Cell Structure and Function

In cystic fibrosis, one amino acid is missing from a membrane protein called CFTR, which should carry chloride ions out of lung cells. The faulty protein is made normally, but the cell's quality checks reject it inside the endoplasmic reticulum, so it never reaches the cell surface. Without it, the mucus in the lungs becomes thick and sticky. To understand the disease, and many exam questions built on it, you need to know where proteins are made, how they travel through the cell, and how the membrane controls what crosses it.

A eukaryotic cell divides its work among organelles 细胞器. The endomembrane system 内膜系统 is a connected set that makes, modifies, and ships molecules: the nucleus 细胞核 (holds DNA), rough and smooth endoplasmic reticulum 内质网 (protein and lipid synthesis), the Golgi 高尔基体 (modifies and packages), lysosomes 溶酶体 (digestion), and vesicles that carry material between them. Mitochondria 线粒体 (respiration) and chloroplasts 叶绿体 (photosynthesis) have their own membranes.

What each organelle actually does

The exam asks you to name a structure and say what it does, so learn them as pairs.

Ribosomes 核糖体 are made of ribosomal RNA 核糖体核糖核酸 (rRNA) and protein. They have no membrane, and they are found in every form of life — bacteria, archaea and eukaryotes alike. That shared presence is evidence of common ancestry: a structure this central was already present in the last common ancestor of all living things. A ribosome reads messenger RNA 信使核糖核酸 (mRNA) and builds the protein it codes for. Free ribosomes make proteins for use inside the cell; ribosomes bound to the rough endoplasmic reticulum make proteins for export or for the membrane.

The endoplasmic reticulum (ER) is a network of membranes that runs through the cytoplasm, helps the cell keep its shape and carries materials around inside it. Rough ER 粗面内质网 is covered with ribosomes: the proteins they make are threaded into the ER, the first step for proteins that will go to the membrane, to a lysosome, or out of the cell. Smooth ER 滑面内质网 has no ribosomes; it makes lipids and breaks down (detoxifies) harmful substances.

The Golgi complex 高尔基复合体 is a series of flattened, membrane-bound sacs. A protein arriving from the endoplasmic reticulum is passed through them, and three things happen to it:

• it is folded correctly and chemically modified — most often by glycosylation 糖基化, the attachment of short sugar chains;
• it is sorted and targeted, so the modified sugar or phosphate tag decides which compartment the protein is sent to;
• it is packaged into a vesicle for transport.

So the Golgi is not storage: it is the cell's finishing and dispatch department, and its modifications determine what a protein does and where it ends up.

Lysosomes are membrane-enclosed sacs of hydrolytic enzymes 水解酶 that digest material — worn-out organelles, engulfed bacteria, and food particles taken in by the cell. Their enzymes work best at a low pH, which the lysosome maintains inside itself; keeping them enclosed protects the rest of the cell. (A lysosome is a good example of compartmentalization: an enzyme that would damage the cytoplasm is safe inside its own membrane.) Lysosomes also take part in apoptosis 细胞凋亡, programmed cell death, in which a cell is dismantled deliberately, for example to sculpt fingers from a webbed embryonic hand.

Vacuoles 液泡 are membrane-bound sacs with many different roles: storing water, ions, nutrients or waste, and in a plant cell the large central vacuole also presses outwards on the cell wall to keep the cell firm. Animal cells have many small vacuoles instead of one large one.

A cell wall 细胞壁 surrounds the membrane of plant, fungal, bacterial and archaeal cells, but not animal cells. It gives the cell shape and strength, stops it bursting when it takes in water, and acts as a barrier to some substances.

Mitochondria and chloroplasts each have a double membrane and their own circular DNA and ribosomes — the evidence behind endosymbiotic theory below. The outer membrane of a mitochondrion is smooth; the inner membrane is folded into cristae 嵴, which give it a much larger surface area for the reactions of aerobic respiration. The two membranes also make separate compartments for different stages of respiration. Chloroplasts, found in plants and algae, are where photosynthesis happens.

Following a protein through the cell

A protein that ends up in the plasma membrane, or is secreted, follows one route. Free-response questions often ask you to draw or describe it.

Worked example (where is the faulty protein?). CFTR is a membrane protein, so it is made by ribosomes on the rough ER, not by free ribosomes. From the ER it goes in a vesicle to the Golgi, then in another vesicle to the plasma membrane. In the most common cystic fibrosis mutation the protein folds wrongly, and the ER keeps misfolded proteins back, so the mutant protein is found in the ER, not in the membrane. If instead a mutation changed only the protein's ATP-binding site, the protein could still fold and reach the membrane, but it would not work there.

