C- Interaction and Interdependence

1- Molecules: Enzymes and metabolism, Cell Respiration, and photosynthesis

Metabolism is all the chemical reactions happening in living things. These reactions can be part of a cycle or a straight (linear) pathway.

• There are two types of metabolic reactions:

  • Anabolic: build large molecules from smaller ones (need energy)

  • Catabolic: break down large molecules into smaller ones (release energy)

• Enzymes are special proteins that speed up reactions by lowering the energy needed.

• Enzymes have an active site where they bind to a substrate (the reactant).

• Each enzyme is specific – it only works with one type or a small group of substrates.

• For a reaction to happen, the enzyme and substrate must collide properly.

• Enzymes can stop working (be denatured) if the temperature or pH is too extreme.

• Enzyme activity is affected by temperature, pH, and substrate concentration, and we can measure reaction rates in experiments.

• ATP (adenosine triphosphate) is the main energy source for all cells. It powers things like transport, movement, and building molecules.

• ATP releases energy when its third phosphate is removed, forming ADP (adenosine diphosphate).

• When ATP is made from ADP, energy is stored.

• Cell respiration is how cells make ATP, usually by breaking down glucose.

• Respiration is not the same as breathing. Breathing is gas exchange, while respiration happens inside cells.

• There are two types of respiration:

  • Aerobic respiration: uses oxygen, makes lots of ATP, and produces carbon dioxide and water

  • Anaerobic respiration: happens without oxygen, makes less ATP, and produces lactate (in humans) or ethanol + CO₂ (in yeast)

• The rate of respiration is affected by temperature, pH, and substrate levels.

• A respirometer can be used to measure respiration rates.

• Photosynthesis is how certain organisms (like plants) use sunlight to make chemical energy that cells can use. This energy supports most life on Earth.

• It happens in two stages:

  • Light-dependent reactions: use sunlight to split water, releasing oxygen and hydrogen

  • Light-independent reactions: use hydrogen and carbon dioxide to make glucose

• Splitting water during photosynthesis also released oxygen, which changed Earth’s atmosphere over time.

• Chromatography is a method to identify plant pigments.

• Different pigments absorb different colors of light, which can be shown using an absorption spectrum.

• An action spectrum shows how effective different wavelengths are at driving photosynthesis.

• Three main factors affect the rate of photosynthesis:

  • Carbon dioxide concentration

  • Light intensity

  • Temperature

• Scientists test these using different experiments, like carbon dioxide enrichment or FACE experiments, to see how more CO₂ might affect plant growth in the future.

2- Cells: Neutral Signaling

• Neurons are cells that send electrical messages in the nervous system.

• Resting potential is when a neuron isn’t sending a signal — it’s kept ready by moving sodium (Na⁺) out and potassium (K⁺) in.

• Action potentials are the electrical signals that travel along neurons. They move faster if the axon is thicker or has myelin.
  

• Synapses are gaps between neurons or between a neuron and a muscle/gland. Messages cross these gaps using chemicals called neurotransmitters.

  • When an impulse reaches the end of a neuron, calcium ions enter and cause neurotransmitters to be released into the gap.

  • These chemicals bind to the next cell and can cause depolarisation, making it more likely to send its own signal (EPSPs).

3- Organisms: Integration of body, and Defense against disease

• Multicellular organisms are made of cells, tissues, organs, and systems working together. This creates emergent properties (the whole is more than just the parts).

• Nervous system uses fast, short-lived electrical signals.

• Endocrine system uses slower, longer-lasting hormonal (chemical) signals.

• The brain handles both conscious and unconscious control.

• The spinal cord controls only unconscious actions (like reflexes).

• The hypothalamus and pituitary gland control the endocrine system.

• Learning changes the brain’s structure (brain plasticity).

• Neural circuit:

  • Sensory neurons detect changes,

  • Interneurons process info,

  • Motor neurons carry instructions to effectors (muscles/glands).

• Nerves are bundles of neurons; may be myelinated (faster) or not.

• Reflexes are fast, automatic responses (e.g., pain reflex arc).

• Cerebellum: controls movement coordination, balance, posture, and motor learning.

