Relative sizes of molecules, organelles and cells:
two fundamental types of cell
- Prokaryotes are single-celled organisms with a simple structure of just a single undivided internal area called the cytoplasm (composed of cytosol, which is made up of water, salts and organic molecules).
- Eukaryotic cells make up multicellular organisms like animals, plants, and fungi.
- much more complicated internal structure, containing a membrane-bound nucleus (nucleoplasm) and cytoplasm, which contains many membrane-bound cellular components.
- The ultrastructure of a cell is those features that can be seen using an electron microscope.
Compartments for life:
part 1
Chemical reactions are the fundamental processes of life and in cells they require both enzymes and specific reaction conditions.
Metabolism involves both the synthesis (building up) and the breaking down of molecules.
Different sets of reactions take place in different regions of the ultrastructure of the cell.
The reactions take place in the cytoplasm.
The cell cytoplasm is separated from the external environment by a cell-surface membrane.
Compartments for life:
part 2
In eukaryotic cells the cytoplasm is divided into many different membrane-bound compartments, known as organelles.
These provide distinct environments and therefore conditions for the different cellular reactions.
Membranes are selectively permeable and control the movement of substances into and out of the cell and organelles.
Membranes are effective barriers in controlling which substances enter and exit cells but they are fragile.
There are a number of organelles that are common to all eukaryotic cells.
Each type has a distinct structure and function.
They are clearly seen in animal cells
animal cell diagram
Nucleus:
The nucleus (plural nuclei) contains coded genetic information in the form of DNA molecules.
DNA directs the synthesis of all proteins required by the cell (although this protein synthesis occurs outside of the nucleus at ribosomes).
DNA controls the metabolic activities of the cell, as many of these proteins are the enzymes necessary for metabolism to take place.
the nucleus is often the biggest single organelle in the cell
The nuclear envelope:
- DNA is contained within a double membrane called a nuclear envelope to protect it from damage in the cytoplasm.
- The nuclear envelope contains nuclear pores that allow molecules to move into and out of the nucleus.
- DNA itself is too large to leave the nucleus to the site of protein synthesis in the cell cytoplasm.
- Instead it is transcribed into smaller RNA molecules, which are exported via the nuclear pores.
- DNA associates with proteins called histones to form a complex called chromatin.
- Chromatin coils and condenses to form structures known as chromosomes.
- These only become visible when cells are preparing to divide.
Nucleolus:
The nucleolus is an area within the nucleus and is responsible for producing ribosomes.
It is composed of proteins and RNA.
RNA is used to produce ribosomal RNA (rRNA) which is then combined with proteins to form the ribosomes necessary for protein synthesis.
Mitochondria:
- Mitochondria (singular mitochondrion) are essential organelles in almost all eukaryotic cells.
- They are the site of the final stages of cellular respiration, where the energy stored in the bonds of complex, organic molecules is made available for the cell to use by the production of the molecule ATP.
- The number of mitochondria in a cell is generally a reflection of the amount of energy it uses, so very active cells usually have a lot of mitochondria.
- Mitochondria have a double membrane - the inner membrane is highly folded to form structures called cristae and the fluid interior is called the matrix.
- The membrane forming the cristae contains the enzymes used in aerobic respiration.
- mitochondria also contain a small amount of DNA, called mitochondrial (mt)DNA.
- Mitochondria can produce their own enzymes and reproduce themselves.
Vesicles and lysosomes:
Vesicles are membranous sacs that have storage and transport roles.
They consist simply of a single membrane with fluid inside.
Vesicles are used to transport materials inside the cell.
Lysosomes are specialised forms of vesicles that contain hydrolytic enzymes.
They are responsible for breaking down waste material in cells, including old organelles.
They play an important role in the immune system as they are responsible for breaking down pathogens ingested by phagocytic cells.
They also play an important role in programmed cell death or apoptosis.
The cytoskeleton:
The cytoskeleton is present throughout the cytoplasm of all eukaryotic cells.
It is a network of fibres necessary for the shape and stability of a cell.
Organelles are held in place by the cytoskeleton and it controls cell movement and the movement of organelles within cells.
