Biology - Exchange and Transport

A Level AQA Biology Topic 3

Insect Adaptations

Gas exchange occurs through the tracheal system
Air enters through spiracles and flows through trachea tubes, then narrower tubes called tracheoles
The tracheoles lead to muscle fibres, where their endings provide a large surface area for gas exchange
Movement of gases mainly relies on diffusion gradients; oxygen moves into respiring muscle cells from tracheoles, and carbon dioxide moves out of the muscle cells

Insects can use rapid contractions of abdominal muscles to draw oxygen into the trachea down a pressure gradient

Water loss in Insects

Insects have a waterproof exoskeleton that prevents water loss by evaporation, and spiracles provide openings through which water vapour can be lost

Insects can close their spiracles and hairs around the spiracles to reduce the diffusion of water vapour

Fish Adaptations

Fish extract oxygen from water, and have gills to maximise surface area for gas exchange

Each gill is attached to two stacks of filaments on the surface of each filament. There are rows of lamellae, which consist of a single layer of flattened cells that cover a vast network of capillaries

Gas exchange is maximised by a counter-current system; the blood in the capillaries flows opposite to the direction of the flow of water as it passes over the gills, ensuring a concentration gradient is maintained

Counter-Current Flow

The water that enters the capillary has the highest oxygen concentration, and this flows adjacent to the blood that is already partially oxygenated

Water that exits the capillary has the lowest oxygen concentration, and it is adjacent to the most deoxygenated blood

Adaptations of Dicotyledonous Plants

• Spongy mesophyl layer - Air flows into and around the air spaces, and the surfaces come in contact with the air spaces, creating a large surface area for gas exchange

• Stomata - Pores on the underside of most leaves which allow air to enter and exit the leaf. Guard cells control the opening and closing of the stomata

• Leaves are flat and thin, reducing the diffusion distance for gases

Adaptations of Xerophytic Plants

Plants that live in conditions where fresh water is limited have adaptations to conserve water

• Leaves are reduced to spines to reduce surface area for water loss

• Stem has a thick cuticle to prevent water loss

• Leaves can roll up to reduce exposure of surfaces to wind, provide deep groves which protect stomata

• A large number of hairs - trap water vapour

Human Gas Exchange System

• Alveoli - A Series of tiny sacs with structural adaptations that maximise gas exchange, the site of gas exchange

• Bronchioles - Narrow tubes that connect the alveoli to the bronchi

• Bronchi - A pair of tubes that connect the bronchioles in each lung to the trachea

• Trachea - The airway, or windpipe, that connects the bronchi with the mouth and nose

• Lungs - Two lungs in the chest cavity are organs of gas exchange

Alveolar Epithelium

• Large surface area - The lungs contain many alveoli, increasing surface area

• Thin walls - only one cell thick, flattened in shape, short diffusion distance

• Steep concentration gradient - constant flow of blood through capillaries that are immediately adjacent to the alveolar epithelium very quickly maintains the concentration gradient necessary for diffusion of oxygen to occur

Ventilation

Breathing In:

1. The diaphragm contracts and flattens, and the external intercostal muscles contract

2. The ribcage moves upwards and outwards

3. Thorax volume (chest volume) increases, resulting in a pressure decrease

4. Air moves into the lungs down a pressure gradient

Breathing Out:

1. The diaphragm relaxes and curves upwards, and the external intercostal muscle relaxes

2. The ribcage moves downwards and inwards

3. Thorax volume decreases, resulting in a pressure increase

4. Air moves out of the lungs down a pressure gradient

Forced Exhalation - Internal intercostal muscles contract to pull ribs down and the abdominal muscles contract to push organs upwards against the diaphragm, decreasing thorax volume further, increasing pressure

Pulmonary Ventilation Rate

The volume of air an individual breathes per minute

Tidal Volume - The volume of air inhaled or exhaled in one normal breath
Breathing Rate - The number of breaths taken per minute

PVR = Tidal Volume x Breathing Rate
cm³/min = cm³  x breaths per min

Digestion

Proteins —> amino acis
Carbohydrates —> Simple sugars
Lipids —>, Glycerol and fatty acids

Used to release energy via respiration, build new molecules for cell growth, repair and function

Mouth and Salivary Glands - Amylase digests starch into maltose
Stomach - Protease digests proteins, HCL destroys pathogens, suitable pH for enzymes
Liver - Bile salts produced, aid in the digestion of lipids, neutralise stomach acids
Pancreas - Amylase, protease, and lipase are produced and released into the duodenum
Small intestine: duodenum - Acidic stomach contents are neutralised by bile and become slightly alkaline, enzymes complete chemical digestion
Small intestine: ileum - food and water are absorbed into the blood via villi in the lining of the ileum

Carbohydrate Digestion

Carbohydrate enzymes - amylase, maltase, lactase

Process of digesting starch

1. Amylase ( made in salivary glands, pancreas, and small intestine) hydrolyses starch into the disaccharide maltose inside the lumen of the gut

