Surface Area to Volume Ratio & Adaptations

• BioSnip sponsors the video, providing a free weekly newsletter with biology news relevant to A-levels
• Newsletters include links to A-level specifications and cover a range of topics, aiding in application and critical analysis skills.
• BioSnip also has an Instagram account with quizzes and content.
• Topic 3 covers how adaptations in cells, organs, and organ systems maximize gas exchange and transport across surfaces.

Surface Area to Volume Ratio

• The surface area to volume ratio (SA:V) is calculated by dividing the surface area of an organism by its volume.
• Larger organisms have a smaller SA:V ratio.
• Small organisms, such as amoeba, have a large SA:V ratio, so they don't require specific adaptations for gas exchange.
• Larger organisms require adaptations for mass transport due to their smaller SA:V ratio and higher metabolic needs.
• Villi, microvilli, alveoli, bronchioles, spiracles, tracheals, gill filaments, lamellae, and stomata are adaptations to maximize transport
• Most gas exchange surfaces have a capillary network nearby with the exception of plants.

Key Terminology

• Breathing: Movement of air in and out of the lungs.
• Respiration: A chemical reaction that releases energy in the form of ATP.
• Ventilation: Scientific term for breathing.
• Gas exchange: Diffusion of oxygen and carbon dioxide in and out of cells.
• Alveoli: Only in mammals

Human Gas Exchange System

• Key structures: Alveoli, bronchioles, bronchi, trachea, and lungs.

Human Ventilation

• Ventilation in humans involves the diaphragm and antagonistic muscles surrounding the ribs (external and internal intercostal muscles).
• Antagonistic muscles work in pairs, with one contracting while the other relaxes.
• External intercostal muscles contract, causing the rib cage to move out during inspiration; internal intercostal muscles relax.
• Diaphragm muscle contracts, moving downwards and becoming flatter, increasing the volume of the thorax.

Muscle Contraction

• When breathing in: external intercostal muscles contract, internal intercostal muscles relax, diaphragm contracts and moves down, increasing thorax volume and decreasing pressure. This causes air to flow in.
• When breathing out: external intercostal muscles relax, internal intercostal muscles contract, diaphragm relaxes and domes upwards decreasing thorax volume. This causes air to flow out

Pulmonary Ventilation Calculation

• Pulmonary ventilation = tidal volume x ventilation rate
• Tidal volume: volume of air.
• Ventilation rate: breaths per minute.

Alveolar Epithelium and Gas Exchange

• Gas exchange occurs in the alveoli between the epithelium and the blood.
• Alveoli are tiny air sacs, approximately 300 million per lung.
• Alveoli epithelium cells are very thin to minimize diffusion distance.
• Each alveolus is surrounded by a network of capillaries, maintaining the concentration gradients.

Gas Exchange in Terrestrial Insects

• Insects possess a waterproof exoskeleton meaning gases cannot exchange across them.
• Insects use a tracheal system including trachea, tracheals, and spiracles for ventilation and gas exchange.
• Spiracles: Tiny holes on the abdomen where oxygen and carbon dioxide enter and leave.
• Trachea: Tubes with rings to strengthen and keep them open.
• Tracheals: Smaller tubes branching from the trachea, delivering oxygen to respiring cells.
• Gas exchange methods: diffusion, mass transport via abdominal muscle contraction and relaxation, and anaerobic respiration.

Insect Adaptations for Gas exchange

• Insects have multiple adaptations for gas exchange, they have lots of fine tracheals, thin tracheal walls, and short diffusion distance.
• Oxygen use produces carbon dioxide, maintaining a concentration gradient.

Water loss prevention in insects

• Small surface area of gas exchange surfaces.
• Waterproof exoskeleton.
• Spiracles that can open and close.

Gas exchange in Fish

• Fish obtain oxygen from water, but have 30 times less oxygen than in the air.
• Adaptations for gas exchange surfaces should include: large surface area, short diffusion distance, high concentration gradient
• Gills are the gas exchange surface in fish.

Fish Gill Anatomy

• Four layers of gills on both sides of the head.
• Gills are made up of gill filaments covered in gill lamellae, positioned at right angles to the filament.
• Opening the mouth while swimming the water flows over the gills
• Short diffusion distance due to the network of capillaries in every gill lamellae.

Counter Current Flow Mechanism

• Maintains the concentration gradient in fish.
• Water flows over the gills in the opposite direction to the blood flowing through the capillaries maintaining equilibrium
• Oxygen diffuses from the water into the capillaries across the entire gill lamellae.

Fick's Law

• Diffusion proportional to surface area times difference in concentration, divided by length of diffusion pathway.

