Cell Structure and Function

Cells

• All life consists of cells.
• Light microscopes can see cells and the nucleus, but not subcellular structures.
• Electron microscopes provide finer details and higher resolution of cell organelles.
• Microscopes with a better resolving power can see finer details.
• Magnification = Image size / Object size.
• Actual cell size can be calculated by measuring the image size and dividing by the magnification.
• Eukaryotic cells have a nucleus containing DNA (e.g., plant and animal cells).
• Prokaryotic cells do not have a nucleus.
• Both eukaryotic and prokaryotic cells contain similar organelles or subcellular structures.
• Cell membrane keeps everything inside the cell and is semi-permeable, allowing only certain substances to pass through.
• Plant cells and most bacteria have a cell wall made of cellulose, providing a rigid structure.
• Cytoplasm is the liquid that makes up the cell, in which most chemical reactions take place.
• Mitochondria are where respiration takes place, releasing energy for the cell to function.
• Ribosomes are where proteins are assembled or synthesized.
• Plant cells contain chloroplasts, which contain chlorophyll where photosynthesis takes place.
• Plant cells also contain a permanent vacuole in which sap is stored.

Bacterial Growth & Culture

• Bacteria multiply by binary fission.
• Aseptic technique ensures no contamination of the culture.
• Lift the lid of the dish towards a flame, causing microbes in the air to move away and upwards from the dish, therefore destroying them.
• Sterilized equipment is used to put a drop of bacteria culture in the middle or spread it all around and put spots of different antibiotics on top instead.
• Secure the petri dish with tape, leaving some space for air to enter for aerobic respiration.
• Incubate at 25 degrees Celsius.
• Calculate the size of the culture from an initial drop or the area where bacteria did not grow due to antibiotic effect for comparison.
• Use πr² or πd²/4 to calculate the area of the circles.

Cell Nuclei & Mitosis

• Eukaryotic cell nuclei contain DNA stored in several chromosomes.
• Humans have 23 pairs of chromosomes in every nucleus, making them diploid cells.
• Gametes are not diploid cells but instead haploid cells and have half the number of chromosomes, thus 23 total.
• New cells are made for growth and repair through mitosis, which duplicates cells.
• The genetic material is duplicated in mitosis.
• The nucleus breaks down during mitosis.
• One set of each chromosome pair is pulled to opposite sides of the cell during mitosis.
• A new nucleus forms in each of the new cells to house the copied chromosomes.
• Mitosis results in two identical cells.
• AQA states that "the nucleus divides," which is technically not fully correct, but acceptable answer for marking.

Cell Specialization & Stem Cells

• Cells specialize depending on the function they need to fulfill (e.g., nerve, muscle, root hair, xylem, phloem cells).
• Stem cells haven't specialized yet are found in human and animal embryos and the meristem of plants.
• Stem cells are also made in your bone marrow throughout your life but these stem cells can only specialize into blood cells.
• Use stem cells to combat conditions like diabetes and paralysis.
• Cloning of humans to harvest stem cells is ethically questionable.
• Cloning plants can prevent species extinction or produce crops with specific characteristics.

Diffusion & Osmosis

• Diffusion is the movement of molecules or particles from an area of high concentration to one of low concentration (down the concentration gradient).
• Diffusion doesn't require any energy input and is thus passive.
• Substances move across a semi-permeable membrane if the holes are large enough.
• Water can pass through by diffusion, but glucose cannot.
• Osmosis is the diffusion of water across a semi-permeable membrane.
• If there is a higher concentration of glucose outside a cell, water moves out, decreasing the cell's mass.
• Rate of diffusion and osmosis increases with increasing differences in concentrations, temperature, or surface area.
• Villi in the small intestine, alveoli in the lungs, and root hair cells all apply this principle.

Osmosis Practical

• Cut equalsized cylinders from a potato or other vegetable.
• Weigh potato cylinders and place in test tubes with varying concentrations of sugar solution.
• After a day or so, remove them, dab off water, and reweigh.
• Calculate percentage change in mass: (final mass - initial mass) / initial mass * 100.
• A lighter cylinder indicates a negative change in mass.
• Plot these percentages against sugar concentration and draw a line of best fit
• Where this line crosses the x-axis indicates the concentration at which there is no change in mass/osmosis, which is the same as the concentration inside the potato.

