Summary
Highlights
Optical or light microscopes, commonly used in schools, magnify images to allow observation of structures like nuclei and mitochondria. Magnification is calculated as image size divided by actual size. However, light microscopes are limited in resolution, unable to view smaller organelles like ribosomes. Electron microscopes offer much higher magnification and resolution, enabling detailed viewing of internal cellular structures, subcellular organelles, and even the smallest organisms like bacteria, significantly advancing our understanding of life.
Transpiration rate can be measured using a potometer by observing the movement of an air bubble in a capillary tube, which indicates water uptake by a plant shoot. Factors influencing transpiration include air movement (increases evaporation), light intensity (opens stomata), and temperature (increases evaporation). Humidity decreases transpiration as it reduces water potential gradient.
To prepare a microscope slide, a thin tissue sample (e.g., onion skin) is stained with iodine and covered with a cover slip. The slide is placed on the microscope stage, and focusing begins with the lowest power objective lens using the coarse focus wheel, followed by increasing magnification and fine-tuning with the fine focus wheel. Biological measurements often involve very small units such as millimeters (10^-3 m), micrometers (10^-6 m), and nanometers (10^-9 m). Conversion between these units is crucial for calculations, typically involving multiplying or dividing by 1000. Comparing sizes of organisms or structures involves dividing the larger size by the smaller, ensuring consistent units.
An animal cell consists of a cell membrane controlling substance entry/exit, cytoplasm where chemical reactions occur, ribosomes for protein synthesis, a nucleus controlling cell activities, and mitochondria responsible for aerobic respiration and energy production. These organelles work together to maintain cell function and energy supply.
Plant cells share common features with animal cells like a nucleus, ribosomes, and mitochondria. Unique to plant cells are chloroplasts for photosynthesis (absorbing light energy), a permanent vacuole for cell support, and a cell wall made of cellulose for structural support.
Eukaryotic cells, like animal and plant cells, possess a nucleus containing DNA. Prokaryotic cells, such as bacteria, lack a nucleus; their DNA is free-floating in the cytoplasm. Prokaryotic cells also have a cell membrane, cytoplasm, ribosomes, and a cell wall (not made of cellulose), and some may possess flagella for movement and plasmids containing useful genes like antibiotic resistance. Key differences include the absence of mitochondria, chloroplasts, and a nucleus in prokaryotic cells.
DNA is a coiled double helix structure found in chromosomes within the nucleus. Short sections of DNA are called genes, which code for proteins. The genome is the entire genetic material of an organism. The Human Genome Project, completed in 2003, sequenced the human genome, enabling research into disease-gene links, inherited genetic disorders, and human migration patterns. DNA is a polymer made of nucleotides, each containing a phosphate, sugar, and one of four bases (A, C, G, T). A sequence of three bases codes for a specific amino acid, determining the protein's sequence.
Protein synthesis begins with RNA polymerase creating an mRNA template from a gene sequence, which then travels to a ribosome. tRNA molecules bring specific amino acids to the ribosome, guided by complementary base pairing with the mRNA. This forms a protein chain, which folds into a unique shape dictating its function (enzyme, hormone, antibody, structural protein). Changes in the DNA sequence can alter the amino acid sequence, leading to different protein shapes and functions. Enzyme reactions can be measured by monitoring color changes, product mass/volume, or pH changes, while considering independent and control variables for accurate results.
Digestive enzymes break down large, insoluble molecules (carbohydrates, proteins, lipids) into small, soluble ones (sugars, amino acids, glycerol, fatty acids) for absorption and utilization. Amylase breaks down carbohydrates, protease breaks down proteins, and lipase breaks down lipids. Enzyme activity is influenced by temperature (optimum temperature maximises rate, high temperatures cause denaturation), pH (optimum pH varies by enzyme, extremes cause denaturation), and substrate concentration (increased concentration increases rate until all active sites are saturated).
Respiration is an exothermic process releasing energy from sugars in cells. Aerobic respiration occurs in mitochondria, uses oxygen and glucose to produce CO2, water, and a large amount of ATP. This energy is used for metabolic reactions, active transport, muscle contraction, maintaining body temperature, cell division, and nerve impulses. Anaerobic respiration occurs in the cytoplasm when oxygen is limited, producing less ATP. In animal cells, it produces lactic acid, which is toxic and removed by the blood to the liver. In plant and yeast cells (fermentation), it produces ethanol and CO2, utilized in industries like baking and brewing.
