Cell · 3D structure

Cell overview

This is an animal cell with a quarter cut away, about 20 µm across. On the outside is the plasma membrane, in the center the nucleus, and between them the cytoplasm: various organelles suspended in the translucent cytosol.

Click any structure and the camera flies to it, with its name and function shown on the right; you can also pick from the list below.

At the bottom right you can switch to fluorescence microscopy mode: different fluorescent dyes label the nucleus, mitochondria, ER and so on, which is how researchers observe living cells.

Organelles in detail

Zoom in on each organelle to see its internal structure. Turn on cutaway to see the gap between double membranes, mitochondrial cristae, chloroplast grana and more.

Classified by membrane structure:

Double membrane: mitochondria, chloroplasts (plus the nuclear envelope)Single membrane: ER, Golgi apparatus, lysosomes, vacuolesNo membrane: ribosomes, centrosomes

Animal and plant cells

Both animal and plant cells have a plasma membrane, cytoplasm and nucleus, as well as organelles such as mitochondria, ER, Golgi apparatus and ribosomes.

Unique to plant cells: cell wall, chloroplasts (in green tissues), large central vacuole.

Unique to animal cells: centrosome (also in lower plant cells); lysosomes are found mainly in animal cells.

Turn on "Highlight differences" and the unique structures of each will flash.

Prokaryotic and eukaryotic cells

Cells are classified as prokaryotic or eukaryotic by whether they have a nucleus bounded by a nuclear envelope. Bacteria and cyanobacteria (blue-green algae) are prokaryotes; animals, plants and fungi are eukaryotes.

Prokaryotic DNA is concentrated in the nucleoid, with no nuclear envelope or nucleolus; the only organelle is the ribosome.

Switch to "Scale comparison": an E. coli cell is only about 2 µm long, like a grain of rice next to an animal cell.

Fluid mosaic model

Zoomed to the molecular scale (about 100 million times), the basic framework of the plasma membrane is the phospholipid bilayer: the hydrophilic heads of phospholipids face the watery environment on both sides, and the hydrophobic tails point inward toward each other.

Proteins may sit on the surface, be partly or fully embedded, or span the entire bilayer.

On the outer surface, carbohydrates bind to proteins to form glycoproteins and to lipids to form glycolipids, together called the glycocalyx, which is closely involved in cell recognition and signaling.

Watch closely: phospholipids keep moving sideways and most proteins can move too, so the membrane is fluid.

Movement across the membrane

Passive transport: down the concentration gradient, no energy used.· Simple diffusion: O₂, CO₂, glycerol, ethanol, etc. pass directly through the phospholipid bilayer.· Facilitated diffusion: via channel proteins (water, K⁺) or carrier proteins (glucose entering red blood cells).

Active transport: against the concentration gradient, requires carrier proteins and uses ATP, e.g. the sodium–potassium pump.

Endocytosis and exocytosis: macromolecules and particles enter and leave the cell through membrane fluidity, using energy.

The chart below shows how the transport rate of each mode depends on the concentration difference and on O₂ concentration.

Synthesis and transport of secretory proteins

Scientists injected ³H-labeled leucine into guinea pig pancreatic acinar cells and tracked where the radioactivity appeared in sequence (isotope labeling, pulse–chase).

Polypeptide made on ribosomes → enters the rough endoplasmic reticulum for processing and folding → ER buds off vesicles → vesicles fuse with the Golgi apparatus for further modification → Golgi buds off vesicles → vesicles fuse with the plasma membrane and the protein is secreted by exocytosis.

Energy for the whole process comes mainly from mitochondria.

The chart records the radioactivity in each structure and the changes in area of the ER membrane, Golgi membrane and plasma membrane: ER membrane decreases, Golgi membrane first rises then falls (roughly unchanged), and plasma membrane increases.

Cytoskeleton and cytoplasmic streaming

The cytoskeleton is made of three kinds of protein fibers: microtubules (thickest, about 25 nm, radiating from the centrosome), microfilaments (thinnest, about 7 nm, i.e. actin filaments, forming a dense mesh beneath the plasma membrane) and intermediate filaments (about 10 nm, adding strength like ropes).

Motor proteins "walk" along the cytoskeleton carrying cargo: kinesin moves toward the plus end (cell periphery), dynein toward the minus end (centrosome). Each step hydrolyzes one ATP.

In plant cells, myosin drags the cytoplasm along microfilaments, and chloroplasts are carried around the central vacuole; this is the cytoplasmic streaming (cyclosis) seen in the lab.