The cytoskeleton has three components:
Microfilaments
Microtubules
Intermediate fibres
Microfilaments
contractile fibres formed from the protein actin.
These are responsible for cell movement and also cell contraction during cytokinesis, the process in which the cytoplasm of a single eukaryotic cell is divided to form two daughter cells
Microtubules
globular tubulin proteins polymerise to form tubes that are used to form a scaffold-like structure that determines the shape of a cell.
They also act as tracks for the movement of organelles, including vesicles, around the cell.
Spindle fibres, which have a role in the physical segregation of chromosomes in cell division, are composed of microtubules.
Intermediate fibres
these fibres give mechanical strength to cells and help maintain their integrity.
Cell movement:
part 1
- The movement of cells (like phagocytes) depends on the activity of the actin filaments in the cytoskeleton.
- The filament lengths change with the addition and removal of monomer subunits.
- The rate at which these subunits are added is different at each end of a filament.
- The subunits are not symmetrical and can only be added if they are in the correct orientation.
- The subunits have to change shape before they are added to one end [the minus end] of the filament but not the other end (the plus end).
- This means that the subunits are added at a faster rate at the plus end.
Cell movement:
part 2
The filaments therefore increase in length at a faster rate in one particular direction.
Whether subunits are added or removed, at either end, is determined by the concentration of subunits in the cytoplasm.
Due to the different rates of addition at either end, at certain concentrations subunits will be added at one end and removed at the other.
This called treadmilling,
The increasing length of the filaments at one edge of a cell, the leading edge, leads to cells such as phagocytes moving in a particular direction.
Suggest, giving your reasons, which components of the cytoskeleton undergo treadmilling and which components do not.
microtubules (and microfilaments) are involved as they undergo polymerisation and hydrolysis however intermediate fibres are not involved as they have role in cell stability
Centrioles:
Centrioles are a component of the cytoskeleton present in most eukaryotic cells with the exception of flowering plants and most fungi.
They are composed of microtubules.
Two associated centrioles form the centrosome, which is involved in the assembly and organisation of the spindle fibres during cell division.
In organisms with flagella and cilia, centrioles are thought to play a role in the positioning of these structures.
Flagella and cilia:
Both flagella (whip-like) and cilia (hair-like) are extensions that protrude from some cell types.
Flagella are longer than cilia but cilia are usually present in much greater numbers.
Flagella are used primarily to enable cell motility.
In some cells they are used as a sensory organelle detecting chemical changes in the cell’s environment.
cilia:
Cilia can be mobile or stationary.
Stationary cilia are present on the surface of many cells and have an important function in sensory organs such as the nose.
Mobile cilia beat in a rhythmic manner, creating a current, and cause fluids or objects adjacent to the cell to move.
For example, they are present in the trachea to move mucus away from the lungs (helping to keep the air passages clean), and in fallopian tubes to move egg cells from the ovary to the uterus.
Each cilium contains two central microtubules (black circles) surrounded by nine pairs of microtubules arranged like a “wheel”. This is known as the 9+2 arrangement (Figure 6).
Pairs of parallel microtubules slide over each other causing the cilia to move in a beating motion.
Organelles of protein synthesis:
A key function of a cell is to synthesise proteins (including enzymes) for internal use and for secretion (transport out of the cell).
A significant proportion of the internal structure of a cell is required for this process.
The ribosomes, the endoplasmic reticulum, and the Golgi apparatus are all closely linked and coordinate the production of proteins and their preparation for different roles within the cell.
The cytoskeleton plays a key role in coordinating protein synthesis.
Endoplasmic reticulum:
The endoplasmic reticulum (ER) is a network of membranes enclosing flattened sacs called cisternae.
It is connected to the outer membrane of the nucleus.
two types of endoplasmic reticulum
Smooth endoplasmic reticulum is responsible for lipid and carbohydrate synthesis, and storage.
Rough endoplasmic reticulum has ribosomes bound to the surface and is responsible for the synthesis and transport of proteins.
- Secretory cells, which release hormones or enzymes, have more rough endoplasmic reticulum than cells that do not release proteins.
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