Metabolism encompasses all chemical reactions in the body, driven by enzymes and energy from respiration. Glucose, produced by photosynthesis, can be converted to glycogen, starch, or cellulose, or combined with nitrate ions to form amino acids and then proteins. Photosynthesis, an endothermic reaction in plant chloroplasts, uses light energy, CO2, and water to produce glucose and oxygen. Plants use glucose for respiration, storage (starch, lipids), structural components (cellulose), and protein synthesis (with nitrates).
Photosynthesis rate is affected by light intensity, CO2 concentration, temperature, and chlorophyll amount. Increased light intensity, CO2, and temperature (up to an optimum) boost the rate. Chlorophyll content directly relates to light absorption. Practical investigations involve changing one independent variable (e.g., light distance) while controlling others (temperature, water, CO2, light color, pondweed type) and measuring oxygen production (bubbles) over time. The inverse square law applies to light intensity and distance. Optimal conditions in greenhouses (controlled light, temperature, CO2) can increase plant yield, but costs must be balanced.
Diffusion is the passive net movement of particles from high to low concentration. It's crucial for oxygen and glucose entering cells, and CO2 and urea exiting. Factors affecting diffusion rate include concentration gradient, distance, temperature, and surface area. Osmosis is the passive net movement of water molecules from a dilute to a concentrated solution across a semi-permeable membrane. Its rate is affected by temperature, surface area, and concentration gradient. Different solutions cause distinct effects on animal and plant cells: dilute solutions cause animal cells to swell (and burst), plant cells to swell (but not burst due to cell wall); concentrated solutions cause both to shrink/shrivel; isotonic solutions cause no net movement.
An osmosis practical involves placing plant tissue (e.g., potato) in different sugar/salt concentrations and measuring the change in mass. Independent variable is concentration, dependent is mass change. Control variables include solution volume, time, tissue size/volume/surface area, tissue type, and temperature. The concentration at which there is no mass change indicates the cell's cytoplasm concentration. Active transport moves particles against their concentration gradient (low to high) using energy from respiration. It is affected by the rate of respiration, number of mitochondria, and carrier proteins. Examples include mineral ion uptake by root hair cells and glucose absorption by small intestine villi, maximizing nutrient absorption.
DNA coils to form chromosomes, which are located in the nucleus. Humans have 23 pairs of chromosomes. The cell cycle prepares cells for division, with the majority spent in interphase (DNA replication, cell growth, increase in subcellular structures). Mitosis is the division phase where chromosomes are pulled to opposite poles, followed by cell membrane and cytoplasm division, resulting in two genetically identical daughter cells. This process is essential for organism growth and repair of damaged tissues.
Cells become specialized through differentiation, where specific genes are activated or deactivated, altering cell shape and subcellular structures for particular functions. Examples include root hair cells for water/mineral absorption, xylem/phloem for transport in plants, sperm cells for reproduction, nerve cells for impulse transmission, and muscle cells for contraction. Undifferentiated cells are stem cells. Embryonic stem cells can differentiate into almost any cell type. Adult stem cells (e.g., bone marrow) can differentiate into some cell types. Plant stem cells (meristems in roots/shoots) can differentiate into any plant cell throughout their life, enabling cloning. Therapeutic cloning uses embryonic stem cells genetically identical to the patient to replace damaged cells.
Ethical objections to embryonic stem cell use include the inability of the embryo to consent and the view of an embryo as a potential life. Risks include viral infection transfer and potential tumor formation due to rapid division of stem cells.
Exchange surfaces maximize diffusion rate through adaptations like thin walls, large surface area, and good blood/air supply. Examples include alveoli in lungs for gas exchange, villi in the small intestine for nutrient absorption, and leaves for gas exchange in plants. Large organisms have a smaller surface area to volume ratio, making simple diffusion insufficient. They require specialized exchange surfaces and transport systems (like the circulatory system) to distribute substances efficiently to all cells, unlike single-celled organisms with a large surface area to volume ratio.
Arteries carry blood away from the heart at high pressure, characterized by a narrow lumen, thick elastic layer for stretch and recoil, and thick muscle layer to withstand pressure. Veins carry blood to the heart at low pressure, having a wider lumen, thinner muscle/elastic walls, and valves to prevent backflow. Capillaries facilitate substance exchange, with very narrow lumens to slow blood flow and thin, one-cell-thick walls to minimize diffusion distance. They connect arteries and veins.
