the cardiac cycle.

Which small **blood vessels** control the **distribution of blood to capillary beds**?

Arterioles ## Footnote Smooth muscle in their walls allows vasoconstriction and vasodilation.

Which type of **blood vessel** contains **valves** that help **return blood to the heart**?

Veins ## Footnote Blood pressure is low, so valves prevent backflow.

Which **blood vessels** consist of a **single layer of endothelial cells** for efficient **exchange of substances**?

Capillaries ## Footnote Their thin walls allow diffusion between blood and tissues.

What **network of capillaries** within a **tissue** provides a large **surface area for exchange**?

Capillary bed ## Footnote Enables efficient transfer of gases, nutrients and wastes.

What **fluid** surrounds **cells** and forms when **plasma is forced out of capillaries**?

Tissue fluid ## Footnote Formed by hydrostatic pressure at the arterial end of capillaries.

What causes fluid to **leave capillaries at the arterial end** to form **tissue fluid**?

High hydrostatic pressure ## Footnote Driven by hydrostatic pressure exceeding oncotic pressure.

Which **system** returns **excess tissue fluid** to the **blood circulation**?

Lymphatic system ## Footnote Lymph vessels eventually return fluid to the bloodstream.

Which **plant tissue** is responsible for transporting **water and dissolved mineral ions** from the roots to the leaves?

Xylem ## Footnote Xylem vessels are formed from dead cells and provide a continuous column for water movement.

Which **plant tissue** transports **sugars and other organic molecules** around the plant?

Phloem ## Footnote Phloem transports products of photosynthesis such as sucrose to growing or storage tissues.

What **theory** explains how **water moves upward through the xylem** due to evaporation from leaves and the cohesion of water molecules?

Cohesion–tension theory ## Footnote Transpiration creates tension that pulls a continuous column of water upward through the xylem.

What **process at the leaf surface** generates the **pulling force** that moves water up the xylem?

Transpiration ## Footnote Water evaporates from mesophyll cell walls and diffuses out through stomata.

# Fill in the blanks: The movement of dissolved sugars through the **phloem** from **sources to sinks** is explained by the \_\_\_\_\_\_ \_\_\_\_\_\_ hypothesis.

Mass flow ## Footnote The hypothesis proposes that pressure differences drive the bulk movement of sap through sieve tubes.

What type of **molecule** is most commonly transported in the **phloem** as the main product of photosynthesis?

Sucrose ## Footnote Sucrose is the main sugar transported because it is soluble and chemically stable.

What term describes a **region of a plant** that produces or releases **sugars for transport in the phloem**?

Source ## Footnote Examples include photosynthesising leaves or storage organs releasing sugars.

What term describes a **region of a plant** that **uses or stores transported sugars**?

Sink ## Footnote Examples include growing roots, fruits, and developing buds.

# True or False: **Xylem vessels** are made of **living cells** that actively pump water upwards.

False ## Footnote Mature xylem vessels are dead and transport water passively.

# True or False: The **cohesion of water molecules** helps maintain a **continuous water column in the xylem**.

True ## Footnote Hydrogen bonding between water molecules allows the column to resist breaking.

What **experimental method** uses **radioactive or labelled carbon compounds** to track movement of substances through plants?

Tracer experiments ## Footnote Radioactive carbon dioxide can be used to label sugars formed during photosynthesis.

What **experimental method** involves **removing a ring of bark from a stem** to investigate transport pathways?

Ringing experiment ## Footnote Removing bark removes phloem but leaves xylem intact.

# True or False: If a **ring of bark** is removed from a tree, **sugars accumulate above the cut** because phloem transport is interrupted.

True ## Footnote This supports the role of phloem in translocation of organic substances.

In the **mass flow mechanism**, what drives the **movement of phloem sap from source to sink**?

Pressure gradient ## Footnote High hydrostatic pressure at sources pushes sap towards lower pressure at sinks.

2: Exchange & Transport Systems Flashcards

Explain how organisms exchange and transport substances efficiently. (76 cards)