Year 9 Science Comprehensive Revision Guide: Biology, Chemistry, and Physics

Plants and Photosynthesis

Photosynthesis is the fundamental process by which plants produce their own food for energy. This chemical reaction occurs within the chloroplasts of plant cells, which contain a green pigment called chlorophyll. Chlorophyll's primary function is to absorb the energy transferred by light waves from the Sun. The process is critical for maintaining high levels of oxygen and low levels of carbon dioxide in the atmosphere. The chemical equation for photosynthesis is defined as: $\text{carbon dioxide} + \text{water} \rightarrow \text{glucose} + \text{oxygen}$

Plants require specific inputs for this process: carbon dioxide, which is absorbed through the leaves; water, which is taken from the ground through the roots; and light, acting as the energy source. The outputs include oxygen, which is released into the air, and glucose. Glucose is highly versatile; it can be converted into starch and plant oils for energy storage to be released later by respiration, or used to make cellulose for constructing cell walls.

Water absorption occurs in the roots through a biological process called osmosis, which does not require energy. The roots contain specialized cells known as root hair cells. These cells are specifically adapted for water absorption through their large surface area and thin walls that facilitate easy passage of water. Notably, root hair cells do not contain chloroplasts. Once absorbed, water is transported to the leaves via tubes called xylem. In contrast to water, minerals are absorbed into the roots via active transport, a process that requires energy.

Plant leaves have several specialized features to maximize efficiency. A thin structure ensures a short distance for carbon dioxide to diffuse into the leaf. A waxy layer prevents water loss through evaporation. Palisade cells are densely packed with chloroplasts to improve light absorption. Stomata are small pores that allow carbon dioxide to diffuse in and oxygen to diffuse out, and their size is controlled by guard cells. In hot, windy, or dry conditions, guard cells cause the stomata to close to conserve water. A network of tubes containing xylem (for water) and phloem (for food) facilitates substance transport. Inside the leaf, the spongy layer contains gaps between cells to allow gases to diffuse effectively to and from other cells.

Food security is intrinsically linked to plant health and pollination. Pollination involves the transfer of pollen from one plant to another, facilitated either by insects (such as bees) or by the wind. Because our entire food supply depends on plants—either as the direct source of food or as the feed for the animals we consume—pollinating insects are vital to human survival.

Biological Systems and Processes: Respiration and Gas Exchange

Aerobic respiration is the process by which all living cells, including plant and animal cells, release energy from glucose using oxygen. This process takes place within the mitochondria. Energy released is used for growth and repair, movement, and maintaining body temperature in mammals and birds. The chemical equation for aerobic respiration is the inverse of photosynthesis: $\text{glucose} + \text{oxygen} \rightarrow \text{carbon dioxide} + \text{water}$

When oxygen is unavailable, organisms perform anaerobic respiration. In humans, this occurs in the muscles during intense activity, producing lactic acid rather than carbon dioxide and water: $\text{glucose} \rightarrow \text{lactic acid}$ The accumulation of lactic acid causes fatigue, pain, and cramps. This chemical is only broken down when aerobic respiration resumes. In microbes like yeast, anaerobic respiration is called fermentation and follows this equation: $\text{glucose} \rightarrow \text{ethanol} + \text{carbon dioxide}$ Fermentation is essential in the food industry; ethanol is used to produce beer and wine, while carbon dioxide causes bread to rise.

The human gas exchange system is responsible for moving oxygen from the air into the blood and removing waste carbon dioxide. Air travels through the mouth, into the trachea (windpipe), then through two bronchi which branch into smaller bronchioles. At the ends of these tubes are air sacs called alveoli, which provide a massive surface area for gas exchange. Alveoli features include thin walls (one cell thick), a moist surface, and a dense network of capillaries. Gases move by diffusion from high to low concentration. Ventilation (breathing) involves the ribs, intercostal muscles, and the diaphragm.

Exercise impacts this system by increasing the breathing rate and the tidal volume (the amount of air per breath). Regular exercise strengthens the diaphragm and intercostal muscles, increasing the vital capacity (the maximum volume of air that can be forcibly exhaled after a full inhalation).

Biological Systems and Processes: Drugs and Health

Drugs are substances that affect the body's functioning. They are categorized as medicines (for treating pain or disease) or recreational drugs (taken for their effects). Recreational drugs can be legal, such as caffeine (found in cola, coffee, tea), tobacco, and alcohol, or illegal, such as cannabis, ecstasy, and heroin. Drugs are often classified as stimulants (speeding up nervous system messages) or depressants (slowing down messages).

