Round an onion cell sits a stiff wall, and inside it sits life. Between the two lies a film so thin that it still decides what comes in and what goes out. What is it made of, and how many years did scientists take to draw it right?
1. What the cell membrane is
Just below the cell wall, a thin, elastic, differentially permeable, living layered membrane wraps the protoplasm. It is made of lipo-protein. This is the plasma membrane, or cell membrane.
Its other names are plasmalemma and cytomembrane, and many now call it the biomembrane. Carl Nageli and Cramer (1855) first named it the plasma membrane, and J. Q. Plower (1931) used the word plasmalemma.
The membrane folds here and there. A fold is a microvillus (plural microvilli). When a microvillus is pushed deep into the cell, it is called a pinocytic vesicle. Both are well seen in animal cells.
2. Models of the membrane: sandwich to iceberg
Scientists gave model after model for the membrane. Danielli and Davson (1935) gave the first definite one, the sandwich model: protein layers above and below, with a lipid layer in the middle. Robertson (1959) proposed the unit membrane hypothesis: all biological membranes share one molecular plan, a phospholipid bilayer with proteins embedded here and there.
There are more: Benson (1966), and Lenard and Singer (1966). But the most accepted is the fluid-mosaic model (S. J. Singer and G. L. Nicolson, 1972). The protein arrangement in the lipid was likened to icebergs floating at sea, so it is also called the iceberg model. Tap the years on the timeline.
timelinetap a year
Timeline of models
Danielli and Davson, sandwich model. The first definite model: protein layers above and below, a lipid layer in the middle.
Robertson, unit membrane. All biological membranes share one plan: a phospholipid bilayer with proteins embedded here and there.
Benson, and Lenard and Singer. More models arrived this year: one by Benson, another by Lenard and Singer.
Singer and Nicolson, fluid-mosaic. The most accepted model. Proteins sit in the lipid like icebergs floating at sea, hence the iceberg model.
3. Inside the fluid-mosaic model
In this model the membrane is a bilayer of phospholipid. The hydrocarbon tails of both layers face each other, and the water-loving polar parts face the other way, outward. Proteins are scattered through the lipid.
Each phospholipid has one glycerol bound to two non-polar fatty acid tails and one polar phosphate head. The glycerol lies between head and tails. The head loves water and the tails repel it; do not mix these up.
Figure 1.5tap a part
One phospholipid
The phosphate head. Polar and water-loving. In the membrane it faces outward.
Glycerol. Sits between the head and the tails.
Fatty acid tails, two of them. Non-polar and water-repelling. The tails of the two layers face each other.
All three parts seen: one head, one glycerol, two tails.
Drawn by hand after the book's figure, not to scale.
Membrane proteins are of three kinds. Integral proteins span both surfaces of the membrane. Peripheral proteins lie on a surface. Lipid-associated proteins are attached to the lipid core.
Glycolipid and glycoprotein together are the glycocalyx; its carbohydrate chains always lie on the outer face of the membrane. Cholesterol is a lipid that sits between the phospholipid molecules, and it is more abundant in the membranes of animal cells.
Below are two layers of phospholipid, but one is the wrong way round. Heads love water, tails avoid it: set them right, then let the membrane float.
build itflip a layer
Heads out, tails facing
The word “mosaic” says it: proteins are not fixed but float in the phospholipid. A protein may be water-loving at the surface and water-repelling where it mixes with lipid inside. Lipid molecules also move sideways and rotate within their layer, which is called flip-flop movement. Figure 1.6 puts it all together; tap the ten points one by one.
Figure 1.6tap a point
The mosaic of the membrane
Phospholipid bilayer. Two layers of phospholipid. Tails face each other, heads face outward.
Integral protein. Spans both surfaces.
Peripheral protein. Lies on a surface.
Lipid-associated protein. Attached to the lipid core.
Glycoprotein. A protein with a carbohydrate chain. Part of the glycocalyx.
Glycolipid. A lipid with a carbohydrate chain. Also part of the glycocalyx.
Cholesterol. Sits between the phospholipids. More abundant in animal cell membranes.
The outside of the cell. The carbohydrate chains always face here.
The inside of the cell. The face of the membrane that looks toward the protoplasm.
