mod 3 ocr alevel bio

Why do multicellular organisms need specialised exchange surfaces but single celled organisms don't?

single celled organisms can exchange materials across their CSM to meet requirements - metabolic activity low → O2 needs low + CO2 produced is low - large SA:V ratio; Multicellular organisms are the opposite - high metabolic rate (active + maintain temp) - small SA:V ratio

3 main features of an efficient exchange surface

• increased surface area • thin layer • good blood supply/ventilation to maintain gradient

how to calculate surface area to volume ration using a sphere

• calculate SA with 4π² - calculate volume with 4/3πr³ - ratio = surface area/volume; as radius increases, SA:V ratio decreases

How does increased SA aid diffusion?

larger SA:V ratio → bigger area for exchange (e.g. root hair cells, villi)

How do thin layers aid diffusion?

reduces diffusion distance (e.g. alveoli)

How does a good blood supply aid diffusion?

Increases concentration gradient → substances constantly delivered and removed (e.g. alveoli, gills, villi)

Nasal cavity features

• large SA with good blood supply - goblet cells secrete mucus to trap dust + bacteria - moist surface so gases dissolve helping them pass across gas exchange surface

trachea structure

• supported by incomplete rings of strong flexible cartilage - lined with ciliated epithelium (moves bacteria out of lungs) - goblet cells (mucus to trap bacteria) - smooth muscle and elastic fibres

Bronchiole structure

• smooth muscle (contracts to constrict airways) - elastic fibres - ciliated epithelium - goblet cells

alveoli structure

• thin flattened epithelial cells (short diffusion pathway) - elastin + collagen (stretch and recoil) - large surface area - surrounded by capillaries (good blood supply) - surfactant (reduces surface tension, speeds up gas transport) - good ventilation

what happens in the alveoli?

main gas exchange surfaces

what is the purpose of cartilage in the trachea?

prevents it collapsing on itself

describe route taken by air as it is inhaled

• through mouth → trachea → bronchi → bronchioles → alveoli

What is ventilation?

movement of air into and out of the lungs

what happens during inspiration?

• diaphragm contracts, flattens, lowers - external intercostal muscles contract → rib cage upwards and outwards - thorax volume ↑ → pressure ↓ - pressure in thorax lower than atmosphere so air is drawn in

what happens during expiration?

• diaphragm relaxes, dome shape - external intercostals relax → rib cage down + in - thorax volume ↓ → pressure ↑ - pressure in thorax higher than atmosphere so air forced out

Is inspiration active or passive?

Active

Is expiration active or passive?

Passive

What does a spirometer measure?

records volumes of air inspired and expired over time → spirograph

Tidal volume (TV)

volume of air breathed in, in one breath at rest (~500 cm³)

Expiratory reserve volume (ERV)

volume of air you can force out after a normal tidal expiration

Inspiratory reserve volume (IRV)

volume of air you can inspire above a tidal inspiration

Vital capacity (VC)

greatest volume of air in one breath; VC = IRV + ERV + TV

what is vital capacity affected by

age, sex, exercise, posture

residual volume (RV)

volume of air left in lungs after max exhalation (keeps alveoli partly inflated)

Total lung capacity

vital capacity + residual volume

Formula for ventilation rate

Tidal volume × breathing rate (units = dm³ min⁻¹)

how to calculate breathing rate

(no. of breaths × 60) ÷ no. of seconds

What does air enter and leave an insect through?

Spiracles along abdomen

what happens to air after it passes through spiracles?

enters trachea → tracheoles → O2 directly delivered to tissues

How is air drawn into trachea?

• insect pumps thorax + abdomen → changes volume + pressure in trachea → air drawn in or forced out

what does the trachea in insects contain

chitin to strengthen it

What is at the end of the tracheoles in insects?

tracheal fluid → O2 diffuses slower in fluid; during activity, fluid withdrawn so more SA exposed → ↑ O2 diffusion

structure of gills

• thousands of filaments - filaments covered in lamellae (thin, ↑ SA, short diffusion path) - large SA, good blood supply - operculum protects gills

What do fish need to maintain for efficient gas exchange?

