Cell Structure and the Plasma Membrane

This is Unit Two of Biology, Part One of three: how cells came to be seen, the organelles inside them, and the network that routes what they build. A microscope has two properties, not one: magnification enlarges an image, and resolving power separates two structures sitting close together. Those instruments established unified cell theory, whose three claims come next. Then the census. Four components appear in every cell that exists — plasma membrane, cytoplasm, D N A, and ribosomes. The prokaryote and eukaryote split concerns what is added on top. One constraint runs underneath the unit. Volume increases faster than surface area as a cell grows, and past some size the membrane cannot support the diffusion the volume needs. Compartments answer that arithmetic. The part closes with the endomembrane system, following one protein from the network that modifies it to the organelle assembled from its packaging. By the end you will be able to do five things. One: say what magnification and resolving power each contribute, and state cell theory's three claims. Two: name the four components every cell shares, and what each type adds. Three: explain why surface area to volume limits cell size. Four: trace a ribosome from the nucleolus to the protein it builds. Five: route a protein through the endomembrane system.

Three entries on the instrument and what it settled. Magnification and resolving power are separate properties, and the sharper picture an electron microscope gives comes from the second, not from stronger magnification.

The instrument, and the scale.

Bio 0241 Microscopy and Cell Size

A microscope magnifies objects too small for the naked eye; cells are measured in micrometers because most fall below that threshold. A red blood cell spans about eight micrometers — roughly 250 would fit across a pinhead. Magnification enlarges an image; resolving power separates two close structures, and a microscope needs both.

Two kinds of microscope.

Bio 0243 Light and Electron Microscopy

A light microscope uses visible light and can image living cells; an electron microscope uses electrons for far higher magnification but kills the specimen. A lab light microscope resolves detail near 200 nanometers, an electron microscope near 50 picometers. Resolving power, not magnification alone, reveals the extra detail.

The theory that followed.

Bio 0245 Unified Cell Theory

Unified cell theory holds that living things are made of cells, that the cell is life's basic unit, and that new cells arise only from existing cells. Schleiden and Schwann proposed it in the 1830s, and Virchow reinforced the third claim. This is the confirmed sense of theory, not a hunch.

Three entries. The first names four components shared by every cell that exists, which turns the prokaryote and eukaryote split into a question about additions. The next two build the prokaryotic cell outward.

What every cell carries.

Bio 0247 Four Components Every Cell Shares

Every cell, prokaryotic or eukaryotic, has a plasma membrane, cytoplasm, D N A, and ribosomes. A bacterium and a human skin cell differ enormously in complexity and both carry all four. Sharing those four is what makes something a cell at all; the split concerns what else is added on top.

Inside a prokaryote.

Bio 0249 Structure of a Prokaryotic Cell

A prokaryotic cell keeps its D N A in a central region, the nucleoid, and typically carries a peptidoglycan wall outside its membrane. The bacterial chromosome sits free there, enclosed by no membrane — the absence a prokaryote is defined by. The nucleoid is a cytoplasmic region, not an organelle.

The prokaryotic surface.

Bio 0251 Prokaryotic Surface Structures

Some prokaryotes carry a capsule for surface attachment, flagella for locomotion, and pili or fimbriae for genetic exchange or host attachment. A pilus transfers genetic material between bacteria by direct contact, a process called conjugation. A flagellum drives movement and a pilus exchanges material — easy to confuse, entirely different functions.

Two entries, and the second answers the first. Surface area and volume do not grow at the same rate, which puts a ceiling on cell size. Eukaryotic compartments are a response to that ceiling.

Why a cell stays small.

Bio 0253 Why Cell Size Is Limited

As a cell grows, volume increases faster than surface area, so past some size the membrane cannot support the diffusion that volume needs. Doubling a cube-shaped cell's side drops its surface-to-volume ratio from six to one to three to one. A cell reaching that limit does not stop growing — it divides, flattens, or develops specialized organelles.

The eukaryotic answer.

Bio 0255 Structure of a Eukaryotic Cell

A eukaryotic cell adds a membrane-bound nucleus, numerous membrane-bound organelles, and several linear chromosomes to the four components every cell shares. Its D N A sits enclosed by a nuclear membrane while mitochondria and other organelles divide the remaining work. Compartmentalizing function answers the size limit — not a coincidence of complexity.

Three entries moving inward. The membrane is named here; how each substance crosses it is held back for this unit's closing block, two lectures away. The cytoplasm then brings a pair of terms worth separating early.

The boundary itself.

Bio 0257 The Plasma Membrane, Introduced

The plasma membrane is a phospholipid bilayer with embedded proteins that separates a cell's interior from its surroundings. It controls passage of organic molecules, ions, water, and oxygen inward while letting wastes such as carbon dioxide out. This entry names the structure; how each substance crosses is the work of this unit's closing block.

Folding the membrane.

