Definition and Location of the Cell Membrane
The cell membrane has several equivalent names: cytoplasmic membrane and plasmalemma. Using any of these terms in an EGE 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. In an animal cell, there is no cell wall — its outer boundary is formed specifically by the cytoplasmic membrane, surrounded on the outside 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 — the polar part facing the water;
- Hydrophobic tails — non-polar regions hidden inside the bilayer.
Proteins and cholesterol molecules, which give the membrane stability, are embedded into the thickness of the bilayer. 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
According to 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 (penetrating) — pass through the entire bilayer.
Penetrating proteins are especially important: they form channels through which hydrophilic substances pass through 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 changing the substrate composition;
- organization of intercellular contacts within tissues and organs.
Carbohydrate Component and Glycocalyx
سكريات قليلة التعدد للغشاء تكون خارج الخلية الحيوانية طبقة الجليكوكاليكس. دورها الرئيسي هو الوظيفة المستقبلية: فهي تضمن التعرف المتبادل بين الخلايا. على سبيل المثال، تتعرف الخلايا الكبدية (خلايا الكبد) بمساعدة الجليكوكاليكس على الخلايا «الخاصة بها» وتشكّل نسيجاً واحداً. كما تضمن الجليكوبروتينات والجلیکوليبيدات التعرف على جزيئات الإشارات.
Passive Transport of Substances
Passive transport moves 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 — 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 — one-way diffusion of water through a semipermeable membrane toward a more concentrated solution in order to equalize concentrations.
Active Transport, Endocytosis, and Exocytosis
Active transport moves against the concentration gradient and requires ATP energy expenditure. It is carried out by carrier enzymes, a classic example being the sodium-potassium pump: K⁺ is pumped inside the cell, Na⁺ is pumped out.
Endocytosis — engulfment of substances by the cell due to membrane invagination:
- Phagocytosis — capture of solid particles; the resulting vacuole fuses with a lysosome, whose hydrolytic enzymes break down biopolymers into monomers;
- Pinocytosis — uptake of liquids with dissolved substances.
Exocytosis — removal of substances from the cell: a transport vesicle fuses with the cytoplasmic membrane, and its contents are released outside. This is how digestive enzymes, hormones, and cell wall components (e.g., hemicellulose in plants) are secreted.
Functions of the Cytoplasmic Membrane
- Transport — ensuring active and passive transfer of substances;
- Barrier (bounding) — separating the cytoplasm contents from the external environment;
- Semi-permeability — selective passage of molecules depending on size and properties;
- Receptor — recognition of substances and cells due to glycoproteins and glycolipids;
- Contact — formation of intercellular junctions in tissues.
النقاط الرئيسية
- 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) proceeds along the gradient without ATP expenditure.
- Active transport (e.g., the Na⁺/K⁺ pump) proceeds against the gradient with ATP expenditure.
- Endocytosis includes phagocytosis and pinocytosis; exocytosis provides the secretion of substances.
- The main functions of the membrane are transport, barrier, receptor, and contact.