Biology · AP Biology
Cell structure and function
compartments and membranes
Every living thing is made of cells, and every cell is a bag of water and chemicals held apart from the world by a membrane.
The figure is an animal cell, cut open. Inside its plasma membrane is the cytoplasm, and in a eukaryotic cell like this one, organelles: compartments with membranes of their own, each with a job.
Blue vesicles carry newly made protein from the endoplasmic reticulum to the Golgi and on to the membrane, where it leaves the cell.
This chapter names the organelles and their jobs, explains why cells stay small, then follows the membrane: what it is made of, what crosses it and how, how water moves, and where the organelles themselves came from.
ribosomes and the endomembrane system
Organelles divide the work of a eukaryotic cell. Prokaryotes, the bacteria and archaea, have no nucleus and no membrane-bound organelles, but every cell has a plasma membrane, cytoplasm, DNA and ribosomes.
Ribosomes make proteins. Each is rRNA and protein, with no membrane. They are found in all forms of life, a sign of common ancestry. Free ribosomes make proteins for the cytosol; ribosomes bound to the ER make proteins for membranes, for organelles and for export.
The endomembrane system is a set of membranes linked by vesicles: the nuclear envelope, the ER, the Golgi, lysosomes, vacuoles and the plasma membrane.
| organelle | job |
|---|
| nucleus | holds the DNA; a double membrane, the nuclear envelope |
| rough ER | membranes with bound ribosomes: protein synthesis, folding, transport |
| smooth ER | no ribosomes: lipid synthesis, detoxification |
| Golgi complex | modifies proteins chemically (adds carbohydrates, for example), sorts and packages them |
So a protein for export runs rough ER → Golgi, cis face → Golgi, trans face → plasma membrane, carried between them in vesicles.
| typical size | |
|---|
| ribosome | 25 nm | in every cell |
| bacterium (prokaryote) | 1–5 µm | no nucleus, no membrane-bound organelles |
| mitochondrion | 0.5–1 µm across | about the size of a bacterium |
| animal cell | 10–30 µm | |
| plant cell | 10–100 µm | large central vacuole |
energy, storage and recycling
A plant cell has all of an animal cell’s organelles but lysosomes (mostly), and adds chloroplasts, a large central vacuole and a cell wall.
Mitochondria have a double membrane: a smooth outer one and a folded inner one, whose folds are cristae. They make most of the cell’s ATP by cellular respiration (chapter 3).
Chloroplasts, in plants and algae, also have a double membrane. Inside are stacks of membrane sacs, the thylakoids, in a fluid called the stroma. They capture light energy in photosynthesis.
Lysosomes are membrane sacs of hydrolytic enzymes. They digest what the cell takes in, recycle worn-out parts, and take part in apoptosis, programmed cell death.
Vacuoles store and release water, ions, macromolecules and waste.
| plant cell | animal cell |
|---|
| number | usually one, central | several, small |
| size | large: often most of the cell | small |
| main job | holds water: its pressure keeps the cell firm (turgor) | storage, transport, digestion |
In the figure the vacuole fills 27% of the cell, and the chloroplasts and mitochondria ride the moving cytoplasm round it: cytoplasmic streaming.
surface area to volume
A cell exchanges food, oxygen, waste and heat through its surface, but its need for them grows with its volume. As a cell grows, volume outruns surface.
For a cube of side s:
| side (µm) | 1 | 2 | 3 | 4 |
|---|
| surface (µm²) | 6 | 24 | 54 | 96 |
|---|
| volume (µm³) | 1 | 8 | 27 | 64 |
|---|
| SA:V (µm⁻¹) | 6 | 3 | 2 | 1.5 |
|---|
Doubling the side multiplies the surface by 4 but the volume by 8, so it halves the ratio.
A sphere. . A cell of radius 10 µm has SA:V = 0.3 µm⁻¹; at 20 µm, 0.15 µm⁻¹, half as much surface for each unit of volume.
Cut a cube of side 4 into 64 cubes of side 1: the same volume, 4 times the surface. Cells divide, flatten, or fold their membranes (microvilli, root hairs) to keep the ratio high; large organisms build exchange surfaces such as alveoli and villi.
