Movement across Cell Membranes.

Cell membranes are a barrier to most substances, and this barrier allows materials to excluded from cells - that is allows for the intracellular environment to be kept separated from the extracellular environment. Cell membranes also allow for things to be packaged inside the cells - think about all those cellular organelles like mitochondria, ribosomes and nucleic acids such as DNA and RNA.

Cellular compartmentalisation is also essential for life, as it enables all those biochmical reactions to take place that couldn't otherwise take place - remember they cytoplasm of cells is a very busy place - not only providing the cytosol to 'accommodate' all the organelles but it is also the place in which a lot of cellular biochemistry takes place! and don't forget Eukaryotic cells can also compartmentalise materials inside organelles too (e.g. mitochondria and chloroplasts).

So, cells have this barrier... this cell membrane, but obviously things need to be able to get in and out of cells, and there are number of methods by which substances can move across a cell membrane.

  • Lipid Diffusion (aka Simple / Passive Diffusion).

  • Osmosis.

  • Passive Transport (Facilitated Diffusion).

  • Active Transport.

  • Vesicles (e.g. exocytosis and pinocytosis).

 

A Level Biology - Simple / Passive Diffusion

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In this lesson we learned about Passive Diffusion (you may also know this as Lipid Diffusion - since substances diffuse directly through the phospholipid bilayer, or you may know it as plain old simple diffusion... Either way, only a few substances can diffuse directly through the lipid bilayer part of the cell membrane.

 

The only substances that can do this are lipid-soluble molecules such as steroids, or very small molecules, such as water, oxygen and carbon dioxide.

The process of simple, passive diffusion A3 PDF for A Level Biology

For these molecules the cell membrane isn't much of a 'barrier' at all. Since lipid diffusion is (obviously) a passive diffusion process, i.e. NO ENERGY is involved and substances can only move down their concentration gradient.

Passive (Lipid) Diffusion cannot be controlled by the cell, in the sense of it being switched on or off (regulated), like for exampleActive Transport can be regulated by the cell.

You must also begin thinking about diffusion in terms of the the effect Surface Area and Distance have on the rate of diffusion.

 

The relationship between the size of an organism (or structure) and the Surface Area :  Volume ratio - the significance of this for the exchange of substances and of heat is an important one to understand and apply to many areas of biology. To begin with think about the cells of multicellular organisms which may differentiate and become adapted for specific functions. Tissues are aggregations of similar cells, and organs are structures performing specific physiological functions. Adaptations of body shapes in organisms and the development of multicellular systems in larger organisms are adaptations which facilitate exchanges as the Surface Area : Volume ratio reduces.

Surface Area : Volume ratio (SA:V) is important to have in mind as you learn about the  several ways in which substances move across cell membranes and  in particular its is the very reason why  Gas Exchange systems have evolved in larger multicellular organisms.

The development of internal gas exchange surfaces in larger organisms has evolved to maintain adequate rates of exchange.

Organisms with internal gas exchange surfaces need systems for transporting gases between the environment and these surfaces. It is important that you are able to consider these structures and adaptations in terms of function (of the gas exchange surfaces / systems) in relation to the environment in which the organisms have adapted to live.

 

For example, consider comparing and contrasting gas exchange systems of mammals (alveoli, bronchioles, bronchi, trachea, lungs) with the ventilation system of bony fish (gill lamellae and gill filaments - and the counter current principle). Compare how terrestrial insects with their tracheal systems exchange respiratory gases with their environment. Don't forget about the ways in which dicotyledonous plant leaves (mesophyll and stomata), exchange gases too - how do these adaptations compare and contrast with one another (why not create a nice table of show the similarities and differences?)

By now as you learn about the many topics in your A-Level biology you should be realising the scope and depth of this vast and interconnect subject and it is a good idea to start thinking early on about the subject holistically - or "synoptically". Make the connections, mind maps, tables to compare and contrast and list ideas for writing synoptic essays. (Based upon what you have learned so far - you can begin adding 'more' as your understanding and coverage of topics increases).

A Level Biology - Facilitated Diffusion

 

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In this lesson we learned about Facilitated Diffusion. We learned that 

facilitated diffusion is passive process in which the transport of substances across a membrane is assisted (i.e. facilitated ) by a trans-membrane protein.

The process of Facilitated Diffusion A3 PDF for A Level Biology

Facilitated Diffusion is a Passive Diffusion Process. 

