Cells

What is a Cell ?

Cells were first discovered in by 1665 by Robert Hooke who used a microscope he built himself to observe microscopic living things in drops of lake water.

A Cell is the basic building block of life. There are millions of different types of cells. Some living things contain millions of cells, some are just one cell. Not all cells look the same and some cells have different parts to do different jobs.

Animal Cells

There are millions of different types of animal cells, each within a specific Job. However, all animal cells have four parts in common. They all have:-

  1. Cell Membrane - The 'skin' of the cell, The cell membrane controls the movement of substances into and out of the cell.

  2. Nucleus - The 'brain' of the cell, The nucleus controls everything that happens within the cell. The nucleus contains the genetic material.

  3. Cytoplasm - The cytoplasm is a 'jelly-like' substance, most chemical reactions take place here.

  4. Mitochondrion - The mitochondria (plural of mitochondrion) are the site of aerobic respiration within the cell.


Plant Cells

Just like animal cells, there are millions of different types of plant cells, each within a specific job. A plant cell has:-

  1. Cell Wall - Gives the cell is rigid shape.

  2. Cell Membrane - The 'skin' of the cell, the cell membrane controls the movement of substances into and out of the cell.

  3. Nucleus - The 'brain' of the cell, the nucleus controls everything that happens within the cell. The nucleus contains the genetic material.

  4. Vacuole - Filled with cell sap, works with the cell wall to give the cell its shape.

  5. Cytoplasm - The cytoplasm is a 'jelly-like' substance, most chemical reactions take place here.

  6. Chloroplasts - Chloroplasts are only found in leaf cells, they contain the chemical Chlorophyll, which absorbs light energy for Photosynthesis.

  7. Mitochondrion - The mitochondria (plural of mitochondrion) are the site of aerobic respiration within the cell.


Cell Functions - Case Study 1 - Cell Membrane

As stated above, the cell membrane's function is to control the movement of substances into and out of a cell.

But why do cells need to have substances pass in and out of them? There are lots of reasons as to why this happens, the most important being to allow the transfer of chemicals vital to life.

For example , the cells of a plant leaf need to take in the chemicals required for Photosynthesis, and get rid of waste products:-

For large molecules, such as Glucose, the cell membrane has special channels to allow them to enter of leave the cell. But very small molecules, such as water, Carbon Dioxide or Oxygen, can pass through the cell membrane through the process of diffusion.

Diffusion is the process of a substance moving from a high concentration to a low concentration through a permeable barrier. The substance will continue to move through the membrane until the concentration on each side is equal.

The diagram below shows the diffusion of a chemical through a permeable membrane:-

In the lab, we can demonstrate the process of diffusion using gummy bears. Gummy bears are made of a jelly containing sugars and water. By placing the gummy bears in different liquids, the process of diffusion can be seen:-

In the above demonstration, the gummy bear placed in:-

  1. Pure water has expanded. It does this because there is a lower concentration of water within the gummy bear than the solution around it, therefore water moves into the gummy bear, causing it to swell up.

  2. Strong salt solution has contracted. It does this because there is a higher concentration of water within the gummy bear than the solution around it, therefore water moves out of the gummy bear, causing it to shrink.

Cell Functions - Case Study 2 - Mitochondria

Mitochondria are the site of aerobic respiration in a cell. Respiration is used to release energy for use in cells.

When Oxygen is present, Glucose is broken down to release Carbon Dioxide, water and Energy.

The above reaction can be demonstrated using a Screaming Jelly Baby reaction:-

In the screaming jelly baby reaction, a boiling tube is used to represent a mitochondrion. Within the tube, Potassium Chlorate is heating until it becomes molten. .

Potassium Chlorate is an oxidiser, providing a source of Oxygen for the reaction. A Jelly Baby is uses as a source of Glucose and is dropped into the boiling tube.

When the jelly baby enters the molten Potassium Chlorate, the jelly baby undergoes a vigorous reaction, following the above formula. This causes the release of lots of water vapour and Energy in the form of light, heat and sound.


The video belows shows a demonstration of the Screaming Jelly Baby Reaction:-

As can be seen from the image below, this reaction is quite impressive!

Cell Functions - Case Study 3 - Nucleus

The function of the nucleus is to control all processes within the cell. Within the nucleus is found the chemical - DNA (Deoxyribonucleic Acid).

