Cell Structure, Membranes and Transport

The cell is the smallest unit that is alive. A single bacterium can sense its surroundings, take in food, release waste and copy itself, and every organ of your body is an organized community of trillions of cells doing the same jobs in specialized ways. This chapter tours the cell from the outside in: how we see cells at all, how simple and complex cells differ, what each internal compartment does, how the cell holds its shape and attaches to its neighbors, and how its membrane decides what comes in and what goes out.

Section 1Microscopy and the cell theory

Seeing the invisible

Most cells are between 1 and 100 micrometers (µm) across, far below the roughly 0.1 millimeter limit of the unaided human eye. Cell biology therefore began with lenses. In 1665 Robert Hooke looked at thin slices of cork and named the tiny boxes he saw "cells"; they were the empty walls of dead plant cells. A few years later Antoni van Leeuwenhoek, grinding his own small, powerful lenses, became the first to see living single-celled organisms, which he called "animalcules," in pond water, and bacteria scraped from his own teeth.

Three parameters describe any microscope:

• Magnification is the ratio of the image size to the real size of the object.
• Resolution is the minimum distance two points can be apart and still be seen as separate. It measures clarity. Magnifying beyond the resolution limit just enlarges a blur.
• Contrast is the difference in brightness between parts of the sample. Most cells are nearly transparent, so contrast is often enhanced with stains or optical tricks.

Resolution is limited by the wavelength of what is used to form the image. Visible light has wavelengths of about 400 to 700 nanometers (nm), so a light microscope (LM) cannot resolve details much finer than about 0.2 µm (200 nm), roughly the size of a small bacterium. Light microscopes magnify usefully up to about 1,000 times. Their great advantage is that they show living cells.

Light microscopy techniques

In brightfield microscopy light passes straight through the specimen; unstained cells show little contrast, but stained cells show clearly, though staining usually kills them. Phase contrast and differential interference contrast optics convert tiny differences in density into differences in brightness, revealing living, unstained cells. In fluorescence microscopy, specific molecules are tagged with fluorescent dyes or with fluorescent proteins such as green fluorescent protein (GFP), which glow when lit with the right wavelength. Confocal microscopes use a laser and a pinhole to reject out-of-focus light, building sharp optical slices that can be stacked into three dimensional images. Super-resolution methods, developed in the last few decades, use clever control of fluorescent molecules to break the traditional 200 nm limit and resolve structures as small as 10 to 20 nm.

Electron microscopy

An electron microscope (EM) focuses a beam of electrons with magnets. Electrons have a much shorter wavelength than light, so the practical resolution of modern EMs is about 2 nm or better, a hundred times finer than the LM. Specimens must be fixed, dried or frozen and placed in a vacuum, so living cells cannot be observed.

• The scanning electron microscope (SEM) scans a beam across a sample coated with a thin film of metal. Electrons knocked off the surface are detected, producing striking images of surface topography that look three dimensional.
• The transmission electron microscope (TEM) sends electrons through a very thin slice of tissue stained with heavy metals that bind certain structures. It reveals internal structure, which is how most organelles in this chapter were first described.
• Cryo-electron microscopy (cryo-EM) flash-freezes samples in a thin layer of ice without fixatives, preserving molecules in their natural shape. Combining thousands of images, it can determine the structure of individual proteins and complexes at near atomic resolution.

Cell fractionation

Microscopes show structure, but biologists also need to know what each structure does. Cell fractionation breaks cells open and separates their parts by spinning them in a centrifuge at increasing speeds. Large, dense components such as nuclei settle (form a pellet) at low speeds; mitochondria and chloroplasts at higher speeds; small membrane fragments and ribosomes only in an ultracentrifuge spinning at up to 100,000 revolutions per minute or more. Testing the purified fractions showed, for example, that the enzymes of cellular respiration reside in mitochondria.

The cell theory

By the mid-1800s, microscopists had seen enough cells in enough organisms to generalize. Matthias Schleiden concluded in 1838 that plants are made of cells, and Theodor Schwann extended the idea to animals in 1839. In 1855 Rudolf Virchow added the crucial third idea, summed up in the Latin phrase omnis cellula e cellula, "every cell from a cell." Together these form the cell theory:

1. All living organisms are composed of one or more cells.
2. The cell is the basic unit of structure and function in organisms.
3. All cells arise from preexisting cells by division.

