Transport across the cell membrane

The structure of the cell membrane
The cell or plasma membrane determines what enters and leaves the cytoplasm of the cell. It ensures that harmful substances do not enter the cell, while permitting useful substances to enter. The cell membrane maintains the structural integrity of the cell.
How does the membrane determine which substances should enter the cell and which substances should not? What makes the cell membrane semipermeable so that it can determine which substances it can allow to pass through and which substances to keep out?
The answer lies in the nature of the structure of the membrane.
The cell membrane is made up of phospholipid molecules with attached phosphate groups (PO4). In an aqueous medium, the phosphate end is attracted to water and is said to be hydrophilic (water loving), while the other end consisting of two fatty acid chains is repelled by water and is said to be hydrophobic (water hating).
When phospholipids are mixed in water, they rearrange themselves in such a way that their hydrophilic heads face towards the water and the hydrophobic tails face away from the water.
The same situation occurs in the cell. Recall that a cell is surrounded by water on the outer membrane and inner membrane (cytoplasm). This means that the side of the membrane that is facing the interstitial fluid and the side that borders the cytoplasm are hydrophilic while the side that is sandwiched between these two is hydrophobic.

Therefore, we can say that a bilayer is a sandwich-like structure whose outer surface, the surface exposed to the watery medium outside and inner surface, i.e. the surface facing the cytoplasm, are hydrophilic, while the unexposed interior is hydrophobic. The unexposed hydrophobic part is a barrier to most substances that are dissolved in water.
Proteins span the lipid bilayer and form transport channels for water-soluble molecules and ions. Some of the protein molecules associated with the membrane are peripheral proteins and are attached to the outer or inner (cytoplasm) surface of the membrane but more to the latter. Others are transmembrane proteins that traverse the entire width of membrane.
The cell membrane with its associated proteins is extremely dynamic. The peripheral proteins on the inside of the cell (cytoplasm) serve as anchors for the cytoskeleton of the cell. They maintain the shape of the cell.
Transmembrane proteins are mainly transport proteins, regulating the movement of molecules into and out of the cells.

Fluid mosaic model

In animal cells, membrane proteins are often attached to branch like sugar molecules to form glycoproteins that protrude from the membrane’s outer surface.
The proteins and glycoproteins that jut from the membrane create surface characteristics that are so important to a cell’s interactions with other cells.
Proteins within the oily lipid sandwich freely swim laterally like a floating ship at sea. The protein lipid bilayer is often called fluid mosaic because the proteins float in a fluid.
Peripheral proteins on the outside of the cell membrane and some transmembrane proteins, both of which may bind with some Carbohydrate chains, give the cell its identity. For example, they may act as receptors for hormones or determine blood group antigens (ABO, Rh glycoproteins) or distinguish between ‘self’ and ‘ nonself’ in immune systems.
Other signature proteins, as they are sometimes called, enable differentiating embryonic cells to recognise one another and allow them to migrate to appropriate parts of the embryo.
Passive Diffusion
Diffusion is the movement of a substance from a region of high concentration to a region of low concentration. This is known as moving down the gradient. A pinch of table sugar placed at the bottom of a glass containing water will dissolve in water completely to form a solution of uniform concentration. The sugar is referred to as the solute and water as the solvent. Compounds in a tea bag placed in a cup of hot water diffuse uniformly throughout the water until the colour of the water is brown. You can also use potassium permanganate instead of tea leaves.
Diffusion is faster at high temperatures. It is also faster in gases than in liquids and is slowest in solids.

Passive diffudion

Osmosis
Osmosis is defined as the movement of water molecules from a region of higher concentration to a region of lower concentration across a semi-permeable membrane.
The cell membrane is selectively permeable to molecules. Smaller molecules cross the cell membrane more easily than large molecules. Water, carbon dioxide and oxygen cross the membrane easily. Glucose and other large compounds are too large to cross easily.

In osmosis water moves from a region of low solute concentration to a region of high solute concentration. In the diagram above water moves from A to B through the semi-permeable membrane, SP. The glucose molecules in B are too large to pass across the semi-permeable membrane, SP and enter A. Incoming water molecules cause the level of the glucose solution in B to rise while the level of water in A drops.

Osmosis can also be demonstrated using a peeled potato. A hollow is made in a potato; the potato is placed on a petri dish containing distilled water. Sugar crystals are placed in the hollow. Twenty four hours later, it is found that the sugar crystals have formed a solution and that the water level in the petri dish has decreased because water has moved to the hollow by osmosis.
Solutions
Most animal cells are isotonic with their environment. This means that the concentrations of the salts inside the cells are equal to the concentrations of the salts in the blood or body fluids so that there is no net flow of water in either direction. A solution whose osmotic concentration is lower than the salt concentration of the blood or that of the body fluids is referred to as hypotonic. A solution that  has a higher osmotic concentration than that of blood or body fluids is called a hypertonic solution.

If red blood cells are placed in a hypotonic solution, water will leave the solution by osmosis and enter the cells. This water will accumulate in the cells until the cells burst. This is known as hemolysis.  If , on the other hand, the cells are placed in a hypertonic solution, water move from the cells into the solution, causing the cells to shrink, condition known as crenation. However, if the cells are placed in an isotonic solution, nothing happens to the cells because the osmotic pressure inside the cells is the same as the osmotic pressure outside the cell.






Facilitated Diffusion
Some molecules are too big to pass through the cell membrane. In order to cross the membrane, they attach themselves to carrier proteins on the cell membrane, which will carry them across the membrane, just as a river ferry does when it carries people with their properties across a river. Such carrier proteins transport their substances from a region of high concentration to a region of low concentration. Facilitated diffusion does not require energy.

 




Active Transport
A cell will sometimes accumulate a certain compound at higher concentration than it is in the surrounding medium. A liver cell, for example, will continue to pick up glucose from the blood even when the concentration of glucose in the liver far exceeds that present in the blood.

Active transport

This is possible because the cell uses both a carrier protein and the energy provided by ATP to pump molecules of glucose into the liver against their concentration gradient. This is an example of active transport. Active transport is also important in nerve action, muscle contraction and kidney function. Active transport requires energy.
Endocytosis
Large solid particles can enter the cell by phagocytosis and globular molecules such as milk fats enter by pinocytosis. Both processes constitute endocytosis.
Phagocytosis (cellular eating) involves the ingestion of large particles by the cells. When a macrophage or any other phagocytic cell encounters a particle such as an invading microorganism, or dead cell debris, it sends out cytoplasmic projections around it and then encloses it within a vesicle inside the cytoplasm. The vesicle detaches itself from the cell membrane and becomes a food vacuole. Enzymes released by the lysosomes then break down the engulfed food material.
Small particles, especially the fragments left over by phagocytosis, are expelled from the cells by a process called exocytosis. This is actually the reverse of phagocytosis. The material contained in the food vacuole or fluid vesicles is expelled through the cell membrane into the surrounding area.

When the substance to be eaten is a liquid such as fat or protein globules, the process is called pinocytosis (cellular drinking). In Amoeba, for example, the remains of digestion leave the cell by exocytosis.

Study Questions
1. How is osmosis different from diffusion?
2. Define the following terms: hypotonic, isotonic and hypertonic.
3. Describe the structure of a cell membrane and show how it is suited to its function
4. Show how the structure of a cell membrane is suited to its task.