Topic 3: Organisms exchange substances with their environment

Why do larger organisms need specialised exchange surfaces?

They have a smaller SA:V ratio so diffusion alone is insufficient to meet metabolic demands

Describe the tracheal system in insects.

Network of chitin-lined tubes (tracheae) that branch into tracheoles, delivering air directly to respiring cells

How are the tracheoles adapted for gas exchange?

Very thin walls, permeable to gases, and contain fluid at their tips that moves out during exercise

How do fish exchange gases?

Using gills with a counter-current flow system to extract oxygen from water

Describe the structure of fish gills.

Gill filaments (primary lamellae) with many gill lamellae (secondary lamellae) containing capillaries

What is counter-current flow in fish gills and why’s it efficient?

Blood flows through gill lamellae in the opposite direction to water flow over the gills to maintain a concentration gradient for oxygen diffusion along the entire length of the lamellae

How do terrestrial insects prevent water loss?

Spiracles can close, body covered in waxy cuticle, and hairs trap moist air

What are spiracles?

External openings on the insect body that allow air to enter the tracheal system and can be opened or closed

What is the structure of a leaf adapted for gas exchange?

Thin and flat shape, stomata, air spaces, and mesophyll cells with large surface area

What controls the opening of stomata?

Guard cells that become turgid (open) or flaccid (closed) in response to light, CO₂, and water availability

Describe the structure of alveoli.

Tiny air sacs with walls one cell thick, surrounded by capillaries, with large total surface area (~70m²)

What is ventilation?

The process of moving air in and out of the lungs to maintain concentration gradient

Describe the mechanism of inspiration (breathing in).

External intercostal muscles contract, internal intercostal muscles are relaxed because they’re antagonistic, ribs move up and out, diaphragm contracts and flattens and moved down, thoracic volume increases, pressure decreases, air flows in

Describe the mechanism of expiration (breathing out).

External intercostal muscles relax, ribs move down and in, diaphragm relaxes and curves up, thoracic volume decreases, pressure increases, air flows out

During forced exhalation, the internal intercostal muscles contract to move the ribs down and in

What is tidal volume?

The volume of air moved in and out of the lungs in a single breath at rest

What is the main pigment in red blood cells?

Haemoglobin

How many oxygen molecules can one haemoglobin molecule carry?

Four (each haem group binds one O₂)

What is cooperative binding of oxygen to haemoglobin?

Binding of first oxygen changes shape of haemoglobin slightly, making it easier for subsequent oxygen molecules to bind

What is the Bohr effect?

Increased CO₂ concentration decreases haemoglobin's affinity for oxygen because it lowers Ph which changes the shape of the haemoglobin slightly, shifting dissociation curve to the right, oxygen unloaded more readily

What is the advantage of the Bohr effect in active muscles?

Muscles produce more CO₂, lowering pH, causing more oxygen to be released where it is needed most

What is the structure of haemoglobin in foetal mammals?

Foetal haemoglobin has a higher affinity for oxygen than adult haemoglobin

Describe the double circulatory system in mammals.

Blood passes through heart twice per circuit: deoxygenated to lungs (pulmonary) and oxygenated to body (systemic)

What is the advantage of a double circulation?

Oxygenated and deoxygenated blood are separated, allowing higher blood pressure to body tissues

Which side of the heart pumps deoxygenated blood?

Right side (right atrium then right ventricle) to the lungs

Which side of the heart pumps oxygenated blood?

Left side (left atrium then left ventricle) to the body

Why is the left ventricle wall thicker than the right?

It pumps blood much further (to whole body) so needs to generate higher pressure

What are the atrioventricular valves?

Tricuspid valve on the right, bicuspid (mitral) valve on the left; prevent backflow from ventricles to atria

What are the semi-lunar valves?

Aortic and pulmonary valves; prevent backflow from arteries into the ventricles

Where is the SAN (sinoatrial node)?

Located in the wall of the right atrium; the heart's natural pacemaker

What is the role of the SAN?

Generates electrical impulses spontaneously, initiating each heartbeat (myogenic)

What is the role of the AVN (atrioventricular node)?

Delays electrical impulse slightly, allowing atria to contract and empty before ventricles contract

What are Purkinje fibres?

Specialised conducting fibres that rapidly transmit impulse from AVN to ventricles

Describe the cardiac cycle sequence.

Atrial systole (contraction), ventricular systole, then diastole (relaxation of all chambers)

How is cardiac output calculated?

Cardiac output = stroke volume × heart rate

What is stroke volume?

The volume of blood pumped by each ventricle per beat (approximately 70mL at rest)

What are arteries adapted for and what’s their structure?

Carrying high pressure blood away from the heart; thick muscular walls, elastic tissue, narrow lumen

What are capillaries adapted for?

Exchange of substances; walls one cell thick (endothelium), narrow lumen, numerous and well branches

What are veins adapted for?

Carrying low pressure blood back to heart; thinner walls, wide lumen, valves to prevent backflow

What forces cause tissue fluid formation?

Arteries are wider than capillaries so when they branch off into capillaries it creates hydrostatic pressure pushing out small molecules out of the capillaries through ultrafiltration

What is tissue fluid?

Plasma without proteins and blood cells, bathing cells and allowing exchange of substances

How are xylem vessel cells adapted for transport?

Lignin strengthens walls to prevent collapse; cells joined end-to-end with no end walls (continuous tubes)

How do xylem vessels transport water?

By transpiration pull from leaves and cohesion-tension mechanism

What is the cohesion-tension theory?

Water evaporates from leaves, pulling water column up xylem; water molecules cohesive due to hydrogen bonding and polarity, they also adhere to the xylem walls causing a continuous stream of water

How are phloem sieve tube elements adapted for transport?

No nucleus, few organelles, contain phloem sap; alive at maturity

What is the role of companion cells in phloem?

Have lots of mitochondria, load and unload sucrose into sieve tubes via active transport

What substances are transported in phloem sap?

Sucrose (main transport sugar), amino acids, hormones, some ions

What is a source in phloem transport?

Region where sucrose is produced or loaded (e.g. leaves, storage organs exporting stores)

What is a sink in phloem transport?

Region where sucrose is used or stored (e.g. roots, growing shoots, fruits, seeds)

Describe the mass flow hypothesis of phloem transport.

Sucrose actively transported into phloem at source from companion cells, lowers water potential, water enters by osmosis from xylem, high hydrostatic pressure at source, sucrose flows to sink where unload occurs, high water potential in phloem now at sink so water moves back into xylem through osmosis

What are the structural differences between arteries and veins?

Arteries: thick muscular wall, small lumen, no valves; Veins: thin wall, wide lumen, valves present

How does temperature affect the oxygen dissociation curve?

Higher temperature reduces haemoglobin's affinity for oxygen, shifting curve right (Bohr effect intensified)