IB Biology SL 2025 Exam - Unit 6 Study Guide

Covering topics from Unit 6 that could be on the 2025 test!

What is Gas Exchange?

The process where organisms absorb gas from the environment and release another one

Properties of Gas-Exchange Surfaces

Permeable so oxygen and carbon dioxide can diffuse freely

Large so that it can diffuse faster

Moist so gases can dissolve

Thin so gases only diffuse through a short distance

Maintenance of Concentration Gradients at Exchange Surfaces

Since diffusion only happens if there are concentration gradients, there must be an imbalance in order for this to happen

For mammals, blood that flows through the capillary network will always either have a high concentration of oxygen and low of carbon dioxide, or vice versa, resulting in a concentration gradient

How are mammalian lungs adapted for gas exchange?

Large amounts of alveoli provide a large surface area, allowing for more efficient gas exchange

Cells in the wall secrete a pulmonary surfactant, Type II, to reduce surface tension and prevent water from causing the sides of the alveoli to adhere when air is exhaled, preventing the collapse of the lung

Ventilation of Lungs

During ventilation, muscle contractions cause the pressure inside the thorax to drop below atm, so air is drawn into the lungs from the atmosphere until the pressure is equal

Other muscle contractions cause pressure inside the thorax to rise above atmospheric, so air is forced out of the lungs

Tidal Volume

Volume of fresh air that is inhaled, as well as the amount of stale air that is exhaled with each ventilation

Vital Capacity

Total volume of air that can be exhaled after a maximum inhalation, or total volume of air that can be inhaled after a maximum exhalation

Inspiratory Reserve

The amount of air a person can inhale forcefully after normal tidal volume respiration

Expiratory Reserve

The amount of air a person can exhale forcefully after normal tidal volume respiration

Adaptations for gas exchange in leaves

Outer surface of leaf is covered in waxy cuticle, secreted by the epidermis cells

Waxy cuticle has low permeability to gases, but within there are pairs of guard cells that can open up a pore or close it (called the stomata), which allows CO₂ and oxygen to pass through

Walls of spongy mesophyll cells provide large total surface area for gas exchange

Distribution of tissues in a leaf

Transpiration as a consequence of gas exchange in a leaf

Water vapor is used to keep the gas exchange surfaces moist, allowing for oxygen and carbon dioxide to diffuse

Since water vapor will always diffuse out through the stomata because the water vapor concentration outside is less than inside, there will always be water evaporating from the moist surface

Stomata Density

Number of stomata per unit area of leaf surface

Adaptations of capillaries for exchange of materials between blood and the internal or external environment

Since capillaries are extremely narrow, it allows them to have a total surface area and increases the scope of diffusion

Capillary wall consists of one layer of endothelium cells (extracellular fibrous proteins to crosslink and form a gel), which allows small or medium-sized particles to pass through but not macromolecules

Tissue fluid leaks out, containing oxygen, glucose and other substances that flows between cells to let them absorb substances and excrete waste products

Arteries

Carry pulses of high-pressure blood away from the heart to the organs (except the pulmonary artery, which transported deoxygenated blood to the lungs of the heart)

Has a thicker wall, narrower lumen, circular, corrugated inner surface and there are fibers visible in the wall

Veins

Carry a stream of low-pressure blood from the organs to the heart (except the pulmonary vein, which transports oxygenated blood from the lungs to the heart)

Has a thinner wall, wider lumen, circular or flattened, smooth inner surface and few or no fibers in the wall

Adaptations of arteries for the transport of blood away from the heart

The elastic fibers in the artery walls can stretch and recoil, and the collagen has high tensile strength, allowing for the arteries to withstand the high pressure and high velocity without bulging or bursting

Elastic fibers helps reduce the amount of energy expended in transporting blood by storing potential energy when pressure pushes the walls of the arteries out, and then squeezing the blood in the lumen at the end of each heartbeat

