Summary
Highlights
This section details the structure and function of animal cells. Key components include the cell membrane (controls entry/exit of substances), cytoplasm (site of chemical reactions), ribosomes (protein synthesis), nucleus (controls cell activities), and mitochondria (aerobic respiration for energy). Students need to be able to label these parts and understand their functions.
Plant cells share many components with animal cells but have additional structures. These include chloroplasts (absorb light for photosynthesis), a permanent vacuole (contains cell sap, supports the cell), and a cell wall (made of cellulose, provides structural support). The three key differences between plant and animal cells are the presence of a cell wall, vacuole, and chloroplasts in plant cells.
Eukaryotic cells, which include both plant and animal cells, are characterized by having a nucleus that contains DNA. Prokaryotic cells, like bacteria, lack a nucleus; their DNA floats freely in the cytoplasm. Bacterial cells also have a cell membrane, cytoplasm, ribosomes, and a cell wall (not made of cellulose), and some may possess a flagellum for movement and plasmids (small loops of DNA).
Cells differentiate to perform specific functions within an organism. Examples of specialized plant cells include root hair cells (absorb water and minerals, lacking chloroplasts) and xylem and phloem (transport water, minerals, and sugars). Specialized animal cells include sperm cells (for swimming to the egg), nerve cells (for transmitting electrical impulses), and muscle cells (contain many mitochondria for contraction).
Cell differentiation is the process by which cells become specialized, turning specific genes on or off. Undifferentiated cells are called stem cells. Plant cells can differentiate throughout their life, enabling cloning. Animal cells primarily differentiate early in development, forming various tissues and organs, and then primarily divide by mitosis for growth and repair. Embryonic stem cells can differentiate into almost any cell type, while adult stem cells (e.g., from bone marrow) have more limited differentiation potential. Plant stem cells are found in meristems.
Microscopes magnify images to allow observation of small structures. Light microscopes can magnify up to 1500 times, allowing visualization of nuclei and mitochondria. Electron microscopes offer much higher magnification and resolution, revealing finer details within cells, organelles, and even bacteria. Magnification is calculated as image size divided by actual size. A practical on preparing and viewing a microscope slide (e.g., onion cells) involves adding water, thinly spread tissue, staining (e.g., with iodine), and adding a cover slip, then focusing at low and high power.
Bacteria reproduce rapidly via binary fission. They can be cultured in liquid broth or on agar plates, requiring sufficient nutrients, oxygen, and warmth, and aseptic techniques to prevent contamination. Sterilization of media, flaming instruments, disinfecting surfaces, and working near a flame are crucial. Plates are incubated at 25°C (to prevent pathogenic growth) with loosely taped lids and stored upside down. Observing clear zones around antibiotic discs helps determine antibiotic effectiveness.
DNA is a long, twisted double helix, with sections called genes. DNA coils to form chromosomes within the nucleus. Humans have 23 pairs of chromosomes. The cell cycle involves interphase (DNA replication, cell growth, increase in organelles) and mitosis (cell division). Mitosis produces two genetically identical daughter cells, essential for organism growth and tissue repair.
Stem cells offer therapeutic potential. Embryonic stem cells can treat conditions like paralysis and diabetes by replacing damaged cells. Plant stem cells (meristems) enable rapid plant cloning. Ethical concerns surrounding embryonic stem cells include lack of consent from the embryo and destruction of potential life. Risks include viral infection transfer and potential tumor formation due to rapid division.
Diffusion is the net movement of particles from high to low concentration, a passive process requiring no energy. Cells primarily take in oxygen and glucose and release carbon dioxide and urea via diffusion. Factors affecting diffusion rate include concentration gradient, distance, temperature, and surface area. Exchange surfaces like alveoli, villi, and leaves are adapted with thin walls, large surface areas, and good supply to maximize diffusion efficiency.
The surface area to volume ratio decreases as an organism's size increases. Single-celled organisms have a large ratio, facilitating efficient exchange via diffusion. Larger organisms require specialized exchange surfaces (e.g., alveoli, villi) and transport systems (e.g., circulatory system) to meet the needs of all cells due to their smaller surface area to volume ratio.
