Transport Across the Cell Membrane

Lessons

Senior 5 · Biology

Transport Across the Cell Membrane

5 lessons

Lesson 5 of 5

Endocytosis and exocytosis

22 min

Swallowing a whole bacterium

A bacterium is about 2 µm long, millions of times bigger than a glucose molecule. No channel or carrier protein could pass it through a membrane. Yet the white blood cells called neutrophils take in bacteria every minute of your life. They do it by wrapping their membrane around the bacterium and pinching it off inside the cell. Moving large particles or large volumes of fluid inside folds of membrane is called bulk transport. It works in both directions: endocytosis brings material in, and exocytosis sends it out.

Definition

Endocytosis: the bulk transport of material into a cell, in which the cell surface membrane folds inwards around it and pinches off to form a vesicle or vacuole.

Phagocytosis ("cell eating"): endocytosis of solid particles, such as bacteria or cell debris, into a phagocytic vacuole. Pinocytosis ("cell drinking"): endocytosis of droplets of fluid with the substances dissolved in them, into small vesicles.

Receptor-mediated endocytosis: endocytosis of specific molecules after they bind to receptor proteins clustered in pits of the membrane.

Exocytosis: the bulk transport of material out of a cell, in which vesicles fuse with the cell surface membrane and release their contents outside.

Phagocytosis

After digestion, useful products such as amino acids are absorbed into the cytoplasm, and indigestible remains are expelled by exocytosis. Phagocytosis is how neutrophils and macrophages destroy pathogens and old red blood cells, and how Amoeba feeds. Pseudopodia form because the actin filaments of the cytoskeleton reorganise, and this needs ATP.

Pinocytosis and receptor-mediated endocytosis

In pinocytosis the membrane forms tiny inward pockets that close off droplets of extracellular fluid. It is not selective: whatever is dissolved in the fluid comes in. Receptor-mediated endocytosis is selective. Liver cells, for example, take up cholesterol carried in the blood as low-density lipoprotein (LDL). LDL binds to LDL receptors gathered in coated pits, and the pits fold in to form vesicles. In the inherited disorder familial hypercholesterolaemia, the receptors are faulty, LDL stays in the blood, and cholesterol builds up in the arteries, causing heart disease at an early age. The work that explained this earned M. S. Brown and J. L. Goldstein the Nobel Prize in Physiology or Medicine in 1985.

Exocytosis

Examples of exocytosis:

Digestive enzymes leave the cells of the pancreas in vesicles.
Hormones: insulin is secreted from the β cells of the pancreas.
Neurotransmitters such as acetylcholine are released at synapses.
Mucus is secreted by goblet cells in the airways and gut.
Plant cell walls: vesicles from the Golgi body carry wall materials to the surface of a dividing cell.

Both endocytosis and exocytosis need ATP to move vesicles along the cytoskeleton and to fuse or pinch off membranes. They also balance each other: exocytosis adds membrane to the cell surface, and endocytosis removes it.

Worked example 1: how much membrane does a vesicle add?

A secretory vesicle is a sphere of diameter 100 nm. (a) Calculate the area of its membrane. (b) A cell has a surface area of 1 000 µm². How many vesicles must fuse with the surface to double this area? ( $1\,\mathrm{\mu m^2} = 10^{6}\,\mathrm{nm^2}$ )

(a) Radius $r = 50\,\mathrm{nm}$ . Area $= 4\pi r^2 = 4 \times \pi \times 50^2 = 31\,416\,\mathrm{nm^2} \approx 3.14 \times 10^{4}\,\mathrm{nm^2}$ .
(b) Cell surface $= 1\,000 \times 10^{6} = 10^{9}\,\mathrm{nm^2}$ . Vesicles needed $= \frac{10^{9}}{3.14 \times 10^{4}} \approx 3.2 \times 10^{4}$ .

Interpretation: a busy secretory cell can release tens of thousands of vesicles in a short time, yet its surface does not keep growing. Membrane must be recovered by endocytosis at about the same rate.

Worked example 2: does phagocytosis need energy?

Neutrophils were mixed with yeast cells, and the mean number of yeast cells engulfed per neutrophil in 30 minutes was counted (illustrative data).

Condition	Yeast cells engulfed per neutrophil
37 °C	8.4
37 °C with a respiratory inhibitor	1.2
4 °C	0.5

(a) Calculate the percentage reduction caused by the inhibitor. (b) Explain the results.