2. Maltose is hydrolysed into glucose  by maltase (membrane-bound, attached to cell surface membranes) inside the epithelial cells of the small intestine

Lipid Digestion

Lipid digestion includes lipase enzymes and bile salts

1. Emulsification - partially digested food arrives in the small intestine and mixes with bile. Bile salts bind to large lipid droplets and break them down into smaller droplets. The smaller droplets have a large surface area on which lipase enzymes can act on

2. Lipase enzymes in the lumen of the small intestine break down lipids into glycerol, monoglycerides, and fatty acids

Protein Digestion

Protein digestion involves endopeptidases, exopeptidases, and dipeptidases

• Endopeptidase enzymes in the stomach and small intestine hydrolyse peptide bonds within polypeptides, creating shorter polypeptide chains

• Exopeptidases hydrolyse peptide bonds at the ends of polypeptide chains, producing single amino acids

• Dipeptidases are a type of exopeptidase that break down dipeptides into individual amino acids. They can be membrane-bound to the cell surface membrane of epithelial cells in the small intestine

Amino Acid Absorption

Co-Transport

1. Sodium ions are actively transported from epithelial cells into the blood via the sodium-potassium pump, thereby decreasing the concentration of sodium ions within the epithelial cells. This maintains the sodium ion gradient

2. Sodium ions move down their concentration gradient from the intestine into the epithelial cell, carrying an amino acid is transported at the same time by the co-transporter protein - facilitated diffusion

3. The concentration of amino acids in the epithelial cell increases, and amino acids diffuse down their concentration gradient into the blood

Monosaccharide Absorption

Co-Transport

1. Active transport of sodium ions into the blood

2. Facilitated diffusion of sodium and glucose into the epithelial cell via a glucose co-transport protein

3. Facilitated diffusion of glucose into the blood

Lipid Absorption

1. Monoglycerides and fatty acids associate with bile salts to form micelles, which transport these insoluble molecules to the cell surface membrane of the epithelial cells

2. Micelles constantly break up and reform; when they break apart, their lipid-soluble contents can cross the membrane by diffusion as they are non-polar

3. Short fatty acid chains within the epithelial cells can move directly into the blood via diffusion, longer chains recombine with monoglycerides and glycerol to form triglycerides in the endoplasmic reticulum, then are packaged into chylomicrons, which eventually enter the bloodstream

Haemoglobin

A protein has a quaternary structure; it has two alpha globins and two beta globins held together by disulfide bonds

Hydrophobic R groups face inwards, helping to preserve the spherical tertiary structure; hydrophilic R groups face outwards, helping maintain solubility

Each subunit contains a haem group, which can combine with an oxygen molecule reversibly; the haem groups contain an iron II ion, Fe²⁺ 

Each haemoglobin can therefore transport 4 oxygen molecules (4O₂)

No nucleus, maximises space available for haemoglobin, biconcave shape increases surface area, highly flexible, can pass through narrow capillaries, diameter roughly the same size as capillaries, slowing down blood flow to maximise time for diffusion

Oxygen + Haemoglobin —> Oxyhaemoglobin

4O₂ + Hb —> Hb4O₂

pO₂ and Oxygen Affinity

At high pO_2, haemoglobin has a high affinity for oxygen
At low pO_2, haemoglobin has a low affinity for oxygen

At a high pO₂, oxygen binds easily and dissociates slowly
At a low pO₂ , oxygen binds slowly to haemoglobin and dissociates easily

Blood Circulation in Mammals

• Coronary arteries - supply the heart with oxygenated blood

• Aorta - carries deoxygenated blood out of the heart towards the body

• Pulmonary artery - carries deoxygenated blood away from the heart towards the lungs

• Vena Cava carries deoxygenated blood into the heart from the body

• Pulmonary Vein - carries oxygenated blood away from the lungs towards the heart

• Renal Artery - supplies the kidneys with oxygenated blood

• Renal Vein - carries deoxygenated blood away from the kidneys towards the heart

Blood Vessels

Arteries

• Thick walls containing smooth muscle and elastic fibres can withstand high pressure generated by the contracting heart, allow it to stretch to expand around blood when the heart beats, and enable recoil to maintain blood pressure when the heart is relaxed

• Narrow Lumen - helps maintain high blood pressure

• Folded inner lining - can stretch to allow increased blood flow

Arterioles

• Low proportion of elastic fibres and a large number of muscle cells - pressure is lower, so elasticity is less essential, muscles can contract to adjust blood flow

Veins

• Walls are thin with few smooth muscle and elastic fibres - blood is not at a high pressure

• Large Lumen - allows a high volume of blood to flow

• Contains valves - prevent backflow

Cardiac Cycle

Atrial Systole

1. The walls of the atria contract

   • Atrial volume decreases

   • Atrial pressure increases 

2. The pressure in the atria rises above that in the ventricles, forcing the atrioventricular (AV) valves open

3. Blood is forced into the ventricles

   • There is a slight increase in ventricular pressure and chamber volume as the ventricles receive the blood from the atria