Active Transport

• Glucose and other nutrients and minerals can move through a membrane by active transport.
• Carrier proteins use energy to move substances through the membrane.
• This can move substances against a concentration gradient (e.g., mineral ions into plant root hair cells).

Tissues, Organs & Organ Systems

• Similar cells connected form a tissue (e.g., heart tissue).
• Tissues form organs (e.g., your heart).
• Organs work together in an organ system (e.g., circulatory system).
• Your digestive system breaks down food into useful nutrients for your body to use.
• Acid in the stomach breaks down the food.
• Bile and enzymes work together in the small intestine to break down food further.
• Bile is made in the liver and stored in the gallbladder before being used.

Bile & Enzymes

• Bile neutralizes the acid from the stomach.
• Bile emulsifies fats to form droplets, increasing their surface area exposed to enzymes.
• Enzymes are biological catalysts.
• Some enzymes break down larger molecules into smaller ones for absorption.
• Amalayse breaks down starch into glucose (found in the small intestine and saliva).
• Enzymes are specific and only break down only certain molecules.
• Carbohydrases break down carbohydrates into simple sugars (e.g. amylase).

Enzymes Continued

• Proteases break down proteins into amino acids.
• Lipases break down lipids (fats) into glycerol and fatty acids.
• Enzymes work on a lock-and-key principle.
• The substrate (e.g., starch) binds to the enzyme's active site forming a complex.

Enzyme Activity

• The rate of enzyme activity increases with temperature until the active site changes shape (enzyme denatures).
• Maximum rate occurs at the optimum temperature.
• Similar effect with pH; enzymes can denature at too high or too low pH.
• Mix amylase with starch at different temperatures or pH buffer solutions for practical on this.
• Time how long it takes every 10 seconds until iodine no longer turns black, due all of the starch being broken down.
• The substrate will take the shortest time to be broken down in the optimal temperature or pH.

Food Tests

• Food tests can identify what nutrients are in food.
• Iodine turns from orange to black in the presence of starch.
• Benedict's solution turns from blue to orange in the presence of sugars.
• Biuret reagent turns from blue to purple with proteins.
• Cold ethanol will go cloudy with lipids (fats).

Respiratory System

• Breathing provides the necessary oxygen for respiration to happen in our cells.
• Air moves down the trachea into the bronchi, then the bronchioles, and ends up in the alveoli.
• Alveoli (air sacs) are where oxygen diffuses into the blood vessels around them.
• Alveoli are lumpy to have a large surface area, this helps diffusion rate happen faster.
• Oxygen binds to hemoglobin in red blood cells.
• Oxygen is transported to every cell in the body for respiration.
• Carbon dioxide, made from respiration, dissolves into the plasma of the blood, diffuses into the lungs, and is exhaled.

Circulatory System & The Heart

• The heart is at the center of the circulatory system, the transport system of your body called a double circulatory system.
• Deoxygenated blood from the body enters the right side of the heart through the vena cava into the right atrium.
• The valve between the right atrium and the right ventricle stops back flow, just like all valves.
• Heart muscles contract and blood goes through the pulmonary artery to the lungs to be oxygenated.
• Oxygenated blood returns to the heart through the pulmonary vein into the left atrium.
• Blood goes from the left atrium to the left ventricle then out to the body through the aorta.
• The left side of the heart has thicker walls as the left ventricle pumps blood to the whole body.
• Cells create electrical pulses that cause the heart muscles to contract.
• Faulty electrical signals can be corrected with an artificial pacemaker.
• Blood vessels that go away from the heart are arteries.
• Blood vessels going towards the heart are veins.
• Arteries carry oxygenated blood, except for the pulmonary artery.
• Veins carry deoxygenated blood, except for the pulmonary vein.

Blood Vessels

• Arteries have thicker walls with a thinner lumen to withstand higher pressure.
• Veins have thinner walls with valves to stop backflow due to lower blood pressure.
• Arteries split and get smaller, ending as tiny capillaries with one-cell-thick walls for fast diffusion.
• The heart needs its own oxygen supply delivered by the coronary artery.
• Blockage of the coronary artery by fatty deposits can cause a heart attack (CHD).
• Stents are tubes inserted into blood vessels to keep them open.
• Statins are drugs that reduce cholesterol and fatty deposits.
• Faulty heart valves can be replaced with artificial ones.
• Blood also carries white blood cells (combat infections) and platelets (clot wounds).
• CVD (cardiovascular disease) is an example of a non-communicable disease.
• Non-communicable diseases come from inside your body.
• Examples of non-communicable diseases include autoimmune conditions like allergic reactions and cancer.
• A communicable disease is caused by a pathogen that enters your body, causing a viral, bacterial, or fungal infection.