(a) $\frac{8.4 - 1.2}{8.4} \times 100 = 86\,\%$ .
(b) Phagocytosis needs ATP to move the cytoskeleton and form pseudopodia; the inhibitor stops ATP production, so engulfment almost stops. At 4 °C, respiration is very slow and the membrane is less fluid, so the membrane cannot flow around the yeast.

All the ways across: a summary

Process	Direction	Energy from ATP?	Proteins or vesicles	Example
Simple diffusion	Down a concentration gradient	No	Through the bilayer	O₂ into alveolar capillaries
Facilitated diffusion	Down a concentration gradient	No	Channel or carrier proteins	Glucose into red blood cells
Osmosis	From higher to lower water potential	No	Bilayer and aquaporins	Water into root hair cells
Active transport	Against a concentration gradient	Yes	Carrier proteins (pumps)	Na⁺ out and K⁺ in by the Na⁺/K⁺ pump
Endocytosis	Into the cell, in bulk	Yes	Vesicles or vacuoles	Neutrophils engulfing bacteria
Exocytosis	Out of the cell, in bulk	Yes	Vesicles	Pancreas secreting enzymes

NIAID (US National Institute of Allergy and Infectious Diseases): "Phagocytosis of MRSA by a human neutrophil". A short real microscope recording: watch the neutrophil's membrane flow around the bacteria and close over them.

Warning

Common mistakes

"Endocytosis passes particles through the membrane." Nothing crosses the bilayer; the material is wrapped in membrane and stays inside a vesicle until it is digested or released.
Confusing phagocytosis (solids) with pinocytosis (fluids).
Forgetting that bulk transport needs ATP; it is an active process.

Tip

Memory hook: endo = in, exo = out; phago = eat, pino = drink.

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Practice: endocytosis and exocytosis

Define endocytosis and exocytosis.
Describe the stages by which a neutrophil destroys a bacterium.
Distinguish between phagocytosis, pinocytosis and receptor-mediated endocytosis.
Give three examples of exocytosis in the human body.
Explain why cells that secrete a lot of protein have a large Golgi body and many mitochondria.
A vesicle has a diameter of 80 nm. Calculate the area of its membrane in nm².
Explain why a person with faulty LDL receptors has a high blood cholesterol level.
Complete: diffusion, osmosis and facilitated diffusion are ______; active transport, endocytosis and exocytosis are ______. Justify your answer.

Show answersHide answers

Endocytosis: bulk uptake of material into a cell by infolding of the membrane to form a vesicle or vacuole. Exocytosis: bulk release of material from a cell by fusion of vesicles with the cell surface membrane.
The bacterium binds to receptors; pseudopodia extend around it and fuse, forming a phagocytic vacuole; lysosomes fuse with the vacuole and their enzymes digest the bacterium; useful products are absorbed and debris is removed by exocytosis.
Phagocytosis takes in solid particles into large vacuoles; pinocytosis takes in fluid and dissolved substances into small vesicles, non-selectively; receptor-mediated endocytosis takes in specific molecules that bind to receptors in coated pits.
Any three: digestive enzymes from the pancreas; insulin from β cells; neurotransmitters at synapses; mucus from goblet cells.
The Golgi body packages the proteins into secretory vesicles; mitochondria supply the ATP needed for protein synthesis, vesicle transport and exocytosis.
$r = 40\,\mathrm{nm}$ ; area $= 4\pi \times 40^2 \approx 20\,100\,\mathrm{nm^2}$ (to 3 s.f.).
LDL cannot bind to the receptors, so it is not taken into liver cells by receptor-mediated endocytosis; it stays in the blood and cholesterol builds up.
Passive; active. The first three use only the kinetic energy of molecules and move substances down a gradient; the other three need ATP from respiration.

Key points

Endocytosis (phagocytosis, pinocytosis, receptor-mediated) brings material in inside vesicles; exocytosis releases it.
Bulk transport needs ATP for the cytoskeleton and for membrane fusion.
Phagocytes destroy pathogens; exocytosis secretes enzymes, hormones, neurotransmitters and mucus.
Passive processes: diffusion, facilitated diffusion, osmosis. Active processes: active transport, endocytosis, exocytosis.

1Swallowing a whole bacterium

2Phagocytosis

3Pinocytosis and receptor-mediated endocytosis

4Exocytosis

5All the ways across: a summary

Swallowing a whole bacterium

Phagocytosis

Pinocytosis and receptor-mediated endocytosis

Exocytosis

All the ways across: a summary