4. The ventricles are relaxed at this point; ventricular diastole coincides with atrial systole

Ventricular Systole

1. The walls of the ventricles contract

   • Ventricular volume decreases

   • Ventricular pressure increases

2. The pressure in the ventricles rises above that in the atria

   • The AV valves are forced to close, preventing backflow of blood

3. The pressure in the ventricles rises above that in the aorta and pulmonary artery

   • The semilunar (SL) valves are forced open, so blood is forced into the arteries and out of the heart

4. During this period:

   • The atria are relaxing; atrial diastole coincides with ventricular systole

   • The blood flow to the heart continues, so the relaxed atria begin to fill with blood again

Diastole

1. The ventricles and atria are both relaxed

2. The pressure in the ventricles drops below that in the aorta and pulmonary artery, forcing the SL valves to close

3. The atria continue to fill with blood  

   • Blood returns to the heart via the vena cava and pulmonary vein

4. Pressure in the atria rises above that in the ventricles, forcing the AV valves open

5. Blood flows passively into the ventricles without need for atrial systole

6. The cycle then begins again with atrial systole

Tissue Fluid

As blood passes through capillaries, some plasma leaks out through gaps in the walls of the capillary to surround the cells of the body; the resulting fluid is known as tissue fluid

• The composition of plasma and tissue fluid is very similar, though tissue fluid contains fewer large proteins, as these are too large to pass out of the capillaries

• Exchange of substances between cells and the blood occurs via the tissue fluid

  • E.g., waste carbon dioxide leaves the cells, dissolves in the tissue fluid, and then diffuses into the capillary

Tissue fluid forms and returns to the blood due to the balance between the:

• hydrostatic pressure

  • Hydrostatic pressure exerts an outward force on the contents of the capillaries

• osmotic pull generated by dissolved solutes, e.g., plasma proteins

  • Dissolved substances in the blood lower the water potential, exerting an inward force on the tissue fluid due to the resulting water potential gradient

Tissue Fluid Formation

1. Tissue fluid forms as follows:

   • At the arterial end of a capillary, the hydrostatic pressure is greater than the osmotic pull

   • Water and small molecules are forced out of the capillary down a hydrostatic pressure gradient, forming tissue fluid

   • Large molecules, e.g., large plasma proteins, remain in the blood as they are too large to pass out of the capillaries

2. Tissue fluid returns to the capillaries as follows:

   • at the venous end, the osmotic pull is now higher than the hydrostatic pressure

   • Hydrostatic pressure in the capillary has decreased due to loss of plasma volume and flow resistance in the narrow capillary

   • Dissolved proteins in the blood lower the water potential and create a water potential gradient between the capillary and the tissue fluid

   • fluid is drawn back into the capillary down its water potential gradient

Xylem

Transports water and dissolved minerals to the rest of the plant
Hollow tubes with no end walls allow a continuous flow of water
Lignin provides waterproofing to prevent water loss by evaporation
Lignin strenghtens xylem
Made of living cells
Transpiration is the loss of water by evaporation

Transport of water

1. Water diffuses out of leaves into the surrounding air via stomata

2. The loss of water vapour lowers the water potential in the air spaces surrounding the mesophyll cells

3. Water within the mesophyll cell walls evaporates into the leaf air, lowering the water potential of the mesophyll cells

4. Water is drawn from the xylem into mesophyll cells by osmosis

5. Water moves up the xylem vessels in a continuous column to replace this lost water; this upwards movement is the transpiration stream

Cohesion Tension Theory

The upward pulling force acting on water in the system can be so great that the water is under tension, exerting an upward pull on the walls of the xylem vessels; this is known as cohesion tension

Sources and Sinks

Source - The place where assimilates have been produced or stored, e.g, leaves in photosynthesis

Sink - Part of the plant where assimilates are required, e.g, cells in plant storage organs

Translocation

1. Sucrose loading mechanism uses active transport to load sucrose into the phloem at the source - companion cells use ATP to actively pump hydrogen ions out of the cytoplasm into their cell walls, H⁺ moves down its concentration gradient back to the cytoplasm via a co-transporter protein, carrying the sucrose molecules. Sucrose molecules then move into the sieve tubes (phloem) via plasmodesmata

2. High concentration of solutes in the phloem lowers the water potential and causes water to move into the phloem vessels via osmosis

3. This results in increased hydrostatic pressure and generates a hydrostatic pressure gradient between the source and the sink; the contents of the phloem move towards the sink down a concentration gradient

4. At the same time, sucrose is being unloaded from the phloem at the sink, lowering the water potential of the cells of the sink

5. Water follows by osmosis, maintaining the hydrostatic pressure gradient between the source and the sink