The Functional unit of Life
Cell Fractionation
Microscopy and the IAM triangle
How cells divide
The Eukaryotic cell cycle
Mitosis
Mitotic Index
Movement into and out of cells
Simple Passive Diffusion
Facilitated Diffusion
Active Transport
Exocytosis
Osmosis
The Functional Unit of Life
So, what are cells?
"Cells are units of life - the smallest units that can be considered alive".
All living organisms are made of cells: -
When it comes to classifying cells, the biggest division is between the cells of the prokaryotic kingdom (Bacteria) and those of the other four kingdoms (Animals, Plants, Fungi and Protoctista), which are all Eukaryotic cells.
All cells from each of these 5 kingdoms has its fundamental differences.
Like whether it has a cell wall or not, if it has a cell wall what is it composed of?
Does the cell contain organelles?
if so which organelles? etc....
You will have to be able to compare and contrast cells, their structures, functions and organelles.
So, let's run through the generalised structure of a Eukaryotic cell and the structure and function of organelles. Prokaryotic cells are smaller and simpler than eukaryotic cells, and you can find out more about their structure, function and division on the bacteria, viruses and cells of immunity page.
What is Cytoplasm?
(Cyto = cell; plasm = colourless fluid). Cytoplasm is the colourless solution within the cell (enclosed by the plasma membrane). Cytoplasm is the fluid which contains enzymes for important metabolic reactions to take place; for example important biochemical reactions such as glycolysis (part of cellular respiration) take place in they cytoplasm of cells. Also contained in the cytoplasm are sugars, salts, amino acids, nucleotides… just about everything needed for the cell to function.
When discussing cytoplasm, you’ll most likely come across the term cytosol - which is sometimes used synonymously with cytoplasm, - however this is incorrect!
Cytoplasm and Cytosol are not the same!
Cytosol is essentially just the liquid part of the cytoplasm - i.e. cytosol is the intracellular fluid that contains organic molecules and cytoskeleton filaments, water and salts. It is probably easiest to think of cytosol as the fluid part of cells without the organelles. Whereas cytoplasm is the fluid part of cells that contains organelles and where many biochemical reactions take place.
What is a cell Nucleus?
The Nucleus (found in eukaryotic cells) is the largest organelle. The Nucleus is surrounded by a nuclear envelope - a double membrane which contains nuclear pores. Nuclear pores are ‘holes’ within the nuclear membrane. The nuclear pores contain proteins which control the exit of substances from the nucleus, e.g. mRNA and ribosomes.
Since the Nucleus is an organelle, with its own nuclear membrane separating it from the cytoplasm of the cell, the fluid part of the nucleus has its own name -it is called nucleoplasm. The nucleoplasm is full of substance called chromatin - a DNA / protein complex which contains the genes of the organism. During cell division chromatin becomes condensed - it becomes shorter and thickens into discrete observable chromosomes.
Also identifiable within the nucleuses is darker region of chromatin - this is called the nucleolus and is involved in making ribosomes.
What are Ribosomes?
Your textbook answer to this should always be “Ribosomes are the sites of protein synthesis”.
Ribosomes are the smallest and most numerous cellular organelles and are the sites of protein synthesis, and are often found in groups called polysomes. Ribosomes are composed of protein and rRNA (ribosomal RNA) and as previously discussed are manufactured in the nucleolus of the nucleus.
Ribosomes can be found in the cell either ‘free floating’ in the cytoplasm, where they make proteins for the cell's own use. Or ribosomes can be found attached to rER (the Rough Endoplasmic reticulum - so called because of its association with ribosomes). When attached to the rER ribosomes make proteins which are exported from the cell.
Ribosomes are composed of 2 subunits, a larger and smaller subunit. The larger subunit is further organised into sites (known as E, P and A) these sites are where tRNA (transfer RNA ‘docks’ when transferring and connecting amino acids when building polypeptides (during protein synthesis this part of the process is known as Translation). The smaller subunit is the binding site for mRNA (messenger RNA) which contains the codons that complementary base pair with the anticodons located on tRNA. Later in your A-Level biology you’ll have to apply the structure and function of ribosomes and their role in protein synthesis (translation).
All eukaryotic ribosomes are the larger, "80S", type, whereas prokaryotes, mitochondria and chloroplasts contain the smaller “70S” type.
What is Rough Endoplasmic Reticulum (rER)?
Rough Endoplasmic Reticulum is similar to sER (smooth endoplasmic reticulum), however in appearance rER looks ‘studded’ and ‘rough’ due its association with numerous ribosomes. It is the ribosomes that give rER its ‘rough’ appearance. The associated ribosomes are the sites of proteins synthesis and as the polypeptides are made, the proteins are processed in the rER (the proteins are enzymatically modified, adding carbohydrates to the polypeptide chain for example). Once modified the proteins can be “packaged” in the Golgi ready to be exported from the cell.
What is Smooth Endoplasmic Reticulum (sER)?
Smooth Endoplasmic Reticulum (sER) is a series of folded membrane channels which are involved in synthesising and transporting of materials, primarily lipids, needed by the cell.
