OCR Biology Module 3: Exchange Surfaces, Transport in Animals, and Transport in Plants
Exchange Surfaces & Surface Area to Volume Ratio
Exchange surfaces have similar adaptations to efficiently exchange substances like oxygen and carbon dioxide.
Smaller organisms (e.g., amoeba) have a large surface area to volume ratio.
They have a large surface area for transport and a short distance between the outside and the center.
They don't require special organs/systems; simple diffusion meets their metabolic needs.
Larger organisms have a smaller surface area to volume ratio.
They have a larger distance from the outside to the center.
They have higher metabolic rates, requiring more oxygen for respiration to create ATP.
They require adaptations to increase the efficiency of exchange.
Adaptations for Exchange
Focus on gills in fish, alveoli in humans, and the tracheal system in insects.
Key adaptations:
Large surface area (e.g., projections like root hair cells or folded membranes).
Maintaining a concentration gradient (ventilation or good blood supply).
Reducing the diffusion pathway (single layer of squamous epithelial cells).
Mammalian Gas Exchange System
Structures: trachea, bronchi, bronchioles, and alveoli.
Trachea (windpipe):
C-shaped rings of cartilage support the trachea to prevent collapse.
Lined with ciliated epithelial cells and goblet cells.
Ciliated cells sweep away mucus.
Goblet cells produce thick, sticky mucus to trap pathogens/dust particles.
Smooth muscle contracts to constrict the lumen (airway) upon detecting harmful substances, reducing airflow.
Elastic fibers allow stretch and recoil of the lumen.
Bronchi and Bronchioles:
Trachea splits into two bronchi (one for each lung).
Bronchi split into smaller bronchioles, which end in alveoli.
Cartilage supports the bronchi and bronchioles.
Alveoli:
Located at the end of bronchioles; site of gas exchange.
Oxygen diffuses from the alveoli into the blood.
Carbon dioxide diffuses from the blood into the alveoli.
Features of Efficient Gas Exchange in Alveoli
Large Surface Area:
Provided by the millions of alveoli in both lungs.
Short Diffusion Distance:
Alveoli and capillary walls are made of a single layer of squamous epithelial cells (long and flat).
Maintaining Steep Concentration Gradients:
Each alveolus is surrounded by a network of capillaries.
Ventilation in the lungs quickly removes carbon dioxide and brings in oxygen.
Ventilation
Mechanism of breathing involving the diaphragm and antagonistic interactions between external and internal intercostal muscles.
Changes in the thoracic cavity volume lead to pressure changes.
Inhalation (inspiration):
Increase in thorax volume, decreasing pressure, causing air to flow in.
Exhalation (expiration):
Decrease in thorax volume, increasing pressure, forcing air out.
Muscles:
Inhalation: Diaphragm contracts (moves down), external intercostal muscles contract (rib cage up and out), internal intercostal muscles relax.
Exhalation: Diaphragm relaxes (domes upwards), external intercostal muscles relax, internal intercostal muscles contract (rib cage inwards and down).
Spirometer
Measures the volume of inhaled and exhaled air, producing a graph.
Vital capacity: Maximum volume of air inhaled/exhaled during a deep breath.
Tidal volume: Air inhaled/exhaled at rest.
Residual volume: Air that always remains in the lungs to prevent collapse.
Breathing rate: Number of breaths per minute.
Ventilation rate = tidal volume * breathing rate.
Oxygen uptake increases with ventilation rate (e.g., during exercise).
Ventilation and Gas Exchange in Fish
Challenge: Less oxygen dissolved in water than in the atmosphere.
Ventilation in Fish:
Fish swim with mouths open, allowing water to flow over the gills (site of gas exchange).
Fish lower the buccal cavity (mouth), increasing volume and decreasing pressure, causing water to flow in.
Operculum valve closes, the operculum cavity expands, and its pressure decreases.
The fish raises the floor of their buccal cavity, forcing water over the gills and out of the operculum.
Gas Exchange in Fish
Gills: Four layers on each side of the head.
Gill filaments: Longer parts of the gills.
Gill lamellae: Semicircle structures covering gill filaments; location of gas exchange.
Large Surface Area: Many gill filaments covered in gill lamellae.
Short Diffusion Distance: Thin gill lamellae and filaments with a network of capillaries.
Maintaining a Steep Concentration Gradient: Achieved via the countercurrent flow mechanism.
Countercurrent Flow Mechanism
Compensates for lower oxygen concentration in water.