Modern biology adds that cells contain hereditary information (DNA) that is passed from cell to cell during division, and that all cells share the same basic chemistry. The third tenet also rejected spontaneous generation, the idea that life routinely arises from nonliving matter.

Why cells are small

Every substance a cell uses enters through its surface, and every waste leaves the same way. As a cell grows, its volume increases faster than its surface. For a cube with side length s, surface area is 6s² and volume is s³, so the surface area to volume ratio is 6/s. A cube 1 µm on a side has a ratio of 6; a cube 10 µm on a side has a ratio of only 0.6. A large cell would have too little membrane to supply its interior. Cells and organisms solve the problem in several ways: they stay small, they become long and thin (nerve cells), they fold their surfaces (the microvilli of intestinal cells increase surface area some twentyfold), and large organisms are built from many small cells rather than a few huge ones.

Section 2Prokaryotic and eukaryotic cells

Features shared by all cells

Every cell, whatever its type, has four things in common. It is bounded by a plasma membrane, a selective barrier that lets oxygen, nutrients and wastes pass while keeping the cell's contents in. Inside is the cytosol, a semifluid, jellylike substance in which subcellular components are suspended. Every cell carries chromosomes made of DNA that hold its genes. And every cell has ribosomes, the small complexes of RNA and protein that make proteins according to instructions from the genes.

Beyond these essentials, cells come in two fundamentally different organizations. Organisms of the domains Bacteria and Archaea are made of prokaryotic cells. Protists, fungi, plants and animals, which make up the domain Eukarya, are made of eukaryotic cells. The names come from the Greek karyon, kernel, meaning the nucleus: pro means before and eu means true.

Prokaryotic cells

The defining feature of a prokaryotic cell is that its DNA is not enclosed in a membrane-bounded nucleus. Instead, the main chromosome, usually a single circular DNA molecule, is concentrated in a region called the nucleoid, which has no membrane around it. Many prokaryotes also carry plasmids, small circles of extra DNA that often carry genes for antibiotic resistance and can be passed from cell to cell. Prokaryotes lack membrane-enclosed organelles; the region between the nucleoid and the plasma membrane is the cytoplasm, and their ribosomes are smaller (70S) than those of eukaryotes (80S). Most prokaryotic cells are 1 to 5 µm long, about a tenth the diameter of a typical eukaryotic cell.

Outside the plasma membrane, nearly all prokaryotes have a rigid cell wall. In bacteria the wall is made of peptidoglycan, a network of sugar polymers cross-linked by short peptides; archaeal walls lack peptidoglycan. Many prokaryotes also secrete a sticky outer layer of polysaccharide or protein called a capsule, which helps them stick to surfaces and resist the host's immune defenses. Hairlike appendages called fimbriae attach cells to surfaces and to each other, and longer pili (sex pili) pull two cells together to transfer DNA. Many prokaryotes swim with flagella, whip-like structures that rotate like propellers. Bacterial flagella are built from different proteins and work by a different mechanism than eukaryotic flagella, an example of similar functions evolving independently.

Eukaryotic cells

A eukaryotic cell has its DNA inside a nucleus bounded by a double membrane, and it is divided internally by membranes into many organelles, each with a specialized job. Compartmentalization lets incompatible processes run side by side: the acidic, enzyme-filled interior of a lysosome can digest material without harming the cytosol, and fatty acids can be built in one compartment while they are broken down in another. Internal membranes also provide huge surface area for enzymes and transport proteins. Eukaryotic cells are typically 10 to 100 µm in diameter, and they have a cytoskeleton of protein fibers that shapes them, organizes their interior and moves them.

Animal cells and plant cells

Animal and plant cells share most organelles: a nucleus, ribosomes, endoplasmic reticulum, Golgi apparatus, mitochondria, peroxisomes, a cytoskeleton and a plasma membrane. They differ in a few telling ways. Plant cells have a rigid cell wall of cellulose outside the plasma membrane, chloroplasts for photosynthesis, a large central vacuole that can fill most of the cell's volume, and plasmodesmata, channels through the walls that connect neighboring cells. Animal cells have lysosomes as prominent digestive organelles (plants use their vacuole for much of this job), centrosomes with a pair of centrioles (absent from most plant cells), and, in some cells, flagella or cilia. Without a cell wall, animal cells rely on an extracellular matrix and on junctions with their neighbors for support.

