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AS Cells and Membranes
Transcript of AS Cells and Membranes
AS: Cells and Membranes
Year 1 - AVD 3 Lessons per week
This section is going to look at how cells were first discovered and consider some of the defining characteristics of cells.
All living organisms are made up out of one or more cells. An Amoeba consists of one cell, and is unicellular, whereas we contain over a trillion cells and are multicellular
All living organisms, be they uni- or multicellular, carry out all the essential processes that define what we term alive.
You may remember the mnemonic MRS GREN.
What do the different letters stand for?
Only some cells are large enough to be seen with the naked eye. Most are microscopic, and as a result cells were not discovered until the development of the microscope in the 17th century
A compound light microscope
You need to be aware of the SI units of measurement that are used when describing cells and cell structures.
These include millimetres (10-3 m), micrometres (10-6 metres) and nanometres (10-9 metres).
A bacterial cell is typically between 0.5-10 micrometres in size, whereas a eukaryotic cell are typically between 50-150 micrometres. Some are larger.
A light microscope can view structures that are about 200nm or larger. Smaller structures can only be seen with an electron microscope. They can separate structures of as small as 0.1nm.
The first cells to be seen by Hooke in 1662 were cork cells. He was the first to use the term cells as the structures reminded him of cells in a monastery or prison.
In 1680 Van Leeuwenhoek identified living organisms in pond water that were not visible by the naked eye. He called these "animalcules". He was also the first person to look at blood and other bodily fluids.
Schwann and Schleiden in the 1830's first postulated that all living organisms were made up out of cells, and that nothing could be alive it wasn't made up out of cells.
Cells were found to have these defining characteristics (with the latter two added in the 20th century):
Cells are the building blocks of all living organisms
Cells are the smallest unit of life
Cells are derived from other cells by division
Cells contain information for their growth, development and behaviour
Cells are the site of metabolism - the chemical reactions that life carries out.
1. Cells are the building blocks of all living organisms
All multi-cellular organisms consists of many individual, specialised cells
Viruses do not consist of cells
2. Cells are the smallest component of life
Individual organelles isolated by cellular fractionation can carry out some of the tasks that are essential to life, but won't be able to carry them all out
3. Cells are derived from other cells by division
Life does not generate spontaneously (Pasteur's experiment)
Cells are generated through mitosis
Except for gametes which are generated by meiosis
4. Cells carry all the information required:
All cells carry the entire genome of the organism (with some exceptions where no nucleus is present such as erythrocytes)
Specialised cells have their genome modified by epigenetic modifications to control which genes are transcribed.
These can revert back to a totipotent state through chemical intervention.
5. Cells are the site of metabolic reactions:
Some reactions happen outside of cells but the enzymes required will have been secreted by cells.
Respiration, DNA replication and protein synthesis occurs in all cells.
You need to be able to calculate the actual size or the magnification from an image. You use this formula which you might need to rearrange depending on which variables are given.
You need to remember a few guidelines in this process:
You might need to convert units so you have the same unit in image and actual size.
Any measurements should be taken at the widest/longest point of a sample.
For organisms to grow above a certain size they need to be made up of more than one cell.
This is because increasing cell size leads to issues with the surface area to volume ratio.
Work out the surface area, volume and SA:V for cubes with sides of 1, 2, 3, 4, 5, 6, 7, 8.
Sketch a graph of your results
SA:V affects the rate of absorption of substances into a cell (it can only come in through the surface) and it increases the diffusion distance of substances within a cell, affecting the rate of chemical reactions.
Use your previous calculations to work out the improvement in the SA:V ratio if a 4x4x4 cube splits into two 4x4x2 boxes.
Cells of multi-cellular organisms do not all look identical. Cells will share certain common characteristics but be adapted to fulfill a specific function.
This is called specialisation and happens through the differentiation of stem cells into specialised cells.
Common features of almost all cells include:
A sac of fluid material (cytoplasm)
Surrounded by a membrane
Containing a nucleus
Cells are different by the number and types of other structures that are found in the cell.
