Unit 3 - Exchange of Substances
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AQA GCE A-Level Biology - Unit 3 - Exchange of Substances
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\[3.3.1\] What is the equation for **surface area to volume ratio**?
Surface Area: Volume = Surface Area / Volume
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\[3.3.1\] What happens to the surface area to volume ratio as an organism increases in size?
It **decreases** (which means less molecules can be exchanged).
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\[3.3.2\] Where does **gaseous exchange** occur in plants?
Within the **spongy mesophyll**.
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\[3.3.2\] Explain how gases are exchanged in plants.
* CO₂ enters the leaf via the **stomata** and travels through **air space** in the **spongy mesophyll**.
* The CO₂ eventually enters the **palisade cells** within the **palisade mesophyll** where it is used in **photosynthesis**.
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* **Photosynthesis** produces O₂ as a waste product which exits the **palisade mesophyll** into the **spongy mesophyll**.
* The O**₂** travels through the **air space** and exits the leaf via the **stomata**.
* The CO₂ eventually enters the **palisade cells** within the **palisade mesophyll** where it is used in **photosynthesis**.
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* **Photosynthesis** produces O₂ as a waste product which exits the **palisade mesophyll** into the **spongy mesophyll**.
* The O**₂** travels through the **air space** and exits the leaf via the **stomata**.
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\[3.3.2\] How do the stomata open and close?
* Water enters the **guard cells**, making them **turgid**, which then **opens** the stomatal pore.
* When the plant **dehydrates**, the guard cells become **flaccid**, which **closes** the stomatal pore.
* When the plant **dehydrates**, the guard cells become **flaccid**, which **closes** the stomatal pore.
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\[3.3.2\] What are the main adaptations of a leaf?
* **Large Surface Area**
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* **Short Diffusion Pathway**
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* Many **stomata** for rapid diffusion of gases.
→ *this maintains the diffusion gradient as photosynthesis quickly uses up the CO₂*
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* **Short Diffusion Pathway**
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* Many **stomata** for rapid diffusion of gases.
→ *this maintains the diffusion gradient as photosynthesis quickly uses up the CO₂*
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\[3.3.2\] What is a **xerophyte**?
A plant that has adaptations that f**acilitate efficient gas exchange** and enable it to survive on **very little water**.
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\[3.3.2\] What are the main adaptations of xerophytes?
* **Sunken Stomata**
→ *Shelters the xerophytic plant from* ***air currents****, indirectly* ***reducing the concentration gradient of water*** *between the leaf and the air.*
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* **Hairs on the Epidermis**
→ ***Traps humid air*** *which* ***reduces the concentration gradient of water vapour*** *and therefore* ***lowers the rate of transpiration****.*
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* **Spiny Leaves**
→ ***Slows movement of air*** *over the leaf surface,* ***reducing concentration gradient of water****, they can also* ***deter predators*** *from feeding on the plant (which means less growth and repair is needed). They can also sometimes* ***reflect light*** *to reduce the internal temperature of a plant, reducing the rate of transpiration.*
→ *Shelters the xerophytic plant from* ***air currents****, indirectly* ***reducing the concentration gradient of water*** *between the leaf and the air.*
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* **Hairs on the Epidermis**
→ ***Traps humid air*** *which* ***reduces the concentration gradient of water vapour*** *and therefore* ***lowers the rate of transpiration****.*
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* **Spiny Leaves**
→ ***Slows movement of air*** *over the leaf surface,* ***reducing concentration gradient of water****, they can also* ***deter predators*** *from feeding on the plant (which means less growth and repair is needed). They can also sometimes* ***reflect light*** *to reduce the internal temperature of a plant, reducing the rate of transpiration.*
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\[3.3.2\] How do **insects** exchange gases?
* **Spiracles** are holes in the **exoskeleton** that allow gases to enter and exit the insect.
* Spiracles open into **tracheae**, which branch further into **tracheoles**.
* The **movement of the abdomen** move gases in and out of the spiracles.
* Spiracles open into **tracheae**, which branch further into **tracheoles**.
* The **movement of the abdomen** move gases in and out of the spiracles.
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\[3.3.2\] How are insects adapted for **water conservation**?
* Spiracles open/close to alter the **level of ventilation** (valves allow the regulation of inflow/outflow of air).