Section 3The endomembrane system, mitochondria and chloroplasts

The nucleus: the cell's information center

The nucleus holds most of the cell's genes and is usually the most conspicuous organelle, averaging about 5 µm across. It is enclosed by the nuclear envelope, a double membrane perforated by nuclear pores about 100 nm wide. Each pore is lined by a large pore complex of proteins that regulates traffic: mRNA and ribosomal subunits move out, while proteins needed in the nucleus, such as DNA polymerases and histones, move in. On the inner side of the envelope, a meshwork of intermediate filaments called the nuclear lamina supports the nucleus and helps organize the DNA.

Inside, DNA is organized into chromosomes. Each chromosome is a single long DNA molecule wrapped around proteins, chiefly histones; the DNA-protein complex is called chromatin. A human body cell has 46 chromosomes. In a cell that is not dividing, chromatin is diffuse; as a cell prepares to divide, it condenses into the thick, visible chromosomes seen in mitosis. A dense region called the nucleolus is where ribosomal RNA (rRNA) is made and joined with proteins imported from the cytoplasm to assemble ribosomal subunits, which then exit through the pores.

Ribosomes: protein factories

Ribosomes are not membrane-bounded; each is a complex of rRNA and protein with a large and a small subunit. Cells that make lots of protein, such as pancreas cells that secrete digestive enzymes, have millions of ribosomes and prominent nucleoli. Ribosomes work in two places. Free ribosomes, suspended in the cytosol, make proteins that function there, such as the enzymes of glycolysis. Bound ribosomes, attached to the outside of the endoplasmic reticulum or nuclear envelope, make proteins destined for membranes, for packaging into organelles such as lysosomes, or for secretion from the cell. Bound and free ribosomes are identical; a ribosome binds the ER only when the protein it is making begins with a signal peptide that directs it there.

The endomembrane system

The endomembrane system includes the nuclear envelope, the endoplasmic reticulum, the Golgi apparatus, lysosomes, vacuoles and the plasma membrane. These membranes are related either through direct physical continuity or by the transfer of membrane segments as small vesicles, sacs made of membrane that bud from one compartment and fuse with another. Together they synthesize proteins, modify and sort them, make lipids, metabolize carbohydrates and detoxify poisons.

The endoplasmic reticulum (ER) is an extensive network of membrane-enclosed tubules and sacs called cisternae, often accounting for more than half the total membrane in a eukaryotic cell. Its internal space, the ER lumen, is continuous with the space between the two membranes of the nuclear envelope. The ER has two regions:

• Smooth ER lacks ribosomes. It synthesizes lipids, including phospholipids and steroids; the cells of the testes and ovaries that make sex hormones are packed with it. In liver cells, smooth ER enzymes detoxify drugs and poisons, mostly by adding hydroxyl groups that make them more soluble and easier to excrete. Repeated exposure to drugs such as alcohol makes smooth ER proliferate, increasing tolerance. In muscle cells, a specialized smooth ER (the sarcoplasmic reticulum) stores calcium ions and releases them to trigger contraction.
• Rough ER is studded with bound ribosomes. As a polypeptide grows, it is threaded into the lumen, where it folds into its shape. Many become glycoproteins as ER enzymes attach short carbohydrate chains. Rough ER also makes new membrane, inserting proteins into its own membrane and adding phospholipids. Finished proteins leave in transport vesicles that bud from a specialized region.

The Golgi apparatus: shipping and receiving

Transport vesicles from the ER travel to the Golgi apparatus, a stack of flattened membranous sacs (cisternae). The Golgi has a distinct polarity. The cis face, usually near the ER, receives vesicles; the trans face ships them out. As products move from cis to trans, mainly because the cisternae themselves mature, Golgi enzymes modify them, trimming and changing the carbohydrate chains of glycoproteins and making some polysaccharides, such as the pectins of plant cell walls. The Golgi then sorts products and targets them by attaching molecular tags, such as phosphate groups, that act like postal zip codes. Vesicles from the trans face go to lysosomes, to the plasma membrane for insertion, or to the surface for secretion.