The heart has four chambers: two atria (top) and two ventricles (bottom). Deoxygenated blood from the body enters the right atrium via the vena cava, flows to the right ventricle, and is pumped to the lungs via the pulmonary artery. Oxygenated blood from the lungs enters the left atrium via the pulmonary vein, flows to the left ventricle, and is pumped to the rest of the body via the aorta. The left ventricle has a thicker muscular wall to pump blood at higher pressure over a longer distance. The human circulatory system is a double circulation system where blood passes through the heart twice. The heart rate is controlled by pacemaker cells in the right atrium; artificial pacemakers can regulate irregular heartbeats.
The lungs are responsible for oxygenating blood and removing carbon dioxide. The trachea branches into bronchi, which further divide into bronchioles, ending in alveoli. Alveoli, alongside capillaries, have one-cell-thick walls for efficient gas exchange. Deoxygenated blood arriving at alveoli releases CO2 and takes up O2, which binds to red blood cells for transport back to the heart.
Blood is a tissue comprising plasma (mostly water, dissolving transported substances like glucose, antibodies), red blood cells, white blood cells, and platelets. Platelets clot open wounds to prevent infection and excessive bleeding. Red blood cells are adapted for oxygen transport: no nucleus, rich in hemoglobin, biconcave shape for large surface area, and flexible to pass through capillaries. White blood cells (phagocytes and lymphocytes) defend the body against pathogens. Blood donations provide life-saving products like red blood cells, platelets, plasma, and antibodies, but require screening for pathogens and blood type matching.
Living organisms are organized hierarchically: cells (basic unit of life) form tissues (similar cells working together, e.g., palisade tissue), tissues form organs (groups of tissues with a function, e.g., heart, leaf), organs form organ systems (organs working together, e.g., nervous system), and organ systems form the whole organism (e.g., human, plant).
A leaf is a plant organ adapted for photosynthesis and gas exchange. Key tissues include xylem (transports water and mineral ions), palisade mesophyll (contains chloroplasts for photosynthesis), upper epidermis (transparent, allows light through), spongy mesophyll (air spaces for gas diffusion), lower epidermis (contains stomata), and phloem (transports sugars). Guard cells control stomatal opening/closing to balance gas exchange with water loss.
The nervous system, comprising the central nervous system (brain and spinal cord) and nerves, allows reactions to surroundings and coordinates responses via electrical impulses. A stimulus is detected by a receptor (in sense organs), sending impulses through sensory neurons to a coordinator (CNS), which then sends impulses via motor neurons to an effector (muscle or gland) for a response. Nerve cells have dendrons/dendrites for receiving impulses, a long myelinated axon for rapid long-distance transmission, and nerve endings connecting to other neurons/effectors via synapses (chemical release across a gap). Reflex actions are involuntary, rapid responses (e.g., knee jerk, blinking) that do not involve the brain, protecting the body from harm. They involve a reflex arc: sensory neuron, relay neuron (in spinal cord), and motor neuron.
Reaction time can be measured using a ruler drop test, where a participant catches a falling ruler, and the distance fallen is converted to time. Independent variables (e.g., practice, sleep, caffeine) can affect reaction time, while dependent variable is the distance the ruler falls. Control variables ensure only the intended factor is tested. A lower reaction time (smaller number) indicates a faster response.
The eye is a sense organ with receptors for light and color. Key structures include the optic nerve (transmits impulses to brain), retina (contains rods and cones), sclera (supports/protects), iris (muscles controlling pupil size), cornea (transparent, focuses light), ciliary muscles and suspensory ligaments (control lens shape for focusing). The eye adapts to light intensity (pupil constriction/dilation) and accommodates for near/far vision (lens changes shape). Hyperopia (long-sightedness) is where the lens is too flat; fixed with convex lenses. Myopia (short-sightedness) is where the lens is too curved; fixed with concave lenses. Other treatments include laser eye surgery and cataract replacement.
The brain, a complex organ of interconnected neurons, controls human behavior. Different regions have specific functions: the cerebral cortex for conscious thought and higher functions, cerebellum for coordination and balance, and medulla oblongata for unconscious behaviors (heart/breathing rate). Brain function is studied through analyzing brain damage, scanning (MRI, CT, PET scans visualize structures, PET shows active areas), and electroencephalography (EEGs). Investigating brain disorders is challenging due to the brain's complexity, delicate nature, inaccessibility, limited repair capabilities of nervous tissue, and ethical concerns. Brain damage can result from injury, stroke, alcohol, or tumors, causing varied symptoms and often requiring careful treatment decisions due to associated risks.