Cells stay small because of the surface-area-to-volume ratio 表面积与体积比. As a cell grows, its volume rises faster than its surface area, so its membrane cannot exchange materials fast enough for the interior. Staying small (or being flat or folded) keeps enough surface to serve the volume.

Worked example. A cube-shaped cell $2\ \mu\text{m}$ on a side has surface area $6\times2^2=24\ \mu\text{m}^2$ and volume $2^3=8\ \mu\text{m}^3$, so its surface-area-to-volume ratio is $\tfrac{24}{8}=3$. Double the side to $4\ \mu\text{m}$: surface area $=6\times4^2=96$, volume $=4^3=64$, ratio $=\tfrac{96}{64}=1.5$. Doubling the size halved the ratio – the larger cell has far less membrane per unit of interior, which is why cells stay small.

Worked example (a sphere). A spherical cell of radius $3\ \mu\text{m}$ has $SA=4\pi r^2=113\ \mu\text{m}^2$ and $V=\tfrac43\pi r^3=113\ \mu\text{m}^3$, a ratio of 1.0. For any sphere the ratio is $3/r$, so it falls as the radius grows. Cells that need a lot of exchange increase their surface without growing: microvilli 微绒毛 on gut cells, root hairs on plant roots, and the cristae inside mitochondria are all folds or projections that add membrane.

The same rule works for whole organisms. A mouse has far more surface per kilogram than an elephant, so it loses heat much faster and must burn food faster to stay warm. In general, the smaller the animal, the higher its metabolic rate per unit of body mass.

The cell membrane is a phospholipid bilayer 磷脂双分子层 – polar heads facing the water, nonpolar tails inside – studded with proteins. The fluid mosaic model 流动镶嵌模型 pictures it as a fluid sheet in which lipids and proteins drift. Cholesterol and the degree of unsaturation tune its fluidity.

Membrane proteins follow the same rule as phospholipids. The part of a protein that crosses the bilayer has mainly nonpolar (hydrophobic) R groups, which touch the fatty-acid tails. The parts that stick out into the cytoplasm or the outside fluid have polar and charged R groups, which touch water. Short sugar chains attached to some proteins (glycoproteins) and lipids (glycolipids) face the outside and let cells recognise each other.

The membrane is selectively permeable 选择透过性. Small nonpolar molecules ($\text{O}_2$, $\text{CO}_2$) cross freely. Water, small but polar, crosses the bilayer only in small amounts and moves mostly through channel proteins called aquaporins 水通道蛋白. Large or charged/polar particles (ions, glucose) cannot pass the nonpolar core without help. This selectivity lets the cell control its internal environment.

The nonpolar core is the reason. Steroid hormones such as oestrogen are nonpolar, so they dissolve in the core and cross the membrane by simple diffusion, then bind receptors inside the cell. Ions such as $\text{Na}^+$, $\text{K}^+$ and $\text{Cl}^-$ are charged, so the hydrophobic tails stop them; they need channel proteins 通道蛋白 even to move down their gradients. Because ions carry charge, moving them across a membrane makes one side more positive than the other: the membrane becomes polarized.

• Passive transport 被动运输 moves substances down their concentration gradient (high → low) with no energy – diffusion 扩散.
• Active transport 主动运输 moves substances against the gradient (low → high), requiring energy (ATP), via protein pumps like the sodium–potassium pump.

The sodium-potassium pump 钠钾泵 uses one ATP to move 3 $\text{Na}^+$ out of the cell and 2 $\text{K}^+$ in, both against their gradients. Because more positive charge leaves than enters, it helps keep the inside of the cell negative: the membrane potential that nerve and muscle cells depend on. Active transport always needs a membrane protein and a source of energy; when a cell runs out of ATP, the gradients slowly run down.

Facilitated diffusion 易化扩散 is passive transport through a membrane protein – a channel or carrier 载体 – for particles that cannot cross the lipid alone. It still goes down the gradient and needs no energy, but its rate can saturate when all the proteins are busy.

Osmosis 渗透 is the diffusion of water across a membrane. Tonicity 张力 compares solute concentrations: in a hypotonic 低渗 solution a cell gains water (may burst); in a hypertonic 高渗 one it loses water (shrinks); in an isotonic 等渗 one there is no net change. Water potential 水势 predicts the direction of water movement (water moves to lower water potential), and is the sum of a pressure term and a solute term: $\Psi=\Psi_p+\Psi_s$, where the solute potential $\Psi_s=-iCRT$. Osmoregulation 渗透调节 is how organisms control this balance.

