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Plasma Membrane

Plasma Membrane

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The Fluid Mosaic Model

“The fluid mosaic model of the plasma membrane describes the plasma membrane as a fluid combination of phospholipids, cholesterol, and proteins.”


Organelle containing structure membranes:

Nucleus Golgi apparatus Vesicle
Mitochondria Endoplasmic reticulum Lysosome
Chloroplast Plasma Membrane Vesicle


Prokaryotic cells perform all of what a eukaryotic cell can do, for in prokaryotic cells they use mesosome instead of mitochondria.

The cell membrane

The cell membrane is made up of a phospholipids, which form phospholipid bilayers


Phopholipids are made up of two parts, the ‘head’ of the phospholipid is hydrophilic, and thus loves water, whereas the ‘tails’ are hydrophobic and thus repels water. They organise themselves in the above fashion so that the ‘heads’ are in water, and the tails are not. Phospholipids allow for simple diffusion and osmosis.

Cholesterol is found within the membrane, this helps ‘pack’ phospholipid into membranes which give more rigidity to the membrane. The Steroid Molecule Cholesterol gives the Plasma Membrane in some Eukaryotic Cells stability by reducing the fluidity and making the Bilayer more complete. For example in artic fish, they have large amounts of cholesterol so their cells do not freeze.

Integral proteins act as a transport of various molecules that otherwise would not be able to move across the membrane. The identification of the cell is another function for recognition by other cells; the antigens on the cell.

The ion channels open and close in order to let ions pass through the membrane into the cell. The ion channel also allows for communication to other cells.

Molecules and function

Phospholipids form a bilayer, where the hydrophilic beads of both phospholipids layers to the outside to the cell surface attracts water on both sides. The hydrophilic tails repels water.  This allows for lipid-soluble substances to enter or leave the cell. It prevents water soluble substance entering and leaving as well as make the membrane flexible and self-sealing

Proteins act either to give mechanical support to the membrane or in conjunction with glycoproteins, as cell receptors for molecules such as hormones. Some make up protein channels which form water filled tubes to allow for water soluble ions to diffuse across. Carrier proteins bind to ions or molecules like glucose or amino acids. They provide structural support, channel transporting water soluble, allow active transport, form cell surface receptors for identifying other cells, help adhere cells together and act as receptors

Cholesterol is found within the phospholipid bilayer, it adds strength to the membrane. They are very hydrophobic and thus help with water loss and ions. They pull together the fatty acid tails of the phospholipid molecules, limiting their movement and that of other molecules but without becoming too rigid. It reduces lateral movement of other molecules include g phospholipids, makes the membrane less fluid at high temperatures, prevents leakage of water and dissolved ions from the cell

Glycolipids are made from carbohydrates covalently bonded with a lipid. The carbohydrate portion extends from the phospholipid bilayer. The watery environment outside the cell, acting as a cell surface receptor for specific chemicals. Glycoproteins act as a recognition site, maintains the stability of the membrane and helps cells to attach to one another thus forming tissues

Glycoproteins are made from carbohydrate chains that are attached to many extrinsic proteins on the outer surface of the cell membrane. Glycoproteins also act as a cell surface receptor, especially for hormones and neutralisation. They help other cells bond together and allow cells to recognise each other.



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Mitosis is the division of a cell that results in 2 daughter cells which have identical copy of the DNA of the parent cells. Mitosis occurs only in ‘body cells’ (e.g. muscle, epithelial tissue etc.) compared to meiosis which only occurs in the production of gametes. Mitosis produces diploid cells compared to meiosis which produced haploids.

