7.1 Cells, Photosynthesis, and Genetics
Key Takeaways
- The cell theory states that all living things are made of cells, the cell is the basic unit of life, and all cells come from pre-existing cells
- Plant cells have three structures animal cells lack: a cell wall, chloroplasts, and a large central vacuole
- Photosynthesis (6CO2 + 6H2O -> C6H12O6 + 6O2) occurs in the chloroplast, while cellular respiration occurs mainly in the mitochondrion with glycolysis in the cytoplasm
- DNA bases pair strictly: adenine with thymine, and guanine with cytosine; humans have 46 chromosomes in 23 pairs
- A heterozygote x heterozygote monohybrid cross (Tt x Tt) gives a 1:2:1 genotypic ratio and a 3:1 phenotypic ratio, so a recessive trait appears in 1 of 4 offspring
Why Cells, Energy, and Heredity Matter for the PUPCET
Science makes up about 25% of the PUPCET, and biology questions consistently cluster around three themes: the cell as the basic unit of life, how organisms capture and release energy through photosynthesis and cellular respiration, and how traits pass from parents to offspring through genetics. Mastering these foundations lets you answer most biology items quickly, because many are direct recall of structures, sites, and ratios.
The Cell Theory
The cell theory, developed in the 1800s by Matthias Schleiden, Theodor Schwann, and Rudolf Virchow, states three core ideas:
- All living things are composed of one or more cells.
- The cell is the basic unit of structure and function in living organisms.
- All cells arise from pre-existing cells through cell division.
Organisms may be unicellular (one cell, like an amoeba or bacterium) or multicellular (many cells, like humans and mango trees).
Prokaryotic vs. Eukaryotic Cells
| Feature | Prokaryote | Eukaryote |
|---|---|---|
| Nucleus | None; DNA floats in a nucleoid region | Has a membrane-bound nucleus |
| Membrane-bound organelles | Absent | Present (mitochondria, ER, etc.) |
| Size | Smaller (about 1-10 micrometers) | Larger (about 10-100 micrometers) |
| Examples | Bacteria, archaea | Plants, animals, fungi, protists |
All cells — even prokaryotes — have a cell membrane, cytoplasm, ribosomes, and DNA as their genetic material.
Plant vs. Animal Cell Organelles
| Organelle | Function | Plant | Animal |
|---|---|---|---|
| Nucleus | Control center; holds DNA | Yes | Yes |
| Cell membrane | Controls what enters and exits | Yes | Yes |
| Mitochondrion | Powerhouse; makes ATP via respiration | Yes | Yes |
| Ribosome | Makes proteins | Yes | Yes |
| Endoplasmic reticulum (ER) | Transports materials; rough ER carries ribosomes | Yes | Yes |
| Golgi apparatus | Packages and ships proteins | Yes | Yes |
| Cell wall | Rigid support and protection | Yes | No |
| Chloroplast | Site of photosynthesis | Yes | No |
| Large central vacuole | Stores water; keeps the cell firm | Yes | No (small vacuoles only) |
Exam tip: the classic PUPCET trap asks which structures plant cells have but animal cells lack. Memorize the trio: cell wall, chloroplast, and large central vacuole.
Photosynthesis: Capturing the Sun's Energy
Photosynthesis is the process by which plants, algae, and some bacteria convert light energy into chemical energy stored in glucose. It happens in the chloroplast, whose green pigment chlorophyll absorbs sunlight.
Overall equation: 6CO2 + 6H2O + light energy -> C6H12O6 + 6O2
| Stage | Site in the chloroplast | What happens |
|---|---|---|
| Light-dependent reactions | Thylakoid membrane | Light splits water, releasing oxygen; ATP and NADPH are produced |
| Calvin cycle (light-independent) | Stroma | Carbon dioxide is fixed into glucose using ATP and NADPH |
Key recall: the oxygen released comes from the splitting of water, not from carbon dioxide.
Cellular Respiration: Releasing Energy
Cellular respiration breaks down glucose to release energy as ATP (adenosine triphosphate). Aerobic respiration uses oxygen:
Overall equation: C6H12O6 + 6O2 -> 6CO2 + 6H2O + ATP (about 36-38 ATP per glucose)
| Stage | Site | Highlight |
|---|---|---|
| Glycolysis | Cytoplasm | Glucose is split into pyruvate; yields 2 ATP; needs no oxygen |
| Krebs cycle | Mitochondrial matrix | Produces CO2 and electron carriers |
| Electron transport chain | Inner mitochondrial membrane | Produces the bulk of ATP; oxygen is the final electron acceptor, forming water |
When oxygen is absent, cells use anaerobic respiration (fermentation): tired muscle cells produce lactic acid (that post-race soreness), while yeast produces alcohol and carbon dioxide — the chemistry behind tuba and risen puto.
Notice the mirror pattern: the products of photosynthesis are the reactants of respiration, and vice versa. The PUPCET loves asking where each process occurs — remember: photosynthesis in the chloroplast, respiration mainly in the mitochondrion, with glycolysis always in the cytoplasm.
A Grade 12 student views a leaf cell under a microscope. In which organelle does photosynthesis take place?
DNA, Genes, and Chromosomes
DNA (deoxyribonucleic acid) is the molecule that stores hereditary information. James Watson and Francis Crick described its double helix shape in 1953, building on Rosalind Franklin's X-ray data. DNA is built from nucleotides carrying four bases: adenine (A), thymine (T), guanine (G), and cytosine (C). Base pairing is strict: A pairs with T, and G pairs with C.
From smallest to largest: a gene is a DNA segment that codes for a trait; genes sit on chromosomes, which are coiled DNA-protein structures inside the nucleus. Humans have 46 chromosomes (23 pairs) — 23 from each parent.
Mendelian Genetics
Gregor Mendel, an Austrian monk who experimented on pea plants, uncovered the basic rules of inheritance:
- An allele is a version of a gene. A dominant allele (capital letter, e.g., T) masks a recessive allele (small letter, t).
- Genotype = the allele combination (TT, Tt, tt); phenotype = the observable trait (tall or short).
- Homozygous = two identical alleles (TT or tt); heterozygous = two different alleles (Tt).
- A recessive trait appears only in homozygous recessive (tt) individuals.
Worked Example: Monohybrid Cross
Cross two heterozygous tall pea plants: Tt x Tt. Each parent can pass on T or t. The Punnett square:
| T | t | |
|---|---|---|
| T | TT (tall) | Tt (tall) |
| t | Tt (tall) | tt (short) |
Genotypic ratio: 1 TT : 2 Tt : 1 tt. Phenotypic ratio: 3 tall : 1 short. So the chance of a short offspring is 1/4, or 25%.
Common Traps
- Confusing genotype with phenotype — a Tt plant is tall even though it carries t.
- Forgetting that each offspring is an independent event; earlier children do not change the odds.
- Assuming dominant means "more common" — dominance only means the trait is expressed whenever the allele is present.
Takeaway: memorize the 3:1 phenotypic ratio for heterozygote x heterozygote crosses, and the 1:1 ratio for a heterozygote x homozygous recessive test cross.
In pea plants, tall stems (T) are dominant over short stems (t). If two heterozygous tall plants (Tt x Tt) are crossed, what fraction of the offspring is expected to be short?