Health & Lifestyle

• Obesity and too much sugar can cause type 2 diabetes.
• A bad diet, smoking, and lack of exercise can affect heart disease risks.
• Alcohol can cause liver diseases.
• Smoking can cause lung disease or cancer.
• A carcinogen is anything that increases the risk of cancer (e.g., ionizing radiation).
• Cancer results from damaged cells dividing uncontrollably, leading to tumors.
• Benign cancers don't spread and are easier to treat.
• Malignant cancers are when cancerous cells spread through the body and are much worse.

Plant Organs and Function

• Plants also have organs.
• Leaves are where photosynthesis takes place, producing food for the plant.
• Water also leaves the plant through them, allowing transpiration to take place.
• Roots let water and mineral ions enter the plant.
• The meristem is where new cells are made.
• Xylem are long continuous tubes, where water rises unidirectional (transpiration).
• Phloem are the conveyor belts of cells that transport sugars (food and sap) bidirectional (translocation).
• The transpiration rate increases with increasing temperature, decreasing humidity, and increasing air movement.

The Plant Structure (Triple Science Only)

• Lack of nitrate ions stunts growth as the plant can't synthesize proteins effectively and affects plant health.
• Chlorosis (yellowing of leaves) can be due to magnesium deficiency, as it is needed to make chlorophyll.
• Waterproof waxy cuticle on top to stop water from evaporating from the top and causing the leaf to dry out.
• The upper epidermis consists of transparent cells that allow light to pass through to the palisade mesophyll layer.
• Palisade mesophyll layer (in the middle); full of chloroplasts, where the majority of photosynthesis takes place.
• Spongy mesophyll layer has lots of gaps around the cells to increase the surface area through which gas exchange can occur.
• Carbon dioxide diffuses into the cells, while oxygen and water diffuse out.
• Vascular bundle includes the xylem and phloem.
• Lower epidermis is the bottommost layer of the leaf and it has stomata (holes), which is how gases enter and exit the leaf.
• Stomata control the rate at which gases enter and leave.
• The size of a stoma is controlled by the guard cells that flank the hole, changing size accordingly.
• The stomata close the holes at night to reduce water loss as less water is needed for photosynthesis.

Infection & Response

• Communicable diseases are caused by pathogens (viruses, bacteria, fungi, or protists).
• All pathogens reproduce in your body and cause damage.
• Viruses can't reproduce by themselves; they are protein casings that surround genetic code that it injects into a cell.
• The infected cell will start to produce copies of the specific virus.
• Measles is a virus that causes a rash and can be deadly, spread by droplets from sneezes or coughs.
• HIV is a sexually transmitted disease (STD/STI) that compromises the immune system (AIDS) and may be spread by shared needles.

Bacteria and Fungi

• Bacteria release toxins that damage your body's cells (e.g., salmonella, gonorrhea).
• Fungi do something similar (e.g., athlete's foot).
• Protists do all sorts of different things.
• Malaria is caused by a protist that burrows into red blood cells to multiply, then bursts out, destroying the red blood cell in the process.
• Mosquitoes are the vector for malaria.
• Animals are not the only organisms that can get sick; plants also are particularly susceptible to fungal infections like rose black spot.
• Rose black spot results in purple/black spots on the leaves and then they fall off: can be treated with fungicides.

Plant Infections

• Tobacco mosaic virus affects plants by discoloring leaves due to inhibiting chlorophyll production: less photosynthesis occurs, which causes stunted growth
• Skin is the first barrier to pathogens entering.
• Pathogens entering your nose and trachea can be trapped by mucus.
• Acid and enzymes in the digestive system will destroy pathogens.
• White blood cells are ready to combat pathogens that enter the bloodstream.