Lysosomes and vacuoles

A lysosome is a membranous sac of hydrolytic enzymes that digest proteins, fats, polysaccharides and nucleic acids. These enzymes work best at about pH 5, which the lysosome maintains by pumping hydrogen ions in from the cytosol. If a lysosome leaks, its enzymes are not very active at the neutral pH of the cytosol, a built-in safeguard. Lysosomes carry out intracellular digestion. In phagocytosis, a cell engulfs a food particle or bacterium in a vacuole that fuses with a lysosome; macrophages destroy bacteria this way. In autophagy, a damaged organelle is wrapped in a membrane and digested, and its building blocks are recycled.

Vacuoles are large vesicles derived from the ER and Golgi. Food vacuoles form by phagocytosis. Many freshwater protists have contractile vacuoles that pump excess water out of the cell. Mature plant cells have a central vacuole enclosed by a membrane called the tonoplast. It stores ions, holds pigments that color petals, accumulates poisons that deter herbivores, and, most importantly, takes up water so that the cell grows larger with little investment in new cytoplasm.

Mitochondria and chloroplasts: energy converters

Mitochondria are the sites of cellular respiration, the oxygen-using process that releases energy from sugars, fats and other fuels and stores it in ATP. Nearly all eukaryotic cells have them, from one large mitochondrion to thousands, roughly matching the cell's activity; heart muscle cells are especially rich in them. A mitochondrion is enclosed by two membranes. The outer membrane is smooth. The inner membrane is folded into cristae, which greatly increase its surface area for the enzymes of the electron transport chain and ATP synthase. The inner membrane divides the organelle into an intermembrane space and the mitochondrial matrix, which contains enzymes of the citric acid cycle, mitochondrial DNA and ribosomes.

Chloroplasts, found in plants and algae, are the sites of photosynthesis, which converts light energy to chemical energy stored in sugar. They are members of a family of plant organelles called plastids, which also includes amyloplasts that store starch. A chloroplast is enclosed by two membranes separated by a narrow space. Inside lies a third membrane system of flattened sacs called thylakoids, often stacked like coins into grana (singular granum). The thylakoid membranes hold the green pigment chlorophyll and capture light. The fluid outside the thylakoids, the stroma, contains the chloroplast's DNA, ribosomes and the enzymes that build sugar.

The endosymbiont theory

Mitochondria and chloroplasts are not part of the endomembrane system; their proteins are made mostly on free ribosomes in the cytosol and on their own ribosomes inside. Their peculiarities point to a remarkable origin. According to the endosymbiont theory, an early ancestor of eukaryotic cells engulfed an oxygen-using bacterium that was not digested but became an endosymbiont, a cell living inside another cell. Over time the two merged into a single organism, and the endosymbiont became the mitochondrion. Later, one of these cells engulfed a photosynthetic cyanobacterium, which became the chloroplast of the lineage that gave rise to algae and plants. The evidence is strong:

• Both organelles are enclosed by two membranes, consistent with an engulfed cell inside a vesicle.
• Both contain their own circular DNA without histones, like bacterial chromosomes.
• Both have 70S ribosomes resembling bacterial ribosomes, sensitive to some of the same antibiotics.
• Both reproduce by a splitting process similar to binary fission, and cells cannot make them from scratch.
• Their DNA sequences are most similar to those of living bacteria: mitochondria to alphaproteobacteria and chloroplasts to cyanobacteria.

Peroxisomes

A peroxisome is a single-membrane compartment with enzymes that remove hydrogen atoms from various substrates and transfer them to oxygen, producing hydrogen peroxide (H₂O₂). Another enzyme, catalase, immediately converts the toxic H₂O₂ to water and oxygen. Peroxisomes break down long fatty acids into pieces that mitochondria can burn, and in the liver they detoxify alcohol and other harmful compounds.

Section 4The cytoskeleton and cell junctions

A dynamic scaffold

Early electron micrographs made the cytosol look like an empty soup in which organelles floated. Better methods revealed instead a network of protein fibers extending throughout the cytoplasm: the cytoskeleton. It gives mechanical support and maintains shape, which matters most in animal cells that lack walls. It anchors organelles in place, and it provides tracks along which motor proteins carry vesicles and organelles. It is constantly disassembled and rebuilt, letting cells change shape, crawl and divide. It has three kinds of fibers.

Microtubules

Microtubules, the thickest fibers (about 25 nm in diameter), are hollow tubes whose walls are made of the globular protein tubulin. Each tubulin unit is a dimer of alpha tubulin and beta tubulin. Microtubules have polarity: dimers are added and removed much faster at the plus end. They shape cells, serve as tracks for vesicles moving from the Golgi to the plasma membrane, separate chromosomes during cell division (as the mitotic spindle) and form the core of cilia and flagella. The drug colchicine, used to treat gout, binds tubulin and blocks microtubule growth; taxol, a cancer drug, does the opposite, freezing microtubules so dividing cells cannot complete mitosis.