The ratio also sets heat loss. A small animal has more surface per gram, loses heat faster, and so has a higher metabolic rate per gram than a large one.
a sheet of lipid with proteins afloat
The plasma membrane is a phospholipid bilayer with proteins embedded in it and carbohydrates attached to its outer face.
A phospholipid is amphipathic: a hydrophilic phosphate head and two hydrophobic fatty-acid tails. In water they form two layers, heads out to the water on each side, tails hidden in the middle.
Membrane proteins sit by the same rule. Integral proteins span the bilayer: their nonpolar (hydrophobic) amino acids face the lipid tails, their polar and charged (hydrophilic) ones face the water. Peripheral proteins are held loosely to a surface.
The membrane is fluid: lipids and most proteins drift sideways, as the figure shows. A lipid flipping from one layer to the other is rare. The proteins scattered through the lipid make the mosaic.
Cholesterol, among the lipids of vertebrate animal cells, regulates fluidity: it steadies the membrane when warm and keeps it from packing solid when cold.
Glycoproteins and glycolipids carry short carbohydrate chains on the outer face, used in cell recognition.
the hydrophobic core decides
A membrane lets some substances through and not others. The barrier is the bilayer’s hydrophobic middle.
| molecule | kind | crosses the bilayer |
|---|
| O2 | small, nonpolar | freely |
| CO2 | small, nonpolar | freely |
| H2O | small, polar | slowly; fast through aquaporins |
| glucose | large, polar | not without a transport protein |
| Na+ | ion (charged) | not without a channel or pump |
Small nonpolar molecules (O2, CO2, N2) dissolve in the tails and pass straight through. Water is small enough to trickle through, but most of it goes through protein channels. Large polar molecules and ions, which are held in shells of water, cannot enter the nonpolar core: they need transport proteins.
So the cell controls what crosses by which proteins it puts in its membrane, and can keep its inside different from its outside.
Cell walls. Plants, fungi and most prokaryotes have a wall outside the plasma membrane: cellulose in plants, chitin in fungi, peptidoglycan in bacteria. It gives shape and support, is a further barrier to some substances, and resists the pressure of water coming in, so the cell does not burst (osmotic lysis).
passive, active and in bulk
A substance crosses a membrane either down its concentration gradient, for free, or up it, at a cost.
Passive transport is the net movement of molecules from high concentration to low, with no input of metabolic energy. Molecules cross both ways (red, left), but more cross from the crowded side, so the net flow is down the gradient until the concentrations even out. Diffusion through the bilayer, and facilitated diffusion through proteins (2.6), are passive.
Active transport moves molecules from low concentration to high. It needs energy put in directly, usually from ATP, and a transport protein: the pump (blue, centre). The sodium–potassium pump (2.8) is the example to know.
Selective permeability plus active transport is how a cell builds and keeps concentration gradients across its membranes.
Large molecules and particles go in bulk, in vesicles, and that also takes energy. In exocytosis a vesicle fuses with the plasma membrane and releases its contents outside: how cells secrete proteins such as hormones. In endocytosis the membrane folds in around material and pinches off as a vesicle: phagocytosis (particles, whole cells), pinocytosis (fluid), receptor-mediated endocytosis (specific molecules bound to receptors).
downhill, through proteins
Ions and large polar molecules need a membrane protein to cross. When they move down their gradient through one, it is facilitated diffusion: passive, no ATP.
Channel proteins form pores. Charged ions, including Na+ and K+, need channels to cross. Many channels pass one kind of ion only: in the figure the K+ channel lets K+ (blue) out, down its gradient, but not Na+ (red). Many channels are gated, opening only in response to a signal.
Aquaporins are channels for water. Large amounts of water cross the membrane through them, single file, by osmosis.
Carrier proteins bind a molecule such as glucose on one side, change shape, and release it on the other. Moving down the gradient this way is still passive.
Because ions carry charge, their movement through channels can leave one side of the membrane more positive than the other: the membrane becomes polarized.
which way water moves
Osmosis is the diffusion of water across a selectively permeable membrane. Water moves toward the side with more dissolved solute.