• Melatonin (sleep hormone):

  • Made by the pineal gland.

  • Released in darkness, stopped by light → sets circadian rhythms.

• Epinephrine (adrenaline):

  • Made by adrenal glands.

  • Helps body react to danger (“fight or flight”).

  • Triggered by the hypothalamus.

• Mechanoreceptors (like baroreceptors and chemoreceptors) help control heart and breathing rate.

• Enteric nervous system: controls digestion.

  • Only swallowing and pooping are controlled by the CNS.

• Skin and mucous membranes are the first line of defense – they block pathogens physically and chemically.

• Blood clotting seals wounds to stop pathogens from entering. It’s triggered by clotting factors from platelets and damaged tissue.

• The immune system has two parts:

  • Innate immunity: fast, general defense (e.g., phagocytosis – white blood cells eat pathogens).

  • Adaptive immunity: slower but specific and long-lasting.

• Lymphocytes control adaptive immunity:

  • B-cells make antibodies to neutralize antigens.

  • T-cells help (helper T-cells) or kill infected cells (cytotoxic T-cells).

• ABO blood groups are based on antigens on red blood cells.

• When B- and T-cells meet their matching antigen, they activate.

  • B-cells need help from helper T-cells to activate.

  • They form plasma cells (make antibodies) and memory cells (for long-term immunity).

• HIV is a virus spread by blood, sex, shared needles, and from mother to child.

  • It attacks helper T-cells, weakening the immune system.

  • HIV infection can lead to AIDS.

• Antibiotics kill or stop bacteria from growing — but antibiotic resistance is a growing global problem.

• Zoonotic diseases (zoonoses) are diseases that spread from animals to humans.

• Vaccines (made from antigens, RNA, or DNA) train the immune system to fight specific diseases.

• Herd immunity happens when most people are immune, helping protect those who can’t be vaccinated.

4- Ecosystems: Population and Communities, and Transfers of energy and matter

• Population size can be estimated using random sampling.

  • Different methods are used for sessile (non-moving) and motile (moving) organisms.

• Carrying capacity = the maximum population an environment can support.

• Limiting factors affect population size:

  • Density-dependent (e.g. disease, competition – affected by population size).

  • Density-independent (e.g. weather, natural disasters – not affected by population size).

• Exponential growth: happens when there are unlimited resources and no competition.

• Sigmoidal (S-shaped) growth: happens when resources are limited and competition increases.

• Within a species, competition or cooperation can affect survival and growth.

• Species interactions:

  • Mutualism – both benefit

  • Parasitism – one benefits, one harmed

  • Pathogenicity – disease-causing

  • Competition – both harmed

  • Predation – one eats the other

  • Herbivory – eating plants

• Invasive species: non-native organisms that disrupt ecosystems.

• Chi-square test: used to check if two species are associated.

• Population control:

  • Top-down: controlled by predators

  • Bottom-up: controlled by resources

• Allelopathy and antibiotic secretion: release of chemicals to stop competitors.

• Sunlight is the main energy source for most ecosystems.

• Food chains and food webs show how energy moves through ecosystems.

• Autotrophs (like plants) make their own food using sunlight or inorganic compounds.

• Heterotrophs eat other organisms to get energy and nutrients.

• Decomposers break down dead organisms and return nutrients to the environment.

• Energy flows one way through ecosystems.

• Trophic levels group organisms by their place in the food chain (e.g. producer, consumer).

• Energy pyramids show how energy decreases at each trophic level.

• Energy transfer is inefficient because of:

  • Not all food being eaten

  • Poor digestion

  • Energy used in metabolism

  • Energy lost as heat

• This inefficiency limits the number of trophic levels in a food chain.

• Primary production: how fast producers make carbon-based compounds.

• Secondary production: how fast consumers build biomass from what they eat.

• Carbon cycle: carbon is recycled between the atmosphere, organisms, and the Earth.

• Ecosystems can be carbon sinks (store carbon) or carbon sources (release carbon), depending on photosynthesis vs. respiration balance.

• Burning fossil fuels adds too much CO₂, upsetting the natural carbon cycle.

• Photosynthesis (removes CO₂) and aerobic respiration (adds CO₂) work together to keep life going.