Measurement of Pulse Rates

Wrist (radial pulse) or on the neck

Cannot use thumb as that also has a pulse

Adaptations of veins for the return of blood to the heart

Veins contain pocket valves that catch blood if it flows backwards and closes the valve, blocking the lumen of the vein. When blood flows back to the heart, it pushes the flaps to the sides of the vein, letting the blood flow freely

Contractions help veins flatten to push blood

Causes and consequences of occlusion of the coronary arteries

Coronary arteries can be narrowed or blocked (occlusion) by fatty deposits (called atheroma), which consist of a variety of lipids and restrict blood flow, causing pain in the chest or shortness of breath

Fatty deposits can become impregnated with calcium salts, which harden the artery and make the surface tough, which can lead to blood clots (thrombosis). Blood clots block blood flow to part of the muscular wall of the heart, depriving it of oxygen and preventing normal contractions, leading to heart attacks

Transport of water from roots to leaves during transpiration

Xylem vessels are normally filled with sap, which consists of water and other ions.

Since water is lost by evaporation from the cell walls of spongy mesophyll cells and then diffusion of water vapor through stomata, this causes water to be drawn through the interconnected leaf cell walls in the pores between cellulose molecules

As the cell walls draw water from the xylem, they generate tension. As long as there is a continuous column, this tension is transmitted from the leaves down to the roots, allowing for water to be drawn from roots to the leaves

Adaptations of xylem vessels for transport of water

Cell wall material between adjacent cells in the column are largely removed, as well as plasma membranes and contents of the cell break down to create long, continuous tubes

Vessel walls are thickened, with the thickenings impregnated with lignin, strengthening the walls and preventing them from collapsing due to the tensions

Lignified wall thickenings are impermeable to water but have small pores through which water can enter and exit

Neurons

Part of the nervous system that transmits nerve impulses to communicate with the body

Have a cell body with cytoplasm and a nucleus

Generation of the resting potential by pumping to establish and maintain concentration gradients of sodium and potassium ions

There is an imbalance of K+ ions and Na+ ions across the membrane (3 Na+ ions are pumped out while 2 Na+ are pumped in, resulting in around -70mV resting potential)

The ions will diffuse back, albeit very slowly. Additionally, the membrane is about 50 times more permeable to K+ than Na+, further causing an imbalance

Nerve Impulses

An action potential that starts at one end of a neuron and is propagated along the axon to the other end of the neuron

Variation in the speed of nerve impulses

Increase in diameter of nerve fibers speed up nerve impulses

Myelination consists of Schwann cells that cover the fiber (gaps are called nodes of Ranvier). The impulses jump from gap to gap, speeding up transmission up to 100 meters per second

Synapse

Junction between two cells in the nervous system. Signals can only pass in one direction across a synapse

Release of neurotransmitters from a presynaptic membrane

Nerve impulse is propagated along the presynaptic neuron until it reaches the end of the neuron and the presynaptic membrane

Depolarization of presynaptic membrane causes calcium ions to diffuse through channels in the membrane into the neuron

Influx of calcium causes vesicles containing neurotransmitters to move to the presynaptic membrane and fuse with it

Neurotransmitters are released into the synaptic gap (only about 20nm wide) by exocytosis

Generation of an excitatory postsynaptic potential

Once the neurotransmitters pass the synaptic gap, they bind to receptors in the postsynaptic membrane, causing channels to open

Ions diffuse down the concentration gradient into the postsynaptic neuron, causing the membrane potential to go from positive to negative

If the excitatory postsynaptic potential is strong enough, it triggers an action potential which propagates away from the synapse. Once this happens, the neurotransmitters break down and removed from the synaptic gap

System Integration

Systems that communicate with each other in order for the organism to function properly

Hierarchy of Subsystems

Cells, Tissues, Organs, Organ Systems, Organisms

Integration of organs in animal bodies by hormonal and nervous signalling and by transport of materials and energy

Endocrine system and nervous system work together in the human body to help organs perform their specific processes

The brain as a central information integration organ

The brain receives information, processes it, store information short or long term, and sends instructions to all parts of the body to coordinate life processes