In animal cells, microtubules grow out from a centrosome near the nucleus, a microtubule-organizing center that contains a pair of centrioles. Each centriole is a ring of nine sets of triplet microtubules. Plant cells and many fungi organize microtubules without centrioles.

Cilia and flagella are microtubule-containing extensions of some eukaryotic cells. They share a common structure: a core of nine doublets of microtubules arranged in a ring around two single microtubules in the center, the 9 + 2 pattern, covered by the plasma membrane. The assembly is anchored by a basal body whose structure (nine triplets, no center) is identical to a centriole. Arms of the motor protein dynein on each doublet "walk" along the neighboring doublet, using ATP; because the doublets are held together by cross-linking proteins, this walking makes the whole structure bend rather than slide apart. Flagella are usually few and long and move with a snake-like undulation (sperm); cilia are numerous and short and beat in an oar-like power stroke (the lining of the airways, which sweeps mucus out of the lungs, and the oviduct, which moves an egg toward the uterus).

Microfilaments

Microfilaments are the thinnest fibers (about 7 nm), solid rods built from a twisted double chain of actin subunits. They bear tension (pulling forces). A three dimensional network of microfilaments just inside the plasma membrane, the cortex, gives the outer cytoplasm a gel-like consistency and supports the cell's shape. Bundles of microfilaments form the core of microvilli, which increase the surface of intestinal cells. Microfilaments are best known for their role in movement. In muscle cells, thousands of actin filaments interdigitate with thicker filaments of the motor protein myosin, and myosin heads walk along the actin to contract the cell. In dividing animal cells, a ring of actin and myosin pinches the cell in two (the cleavage furrow). Cells such as amoebas and white blood cells crawl by extending pseudopodia as actin assembles at the leading edge, and in plant cells actin and myosin drive cytoplasmic streaming, a circular flow that distributes materials in large cells.

Intermediate filaments

Intermediate filaments range from 8 to 12 nm in diameter, between the other two. They are the most permanent part of the cytoskeleton and are specialized for bearing tension, like rope. Unlike the other two, they are built from a diverse family of proteins, and different cell types use different ones: keratins in epithelial cells (and in hair and nails), neurofilaments in nerve axons, vimentin in fibroblasts and lamins forming the nuclear lamina. They reinforce cell shape, anchor the nucleus, and in skin they connect to junctions between cells to spread mechanical stress. They are found only in some animals, including vertebrates.

Outside the cell: walls and matrix

A plant cell wall protects the cell, maintains its shape and prevents excessive uptake of water. It is built from cellulose microfibrils embedded in a matrix of other polysaccharides and protein. A young cell secretes a thin, flexible primary wall; between neighboring cells lies the middle lamella, rich in sticky pectins that glue cells together. When growth stops, many cells add a strong, often woody secondary wall between the plasma membrane and the primary wall.

Animal cells lack walls but secrete an elaborate extracellular matrix (ECM). Its main ingredients are glycoproteins and other carbohydrate-containing molecules. The most abundant is collagen, about 40 percent of the total protein in the human body, which forms strong fibers embedded in a network of proteoglycans, molecules with a small protein core and many carbohydrate chains that trap water. Glycoproteins called fibronectin bind the ECM to integrins, receptor proteins that span the plasma membrane and connect on the inside to microfilaments. Integrins transmit signals in both directions, so changes in the matrix can alter gene expression inside the cell and coordinate the behavior of all cells in a tissue.

Cell junctions

Neighboring cells in tissues adhere, interact and communicate through specialized junctions.

• Plasmodesmata are channels that pass through plant cell walls. The plasma membranes of adjacent cells line each channel, so the cytosol of the cells is continuous. Water, small solutes and even some proteins and RNA move from cell to cell this way.
• Tight junctions press the plasma membranes of neighboring animal cells tightly together, bound by specific proteins, forming a continuous seal around the cells that prevents leakage of fluid between them. Tight junctions between skin cells make us watertight, and in the gut and kidney they force substances to pass through cells rather than around them.
• Desmosomes (anchoring junctions) work like rivets, fastening cells together into strong sheets. Intermediate filaments of keratin anchor them on the inside. They are abundant in skin and heart muscle, tissues that are stretched and pulled.
• Gap junctions (communicating junctions) are channels formed by membrane proteins that connect the cytoplasm of adjacent animal cells, letting ions, sugars and other small molecules pass. They are functionally similar to plasmodesmata and are essential in heart muscle, where they let electrical signals spread quickly so the cells contract in unison.