The 4 phases of mitosis include:


  • Prophase: Chromosomes become visible, nuclear envelope disintegrates, nucleolus disappears
    • Nucleus disappears, the chromosomes become visible in their super coiled state
    • Spindle fibres produced by the centrioles
  • Metaphase: Chromosomes line up on the centre of the cell
    • Chromosomes are seen to be made up of 2 chromatids. Each chromatid is identical to the parent cell
    • Chromatids are joined by centromere
    • The chromatids are aligned across the equator of the cell
  • Anaphase: Spindle fibres attach to the chromatids. Chromatids are pulled together towards the poles
    • Centromere divides into 2
    • Spindle fibres pull individual chromatids making up the chromosome parts
    • The chromatids move rapidly to the opposite poles of the cells, they are now referred to as chromosomes
  • Telophase: Chromosomes reach poles become indistinct. Nuclear envelope forms as well as nucleolus. Spindle fibres disintegrate
    • Chromosomes become longer and thinner widely spread chromatids
    • The spindle fibres disintegrate and the nucleus envelope and nucleolus form
    • The cytoplasm divides in a process called cytokinesis


Prokaryotic Cell Division

  • Prokaryotic reproductions is called Binary Fission.
  • This involves:
  • The circular DNA molecules replicates and both copies attach to the cell membrane
  • The plasmids replicate
  • The cell membrane begins to grow between the two molecules of NDA, dividing the original cells into the 2 daughter cells. This also divides the cytoplasm as well.
  • A new cell wall forms between the two molecules of DNA, dividing the original cells into two identical daughter cells, each with a single copy of circular DNA and a variable number of copies of the plasmids.


Virus Replication

  • Viruses are non-living, so they do not undergo cell division
  • This involves:
  • Viruses attach themselves with attachment proteins on their surface
  • They inject nucleic acids into the host cell. Into the host cell
  • Genetic Information is injected with viral nucleic acid then provides the instructions for the host cell metabolic processes to start producing the viral components, nucleic acid, enzymes and structural proteins
  • They are then assembled into new viruses.



Eukaryotic Cells
Eukaryotic Cells

Eukaryotic Cells

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 Eukaryotic Cells (summary)

Animal Cell

An Animal Cell



In most cells the nucleus is the largest organelle. It contains the DNA from the entire cell as well as the nucleolus within it. It is a membrane bound structure.  The nucleus maintains the integrity of the cell as well as controlling all activities of the cell


The nucleolus is a small, non-membrane bound sub organelle found within the nucleus it contains RNA, an important part of the production of proteins and nucleic acids with the ribosomes.


The cytoplasm fills the cell, it is made up of 80% water as well as nutrients. All of the cells organelles are suspended within the cytoplasm. Cytoplasm aids in the dissolving of nutrients and the removal of waste.


Mitochondria convert oxygen and nutrients into Adenosine Triphosphate (ATP) through a process of aerobic respiration. Mitochondria are separate from the animal cell, they reproduce on their own and store their own, separate DNA. ATP provides the energy for all actions to take place.

Plasma Membrane

Plasma membrane surrounded the cell, keeping everything contained within its borders. The membrane has selective permeability, it can decide what enters or leaves the cell.

Rough Endoplasmic Reticulum

The Rough endoplasmic reticulum (rough ER) is covered in ribosomes, their role is to synthesis and package proteins. They are attached around the nucleus. When mRNA moves from the nucleus to a ribosomes, this is behind the process of making the protein. The amino acid chains are pushed into internal space. When complete, they are collected into vesicles and pinched off.

Smooth Endoplasmic Reticulum

Smooth Endoplasmic Reticulum (smooth ER) contains no ribosomes on its surface and therefore smooth. It is a storage organelle containing lipids and steroids.


Ribosomes are the main organelle in protein synthesis. Ribosomes found free in the cytoplasm are going to produce proteins for use within the cell. Their key job is to covert amino acids into polypeptides

Golgi Apparatus

 Golgi Apparatus is made up of layers of membranes called Golgi bodies. Within the Golgi Apparatus it further process proteins, instructing them where to go as well as converting proteins to hormones are bonding proteins with carbohydrates to form different molecules.


The lysosomes are filled with enzymes with the task of breaking down cellular waste. The enzymes found within are produced by the rough Endoplasmic Reticulum. When enzymes are in the Golgi Apparatus a vesicle filled with these enzymes is produced, known as the lysosomes. They float in the cytoplasm until needed, able to restore useful material back into the cytoplasm for further use.

