GCSE Biology Complete Notes Flashcards
Characteristics and Classification of Eukaryotic and Prokaryotic Cells
Eukaryotic cells are defined as those which contain a nucleus. This category includes both animal and plant cells. Within an animal cell, several specific structures carry out vital functions: the nucleus contains the genetic material and exerts control over the cell's activities; the cytoplasm serves as the site where the majority of chemical reactions occur; the cell membrane regulates the movement of substances into and out of the cell; the mitochondria act as the site of aerobic respiration, releasing essential energy; and ribosomes are the specific site for protein synthesis.
Plant cells possess all the structures found in animal cells but also include additional components. These include a cell wall made of cellulose, which functions to strengthen and support the cell structure; chloroplasts, which contain chlorophyll to absorb light for the process of photosynthesis; and a permanent vacuole containing cell sap, which helps maintain the internal pressure of the cell.
Prokaryotic cells, exemplified by bacteria, are notably smaller and simpler in structure than eukaryotic cells. Unlike eukaryotes, they do not possess a nucleus. Their components include a cell membrane, cytoplasm, ribosomes, and a cell wall. Their genetic material is organized into a circular chromosome of DNA, and they may also contain plasmids, which are small, distinct rings of DNA.
Cell Specialisation and Mitosis
Cells undergo a process called differentiation to become specialised for specific functions. In sperm cells, a flagellum facilitates movement, numerous mitochondria provide the necessary energy for travel, and an acrosome contains enzymes required to penetrate the egg. Nerve cells feature a long axon to carry electrical impulses over distances and branched ends to establish connections with other cells. Root hair cells are designed with a large surface area to maximize the absorption of water and minerals.
Cell division occurs through a process called mitosis, which is essential for growth, repair, replacing damaged cells, and asexual reproduction. Before mitosis begins, the cell must grow, increase the number of its sub-cellular structures, and replicate its DNA. During the actual process of mitosis, chromosomes separate and the nucleus divides. This is followed by the division of the cytoplasm and the cell membrane to create two daughter cells.
The Function and Potential of Stem Cells
Stem cells are undifferentiated cells capable of dividing and developing into various types of specialised cells. Embryonic stem cells have the capacity to differentiate into many different cell types. Adult stem cells, which are found in specific tissues like bone marrow, possess a more limited potential for differentiation. While stem cells offer great potential for replacing damaged cells and treating various diseases, they also carry risks. These include the possibility of uncontrolled division leading to tumours, as well as risks associated with rejection by the host immune system or the transmission of infections.
Principles of Passive and Active Transport
Diffusion consists of the net movement of particles from an area of higher concentration to an area of lower concentration. Because it is a passive process, it does not require energy from respiration. The rate at which diffusion occurs is increased by a greater concentration gradient, higher temperatures, a larger surface area, and a shorter distance for the particles to travel.
Osmosis is a specific type of diffusion involving the net movement of water molecules from a dilute solution to a more concentrated solution across a partially permeable membrane. In plant cells, being placed in a dilute solution causes water to enter, making the cell turgid. Conversely, in a concentrated solution, water leaves the cell, causing it to become plasmolysed.
Active transport involves the movement of substances against a concentration gradient, moving from a low concentration to a high concentration. This process is active and requires energy released from respiration. A primary example of active transport is seen in root hair cells, which absorb mineral ions from the soil against the concentration gradient.
Biological Organisation and the Digestive System
The hierarchy of biological organisation proceeds from cells to tissues, then to organs, organ systems, and finally the whole organism. A tissue is defined as a group of similar cells working together, while an organ contains different tissues working in coordination. An organ system consists of various organs working together. Examples include muscular, nervous, and epithelial tissues.
The digestive system is an organ system designed to break down large, insoluble food molecules into small, soluble molecules that can be absorbed into the bloodstream. This process begins in the mouth, where teeth mechanically break down food and saliva provides amylase. Amylase is responsible for breaking starch into sugars. The oesophagus then moves food toward the stomach using a process called peristalsis.
In the stomach, hydrochloric acid is produced to provide a low pH environment for protease enzymes and to kill many microorganisms. The stomach also produces protease to digest proteins. Digestion is completed in the small intestine, where soluble products are absorbed into the blood. The large intestine is responsible for absorbing excess water and producing faeces. The liver produces bile, processes nutrients, and stores glycogen, while the gall bladder stores the bile. The pancreas produces digestive enzymes and releases them into the small intestine.