Smoking is significantly detrimental to health due to three primary substances:

1. Tar: Causes cancer of the mouth, throat, and lungs, damages alveoli, and coats the lungs, causing coughing.
2. Nicotine: An addictive stimulant that increases heart rate and blood pressure while narrowing blood vessels, leading to heart disease.
3. Carbon Monoxide: A toxic gas that replaces oxygen in red blood cells, forcing the circulatory system to work harder. Furthermore, smoke damages the cilia (hair-like structures that move mucus out of the lungs), leading to mucus buildup and bronchitis. Smoking during pregnancy increases the risks of complications, stillbirth, premature birth, and low birth weight.

Depressants like alcohol and heroin can cause short-term feelings of well-being and slowed muscular activity, but long-term use leads to brain, liver, and heart damage. Stimulants like cocaine or amphetamines increase energy but damage the heart and increase the risk of mental illness.

Asthma is a respiratory condition affecting the bronchioles, causing them to become inflamed, swollen, and constricted. During an attack, excess mucus is produced and muscles around the bronchioles contract, leading to wheezing and difficulty breathing. This is treated with drugs called relievers (usually via an inhaler) that relax and open the airways.

Biological Systems and Processes: The Skeleton and Movement

Bone is a living tissue with its own blood supply, capable of constant regeneration and self-repair. The skeleton serves four primary functions: support (allowing us to stay upright), protection of vital organs (skull for brain; ribcage for heart and lungs; backbone for spinal cord), movement (via joints), and blood cell production. Both red blood cells (oxygen carriers) and white blood cells (pathogen destroyers) are manufactured in the bone marrow.

Joints facilitate movement. Synovial joints are common in the human skeleton and feature cartilage to protect bone ends. This cartilage is lubricated by synovial fluid. Ligaments hold the bones together at the joint. If cartilage wears away, the bones rub together, a painful condition known as arthritis.

Movement is achieved through muscle contraction. Muscles are attached to bones via strong tendons. Because muscles can only pull and not push, they work in antagonistic pairs. For example, to raise the forearm, the biceps contract and the triceps relax; to lower it, the triceps contract and the biceps relax. Biomechanics is the study of how muscles and bones work together to exert force.

Genes, DNA, and Microscopy

Heredity is the process by which genetic information is passed from parents to offspring. This information is stored in DNA (Deoxyribonucleic acid). The structure of DNA was modeled by James Watson and Francis Crick in the 1950s, supported by the X-ray diffraction images of Rosalind Franklin and the work of Maurice Wilkins. DNA consists of two strands twisted into a double helix, held together by bonds between complementary base pairs.

DNA is organized into genes, which are short sections coding for specific proteins. These are coiled into chromosomes found in the cell nucleus. Human body cells contain 23 pairs of chromosomes ($46$ total), while gametes (sperm and egg) contain only $23$ individual chromosomes. Upon fertilization, the resulting cell has $23$ pairs. A genome refers to one full copy of all an organism's chromosomes.

Biology uses microscopes to study these structures. Light microscopes have developed over 200 years. To observe plant cells (RP1), an onion membrane is stained with iodine, placed on a slide, and viewed under an objective lens using coarse and fine focus. Magnification is calculated using: $\text{magnification} = \frac{\text{image size}}{\text{actual size}}$ Units must be consistent, often requiring conversion ($1\,\text{mm} = 1000\,\mu\text{m}$).

Eukaryotic cells (animals, plants, fungi, protists) contain a nucleus. Specialized cells have unique features: sperm cells have high mitochondria counts for energy; nerve cells have long branches (dendrites); and xylem walls are thickened with lignin for strength.

Physical Sciences: Sound and Waves

All waves transfer energy. There are two types:

1. Longitudinal: vibrations are parallel to the direction of travel (e.g., sound waves).
2. Transverse: vibrations are perpendicular (90 degrees) to the direction of travel (e.g., light and water waves).

Sound waves require a medium (solid, liquid, or gas) and cannot travel through a vacuum. They are produced by vibrations and travel at approximately $340\,\text{m/s}$. Sound reflects off hard, smooth surfaces to create echoes, while soft, rough surfaces absorb sound. Humans hear frequencies between $20\,\text{Hz}$ and $20,000\,\text{Hz}$. Ultrasound is defined as sound with a frequency higher than $20,000\,\text{Hz}$.

Key wave properties include amplitude (maximum height from rest; determines loudness), wavelength (distance between two crests), and frequency (waves per second; determines pitch). When two waves meet, superposition occurs—they can reinforce (constructive interference) or cancel each other out (destructive interference).

In the ear, sound vibrations hit the eardrum, which moves the ossicles (three small bones), then the cochlea, which sends electrical signals to the brain via the auditory nerve. Microphones use a diaphragm to perform a similar function, converting vibrations into electrical impulses.

Matter, Pressure, and Diffusion

Diffusion is the random movement of particles from an area of high concentration to an area of low concentration. It occurs in fluids (liquids and gases) but not solids. The rate of diffusion increases with temperature and steeper concentration gradients. In gases, particles collide frequently and change direction randomly, a phenomenon called Brownian motion.