Section 5Membrane structure, passive and active transport, bulk transport

The fluid mosaic model

The plasma membrane is only about 8 nm thick, yet it controls traffic into and out of the cell. It shows selective permeability: some substances cross easily and others hardly at all. The accepted description of its structure is the fluid mosaic model: a fluid phospholipid bilayer in which a mosaic of diverse proteins is embedded or attached. The phospholipids are amphipathic (Chapter 1), with hydrophilic heads facing the watery cytosol and extracellular fluid and hydrophobic tails facing each other in the interior.

The membrane is held together mainly by hydrophobic interactions, which are much weaker than covalent bonds. Phospholipids can therefore drift sideways within their layer, swapping places with neighbors millions of times per second, though they rarely flip from one layer to the other. Many proteins also drift, more slowly, and some are held in place by attachments to the cytoskeleton or the ECM. Membrane fluidity is essential: a membrane that is too fluid cannot hold proteins in position, and one that solidifies cannot function. Fluidity depends on:

• Temperature: as temperature falls, the membrane eventually solidifies.
• Fatty acid tails: unsaturated tails, kinked by cis double bonds, cannot pack closely and keep the membrane fluid at lower temperatures. Organisms adapt: winter wheat and cold-water fish increase the proportion of unsaturated phospholipids in their membranes as temperatures drop.
• Cholesterol, wedged between phospholipids in animal membranes, acts as a fluidity buffer. At warm temperatures it restrains phospholipid movement, and at cool temperatures it keeps them from packing tightly.

Membrane proteins and carbohydrates

Proteins determine most of a membrane's functions. Integral proteins penetrate the hydrophobic interior; most are transmembrane proteins that span the whole bilayer, with hydrophobic stretches (usually alpha helices of nonpolar amino acids) inside the membrane and hydrophilic regions exposed to the water on each side. Peripheral proteins are not embedded at all; they are loosely bound to the surface, often to exposed parts of integral proteins. Membrane proteins carry out six broad functions: transport, enzymatic activity, signal transduction (receptors), cell to cell recognition, intercellular joining and attachment to the cytoskeleton and ECM.

Cells recognize one another by binding to molecules, often containing carbohydrates, on the outer surface of the plasma membrane. Short, branched carbohydrate chains are covalently bonded to lipids, forming glycolipids, or to proteins, forming glycoproteins. Together they make a sugar coat, the glycocalyx, that differs from species to species, from individual to individual and from one cell type to another. The A, B, AB and O human blood types reflect variation in the carbohydrate part of glycoproteins on red blood cells. Membranes are therefore asymmetric: the inner and outer layers differ in their lipids, and carbohydrates face only the outside.

Permeability of the lipid bilayer

The hydrophobic core of the bilayer controls what can cross on its own. Small nonpolar molecules, such as O₂, CO₂ and steroid hormones, dissolve in the lipid and cross easily. Very small polar molecules such as water cross slowly. Ions and larger polar molecules such as glucose barely cross at all, because they are surrounded by water and repelled by the hydrophobic core. These substances cross through transport proteins. Channel proteins provide hydrophilic tunnels; aquaporins, for example, let up to 3 billion water molecules per second pass through each channel, and many ion channels open and close in response to signals (gated channels). Carrier proteins bind a specific substance and change shape to shuttle it across.

Passive transport: diffusion and osmosis

Diffusion is the net movement of a substance from where it is more concentrated to where it is less concentrated, the result of random thermal motion. Any substance diffuses down its own concentration gradient, independent of other substances, until it reaches dynamic equilibrium, when as many molecules cross one way as the other. Because no cellular energy is spent, diffusion across a membrane is called passive transport. Oxygen diffuses into a cell that is using it, and CO₂ diffuses out.