Osmosis is the net movement of water molecules from a dilute to a concentrated solution across a partially permeable membrane, also a passive process. Water movement into or out of cells depends on the concentration difference between the cytoplasm and the external solution. Factors affecting osmosis are temperature, surface area, and concentration gradient. In a dilute solution, animal cells swell and burst, while plant cells swell but are protected by their cell wall. In a concentrated solution, animal cells shrink, and plant cell vacuoles shrink, pulling the cytoplasm from the cell wall.
A practical to observe osmosis involves placing plant tissue (e.g., potato) in various concentrations of salt/sugar solution. The independent variable is solution concentration, and the dependent variable is the change in tissue mass. Control variables include solution volume, time, tissue size/volume, tissue type, temperature, and ensuring no skin on the tissue. Blotting tissue dry before weighing is crucial. Graphs of percentage change in mass against concentration can identify the cell's cytoplasm concentration where no net water movement occurs (x-intercept).
Active transport is the movement of particles against their concentration gradient (from low to high concentration), requiring energy from respiration. Its rate is influenced by respiration rate, number of mitochondria, oxygen availability, and the number of carrier proteins. Examples include root hair cells absorbing mineral ions from the soil and small intestine cells absorbing glucose into the blood. These cells have many mitochondria to supply the necessary energy.
Living organisms are organized hierarchically: cells (smallest living unit) form tissues (similar cells working together), which form organs (tissues working together), which form organ systems (organs working together), culminating in a whole organism.
The digestive system involves organs like salivary glands (secrete amylase for carbohydrate digestion), stomach (hydrochloric acid, protease for protein digestion), and small intestine (absorbs small molecules, contains amylase, protease, lipase from pancreas for complete digestion of carbohydrates, proteins, and lipids). Enzymes work via a lock-and-key mechanism, with specific active sites for substrates. Digested molecules like amino acids, glucose, glycerol, and fatty acids are absorbed and used for growth, energy, and insulation.
Enzyme activity is affected by temperature (optimum temperature, then denaturation) and pH (optimum pH, then denaturation). Increasing substrate surface area or concentration also increases reaction rate. Denaturation means the active site changes shape, preventing substrate binding. Bile, produced by the liver and stored in the gallbladder, neutralizes stomach acid in the small intestine and emulsifies fats (breaking large droplets into smaller ones), increasing surface area for lipase action, but it is not an enzyme.
Chemical tests identify different molecules in food: starch (iodine, yellow-brown to blue-black), proteins (Biuret reagent, blue to purple), glucose (Benedict's reagent, heated, light blue to brick red), and lipids (ethanol then water, cloudy white emulsion). A negative result shows no color change or emulsion.
Enzyme reaction rates can be measured by changing independent variables (pH, temperature, substrate concentration) and observing dependent variables (color change with indicators, mass/volume of product, pH change over time). For example, testing amylase activity at different temperatures by measuring the time for iodine to stop changing color (indicating starch digestion).
The circulatory system comprises arteries (transport blood away from the heart at high pressure: narrow lumen, thick elastic and muscle layers), veins (transport blood to the heart at low pressure: wider lumen, thinner walls, valves to prevent backflow), and capillaries (transport blood close to cells for exchange: very narrow lumen, one-cell-thick walls).
Blood is a tissue composed mainly of plasma (transports substances like glucose, antibodies, amino acids), red blood cells (no nucleus, contain hemoglobin, biconcave for oxygen transport), white blood cells (phagocytes engulf pathogens, lymphocytes produce antibodies/antitoxins), and platelets (involved in clotting).
The heart has four chambers: right and left atria (top) and ventricles (bottom). Deoxygenated blood 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 body via the aorta. The left ventricle has a thicker muscle wall to pump blood at higher pressure. The heart is a double pump, and its rate is controlled by pacemaker cells in the right atrium.