Gas Exchange in Plants

• Palisade mesophyll: primary site of photosynthesis.
• Spongy mesophyll: contains air spaces.
• Stomata: where gases diffuse in and out of plants
• Stomata close at at night and open in the daytime due to light
• Xerophytic plants have adaptations to minimize water loss, as they're adapted to dry environments

Xerophytic adaptations

• Rolled leaves.
• Sunken stomata.
• Hairs.
• Thicker cuticle.
• Longer root network.

Digestion and Absorption

• Large biological molecules are hydrolyzed into smaller soluble molecules.
• The biological molecules are carbohydrates lipids and proteins

Carbohydrate Digestion

• Amylases and membrane-bound disaccharidases.
• Digestion begins in the mouth, continues in the duodenum, and ends in the ileum.
• Amylase, from the salivary glands and pancreas, hydrolyzes polysaccharides into maltose by breaking glycosidic bonds.
• Sucrase and lactase(membrane bound that hydrolyze sucrose and lactose into monosaccharides)

Protein Digestion

• Endopeptidases hydrolyze peptide bonds in the middle of a polymer chain.
• Exopeptidases hydrolyze peptide bonds at the end of chains.
• Membrane-bound dipeptidase break the peptide bonds between two amino acids

Lipid Digestion

• Digestion by lipase and bile salts.
• Lipase is produced in the pancreas and hydrolyzes ester bonds in triglycerides, forming monoglycerides and fatty acids.
• Bile salts, produced by the liver, emulsify lipids to form micelles.
• This emulsification increases the surface area available for lipase binding.
• Micelles aid in the absorption of lipids.

Micelle Formation

• Lipids are coated in bile salts to create an emulsion
• The formation of droplets of lipids provides a larger surface area to make hydrolysis faster
• Micelles transport lipids in a water-soluble form to the intestinal epithelium, where the lipids are absorbed.
• Micelles are vesicles formed of fatty acids, glycerol, monoglycerides, and bile salts.
• Micelles diffuse to the ilium epithelial cells and enter the epithelial cells of the ilium.
• Lipids are modified back into triglycerides inside of the endoplasmic reticulum and the golgi body
• They then form vesicles and are released from the cell into the lacteal and be transported around the body.

Absorption in Mammals

• Occurs in the ilium.
• Villi and microvilli increase surface area.
• Thin walls provides a short diffusion distance.
• Capillaries maintain concentration gradients.
• Monosaccharides and amino acids are absorbed by active transport, in the form of co-transport

Hemoglobin & Mass Transport

• Hemoglobin is transporting oxygen around the body
• Hemoglobin is an example of a quaternary structure protein.
• Myoglobin is found in the muscle tissue in vertebrates and also in fetuses.
• In regions with a low partial pressure of oxygen the hemoglobin unloads the oxygen.

Oxyhemoglobin dissociation curve

• Demonstrates cooperative binding of oxygen.
• Hemoglobin changes shape when the first oxygen binds, making it easier for subsequent oxygens to bind.
• High carbon dioxide concentration causes the oxyhemoglobin curve to shift to the right (Bohr effect).
• The affinity for oxygen decreases (Bohr effect).
• This occurs due to the acidic carbon dioxide changing the shape of the hemoglobin slightly
• Makes hemoglobin more likely to unload the oxygen even at the same partial pressures
• A low partial pressure of carbon dioxide would typically happen in the alveoli
• A high partial pressure of carbon dioxide would happen at respiring tissues
• The bore effect means that at respiring tissues the oxygen is unloaded

Animal Adaptations

• Fetal hemoglobin has a higher affinity for oxygen.
• Llamas have hemoglobin with a higher affinity for oxygen at low partial pressures.
• Doves have hemoglobin with a lower affinity for oxygen for faster metabolism.
• Earthworms have hemoglobin with high affinities even at low partial pressures.

Mammalian Circulatory System

• Described as closed and double.
• Double meaning the blood passes through the heart twice in each circuit
• One circuit delivers blood from the heart to the lungs, while the other delivers blood from the heart to the rest of the body.
• Blood flows through the lungs at lower pressure, and the blood flows at a slow speed
• Blood is pumped out at a higher pressure to the rest of the body.

Key blood Vessels

• Coronary arteries supply the heart muscle with oxygenated blood.
• Vein cava, aorta, pulmonary artery, and pulmonary vein deliver blood into and out of the heart.
• Pulmonary refers to the lungs.
• Pulmonary artery carries blood away from the heart to the lungs.
• Pulmonary vein delivers blood back into the heart.
• Renal artery and renal vein are attached to the kidneys.
• Major blood vessels are connected via arteries, arterioles, capillaries, and veins.

Cardiac Muscle Features

• Walls of the hearts have a thick muscular layer.
• Myogenic: can contract and relax without nervous or hormonal stimulation.
• Never fatigues: as long as it has a constant supply of oxygen and glucose it will be able to respire aerobically.
• Myocardial infarction, or heart attack is when one of the coronary arteries is blocked, meaning the cardiac muscle is not receiving oxygen or glucose.