Water flows over gill lamellae in the opposite direction to blood flow in capillaries.
Equilibrium is never reached, maintaining a steep concentration gradient and maximizing diffusion along the entire gill lamellae.
Gas Exchange in Insects
Terrestrial insects have a tracheal system: spiracles, trachea, and tracheoles.
Spiracles: Valve-like structures along the abdomen that open/close to allow gas exchange and prevent water loss.
Trachea: Attached to spiracles, branching into many tracheoles.
Tracheoles: Site of gas exchange in insects.
Features of Gas Exchange in Insects
Ventilation: Insects contract/relax abdominal muscles to pump gas in/out.
Large Surface Area: Maintained by many branching tracheoles.
Short Diffusion Distance: Thin tracheole walls reaching across the abdomen.
Maintaining Steep Concentration Gradient:
Cells respire, using oxygen and producing carbon dioxide.
Abdominal muscles pump new air in and old air (with carbon dioxide) out.
During Flight:
Muscles contract/relax more rapidly, leading to anaerobic respiration and lactate production.
Lactate dissolves to form lactic acid, lowering the water potential of the cell.
Water moves from the trachea (tracheal fluid) into abdominal cells by osmosis.
Decreased liquid volume in the trachea reduces pressure, causing air to move in through the spiracles.
Circulatory Systems
All circulatory systems transport gasses and nutrients around an organism in transport liquid.
The liquid is transported around in vessels, and there's usually a pump as well to help move that liquid.
Open Circulatory System:
Found in invertebrates like insects.
Transport medium is hemolymph, which is pumped directly to the body cavity (hemocoel).
Few transport vessels.
Pumped at low pressure.
Transports food and nitrogenous waste but not gases (gases are transported via the tracheal system).
The transport medium returns to the heart through an open-ended vessel.
Closed Circulatory System:
Found in vertebrates and some invertebrates like annelid worms.
The transport medium is blood, which always remains inside blood vessels.
Gas and small molecules can leave the blood by diffusion or high hydrostatic pressure.
Transports oxygen and carbon dioxide (oxygen is usually transported by pigmented protein, e.g., hemoglobin).
Types of Closed Circulatory Systems
Single Closed Circulatory Systems:
Blood passes through the heart once per cycle; there is only one circuit (e.g., fish).
Blood flows through two sets of capillaries immediately after being pumped out of the heart.
Blood flows through capillaries in the gills to become oxygenated.
Then, it flows through capillaries delivering blood to the body before returning to the heart.
Not efficient for mammals but works for fish due to the countercurrent flow mechanism.
Double Closed Circulatory System:
Blood passes through the heart twice per cycle; there are two separate circuits (e.g., birds and mammals).
Pulmonary circuit: Carries blood from the heart to the lungs for gas exchange.
Systemic circuit: Carries blood from the heart to the rest of the body to deliver oxygen/nutrients and collect waste.
Blood Vessels
Arteries, arterioles, capillaries, venules, and veins.
*Note: The transcript suggests pausing the video and taking notes on a slide that has a table comparing the characteristics of the different blood vessels.
Capillaries
Form capillary beds (many branched capillaries are connected), typically at exchange surfaces (e.g., alveoli).
Narrow diameter slows down blood flow; red blood cells are squashed against walls, maximizing diffusion.
Made up of a single layer of squamous epithelial cells in their endothelium.
Small gaps between cells allow tissue fluid to form.
Tissue Fluid Formation
Due to small gaps, liquid and small molecules can be forced out of the capillaries due to high pressure, forming tissue fluid.
Hydrostatic pressure: Pressure exerted by a liquid.
Oncotic pressure: The tendency of water to move into the blood by osmosis.
Formation of Tissue Fluid
As blood enters capillaries from arterioles, the arteriole's wider diameter compared to the capillary causes high hydrostatic pressure.
High hydrostatic pressure forces water and small molecules (e.g., glucose, amino acids, fatty acids, ions, oxygen) out.
This liquid surrounding tissues is called tissue fluid.
Substances forced out can move into cells, and waste products from cells can move out to be picked up and transported away.
Hydrostatic pressure is higher than oncotic pressure at the arteriole end, resulting in net movement out of the capillaries to form tissue fluid.
Reabsorption of Water
Large molecules (e.g., soluble plasma proteins) remain in capillaries, lowering the water potential of the blood (higher oncotic pressure).
The hydrostatic pressure has decreased because liquid has been forced out.
There is a net movement of liquid back into capillaries by osmosis at the venule end.