Animal and plant cells, whilst all sharing the three common features, have some significant differences:
Plants contain a cellulose cell wall whereas animal cells do not.
Some plant cells contain chloroplasts, no animal cells do
Plant cells have a large, permanent, sap-filled vacuole to allow it to control its turgidity without affecting the water potential of the cytoplasm
Plant cells do not have a centrosome, which controls the formation of microtubules of the cytoskeleton.
Carbohydrates are stored as glycogen in animal cells, but starch in plants
Cells in most multi-cellular organisms are organised into tissues carrying a specific function.
These tissues can be organised into groups of different tissues carrying out a specific function. These are called organs.
Each cell contains the same number of chromosomes carrying the same genes.
In cells that are adapted to a different functions different genes will be expressed. When the cell first differentiates these expression patterns are fixed.
This means that a muscle cell will always remain a muscle cell and not suddenly turn into a cone cell of the eye.
These expression patterns are typically kept the same when the cell divides, so that a muscle cell divides into two muscle cells.
Cells that have not differentiated and can still turn into any other type are known as stem cells.
Stem cells that have the possibility to turn into any cell type are known as totipotent. They are only found in the early embryo.
Stem cells that can turn into a number of cell types (such as the haemopoietic stem cells in the bone marrow) are known as pluripotent.
Stem cells have a large number of possible therapeutic applications, as theoretically they would allow us to repair any damaged tissue. This could include repairing heart muscle after a heart attack, regrowing severed nerves in the spinal cord or even growing entire organs in the lab.
There are two main stem cell types that this course requires you to know, though there have been significant developments in this field and I would recommend you look at third.
Embryonic stem cells
Adult stem cells
Induced pluripotent stem cells (iPS), which can turn into any cell type except the placenta
Light microscopes allow you to magnify a specimen by up to 1500x. This allows you to see some of the structures inside the cell, but many are too small to be seen even then.
To view specimens at even higher magnification you need to use a electron microscope. These can magnify a specimen by up to 500000x.
This is because the wavelength of an electron is much smaller than that of light.
Transmission electron microscopes work similarly to a light microscope by sending electrons through a sample, which has to be cut very thinly.
Scanning electron microscopes work by bouncing electrons off of a gold-covered specimen, allowing you to look at the surface of a specimen.
Using an TEM allows you to visualise more of the organelles of the cell.
We will first look at the organelles of a typical animal cell (mammalian liver cell)
The nucleus of the cell is surrounded by the nuclear membrane. This protects the DNA, packaged in chromosomes, from the enzymes in the cytoplasm of the cell.
There are pores in the nuclear membrane that allow mRNA out of the nucleus and raw materials into it.
The nucleolus is where most of the transcription occurs.
All organelles are suspended in the cytoplasm, which is a fluid matrix given shape by an internal cytoskeleton made up out of protein strands.
Mitochondria are the site of aerobic respiration and the formation of ATP through oxidative phosphorylation.
They are surrounded by a double membrane and can be identified by the cristae formed by the inner membrane into the matrix (cytoplasm equivalent).
Mitochondria carry their own DNA.
Ribosomes are responsible for translation of the mRNA and the formation of polypeptides.
Some ribosomes are free in the cytoplasm and they produce proteins that remain within the cell.
Rough endoplasmic reticulum (rER) is the site of the synthesis of proteins that are secreted from the cell. Ribosomes are bound to membrane structures and the polypeptide is packaged into a membrane vesicle.
The smooth endoplasmic reticulum also consists of membranes, but no ribosomes are attached.
It carries out some of the aspects of the synthesis of lipids, hormones and phospholipids.
Lysosomes are membrane-bound vesicles that contains enzymes within the cell. The enzymes are stored in these vesicles to prevent them from interfering with the cell's processes until needed.
The Golgi apparatus packages proteins and other macromolecules for transport within and out of the cell. The microtubules transport the vesicles that bud off to their destination.
The cell membrane forms the outer boundary of the cell. We will look at it in more detail soon.