Lysosomes are produced by a vesicle on the Golgi apparatus which contains protease and lipase. They also contain materials that hydrolyse the cell wall of certain bacteria called lysozymes and enzymes.
As many as 50 enzymes can be found in each lysosome, each lysosome being up to 1µm in diameter. Lysosomes isolate these enzymes from the rest of the cell before releasing them, into either th e outside or into a phagocyte vesicle within the cell

Lysosome functions include:

  • Hydrolyse materials ingested by phagocytic cells (e.g. white blood cells and bacteria)
  • Release enzymes to the outside of the cell (exocytosis ) in order to destroy materials around the cell
  • Digest worn out organelles so that the useful chemicals they are made of can be reused within the cell
  • The complete breakdown of cells after they have died (autolysis)
  • They are especially abundant in secretory cells such as epithelial cells and in phagocytic cells.


The centriole is used for the division of cells both in mitosis and meiosis They are made up of microtubules arranged into a group when. When the cell divides, threads to the centriole called mitotic spindles.

Cell Wall

Found in all plant cells, a cell wall contains microfibrils of the polysaccharide Cellulose, embedded into a matrix. Cellulose microfibrils have considerable strength and so contributes greatly to the strength of the cell wall.

Cell walls have the following features:

  • They include a number of polysaccharides including cellulose
  • The boundary mark between adjacent cell walls is called the middle lamella. This thin layer also “cements” adjacent cells together.

The function of the cellulose cell wall are:

  • To provide mechanical strength in order to prevent the cells bursting under the pressure created by the osmotic entry of water.
  • To give mechanical strength to the plant
  • To allow water to pass along it and so contributes to the movement of water through the plant

The cell wall of algae are made up of cellulose or glycoproteins of a mixture of both.

The cell wall of fungi does not contain cellulose but comprises a mixture of nitrogen containing polysaccharides call chitin, a polysaccharide call glycan and glycoproteins.


Measuring Cells

Measuring Cells

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Light Microscopes are limited due to the long wavelength of light it limits the ability to view small parts of the cell, for example like the Ribosomes.

Electron Microscopes are 2000x better than light microscopes. They can see objects down to 0.1nm. Electrons are asorbed or deflected in air therefore a vacuum must be used. As a result the specimen must be dead before it can be taken.

There are 2 types of Electron Microscopes:

  • Transmission Electron Microscopes (TEM)
  • Scanning Electron Microscopes (SEM)

The Transmission Electron Microscope is an electron gun that produces a beam that is focussed on to the specimen, by a condenser electromagnet. The electrons passes through a thin section.  Parts of the specimen absorbs electrons and therefore appears dark. An image is produced called a photomicrograph. The resolving power is 0.1nm, although problems with specimen preparations means that this cannot always be achieved. The main limitations of TEMs:

  • The entire system must be in a vacuum, so dead specimens must be used
  • Complex ‘staining’ process is required
  • The resulting image is in Black and White
  • The image may contain artefacts

The specimens must be extremely thin to allow for the electrons to penetrate for it. The images are 2D, however multiple samples can build up a 3D image.

The Scanning Electron Microscope shares many of the same flaws as the SEM

  • The entire system must be in a vacuum, so dead specimens must be used
  • Complex ‘staining’ process is required
  • The resulting image is in Black and White
  • The image may contain artefacts
  • Not as high of a magnification as TEM (highest magnification yields of TEM 0.1nm, compared to SEM 20nm)

The SEM directs its beam of electrons on to the surface of the specimen from above, compared to the TEM which fires electron beam from below the specimen upwards. This difference allows for the electrons to be passed back and forth across a portion of specimen in a regular pattern. The electrons are scattered by the specimen and the pattern of this scattering depends on the countours of the specimen’s surface. We can build 3D images of these images.

Units to remember:

Unit How many cm in each
Centimetre (cm) 1
Millimetre (mm) 10
Micrometre (µm) 10,000
Nanometre (nm) 1,000,000