7.1 Cell Biology, Transport, and Cellular Energetics

7.1 Cell Biology, Transport, and Cellular Energetics

Adamson University publishes no content outline for the AdUCET, so the sensible preparation target for the science items is the Grades 11 to 12 K-12 core — and no part of that core is denser than the cell. What follows is standard senior-high material that rewards precise vocabulary and a few exact numbers far more than it rewards memorised paragraphs.

1. Cell theory and the two great cell types

Cell theory is the organising idea of biology and rests on three statements:

1. Every living thing is built from one or more cells.
2. The cell is the smallest unit that is both structural and functional in a living thing.
3. New cells arise only from the division of existing cells; they do not appear spontaneously.

Cells come in two architectures. A prokaryote (Greek for “before the nucleus”) keeps its DNA loose in a region called the nucleoid. A eukaryote (“true nucleus”) seals its DNA inside a membrane.

Plant cell versus animal cell

Both are eukaryotic, so both carry a nucleus, mitochondria, ribosomes, endoplasmic reticulum, Golgi and a cell membrane. Four differences separate them:

• A cell wall of cellulose sits outside the membrane of a plant cell, giving it a fixed boxy outline and preventing it from bursting.
• Chloroplasts allow the plant cell to manufacture its own food; animal cells have none.
• One large central vacuole fills most of a mature plant cell and supplies internal pressure, whereas animal cells carry only small scattered vacuoles.
• Animal cells contain centrioles that organise the spindle during division, and their lysosomes are far more prominent than a plant's.

2. The organelle roll call

3. The fluid mosaic membrane

The fluid mosaic model describes the membrane as a double sheet of phospholipids whose water-loving phosphate heads face the watery fluid on either side and whose water-fearing fatty-acid tails point inward at each other. It is fluid because the phospholipids drift sideways like boats on a pond, and a mosaic because proteins are scattered irregularly through it: channel proteins form pores, carrier proteins change shape to ferry one specific solute, receptor proteins bind hormones, and cholesterol in animal cells stops the sheet becoming too runny when hot or too rigid when cold.

           OUTSIDE THE CELL
   ooooooooo ooooo ===== oooooooooo      o  = phosphate head (water-loving)
   ||||||||| ||||| =====  ||||||||||     |  = fatty-acid tail (water-fearing)
   ||||||||| ||||| =====  ||||||||||    === = transport protein spanning
   ooooooooo ooooo ===== oooooooooo           the whole bilayer
           INSIDE (CYTOPLASM)

Only small uncharged molecules — oxygen, carbon dioxide, and to a limited extent water — slip straight between the tails. Glucose, amino acids and every ion need a protein.

4. Passive transport and tonicity

Passive transport costs no ATP; particles move down their concentration gradient until evenly spread.

1. Simple diffusion — small non-polar molecules such as O₂ and CO₂ pass directly through the bilayer.
2. Facilitated diffusion — polar or charged particles such as glucose and Na⁺ still move down their gradient, but through a channel or carrier protein.
3. Osmosis — the diffusion of water across a selectively permeable membrane, from the side with more water (fewer dissolved particles) to the side with less.

Tonicity always describes the outside solution relative to the inside of the cell.

Worked example 1 — settle tonicity by counting particles

A cell whose cytoplasm behaves like 0.30 osmol/L is dropped into 0.20 M sodium chloride. Does it swell or shrink?

1. Sodium chloride is ionic and dissociates completely: each formula unit yields one Na⁺ and one Cl⁻.
2. Particle concentration outside = 0.20 × 2 = 0.40 osmol/L.
3. Compare: 0.40 outside is greater than 0.30 inside, so the solution is hypertonic.
4. Water therefore leaves by osmosis. An animal cell crenates; a plant cell plasmolyses.

The trap is comparing “0.20 M” with “0.30 osmol/L” and calling the solution dilute. Convert to particles first. Had the solute been 0.20 M glucose, which does not dissociate, the solution would have been hypotonic and the cell would have swollen instead.

5. Active transport, endocytosis and exocytosis

Active transport spends ATP to drive a solute against its gradient, from low concentration to high. The standard case is the sodium-potassium pump (Na⁺/K⁺-ATPase) present in every animal cell membrane: for each ATP hydrolysed it ejects 3 Na⁺ and imports 2 K⁺. Since three positive charges leave for every two that enter, the interior is left slightly negative — the resting condition a nerve cell needs before it can fire.