It is made up out of a phospholipid bilayer. Various protein are embedded into this, including transport proteins, enzymes and glycoproteins that act as receptors.
Plant cells contain a number of extra components:
Chloroplasts are membrane-bound organelles found in the green cells of a plant. They are the site of the light-dependent and light-independent reactions of photosynthesis.
A cellulose cell wall gives the plant cell shape and takes the place of an internal or exoskeleton. It prevents the cell from bursting when the cell is turgid.
Water can also move through the cell wall.
Plant cells have a large internal vacuole that in which it stores cell sap (a mixture of water, sugars and amino acids)
Prokaryotic cells are cells without any membrane-bound organelles. They are Bacteria or Archaea.
Bacteria are surrounded by a cell wall, to prevent them from bursting. This wall is made from peptidoglycan.
The bacterial cell has a cell membrane underneath the cell wall, fulfilling the same function as in eukaryotic cells
Bacterial ribosomes are smaller and never membrane-bound. They are 70S ribosomes rather than 80S ribosomes.
Bacteria do not have a membrane surrounding their genetic material.
It consists of a single circular chromosome called the nucleoid. Bacterial cells often have small extra chromosomes known as plasmids.
Pili allow bacteria to attach to other bacteria, exchange plasmids or form aggregates.
Many bacteria can use a flagellum to move through their environment. Bacterial flagella are 20nm in diameter whereas eukaryotic flagella are 200nm in diameter.
Bacterial DNA is not associated with histones.
Draw a table comparing the features of prokaryotic, animal and plant cells.
The cell membrane is a key component of all cells. As we saw earlier, it controls which substances can move into, and out of the cell as well as giving the cell shape.
All of these functions are a result of its structure.
The structure of the membrane is often described as the fluid mosaic model.
It is fluid because the components are not fixed in one place, and mosaic because it consists of a number of different components.
These components are:
Phospholipids form the bulk of the lipids in the membrane.
It consists of a glycerol which two fatty acid tails. The third carbon of the glycerol is bound to a phosphate group (which has a choline attached to it).
The fatty acid tails cannot form hydrogen bonds with water and are thus called hydrophobic.
The oxygen atoms on the phosphate however can form hydrogen bonds with the water and is classed as hydrophillic.
A single layer of phospholipids on water will therefore orientate themselves with the phosphate heads into the water and the fatty acid tails sticking out.
When mixed into a aqueous environment the phospholipids form bilayers where the hydrophobic regions are attracted to each other, pointing away from the water surrounding them and the hydrophillic region is pointed outwards.
Proteins are embedded into the phospholipid bilayer.
These can be trans-membrane, integral or peripheral.
They can act as transport channels, enzymes, carriers or receptors.
A trans-membrane protein is a type of integral protein which spans the entire bilayer and can interact with the outside and the inside of the cell. Examples include transport channels.
Not all integral proteins span the entirety of the plasma membrane. Some are embedded into the inside or outside layer of the bilayer.
Peripheral proteins are associated with the membrane but not embedded in them. They can be bound to other proteins or to the phospholipids
Carbohydrate chains can be attached to both the phospholipids and the membrane proteins in the outer layer of the membrane. This forms glycolipids or glycoproteins respectively.
These acts as receptor sites, are used in cell-to-cell interactions and will form part of the extracellular matrix.
Functions of membrane proteins
Channels that allow specific molecules to diffuse through - this process is called facilitated diffusion.
Pump proteins, such as the sodium-potassium pump, require energy in the form of ATP to move substances against their concentration gradient. This is an example of active transport.
Electron carriers such as those used in the light-dependent reaction of photosynthesis and the electron transport chain of respiration
Membrane proteins can be enzymes, catalysing reactions within, outside or inside of the cell depending on where the active site is located.
Some hormones cannot travel through the membrane (steroid hormones can) and will have receptor (typically a glycoprotein) on the outside of the membrane which sets of a chain of events inside the cell.
Other glycoproteins (as well as glycolipids) help with interactions between neighbouring cells as well as with cells from the immune system.