Bio 0259 Microvilli

Microvilli are fingerlike foldings of the plasma membrane that increase a cell's surface area for absorption. Cells lining the small intestine carry dense microvilli, matching their job of absorbing nutrients from digested food. The added area appears on one face only — the side facing the substances to be absorbed.

The interior, named.

Bio 0261 Cytoplasm and Cytosol

Cytoplasm is the entire region between the plasma membrane and the nuclear envelope: organelles suspended in the gel-like cytosol. Though cytoplasm is 70 to 80 percent water, dissolved proteins give it a semi-solid consistency rather than a watery one. Cytosol names the fluid alone; cytoplasm names that fluid with everything suspended in it.

Five entries running as one route. D N A sits inside the envelope as chromatin. A region in the nucleus assembles ribosomal subunits, they leave through the pores, the ribosome makes protein in the cytoplasm, and the last entry names the energy source.

The nucleus.

Bio 0263 The Nucleus and Nuclear Envelope

The nucleus houses a eukaryotic cell's D N A and directs ribosome and protein synthesis, enclosed by a double-membrane envelope studded with pores. Those pores control passage of ions, molecules, and R N A between nucleoplasm and cytoplasm. The envelope is two phospholipid bilayers, an inner and an outer, continuous with the endoplasmic reticulum.

What the envelope holds.

Bio 0265 Chromatin and Chromosomes

A chromosome is a nuclear structure containing D N A; chromatin is that same material, D N A bound to protein, condensed or loosely unwound. A human cell's 46 chromosomes look distinct only when the cell readies to divide; otherwise they exist as decondensed chromatin. One material at two packing states, not two substances.

Where ribosomes are assembled.

Bio 0267 The Nucleolus

The nucleolus is a darkly staining region within the nucleus where ribosomal R N A and protein assemble into ribosomal subunits. Those subunits pass out through the envelope's pores into the cytoplasm, where they complete a functioning ribosome. It is a region within the nucleus, not a membrane-bound organelle of its own.

Where protein is made.

Bio 0269 Ribosomes

A ribosome, built of large and small protein-and-R N A subunits, carries out protein synthesis by translating messenger R N A into a sequence of amino acids. Ribosomes float free in the cytoplasm or attach to the plasma membrane or endoplasmic reticulum. A ribosome is not membrane-bound — it is a molecular complex, present in prokaryotes and eukaryotes alike.

What powers the work.

Bio 0271 Mitochondria

A mitochondrion is a double-membrane organelle that produces most of a cell's A T P by cellular respiration, its inner membrane folded into cristae. Muscle cells, demanding large amounts of A T P for contraction, pack an especially high concentration. Mitochondria carry their own D N A and ribosomes, distinct from the nucleus's.

Three entries. The chloroplast shares the mitochondrion's double membrane, own D N A and own ribosomes while doing the opposite energy job, a theory explains that shared inheritance, and a single-membrane organelle for breakdown follows. Membrane count separates them.

The plant counterpart.

Bio 0273 Chloroplasts

A chloroplast is a double-membrane plant organelle with its own D N A and ribosomes, carrying out photosynthesis using stacked thylakoids within a stroma. Chlorophyll in the thylakoid membranes captures light energy, while sugar synthesis happens in the surrounding stroma. Chloroplasts belong to plant cells and some protists only — animal cells never carry them.

Why both carry D N A.

Bio 0274 Endosymbiosis

Endosymbiotic theory holds that mitochondria and chloroplasts descend from free-living bacteria an ancestral host cell engulfed and kept rather than digested. Margulis's 1967 argument cited the size, D N A, and ribosomes these organelles share with bacteria. The course teaches this as the field's well-supported account, not one option among equals.

An organelle for breakdown.

Bio 0275 Peroxisomes

A peroxisome is a small, single-membrane organelle that breaks down fatty acids and amino acids through oxidation and detoxifies poisons that enter the cell. Peroxisomes in liver cells convert reactions releasing hydrogen peroxide into safely broken-down oxygen and water. A peroxisome has a single membrane, unlike a mitochondrion or chloroplast.

Five entries. Storage sacs both cell types carry come first, then two structures an animal cell has that most plant cells lack, then two a plant cell has instead. The centrosome entry carries this part's sharpest unsettled question.

Sacs for storage and transport.

Bio 0277 Vesicles and Vacuoles

Vesicles and vacuoles are membrane-bound sacs for storage and transport, differing mainly in that a vacuole is larger and does not fuse with other membranes. A transport vesicle buds from one organelle's membrane and fuses with another's, carrying cargo between them. A vesicle's membrane fuses readily; a vacuole's does not.

An animal-cell structure.

Bio 0279 The Centrosome

The centrosome is a microtubule-organizing structure near an animal cell's nucleus, built from a pair of centrioles at right angles. It duplicates before division, and its centrioles help pull duplicated chromosomes toward opposite ends. A centriole's exact role stays unclear: cells missing the centrosome, and plant cells that lack it, still divide.