* **Tiny hairs** around the spiracles which trap humid air, **reducing the concentration gradient** of water to reduce the rate of water loss.
* The **waxy cuticle** is water proof which reduces evaporation.
* **Tiny hairs** around the spiracles which trap humid air, **reducing the concentration gradient** of water to reduce the rate of water loss.
* The **waxy cuticle** is water proof which reduces evaporation.
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\[3.3.2\] What is the link between insect gaseous exchange and major activity?
* During periods of major activity, the muscle cells around the tracheoles **respire anaerobically**, producing **lactic acid**.
* This **reduces the water potential**, meaning that water moves into the muscle cells via osmosis.
* Air is therefore drawn further into the muscle cells, **increasing the rate of diffusion** as oxygen will diffuse faster through air than through water.
* This **reduces the water potential**, meaning that water moves into the muscle cells via osmosis.
* Air is therefore drawn further into the muscle cells, **increasing the rate of diffusion** as oxygen will diffuse faster through air than through water.
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\[3.3.2\] How are fish adapted to efficient gas exchange?
* **Filaments** are **stacked** to provide a **large surface area** for diffusion.
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* **Lamellae** cover the filaments (water flows over them)
→ *Contains many* ***capillaries*** *and a* ***thin surface layer*** *to speed up the rate of diffusion*.
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* **Counter-current Flow**.
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* **Lamellae** cover the filaments (water flows over them)
→ *Contains many* ***capillaries*** *and a* ***thin surface layer*** *to speed up the rate of diffusion*.
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* **Counter-current Flow**.
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![\[3.3.2\] What is **counter-current flow**?](https://knowt-user-attachments.s3.amazonaws.com/910d78f4d2ba4b6e9213656cad978be9.jpeg)
\[3.3.2\] What is **counter-current flow**?
Blood flows through the lamellae in one direction and water flows over in the opposite direction.
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This maintains a **large concentration gradient** so that oxygen is always diffusing from the water into the blood.
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This maintains a **large concentration gradient** so that oxygen is always diffusing from the water into the blood.
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\[3.3.2\] Describe the pathway that air takes from the atmosphere to the bloodstream in mammals.
Air enters the airways via the **nasal cavity** and travels down the **trachea**, **bronchi** and **bronchioles** before entering the **alveoli**. Oxygen then diffuses from the **alveoli** into the **capillaries**.
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\[3.3.2\] How is the **trachea** adapted to its function?
* Covered by the **epiglottis** during swallowing of food or liquids.
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* Has **incomplete c-shaped rings of cartilage** which hold the trachea open, preventing collapse during inhalation.
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* Has **incomplete c-shaped rings of cartilage** which hold the trachea open, preventing collapse during inhalation.
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\[3.3.2\] How are the **bronchioles** adapted to their function?
* Contains **muscle** and **elastic fibres** for **contraction and relaxation** during ventilation.
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![\[3.3.2\] How are **alveoli** adapted to their function?](https://knowt-user-attachments.s3.amazonaws.com/1bff3b41291f4f5996eb57973dab0279.jpeg)
\[3.3.2\] How are **alveoli** adapted to their function?
* **Elastic Fibres** help to **dilate and widen the airways**.
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* Contraction of the smooth muscle **deforms** them and when it relaxes, the fibres **recoil** to their original shape.
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* **Lined with a thin layer of epithelium** reduces the diffusion distance of gases, increasing the rate of diffusion.
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* Extensive capillary network which quickly **removes oxygen** to **maintain** the **steep concentration gradient**.
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* **Large number** to provide a **large surface area to volume ratio**.
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* Contraction of the smooth muscle **deforms** them and when it relaxes, the fibres **recoil** to their original shape.
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* **Lined with a thin layer of epithelium** reduces the diffusion distance of gases, increasing the rate of diffusion.
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* Extensive capillary network which quickly **removes oxygen** to **maintain** the **steep concentration gradient**.
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* **Large number** to provide a **large surface area to volume ratio**.
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\[3.3.2\] What is the equation for **pulmonary ventilation**?
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**Pulmonary Ventilation (dm³min⁻¹) = Tidal Volume (dm³) x Ventilation Rate (min⁻¹)**
**Pulmonary Ventilation (dm³min⁻¹) = Tidal Volume (dm³) x Ventilation Rate (min⁻¹)**
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\[3.3.2\] What is **pulmonary ventilation**?