The trachea branches into bronchi, then bronchioles, ending in alveoli. Gas exchange occurs in the alveoli, which are surrounded by capillaries. Both alveoli and capillary walls are one cell thick. Carbon dioxide diffuses from deoxygenated blood into the alveoli, and oxygen diffuses from alveoli into the blood, binding to red blood cells for transport back to the heart.
Coronary arteries supply blood to the heart muscle. Blockage by fatty plaques reduces blood flow, limiting oxygen and glucose to the heart muscle, leading to reduced respiration and potential heart attack. Risk factors include poor diet, lack of exercise, high blood pressure, high cholesterol, and obesity. Treatments include statins (lower cholesterol), stents (widen arteries), and in severe cases, heart transplant, heart bypass, or artificial heart.
Health is physical and mental well-being. Ill health can be caused by communicable diseases (spread via pathogens), non-communicable diseases (not spread by pathogens, often long-term), diet, and stress. Diseases can interact; e.g., viruses (HPV) can cause cancer, infections trigger allergic reactions (asthma), and physical ill health can lead to mental illness (depression). Non-communicable diseases have risk factors like genetics, lifestyle choices (alcohol, high-fat diet, smoking), and exposure to ionizing radiation.
Cancer is a non-communicable disease characterized by uncontrolled cell division, forming tumors. Damage to DNA leads to this uncontrolled growth. Benign tumors are slow-growing and contained, not spreading. Malignant tumors are fast-growing, cancerous, and can spread (metastasis) to form secondary tumors. Risk factors include smoking, UV radiation exposure, obesity, viral infections (HPV), and genetic factors (e.g., BRCA gene).
A leaf is a plant organ with tissue layers: xylem (transports water/minerals), palisade mesophyll (photosynthesis), upper epidermis (transparent), spongy mesophyll (gas exchange), lower epidermis (stomata), phloem (transports sugars), and guard cells (control stomata). Roots, stems, and leaves form an organ system for substance transport. Transpiration (water evaporation from stomata) drives the transpiration stream through xylem. Phloem transports sugars via translocation.
Transpiration is the evaporation of water vapor from leaves, primarily through stomata. Its rate can be measured using a potometer. Factors affecting transpiration rate are air movement (wind, increases evaporation), light intensity (more stomata open), and temperature (increases evaporation). Humidity is inversely related; higher humidity decreases transpiration.
Communicable diseases spread between organisms due to pathogens: viruses (acellular, replicate inside cells), bacteria (prokaryotic, release toxins), fungi (feed on living tissue), and protists (single-celled, often parasitic). Transmission occurs via air (droplet infection), direct contact (contaminated food/water, infected tissue), or vectors (e.g., mosquitoes for malaria).
Measles (red rash, high temperature, cough) is spread by droplet infection and prevented by vaccination. HIV (infects and kills white blood cells, leading to AIDS) is transmitted through sexual contact or bodily fluids and prevented by barrier methods. Tobacco mosaic virus (plant disease, discolors leaves, reduces growth) is spread by insects and preventable by burning infected plants and sterilizing tools.
Bacterial diseases include gonorrhea (STI, discharge, pain, fever, prevented by condoms) and salmonella (food poisoning, sickness, fever, diarrhea, prevented by hygiene and cooking). Both cause symptoms from toxins. Antibiotics kill bacteria. Rose black spot (plant fungal disease, black/purple spots on leaves, reduced growth) is spread by spores and prevented by fungicides, removing infected leaves, and burning.
Malaria, caused by a protist parasite transmitted by mosquitoes, causes fever and can be fatal. Prevention involves avoiding mosquito bites and controlling mosquito populations. Human non-specific defenses include the nose (hairs, mucus), trachea/bronchi (cilia, mucus), stomach (hydrochloric acid), and skin (physical barrier). Specific defenses involve white blood cells: phagocytes engulf pathogens, lymphocytes produce antibodies and antitoxins.
Vaccines contain dead or weakened pathogens, triggering white blood cells to produce antibodies and memory cells against specific antigens. This leads to immunity, as subsequent exposures trigger a faster, stronger secondary immune response, preventing illness. Vaccinating a large population (herd immunity) prevents disease spread.