Heart Structure

• Four chambers: two atria and two ventricles.
• Atria have thinner muscular walls.
• Ventricles have thicker muscular walls.
• The right ventricle pumps blood to the lungs at a lower pressure.
• The left ventricle pumps blood at high pressure around the body.
• Vena cava carries deoxygenated blood from the body into the right atrium.
• Pulmonary vein is carrying oxygenated blood from the lungs into the left atrium.
• Pulmonary artery is carrying deoxygenated blood from the right ventricle to the lungs.
• Aorta carries oxygenated blood from the left ventricle to the rest of the body.
• The semilunar valves are located in the aorta and pulmonary artery and the atrioventricular valves are between the atria and the ventricles
• Valves prevent the backflow of blood.
• The septum separates the blood on the deoxygenated and the oxygenated side.

Blood Vessels

• Arteries carry blood away from the heart towards the arterioles.
• Arterioles are smaller than arteries and connect to the capillaries.
• Capillaries connect the arterioles to the veins.
• Veins carry blood back into the heart.

Structure of arteries and veins

• Compared to veins, arteries have thicker muscular layers for constriction and dilation.
• Arteries have ticker elastic layers to help maintain the blood pressure.
• Arteries do not have valves.
• Veins require valves because the blood is at a lower pressure.
• Capillaries are very narrow in diameter and only one cell thick.

Arterioles and capillaries

• Arterioles have thicker muscular layers than in the arteries (restricts the blood flow)
• Arterioles elastic layer is thinner than arteries.
• Arterioles do not have valves.
• Capillaries do not have any muscular or elastic tissue layer
• Capillaries function is were exchange of materials occurs

Cardiac Cycle

• 3 stages: diastole, atrial systole, and ventricular systole.
• In diastole, the atria and ventricle muscles are relaxed and blood will enter the atria through the vena cava and the pulmonary vein
• In atrial systole, the atria muscles contract, and atrioventricular valves open.
• In ventricular systole, the ventricle muscles contract, and semilunar valves open where blood is pumped out
• Cardiac output = heart rate x stroke volume
• Heart rate: beats of the heart in one minute
• Stroke volume: amount of blood which leaves the heart each beat

Tissue Fluid Formation

• Tissue fluid contains water, glucose, amino acids, fatty acids, ions, and oxygen.
• Hydrostatic pressure forces water and small molecules out of the capillaries (ultrafiltration), forming tissue cell fluid.
• Large proteins cannot get out of the tiny gaps and remain in the blood.
• By the time the venule end of the capillary is reached the hydrostatic preceptors decreased and the water reenters by osmosis, and absorbs carbon dioxide and urea from the bodily wastes.
• Lymphatic system is what absorbs re-absorption of water and nutrients in the tissue cell fluid due to its water equilibrium and high concentration of nutrients.

Mass transport in plants

• Transpiration: the loss of water vapor from the stomata.
• These factors will affect the rate of transpiration: light intensity, temperature, humidity and wind

Cohesion tension theory

• The movement of water up
• Cohesion links to water molecules and different charges
• The capillary effect states with narrow xylem easier to increase transport of liquid up the xylem vessels
• Root pressure is the factor when the water moves into the roots by osmosis

Organic Molecule Transport

• Organic molecule transport happens in the pholem
• Pholem transports sugars
• Sieve tube in the pholem are hollow in order to make it easier for transport, but does not contain any nuclei.
• Companion cells supply ATP for active transport of the organic substances, for pholem to transport from source (photosynthesis) and sink respirating
• Source to sink model: the sucrose or glucose that have been made in photosynthesis will affect the water volume of cells, creating increased hydrostatic pressure, the liquids that are within that source cell will then be forced by that high hydrostatic pressure through the xylem from the source (photosynthesis) all the way to the sink (respiring to zinc

Translocation steps

• Photosynthesis creates a high concentration of sucrose.
• Sucrose diffuses into the companion cell.
• Active transport of protons from the companion cell into the space within the cell walls.
• Protons move down their gradient via carrier proteins into the sieve tube elements.
• Movement of sucrose within the phloem sieve tube element will lower that water potential

Two key investigations that prove traslocation

• Tracers: Thin slices of radioactive carbon and used.
• Ringing experiments is when a ring of bark and phloem are peeled and removed off the trunk this proves the flow transports sugars

Explore the surface area to volume ratio (SA:V) and its impact on gas exchange needs in organisms. Learn how various adaptations like villi, alveoli, and gill filaments maximize transport across surfaces in larger organisms. BioSnip sponsors weekly biology news relevant to A-levels.