Continuous flow of water over gills

process of ventilation in fish

• mouth opens → buccal cavity lowers → volume ↑ → pressure ↓ → water in - mouth closes → buccal cavity raises → pressure ↑ → water pushed into gill cavity - pressure in gill cavity forces operculum open → water out

What is counter current flow in fish

• water and blood flow in opposite directions → blood always meets water with higher O₂ conc → maintains diffusion gradient

How do fish slow the movement of water to allow more time for gas exchange?

Tips of adjacent filaments overlap

How does a countercurrent exchange system help fish?

Maintains steep concentration gradient

purpose of transport systems

• supply nutrients + oxygen - remove waste - hormone circulation - temperature maintenance - immune responses

Why do multicellular animals need transport systems?

• high metabolic demands (more O₂ + waste removal) - SA:V smaller in larger animals → diffusion too slow - longer diffusion pathways

Which circulatory system is found in insects?

Open system → haemolymph pumped into haemocoel, direct contact with cells, low pressure, no O₂/CO₂ transport

What is a closed circulatory system?

Blood confined to vessels, no direct contact with cells, distribution can be adjusted

Which circulatory system is found in fish?

Single closed system → blood passes heart once per full circuit: atrium → ventricle → gills → body → heart

How can fish be active with an inefficient single closed system?

• countercurrent gaseous exchange → efficient O₂ uptake - don’t maintain body temperature → lower metabolic demand

Which circulatory system is found in mammals?

Double closed system → right heart → lungs → left heart → body; blood passes heart twice per circuit; 4 chambers

Why do blood vessels contain elastin?

Stretch and recoil, flexibility

Why do blood vessels contain smooth muscle?

Contracts/relaxes to change lumen size

Why do blood vessels contain collagen?

Structural support for shape

Where do arteries take blood?

Arteries = away from heart (oxygenated except pulmonary/umbilical)

Where do veins take blood?

Veins = to heart (deoxygenated except pulmonary/umbilical)

Structure of artery

• endothelium reduces friction - elastic fibres + smooth muscle maintain high pressure - collagen prevents over-stretching - small lumen - no valves

Structure of veins

• endothelium reduces friction - thin smooth muscle + elastic tissue (low pressure) - collagen for strength - large lumen (low resistance) - valves prevent backflow

How do muscles support blood flow in veins in arms and legs?

Contracting muscles squeeze veins, forcing blood up; valves prevent backflow

Structure of capillaries

• walls one cell thick (squamous endothelium) - small lumen (slows blood, time for exchange) - gaps allow diffusion - large SA

Which vein does NOT carry deoxygenated blood?

Pulmonary vein

Which artery does NOT carry oxygenated blood (apart from umbilical)?

Pulmonary artery

How many red blood cells can fit through a capillary side by side?

1

Plasma vs tissue fluid vs lymph

Plasma (in vessels, contains proteins, cells, solutes); Tissue fluid (plasma without proteins, surrounds cells); Lymph (similar to tissue fluid, less O₂/nutrients, more fatty acids)

What does plasma contain?

Glucose, amino acids, mineral ions, hormones, plasma proteins (albumin, fibrinogen, globulin), RBCs, WBCs, platelets

Functions of blood

Transports O₂, CO₂, waste, hormones, antibodies, WBCs; buffer; distributes heat

How does tissue fluid form from plasma?

Plasma forced out of capillaries (via fenestrations) → tissue fluid (no plasma proteins); supplies cells; returns via capillaries or lymph

What is osmotic pressure?

Tendency of water to move into blood by osmosis

Arterial end of a capillary

High hydrostatic pressure → plasma forced out; osmotic pressure lower; net movement out

Venule end of a capillary

Low hydrostatic pressure; blood has low Ψ (proteins, RBCs) → water moves back in by osmosis; net re-entry

What is lymph?