Enzyme Function and Digestive Chemistry
Enzymes act as biological catalysts, speeding up chemical reactions without being consumed. They are proteins featuring a specific active site that matches a substrate's shape, a concept known as the lock-and-key model. As temperature increases, particles gain kinetic energy, leading to more frequent collisions and a higher reaction rate. However, if the temperature exceeds the optimum level, the bonds maintaining the enzyme's shape break, leaving the enzyme denatured. Similarly, each enzyme has an optimum pH, and extreme acidity or alkalinity can change the active site shape, leading to denaturation.
Specific digestive enzymes include amylase, which converts starch into sugars and is produced by the salivary glands and the pancreas. Protease breaks proteins into amino acids and is produced by the stomach and the pancreas. Lipase breaks lipids into fatty acids and glycerol and is produced by the pancreas and small intestine. Bile, produced by the liver and stored in the gall bladder, neutralises stomach acid to create alkaline conditions and emulsifies fats. This emulsification breaks large fat droplets into smaller ones, increasing the surface area for lipase to act upon and increasing the rate of digestion.
The Heart, Blood Vessels, and Blood Composition
The heart is a muscular organ that pumps blood through a double circulatory system consisting of the pulmonary circuit (heart to lungs and back) and the systemic circuit (heart to body and back). It has four chambers: the right atrium receives deoxygenated blood from the body; the right ventricle pumps it to the lungs; the left atrium receives oxygenated blood from the lungs; and the left ventricle pumps it to the rest of the body. The left ventricle has a thicker muscular wall to generate the high pressure needed for systemic circulation. Valves are present throughout the heart to prevent the backflow of blood.
Blood vessels vary by function. Arteries carry blood away from the heart under high pressure and have thick muscular and elastic walls with a small lumen. Veins carry blood toward the heart under lower pressure, featuring thinner walls, a large lumen, and valves to prevent backflow. Capillaries are tiny vessels with walls only one cell thick, forming large networks that provide a massive surface area and short diffusion distance for efficient exchange.
Blood is composed of several key elements. Red blood cells carry oxygen using haemoglobin and feature a biconcave shape for a large surface area, lacking a nucleus to provide more space for haemoglobin. White blood cells defend against pathogens through phagocytosis (engulfing pathogens), the production of antibodies, and the production of antitoxins. Platelets are involved in blood clotting to prevent blood loss. Plasma is the liquid component that transports cells, platelets, and nutrients.
Pathogens and Human Immunity
Pathogens are microorganisms that cause disease, categorized into four main types: bacteria (living single-celled organisms that produce toxins); viruses (smaller than bacteria, reproducing inside and destroying host cells); fungi (which may produce spores); and protists (mostly single-celled organisms, some of which cause diseases like malaria).
Human defence mechanisms include the skin as a physical barrier, mucus to trap pathogens, and cilia to move mucus out of the airways. Stomach acid kills many ingested pathogens. White blood cells provide immunity through phagocytosis, producing specific antibodies that bind to antigens on pathogens, and producing antitoxins to neutralise bacterial toxins.
Vaccination involves introducing a harmless form of a pathogen or its antigens to stimulate white blood cells to produce antibodies and memory cells. If the pathogen returns, memory cells recognise it, and antibodies are produced rapidly to destroy it. This process reduces the spread of communicable diseases. Antibiotics are used to kill bacteria or stop their reproduction but are ineffective against viruses because viruses live inside cells. Antibiotic resistance can occur through random mutations and natural selection; to combat this, antibiotics should only be used when necessary and the full course should always be completed. Painkillers are used to relieve symptoms but do not eliminate the underlying pathogens.
Bioenergetics: Photosynthesis and Respiration
Photosynthesis is a process that converts light energy into chemical energy stored in glucose according to the following equation:
This reaction requires light energy and chlorophyll. The rate of photosynthesis can be limited by light intensity, carbon dioxide concentration, and temperature. Glucose produced can be used in respiration, stored as starch, converted into cellulose for cell walls, used to make fats and oils, or combined with nitrate ions to produce amino acids.
Aerobic respiration releases energy from glucose using oxygen, occurring primarily in the mitochondria:
The energy released is utilized for muscle contraction, active transport, building large molecules, and maintaining body temperature. When oxygen is insufficient, anaerobic respiration occurs. In animals, the process is as follows:
Anaerobic respiration releases less energy than aerobic respiration. Lactic acid build-up contributes to muscle fatigue, resulting in an oxygen debt, which is the extra oxygen needed after exercise to break down the lactic acid. In plants and yeast, anaerobic respiration (fermentation) follows this equation: