INTRODUCTION TO CELLS Flashcards
Seven basic functions to survive
Metabolism: undertakes essential chemical reactions.
Reproduction: must produce offspirng, either sexually or asexually.
Homeostatis: maintain an internal stable chemistry.
Sensitivity (response to stimule) : are responsive to internal or external stimuli.
Excretation: exhibit the removal of watse products of the metabolism.
Nutrition: exhange material and gas with the environment in order to supply the nutrients for growth, repair and energy.
Growth/movement: can move and they change in size or shape.
CELL THEORY:
The cell theory consists of three basic tenets that function to describe the organization of life. According to the cell theory:
* The cell is the smallest unit of life
* Cells only arise from pre-existing cells
* All living things are composed of cells (or cell products)
CELL STRUCTURE:
There are four basic structures common to all cells:
Plasma Membrane:
* All cells must have an outer border (which is the plasma membrane) to maintain an internal chemistry that is different to the exterior (homeostasis).
* It controls the entry and exit of substances (it can pump substances in and prevents the entry of unwanted or toxic substances). It encloses all its contents
* The permeability of the plasma membrane relies on a structure based on lipids.
* Occasionally the plasma membrane bursts (it is known as lysis). It can be caused by excess pressure of viruses.
* It can be killed by the cell itself (autolysis). As it always leads to death.
Genetic Material
* All cells must contain coded instructions/ information (DNA) that function to control internal activities within a cell (metabolism).
* Many hold the instructions to make proteins, which are needed for growth and repair.
* Other act as enzymes, which enables the cell to control chemical reactions and to have a functioning metabolism.
* DNA can be copied and passed on to daughter cells (heritable)
Ribosomes
* All cells must contain ribosomes to translate the cell’s coded instructions into functional elements (proteins)
Cytosol (cytoplasm)
* All cells must contain an internal fluid (made of mainly water) that functions as a reaction medium for all necessary metabolic processes (which provides energy, proteins and other substances that create the structure of the cell).
* Proteins get damaged easily so the cytoplasm must break down and replace its proteins.
What are atypical cell structures?
Certain types of eukaryotic cells and tissues do not conform to the standard organization of a typical cell. These cells have developed unique characteristics to better to support their specific cellular activities
STRIATED MUSCLE FIBRES:
- Individual muscle cells (formed by cell division) fuse together to form long striated muscle fibers (this type is called syncytium)
- These fibers are surrounded by a continuous plasma membrane and possess multiple nuclei
- They challenge the idea that all living things are comprised of discrete cell units
ASEPTATE FUNGAL HYPHAE:
- In some growing cells, the nucleus divides repeatedly without subsequent cell division.
- This results in an unusually large multinucleate structure (coenocyte)
- Fungi may have filamentous structures called hyphae, which are used for nutrient absorption and growth
- Hyphal cells are typically separated by internal walls (septa- uninucleate cells), but some hyphae are not partitioned and have a continuous cytoplasm (with multiple nuclei)
- They challenge the idea that living structures are composed of autonomous cells
- Hyphae without these divisions are aseptate
SIEVE TUBE ELEMENTS:
- Plants move sap through tubular vessels, made from columns of cylindrical cells. The flow of sap would be hold back if these cells had a typical structure.
- In xylem vessels (conduct watery sap) all the dividing walls between adjacent cells are removed, thus all cell contents break down. This creates a hollow tube that no longer consist of cells.
- Phloem (conducts sugary sap) the conducting vessels are sieve tubes.
- The nucleus and most other cell contents break down relying on local companion cells for survival.
- the plasma membrane remains as it is essential for phloem transport.
- Sieve elements that line the phloem in plants are interconnected by plasmodesmata into supracellular assemblies that transverse the length of a plant
- Phloem sieve tube elements challenge the idea that multicellular structures are composed of anatomically independent cells
- The subunits in a sieve tube are called elements
- They are connected to adjacent cells, that have nucleus and mitochondria, to help sieve tubes to survive and carry out their functions
RED BLOOD CELLS:
- Red blood cells have no nucleus or mitochondria when they are mature (the organelles are ejected to allow more haemoglobin to be stored as well as it is destroyed by a phagocyte)
- Without any genetic material, red blood cells cannot independently replicate, and new cells must be continually produced within the bone marrow.
- Red blood cells challenge the traditional definition of a eukaryotic cell as they lack critical structures needed for autonomous survival
- They cannot repair themselves (short life span)
MICROSCOPES:
Microscopes are scientific instruments that are used to visualize objects that are too small to see with the naked eye
* There are two main types of microscope – optical (light) microscopes and electron microscopes
LIGHT MICROSCOPY
Light microscopes can be used to view living specimens in their natural colors
* These microscopes use glass lenses to bend light in order to magnify images (extent of magnification is determined by the lenses used)
The clarity of cellular sub-structures can be improved via the use of fluorescent labelling
* Synthetic dyes can be used to bind particular cellular compounds in order to resolve specific structures (e.g., DAPI is a fluorescent dye that stains DNA)
* Immunofluorescence staining uses antibodies that are conjugated to fluorescent probes to specifically target a cellular component of choice
ELECTRON MICROSCOPY
Electron microscopes can generate images at a much higher magnification and resolution, however, cannot view living specimens in natural color
* These microscopes use electromagnets to focus electrons and produce monochromatic images (to which false color may be applied)
There are two main types of electron microscopes that allow for different visual representations of a biological specimen
* Transmission electron microscopes (TEMs) pass electrons through a specimen to generate a cross-section image
* Scanning electron microscopes (SEMs) scatter electrons over a surface to differentiate depth and map in 3D
Cryogenic electron microscopy involves freezing samples prior to viewing to generate images of a comparable standard to X-ray crystallography
* This allows for the determination of molecular structures at near atomic resolution without requiring the crystallization of the specimen
* If the frozen specimen is cracked along a plane via freeze fracturing, then internal cellular structures can be studied
* Freeze fracturing was used to demonstrate the presence of integral membrane proteins within the plasma membrane
MICROSCOPES SKILLS:
Light microscopes use visible light and a combination of lenses to magnify images of mounted specimens
* Most light microscopes include both an ocular lens (~10×) and objective lens (~10×; 40×; 100×)
Using a Light Microscope
When using a light microscope to view biological specimens, the following conventions should be followed:
* The image should initially be resolved at the lowest magnification using the coarse focus mechanism
* Higher magnifications are then obtained by changing the objective lens (via the revolving nosepiece) and making fine focus adjustments
* The total magnification of the image is calculated by multiplying the magnification of both lenses (ocular and objective) together
* If the eyepiece has a measurement scale on its surface (eyepiece graticule), this can be used to determine sizes of biological structures
CALCULATING MAGNIFICATION
To calculate the linear magnification of a drawing or image, the following equation should be used:
* Magnification = Image size (with ruler) ÷ Actual size (according to scale bar)
In order to calculate magnification, both image size and actual size must be in the same units
PROKARYOTES:
Organisms can be divided by two groups: eukaryotes and prokaryotes.
Prokaryotes are single-celled organisms that have a simple cell structure without compartmentalization (they do not possess any membrane-bound organelles). They are found almost everywhere.
* Prokaryotic cells can have several different shapes: rods (bacilli), spheres (cocci), spirals (spirilla), commas (vibrio) or corkscrews (spirochetes)
Prokaryotes have been classified into two different domains, based on key differences in structure and genetics
* Bacteria: A large and diverse range of organisms including many pathogenic (disease-causing) forms
* Archaea: Include a variety of extremophiles (organisms living in extreme environments), but also exist in normal habitats
PROKARYOTIC CELL STRUCTURE:
All prokaryotic cells share several key cellular components:
* The genetic material (lighter than the rest, when looking through microscope) is found within a region of the cytosol called the nucleoid (the single DNA strand is called the genophore)
* Prokaryotes may contain additional DNA molecules (plasmids) that can be exchanged via bacterial conjugation (horizontal gene transfer)
* The DNA is naked in the cytoplasm, it isn’t associated with proteins, it is called nucleoid (it is similar to a nucleus because it contains DNA but it isn’t a true nucleus)
* The ribosomes within the cell that are responsible for protein synthesis are characteristically small in size (70S)
* Prokaryotic cells all possess a cell wall (the structures is thicker stronger than membrane) and may possess an additional outer covering (a slime capsule called a glycocalyx)
* The cell wall (which contains peptidoglycan) protects the plasma membrane from bursting and maintains its shape.
* They may possess hair-like extensions called pili, that aid in adhesion (attachment pili) or plasmid exchange (sex pili)
* Additionally, many prokaryotes may possess several whip-like projections called flagella, which facilitate movement
* As there is no nucleus the cell is filled with cytoplasm. The cytoplasm is not divided into compartments by membranes, instead, it is one uninterrupted chamber. They only have ribosomes as their cytoplasmic organelles.
EUKARYOTES:
Eukaryotes are organisms whose cells contain a nucleus and are compartmentalized by numerous membrane-bound organelles
* They have a greater level of structural complexity and are believed to have evolved from prokaryotic cells via endosymbiosis
Eukaryotes have been classified into distinct kingdoms, based on key structural and functional differences
* Animal: Have no cell wall and undertake heterotrophic nutrition (via ingestion)
* Plant: Have a cell wall (made of cellulose) and undertake autotrophic nutrition (via photosynthesis)
* Fungi: Have a cell wall (made of chitin) and undertake heterotrophic nutrition (via absorption)
* Protist: Any eukaryotic organism that does not belong to the animal, plant or fungal kingdoms
EUKARYOTIC CELL STRUCTURE
All eukaryotic cells share several key cellular components:
* The genetic material is found within a double-membrane structure called the nucleus. This compartment helds chromosomes. Each chromosome consists of one long DNA molecule attached to proteins, except when the cell is about to divide, and the DNA is replicated.
* Keeping chromosomes inside the nucleus safeguards the DNA.
* DNA molecules are linear rather than circular.
* The ribosomes within the cell that are responsible for protein synthesis are larger in size (80S)
* Ribosomes in eukaryotic cells sink more quickly when centrifuged.
* Eukaryotes all share a few membrane-bound organelles – including mitochondria, endoplasmic reticulum, Golgi apparatus and vesicles
* Plant cells possess chloroplasts (for photosynthesis) and have a large, fluid-filled vacuole surrounded by a tonoplast membrane
* Multicellular fungi form filamentous hyphae that are typically separated by internal walls called septa
* A mitochondrion is surrounded by a double membrane, they carry out aerobic cell
SEPARATION OF THE NUCLEUS AND CYTOPLASM:
One of the key distinguishing features of a eukaryotic cell is the presence of a nucleus (prokaryotic cells do not have nuclei)
* The nucleus is a double membrane structure with pores that stores the genetic material of the cell
The presence of a nucleus allows eukaryotes to separate the processes of transcription (nucleus) and translation (cytoplasm)
* Transcription is the process by which specific DNA instructions (genes) are converted into RNA transcripts (mRNA)
* Translation involves the synthesis of polypeptide chains (proteins) from the RNA transcripts by ribosomes
Separating the processes of transcription and translation allows for the post-transcriptional modification of mRNA before it is translated by ribosomes
* These modifications help to stabilize the mRNA transcript (via capping and polyadenylation) and remove non-coding sequences (introns) via splicing
* This greatly improves the efficiency of protein synthesis and allows for tighter control of gene expression than is possible in prokaryotic cells
ADVANTAGES OF COMPARTMENTALISATION:
Another defining feature of eukaryotic cells is the presence of membrane-bound organelles in the cytoplasm (prokaryotic organelles are not membrane-bound)
* This enables the organelles to maintain an internal chemistry that is different to the cytoplasm (and suitable to its specific function)
* It also allows for the concentration of key enzymes and metabolites needed to optimize the function of the organelle.
* Enzymes and substrates for a particular process can be much more concentrated than if they were spread throughout the cytoplasm.
* Substances that could cause damage to the cell can be kept outside the membrane of an organelle.
* Conditions such as pH can be maintained at an ideal level for a particular process, which may be different from the levels needed for other processes in a cell.
* Organelles with their contents can be moved around within the cell.
* There is a larger area of membrane available for processes that happen within or across membranes.
Lysosomes and phagocytic vacuoles provide evidence for the advantage of compartmentalizing the cytoplasm into discrete sections
* These organelles contain hydrolytic enzymes that are responsible for digesting cellular debris or engulfed pathogenic materials
* If these enzymes were not contained within a specific compartment, they would freely digest the contents of the cell (autophagy)
ORGANELLES:
Organelles are the discrete subunits of a cell that are adapted to perform specific functions
* The plasma membrane and ribosomes are universal organelles that are present in every living cell
* Complex cells (eukaryotes) possess additional membrane-bound organelles that provide further functionality
NUCLEUS
- Double membrane structure that stores genetic material / DNA
- A nucleolus is a dark region in a nucleus that makes ribosomes
- The nucleus contains chromosomes consisting with DNA associated with histone proteins.
- The edge of the nucleus contains chromosomes that remain condensed
- The DNA is replicated and transcribed to form mRNA, which is exported via the nuclear pores to the cytoplasm.
MITOCHONDRIA
- Responsible for ATP production (via aerobic cell respiration)
- The inner membrane is highly folded to increase SA:Vol ratio
- A double membrane surrounds mitochondrion.
- The inner membrane is invaginated to form structures called cristae. The fluid inside is called matrix.
- The shape is usually spherical or ovoid.
- Fat is being digested here if it is being used as an energy source.