The spinal cord as an integrating center for unconscious processes

The spinal cord is part of the central nervous system and contains white matter (which contains myelinated axons and other nerve fibres) and grey matter (which contains cell bodies of motor neurons and interneurons, with many synapses between said neurons)

Since the grey matter is used for processing information and for decision making, the spinal cord is an integrating center. However, it only coordinates unconscious processes

Input to the spinal cord and cerebral hemispheres through sensory neurons

Sensory neurons sense a change in the external environment. Receiving from receptor cells or their own sensory nerve endings, they act as stimuli to the nervous system

The signals are conveyed to the central nervous system via nerve impulses. They are received in the cerebral hemispheres of the brain

Output from the cerebral hemispheres to muscles through motor neurons

Primary motor cortex sends signals via motor neurons from the brain to each striated muscle in the body

The cell body and dendrites of many motor neurons are located in the grey matter of cerebral hemispheres. One axon leads from the cell body out of the brain and down the spinal cord, where it forms a synapse with another to the striated muscle

What are nerves composed of?

Bundle of nerve fibres enclosed in a protective sheath

Most include both sensory and motor neurons, but some nerves only have one type

Pain reflex arcs as an example of involuntary responses with skeletal muscle as the effector

Pain reflex arcs are rapid, involuntary response to a specific stimulus. Some are coordinated by the spinal cord (pain reflex), others are coordinated by the brain (constriction of the pupil)

They pass through the smallest number of neurons, helping to speed up reflexes

Cerebellum

Responsible for controlling skeletal muscle contraction and balance, which allows for very precise coordination of movements and helps maintain posture

It integrates sensory input with motor commands to produce adaptive motor coordination

If it is damaged, it can interfere with balance, timing, accuracy, or coordination of movements.

Circadian Rhythms

The adaptation in which humans live in a 24-hour cycle and have rhythms in behavior that fit this cycle

Secretion of melatonin is an example

Epinephrine (adrenaline) secretion by the adrenal glands to prepare the body for vigorous activity

Adrenaline is responsible for increasing the organism’s speed of processes to help meet higher demands

Muscle cells break down glycogen into glucose

Bronchi and bronchioles dilate, allowing for larger airways and faster respiration

Sinoatrial node speeds up heart rate, so cardiac output increases

Arterioles that carry blood to muscles and liver widen, while less important organs constrict

Control of the endocrine system by the hypothalamus and pituitary gland

Within the hypothalamus, there are nuclei that operate one or more specific control systems. Many of them receive signals from sense organs via the cerebral hemispheres, but sometimes receive from the medulla or hippocampus

The pituitary gland secretes hormones under the direction of nuclei in the hypothalamus

Feedback control of heart rate following sensory input from baroreceptors and chemoreceptors

If both receptors note there is high blood pressure, heart rate is slowed down to lower blood pressure

If both receptors note there is low blood pressure, heart rate increases to increase blood flow and increase blood pressure

Feedback control of ventilation rate following sensory input from chemoreceptors

If the chemoreceptors detect too low of a pH (too much CO2), ventilation rate increases to balance this out

If the chemoreceptors detect too high of a pH (too little CO2), ventilation rate decreases to lower pH

Pathogens

Disease-causing organisms

Skin and mucous membranes as a primary defense

Outermost layer provides a physical barrier

Most of the body is covered by a tough layer of dead cells that have the protein keratin, making it hard for pathogens to pass through

Certain sebaceous glands secrete a chemical called sebum, maintaining skin moisture and lowering pH

Secrete mucus, which is a physical barrier that traps pathogens and swallows or expels them

Have antiseptic properties because of the presence of lysozyme

Blood Clotting

Happens when blood vessels erupt due to cuts

First, platelets will form a plug at the site to stop bleeding, prothrombin helps with this

Then, platelets will produce thrombin to convert fibrinogen to fibrin, forming a mesh in the cuts