Osmosis is the diffusion of free water across a selectively permeable membrane. Water moves from the side with lower solute concentration (more free water) to the side with higher solute concentration, because solutes bind water molecules and reduce the concentration of free water. To predict osmosis, compare the solution outside a cell to the cell's interior using tonicity, the ability of a surrounding solution to make a cell gain or lose water. Tonicity depends on the concentration of solutes that cannot cross the membrane.

• An isotonic solution has the same nonpenetrating solute concentration as the cell; there is no net water movement. For human cells, 0.9 percent NaCl (normal saline) is isotonic.
• A hypertonic solution has more nonpenetrating solute than the cell; the cell loses water. An animal cell shrivels (crenation); in a plant cell, the plasma membrane pulls away from the wall in plasmolysis, usually lethal.
• A hypotonic solution has less solute than the cell; water enters. An animal cell swells and may burst (lysis). A plant cell swells until its wall pushes back, becoming firm, or turgid, which is the healthy state for most plant cells. If water is lost, plants wilt as cells become flaccid.

Organisms without walls that live in hypotonic or hypertonic environments must practice osmoregulation, the control of solute concentrations and water balance. The freshwater protist Paramecium lives in water strongly hypotonic to its cytoplasm and uses a contractile vacuole to pump the incoming water back out.

Facilitated diffusion

In facilitated diffusion, polar molecules and ions diffuse passively down their gradients with the help of transport proteins, either channels or carriers. It is still passive: no energy is spent, and the direction is always down the gradient. Because carrier proteins have a limited number of binding sites, facilitated diffusion saturates; once every carrier is busy, adding more solute outside does not speed transport. The GLUT family of glucose carriers moves glucose into most of your cells by facilitated diffusion.

Active transport

Some transport proteins move solutes against their concentration gradients, from low to high. This active transport requires energy, usually from ATP, and lets a cell keep internal concentrations very different from its surroundings. All active transport uses carrier proteins, not channels.

The best-known example is the sodium-potassium pump (Na⁺/K⁺ ATPase) of animal cells. In each cycle it binds 3 Na⁺ on the cytoplasmic side, is phosphorylated by ATP, changes shape and releases the sodium outside; it then binds 2 K⁺ outside, loses the phosphate, returns to its original shape and releases the potassium inside. The result is high K⁺ and low Na⁺ inside the cell. Because it moves 3 positive charges out for every 2 in, the pump also makes the inside of the cell negative relative to the outside.

That voltage across the membrane is the membrane potential, typically about −50 to −70 millivolts. It favors the movement of cations into the cell and anions out. Ions therefore respond to a combined electrochemical gradient: the chemical force of their concentration gradient plus the electrical force of the membrane potential. A transport protein that generates voltage is an electrogenic pump; in animals the main one is the sodium-potassium pump, and in plants, fungi and bacteria it is a proton pump that moves H⁺ out of the cell.

In cotransport (secondary active transport), a gradient created by a pump drives the uphill transport of another solute. A cotransporter lets an ion flow down its gradient and harnesses that energy to move a second solute against its gradient. In your intestine, a sodium-glucose cotransporter uses the inward Na⁺ gradient to pull glucose into cells even when glucose is already more concentrated inside. Plant cells use the H⁺ gradient from their proton pumps to load sucrose into the phloem the same way.

Bulk transport: exocytosis and endocytosis

Large molecules such as proteins and polysaccharides, and particles such as bacteria, cross the plasma membrane in bulk, packaged in vesicles. This also costs energy. In exocytosis, a transport vesicle from the Golgi moves along microtubules to the plasma membrane, the two membranes fuse, and the contents spill outside while the vesicle membrane becomes part of the plasma membrane. Pancreatic cells secrete insulin this way, and nerve cells release neurotransmitters.

In endocytosis, the cell takes in molecules and particles by forming new vesicles from the plasma membrane. There are three types:

• Phagocytosis ("cellular eating"): the cell wraps pseudopodia around a large particle, such as a bacterium, and packages it in a food vacuole, which fuses with a lysosome.
• Pinocytosis ("cellular drinking"): the membrane folds inward to take in droplets of extracellular fluid with whatever solutes they contain. It is nonspecific.
• Receptor-mediated endocytosis: specific receptors, clustered in coated pits lined with the protein clathrin, bind particular molecules (ligands), and the pit pinches off as a vesicle. This lets cells take up large amounts of specific substances even when they are scarce. Human cells take in cholesterol this way, as low-density lipoprotein (LDL) particles bound to LDL receptors.

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