Final parts of the tissue fluid are absorbed into the lymphatic system, becoming lymph.
Lymph is similar to plasma but doesn't contain large plasma proteins and some blood cells.
Mammalian Heart
Organ made of cardiac muscle responsible for pumping blood around the body.
Cardiac muscle is myogenic (automatically contracts/relaxes) and doesn't fatigue.
Coronary arteries on the outside supply the cardiac muscle with oxygenated blood for aerobic respiration to create ATP.
Pericardial membranes (inelastic) prevent the heart from filling and swelling with blood.
Internal Structures of the Heart
Left ventricle: Thicker muscular wall for stronger contraction and higher-pressure blood pumping to the rest of the body.
Right ventricle: Thinner cardiac muscle wall because it doesn't need to contract with as much force (blood is pumped to the lungs).
Blood needs to flow through the lungs at lower pressure to prevent damage to lung capillaries and allow more time for gas exchange.
Atria (two chambers at the top): Thinner cardiac muscle because they don't need to contract with much force (blood moves to ventricles, moving down with gravity).
Cardiac Cycle
Split into three stages: diastole, atrial systole, and ventricular systole.
Atrial systole: Atria are contracting.
Atrial diastole: Atria are relaxing.
Ventricular systole: Ventricles contract.
Ventricular diastole: Ventricles relax.
Details of the cardiac cycle at each stage are included in the original transcript.
Cardiac Output
Volume of blood that leaves one ventricle in one minute.
Calculated using the formula: Cardiac output = heart rate * stroke volume.
Heart rate: Beats per minute.
Stroke volume: Volume of blood that leaves the heart each beat (typically in decimeters cubed).
Controlling the Cardiac Cycle
Cardiac muscle is myogenic, contracting on its own accord.
The rate of contraction is controlled by a wave of electrical activity.
Sinoatrial node (SAN): Located in the right atrium; known as the pacemaker.
Atrioventricular node (AVN): Located near the border of the right and left ventricle.
Bundle of His: Runs through the septum.
Purkinje fibers: Branch into the walls of the ventricles.
Details of how these structures are key in the control of the cardiac cycle are included in the original transcript.
Electrocardiogram (ECG)
Measures waves of depolarization.
Doesn't directly measure electrical activity of the heart but measures differences in electrical activity in your skin that are caused by the electrical activity of the heart.
Electrodes are placed on the skin to detect electrical activity.
Can be used to diagnose irregularities in heart rhythms.
Abnormal Heart Rhythms
Tachycardia: Heart is beating over 100 beats per minute (abnormally high at rest).
Bradycardia: Heart is beating at less than 60 beats per minute (common in athletes, but an artificial pacemaker may be needed if the heart rate drops too low).
Fibrillation: Irregular heartbeat or chaotic rhythm of the heart.
Ectopic heartbeat: Additional heartbeats that are not in rhythm (common once a day, but more regular occurrences could indicate a serious health condition).
Hemoglobin
Globular proteins that many organisms have; has a quaternary structure (more than one polypeptide chain).
Responsible for transporting oxygen.
Oxyhemoglobin dissociation curve shows the percentage saturation of hemoglobin with oxygen against different partial pressures of oxygen.
At high partial pressures of oxygen, hemoglobin is at approximately 100% saturation.
At low partial pressures of oxygen, there are much lower percentage saturations.
Change in affinity: Attraction; at high partial pressures of oxygen, hemoglobin has a high affinity for oxygen, and at low partial pressures of oxygen, hemoglobin has a low affinity.
Cooperative Binding
The cooperative nature of oxygen binding to hemoglobin is due to the hemoglobin changing shape.
When the first oxygen binds, it causes the shape of that protein, the hemoglobin, to change, exposing the further binding sites of oxygen more.
Bohr Effect
High carbon dioxide concentration causes the oxyhemoglobin curve to shift to the right.
Indicates that the affinity hemoglobin has for oxygen is decreasing.
If you have high levels of carbon dioxide, it indicates it's the site of respiration, so if you're unloading oxygen at that location, there will be a constant supply of oxygen for aerobic respiration to continue.
Comparing Hemoglobins of Different Organisms
Human Fetus:
HBA (adult hemoglobin) and HBF (fetal hemoglobin).
Fetal hemoglobin has the curve shifted to the left, demonstrating that even at the same partial pressure of oxygen, it is more saturated with oxygen.
