GCSE Biology: Movement into and out of Cells

So, in the previous sections you’ve revised who the main people involved in the development of cell theory, the structures and functions of prokaryotes (bacteria) and eukaryotes (plant and animal cells). You should have created posters of these cells and a table to compare and contrast them (don’t forget to download the revision PDFs (coming soon) to help you learn, revise and keep your notes nice and organised!

Now you know about the cells you need to know the different ways things can get in and out of them

What is Diffusion?

Molecules in liquids and gases are in constant random motion.

Molecules spread from areas of high concentration to low concentration. Hence there is a net movement of molecules. This is called diffusion.

The rate of diffusion depends on several factors…

1. The concentration gradient – the higher it is the faster the rate of diffusion

2. The thickness of the exchange surface – the thinner the exchange surface (I.e. the shorter the distance the particles have to travel), the faster the rate of Diffusion.

3. The surface area – the larger the surface area (e.g. of the cell membrane), the faster the rate of diffusion.

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The greater the difference between the regions of high concentration and low concentration, the faster the substances rate of diffusion.

Substances move into and out of cells by diffusion.

A good example of this gas exchange. The process by which oxygen enters the lungs and carbon dioxide leaves. This exchange takes place across the cell membranes of the alveoli (air sacs) in the lungs.

Concentration gradient.

This is the difference in concentration between the substance in the high and low regions.

The greater the difference in these regions the greater the concentration gradient, resulting in an increased rate of diffusion.

Active Transport – movement across a cell membrane against its concentration gradient – thus, ATP (energy currency) is needed. 

Example:

The rate of diffusion of glucose and oxygen to respiring cells relies on a high concentration gradient between the cells and the blood capillaries. The concentration gradient is maintained because the cells are constantly respiring, breaking down the glucose and oxygen, therefore continually lowering their concentration within the cells.

Osmosis is a type of diffusion.

What is osmosis?

Osmosis is the movement of water (H2O) from a region of high water concentration to low water concentration across a semi permeable membrane. A semi permeable membrane only allows H20 across (not the solute – i.e. salts and sugars that are dissolved in water (H20 is the solvent).

Remember!!

A solution that is highly concentrated, e.g. has a high sugar content, will have a low water concentration!

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Example: Root hair cells.

Substances in the soil are taken up by plant root hair cells.

The diagram shows the direction of movement of two substances A and B across the cell membrane of a root hair cell.

GCSE Biology: Cell Division and differentiation

Cell Differentiation

In Humans tissues are an organisation of cells  working together to carry out specific functions. The precise tissue function within an organism is dependant upon the type of cells it contains.

Cell types:

…cells come in many different shapes & sizes!

Nerve cell…

Sperm cell…

Red blood cells

Muscle cells

It is not just the shape of different cells that varies, but also the numbers of each of there organelles…

e.g. a muscle or sperm cell will have many mitochondria, while a bone cell has very few.

How do cells / tissues become “Specialised” ?

1. Tissues maintain stem cells to serve as a reservoir of undifferentiated cells. 

2. Stem cells typically have the capacity to mature into many different cell types

Adult stem cells are in the bone marrow! But these are not quite as useful as embryonic stem cells… Which may be able to cure many diseases!

Stem cells can “become” any type of cell. 

1. Fertilised egg (zygote) divides by Mitosis forming an embryo.

2. All embryonic ‘stem’ cells are undifferentiated.

3. Stem cells divide producing more stem cells or different cells (Specialised cells – e.g. epithelial, blood, liver etc.) 

4. Differentiation is the term given to stem cells dividing and becoming specialised.

5. Differentiated stem cells develop into the recognisable organism, complete with its organs and organ systems.

Cells make up Tissues 

Which are communities of cells that have functions beyond what any single cell type could accomplish…

So, cells differentiate, becoming specialised. These cells form tissues (a group of similar cells working together to carry out a particular function).