Pressure in fluids acts at $90^{\circ}$ (normal) to surfaces. Atmospheric pressure decreases with altitude because the weight of the air above decreases. Conversely, liquid pressure increases with depth. This pressure creates upthrust on objects submerged in liquids. An object sinks if its weight is greater than the maximum upthrust, and floats if the upthrust equals its weight.

Substances change state through heating or cooling. These changes are reversible and involve no new substances.

• Solids: Particles are touching, ordered, and vibrate in fixed positions. They have the highest density and lowest internal energy.
• Liquids: Particles are mostly touching but arranged randomly and move freely.
• Gases: Particles are far apart, move faster, and have the highest internal energy and lowest density.

Chemistry: Reactivity and Energetics

Metals react to achieve a full outer electron shell by losing electrons to become positive ions. Non-metals gain electrons to become negative ions. The Reactivity Series ranks elements by how easily they lose electrons, ranging from Potassium (most reactive) to Platinum (least reactive). Carbon is included as a reference; metals less reactive than carbon (e.g., Zinc, Iron, Copper) can be extracted from their oxides by heating them with carbon through reduction.

Acids react with metals to produce a salt and hydrogen gas (tested with a "squeaky pop" from a lighted splint). Acid-base neutralization produces salts:

• Hydrochloric acid produces chlorides.
• Sulfuric acid produces sulfates.
• Nitric acid produces nitrates.

Relative formula mass ($M_r$) is calculated by summing the mass numbers ($A_r$) of all atoms in a formula (e.g., $MgCl_2 = 24 + (2 \times 35.5) = 95$). Pure metals have layers of atoms that slide, making them soft; alloys involve different-sized atoms that disrupt these layers, making the material harder.

Chemical reactions involve energy changes. Exothermic reactions (like combustion) release energy, causing a temperature rise. Endothermic reactions (like thermal decomposition) take in energy, causing a temperature drop. Combustion of fuels (hydrocarbons) results in complete combustion (if oxygen is plentiful) producing $CO_2$ and $H_2O$, or incomplete combustion producing carbon monoxide ($CO$) and soot ($C$).

Atomic Structure and the Periodic Table

Atoms consist of protons (mass $1$, charge $+1$), neutrons (mass $1$, charge $0$), and electrons (mass nearly $0$, charge $-1$). Protons and neutrons are in the nucleus. Elements contain one type of atom, while compounds contain two or more elements chemically combined. Mixtures can be separated physically via methods such as filtration (insoluble solids from liquids), chromatography (liquids like ink), evaporation/crystallization (solutes from solutions), or distillation (liquids based on boiling points).

The Periodic Table is ordered by atomic (proton) number. Groups (columns) indicate electrons in the outer shell, while Periods (rows) indicate the number of electron shells. Group 1 (Alkali metals) are highly reactive, soft, and form $+1$ ions. Group 7 (Halogens) are non-metals that form molecules (e.g., $Cl_2$) and form $-1$ ions. Reactivity increases down Group 1 but decreases down Group 7.

Questions & Discussion

Respiration v photosynthesis

1. Write the word equation for respiration: $\text{glucose} + \text{oxygen} \rightarrow \text{carbon dioxide} + \text{water}$
2. How is photosynthesis different to respiration? Photosynthesis produces glucose and oxygen while using carbon dioxide and water; respiration does the opposite.
3. Which process uses carbon dioxide and water? Photosynthesis.
4. Which process uses glucose and oxygen? Respiration.

Photosynthesis Essentials

1. Where does photosynthesis take place? In the chloroplasts.
2. Name the green chemical inside chloroplasts: Chlorophyll.
3. What is the function of this green chemical? To absorb energy from light.
4. Describe 3 ways plants use glucose: Stored as starch/oils, released by respiration for energy, used to make cellulose for cell walls.

The Skeleton and Muscles

1. State the 4 functions of the skeleton: Support, Protection, Movement, Making blood cells.
2. Where are blood cells made? In the bone marrow.
3. What joins two bones in a joint? Ligaments.
4. What attaches muscles to bones? Tendons.
5. Describe antagonistic muscles: They work in pairs where one contracts while the other relaxes to cause movement.

DNA and Genetics

1. What does 'hereditary' mean? Genetic information passed from one generation to another.
2. Name the cell that does not contain DNA: Red blood cell.
3. Where are chromosomes found? In the cell nucleus.
4. How many pairs of chromosomes in a body cell? 23 pairs.

Chemistry and Matter

1. What is a catalyst? A substance that speeds up a reaction without being used up by lowering activation energy.
2. State the law of conservation of mass: Total mass of reactants equals the total mass of products.
3. Define Brownian motion: The random motion of particles due to frequent collisions.
4. What is the test for Carbon Dioxide? Bubbling through limewater turns it cloudy/milky.