Fetal hemoglobin has a higher affinity than adult hemoglobin, so it is able to remove oxygen from the hemoglobin of an adult.
Llamas:
Llamas live at very high altitudes, so their hemoglobin has a higher affinity for oxygen so that it can still load up oxygen.
Transport of Carbon Dioxide
Dissolved and transported in the blood plasma.
As carbaminohemoglobin (carbon dioxide combines with hemoglobin).
In the cytoplasm of red blood cells in the form of hydrogen carbonate ions.
Details about the hydrogen carbonate ions process are described in the original transcript.
Transport in Plants
Main substances transported on mass are water and organic substances (containing carbon) by the xylem or phloem (vascular bundle).
Vascular Bundles in Roots: The xylem is found at the center of the root, often with a star shape, and the phloem is found between each of those star shapes.
Stem: There are vascular bundles with xylem and phloem tissues. Xylem is on the inner edge, closest to the center, and phloem is closest to the surface of the stem. Cambium is between the xylem and the phloem.
Leaf: the vascular bundle runs down the center of the leaf, and the xylem is towards the top of the leaf, and the phloem is towards the bottom of the leaf.
Structure of the Xylem and Phloem
Phloem:
Sieve tube elements: Living cells with no nucleus, few organelles, and perforated end walls.
Companion cells: Provide ATP for the active transport of organic substances into the sieve tube element.
Xylem:
Dead, hollow cells with no organelles or end walls that stack up to create a continuous column.
The wall is strengthened with lignin (a waterproof chemical).
Transport of Water in Plants
Absorbed from the soil into root hair cells by osmosis.
Root hair cells are adapted with thin walls to reduce the diffusion rate and long protrusions providing a large surface area.
Symplast pathway: Water moves through the cytoplasm of cells cell-to-cell towards the xylem by osmosis through the cytoplasm, going through gaps in the cell wall (plasmodesmata). Each successive cell's cytoplasm has a lower water potential.
Apoplast pathway: Water moves through the cell walls due to cohesive forces, moving towards the xylem. This pathway transports water much faster as there is little resistance to the water in the cell wall.
Gas Exchange in Plants
Gas exchange happens through the stomata; the stomata are tiny pores mainly on the lower sides of the leaves.
The size of the opening is determined by guard cells.
Xerophytes
Extreme plants that have adaptations to reduce water loss and are found in environments with limited water, for example, the desert or mangrove grass, which is also found on the sand dunes.
Mangrove Grass Adaptations:
Curled leaves trap moisture, increasing humidity and decreasing the water potential gradient.
Hair-like structures trap moist air, increasing humidity.
Sunken stomata increase local humidity.
A thicker cuticle reduces water loss by evaporation.
Longer root network to absorb more water by osmosis.
Hydrophytes
Plants that live in or on water, requiring adaptations to survive in excess water.
Adaptations:
Short roots
Very thin or no waxy cuticle
Stomata permanently open on the top surface of the leaf.
Adaptations to ensure enough light is still absorbed for photosynthesis: Leaves being large and wide on the surface of the water.
Transpiration
Water vapor is lost from a leaf via the stomata.
Conditions that affect the rate of transpiration: light intensity, temperature, humidity, and the amount of wind, which is measured with a potometer.
*Details of how different light intensity, temperature, humidity, and wind rates increase or decrease the rate of transpiration are mentioned in the original transcript.
Movement of Water Up the Xylem
Water moves up a plant from the roots against gravity because of the cohesion-tension theory.
Cohesion of water molecules
Adhesion of water molecules to the xylem wall
Root pressure
*Details of cohesion, adhesion, and root pressure are explained in the original transcript.
Cohesion-Tension Theory
Water evaporates out of the stomata of the leaves, causing a lower volume of liquid in the leaves, and the pressure decreases.
Water moves to fill its space that moves up the xylem to replace it because of that negative pressure.
The continuous column of water is pulled up the xylem, replacing the water as it transpires.
water molecules adhere to the lignin in the xylem walls
Root pressure pushes up all of the water above it.
Transport Using the Pholem
The second type of transport is the transport of the organic substances from photosynthesis and the organic substances are transported through the phloem.
Translocation: The transport of organic substances in plants requires energy; it is an active process.
It revolves around that idea that there is mass flow from the source, and the source is where organic substances are made, like sucrose (created in photosynthesis). And there is also mass flow from that source to the sink, and the sink is the site where the organic substances are used in repiration, like in respiring tissues, where amino acids are used up.
Details for translocation are explained in the original transcript.