Painkillers (e.g., ibuprofen, paracetamol) treat disease symptoms (pain, fever) but do not affect pathogens. Antibiotics (e.g., penicillin) kill bacteria or inhibit their growth, treating the cause of bacterial infections. They are ineffective against viruses because viruses replicate inside cells. Overuse of antibiotics leads to antibiotic resistance through natural selection, emphasizing the need for responsible prescription, full course completion, and reduced use in animals.
Drugs were originally derived from plants (aspirin from willow bark, digitalis from foxglove) and microorganisms (penicillin from mold). New synthetic drugs undergo rigorous testing for toxicity (side effects), efficacy (effectiveness), and dosage. Drug trials involve preclinical testing (cells, tissues, animals) and clinical trials (healthy volunteers, then sick patients). Blind and double-blind trials, often with placebos, are used to minimize bias and confirm drug effectiveness.
Monoclonal antibodies (MAbs) are identical antibodies produced from a single cloned cell, binding to specific antigens. They are produced by injecting an antigen into a mouse to stimulate lymphocyte antibody production, then fusing lymphocytes with tumor cells to create hybridoma cells which rapidly divide and produce MAbs. Uses include diagnostic tests (e.g., pregnancy), identifying molecules in cells/tissues, and targeted cancer treatment (delivering drugs directly to cancer cells). Advantages: rapid production, highly specific. Disadvantages: expensive, side effects, ethical concerns (animal use), time-consuming initial production.
Plant diseases can be caused by bacteria, fungi, viruses, or insects (e.g., aphids, which can also be vectors). Deficiency diseases (e.g., lack of nitrates for protein, magnesium for chlorophyll) also impair growth. Symptoms include stunted growth, spots, decay, or discoloration. Identification methods include visual comparison, lab testing, or home test kits with MAbs. Plants have physical (cellulose cell walls, waxy cuticle, bark), chemical (antibacterial, poisons), and mechanical (thorns, hairs, mimicry) defenses.
Photosynthesis is an endothermic chemical reaction in plant cells using light energy, carbon dioxide, and water to produce oxygen and glucose. Glucose is used for respiration, storage (starch, lipids), cell wall formation (cellulose), and protein synthesis (with nitrates). Factors affecting photosynthesis rate include light intensity, carbon dioxide concentration, temperature (enzyme-controlled), and chlorophyll amount. Limiting factors apply when one factor constrains the rate despite others being optimal.
Photosynthesis rate can be measured in a lab by changing light intensity (distance from lamp), light color, temperature (water bath), or CO2 concentration (sodium hydrogen carbonate). Control variables must be kept constant. A common practical involves counting oxygen bubbles produced by pondweed. The inverse square law demonstrates that as the distance from a light source increases, light intensity decreases proportionally to 1/distance². Optimizing plant growth in a greenhouse involves controlling light, temperature, CO2, water, and mineral ions, balancing costs with increased yield.
Respiration is an exothermic chemical reaction releasing energy from sugars. Aerobic respiration (in mitochondria) uses oxygen to break down glucose into CO2 and water, releasing much energy for chemical reactions, active transport, movement, and heat. Cells needing lots of energy have many mitochondria. Anaerobic respiration (in cytoplasm, without oxygen) releases less energy. In animal cells, glucose breaks down into lactic acid (toxic, removed by liver). In plant/yeast cells (fermentation), glucose breaks down into CO2 and ethanol, important in food/drink industries.
During exercise, increased energy demand triggers higher breathing rate/volume (more oxygen), and increased heart rate (faster delivery of oxygen/glucose to muscles). Muscle cells also use glycogen stores. Intense exercise can lead to oxygen debt due to anaerobic respiration producing lactic acid, causing muscle fatigue. Lactic acid is transported to the liver and converted back to glucose, requiring oxygen (oxygen debt). Metabolism is the sum of all enzyme-controlled chemical reactions in the body, requiring energy from respiration, including synthesis and breakdown of molecules like glucose, glycogen, starch, cellulose, amino acids, proteins, glycerol, and lipids.