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OCR Biology Revision - Part 3 - Cell Membranes

Updated on December 5, 2012

Cell Membranes

Functions of Membranes:

  • Separating the cell contents from the outside environment.
  • Separating organelles from the cell's cytoplasm.
  • Cell recognition and signalling.
  • Holding organelles that are involved in metabolic processes in place.
  • Regulating what materials come in and out of the cell.

Phospholipid Bilayers:

Definition: A phospholipid bilayer is the structural component of a plasma membrane. It is made of two layers of phospholipid molecules with embedded proteins.

A phospholipid molecule consists of a hydrophillic head and a hydrophobic tail. This is caused by an uneven distribution of charges throughout the molecule. If you mix phospholipids with water then it will form a layer on the top of the water - with the head's sticking in the water and the tails sticking out.

If you completely surround phospholipids with water then they will form a bilayer. This is when the heads of the molecules stick in the water and the tails point towards each other away from the water molecules.

Despite the fact the molecules are not actually bonded together, the fact that the hydrophillic head can't easily pass through the hydrophobic middle section of the bilayer gives the membrane some stability.

This is the basic structure of all biological membranes, however some cell membranes have various adaptations depending on what function they are needed to carry out. For example, Muscle cell membranes contain a lot of channel proteins to increase the amount of glucose that is taken up because the muscle needs a lot of energy (in the form of glucose) to contract.


The Fluid Mosaic Model

This is the model of a cell membranes structure.

The main features of this model are:

  • A phospholipid bilayer.
  • Protein molecules within the bilayer.
  • Proteins embedded in the bilayer (extrinsic) or completely spanning the bilayer (intrinsic).

Roles of each component:

Cholesterol - Gives the membranes stability. The molecules fit between the phospholipid tails and make it harder for substances such as water molecules and ions to pass through into the cell.

Channel Proteins - Allow some substances, that are either too big or too hydrophillic, to cross the membrane. An example of such a substance is glucose.

Carrier Proteins - Actively transport substances across the membrane. For example in plant roots, carrier proteins actively transport magnesium ions from the surrounding soil into the root hair cells. This is because magnesium is essential for chlorophyll production.

Receptor Sites - Allow hormones to bind with a cell so the cell can respond to it, however this can only happen if it has a receptor for that hormone on the cell surface membrane. Drugs can bind to receptor sites.

Glycoproteins (protein molecules with a carbohydrate part) - Can bind cells together in tissues and can be a hormone receptor. They can also be involved in cell signalling - allowing recognition from the immune system.

Glycolipids (phospholipid molecules with a carbohydrate part) - Involved in cell signalling and can act as a hormone receptor.

Enzymes and Coenzymes - Organic catalysts that are found in the cell membranes of organelles such as chloroplasts and mitochondria.

Type of Diffusion
Examples of substance
Simple Diffusion
Oxygen, Carbon Dioxide, Steroid Molecules and other lipid-based molecules.
Facilitated Diffusion - Using Channel Proteins
Ions such as Sodium and Calcium
Facilitated Diffusion - Using Channel Proteins
Larger molecules such as glucose and amino acids

Passive Processes

Passive processes are processes that depend only on the kinetic energy of the molecules that are trying to move across a membrane, an example of this is diffusion.

Diffusion:

Definition - The movement of molecules from a high concentration of the molecules to a low concentration. Molecules will continue to diffuse until they are evenly distributed. The state in which molecules no longer move around in one direction (no net movement) is called an equilibrium.

Diffusion rate is affected by the following things:

  • Temperature - An increasing temperature gives molecules more kinetic energy and because diffusion relies on kinetic energy the more there is the faster the rate of diffusion will be.
  • Concentration Gradient - If the concentration gradient is high (there are a lot more molecules on one side of the membrane than the other) then the rate of diffusion will be quicker.
  • Surface Area - The larger the surface area the faster diffusion occurs.
  • Distance/Thickness of the Membrane - Diffusion is quicker across thinner membranes because there is less distance to travel.
  • Size - The smaller the molecule that is trying to diffuse, the faster diffusion will be.

How do molecules diffuse through the phospholipid bilayer?

Phospholipid bilayers (membranes) have a hydrophobic region. Steroid hormones can diffuse through this region because it they are lipid based.

Molecules such as carbon dioxide and oxygen are small enough to be able to pass between the individual phospholipids that make up the bilayer.

Charged particles (such as sodium ions) or larger molecules (such as glucose molecules) can't diffuse through the phospholipid bilayer. There are two types of protein molecules (the channel proteins and the carrier proteins) that allow these kind of substances to pass through. This is called 'facilitated diffusion'.

Channel proteins are basically pores in the membrane that are shaped to allow only one specific type of ion through.

Carrier proteins are shaped so that only a specific molecule can pass through. When the molecule fits with the carrier protein's shape, then it will allow the molecule through to the other side of the membrane.


Active Processes

Sometimes a cell will need more of a substance in the cytoplasm than is present outside the cell or it might just need those substances quicker than diffusion would allow.

Active transport:

Definition: The movement of molecules against a concentration gradient with the aid of ATP for energy for the protein pumps.

Active transport requires channel proteins that are within the membrane to act as pumps to transport specific molecules through the membrane and into or out of the cell.

These carrier proteins are significantly different from the carrier proteins that aid facilitated diffusion in the following ways:

  • They carry specific molecules one way across the membrane.
  • They use ATP as energy for this.
  • They can transport molecules against the concentration gradient.

Bulk Transport:

If a cell transports large amounts of substances into the cell it is called endocytosis and if it transports large quantities of materials out of the cell it's called exocytosis.

Bulk transport is possible because vesicles can fuse with or break from the cell surface membrane. Bulk transport also requires energy in the form of ATP, but unlike active transport, the energy goes into move the membranes around to form vesicles and to move these vesicles around the cell.

Different names are given to bulk transport depending on whether the substance being transported is a solid or a liquid and whether it's going into or out of the cell.

Phago is solid material.
Pino is liquid material.
Exo is outwards.
Endo is inwards.

Using this, the bulk movement of liquid material into the cell would be called 'endopinocytosis'.

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    • SouradipSinha profile image

      Souradip Sinha 

      3 years ago from Calcutta

      I have an exam next month and it was a nice and quick revision online. :)

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      biochemi 

      4 years ago

      Dear Anthropophobia, this hub is really appreciative and nice.

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