The **total volume of air** that is moved into the lungs during **one minute**.
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\[3.3.2\] What is meant by the **tidal volume**?
The **amount of air moving in and out** of the lungs during **one breath** at **rest**.
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\[3.3.2\] What is meant by the **ventilation rate**?
The **volume of air** **ventilating the lungs each minute**.
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\[3.3.2\] What is meant by the **vital capacity** on a **spirometer**?
The **largest possible volume change**.
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\[3.3.2\] What is meant by the **residual volume** on a **spirometer**?
The volume remaining after **maximum expiration**.
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\[3.3.2\] What is meant by the **expiratory reserve** and **inspiratory reserve volume** on a **spirometer**?
* Air out in forced expiration.
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* Air in in forced inspiration.
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* Air in in forced inspiration.
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\[3.3.2\] What is **Fick’s Law**?
**Diffusion** is **proportional** to:
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**(Surface Area** x **Difference in Concentration)** / **Length of Diffusion Path**
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**(Surface Area** x **Difference in Concentration)** / **Length of Diffusion Path**
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\[3.3.3\] What is meant by **ingestion**?
The process of taking in food through the mouth.
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\[3.3.3\] What is meant by **digestion**?
The process of breaking **larger**, **insoluble** molecules down into **smaller**, **more soluble** molecules.
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\[3.3.3\] What is meant by **egestion**?
The **removal of undigested food**, **dead intestinal cells** and **bacteria**.
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\[3.3.3\] What occurs for food to end up in the stomach?
Mechanical and Chemical Digestion in the mouth form a **bolus** of food, which then moves down the **oesophagus** by **peristalsis** into the stomach.
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\[3.3.3\] What is the function of the **mouth**?
**Mechanical** and **Chemical** Digestion.
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\[3.3.3\] What is the function of the **oesophagus**?
Moves food by **peristalsis** (*involuntary wave-like muscle contractions*).
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\[3.3.3\] What is the function of the **stomach**?
Lined with **mucus** due to stomach acid and to prevent peptidases digesting the stomach tissues. It is responsible for **chemical** and **mechanical digestion**, **food storage** and **disinfection** of food.
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\[3.3.3\] What is the function of the **small intestine**?
**Absorption** of digestive products (beings in **ileum**) and **chemical digestion** in the **duodenum**.
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\[3.3.3\] What is the function of the **large intestine**?
**Absorption of water** and secretion of **vitamin B** and **vitamin K**.
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\[3.3.3\] What is the function of the **rectum**?
**Egestion**.
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\[3.3.3\] What is the function of the **salivary glands**?
Secretes **saliva** which contains **amylase**.
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\[3.3.3\] What is the function of the **pancreas**?
An **accessory organ** which secretes **enzymes** to aid in digestion.
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\[3.3.3\] What is the function of the **liver**?
Produces **bile** which **emulsifies lipids** (to increase their surface area) and **neutralises acidic food**.
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\[3.3.3\] What is the function of the **gallbladder**?
**Stores bile** produced by the liver.
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\[3.3.3\] What is the function of the **bile duct**?
Connects the liver, gallbladder and the duodenum.
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\[3.3.3\] What is the function of the **epiglottis**?
A **flap of cartilage** which **closes the trachea** to ensure that food goes down the oesophagus.
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\[3.3.3\] What is the function of the **pyloric sphincter**?
Responsible for **allowing semi-digested food** into the small intestine.
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\[3.3.3\] What is the function of the **duodenum**?
* The site of the last part of digestion.
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* **Secretes maltase** and **proteases**.
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* **Secretes alkaline juices** to neutralise acidic food.
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* **Secretes maltase** and **proteases**.
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* **Secretes alkaline juices** to neutralise acidic food.
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\[3.3.3\] What are **endopeptidases**?
**Endopeptidases** are produced and act in the **duodenum**, **stomach** and **small intestine**.
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They hydrolyse the peptide bonds within the **internal sections** of the amino acid chain.
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They hydrolyse the peptide bonds within the **internal sections** of the amino acid chain.
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\[3.3.3\] What are **exopeptidases**?
**Exopeptidases** are produced and act in the **pancreas** and the **membranes of epithelial cells**.
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They hydrolyse the peptide bonds the **external amino acids** of the polypeptide chain.
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They hydrolyse the peptide bonds the **external amino acids** of the polypeptide chain.