The endocrine system consists of glands secreting hormones directly into the bloodstream, which travel to target cells. This system uses chemical signals, is slower than the nervous system, but can affect multiple organs globally. Key glands include the pituitary (master gland, controlling other glands, FSH, LH), adrenal (adrenaline), pancreas (insulin, glucagon), ovaries (estrogen, progesterone), testes (testosterone), and thyroid (thyroxine). Hormones initiate puberty, causing secondary sexual characteristics and gamete production (sperm, egg maturation, release).
Thyroxine, from the thyroid gland, controls metabolic rate, growth, and development, regulated by a negative feedback loop involving the pituitary gland's TSH. Adrenaline, from adrenal glands, prepares the body for 'flight or fight' in stress, increasing heart and breathing rates and diverting blood flow. The menstrual cycle (approx. 28 days) is controlled by FSH, LH, estrogen, and progesterone, which interact to mature an egg, prepare the uterus lining, and trigger ovulation (day 14). Increases and decreases in these hormones regulate the cycle and can inhibit further egg maturation during potential pregnancy.
Contraception methods include hormonal options (pill, patch, implant, IUDS with progesterone/estrogen) that inhibit FSH/LH to prevent egg maturation/release, and non-hormonal barrier methods (condoms, physical barriers, spermicides) that block sperm or render them inactive. Surgical sterilization offers permanent contraception. Evaluation of methods considers benefits (STI protection, long-term) and disadvantages (side effects, infection risk, ethical concerns). Infertility can be treated with fertility drugs (FSH, LH to stimulate egg production) or IVF (in vitro fertilization), where eggs and sperm are fertilized in the lab and embryos implanted into the uterus. IVF offers biological parenthood but with physical/emotional stress, no increased success rate, and ethical concerns regarding unused embryos.
Plant hormones control growth responses to light (phototropism) and gravity (geotropism). Auxin, a key hormone, causes unequal growth in roots and shoots. In shoots, more auxin on the shaded side causes increased cell elongation, leading to bending towards light (positive phototropism). In roots, auxin inhibits growth, leading to bending away from light (negative phototropism) and towards gravity (positive geotropism). Phototropism can be investigated by varying light direction and measuring shoot growth. Geotropism can be studied in darkness by positioning seedlings at different angles. Plant hormones have agricultural applications: auxins as weed killers or rooting powders, ethene for fruit ripening control, and gibberellins to promote seed germination, flowering, and fruit size.
Homeostasis maintains internal body conditions (temperature, blood glucose, water levels) at an optimum, controlled by the nervous and endocrine systems via negative feedback loops: receptors detect stimuli, coordinators process information, and effectors trigger responses. Temperature control by the thermoregulatory center in the brain involves vasoconstriction/vasodilation, muscle contraction (shivering), and sweating to regulate heat loss/gain. Blood glucose is regulated by the pancreas: insulin lowers high glucose by promoting absorption and glycogen storage in the liver, while glucagon raises low glucose by breaking down glycogen in the liver. Diabetes (type 1: insufficient insulin, type 2: insulin resistance) results from improper blood glucose control, treated with insulin injections or diet/exercise.
Maintaining water balance is vital to prevent cell damage from osmosis. Excess water, ions, and urea are removed via excretion (sweat, exhale, urine). Kidneys filter blood, selectively reabsorbing useful substances (glucose, some ions, water) and excreting waste in urine. Kidney failure leads to urea buildup, treated by dialysis (filtering blood externally) or kidney transplant. ADH (antidiuretic hormone), secreted by the pituitary gland, controls water reabsorption in the kidneys, regulating urine volume. High ADH leads to less water in urine (more reabsorbed), low ADH leads to more water in urine (less reabsorbed).
Hyperthermia (body temperature too high) can be caused by heat stroke, treated by slow temperature decrease. Hypothermia (body temperature too low) can be acute or chronic, treated by slow temperature increase. Both are more dangerous for babies and the elderly. Excess water can cause cells to swell, potentially affecting the brain. Dehydration occurs when fluid loss (sweat, urine) exceeds intake, leading to thirst and concentrated urine. High salt intake (from diet or excessive fluid loss) causes thirst and can damage kidneys. Athletes and the elderly are especially vulnerable to dehydration and salt imbalances.
Root hair cells are specialized for absorption with abundant mitochondria for active transport and a large surface area for osmosis. Xylem tissue, made of lignified dead cells, forms continuous tubes for one-way water and mineral ion transport (transpiration). Phloem tissue, comprising living cells and companion cells, transports sugars around the plant in a two-way process called translocation, which is an active process requiring energy from companion cells. Transpiration involves water vapor evaporating from stomata, creating a continuous pull of water from roots to leaves (transpiration stream), aided by water molecule cohesion.