biology

• Photosynthesis is a reaction where light energy is converted to chemical energy in the form of glucose, with oxygen released as a waste product
• Photosynthesis occurs in chloroplasts, which are adapted for photosynthesis in ways such as containing stacks of thylakoid membranes called grana and stroma which contains all the enzymes required for the light-independent stage of photosynthesis
• Leaves of C4 plants like maize and sorghum are adapted to work at high temperatures by fixing carbon dioxide into a four carbon organic acid called malate in mesophyll cells before transporting it to photosynthetic cells, ensuring a high concentration of carbon dioxide for efficient photosynthesis
• Photosynthetic pigments like chlorophylls absorb light for photosynthesis, with chlorophyll a and chlorophyll b being the main types, while carotenoids prevent chlorophyll damage and are present in forms like beta carotene and xanthophyll
• An absorption spectrum determines the wavelengths absorbed by pigments, while an action spectrum shows the relationship between the rate of photosynthesis for a given wavelength
• Photosynthetic pigments can be separated by extracting them from a leaf and carrying out chromatography, where the Rf value can be used to identify which pigments are present
• Photosynthesis has two stages:

  • Light-dependent reaction involves photoionisation, electron transport chain, ATP production, photolysis of water, and generation of reduced NADP and ATP
  • Light-independent reaction (Calvin cycle) uses ATP and reduced NADP to produce glucose in the stroma
• Limiting factors of photosynthesis include carbon dioxide concentration, light intensity, light wavelength, and temperature, with the rate increasing as these factors increase but slowing down at high intensities and temperatures due to enzyme denaturation
• Communication is essential for the survival of organisms as they must detect and respond to changes in both their internal and external environments
• In multicellular organisms, changes necessary for survival are triggered by nervous and endocrine systems
• Cell signalling involves communication between cells through electrical signals carried by neurones or hormones
• Neuronal cell signalling is faster and short term, while chemical signalling is slower and long term
• Endocrine signalling is used for long-distance signalling, paracrine signalling occurs between close cells, and autocrine signalling stimulates a cell's own receptors triggering a response within itself
• Homeostasis maintains a constant internal environment despite external changes, involving factors like temperature, water potential, pH, and blood glucose level
• Negative feedback in homeostasis counteracts changes in internal conditions to restore optimum conditions, requiring sensory receptors and effectors
• Positive feedback, less common than negative feedback, increases the original change in conditions, like the dilation of the cervix during childbirth
• Ectotherms regulate body temperature with external sources, while endotherms maintain a constant body temperature independently of external temperature
• Endotherms control body temperature through actions like shivering, sweat production, adjusting hairs on the skin, and dilation/constriction of arterioles
• The liver breaks down excess amino acids through deamination, converting them to ammonia and organic acids, then to urea in the ornithine cycle, which is released as urine
• The kidneys excrete waste products like urea in urine, with blood entering through the renal artery and exiting through the renal vein after filtration
• Selective reabsorption in the proximal convoluted tubule involves reabsorbing useful substances like amino acids, glucose, and vitamins back into the blood
• In dehydration, less water is reabsorbed into the blood by osmosis from the loop of Henle, the distal convoluted tubule, and collecting duct, leading to more concentrated blood
• In cases of dehydration, where the water content of blood is too low, less water is reabsorbed into the blood by osmosis from the loop of Henle, the distal convoluted tubule, and collecting duct, leading to the production of more concentrated urine
• Hormones play a crucial role in controlling the reabsorption of water in the body
• Osmoreceptors in the hypothalamus control the water potential and content in the blood
• Vesicles' membranes contain aquaporins, protein-based water channels that increase membrane permeability to water, allowing water to move out of the kidney tubule
• The balance of the water potential of the blood is maintained through osmoregulation
• Blood glucose regulation is crucial to maintain essential processes like brain cell respiration
• Insulin, secreted by beta cells in the pancreas, stimulates the opening of glucose channels in target cells like hepatocytes in the liver, fat, and muscle cells, allowing more glucose to enter the cells for conversion to glycogen or fats
• In cases of low blood glucose concentration, alpha cells secrete glucagon, which stimulates hepatocytes to convert glycogen to glucose for release into the blood
• Adrenaline triggers physiological changes like pupil dilation, increased heart rate, and increased metabolic rate in response to a perceived threat
• Adrenaline interacts with cell receptors, activating adenyl cyclase, which converts ATP to cyclic AMP (cAMP), acting as a second messenger to trigger the breakdown of glycogen into glucose for energy
• Stomatal aperture in plants is regulated to balance carbon dioxide uptake for photosynthesis and water conservation
• Guard cells control stomatal opening and closing by inflating to allow water and gas exchange or deflating to prevent water loss
• Stomata close following excess water loss, usually in response to a drop in light levels and lower rates of photosynthesis
• Abscisic acid, produced in plant roots in response to decreased water potential or stress, activates calcium ions as a secondary messenger, leading to stomatal closure