Movement across the membrane
Whilst the membrane controls the movement of substances in and out of the cell, many substances do need to move through the membrane
These substances include:
respiratory gases (oxygen, carbon dioxide)
nutrients (sugars, amino acids, fatty acids, vitamins)
ions (including Na , K , Ca )
waste materials (urea (animals only), ammonium)
substances that are secreted (hormones, digestive enzymes, neurotransmitters etc.)
proteins such as collagen that make up the extracellular matrix
cellulose and hemicellulose to make cell walls
There are four main mechanisms for the movement of substances across the cell membrane:
diffusion (including facilitated diffusion)
All particles (atoms, molecules and ions) in fluids (liquids and gases) undergo continuous random movement.
As a result, if left for enough time, constituent parts of a gas mixture or solutes in solution will be evenly spread through the available space.
Diffusion is the free passage of particles from a region of their high concentration to a region of low concentration.
The energy required for this process is provided by the kinetic energy of the molecules.
Molecules have a greater chance of moving from an area of high concentration to an area of low concentration than vice versa, so over time the concentrations will be equal throughout the fluid.
Diffusion happens in cells all the time. It can't however occur for all substances across the cell membrane.
Diffusion across the cell membrane can only occur when:
The plasma membrane is fully permeable to the solute. The bilayer is permeable to non-polar molecules such as glycerol, steroid hormones, oxygen and carbon dioxide.
Non-specific protein pores can allow certain polar substances such as water to diffuse through (as we'll see in osmosis).
Facilitated diffusion is a special case of diffusion, where a substance can only diffuse through the membrane if a specific protein channel is present.
This allows polar and/or charged molecules to cross the plasma membrane.
Facilitated diffusion is a passive process requiring no energy input. Examples include the movement of ADP into mitochondria and ATP out of mitochondria
Osmosis is the diffusion of water across a partially permeable membrane.
A partially permeable membrane lets certain substances through but prevents others.
The cell membrane is an example of a partially permeable membrane.
Water potential depends on the concentration of solutes in the solution.
Dissolved substances attract water molecules that are bound to them by hydrogen bonds and weak chemical bonds.
As a result the movement of water molecules is restricted and they move slower or are held stationary.
In a more concentrated solution, more of the water molecules are prevented from moving freely.
In pure water all water molecules can move freely.
As a result water molecules will diffuse from a dilute solution (with a higher water potential) to an area with a lower water potential (the more concentrated solution) across a partially permeable membrane.
This is osmosis, and is also a passive process requiring no energy to be put into the system..
Water molecules that are hydrogen-bonded to solute molecules are far less likely to diffuse across a partially permeable membrane than free water molecules.
The processes we have looked at so far have been passive and reliant on concentration gradients.
Cells will however need materials that need to move against the concentration gradient.
Substances can only move against concentration if energy is used to "pump" the substances up the gradient.
This process is known as active transport. In cells the energy is supplied by the dephosphorylation of ATP.
Active transport occurs against a concentration gradient. Cells store reserves of essential ions and molecules but will still look to absorb more.
Examples include ions in the root hair cells of plants, or calcium ions in skeletal muscle.
Active transport is highly specific. Only those substances that are needed by the cell will be absorbed in this fashion, as otherwise the cell would be wasting energy.
Active transport requires pump proteins (or carrier proteins) in the cell membrane.
Most of these are specific to a specific substance.
Examples of an antiport pump protein includes the sodium-potassium pump essential for establishing precise charge differentials across membranes.
An example of a symport pump is the sodium-glucose co-transport protein in the epithelial cells lining the small intestine.
The active transport in this process is carried out by a sodium-potassium pump however and this is more of an example of facilitated diffusion.
The bulk transport of material can be used to move entire pathogens into a macrophage or to move neurotransmitter out of a synaptic knob in the nervous system.
The process of moving a substance into a cell by bulk transport is known as endocytosis, moving out is exocytosis.
These processes takes a lot of energy.
Both of these methods rely on the structure of the membrane.
