Grade 7 Biology & Chemistry – Comprehensive Bullet-Point Study Notes

Cells – The Building Blocks of Life

• Everything living––from microscopic bacteria to towering redwood trees––is made up of tiny structural units called cells.
• Studying cells is foundational for understanding ALL of biology because every body system, tissue, and biochemical process begins with what each cell can or cannot do.

Big-Picture Functions of a Cell

• Obtain and convert energy.
• Manufacture needed molecules (proteins, lipids, carbohydrates, nucleic acids).
• Maintain internal balance (homeostasis).
• Reproduce itself so the organism can grow, repair, or create offspring.

Plant Cells

• Cell Wall
  • Rigid outermost layer made primarily of cellulose.
  • Provides support, maintains square/rectangular shape, prevents bursting when water enters.
  • Ecological significance: gives wood its strength; source of dietary fiber.
• Chloroplasts
  • Green organelles containing chlorophyll.
  • Site of photosynthesis: $(6\,CO<em>2 + 6\,H</em>2O \xrightarrow{\text{light}} C<em>6H</em>{12}O<em>6 + 6\,O</em>2)$
  • Practical tie-in: basis of almost every food web on Earth.
• Large Central Vacuole
  • Single membrane-bound sac occupying up to $90\%$ of the cell’s volume.
  • Stores water, dissolved minerals, pigments (e.g.
  anthocyanin in petals), and waste.
  • When full, it exerts turgor pressure, keeping leaves firm.
• Shared Organelles (with animal cells)
  • Nucleus, mitochondria, cell membrane, endoplasmic reticulum, Golgi bodies, ribosomes.

Animal Cells

• No Cell Wall
  • Flexible—enables specialized shapes (e.g.
  neuron’s axon, red blood cell’s concavity).
  • Makes animals vulnerable to dehydration → behavior (drinking) and protective structures (skin) evolved.
• No Chloroplasts
  • Animals are consumers; must ingest food for energy.
  • Energy extracted by cellular respiration inside mitochondria: $(C<em>6H</em>{12}O<em>6 + 6\,O</em>2 \rightarrow 6\,CO<em>2 + 6\,H</em>2O + \text{ATP})$
• Many Small Vacuoles rather than one gigantic one.
• Still possess nucleus, mitochondria, and selective cell membrane.

Key Cell Parts & Functions

• Nucleus
  • "Control center"; stores DNA, coordinates growth, metabolism, and reproduction.
  • Surrounded by a nuclear envelope dotted with pores.
• Mitochondria
  • "Powerhouse"; converts food into ATP via respiration.
  • Double membrane & own circular DNA → evolutionary clue (endosymbiotic theory).
• Cell Membrane
  • Phospholipid bilayer; acts like a security gate regulating entry/exit of molecules.
  • Exhibits selective permeability via protein channels, pumps, and receptors.
• Cytoplasm
  • Jelly-like cytosol + suspended organelles; site for thousands of metabolic reactions.
• Vacuole
  • Storage for water, nutrients, toxins; size & number differ in plants vs.
  animals.

Human Body Systems

Studying body systems reveals how specialized cell groups cooperate to keep the organism alive and balanced.

Digestive System – How Food Is Broken Down

• Mouth
  • Teeth mechanically tear food; saliva (from salivary glands) starts chemical digestion with enzyme amylase.
• Esophagus
  • Muscular tube; peristalsis pushes food downward regardless of gravity (helpful example: swallowing while upside-down).
• Stomach
  • Acidic (pH ≈ $1.5$–$3$) HCl and enzyme pepsin attack proteins.
  • Churns food into semi-liquid chyme.
• Small Intestine
  • ~$6$ meters long; main site of nutrient absorption via villi & microvilli → massive surface area.
  • Enzymes from pancreas & bile from liver assist fat breakdown.
• Large Intestine
  • Reclaims water, minerals; houses beneficial bacteria producing vitamin K.
  • Forms and expels feces.

Respiratory System – Breathing

• Nose/Mouth – air entry; nasal hairs & mucus filter particles.
• Trachea (Windpipe) – C-shaped cartilage rings keep airway open.
• Lungs – millions of alveoli maximize diffusion surface.
  • Gas exchange: oxygen moves to blood, carbon dioxide leaves via concentration gradients.
• Diaphragm – dome-shaped muscle; contracting → lung expansion (inhalation); relaxing → lung shrinkage (exhalation).

Circulatory System – Moving Blood Around

• Heart – four-chambered pump; right side → lungs (pulmonary circuit), left side → rest of body (systemic).
• Blood Vessels
  • Arteries (thick walls, high pressure) carry blood away from heart.
  • Veins (valves, lower pressure) bring blood back.
  • Capillaries (one-cell-thick) perform exchange.
• Blood Transports
  • $O<em>2$ from lungs, nutrients from intestines, hormones, heat. • Removes $CO</em>2$ to lungs, urea to kidneys.