Cytoplasm. Lies just inside the membrane.
All ten seen. Remember: the carbohydrate chains always face outward.
Drawn by hand after the book's figure, not to scale.
Now match each description to its kind of protein.
sort thempick a description, then a box
Which protein?
Pick up a description, then drop it in a box.
Right. Next one.
Not yet. Ask yourself: how deep into the membrane does the protein go?
All right. Integral spans both surfaces, peripheral lies on a surface, lipid-associated sits in the lipid core.
4. Makeup, jobs, and how it differs from the wall
The membrane holds protein, lipid and sometimes polysaccharide. About 75% of its dry weight is lipid, mostly phospholipid.
Five phospholipids have been identified, and the simplest is phosphatidic acid. Of the complex ones the chief is lecithin, which makes up more than half of the membrane phospholipid.
Membrane proteins work as structural elements, as enzymes and as carrier proteins. Sometimes RNA may be present too, as in onion cells.
The membrane has many jobs: it encloses the cell contents, protects them, controls and coordinates transport and diffusion, gives a frame for holding enzymes, synthesises macromolecules, serves as a source of information, and takes part in adhesion, growth and movement.
Do not mix it up with the cell wall. The cell wall is non-living, rigid, and unique to plant cells. The cell membrane is living, elastic, made of lipo-protein and differentially permeable, and it is present in every living cell.
5. Check yourself
The first question is in the exam's style.
check yourselfpick one answer
Question 1
Which part of the fluid-mosaic model of the plasma membrane is fluid?
Right. In the fluid-mosaic model the phospholipid layer behaves as a fluid in which proteins float (the mosaic).
Proteins float in or sit on the phospholipid layer (the mosaic), not the fluid part.
Carbohydrate lies on the outer surface as glycolipid or glycoprotein, not as the fluid part.
Enzymes are proteins and are not the fluid part.
In the second, match names and years.
check yourselfpick one answer
Question 2
Which protects the organelles inside the cell and controls the movement of substances?
Protoplasm is the living matter and does not guard the organelles.
The cytoplasm holds organelles; it does not control movement of substances.
Right. The cell membrane protects internal contents and regulates passage of substances.
The nucleus controls the activities of the cell.
Read the last one carefully.
check yourselfpick one answer
Question 3
The carbohydrate layer on the plasma membrane, the glycocalyx, has the following components:
i. Glycoprotein ii. Glycocephalin iii. Glycolipid
i and ii: ii is wrong, since glycocephalin is not a component of the glycocalyx.
ii and iii: ii is wrong.
Right. Glycoproteins and glycolipids together form the glycocalyx.
Not all three, because ii is wrong.
The whole note on eight cards.
cut from the notetap to turn over
Eight cards
QuestionDefine the plasma membrane.
AnswerThe thin, elastic, differentially permeable living layered membrane of lipo-protein around protoplasm, just below the cell wall.
QuestionWhich was the first definite membrane model, by whom and when, and what is its structure?
AnswerThe sandwich model; Danielli and Davson (1935). Protein layers above and below, lipid layer in the middle.
QuestionWho proposed the fluid-mosaic model and when? Why is it called the iceberg model?
AnswerS. J. Singer and G. L. Nicolson, 1972. Proteins in lipid were likened to icebergs floating at sea.
QuestionWhat is a phospholipid molecule made of?
AnswerOne glycerol, two non-polar fatty acid tails and one polar phosphate head.
QuestionName the three kinds of membrane protein and their positions.
AnswerIntegral: across both surfaces; peripheral: on the surface; lipid-associated: in the lipid core.
QuestionWhat is the glycocalyx? On which face are its carbohydrate chains?
AnswerThe glycolipid and glycoprotein together. Its carbohydrate chains always lie on the outer face.
QuestionState five functions of the cell membrane.
AnswerEnclose contents; protect them; control transport and diffusion; frame for enzymes; synthesise macromolecules.
QuestionState the main difference between the cell wall and the cell membrane.
AnswerThe wall is non-living, rigid and unique to plant cells. The membrane is living, elastic, of lipo-protein, differentially permeable and in every living cell.
Source: Biology 1st Paper (Botany), Classes 11-12 (NCTB), Chapter 1 “Cell and Its Structure”, section 1.2.