Cargo too large for any protein forces the membrane itself to move:

• Endocytosis folds the membrane inward and pinches off a vesicle into the cell. Phagocytosis (“cell eating”) takes in solids, as when a white blood cell swallows a bacterium; pinocytosis (“cell drinking”) takes in droplets of fluid.
• Exocytosis fuses a vesicle with the membrane and empties it out of the cell: insulin from pancreatic cells, neurotransmitter at a synapse, mucus from goblet cells.

6. Photosynthesis — banking the energy

$6\,\text{CO}_2 + 6\,\text{H}_2\text{O} + \text{light energy} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6\,\text{O}_2$

It runs in two stages inside the chloroplast:

• Light-dependent reactions, in the thylakoid membranes: chlorophyll absorbs light, water is split by photolysis, oxygen escapes as waste, and the captured energy is parked in ATP and NADPH.
• Calvin cycle, or light-independent reactions, in the stroma: carbon dioxide is fixed by the enzyme RuBisCO and reduced to sugar using the ATP and NADPH made in stage one. It needs no light directly, yet it halts within minutes of darkness because its supply of ATP and NADPH dries up.

Remember that the oxygen you breathe was pulled out of water, not out of carbon dioxide.

7. Aerobic respiration — spending it

$\text{C}_6\text{H}_{12}\text{O}_6 + 6\,\text{O}_2 \rightarrow 6\,\text{CO}_2 + 6\,\text{H}_2\text{O} + \text{energy}$

Worked example 2 — the ATP ledger and the price of losing oxygen

Using the senior-high convention that one NADH is worth 3 ATP and one FADH₂ is worth 2 ATP:

1. Direct (substrate-level) ATP: 2 from glycolysis + 2 from the Krebs cycle = 4 ATP.
2. Total NADH: 2 + 2 + 6 = 10, so 10 × 3 = 30 ATP.
3. Total FADH₂: 2, so 2 × 2 = 4 ATP.
4. Grand total = 4 + 30 + 4 = 38 ATP per glucose. Where the two glycolytic NADH must be shuttled into the mitochondrion at a cost, the figure falls to 36; careful modern measurements put the real yield closer to 30 to 32.

Now price the loss of oxygen. A sprinter's thigh fibre burns 5 glucose molecules. Aerobically that is 5 × 38 = 190 ATP. If the oxygen supply fails and the fibre switches entirely to fermentation, it collects 5 × 2 = 10 ATP — a shortfall of 180 ATP, which is why all-out anaerobic effort lasts seconds rather than minutes.

8. Anaerobic respiration and the two fermentations

Both pathways begin with the same glycolysis and both net only 2 ATP. Their real job is to regenerate NAD⁺ so that glycolysis can keep turning without oxygen.

• Lactic acid fermentation: pyruvate becomes lactic acid. It runs in overworked human muscle and in the bacteria that sour milk into yoghurt or ferment rice for burong isda.
• Alcoholic fermentation: pyruvate becomes ethanol plus CO₂. Yeast does this; the carbon dioxide raises pandesal dough while the ethanol produces tuba and lambanog.

9. ATP, and mitosis against meiosis

ATP (adenosine triphosphate) is adenine plus ribose plus three phosphate groups. Snapping off the terminal phosphate releases usable energy:

$\text{ATP} + \text{H}_2\text{O} \rightarrow \text{ADP} + \text{P}_i + \text{energy}$

ATP is a rechargeable battery rather than a fuel tank: a cell holds only seconds' worth at a time and rebuilds it continuously.

Common traps

• Calling the cell hypertonic when the item asks about the solution. Tonicity describes the outside relative to the inside.
• Believing osmosis moves the solute. Osmosis moves water; the solute is usually the thing that cannot cross.
• Assuming active transport always means inward. The sodium pump drives Na⁺ out.
• Claiming plants only photosynthesise. Plants respire day and night, and at night they are net consumers of oxygen.
• Placing the Calvin cycle in the thylakoid, or glycolysis in the mitochondrion. Calvin cycle equals stroma; glycolysis equals cytoplasm.
• Writing that fermentation yields no ATP. It yields 2 — merely far fewer than 38.
• Forgetting that the cell wall is freely permeable. Selection is the job of the membrane.