The plasma membrane, cell membrane or plasmalemma is a thin, porous and microscopic envelope that coats the cells of prokaryotic and eukaryotic beings.
It is a semi-permeable structure, responsible for the transport and selection of substances entering and leaving the cell.
Only with the development of the electron microscope was it possible to observe the plasma membrane.
Roles
The functions of the plasma membrane are:
- Selective Permeability, control of the entry and exit of substances from the cell;
- Protection of cell structures;
- Delimitation of intracellular and extracellular content, ensuring cell integrity;
- Transport of substances essential to cell metabolism;
- Recognition of substances, thanks to the presence of specific receptors on the membrane.
See too: prokaryotic and eukaryotic cells
Structure and Composition
Plasma Membrane Structure
The plasma membrane has the so-called "fluid mosaic model”. It was unveiled by American biologists Seymour Jonathan Singer and Garth L. Nicolson, in 1972.
The name "fluid mosaic" is due to the presence of flexible and fluid structures, with great regeneration power.
The plasma membrane is chemically made up of lipids (glycolipids, cholesterol and phospholipids) and proteins. Therefore, it is recognized for its lipoprotein composition.
Phospholipids are arranged in a double layer, the lipid bilayer. They are connected to the fats and proteins that make up cell membranes.
Phospholipids have a polar and a non-polar portion. The polar portion is hydrophilic and faces outwards. The nonpolar portion is hydrophobic and faces the interior of the membrane.
Phospholipids move, however, without losing contact. This allows for flexibility and elasticity of the membrane.
Proteins are represented by enzymes, glycoproteins, transport proteins and antigens. Proteins can be transmembrane or peripheral.
- transmembrane proteins: cross the lipid bilayer side by side.
- Peripheral proteins: are located on only one side of the bilayer.
Enzymes that are present in the plasma membrane have several catalytic functions, responsible for facilitating intracellular chemical reactions.
Learn more, read too:
- Cell
- Cytology
- Cytoplasm
- Cell Core
- Cell Organelles
- Glycolyx
Substance Transport
The membrane acts as a filter, allowing the passage of small substances and preventing or hindering the passage of large substances. This property is called Selective Permeability.
The transport of substances across the plasma membrane can be passively or actively:
O passive transport takes place without energy expenditure. Substances move from the more concentrated medium to the less concentrated one. Examples are:
- Simple Broadcast - It is the passage of particles from where they are more concentrated to regions where their concentration is lower.
- Diffusion facilitated - It is the passage, through the membrane, of substances that do not dissolve in lipids, with the help of proteins from the lipid bilayer of the membrane.
- Osmosis - It is the passage of water from a less concentrated medium (hypotonic) to a more concentrated one (hypertonic).
O active transport occurs with energy expenditure (ATP). Substances move from lower to higher concentration. Examples are:
- Block Transport: Endocytosis and Exocytosis - Occurs when the cell transfers large amounts of substances into or out of its intracellular environment.
- Sodium and Potassium Pump - Passage of sodium and potassium ions to the cell, due to differences in their concentrations.
know more:
- Active Transport
- Passive Transport
Entrance Exam Exercises
1. (PUC RJ-2007) In relation to cell envelopes, we can state that:
a) all cells in living beings have a cell wall.
b) only plant cells have a cell membrane.
c) only animal cells have a cell wall.
d) all cells in living beings have a cell membrane.
e) fungi and bacteria do not have a cell wall.
d) all cells in living beings have a cell membrane.
2. (Mack-2005) Tick the correct alternative regarding the lipoprotein membrane.
a) In bacteria, it has a different organization from that found in eukaryotic cells.
b) It exists only as an outer shell of the cells.
c) It is formed by a double layer of glycoproteins, with several lipid molecules embedded in it.
d) It is rigid, ensuring cell stability.
e) It is involved in processes such as phagocytosis and pinocytosis.
e) It is involved in processes such as phagocytosis and pinocytosis.
3. (VUNESP-2010) Due to its chemical composition – the membrane is formed by lipids and proteins – it is permeable to many substances of a similar nature. Some ions also enter and exit the membrane easily, due to their size... However, certain large molecules need a little extra help to get into the cell. This little help involves a kind of doorman, who examines what's outside and helps you inside. (Solange Soares de Camargo, in Biology, High School. 1st series, volume 1, SEE/SP, 2009.) In the text, and in the order in which they appear, the author refers:
a) the mosaic-fluid model of the plasma membrane, diffusion and active transport.
b) the mosaic-fluid model of the plasma membrane, osmosis and passive transport.
c) selective permeability of the plasma membrane, active transport and passive transport.
d) the pores of the plasma membrane, osmosis and facilitated diffusion.
e) the plasma membrane pores, diffusion and selective membrane permeability.
a) the mosaic-fluid model of the plasma membrane, diffusion and active transport.
For more questions on the topic, see: Exercises on Plasma Membrane.