What is the Golgi Apparatus (Golgi Body)?
Golgi is another series of flattened membrane vesicles, formed from the endoplasmic reticulum. However, the role of Golgi bodies is to transport proteins from the rER to the cell membrane for export. Parts of the rER containing proteins fuse with one side of the Golgi body (the part of the Glogi that is closely associated with rER is known as the “Cis face” of the Golgi), whilst at the other side (called the “Trans face”) small Golgi vesicles are released. The Golgi vesicles, which appear to “bud off” from the Golgi move towards the cell membrane. At the cell membrane these small Golgi vesicles fuse with the cell membrane and release their contents via exocytosis.
What are Lysosomes?
Lysosomes are small membrane-bound vesicles which are formed in the rER. Lysosomes contain digestive enzymes their function is to break down unwanted chemicals, toxins, organelles and even whole cells, so that the materials can be recycled by the cell. Lysosomes also fuse with vacuoles and digest the contents contained with them.
What are Vacuoles?
Vacuoles are membrane-bound organelles which resemble little ‘sacs’ containing water, dilute solutions of salts and other solutes. Most cells can have small vacuoles that are formed as required, but plant cells typically have one large permanent vacuole that can fill most of the cell. These large permeant vacuoles found in plant cells are filled with cell sap (water, sugars, and mineral salts). They are very important in keeping the cell turgid. Some unicellular protoctists have feeding vacuoles for digesting food, or contractile vacuoles for expelling water.
What is the Cytoskeleton?
A cells cytoskeleton is a network of protein fibres which extend throughout all eukaryotic cells. The Cytoskeleton is used for support, transport and motility. Attached to the cell membrane, the cytoskeleton provides the cell with structural integrity, helping maintain the overall shape and structure of the cell. Additionally, the cytoskeleton plays an important role holding organelles in position.
The cytoskeleton is a ‘network’ of protein fibres, of which there are 3: -
1) Microfilaments,
2) Intermediate filaments and
3) Microtubules.
Each of these protein fibres has a corresponding motor protein that can move along the protein fibre carrying cargo such as organelles, chromosomes or other cytoskeleton fibres.
It is these cytoskeletal motor proteins which are responsible for cellular actions such as: -
- chromosome movement during mitosis.
- cytoplasm cleavage during cell division.
- cilia and flagella movements,
- cell crawling (e.g. amoeba movement due to pseudopodia) and
- muscle contraction in animals.
- cytoplasmic streaming (cytoplasmic streaming is the movement or flow of cytosol in cells - particularly in in plant cells - to allow for the transport of molecules around the cell).
What are centrioles?
Centrioles are a pair of short microtubules involved in cell division. Before cell division the
centriole replicates itself and the two centrioles move to opposite poles (opposite ends) of the cell. Here at each pole of the cell the centrioles initiate spindle formation which organises and separates the chromosomes during cell division. (see mitosis for more detail).
What are Mitochondria?
Mitochondria are “sausage-shaped organelles” typically about 8μm long. Within the mitochondria is the “space” enclosed by the inner membrane - this “space” is called the mitochondrial matrix, and it is here within the mitochondrial matrix where 70S ribosomes and small circular strands of mitochondrial DNA(mtDNA) can be found. Structurally mitochondria are surrounded by a double membrane: the outer membrane is relatively simple and permeable. Whilst the inner membrane is slightly more complex. This highly folded inner membrane, forms the inner membrane structures called cristae. Cristae of course provide the inner mitochondria membrane with a large surface area - and it is here where ATP synthesis occurs. Mitochondria are where aerobic respiration takes place in all eukaryotic cells.
What are Chloroplasts?
Chloroplasts are where photosynthesis takes place. Thus, chloroplasts are only found in photosynthetic organisms (e.g. plants and algae). Similar to mitochondria, chloroplasts are enclosed by a double membrane. However, chloroplasts also have a third membrane called the thylakoid membrane. The thylakoid membrane is folded into thylakoid disks, which are then stacked into structures knows as grana. The thylakoid membrane contains chlorophyll and other photosynthetic pigments arranged in photosystems, and is the site of photosynthesis and ATP synthesis. The space between the inner membrane and the thylakoid is called the stroma. Chloroplasts also contain starch grains, ribosomes (70S) and circular DNA.
What is a Cell Membrane?
The cell membrane (aka. the Plasma Membrane or phospholipid bilayer) is a thin, flexible layer around the outside of all cells. Cell membranes are composed of phospholipids and proteins. Cell membranes separate the contents of the cell (intracellular environment) from the outside (extracellular environment). Cell membranes regulate, that is they control the entry and exit of materials. The cell membrane (i.e. The fluid mosaic model) will be discussed in greater detail later.
What is a Cell Wall?
Cell walls are a thick layer consisting of a network of fibres - outside of the cell membrane. Cell walls are there to provide strength and rigidity, and whilst the cell wall gives a cell strength it is still freely permeable to solutes (unlike cell membranes).