Compared with a cell, a solution is hypotonic (less solute: water moves in), isotonic (equal: no net movement) or hypertonic (more solute: water moves out). In the figure the bath swings from 0.15 M to 0.6 M around the cells’ 0.3 M.
Water potential, Ψ, puts this in numbers. Water moves from higher Ψ to lower Ψ.
i is the ionization constant (1 for sucrose, 2 for NaCl), C the molar concentration, R = 0.0831 L·bar/(mol·K), T in kelvins. Pure water in an open beaker has Ψ = 0; solute makes Ψs negative; pressure Ψp raises Ψ.
Example. A 0.2 M sucrose solution, open to the air, at 22 °C:
A plant cell with Ψs = −7.0 bar and Ψp = +3.0 bar has Ψ = −4.0 bar. Put in this solution, it loses water: −4.0 is higher than −4.90.
An animal cell has no wall: in a hypotonic solution it swells and can burst; in a hypertonic one it shrinks. A plant cell in a hypotonic solution presses its membrane against the wall and becomes turgid, its healthy state; in a hypertonic one the cytoplasm pulls away from the wall: plasmolysis.
Osmoregulation keeps water and solutes in balance: freshwater protists pump water out with contractile vacuoles; animals regulate the solutes in their blood.
active transport, one ATP at a time
The Na+/K+ ATPase moves 3 Na+ out of the cell and 2 K+ in for each ATP it uses, both against their gradients.
| step | |
|---|
| 1 | open to the cytoplasm: 3 Na⁺ bind |
| 2 | ATP phosphorylates the pump |
| 3 | the pump changes shape |
| 4 | open outside: 3 Na⁺ released |
| 5 | 2 K⁺ bind from outside |
| 6 | the phosphate leaves |
| 7 | the pump changes back |
| 8 | open inside: 2 K⁺ released |
The energy goes into the pump’s shape: ATP transfers its last phosphate to the pump, which changes shape and opens to the outside; when the phosphate leaves, it changes back.
| typical | outside | inside |
|---|
| Na+ (mM) | 145 | 15 |
| K+ (mM) | 5 | 140 |
Each cycle moves 1 more positive charge out than in. With K+ leaking out through channels, this keeps the inside of the cell negative: a membrane potential, about -70 mV in a resting neuron.
Without ATP the pump stops and the gradients run down by diffusion.
membranes keep reactions apart
Internal membranes divide a eukaryotic cell into compartments, each with its own conditions and enzymes.
Fewer competing reactions. A lysosome keeps its hydrolytic enzymes, which would digest the cell, behind a membrane, at pH 5 against the cytosol’s 7.2. That is times the H+ concentration. The enzymes work best at acid pH, so any that leak into the cytosol do little harm.
More surface. Many reactions run on membranes, so a membrane’s area limits them. The mitochondrion’s inner membrane is folded into cristae; in the figure the folds make it 1.8 times the area of the smooth outer membrane. Chloroplasts stack thylakoids the same way.
Two membranes also make two spaces: the mitochondrion has an intermembrane space and an inner matrix, and chapter 3 uses the difference between them.
endosymbiosis
Mitochondria and chloroplasts were once free-living prokaryotes, taken in by an ancestral host cell and kept: endosymbiosis.
In the figure a host cell engulfs an aerobic bacterium. It is not digested; it lives on inside a membrane from the host, and divides. Its descendants are mitochondria. Later, in the line that led to plants and algae, a cell that already had mitochondria took in a photosynthetic bacterium (a cyanobacterium): the ancestor of chloroplasts.
| evidence | mitochondria and chloroplasts |
|---|
| membranes | two: the inner from the bacterium, the outer from the host |
| DNA | their own, circular, like a bacterium’s |
| ribosomes | their own, similar to bacterial ribosomes |
| reproduction | divide by fission, as bacteria do, apart from the cell’s division |
| size | about that of a bacterium |
Prokaryotes have no internal membrane-bound organelles, but they have regions inside with special structures and jobs, such as the nucleoid where their DNA is. Eukaryotic cells keep their DNA in a nucleus and their reactions in membrane-bound organelles.
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