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School Biology revision notes: Cell specialisation 2. Examples described

Cell specialisation: 2. Examples of specialised animal cells adapted to their functions

Doc Brown's GCSE level Biology exam study revision notes

There are various sections to work through, after 1 they can be read and studied in any order.

Sub-index for notes on stem cells, cell differentiation and cell specialisation


(2) Examples of specialised animal cells adapted to their functions

On the right is a diagram of the basic structure of an animal stem cell which has the features characteristic to most cells e.g. membrane, nucleus of DNA/RNA for instructions, mitochondria for respiration, cytoplasm, vacuoles, ribosomes for making proteins etc.

In most animal cells, this ability to differentiate is lost at early stage of development once the cells have become specialised. But, most plant cells retain the ability to differentiate throughout the life of the plant.

The cells that differentiate in mature animals are mainly used for replacing damaged/dead cells e.g. skin or blood cells.

Skin cells live for 14 to 21 days and blood cells have a lifetime of 80 to 120 days.

Reminder: Other cells, stem cells, are undifferentiated.

For more on stem cells see later section on stem cells and their potential uses

Note that stem cells are undifferentiated and have not changed into a specialised cells in the developing embryo.

Note that you can often work out how the structure of the cell relates to its function in the organism.

e.g. consider its shape, size and surface area AND the subcellular structure it contains e.g. organelles like ribosomes and mitochondria, and also food stores like fat cells.

In other words note how specialised cells have evolutionary adaptations to carry out their function.

Several examples of specialised cells are described below


Gamete cells - the function of egg cells and sperm cells in sexual reproduction - adaptations described in detail

Its important to compare male and female gametes in terms of size, structure, motility, numbers and adaptations

Egg cells and sperm cells are the specialised cells of sexual reproduction.

In sexual reproduction the nucleus of an egg cell fuses with the nucleus of a sperm cell to produce a fertilised egg.

The fertilised egg develops into an embryo.

Both the sperm cell and an egg cell are referred to as being haploid, because their nuclei only contain half the number of chromosomes that you find in a normal body cell.

This ensures that when the egg and sperm nuclei combine at fertilisation the created cell will have the right number of chromosomes (now referred to as a diploid cell see diagram below).

With reference to the human reproduction diagram above:

The zygote is a diploid cell resulting from the fusion of two haploid gametes (sperm + egg cells) to give a fertilized ovum.

For the first 5 days the cells from the fertilised egg divide by mitosis producing identical cells.

The egg cell - its structure and its adapted functions

  Simple diagram of human egg cell

The principal function of the female egg cell is to convey the female DNA and to provide nutrients for the developing embryo in the early stages of the organism's development.

The egg cell has a haploid nucleus (half set of 23 chromosomes) and the cytoplasm contains the nutrients to feed the growing embryo - its nourishment.

Unlike the sperm cell, it has no flagellum to move independently.

Adaptations:

(i) The egg cell has a substantial energy store to 'fuel' the fertilised egg development and lots of accompanying mitochondria to power the developing cell's chemistry.

(ii) Immediately after fertilisation the egg cell's extra jelly coating (a gelatinous layer outside the cell membrane) changes chemically in structure to stop another sperm getting into the egg cell and this ensures the right amount of chromosomal DNA is present in the fertilised cell.

The human egg cell is bigger than most body cells and 10,000 times bigger than the male sperm below!

The egg cells need to accumulate lots of nutrients to support a growing embryo after fertilization, plus many mitochondria to power the growth of the embryo.

At birth, the normal female ovary contains about ~1.5 million immature eggs.

Females cannot make new eggs, and there is a continuous decline in the total number of eggs each month.

By the time a girl enters puberty, only about 25% of her lifetime total egg pool remains, around 300,000.

This contrasts with the millions of sperm cells the male can produce.

See also CELL DIVISION - cell cycle - mitosis and meiosis in sexual reproduction

 

The sperm cell - its structure and its adapted functions

 

Simplified diagram of human sperm cell

The sperm cell, like the egg cell has a haploid nucleus (half of the full set of required chromosomes).

It is about 55 m long and about 3 m wide.

The principal function of a male sperm cell is to convey the male's DNA and combine with the female's DNA in the egg cell, to fertilise the egg in the female reproductive system - to produce the zygote.

The sperm cell has a long tail (flagellum) and streamlined head to enable it to swim efficiently over relatively large distances to meet the female egg (the egg cell  cannot move of its own volition..