Plant cell walls are made mainly of cellulose, (plant cell walls can also contain hemicellulose, pectin, lignin and other polysaccharides). Plant cell walls are built up in three layers called the primary cell wall, the secondary cell wall and the middle lamella. Channels through plant cell walls called plasmodesmata link the cytoplasm of adjacent cells.
Fungal cell walls are made of chitin (pronounced Kai-Tin - Not Shittin’ as many of my students liked to say!). Chitin is a complex of fibrous polysaccharides and also forms the exoskeleton of arthropods.
Animal cells DO NOT have a cell wall (though they do have a layer of carbohydrate outside the cell membrane called glycocalyx).
What is Microvilli?
Microvilli are small finger-like extensions of the cell membrane found in certain cells, for example epithelial cells of the intestine. Microvilli increase the surface area for absorption of materials. They are just visible under the light microscope and are described a as a “brush border.”
What is Undulipodium?
Undulipodium is a long flexible tail present in some eukaryotic cells. The Undulipodium is used for motility. A undulipodium is an extension of the cytoplasm, surrounded by the cell membrane, and is full of microtubules and motor proteins so is capable of complex swimming movements.
Cilia are identical in structure to undulipodia, but are much smaller and there are usually very many of them - cilia typically resemble short microscopic hairlike structures which vibrate allowing for movement (e.g. in ciliated protists) or “beating” and “vibrating” creating a flow / propulsion in the surrounding fluid - e.g. in the respiratory tract where they help filter out microorganisms, dust and other particles breathed in from the air.
The Cell Membrane
The Cell Membrane - The Fluid Mosaic Model.
The cell membrane / plasma membrane surrounds all living cells. It cannot be overemphasised just how important cell membranes are, since they control how substances can move into and out of cells. Cell membranes are also responsible for many other things too. Membranes that surround the nucleus and other organelles (e.g. mitochondria) are almost identical to the cell membrane, and are composed of phospholipids, proteins and carbohydrates arranged what is commonly known as the fluid mosaic model (conceived in 1972 by Nicolson and Singer).
The phospholipids that form cell membranes arrange themselves in a particular way, due to their amphiphilic nature, remember from the phospholipids lesson how phospholipids form cell membranes. The phospholipids form a bilayer the with their polar (hydrophilic) phosphate heads facing outwards, and their non-polar (hydrophobic) fatty acid tails facing each other in the middle of the phospholipid bilayer. It is the hydrophobic fatty acid tails that form a “layer” in the middle of the bilayer which acts as a barrier to all but the smallest molecules. It is this 'barrier' which effectively isolates the two sides of the cell membrane.
Cell membranes can have phospholipids with different fatty acid tails too, which affects the rigidity / flexibility of the membrane. Animal cell membranes also contain cholesterol which can link to the fatty acids and provide stability, thus, strengthening the cell membrane.
The phospholipid bilayer is thin, flexible and in constant motion - the phospholipids move in a range of ways, rotating in situ, swapping positions with neighbours and even “jumping” from one side of the membrane to the other - in a manoeuvre called the “flip flop”. It is of course this “fluidity” of the phospholipid bilayer that gave rise its “fluid” name in Singer and Nicholson's "fluid mosaic model".
The “mosaic” part is simply due to the fact that embedded within the phospholipid bilayer are proteins. The proteins "float" in the phospholipid membrane forming channels and receptors for molecules that warrant entry or exit from cells, and don’t forget the carbohydrates that extend out from the proteins too. These carbohydrates have functional parts to play as cell surface receptors, and other roles which we’ll get to shortly.
The proteins embedded within the phospholipid bilayer usually span from one side of the phospholipid bilayer to the other (integral proteins), but can also sit on one of the surfaces of cell membranes (peripheral proteins). The proteins can slide around the membrane, moving very quickly and colliding with one another, but can never flip from one side to the other (like phospholipids can!) Cell membrane proteins have hydrophilic regions in contact with the water on the outside of cell membranes (these hydrophilic regions are composed of hydrophilic amino acids). The proteins also have hydrophobic regions (composed of hydrophobic amino acids) in contact with the fatty chains which form the inside part of the cell membrane. Proteins make up approximately 50% of the mass of cell membranes and are responsible for most of the membrane's functional properties (i.e. regulation / transport of substances into and out of the cell).
Proteins that span the membrane are usually involved in transporting substances across the membrane.
Proteins on the inside surface of cell membranes are often attached to the cytoskeleton and are involved in maintaining the cell's shape and aid in cell motility. Some of these proteins maybe enzymes that catalyse specific reactions.
Proteins on the outside surface of cell membranes typically act as receptors -
These proteins have specific binding sites where hormones or other molecules bind - triggering cellular events. These proteins may also be involved in cell signalling, cell recognition and enzyme activities - like in digestive processes, e.g. maltase is a membrane bound enzyme that hydrolyses the disaccharide maltose into two alpha-glucose molecules.