Two membranes can fuse together and little vesicles can bud from the membrane.
In exocytosis the vesicle inside the cell fuses with the cell membrane, releasing its contents into the tissue fluid surrounding the cell.
In endocytosis the cell moves its membrane around the substance to be absorbed.
This is most obvious in phagocytosis (the bulk transport of solid particles) carried out by macrophages.
The absorption of liquids in this process is known as pinocytosis.
All new cells arise from existing cells.
To ensure that daughter cells are identical to the parent cell it is essential that the genetic information is preserved during the division.
Cell division resulting in identical daughter cells is known as mitosis.
The cycle of growth and division in cells is known as the cell (division) cycle which has three main stages: interphase, mitosis and cytokinesis.
Interphase is the longest part of the cell cycle and is the time when the cell carries out its normal activities.
Interphase is divided into three phases:
G1 - the first growth phase. During this time proteins are produced, organelles are replicated and the cell is carrying out all the various metabolic activities that it is specialised to do.
S - DNA synthesis phase. During this phase each chromosome replicates, forming two identical chromatids attached by a centromere.
G2 - the second growth phase. The cell continues to grow and prepares itself for mitosis.
Mitosis can also be subdivided into mulitple phases. These are:
At the end of mitosis cytokinesis occurs.
During prophase the chromosomes become visible as long, thin threads.. By the end of prophase you end up with the distinctive chromosomes consisting of two chromatids joined by a centromere.
The nucleolus breaks down and the nuclear membrane disintegrates.
The centrioles replicate and move to opposite ends of the cell.
During metaphase the centrioles form spindle fibres from microtubules which attach to the centromere of each chromosome.
All chromosomes align themselves on the equator of the cell.
During anaphase the centromeres divide and the paired chromatids split and are pulled towards the opposing poles by the microtubules attached to the centrioles.
During telophase the chromosomes start to decondense at the poles and the spindle will disappear.
The nuclear membrane and nucleolus will reappear.
During cytokinesis the animal cell membrane invaginates and 'pinches' the cell in half, equally dividing the cytoplasm and organelles.
Mitosis ensures that the daughter cells are identical by:
Making an exact copy of each chromosome during interphase
Chromatids remaining attached to each other by the centromere during metaphase.
Each chromatid being pulled to an opposing pole during anaphase.
Two nuclei forming containing one copy of each chromatid prior to cytokinesis.
Cytokinesis occurring through invagination at the equator of the cell, equally dividing the cell in two.
Why is this important?
Cancer is caused by cells dividing when they should not.
As they divide uncontrollably by mitosis the cancerous cells form an irregular mass of cells known as a tumour
Most cancer are caused by mutations in genes that control the cell cycle.
These mutations can happen naturally as cells divide, be passed on through generations or be acquired as a result of exposure to mutagens such as ionising radiation or components of tar in cigarette smoke.
For example, the protein encoded by the gene p53 normally prevents the cell from dividing when the DNA is damaged.
An electron microscope has a higher resolution than a light microscope because the wave length of an electron beam is much shorter than that of a light beam.
The level of detail that is discernible with any microscope is 1/2 the wavelength of the beam used.
You can isolate different components of cell by using a technique called centrifugal cell fractionation.
First you break open the cells, often by mechanical shearing sometimes with a mild detergent.
You then use a high-powered centrifuge and spin the cell mixture, first at a relatively low speed.
This will cause the larger fragments to form a pellet at the bottom whereas the smaller fragments remain in the supernatant (the liquid)
Remove the supernatant and spin it again, at a slightly higher speed.
Keep doing this until no pellet is left.
Each pellet will contain components of the cell of different sizes.
In intestinal epithelial cells on the villi in the small intestine the membrane is folded into microvilli to dramatically increase the surface area.
Oils and fats are made up from 1 glycerol molecule and 3 fatty acid molecules
The glycerol and the three fatty acids bond together using a condensation reaction, forming ester bonds.
Fatty acids are acidic as the carboxyl group ionises to form H+ ions.