This DNA is a 'Blueprint' or set of instructions for how the cell should be constructed and function. Every cell within an organism contains the full DNA sequence for the entire organism.

The DNA can be extracted from the nucleus of a cell using the following procedure:-

The image below shows the DNA layer within the boiling tube:-

The Structure of DNA

A gene is a section of DNA which controls specific characteristics in an individual. Every individual has a unique combination of genes, which are located on chromosomes in the nucleus of a cell:-

Key Definitions in Genetics


  1. Cell - Basic unit of life.

  2. Nucleus - Structure in the cell that controls cell activities.

  3. DNA - Chemical that carries the genetic code.

  4. Chromosome - Thread-like structures that are made from long strands of DNA.

  5. Gene - Section of the DNA sequence that codes for a single characteristic.


DNA Structure

The diagram below shows the structure of DNA:-

DNA consists of a double-helix (a type of spiral) made of a sugar Phosphate backbone, connected together by combinations of four DNA Bases.

These bases always form two pairs:-

A binds to T

C binds to G


The specific order of these base pairs within the DNA is what controls what the genes code for.

Chromosomes

Different organisms have different numbers of Chromosomes that make up their genetic sequence, but all organisms have their Chromosomes organised in pairs.


For example:-

Jack Jumper Ant - 2 Chromosome Pairs.

Fruit Fly - 8 Chromosomes Pairs.

Carrot - 18 Chromosome Pairs.

Humans - 23 Chromosome Pairs.

Cat - 38 Chromosome Pairs.

Hermit Crab - 127 Chromosome Pairs.

Adder's Tongue Fern - 630 Chromosome Pairs.


Human Genetics

The diagram below shows the 23 pairs of chromosomes which make up the human genetic sequence:-

Each of the chromosomes above contain many different genes which control all functions within the human body.

For example:-

Chromosome pair 4 carries the genes for making blood plasma proteins.

Chromosome pairs 2 and 12 carry the genes for making T cells which form part of the immune system.

Chromosome pair 21 can sometimes have an extra chromosome, creating a chromosome triplet 21. People with this chromosome triplet have Down Syndrome.

Chromosome pair 23 are the sex chromosomes. Their combination determines the biological gender of the human:-

Genetic Engineering

The understanding of the genetic sequence that has been developed over the last 70 years has allowed scientists to make changes to the genetics of an organism to produce a new organism with desirable characteristics.

An organism made in this way in known as a Transgenic Organism.

For example:-

Lots of animals have been genetically altered in this way, for a variety of reasons:-

Specialised Cells

All animal and plant cells contain the parts shown above, but these cells all look very different, depending on the jobs they do. Below is some examples of the ways that cells can be specialised:-


Red Blood Cell

The image above shows red blood cells. Red blood cells carry Oxygen around the body. A red blood cell is specialised in two ways:-

  1. Contains Haemoglobin, a chemical that can carry Oxygen.

  2. Has a biconcave shape, giving it a big surface area.


White Blood Cell

The image above shows a white blood cell. White blood cells are part of the body's immune system. A white blood cell is specialised in two ways:-

  1. Makes antibodies, chemicals which attack virus' and bacteria in the body.

  2. Can change shape, to engulf bacteria and digest them.


Nerve Cell

The image above shows a nerve cell. Nerve cells transmit signals around the body. A nerve cell is specialised:-

  1. Long and thin, to quickly send signals long distances in the body.


Root Hair Cell

The image above shows a plant's root hair cell. A root hair cell is specialised:-

  1. Long and thin, to give a large surface area for the transfer of water and nutrients from the soil.


Guard Cell

The image above shows a plant's leaf guard cell. The guard cells allow water to leave the leaf. A guard cell is specialised:-

  1. Can open and close, to let water leave the leaf, or to stop water leaving when needed.


Egg Cell

The image above shows an egg cell. Egg cells are part of the reproductive system. An egg cell is specialised in two ways:-

  1. Nucleus contains only 1/2 the DNA required to create a new organism.

  2. Has a lot of cytoplasm containing nutrients to keep the cell alive until implantation.


Sperm Cell

The image above shows a sperm cell. Sperm cells are part of the reproductive system. A sperm cell is specialised in two ways:-

  1. Nucleus contains only 1/2 the DNA required to create a new organism.

  2. Has a tail, to help it 'swim' to the egg cell.