Carbohydrates are not just digested either… some carbohydrates play functional roles and are found on the outer surface of all eukaryotic cell membranes. These carbohydrates are attached to the cell membrane proteins and sometimes directly to the phospholipids. Proteins with carbohydrates attached have specific names - they are called glycoproteins, (“glyco” referring to the carbohydrate portion and “protein” referring to, well the protein part)…
Phospholipids with carbohydrates attached also have special names - these are called glycolipids. (the same convention of naming applies - “glyco” meaning the carbohydrate bit and “lipid” meaning the phospholipid part.
The carbohydrates that associate with cell membranes are short polysaccharides composed of a variety of different monosaccharides. Together they form a cell “coat” or glycocalyx outside the cell membrane (once again naming conventions remain consistent here too - the “glyco” meaning carbohydrate and “calyx” is of Greek origin meaning “case” or “enclosure”. So, the glycocalyx is involved in protection and cell recognition - Antigens, for example the ABO antigens on blood cells are usually cell-surface glycoproteins.
The important thing to remember is that cell membranes are more complex that just a simple lipid bilayer allowing things into and out of cells. Rather, cell membranes are “Fluid” - that is to say they are in constant motion and they are “mosaic” - that is, cell membranes are embeddedwith a variety of proteins, enzymes, glycoproteins and glycolipids - all playing essential roles in cell regulation cell recognition, cell signalling, and many many more important cellular events.
Cell Fractionation and Differential Centrifugation
When studying the structure and function of cells, biologists use many techniques.
Microscopy / electron microscopy being at the top of the essential skills list. Microscopy of course allows scientists to gain information about the relative size and structure of cells and their organelles.
But how do biologists isolate organelles and study their structures and functions?
Cell fractionation and differential centrifugation is used to isolate sub-cellular organelles in order to study their structures and functions… Here you'll learn exactly how and why cell fractionations is done and why certain steps in the procedure are necessary.
Firstly, as with all a-level biology topics it’s a good idea to familiarise yourself with the vocabulary used - and an understanding of the key terms used here is essential.
So, to begin - Create a flashcard to learn and revise each of the following key words.
Define the following terms: -
Cell | Tissue | Fractionation | Differential,
Centrifugation | Homogenate | Supernatant,
Precipitate | Buffer | Isotonic
If you need help define any of these (check out the glossary) and of course keep reading as you’ll come across these terms in context as we go through cell fractionation and differential centrifugation.
What’s in a name?
Well by now you know how to define “cell” (“the basic unit of living organisms”) and you most likely know what “fractionation” means, but let’s break it down anyway.
Fraction - “a small (or tiny) part of ‘something’
Fractionation - “a procedure which separates ‘something’ into smaller parts (i.e. into “fractions”).
So, in this case, cell fractionation is literally “a procedure that separates cells into smaller parts” i.e. breaking cells down into their constituent parts, such as mitochondria, endoplasmic reticulum, chloroplasts, ribosomes, etc…
Easy right… so how is cell fractionation done?
Let’s take a look at each step of the cell fractionation and centrifugation procedure… (by the way you can download the cell fractionation worksheet help with your revision).
Cell fractionation - The Procedure.
Cell Fractionation and Ultracentrifugation Step 1:
The sample is chilled in ice-cold, buffered, isotonic solution.
Note: “The Sample “ could be any tissue sample - i.e. liver tissue, lung, tissue, pancreatic tissue, plant tissue… whatever - so its a good idea you can define “tissue” as its a question commonly associated with this procedure in exams.
Cell Fractionation and Ultracentrifugation Step 2:
The tissue (e.g. liver, heart, leaf, etc) is homogenised in an ice-cold, isotonic, buffered solution.
Note: The sample is homogenised in order to “break open” the cells and release the organelles into the solution.
Cell Fractionation and Ultracentrifugation Step 3:
The mixture (homogenate) is filtered to remove any larger pieces of tissue not broken up by homogeniser (the blender) and any cell debris. The result is a solution consisting ‘free' organelles.
Cell Fractionation and Ultracentrifugation Step 4:
This solution (the homogenate) is now centrifuged at low speed. Any cellular debris is forced to the bottom of the tube. This is called "the pellet" (or sediment). The solution above this pellet is the supernatant, and consists of all the other (less dense) organelles. The supernatant is decanted (carefully poured) into a new tube. The pellet can be resuspended if needed (i.e. if the pellet consists of mitochondria we would want to re-suspend it in cold, buffered, isotonic solution for isolation and further study.
Cell Fractionation and Ultracentrifugation Step 5:
The solution (supernatant) is now centrifuged at a higher speed. The larger (more dense) organelles are forced to the bottom of the tube, again forming “the pellet”. The supernatant is decanted ready for the next round of centrifugation and the pellet can be removed and can be resuspended too if required.
Cell Fractionation and Ultracentrifugation Step 6... 7, 8
Repeat.
The above process can be repeated many times, each time increasing the speed of the centrifuge (until centrifugation is ultra fast! (ultracentrifugation). This differential centrifugation technique allows us to separate (that is: differentiate) each of the sub-cellular organelles, based upon size (density).
So, which each step centrifugation separates the organelles according to their size (density).
