Definition and Location of the Cell Membrane
The cell membrane has several equivalent names: cytoplasmic membrane and plasmalemma. The use of any of these terms in the Unified State Exam answer is considered correct.
In eukaryotic cells (animal, plant, fungal), the outer covering is structured differently. In plant and fungal cells, a cell wall is located outside the membrane, providing protection against mechanical stress. An animal cell lacks a cell wall — its outer boundary is formed specifically by the cytoplasmic membrane, surrounded externally by a layer of glycocalyx.
Chemical Composition and Structure of the Membrane
The basis of the membrane is the lipid bilayer — two rows of lipid molecules, predominantly phospholipids. Each phospholipid molecule has:
- A hydrophilic head — a polar part facing the water;
- Hydrophobic tails — non-polar regions hidden inside the bilayer.
Proteins and cholesterol molecules are embedded within the thickness of the bilayer, conferring stability to the membrane. Carbohydrates account for up to 10% of the membrane mass — they are part of glycoproteins (carbohydrate + protein) and glycolipids (carbohydrate + lipid). Carbohydrate "horns" located on the outside of the animal cell membrane form the glycocalyx layer.
Classification of Membrane Proteins
Based on their location relative to the bilayer, proteins are divided into three groups:
- Peripheral — lie on the membrane surface;
- Semi-integral — partially embedded in the bilayer;
- Integral (transmembrane) — span the entire bilayer.
Transmembrane proteins are especially important: they form channels through which hydrophilic substances pass across the hydrophobic layer. The functions of membrane proteins are diverse:
- selective transport of substances into and out of the cell;
- transmission of hormonal signals;
- formation of coated pits during endocytosis and exocytosis;
- participation in immunological reactions;
- enzymatic acceleration of reactions without altering the substrate composition;
- organization of intercellular contacts within tissues and organs.
Carbohydrate Component and Glycocalyx
Oligosaccharides of the membrane form a glycocalyx layer on the outside of the animal cell. Its main role is receptor function: it ensures cell recognition of each other. For example, hepatocytes (liver cells) use the glycocalyx to recognize "self" cells and form a single tissue. Glycoproteins and glycolipids also provide recognition of signal molecules.
Passive Transport of Substances
Passive transport proceeds along the concentration gradient (from an area of high concentration to an area of low concentration) and does not require ATP energy expenditure. It includes:
- Simple diffusion — the transport of small, non-polar molecules soluble in lipids: O₂, CO₂.
- Facilitated diffusion — transport along the gradient involving carrier proteins; this is how glucose, amino acids, water, and other hydrophilic substances pass.
- Osmosis — the unidirectional diffusion of water through a semipermeable membrane toward a more concentrated solution in order to equalize concentrations.
Active Transport, Endocytosis, and Exocytosis
Active transport proceeds against the concentration gradient and requires the expenditure of ATP energy. It is carried out by carrier enzymes, with the classic example being the sodium-potassium pump: K⁺ is pumped into the cell, and Na⁺ is pumped out.
Endocytosis — the uptake of substances by the cell through membrane invagination:
- Phagocytosis — the engulfment of solid particles; the resulting vacuole fuses with a lysosome, whose hydrolytic enzymes break down biopolymers into monomers;
- Pinocytosis — the uptake of liquids containing dissolved substances.
Exocytosis — the expulsion of substances from the cell: a transport vesicle fuses with the cytoplasmic membrane, and its contents are released outward. Digestive enzymes, hormones, and cell wall components (such as hemicellulose in plants) are secreted in this manner.
Functions of the Cytoplasmic Membrane
- Transport — ensuring the active and passive transfer of substances;
- Barrier (boundary) — separating the cytoplasm contents from the external environment;
- Semipermeability — selective passage of molecules depending on size and properties;
- Receptor — recognition of substances and cells via glycoproteins and glycolipids;
- Contact — formation of intercellular junctions in tissues.
Key points
- Synonyms: cell membrane = cytoplasmic membrane = plasmalemma.
- The basis of the membrane is a lipid bilayer made of phospholipids with hydrophilic heads and hydrophobic tails.
- Membrane proteins are divided into peripheral, semi-integral, and integral.
- The glycocalyx of an animal cell performs a receptor function and ensures cell recognition.
- Passive transport (diffusion, facilitated diffusion, osmosis) occurs along the gradient without ATP expenditure.
- Active transport (e.g., the Na⁺/K⁺ pump) occurs against the gradient with ATP expenditure.
- Endocytosis includes phagocytosis and pinocytosis; exocytosis ensures the secretion of substances.
- Main membrane functions: transport, barrier, receptor, and contact.