Eukaryotic cells, cell membrane and cell wall

Eukaryotic cells are found in protists, plants, animals and fungi. Their cytoplasm is divided into compartments by membrane-bound organelles; they have a nucleus with an envelope, and a supporting framework (the cytoskeleton). Plant cells have a cell wall, plastids and a large central vacuole. Animal cells have centrioles, which plant cells lack.

The cell membrane is made mainly of lipids and proteins. The lipids form a double layer (bilayer) of phospholipids: their water-loving heads face outwards and their water-hating tails face inwards. Cholesterol, proteins and some carbohydrate are also present. In a human red blood cell membrane, about 52% is protein and 40% lipid. Proteins buried in the membrane are integral; those on its surface are peripheral.

The accepted picture is the fluid mosaic model of Singer and Nicolson (1972): the lipid layer is semi-fluid, like a sea, and proteins float in it like icebergs and can move sideways. This fluidity is important for cell growth, forming junctions, secretion, taking things in (endocytosis) and cell division.

How things cross the membrane:

• Passive transport (no energy used), always from more to less concentration: simple diffusion of neutral substances; osmosis, the diffusion of water; and help from carrier proteins for charged or polar molecules.
• Active transport: moving substances against the concentration gradient, which needs energy from ATP. Example: the Na⁺/K⁺ pump.

The cell wall (in plants and fungi) is a non-living layer outside the membrane. It gives shape, protects against damage and germs, and helps cells interact. In algae it is made of cellulose, galactans, mannans and calcium carbonate; in other plants, of cellulose, hemicellulose, pectins and proteins. A young cell has a growing primary wall; later a stronger secondary wall forms inside it. Neighbouring cells are glued by the middle lamella, made mainly of calcium pectate, and connected through fine channels, the plasmodesmata.