Remember! The organelle(s) forced into the pellet through centrifugation is always dependent upon the sample you start with (i.e. animal or plant tissue - and what already exists in the sample). Make sure you read any exam questions carefully as your answers must be in context.
A Level Biology - Microscopy
A Level Biology - How Eukaryotic cells divide.
A Level Biology - The Eukaryotic Cell Cycle
The cell cycle
There are two ways in which a cell can divide:
1. Mitosis.
2. Meiosis.
Mitosis is 'normal' cell division in which the DNA replicates and each daughter cell has an identical copy of the original DNA (unless a gene mutation has taken place).
Organisms grow and repair by Mitosis.
Whereas Meiosis is a more complex type of cell division with 2 essential things to understand:
-
The (diploid) chromosomal number is halved
-
Genes are randomly assorted (i.e. ‘shuffled’) so that each daughter cell contains different combinations of alleles.
Meiosis plays an essential role in the sexual reproduction for most species (but not all!). Meiosis is the process that produces haploid gametes (Sperm and Ova).
A Level Biology - Mitosis
A Level Biology - Practical Skills: Identifying the stages of Mitosis and Calculating the Mitotic Index
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).
-
Passive Transport (Facilitated Diffusion).
-
Active Transport.
-
Vesicles (e.g. exocytosis and pinocytosis).
A Level Biology - Simple / Passive Diffusion
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.
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 example, Active 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
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.
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
A Level Biology - Active Transport
Active Transport: -
Active transport is the ‘pumping’ (ie. the “active” movement) of substances across a membrane by a trans-membrane protein pump molecule.
The trans-membrane protein binds a molecule of the substance to be transported on one side of the membrane, changes shape, and releases it on the other side.
Active transport is energy dependent!
Trans-membrane protein protein pumps are ATPase enzymes.
ATPase catalyses the hydrolysis of ATP —> ADP + phosphate (Pi) releasing “energy” which can be put to use. (Revise the structure of ATP and ADP-ATP cycle).
Energy ‘released’ from ATP hydrolysis is used to: -
Change the shape of the protein and ‘pump’ the molecule across the membrane (hence these molecules are also called protein pumps).
Active transport “Pumping” is therefore an “Active Process’ meaning it is Energy dependent, and is the only transport mechanism that can transport substances uphill, meaning: -
“Active transport moves molecules against their concentration gradient”
Transmembrane proteins are highly specific.
Transmembrane proteins are highly specific which means there is a different protein pump for each molecule being transported across the membrane. A common example and one you must know is the sodium-potassium pump. (Na+ K+ Pump).
The Sodium - Potassium Pump:
The sodium-potassium pump (Na+ K+ pump) is a transport protein is present in the cell membranes of all animal cells and is the most abundant and important of all membrane pumps.
[Insert image of (Na+ K+ Pump)].
The Na+K+ pump is a complex pump, simultaneously pumping three sodium ions out of the cell
and two potassium ions into the cell for each molecule of ATP hydrolysed.
Why is this important? Well apart from moving ions, the Na+K+ pump also generates a “potential difference” across the cell membrane. This “potential difference” is called the membrane potential, and all animal cells have it.
Membrane potential varies from 20mV to 200mV, but and is always negative inside the cell.
In most cells the Na+K+ pump runs continuously and uses 30% of all the cell's energy (70% in nerve cells). (We’ll discuss membrane potential in more detail when you learn about nerve impulses).
[Insert Graph to compare simple, facilitated and active transport]
The graph shows the rate of “transport” (movement of a given molecule across a cell membrane in relation to the concentration difference inside and outside a cell.
What the graph shows is that the rate of (simple/passive/lipid) diffusion of a substance across a membrane increases as its concentration gradient increases. Thus, lipid diffusion shows a linear relationship between rate of transport and concentration.
Facilitated diffusion has a curved relationship, as the line begins to flatten a maximum rate of diffusion will be realised. This is due to the rate of diffusion being limited by the number of transport proteins, etc. (see factors limiting the rate of facilitated diffusion).
The graph shows that the rate of active transport also increases with concentration gradient, however, and most importantly: -
Active transport has a high rate of transport even when there is No concentration difference across the membrane.
Remember:
Active transport is energy dependant, and is the only transport method that can transport molecules against their concentration gradient. If cellular respiration stops, so does does active transport, since of course there is no energy to “power” the energy dependant protein pumps!
Active transport:
-
Uses Energy (energy release for ATP hydrolysis).
-
Requires trans-membrane proteins (e.g. the Na+K+ pump).
-
Is very Specific (different protein pump for each molecule being transported across a cell membrane).
-
It is controllable.
A Level Biology - Endocytosis and Exocytosis
Endocytosis and Exocytosis:
The way in which substances are transported across cell membranes described above, only apply to small molecules.
So, how do large molecules such as proteins, polysaccharides, nucleotides (i.e. mRNA), and even whole cells enter and exit cells across a phospholipid bilayer?
The answer is via membrane vesicles. Large molecules (and even whole cells) are moved into and out of cells by using membrane vesicles. Two methods of membranes transport via vesicles: Endocytosis and Exocytosis will be considered subsequently.
