Welcome to Topic 2: Membranes, Proteins, DNA and Gene Expression!
In this chapter, we are going to dive deep into the microscopic world of the cell. We'll explore how cells "breathe," how they control what enters and leaves, and how the "instruction manual" of life (DNA) is used to build the proteins that make you, you. Don't worry if some of these terms sound scary at first—we will break them down step-by-step with simple analogies and clear explanations!
1. Gas Exchange and Fick’s Law
Every living cell needs to take in "good stuff" (like oxygen) and get rid of "waste stuff" (like carbon dioxide). This is gas exchange.
Properties of Gas Exchange Surfaces
To be really good at swapping gases, a surface needs three things:
- Large Surface Area to Volume Ratio: Think of it like a crowded shop; the more doors you have, the faster people can get in and out.
- Thinness: The shorter the distance the gas has to travel, the faster it gets there.
- Concentration Gradient: There needs to be a big difference between the amount of gas on one side versus the other. Gases always want to move from where there is "lots" to where there is "a little."
Fick’s Law of Diffusion
We can actually calculate how fast gas moves using Fick’s Law:
\( \text{Rate of Diffusion} \propto \frac{\text{Surface Area} \times \text{Difference in Concentration}}{\text{Thickness of Gas Exchange Surface}} \)
Memory Tip: To get a high rate, you want the top numbers to be big and the bottom number to be small!
The Mammalian Lung
Our lungs are perfectly adapted for this. They have millions of tiny air sacs called alveoli which provide a massive surface area. The walls of these alveoli are only one cell thick, making the "trip" for oxygen very short!
Quick Review: The lungs maximize diffusion by being huge (surface area), thin (distance), and having a constant blood flow (maintaining the gradient).
2. The Cell Membrane: The Gated Community
The cell membrane isn't just a plastic bag; it's a complex, "intelligent" barrier known as the Fluid Mosaic Model.
Structure of the Membrane
- Phospholipid Bilayer: Imagine two layers of matches. The "heads" love water (hydrophilic) and face outwards, while the "tails" hate water (hydrophobic) and hide in the middle.
- Fluid: The molecules can move around; it's not a solid wall.
- Mosaic: It’s a mix of different things like proteins, cholesterol, and carbohydrates stuck into the bilayer.
How Things Move (Transport)
Cells have different ways of moving things across this membrane:
- Diffusion: Particles moving from high to low concentration. This is passive (it uses no energy).
- Facilitated Diffusion: Some things are too big or "wrongly shaped" to pass through the lipids. They need channel proteins or carrier proteins to help them through. This is still passive!
- Osmosis: The movement of water molecules from a region of high water potential to low water potential through a partially permeable membrane.
- Active Transport: Moving things against the gradient (from low to high). This requires ATP (the cell's energy currency).
- Exocytosis and Endocytosis: For really big stuff. The cell membrane "gulps" it in (endo) or "spits" it out (exo) using vesicles (bubbles of membrane).
Key Takeaway: Passive transport is like rolling a ball downhill (no energy), while active transport is like pushing it uphill (needs ATP).
3. Proteins and Enzymes
Proteins do almost everything in your body. They are made of building blocks called amino acids.
Protein Structure
- Primary Structure: The basic sequence of amino acids in a chain.
- Secondary Structure: The chain coils up into shapes like an alpha helix or beta-pleated sheet.
- Tertiary Structure: The whole thing folds into a specific 3D shape held by bonds (hydrogen, ionic, and disulfide). This shape is vital for how it works!
- Globular vs. Fibrous: Globular proteins (like haemoglobin) are round and soluble. Fibrous proteins (like collagen) are long, tough, and insoluble.
Enzymes: The Biological Catalysts
Enzymes are special globular proteins that speed up reactions by lowering the activation energy (the "start-up cost" of a reaction).
They are highly specific. Because of their 3D shape, only one specific substrate molecule fits into their active site. Think of it like a lock and a key!
Quick Review: If you change the shape of the enzyme (denature it), the key won't fit the lock anymore, and the reaction stops.
4. DNA and the Genetic Code
DNA is the master blueprint. It is made of units called mononucleotides.
Nucleotide Structure
Each nucleotide has three parts: a sugar (deoxyribose in DNA, ribose in RNA), a phosphate, and a base.
- DNA Bases: Adenine (A), Guanine (G), Cytosine (C), and Thymine (T).
- RNA Bases: A, G, C, and Uracil (U) instead of Thymine.
The Double Helix
DNA is two strands twisted together. They are held by hydrogen bonds between complementary base pairs: A always pairs with T, and C always pairs with G.
The Genetic Code
The code is:
1. Triplet Code: Three bases code for one amino acid.
2. Non-overlapping: Each base is part of only one triplet.
3. Degenerate: There are more combinations than amino acids, so some amino acids are coded for by more than one triplet.
Key Takeaway: A gene is just a sequence of bases on DNA that tells the cell how to make a specific protein.
5. Protein Synthesis: From Gene to Protein
How does the cell turn a DNA code into a physical protein? Two main steps:
Step 1: Transcription (In the Nucleus)
- The DNA unzips.
- An enzyme called RNA polymerase uses one DNA strand (the antisense strand) as a template.
- Free RNA nucleotides line up to make a copy called mRNA.
Step 2: Translation (In the Ribosome)
- The mRNA travels to a ribosome.
- Molecules called tRNA bring the correct amino acids.
- The anticodon on the tRNA matches the codon on the mRNA.
- Amino acids join together with peptide bonds to form a polypeptide chain.
Analogy: Transcription is like copying a recipe from a huge cookbook (DNA) onto a small piece of paper (mRNA). Translation is like taking that paper to the kitchen (ribosome) to actually cook the meal (protein).
6. Genetics, Mutations, and Cystic Fibrosis
Sometimes, errors happen when DNA is copied. These are mutations (substitutions, insertions, or deletions of bases).
Genetics Vocabulary
- Allele: A different version of a gene.
- Genotype: The alleles you have (e.g., Bb).
- Phenotype: The physical characteristic you see (e.g., Brown eyes).
- Homozygote: Two of the same alleles (BB).
- Heterozygote: Two different alleles (Bb).
Cystic Fibrosis (CF)
CF is caused by a mutation in a protein that transports chloride ions. This leads to thick, sticky mucus.
- Lungs: Mucus blocks airways and traps bacteria (infections).
- Digestion: Mucus blocks the tubes from the pancreas, so enzymes can't reach food.
- Reproduction: Mucus blocks the tubes that carry sperm or eggs.
Quick Review: CF is a recessive disorder, meaning you need two copies of the faulty gene to have the disease.
7. Genetic Screening and Ethics
We can now test for genetic disorders using genetic screening.
- Amniocentesis: Taking a sample of the fluid around a baby (around 15 weeks).
- Chorionic Villus Sampling (CVS): Taking a sample from the placenta (earlier, around 10 weeks).
- PGD: Testing embryos created via IVF before putting them in the womb.
The Big Questions (Ethics)
Genetic testing brings up tough choices. Is it right to end a pregnancy? Who should have access to this data? Could it lead to "designer babies"? There are no easy answers, and you should be able to discuss different viewpoints (religious, moral, and social).
Final Key Takeaway: Science gives us the tools to understand life, but society must decide the ethics of how to use them!