White Blood Cells

• Lymphocytes produce antitoxins to neutralize poisons and antibodies.
• Antibodies stick to the antigen on an pathogen and stop them from being able to infect more cells, making them clump together.
• Phagocytes ingest them and destroy pathogens.
• An antigen on an pathogen will have a specific shape.
• A antibody that fits the complementary antigen will neutralize it the most effectively.
• If pathogens are unknown to the immune system, lymphocytes will start making antibodies until one fits.
• The immune system then stores a copy of this antibody and antigen for future infections, therefore have immunity.
• Vaccines are dead or inert versions of pathogens (viruses) to expose your immune system without infecting you so it can produce the antibody.
• The COVID mRNA jab injected the DNA "instructions" needed to trick your cells into synthesizing part of the virus, including the antigen.
• Antibiotics kill bacteria, but they don't kill viruses. Penicillin was the first one.
• Because there are good bacteria in our bodies, antibiotics are designed to be as specific against harmful bacteria as possible.
• As bacteria mutate, they can become resistant to antibiotics, making them less effective.
• Drugs used to be extracted from plants and organisms (e.g., aspirin from willow trees, penicillin from mold).
• Synthesizing drugs is one of the biggest industries.
• New drugs have to be trialed to see how effective they are and to check for side effects.
• Phase One: Lab trials on cell tissue.
• Phase Two: Trials on animals.
• Phase Three: Human trials.
• Give the drug to a group of people, but also give a placebo to a control group without telling them (blind trial).

Blind & Double Blind Trials

• A blind trial is when the test subjects don't know what they're taking, placebo or drug.
• A double-blind trial is when even those analyzing the results from the tests are not aware of which group is which to eliminate bias.
• Monoclonal antibodies are made from clones of a cell, which can produce a specific antibody to combat a disease; used in triple science.
• Antibodies made from mice are combined with tumor cells to create a hybrid cell.
• This hybrid cell is then cloned to produce many antibodies ready to treat a patient.
• These monoclonal antibodies can also be used for medical diagnosis, pathogen detection in a lab, or even identifying molecules in tissue by binding them to a dye so they glow when grouped together.

Photsynthesis - Plants

• Photosynthesis happens in chlorophyll and chloroplast in plant cells to provide food for them.
• The chemical equation for photosynthesis is energy (in the form of light) + carbon dioxide + water → glucose + oxygen..
• Energy is needed in the form of light to make the reaction happen which is an endothermic reaction.
• Glucose made from photosynthesis is either used for respiration, turned into starch, or fat as a store of energy.
• Plants then use celluose to produce cell walls or amino acids to synthesize proteins.
• The rate of photosynthesis is increased with a higher temperature unless it's so high that enzyme denaturing occurs by changing the enzymes shape.

CO2 Concentration and Limiting Factors

• Increasing light intensity or increasing carbon dioxide concentration can increase the rate of photosynthesis.
• Any one of these can be a limiting factor.
• The variable on the x-axis has to be the limiting factor before the graph plateaus levels out, and the other two at the end instead.
• Light intensity follows an inverse square relationship, in other words, if you double the distance, the light intensity quarters.
• Measure the rate of photosynthesis by submerging pondweed in an inverted measuring cylinder. Measure the volume of oxygen made over time.
• Instead of cylinder measurements, you can also count the bubbles, but it's less accurate.
• The independent variable could be the increasing changing distance from the light source, thereby changing the light intensity.

Repsirartion Overview

• Every cell has mitochondria (except for red blood cells), where respiration takes place to provide energy for every organism.
• Also needed For other chemical reactions to take place for movement and warmth.
• Aerobic respiration means with oxygen.
• The chemical equation for aerobic photosynthesis is glucose + oxygen → carbon dioxide + water.
• During exercise, your breathing rate and heart rate increase to increase the rate of oxygen delivered to cells for respiration and to remove the carbon dioxide.
• Anaerobic respiration occurs when there's a lack of oxygen: glucose is converted straight into lactic acid, which releases less energy.
• Build-up of lactic acid causes aching muscles due to the intensity of work done.
• More oxygen is needed afterward to break it down in the liver where it's turned back into glucose and is due to an oxygen debt: breathing and heart rate take longer to return to normal than just exercising.
• Plant and yeast cells respire anaerobically turning glucose into ethanol and carbon dioxide.
• Yeast is added when baking: the carbon dioxide bubbles made cause the bread or the cake to rise.
• Plant and Yeast Anarobic respiration is also called fermentation and is how alcoholic drinks are made as ethanol is produced.
• Metabolism is defined as the sum of all reactions in a cell or organism.

Glucose

• Conversions of glucose into glycogen and cellulose occurs during metabolism.
• Glucose can also be built into cellulose, which is used to make cell walls.
• Glucose building blocks and nitrates are used to make amino acids for protein synthesis.
• Fatty acids and glycerol are built up into lipids.
• Excess proteins are turned into urea (more about that in paper two).