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\[3.3.3\] What are **dipeptidases**?
**Dipeptidases** are produced in and act in the **membranes of epithelial cells**.
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They hydrolyse the peptide bonds within **dipeptides**.
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They hydrolyse the peptide bonds within **dipeptides**.
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\[3.3.3\] Describe the **co-transport of amino acids**.
1. Amino Acids move into the **epithelial cell** with sodium ions via a carrier protein (using **facilitated diffusion**).
2. The sodium ions are then removed by **active transport** or the **sodium-potassium pump** into the bloodstream.
3. This means that the **lower concentration of sodium ions i**nside the cell is maintained, helping to **maintain the concentration gradient** between the lumen and the cell.
4. The amino acids then move out of the cell and into the bloodstream by **facilitated diffusion**.
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\[3.3.3\] How are **lipids** digested?
* Bile salts **emulsify** the lipids into smaller droplets to **increase their surface area** for **lipase** to act on
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* Lipase can then break down the lipids into **fatty acids**, **glycerol** and **monoglycerides** by **hydrolysing the ester bonds**.
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* Lipase can then break down the lipids into **fatty acids**, **glycerol** and **monoglycerides** by **hydrolysing the ester bonds**.
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\[3.3.3\] Explain how **lipids** are **absorbed** into the body.
* Monoglycerides and Fatty Acids stick to the **bile salts**, forming a **micelle**. These micelles move the monoglycerides and fatty acids towards the **epithelium**.
* The micelles are constantly breaking up and reforming, meaning that the products can be released and absorbed (they are **lipid-soluble** so can cross the **phospholipid bilayer**).
* In the cell, the monoglycerides and fatty acids recombine in the **smooth endoplasmic reticulum** to form **triglycerides**.
* The **golgi body** associates these triglycerides with **cholesterol** and **proteins** to form a **chylomicron complex**.
* The chylomicrons move out of the epithelial cells by **exocytosis** into the **lacteals** of the lymphatic system.
* They are then hydrolysed and the products diffuse into cells.
* The micelles are constantly breaking up and reforming, meaning that the products can be released and absorbed (they are **lipid-soluble** so can cross the **phospholipid bilayer**).
* In the cell, the monoglycerides and fatty acids recombine in the **smooth endoplasmic reticulum** to form **triglycerides**.
* The **golgi body** associates these triglycerides with **cholesterol** and **proteins** to form a **chylomicron complex**.
* The chylomicrons move out of the epithelial cells by **exocytosis** into the **lacteals** of the lymphatic system.
* They are then hydrolysed and the products diffuse into cells.
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\[3.3.3\] How are the epithelial cells of the ileum adapted to the absorption of molecules?
* **Villi** and **Microvilli** to **increase the surface area** for diffusion.
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* Thin to allow for a **short diffusion pathway.**
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* Contains **muscle** for movement (to ***maximise the concentration gradient***).
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* **Good Blood Supply** to maintain a diffusion gradient.
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* Thin to allow for a **short diffusion pathway.**
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* Contains **muscle** for movement (to ***maximise the concentration gradient***).
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* **Good Blood Supply** to maintain a diffusion gradient.
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\[3.3.3\] Explain the mechanism of the co-transport of carbohydrates?
1. **Sodium** **ions** diffuse out of the cell from the intestinal lumen through a co-transporter protein.
2. This maintains a **higher concentration** of sodium ions in the small intestine than in the cell.
3. Sodium ions diffuse into cells **down the concentration gradient** through a co-transporter protein and brings the monosaccharide with it.
4. Monosaccharide diffuses into the blood via **facilitated diffusion**.
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\[3.3.4.1\] What is meant by **mass transport**?
The **bulk movement** of substances in **one direction**, usually by a system of vessels or tubes.
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\[3.3.4.1\] What are the characteristics of an **open circulatory system**?
Consists of a heart which pumps **haemolymph** through short vessels into a cavity called the **haemocoel**. The haemolymph bathes the **haemocoel** for diffusion and when the heart relaxes, the haemolymph is sucked back in via pores called **ostia**.
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\[3.3.4.1\] What are the characteristics of a **closed circulatory system**?
Blood stays in the vessels. It is pumped through **arteries**, **arterioles** and **capillaries** before diffusion occurs, then from the **capillaries** to the **venules**, **veins** and the heart after diffusion.