Endocytosis is the transport of materials into a cell. (Endo = “inside” and Cyto = cell).
Large molecules/cellular material are enclosed by a folding of the cell membrane. This portion of the cellular membrane encloses, encapsulating the ‘material’ and pinches shut to form a closed vesicle.
Technically the ‘material’ has not yet crossed the membrane, so it is typically ‘digested’ which subsequently releases the ‘digested material’ . Now, smaller ‘products of digestion’ i.e. smaller molecules can be absorbed by the methods outlined above.
When the materials and the vesicles are relatively small (such as a protein molecule) the process is known as pinocytosis (cell drinking). However, if the materials are relatively large (for example a white blood cell ingesting a bacterial cell) the process is known as phagocytosis (cell eating).
[Insert Endocytosis images]
Exocytosis:
Exocytosis is the transport of materials out of a cell (Exo = "outside" and cyto = cell).
It is the exact same process of endocytosis, but in reverse!
Cellular products/materials which are to be exported from a cell must first be enclosed in a membrane vesicle (Golgi vesicles), usually from the rER and Golgi Body. For example, Hormones and digestive enzymes are secreted by exocytosis from the secretory cells of the intestine and endocrine glands.
A Level Biology - An Overview of fundamentals of Osmosis
Osmosis is the diffusion of water across a membrane, in fact, osmosis is just simple, passive diffusion. However, since water is so important to life and such an abundant molecule in cells, the diffusion of water has been given its own name - osmosis.
When explaining, discussing and answering questions about osmosis it is super important that you’re able to use the appropriate terminology (and understanding water potential is a fundamental expectation, since osmosis can be quantified using water potential; The short hand symbol for water potential is Ψ the Greek letter psi, pronounced “sy").
Water potential is simply the effective concentration of water which is measured in units of pressure i.e. measured in Pascals (Pa) (or usually kPa kilopascals). The rule is very simple, just like that of passive diffusion:-
“Water always "moves" from a high water potential to a lower water potential”.
The key thing to remember is that 100% pure water has a water potential (Ψ) of zero (0), and zero is the highest possible water potential. Thus, ALL solutions have a water potential (Ψ) Less than zero (0).
You cannot have a water potential (Ψ) great than zero (0).
Another term term you may come across when learning osmosis is osmotic pressure (OP).
You just need to know that the more concentrated a solution, the higher the osmotic pressure. Therefore, OP is the opposite of water potential (Ψ), meaning water will move from a low OP to a high OP.
Now that you know the difference between OP and Ψ you’ll be expected to apply your understanding of osmosis to cells. and once again use the appropriate terminology.
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A Level Biology - Osmosis: Key terms you must know, understand and use when explaining Osmosis
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A Level Biology - Osmosis and Aquaporins
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A Level Biology - Factors affecting the rate of Osmosis
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★ AQA Specification Reference: - 3.2 Cells: nucleus (containing chromosomes, consisting of protein-bound, linear DNA). 3.4.1 DNA, genes and chromosomes. In the nucleus of eukaryotic cells, DNA molecules are very long, linear and associated with proteins, called histones. Together a DNA molecule and its associated proteins form a chromosome. A gene occupies a fixed position, called a locus, on a particular DNA molecule.
★ CIE Specification Reference: - 5 The mitotic cell cycle: describe the structure of a chromosome, limited to DNA, histone proteins, chromatids, centromere and telomeres.
★ Edexcel (Biology A – Salters-Nuffield) Specification Reference: - Topic 3: Voice of the Genome. 3.8 i) Know that a locus (plural = loci) is the location of genes on a chromosome. ii) Understand the linkage of genes on a chromosome.
★ Edexcel (Biology B) Specification Reference: - Topic 2: Cells, Viruses and Reproduction of Living Things; Topic 7: Modern Genetics; Topic 8: Origins of Genetic Variation.
★ OCR (Biology A) Specification Reference: - Terminology is expected knowledge throughout. topics: -2.1 Foundations in biology; 2.1.6 Cell division, cell diversity and cellular organisation & Module 6 – Genetics, evolution and ecosystems).
★ OCR (Biology B) Specification Reference: - Terminology is expected knowledge throughout.: 3.1.1 The developing cell: cell division and cell differentiation; 3.1.2 The developing individual: meiosis, growth and development; 3.3.1 The cellular basis of cancer and treatment; Module 5: Genetics, control and homeostasis - e.g. To include the correct usage of the terms gene, allele (gene variant), locus, phenotype, genotype, dominant and recessive, heterozygous and homozygous and codominant.
★ WJEC Specification Reference: - Terminology is expected knowledge throughout: i.e. Core Concepts 5. Nucleic acids and their functions. CONTINUITY OF LIFE: 2. Genetic information is copied and passed on to daughter cells. 5. Inheritance: (a) alleles as different forms of the same gene. 7. Application of reproduction and genetics.