Reproduction

Reproduction ensures genetic information passes to the next generation, sustaining species.

Human Female Reproductive Cycle (Menstrual Cycle)

• Begins during puberty (≈ age $9$ onward).
• Average length: $28$ days; healthy range $21$–$35$.
• Governed by hormone interplay: FSH, LH, estrogen, progesterone.

1. Menstrual Phase (Days 1–5)

• Uterine lining (endometrium) sheds → menstrual bleeding.
• Uterine muscle contractions → cramps; NSAIDs often relieve pain.

2. Follicular Phase (Days 1–13)

• While menstruation occurs, ovaries recruit follicles.
• Estrogen rises → thickens new uterine lining; cervix mucus thins (sperm-friendly).
• Biologically analogous to “reset & rebuild.”

3. Ovulation (≈ Day 14)

• Surge in LH triggers release of a mature egg from ovary.
• Fertile window ~$5$ days (sperm can survive) + egg’s $24$ hours.

4. Luteal Phase (Days 15–28)

• Empty follicle → corpus luteum; secretes progesterone to maintain lining.
• If fertilization fails, hormones fall → lining breaks → new period.
• PMS symptoms (mood swings, bloating) linked to hormone drop.

Plant Reproduction

• Flower houses reproductive parts.
  • Stamen (♂) – anther + filament; produces pollen (sperm cells).
  • Pistil (♀) – stigma, style, ovary containing ovules (eggs).
• Pollination – pollen transferred to stigma via wind, water, insects, birds, bats (example: bees & clover mutualism).
• Fertilization – pollen tube delivers sperm to ovule → zygote → seed.
• Seeds disperse (wind, animals) → germinate into new plants.

Chemistry Foundations

Understanding matter and its interactions underpins everything from cooking to climate science.

States of Matter

• Solid – fixed shape & volume; particles vibrate in place (ice, quartz).
• Liquid – variable shape, fixed volume; particles slide past each other (water, oil).
• Gas – no fixed shape/volume; particles move freely, fill container (air, steam).

Phase Changes (Physical, not chemical)

• Solid → Liquid: Melting (absorbs heat; $\text{ice} \rightarrow \text{water}$).
• Liquid → Gas: Evaporation/Boiling (latent heat of vaporization).
• Gas → Liquid: Condensation (cloud formation, fogged mirrors).
• Liquid → Solid: Freezing (exothermic; lakes freeze top-down, protecting fish).

Composition of Air

• $78\%$ Nitrogen – relatively inert, dilutes oxygen.
• $21\%$ Oxygen – essential for aerobic respiration & combustion.
• $1\%$ Other Gases – $CO<em>2$, Argon, water vapor, neon etc. • Greenhouse effect linked to rising $CO</em>2$.
• Ecological reciprocity:
  • Plants during photosynthesis consume $CO<em>2$, release $O</em>2$.
  • Humans/animals inhale $O<em>2$, exhale $CO</em>2$.

Acids and Bases

• Acids
  • Taste sour (lemon juice, vinegar).
  • pH <7.
  • Turn blue litmus red.
  • Key role: stomach’s HCl aids protein digestion; industrially used in batteries (sulfuric acid).
• Bases (Alkalis)
  • Feel slippery/soapy (soap, baking soda).
  • pH >7.
  • Turn red litmus blue.
  • Used in cleaning agents, antacids (to neutralize stomach acid).
• Neutralization Reaction
  • Acid + Base → Salt + Water
  • Everyday example: using baking soda to treat bee stings (acidic venom).

Indicators – Detecting pH

• Litmus Paper
  • Blue → Red in acid; Red → Blue in base.
• Universal Indicator
  • Full color scale: red (strong acid, pH $0$) → green (neutral, pH $7$) → purple (strong base, pH $14$).
  • Helpful for titrations in labs and aquarium water testing.

Inter-Topic Connections & Applied Context

• Cellular respiration (biology) depends on oxygen levels described in air composition (chemistry).
• Photosynthesis (plant cells) both drives ecological oxygen production and underlies the carbon cycle influencing climate change (chemistry).
• Digestive enzymes are proteins encoded by DNA stored in the nucleus (cell biology), showing multi-scale integration.
• Acid–base neutralization mirrors the stomach’s pH regulation: antacid tablets (basic) ease heartburn by neutralizing excess acid.
• Phase changes of water are critical in bodily processes (e.g.
  sweat evaporation removes heat; blood freezes at lower temperatures due to salts).

Ethical, Philosophical & Practical Notes

• Understanding cell structures has enabled genetic engineering and stem-cell research, raising debates over bioethics and consent.
• Knowledge of menstrual cycles empowers reproductive choices and debunks myths, promoting gender equality and menstrual health advocacy.
• Air quality chemistry links directly to public-health policies (industrial emissions, urban planning).
• Sustainable farming leverages plant reproduction mechanisms (selective breeding, pollinator conservation) for food security.