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\[3.3.4.1\] What are the characteristics of a **single circulatory system**?
Blood passes through a **two-chambered heart** where it receives deoxygenated blood and pumps it to the gas exchange surface.
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Organisms with this type of circulatory system tend to have low activity so require less energy. They have **low pressure**, **slow flow** blood.
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Organisms with this type of circulatory system tend to have low activity so require less energy. They have **low pressure**, **slow flow** blood.
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\[3.3.4.1\] What are the characteristics of a **double circulatory system**?
Blood passes through a **four-chambered heart twice in every complete circuit**. The blood flows fast due to the blood pressure created by the heart, increasing the **systemic pressure** without the **pulmonary pressure** being increased.
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\[3.3.4.1\] What is meant by **pulmonary circulation**?
Carries **deoxygenated blood** to the lungs.
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\[3.3.4.1\] What is meant by **systemic circulation**?
Carries **oxygenated blood** to body tissues.
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\[3.3.4.1\] What is an **artery**?
A blood vessel that carries **oxygenated blood** __**AWAY**__ from the heart at **high pressure**.
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\[3.3.4.1\] What are the main adaptations of arteries?
* **Thick muscle layer** which allows for **constriction** and **dilation** (to control blood volume).
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* **Thick elastic layer** to maintain the high blood pressure - it stretches during **systole** and recoils during **diastole**.
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* There is a **thick wall** which resists bursting under pressure.
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* F**olded endothelium** to allow artery to stretch.
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* **Thick elastic layer** to maintain the high blood pressure - it stretches during **systole** and recoils during **diastole**.
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* There is a **thick wall** which resists bursting under pressure.
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* F**olded endothelium** to allow artery to stretch.
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\[3.3.4.1\] What are **arterioles**?
Blood vessels that **control blood flow** to the capillaries.
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\[3.3.4.1\] What are the main adaptations of arterioles?
* Structurally similar to arteries but have **less elastic tissue** (because of the lower blood pressure) and a **relatively thicker muscle layer** to allow for constriction of the lumen.
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\[3.3.4.1\] What is a **vein**?
Blood vessels that carry **deoxygenated blood** __**TO**__ the heart at a **low pressure**.
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\[3.3.4.1\] What are the main adaptations of **veins**?
* They have a **relatively thin muscle layer** for constriction and dilation, so it cannot control blood volume.
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* **Thin layer of elastic tissue** as there is a **lower blood pressure**, meaning they will not burst or recoil as easily.
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* Can **flatten easily** to aid the flow of blood.
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* Contains **valves** to **prevent the backflow** of blood.
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* **Thin layer of elastic tissue** as there is a **lower blood pressure**, meaning they will not burst or recoil as easily.
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* Can **flatten easily** to aid the flow of blood.
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* Contains **valves** to **prevent the backflow** of blood.
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\[3.3.4.1\] What is a **capillary**?
The blood vessel responsible for **taking blood directly to body cells**.
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\[3.3.4.1\] What are the main adaptations of capillaries?
* **One cell thick** to provide a **short diffusion pathway**.
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* **Narrow** to permeate tissues so every respiring cell has a short diffusion pathway.
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* **Narrow lumen** ***flattens red blood cells*** (erythrocytes) to reduce the diffusion distance.
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* **Fenestrations** are present to allow large molecules to pass through without the need for transportation systems.
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* **Slow** blood flow to allow more time for diffusion.
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* **Narrow** to permeate tissues so every respiring cell has a short diffusion pathway.
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* **Narrow lumen** ***flattens red blood cells*** (erythrocytes) to reduce the diffusion distance.
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* **Fenestrations** are present to allow large molecules to pass through without the need for transportation systems.
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* **Slow** blood flow to allow more time for diffusion.
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\[3.3.4.1\] What is **tissue fluid**?
The fluid that surrounds cells in tissues.
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\[3.3.4.1\] Explain how **tissue fluid** is formed.
* Water is forced out of the capillaries by the **high hydrostatic pressure** created by the contraction of ventricles in the heart.
* Other substances, such as sugars and mineral ions, move into the tissue fluid by **facilitated diffusion** because their concentration in the plasma is higher than the surrounding tissues.
* White blood cells can also enter the tissue fluid because they can pass through the **fenestrations** between endothelial cells lining the capillaries.