Innate Immune System

Respond to a broad range of pathogens

Does not change over the course of an organism’s lifespan

Involve phagocytes

Adaptive Immune System

Responds in a specific way to particular pathogens

Change over the course of an organism’s lifespan

Phagocytes

Use amoeboid movement to move from blood to site of infection

Use phagocytosis (the process where phagocytes engulf pathogens and digest and kill them with lysosomes)

Lymphocytes

Cells with a rounded nucleus and a small amount of cytoplasm

Produce antibodies to help destroy pathogens

Antigens

Any type of molecule that stimulates an immune response

Composed of glycoproteins or other proteins, and also some large polysaccharides

B-Lymphocytes

Produced in the bone marrow

Synthesize antibodies and can differentiate into antibody-releasing B-plasma cells or B-memory cells, once activated by T cells

T-Lymphocytes

Produced in bone marrow, processed in the thymus gland, often found in lymph nodes of the lymphatic system

Can differentiate into Helper T-cells or Cytotoxic T-cells

Multiplication of activated B-lymphocytes to form clones of antibody-secreting cells

When B-lymphocytes are activated, they do not immediately start to produce antibodies

They divide repeatedly by mitosis to form a clone of cells that all produce the same antibody

Once they grow in size, they develop an extensive rough endoplasmic reticulum with many ribosomes attached to it, along with a large Golgi apparatus, helping them to secrete multiple antibodies as quickly as possible

Immunity as a consequence of retaining memory cells

When B-lymphocytes divide repeatedly, most of them turn into active cells. However, a small portion turn into memory cells, which can create the same antibody to fight the same pathogen if the organism is re-infected

Immunity rises as more B memory cells are created

HIV

Virus that invades and destroys T helper cells, which can cause AIDS (Acquired Immunodeficiency Syndrome)

HIV can come from blood, semen, vaginal fluids, rectal secretions and breast milk

Can happen through unprotected sex or infected transfused blood

Antibodies

Large proteins that help to fight and kill diseases

They either make a pathogen more recognizable to phagocytes or prevent viruses from docking to host cells

Zoonoses

Diseases that can be transmitted to humans from other animals in natural circumstances

1 - Tuberculosis, comes from cattle and drinking unpasteurized milk

2 - Rabies, comes from dogs and bites or scratches

3 - Japanese Encephalitis, comes from pigs or birds, comes from mosquito bites

4 - COVID-19, thought to come from bats

Vaccines & Immunization

Vaccines contain antigens that allow a pathogen to be recognized by the immune system or nucleic acids from which antigens can be made

Vaccines can include: live but attenuated version of a pathogen, a killed form of a pathogen, or subunits of a pathogen that act as antigens

Vaccines trigger a primary immune response, building B-memory cells, therefore increasing immunity

Herd Immunity

When enough of the population is vaccinated so that those that are unvaccinated will not be infected by the disease

Homeostasis

State when the internal environment is regulated within narrow limits

The 4 homeostatic variables in humans are: blood glucose concentration, blood pH, blood osmotic concentration, and core body temperature

Negative feedback loops in homeostasis

Negative feedback helps to keep internal conditions in the body within narrow limits, in turn helping the human to maintain homeostasis

Regulation of Blood Glucose

Most of the pancreas, as well as the Islets of Langerhans, secrete certain substances to maintain the proper blood glucose concentration

If there is too much glucose, it is called hyperglycemia

If there is too little glucose, it is called hypoglycemia

Insulin

Comes from beta cells

Stimulates the uptake of glucose by many target cells in multiple tissues when the glucose concentration is too high. Glucose will convert back to glycogen

Glucagon

Comes from alpha cells

Stimulates the breakdown of glycogen to glucose if blood glucose falls below a certain point

Type 1 Diabetes

When the organism is unable to produce sufficient insulin

Symptoms are: Frequent urination, excessive thirst, extreme hunger, fatigue

Treated by testing blood glucose concentration regularly and injecting insulin when it is too high or likely to become too high. These are usually done before a meal.