Different tissues form organs (e.g. liver, heart etc.)  these tissues work together to carry out a particular function).

Organs form organ systems – e.g. Digestive system; a group of organs working together to carry out a particular function.

Cells —> Tissues —> Organs —> Organ system —> Organism

Similar cells —> grouped into to tissues —> organs & —> organs into systems for increased efficiency.

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How do cells divide to become anything at all?

Through a process called… Mitosis!

Somatic (body) cells are described as Diploid (from the Greek meaning ‘double’). 

…you will see this written as “2n”

Mitosis is the process of normal cell division. 

In mitosis, the chromosomes are copied and divided equally between the 2 new daughter cells. ..

...So each mitotic division produces 2 cells…

SO... both are diploid, & each has exactly the same genes as the parent cell!! Mitosis produces identical cells

Some cells in the human body are not diploid... 

e.g. Gametes (sex cells) contain only 1 copy of each gene as they have only 1 set of chromosomes.

These cells are Haploid and are produced by a special type of cell division called Meiosis

A male and Female gamete join  together at fertilisation...  forming a.  zygote

This 1 cell then divides  by  Mitosis to produce   a complete  new   organism…

You received one chromatid from your father. 

And one from chromatid your mother!

So, although homologous pairs contain the same genes, they do not necessarily carry the same versions of each gene.

A different version of a gene is called an allele.

Remember... 

This is key to understanding why organisms vary!

So, what are Alternative forms of genes are called?? Alleles!

The Appearance, number and arrangement of chromosomes in the nucleus are referred to as the karyotype.

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 [image of human karyotype]

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The human karyotype consists of 23 pairs of chromosomes. (46, or 2n)

All diploid human cells contain 23 pairs... What does a haploid cell have? (n or 23)

As such, often the notation 2n is given:    

2n = 46 Chromosomes in humans; which simply means that a diploid cell has 46 chromosomes.

[Mitosis step by step image]

Mitosis results in 2 identical cells… and some organisms use mitosis to reproduce! This form of reproduction is called: Asexual reproduction. All offspring have exactly the same genes as the parent. E.g. Aphids and Strawberries…

Mitosis is great for growth and repair… but since all “daughters” are clones – there is NO genetic variation.

Meiosis: The production of Gametes (sex cells)  

Remember sperm and egg (ova) are Haploid – having just “n” half the number of chromosomes.

Meiosis is a special kind of cell division in which there are 2 successive divisions that result in the production of gametes – the sex cells!

[Meiosis step by step image]

GCSE Biology: Cellular Respiration

Cellular Respiration.

All living cells carry out Respiration – EVEN Plants! (They photosynthesise – when its light, and respire when its dark!)

SO, what is Respiration?

Respiration is the process of breaking down (food) glucose to release energy (ATP).

This energy is supplied to living cells. Respiration can be aerobic (with O2) or anaerobic (without O2).

Remember Respiration is not breathing! Breathing is ventilation (breathing in and out) which actually provides the Oxygen (O2) needed for aerobic respiration. Respiration takes place in the cytoplasm & Mitochondria (remember when labelling your cells mitochondria are the organelles that produce energy!)

Aerobic Respiration.

Aerobic respiration uses oxygen (O2) and the most efficient way of getting energy (ATP) from glucose. 

In fact, if we compare to anaerobic respiration, we find that for every 1 molecule of Glucose (C6H12O6) aerobic respiration (with O2) provides 20 times more energy than would be produced anaerobically (without O2)! 

Blood transports O2 and Glucose (C6H12O6) (Food) to our cells, where enzymes (biological catalyst’s) speed up the process of releasing energy.

So What is the equation for Respiration? Remember, respiration uses oxygen and food (glucose) to get energy! So, we have: Oxygen + Glucose = Energy

Now, as a result of respiration carbon dioxide (CO2) is also produced (our waste product) and some water is produced (H2O). So, we must also add these products to our equation.