Tissue fluid that drains into lymph vessels; less O₂/nutrients, more fatty acids

Why does lymph have fatty acids?

Absorbed from villi in small intestine

Where are lymph nodes?

Along lymph vessels; lymphocytes build up, produce antibodies, swell during infection

Do lymph vessels have valves?

Yes, to prevent backflow

How is lymph transported?

Muscle contraction squeezes vessels; one-way flow; drains into subclavian veins

How many O₂ molecules can bind to one haemoglobin?

4

Reaction of Hb with O₂

Hb + 4O₂ ⇌ Hb(O₂)₄

Why does O₂ move into erythrocytes in lungs?

Low O₂ in blood; steep gradient vs alveoli; O₂ diffuses in and binds Hb

Why does O₂ move out of erythrocytes in respiring tissues?

Low O₂ in cytoplasm; O₂ diffuses out down gradient

What is partial pressure of O₂ (pO₂)?

O₂ concentration

Typical oxygen dissociation curve

S-shaped curve showing Hb affinity for O₂

Effect of ↑ CO₂ on Hb affinity

Hb gives up O₂ more easily (Bohr effect)

What is the Bohr shift?

Increased CO₂ → curve shifts right/down → Hb releases O₂ more readily in tissues, still binds easily in lungs

Why is the Bohr shift important?

Ensures Hb releases O₂ at respiring tissues but loads O₂ at lungs

Which type of Hb has the highest O₂ affinity?

Fetal haemoglobin (higher affinity to extract O₂ from mother’s blood in placenta)

Why does fetal Hb change to adult Hb after birth?

If retained, a pregnant woman couldn’t supply O₂ to her own fetus

What happens to most CO₂ in blood?

Converted into HCO₃⁻ (hydrogen carbonate) ions

Other ways CO₂ is transported?

• dissolved in plasma - bound to Hb (carbaminohaemoglobin)

How is carbonic acid formed?

CO₂ + H₂O → H₂CO₃ (catalysed by carbonic anhydrase in RBCs)

How does Hb act as a buffer?

Hb binds H⁺ ions (from H₂CO₃ dissociation) forming haemoglobinic acid

What is the chloride shift?

When HCO₃⁻ leaves RBC, Cl⁻ moves in to maintain charge balance

Why must erythrocytes remove CO₂?

Conversion to HCO₃⁻ maintains steep diffusion gradient

What happens to CO₂ transport at lungs?

Low CO₂ in alveoli → carbonic anhydrase reverses reaction → H₂CO₃ → CO₂ + H₂O; HCO₃⁻ re-enters RBC → CO₂ released, diffuses into lungs

Why do plants need transport systems?

• metabolic demands (cells not near leaves need O₂, glucose, hormones, waste removal) - SA:V too small for diffusion alone

Two main transport systems in plants

Xylem (water + mineral ions) and Phloem (assimilates e.g. sucrose)

What is a dicotyledonous plant?

Flowering plant with two seed leaves (dicots) → vascular tissue in bundles

Distribution of vascular bundles in stem

• around edge for strength/support - xylem inside, phloem outside, cambium between

Distribution of vascular bundles in root

• centrally located (xylem in X-shape, phloem between arms, surrounded by endodermis)

Distribution of vascular bundles in leaf

• midrib main vein with branches - xylem on top, phloem below

Xylem structure

• dead cells form hollow tube (xylem vessels) - lignin thickens walls (spiral, rings) → strength, waterproof, prevents collapse - pits (unlignified areas) allow lateral water movement

Other xylem functions

Structural support

Phloem structure

• sieve tube elements (living but few organelles) - perforated sieve plates (allow flow of sap) - companion cells (many mitochondria, support active loading/unloading) - plasmodesmata (links)

What is turgor pressure?

Pressure of cell contents against cell wall