★ AQA Specification Reference: - 3.2.2 All cells arise from other cells. Eukaryotic cells that do retain the ability to divide show a cell cycle. DNA replication occurs during the interphase of the cell cycle. Mitosis is the part of the cell cycle in which a eukaryotic cell divides to produce two daughter cells, each with the identical copies of DNA produced by the parent cell during DNA replication. Division of the cytoplasm (cytokinesis) usually occurs, producing two new cells.
★ CIE Specification Reference: - 5. The mitotic cell cycle: When body cells reach a certain size they divide into two. Nuclear division occurs first, followed by division of the cytoplasm. The mitotic cell cycle of eukaryotes involves DNA replication followed by nuclear division. This ensures the genetic uniformity of all daughter cells. Candidates will be expected to use the knowledge gained in this section to solve problems in familiar and unfamiliar contexts. c) outline the cell cycle, including interphase (growth in G1 and G2 phases and DNA replication in S phase), mitosis and cytokinesis.
★ Edexcel (Biology A – Salters-Nuffield) Specification Reference: - Topic 3: Voice of the Genome. 3.10 Understand the role of mitosis and the cell cycle in producing identical daughter cells for growth.
★ Edexcel (Biology B) Specification Reference: - Topic 2: Cells, Viruses and Reproduction of Living Things. 2.3 Eukaryotic cell cycle and division. i Know that the cell cycle is a regulated process in which cells divide into two identical daughter cells, and that this process consists of three main stages: interphase, mitosis and cytokinesis. ii Understand what happens to genetic material during the cell cycle, including the stages of mitosis.
★ OCR (Biology A) Specification Reference: - Module 2: Foundations in biology. 2.1.6 Cell division, cell diversity and cellular organisation. Learners should be able to demonstrate and apply their knowledge and understanding of: the cell cycle - To include the processes taking place during interphase (G1, S and G2), mitosis and cytokinesis, leading to genetically identical cells.
★ OCR (Biology B) Specification Reference: - Module 3: Cell division, development and disease control Learners should be able to demonstrate and apply their knowledge and understanding of: the cell cycle - To include the processes taking place during interphase (G1, S and G2), mitosis and cytokinesis, leading to genetically identical cells.
★ WJEC Specification Reference: - 2. Genetic information is copied and passed on to daughter cells. This topic covers cell division. During the cell cycle, genetic information is copied and passed on to daughter cells.
★ AQA Specification Reference: - 3.2 Cells: 3.2.2 All cells arise from other cells. Mitosis is the part of the cell cycle in which a eukaryotic cell divides to produce two daughter cells, each with the identical copies of DNA produced by the parent cell during DNA replication. The behaviour of chromosomes during interphase, prophase, metaphase, anaphase and telophase of mitosis. The role of spindle fibres attached to centromeres in the separation of chromatids. Division of the cytoplasm (cytokinesis) usually occurs, producing two new cells.
★ CIE Specification Reference: - 5 The mitotic cell cycle. Explain the importance of mitosis in the production of genetically identical cells, growth, cell replacement, repair of tissues and asexual reproduction. 5.2 Chromosome behaviour in mitosis: describe, with the aid of photomicrographs and diagrams, the behaviour of chromosomes in plant and animal cells during the mitotic cell cycle and the associated behaviour of the nuclear envelope, cell surface membrane and the spindle (names of the main stages of mitosis are expected).
★ Edexcel (Biology A – Salters-Nuffield) Specification Reference: - Topic 3: Voice of the Genome. 3.10 Understand the role of mitosis and the cell cycle in producing identical daughter cells for growth and asexual reproduction.
★ Edexcel (Biology B) Specification Reference: - Topic 2: Cells, Viruses and Reproduction of Living Things. 2.3 Eukaryotic cell cycle and division. i Know that the cell cycle is a regulated process in which cells divide into two identical daughter cells, and that this process consists of three main stages: interphase, mitosis and cytokinesis. ii Understand what happens to genetic material during the cell cycle, including the stages of mitosis. iii Understand how mitosis contributes to growth, repair and asexual reproduction.
★ OCR (Biology A) Specification Reference: - 2.1.6 Cell division, cell diversity and cellular organisation. The main stages of mitosis. To include the changes in the nuclear envelope, chromosomes, chromatids, centromere, centrioles, spindle fibres and cell membrane.
★ OCR (Biology B) Specification Reference: - 3.1.1 The developing cell: cell division and cell differentiation . The main stages of mitosis. To include the changes in the nuclear envelope, chromosomes, chromatids, centromere, centrioles, spindle fibres and cell membrane.
★ WJEC Specification Reference: - Continuity of Life. 2. Genetic information is copied and passed on to daughter cells. This topic covers cell division. During the cell cycle, genetic information is copied and passed on to daughter cells. (a) interphase and the main stages of mitosis (b) the significance of mitosis as a process in which daughter cells are provided with identical copies of genes and the process of cytokinesis.
★ AQA Specification Reference: - Required practical 2: Preparation of stained squashes of cells from plant root tips; set-up and use of an optical microscope to identify the stages of mitosis in these stained squashes and calculation of a mitotic index.