Making the full Respiration equation look like this:

Oxygen + Glucose —> Energy + Carbon dioxide + Water.

Now we should also be able to write this out “Chemically”, So…

O2 + C6H12O6 —> Energy (ATP) + CO2 + H2O

What is going on?

Oxygen (O2) and Glucose (C6H12O6) are transported in the bloodstream (O2 you get from ventilation – breathing it in) and Glucose (C6H12O6) comes from your diet. 

Energy (ATP – adenosine triphosphate the energy molecule) is used for many life processes.

Carbon dioxide (CO2) is the waste product, and is returned via the blood stream to your lungs (ventilation – Breathing it out). 

Water is also a by product of respiration, which is then lost as sweat; some in your urine; and some as moist breath (ventilation – you breath out some water).

Anaerobic Respiration.

Anaerobic = without oxygen (O2)

Sometimes we need to produce energy very quickly – i.e. explosive activities like sprinting.

So, anaerobic respiration takes place when there isn’t enough oxygen (O2) available to supply the body (e.g. muscles) with the O2 required for ‘normal’ aerobic respiration. 

Anaerobic respiration is very useful in emergencies! (think Fight or Flight – which we’ll cover when we cover the nervous system). On the whole, anaerobic respiration does not produces as much energy as aerobic respiration (about 20 times less remember) but is very rapid. This is because glucose (C6H12O6) is only partially broken down.

When glucose is only partially broken down another waste product is made – Lactic acid!

We can show this with our “word equation” for Anaerobic respiration:

Glucose —> Lactic acid + Energy (ATP)

What’s going on?

Glucose (C6H12O6) is delivered to the cells (muscles) via the blood stream.

Lactic acid is a waste product (that builds up in the muscles – and leads to “cramp”)

Energy (ATP) only a small amount is produced – but is enough for short term / explosive activity.

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A build up of lactic acid requires extra oxygen to break it down.

As you increase physical activity (exercise) your breathing (ventilation) rate will also increase. This is because your cells (your body) needs the extra oxygen in order to respire (carry out aerobic respiration) efficiently, and provide the extra energy needed.

So, as your breathing (ventilation) rate increases larger amounts of O2 enters your bloodstream, your heart must accommodate by speeding up, and supplying this oxygenated blood to all the cells and tissues that require it. 

The quicker you breathe, the quicker your heart rate!

This supplies O2 where needed, but as the rate of respiration is increased so are the waste products.

To deal with this the rate of diffusion of (O2 and CO2) at the Alveoli (in the lungs) and at the Muscle cells increases. (Remember these gases are moving down their concentration gradients).

However, this increased diffusion and supply of O2 / removal of waste (Carbon dioxide CO2) can go on indefinitely. As you perform vigorous exercises (like Insanity, Tapout or sprinting away from crocodiles) your body is unable to supply all the O2 needed / quickly enough.

So, your cells start to respire anaerobically! (but remember this comes with its own waste product – Lactic acid). The upside is that you can carry on exercising and using those muscles a little bit longer!

After all this crazy exercise you’ll most likely be “out of breath”!

Because your breathing rate has increased and you have employed anaerobic respiration you will be “out of breath”! This is called oxygen dept.

So, you will have to repay the O2 dept. – which is why you breathe heavily following vigorous exercise. This is give a special acronym:

EPOC 

Exercise / post-exercise oxygen consumption. 

This means you’ll continue to breathe heavily – taking in lots of O2 into the bloodstream, supplying the cells and muscles. Your heart rate will remain high to deal with this extra O2 demand. The extra O2 is used to convert Lactic acid into CO2 and H2O – where you breathe them out (and sweat them out – H2O). 

*When we cover the circulatory systems and respiratory systems we will investigate the effects of exercise, so, how cellular respiration – i.e. how cells get their energy and utilise it will be revisited. You will also need to remember to connect cellular respiration with Diffusion and Active Transport!