 

Remember! Just because membrane proteins are involved in the diffusion of molecules across the cell membrane does NOT mean Energy is needed. 

 

In Facilitated Diffusion NO Energy is required and substances diffusion across the cell membrane with help of membrane proteins - BUT, the molecules can only move down their concentration gradient

There are two kinds of transport protein:

Channel Proteins form a water-filled pore or channel in the membrane.

This allows charged substances, typically ions to diffuse across cell membranes.

 

Most channels can be gated (opened or closed), allowing the cell to control the entry and exit of ions.

Carrier Proteins have a binding site for a specific molecule, Carrier proteins 'flip' between two states so that the binding site is alternately open to opposite sides of the membrane. Substances bind to the carrier protein on the side of the cell membrane where it is at a high concentration and is released on the side of the cell membrane where concentration is low.

A Level Biology -

Factors Affecting The Rate of Passive and Facilitated Diffusion

 

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Factors affecting the rate of passive and facilitated diffusion A3 PDF for A Level Biology
 
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AQA Specification Reference: - 3.2.3 Transport across cell membranes. Movement across membranes occurs by: simple diffusion (involving limitations imposed by the nature of the phospholipid bilayer). Cells may be adapted for rapid transport across their internal or external membranes by an increase in surface area of, or by an increase in the number of protein channels and carrier molecules in, their membranes.

 

CIE Specification Reference: - 4 Cell membranes and transport. 4.2 Movement of substances into and out of cells: a) describe and explain the processes of diffusion.

 

Edexcel (Biology A – Salters-Nuffield) Specification Reference: - Topic 2: Genes and Health: 2.4 i) Understand what is meant by passive transport (diffusion).

 

Edexcel (Biology B) Specification Reference: - Topic 4: Exchange and Transport. 4.2 Cell transport mechanisms: ii Understand how passive transport is brought about by Diffusion.

 

OCR (Biology A) Specification Reference: - Module 2: Foundations in biology: 2.1.5 Biological membranes. (d) (i) the movement of molecules across membranes: To include diffusion and facilitated diffusion as passive methods.

 

OCR (Biology B) Specification Reference: - Module 2: Cells, chemicals for life, transport and gas exchange. (I) the movement of molecules across plasma membranes. To include diffusion and facilitated diffusion as passive methods of transport across membranes.

 

WJEC Specification Reference: - Core Concepts: 3. Cell membranes and transport. Overview: Cell membranes are essential in the control of the movement of substances into and out of the cell. They also play a vital role in cell recognition. (c) the following transport mechanisms: diffusion and factors affecting the rate of diffusion.

AQA Specification Reference: - 3.2.3 Transport across cell membranes. Movement across membranes occurs by: Facilitated diffusion (involving limitations imposed by the nature of the phospholipid bilayer). Cells may be adapted for rapid transport across their internal or external membranes by an increase in surface area of, or by an increase in the number of protein channels and carrier molecules in, their membranes.

 

CIE Specification Reference: - 4 Cell membranes and transport. 4.2 Movement of substances into and out of cells: a) describe and explain the processes of Facilitated diffusion.

 

Edexcel (Biology A – Salters-Nuffield) Specification Reference: - Topic 2: Genes and Health: 2.4 i) Understand what is meant by passive transport (diffusion and Facilitated diffusion.).

 

Edexcel (Biology B) Specification Reference: - Topic 4: Exchange and Transport. 4.2 Cell transport mechanisms: ii Understand how passive transport is brought about by Diffusion and Facilitated diffusion.

 

★ OCR (Biology A) Specification Reference: - Module 2: Foundations in biology: 2.1.5 Biological membranes. (d) (i) the movement of molecules across membranes: To include diffusion and facilitated diffusion as passive methods.

 

OCR (Biology B) Specification Reference: - Module 2: Cells, chemicals for life, transport and gas exchange. (I) the movement of molecules across plasma membranes. To include diffusion and facilitated diffusion as passive methods of transport across membranes.

 

WJEC Specification Reference: - Core Concepts: 3. Cell membranes and transport. Overview: Cell membranes are essential in the control of the movement of substances into and out of the cell. They also play a vital role in cell recognition. (c) the following transport mechanisms: diffusion, Facilitated diffusion and factors affecting the rate of diffusion.

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