★ CIE Specification Reference: - 4.2 Paper 3. Advanced Practical Skills 1 and Advanced Practical Skills 2.
★ Edexcel (Biology A – Salters-Nuffield) Specification Reference: - Practical skills: In carrying out practical activities, students will be expected to use their knowledge and understanding to pose scientific questions which can be investigated through experimental activities. Such activities will enable students to collect data, analyse it for correlations and causal relationships, and to develop solutions to the questions posed.
★ Edexcel (Biology B) Specification Reference: - Practical skills: In carrying out practical activities, students will be expected to use their knowledge and understanding to pose scientific questions which can be investigated through experimental activities. Such activities will enable students to collect data, analyse it for correlations and causal relationships, and to develop solutions to the questions posed.
★ OCR (Biology A) Specification Reference: - Module 1: Development of practical skills in biology. 1.1 Practical skills assessed in a written examination. Practical skills are embedded throughout all the content of this specification. Learners will be required to develop a range of practical skills throughout their course in preparation for the written examinations. 1.1.3 Analysis: 1.2 Practical skills assessed in the practical endorsement. 1.2.1 Practical skills.
★ OCR (Biology B) Specification Reference: - Module 1: Development of practical skills in biology. 1.1 Practical skills assessed in a written examination. Practical skills are embedded throughout all the content of this specification. Learners will be required to develop a range of practical skills throughout their course in preparation for the written examinations. 1.1.3 Analysis: 1.2 Practical skills assessed in the practical endorsement. 1.2.1 Practical skills.
★ WJEC Specification Reference: - PRACTICAL TECHNIQUE REQUIREMENTS: Use of light microscope at high power and low power, including use of a graticule. 5. produce scientific drawing from observation with annotations. Core concepts 2: Scientific drawing of living cells, Preparation and drawing of cells of root tip. MATHEMATICAL REQUIREMENTS AND EXEMPLIFICATION. Recognise and make use of appropriate units in calculations. Handling data.
★ AQA Specification Reference: - 3.4.3 Genetic diversity can arise as a result of mutation. Gene mutations involve a change in the base sequence of chromosomes. They can arise spontaneously during DNA replication and include base deletion and base substitution. Due to the degenerate nature of the genetic code, not all base substitutions cause a change in the sequence of encoded amino acids. 3.8.1 Alteration of the sequence of bases in DNA can alter the structure of proteins (A-level only). Gene mutations might arise during DNA replication. They include addition, deletion, substitution, inversion, duplication and translocation of bases.
★ CIE Specification Reference: - 6 Nucleic acids and protein synthesis: 6.2 Protein synthesis. b) state that a gene mutation is a change in the sequence of nucleotides that may result in an altered polypeptide. 16 Inherited change: 16.2 The roles of genes in determining the phenotype. e) explain that gene mutation occurs by substitution, deletion and insertion of base pairs in DNA and outline how such mutations may affect the phenotype.
★ Edexcel (Biology A – Salters-Nuffield) Specification Reference: - N/A.
★ Edexcel (Biology B) Specification Reference: - N/A.
★ OCR (Biology A) Specification Reference: - 6.1 Genetics and evolution. Types of gene mutations and their possible effects on protein production and function. To include substitution, insertion or deletion of one or more nucleotides. The possible effects of these gene mutations (i.e. beneficial, neutral or harmful).
★ OCR (Biology B) Specification Reference: - Module 5: Genetics, control and homeostasis 5.1.1 Patterns of inheritance. B) gene mutations (To include cystic fibrosis, sickle cell anaemia, phenylketonuria (PKU) and Huntington’s disease).
★ WJEC Specification Reference: - Continuity of Life 5. Inheritance (f) gene mutation as illustrated by sickle cell anaemia.
★ 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.
★ AQA Specification Reference: - 3.2.3 Transport across cell membranes. Movement across membranes occurs by: Students should be able to explain osmosis (explained in terms of water potential). Explain how surface area, number of channel or carrier proteins and differences in gradients of concentration or water potential affect the rate of movement across cell 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 Osmosis. Explain the movement of water between cells and solutions with different water potentials and explain the different effects on plant and animal cells.
★ Edexcel (Biology A – Salters-Nuffield) Specification Reference: - Topic 2: Genes and Health: 2.3 Understand what is meant by osmosis in terms of the movement of free water molecules through a partially permeable membrane (consideration of water potential is not required).
★ Edexcel (Biology B) Specification Reference: - Topic 4: Exchange and Transport. 4.2 Cell transport mechanisms: Understand how passive transport is brought about by osmosis: CORE PRACTICAL 6: Determine the water potential of plant cells.
★ OCR (Biology A) Specification Reference: - Module 2: Foundations in biology: 2.1.5 Biological membranes. (e) the movement of water across membranes by osmosis and the effects that solutions of different water potential can have on plant and animal cells. Practical investigations into the effects of solutions of different water potential on plant and animal cells. Osmosis to be explained in terms of a water potential gradient across a partially permeable membrane.
★ OCR (Biology B) Specification Reference: - Module 2: Cells, chemicals for life, transport and gas exchange.
★ 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.