The animal cell's digestion.

Bio 0281 Lysosomes

A lysosome is an animal-cell organelle that digests proteins, polysaccharides, lipids, nucleic acids, and worn-out organelles, using enzymes active at low p H. A macrophage engulfs a bacterium into a vesicle that fuses with a lysosome, and the enzymes destroy the pathogen. Most plant cells lack lysosomes and digest inside the vacuole instead.

The plant cell's rigid covering.

Bio 0283 The Plant Cell Wall

The plant cell wall is a rigid covering outside the plasma membrane, built mainly from the polysaccharide cellulose, protecting the cell and giving it shape. Biting raw celery crunches because teeth tear through the rigid cellulose walls of its cells. A plant wall's chief molecule differs from the peptidoglycan of most prokaryotic walls.

What fills a plant cell.

Bio 0285 The Central Vacuole

The central vacuole is a large plant organelle regulating water content and supporting expansion without new cytoplasm. A plant wilts when the vacuole loses water faster than the soil replaces it. The vacuole supports the wall through water pressure; losing that water leaves the wall intact but unsupported.

Five entries that teach better together than apart. The network is named, then its stations in order — rough endoplasmic reticulum, smooth, Golgi, and the lysosome assembled from what the Golgi tags and releases. One protein followed through is the point.

One network, several organelles.

Bio 0287 The Endomembrane System

The endomembrane system is a group of organelles — nuclear envelope, endoplasmic reticulum, Golgi, lysosomes, vesicles, plasma membrane — modifying, packaging, and transporting lipids and proteins. A protein made in the reticulum travels by vesicle to the Golgi, then the plasma membrane, without leaving the network. Mitochondria and chloroplasts sit outside it.

The first stop.

Bio 0289 Rough Endoplasmic Reticulum

The rough endoplasmic reticulum is a membrane network studded with ribosomes that modifies newly made proteins and synthesizes membrane phospholipids. Liver cells, which secrete large amounts of protein, carry an especially dense rough endoplasmic reticulum. Its ribosomes sit on the cytoplasmic surface — the same ribosomes described earlier, positioned differently.

The region without ribosomes.

Bio 0291 Smooth Endoplasmic Reticulum

The smooth endoplasmic reticulum is continuous with the rough but carries few or no ribosomes, and synthesizes lipids and steroid hormones, detoxifies chemicals, and stores calcium. In 1960s electron-microscopy studies of adrenal cortex cells, researchers linked that calcium storage to steroid hormone synthesis. Rough and smooth are one continuous system distinguished by function, not two organelles.

Sorting and packaging.

Bio 0293 The Golgi Apparatus

The Golgi apparatus is a stack of flattened sacs that sorts, tags, and packages the lipids and proteins the endoplasmic reticulum delivers. Vesicles from the reticulum fuse with the cis face; tagged proteins bud from the opposite trans face. Cis and trans name entry and exit points of one continuous stack, not separate structures.

Where the route ends.

Bio 0295 Lysosome Formation within the Endomembrane System

Lysosomes form within the endomembrane system, receiving digestive enzymes that the Golgi apparatus has sorted, tagged, and packaged for them. A macrophage's lysosome uses those Golgi-supplied enzymes to break down an engulfed pathogen. This entry places lysosome formation in the endomembrane pathway; its digestive function was established earlier.

Five things to carry out of Part One. Magnification enlarges the image; resolving power separates two structures sitting close together. An electron microscope resolves detail near 50 picometers against roughly 200 nanometers for a lab light microscope, and that resolution is what shows the extra detail. Cell theory followed: living things are made of cells, the cell is life's basic unit, and cells arise only from existing cells. Four components appear in every cell: plasma membrane, cytoplasm, D N A, and ribosomes. A prokaryote keeps its D N A in the nucleoid, a cytoplasmic region with no membrane of its own. A eukaryote adds a nucleus, many membrane-bound organelles, and several linear chromosomes. Volume grows faster than surface area: doubling a cube-shaped cell's side takes its surface-to-volume ratio from six to one to three to one, and the membrane can no longer supply the interior by diffusion. A cell at that limit divides, flattens, or develops specialized organelles. Microvilli add absorptive area on one face. Inside the nuclear envelope, D N A is packed as chromatin — the same material as a chromosome at a different packing state. The nucleolus assembles ribosomal subunits, they leave through the pores, and the completed ribosome translates messenger R N A into amino acid sequence. Mitochondria supply the A T P. The endomembrane system routes what the ribosome makes. Rough endoplasmic reticulum modifies new proteins, smooth handles lipids and detoxification, the Golgi sorts and tags, and lysosomes form from the enzyme-carrying vesicles it releases. Mitochondria and chloroplasts stand outside. Part Two takes up the cytoskeleton, the junctions between cells, and the fluid mosaic model of the membrane itself.