Type 2 Diabetes

When the organism is unable to process or respond to insulin because of a deficiency of insulin receptors or glucose transporters on target cells

Symptoms are the same as Type 1

Treated by adjusting the diet to reduce the peaks and troughs of blood glucose

Thermoregulation

If a human is cold, blood vessels constrict, shivering occurs to generate heat, brown adipose tissue oxidize fat to produce heat, and hair stands up

If a human is too hot, blood vessels dilate, and sweating occurs to cool the body down thanks to its high latent heat of vaporization

Fenestrated Capillaries

Capillaries that have greater numbers of very large pores

Allow larger volumes of tissue fluid to be produced, speeding up the exchange between tissue cells and blood

Tunica Externa

Tough outer layer of connective tissue with collagen fibers in arteries

Tunica Media

Thick layer containing smooth muscle and elastic fibers made of elastin in arteries

Tunica Intima

Smooth endothelium forms the lining of the artery, sometimes including a layer of elastic fibers

Coronary Heart Disease

Conditions associated with narrowed or blocked coronary arteries due to occlusions

Coronary Heart Disease Risk Factors

Hypertension, smoking, eating too much saturated fat, obesity, high salt intake, drinking excessive alcohol, sedentary lifestyles, old age, genetic predisposition

Xylem

Transport of water from roots to leaves

Phloem

Transport of sugars from leaves to roots

Cambium

Production of more xylem and phloem

Upper and Lower Epidermis

Regulates gas exchange in and out of the leaf. Has a small number of pores in it called stomata

Cortex

Responsible for support and helping with photosynthesis

Pith

Responsible for bulking out the stem

Dicotyledonous Stem Transverse

Dicotyledonous Root Transverse

Dendrites

Short-branched nerve fibers

Those used to transmit impulses between neurons in one part of the brain or spinal cord

Axons

Very elongated nerve fibers

Those that transmit impulses from the tips of toes or fingers to spinal cord

Depolarization

Changing the membrane potential from negative to positive

Due to the opening of the sodium channels, allowing Na+ ions to diffuse down the concentration gradient, raising the membrane potential from about -70mV to +30mV

Repolarization

Changing the membrane potential from positive to negative

Happens rapidly after depolarization and is due to the closing of sodium channels and opening of potassium channels, letting the membrane potential fall down to -70mV

3 Types of Synapses

Between sensory receptor cells and neurons (in sense organs)

Between neurons, in both brain and spinal cord

Between neurons and muscle fibres or gland cells

Presynaptic Neuron

Brings signal to synapse in the form of a nerve impulse or action potential

Postsynaptic Neuron

Carries signal away from the synapse, again in the form of a nerve impulse

Neurotransmitters

Carry signals across a narrow fluid-filled gap (20nm) between the presynaptic and postsynaptic neurons

Vasoconstriction

When the smooth muscle cells contract and the diameter of the lumen is narrowed. It reduces the flow of blood along an artery or arteriole

Vasodilation

When the smooth muscle cells relax and the lumen widens, allowing for more blood to flow

Aorta

Carries blood pumped by the left side of the heart to all organs of the body except the lungs

Inspiration Process

External intercostal muscles moves up and outwards, allowing the thorax to increase

The diaphragm contracts and moves downwards

Pressure in the lungs decreases below atm

Air enters lungs as the atm is greater

Expiration Process

Diaphragm and external intercostal muscles relax while internal intercostal muscles contract. The rib cage moves downwards and inwards, decreasing the volume of the thorax

Pressure in lungs increases compared to atm

Air leaves the lungs

Palisade Cell

Found in the palisade layer. They contain many chloroplasts for maximizing light absorption for photosynthesis

Xylem Tracheids

Long, narrow, tapered cells with thick, lignified walls that provide structural support. The water moves through the pits between adjacent tracheids

Xylem Vessels

Shorter, but wider and more cylindrical that forms end-to-end. The end walls are perforated or absent, allowing for more efficient flow of water

Vertebrate vs Invertebrate Neurons

Vertebrate neurons typically have myelination and a smaller axon diameter, while invertebrate neurons don’t have myelination and therefore a larger axon diameter