A sperm cell contains lots of mitochondria arranged in a spiral in the middle section.

Mitochondria are sites of respiration - releasing energy to provide the energy for it to swim to the egg cell, as well as the rest of the cell chemistry. Three adaptations to 'head' for fertilisation.

At the front of the head of a sperm cell is an acrosome where enzymes are stored.

These enzymes are needed so that the sperm cell can digest its way through the membrane of the egg cell to fertilise it and enable the fusion the of the male and female nuclei.

Sperm cells are produced in much large numbers than egg cells to ensure the fertilisation of the egg cell.

A fertile man may produce an average total of 80 to 300 million sperm per ejaculation.

The total per life time far exceeds the much lower number of female egg cells - no extra of which can be made in the ovary.

See also CELL DIVISION - cell cycle - mitosis and meiosis in sexual reproduction


Ciliated Epithelial Cells - these line the surface of organs

 

Simple diagram of epithelial cells

Organ surfaces are lined with epithelial cells.

Some types of epithelial cells are adapted with hairs called cilia on the top of the cell's surface.

The function of these ciliated epithelium cells is to move substances in one particular direction along the surface of the tissue.

The hair-like structure of the cilia beat in tandem to move the material along.

A good example is the lining of your air passage, the surface of which is covered in lots of epithelial cells.

The 'beating' cilia move mucous and any particles from air trapped on the surface up the throat and away from your delicate lungs.

This allows the mucous to be swallowed or blow out through your nose.

Ciliated epithelial cells contain lots of mitochondria to power the cells chemistry including the energy needed to move cilia to move 'stuff' along! - for most cells this is not an extra energy requirement - but note energy requirements of muscle cells in the next section.

See also Introduction to the organisation of cells => tissues => organs => organ systems (e.g. in humans)


Muscle cells

Muscle cells form soft tissue found in most animals, they are relatively long and must be able to contract quickly.

Muscle cells have a striped appearance.

They contain protein filaments of actin and myosin that can slide past one another.

This adaptation produces a contraction that changes both the length and the shape of the cell.

The contraction can be reversed and allows muscle tissue cells to function in such a way as to produce force and motion.

Muscle cells contain lots of mitochondria to supply the larger amounts of energy from respiration needed to work the muscles.

There are actually three types of muscle cells, all adapted for different movement effects:

(i) heart muscle cells - found in the walls of the heart and under involuntary control - automatically work,

cardiac muscle tend to be more chunky (than the diagram above!).

(ii) skeletal muscle cells - in muscle tissue and are the most striated in appearance,

muscle tissue is under voluntary control, and the fibres join up (in development) to give strength and co-ordinated movement

(iii) smooth muscle cells - these are 'spindle-shaped' (like the diagram above) and found in 'hollow' organs e.g. stomach, intestine, bladder,

these are also under involuntary control - automatically work.

See also Introduction to the organisation of cells => tissues => organs => organ systems (e.g. in humans)

and notes on the skeleton and muscle systems


Nerve cells

Neurones are nerve cells that have adapted to carry information as tiny electrical signals from one part of the body to anther e.g. between the brain and muscles.

On the right is a simplified diagram of a nerve cell.

These sensory neurone cells are elongated and adapted to cover the relatively 'long' distances over which they have to carry the nerve signals from receptors to the spinal cord and brain.

Nerve cells have branched connections at their ends to form a network of connections with other cells throughout the body.

For much more on the structure of nerve cells and how the nervous system works see ...

An introduction to the nervous system including the reflex arc  gcse biology revision notes


Blood Cells

Red blood cells

Red blood cells are adapted to carry oxygen via the haemoglobin molecules inside them.

Their shape is biconcave to give a larger surface area to absorb oxygen molecules more efficiently to combine with haemoglobin and transport them around the body where they needed e.g. in mitochondria.

Without this adapted cell transportation of oxygen you could not have efficient energy releasing respiration in mitochondria.

For much more detail see The human circulatory system - heart, lungs, blood, blood vessels

 

White blood cells

White blood cells are part of the immune system.

White blood cell change shape to engulf a microbe thereby ingest disease-causing bacteria and destroy them.

White cells can produce antibodies to destroy pathogens.

For much more detail see The human circulatory system - heart, lungs, blood, blood vessels


Summary of learning objectives and key words or phrases

Be able to describe examples of specialised animal cells that have become adapted to their specialised functions through the course of evolution.


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