Freshwater protists live in water that is hypotonic to their cytoplasm, so water flows in all the time. A contractile vacuole 收缩泡 collects it and pumps it out, using energy. Put the protist in a medium with even less solute and water enters faster, so the vacuole must fill and empty more often. In a plant cell, water entering the large central vacuole pushes the membrane against the cell wall, giving turgor pressure; the wall stops the cell from bursting.

Worked example. For a $0.1\,\text{M}$ sucrose solution ($i=1$) in an open beaker (so pressure potential $\Psi_p=0$) at $25\,°\text{C}$ ($T=298\,\text{K}$, $R=0.0831\ \text{L}\cdot\text{bar/mol}\cdot\text{K}$): $\Psi_s=-iCRT=-(1)(0.1)(0.0831)(298)\approx-2.48\ \text{bar}$, so $\Psi\approx-2.48\ \text{bar}$. A plant cell whose interior is $\Psi=-1.0\ \text{bar}$ sits in this solution: since the solution is more negative, water moves out of the cell into the solution, and the cell loses turgor.

Large materials move in bulk by vesicles: endocytosis 内吞 brings material in (the membrane engulfs it), and exocytosis 外排 sends material out (a vesicle fuses with the membrane). Both require energy and let cells import and secrete large molecules.

Membranes create separate compartments 区室 so incompatible reactions can run at once and conditions (pH, ion levels) can be tuned locally. This organization boosts efficiency – the internal membranes also add surface area for reactions.

The endosymbiotic theory 内共生学说 explains mitochondria and chloroplasts: they arose when a larger cell engulfed free-living prokaryotes that then lived inside it. The evidence – their own circular DNA, their own ribosomes, and double membranes – supports this shared evolutionary origin.

Prokaryotes 原核生物 (bacteria and archaea) usually have no membrane-bound organelles, but their cells still have specialised regions: the DNA sits in an area called the nucleoid, and ribosomes are spread through the cytoplasm. In bacteria, the enzymes of the Krebs cycle work in the cytosol; in eukaryotes they work inside mitochondria. Eukaryotic cells keep many internal membranes that divide them into specialised regions.

Exam skill (experiments). Many unit 2 free-response questions describe a real experiment. The independent variable is what the researchers changed (for example, the ouabain concentration); the dependent variable is what they measured (the pump's activity). A control is a group treated the same way except for the factor being tested, such as normal kidney cells beside diseased ones, or cells with no inhibitor. When asked to "justify a control", say what it rules out: it shows that any difference is caused by the factor tested, not by something else in the method.

Common mistakes.

• Using "hypertonic" without saying compared with what. Tonicity always compares two solutions: the solution outside is hypertonic to the cell when it has more solute than the cell's cytoplasm.
• Moving water the wrong way. Water moves towards the hypertonic side, which is the side with the lower (more negative) water potential.
• Dropping the minus sign in $\Psi_s = -iCRT$. Adding solute always makes the solute potential more negative.
• Forgetting $i$ for an ionic solute. NaCl gives two particles, so $i = 2$ and $\Psi_s$ is about twice that of sucrose at the same molarity.
• Saying water crosses the membrane only through aquaporins. A little crosses the bilayer directly; aquaporins make it much faster.
• Calling facilitated diffusion active because it uses a protein. Any movement down the gradient is passive, with or without a channel or carrier.
• Saying membrane proteins are made by free ribosomes. Proteins for the membrane or for export are made by ribosomes on the rough ER.
• Saying the Golgi makes proteins. Ribosomes make them; the Golgi modifies, sorts and packages them.
• Saying ions cross the membrane because they are small. They are blocked by the nonpolar core because they are charged, however small.
• Saying the sodium-potassium pump moves equal numbers of ions. It moves 3 Na⁺ out and 2 K⁺ in per ATP.

• Use the two AP-formula skills: surface-area-to-volume ratio (why cells stay small) and water potential $\Psi=\Psi_p+\Psi_s$ ($\Psi_s=-iCRT$).
• Water moves toward the lower (more negative) water potential; predict swelling or shrinking from tonicity (hypo/hyper/isotonic).
• Passive transport and osmosis need no energy (down the gradient); active transport needs ATP (against the gradient).
• Distinguish diffusion, facilitated diffusion (a channel/carrier protein), and active transport.
• Explain membrane selectivity from the phospholipid bilayer — small non-polar molecules cross, large/charged ones need help.