* Proteins do not leave the plasma because they are large molecules, meaning that the water potential is now **more negative** than in the tissue fluid - water therefore moves back into the capillary by **osmosis**.
* Other substances, such as sugars and mineral ions, move into the tissue fluid by **facilitated diffusion** because their concentration in the plasma is higher than the surrounding tissues.
* White blood cells can also enter the tissue fluid because they can pass through the **fenestrations** between endothelial cells lining the capillaries.
* Proteins do not leave the plasma because they are large molecules, meaning that the water potential is now **more negative** than in the tissue fluid - water therefore moves back into the capillary by **osmosis**.
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\[3.3.4.1\] What is the difference between arterial and venous tissue fluid?
* At the arterial end, the hydrostatic pressure is **higher** than the water potential so there is a net movement of fluid into the tissues. As the plasma moves along the capillary, its **volume decreases** due to fluid loss so the hydrostatic pressure will **decrease**.
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* At the venous end, the hydrostatic pressure will be **lower** than the water potential, therefore there will be a net movement of fluid back in the capillary.
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* At the venous end, the hydrostatic pressure will be **lower** than the water potential, therefore there will be a net movement of fluid back in the capillary.
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\[3.3.4.1\] Explain what happens when tissue fluid enters the lymph capillaries.
Some tissue fluid re-enters the capillaries while some enters the **lymph** **capillaries**. The **lymph** moves along the larger vessels by **compression** caused by body movement, with backflow prevented by valves. The **lymph** **eventually re-enters the bloodstream** through veins located close to the heart.
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\[3.3.4.1\] What is **haemoglobin**?
Haemoglobin (**Hb**) is a protein with a **quaternary structure**. It is made up of **4 polypeptides** - 2 alpha and 2 beta.
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\[3.3.4.1\] What are the requirements that haemoglobin needs for oxygen transportation?
1. **Readily associates** with oxygen at the surface where **gas exchange occurs** (in high oxygen environments).
1. **Readily dissociates** with oxygen at tissues requiring it - i.e. respiring cells (in low oxygen environments).
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\[3.3.4.1\] What is **association**?
The process of haemoglobin combining with oxygen.
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\[3.3.4.1\] What is **dissociation**?
The process of haemoglobin releasing its oxygen.
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\[3.3.4.1\] What is meant by **affinity**?
The **ability of haemoglobin to bind to oxygen**.
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\[3.3.4.1\] What is meant by a **high affinity**?
Haemoglobin will **associate** with oxygen easily, but **dissociation** is more difficult.
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\[3.3.4.1\] What is meant by a **low affinity**?
Haemoglobin will **dissociate** with oxygen easily but will struggle to **associate**.
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\[3.3.4.1\] What are **oxygen dissociation curves?**
They show the **relationship between how much oxygen the haemoglobin is carrying and the partial pressure of oxygen** in the surrounding tissues.
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[3.3.4.1] Why are oxygen dissociation curves a sigmoid shape?
The first oxygen molecule associates slowly but the others associate more quickly - they use cooperative binding.
The binding of the first oxygen molecule changes the tertiary structure of haemoglobin, uncovering further binding sites.
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[3.3.4.1] What is characteristic about left shifted dissociation curves?
High affinity for oxygen.
Associates more readily.
Dissociates less readily.
Usually for organisms in low oxygen environments.
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[3.3.4.1] What is characteristic about right shifted dissociation curves?
Low affinity for oxygen.
Associates less readily.
Dissociates more readily.
Usually for organisms in high rates of respiration.
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[3.3.4.1] How does a high partial pressure of carbon dioxide affect the position of the dissociation curve?
The greater the concentration of CO₂, the more readily oxygen from haemoglobin dissociates.
CO₂ is produced by respiring tissues, which dissolves in the blood to form carbonic acid, which lowers the pH, slightly altering the tertiary structure so affinity decreases.
A Bohr shift (right shift) is useful as a higher CO₂ partial pressure leads to oxygen being more readily dissociated to the respiring tissues for use in respiration.
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[3.3.4.1] How does a low partial pressure of carbon dioxide affect the position of the dissociation curve?
The lower the concentration of CO₂, the less readily oxygen from haemoglobin dissociates. The curve shifts to the left in order to have an increased affinity, particularly useful in low O₂ environments (such as for foetal haemoglobin).
Note 
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