The cell is the basic unit of all living things. New cells are formed only by the division of existing ones as these contain instructions for growth to be passed on. All living organisms are capable of these processes (MRS GREN):
- M - Movement
- R - Respiration
- S - Sensitivity
- G - Growth
- R - Reproduction
- E - Excretion
- N - Nutrition
The Light Microscope
Magnification is the degree to which the size of an image is larger than the object itself. Most light microscopes are capable of magnifying up to x1500. TEM are capable of x500,000 and SEM are capable of x100,000.
Resolution is the degree to which it is possible to distinguish between two objects close together. The higher the resolution, the greater the visible detail. The maximum resolution is 200nm, meaning that if two objects are closer than this, they will be seen as one. This is due to the magnitude of wavelength - in the case of the light microscope, two objects can only be distinguished if light waves can pass between them. The resolution of electron microscopes is 0.2nm.
How It Works
Light microscopes use a number of lenses to produce an image that can be viewed directly at the eyepieces. Light passes from a bulb under the stage, through condenser lens, through the specimen and focused through the objective lens before going to the eyepiece.
Four objective lenses are present and can be rotated to change the magnification. These are usually x4, x10, x40 and x100 (the last is an oil immersion lens). The eyepiece lens magnifies this image again, usually x10.
Total magnification: objective magnification x eyepiece magnification
Staining and Sectioning in Light Microscopes
A lot of biological material is not covered, or may distort when cut. To overcome this, chemicals known as coloured stains bind to chemicals in/on the specimen, allowing it to be seen. Some bind to specific cell structures - ex. staining DNA dark red with Acetic orcein, and staining bacterial cell walls violet using Gentian.
Sectioning refers to embedding specimens in wax, before cutting them into thin slices, without distorting the structure. Useful for soft tissue such as brain tissue.
The Graticule and the Stage Micrometer
micrometre (µm) - 1/1,000,000 of a metre
nanometre (nm) - 1/1,000,000,000 of a metre
The eyepiece can be fitted with a small transparent ruler (graticule). The scale is arbitrary as it represents different lengths and magnifications, hence it must be calibrated as appropriate.
The stage micrometer is a special slide containing 1mm (1000µm) long ruler with 100 divisions. This is placed on the stage. The calculate the length of 1 eyepiece unit (epu) at a certain magnification: 1 epu (µm) = 1000µm / total magnification
The relationship between actual size, image size and magnification: actual size = image size / magnification.
The Electron Microscope
Electron microscopes generate a beam of electrons. This beam has a wavelength of 0.004nm. Magnets are used to focus the beam onto a prepared specimen. The image produced is projected onto a screen or photographic paper to make a grayscale image.
- Transmission Electron Microscope (TEM) - The electron beam passes through a very thin prepared sample. Electrons pass through denser parts less easily, giving contrast. It produces a 2D image.
- Scanning Electron Microscope (SEM) - The electron beam is directed onto a sample and 'bounces' off as opposed to passing through. The image is a 3D view of the surface of the sample.
Staining samples with lead salts scatterthe electrons differently to give contrast. The image can be coloured using computer software - product is 'false-colour' electron micrographs.
- Advantages of EM: higher resolution gives more detail, SEM gives 3D so reveals arrangement
- Disadvantages of EM: expensive equipment, preparation requires skill, air deflects electrons so samples must be in vacuum
Refers to the network of protein fibres that provide mechanical strength (structure), aids transport, and enables cell movement.
Some of the fibres - actin filaments - are able to move against each other. These act like muscle fibres and are able to move organelles around within the cell.
Others - microtubules - can move a microorganism through a liquid or waft a liquid past the cell. Microtubules are made of tubulin. Other proteins present (microtubule motors) move organelles and other cell contents along the fibres - this is how chromosomes are moved during mitosis and how vesciles move from the endoplasmic reticulum to the Golgi apparatus. These movements use ATP.
ATP (adenosine triphosphate) is a coenzyme that transports chemical energy. Coenzymes are small organic non-protein molecules that carry chemical groups between enzymes.
Cilia and Undulipodia (Flagella)
Eukaryotes can have hair-like extensions that stick out from the surface. Each one contains nine microtubules arranged in a circle and two in the centre. These are cilia and undulipodia - both are structurally the same.
Cilia often occur in large numbers on a cell, and their sweeping movements can move substances such as mucus across the surfaces of cells in ciliated epithelial tissue. Undulipodia are longer and usually occur alone or in pairs - ex. tail of sperm or bacteria, can propel the whole cell.
Animal Cells vs Plant Cells
The detail of the inside of cells as revealed by an EM is called the cell's ultrastructure.
Each type of organelle has a specific role within the cell, known as the division of labour.
Typical Organelles in...
- An animal cell - Nucleus (with nucleolus and nuclear envelope), cell membrane, Golgi apparatus, cytoplasm, mitochondrion, ribosome, cytoskeleton, lysosome, smooth ER, rough ER, centriole
- A plant cell - Nucleus (with nucleolus and nuclear envelope), cell wall, cell membrane, Golgi apparatus, cytoplasm, mitochondrion, ribosome, cytoplasm, vacuole (with membrane), chloroplast, amyloplast (starch grain), smooth ER, rough ER
Vesicles, Vacuoles and Cell Wall
Vesicles are membrane-bound sacs. They carry many different substances around cells.
The vacuole (found in plant cells) is filled with water and solutes such that it pushes the cytoplasm against the cell wall, making the cell turgid. This helps support the plant.
Plant cell walls are made of cellulose. It forms a sieve-like network that makes the wall strong. It is held rigid by the turgor pressure.
Found in plant cells and some protocists. Most are found in the palisade cells in leaves. Thylakoids are flattened membrane sacs. A granum (pl: grana) is a stack of thylakoids. This increases surface area for light absorption in photosynthesis. Chlorophyll molecules are present on the thylakoids' membranes and intergranal membranes. The stroma is the fluid filling the inner membrane. This is the sight where light-independent reactions take place.
The largest organelle in a cell. When stained, it shows darkened patches consisting of DNA and proteins, known as chromatin. The nuclear envelope surrounds the nucleus, and consists of two membranes with fluid between them. The nucleus is a dense spherical structure inside the nucleus.
The nucleus houses nearly all the cell's genetic material. The chromatin has instructions for making proteins to regulatr the cell's activities. During mitosis, chromatin condenses into visible chromosomes. The nuclear envelope contains large nuclear pores which allow relatively large molecules to pass through. The nucleolus makes RNA and ribosomes, which pass into the cytoplasm.
ER consists of a series of flattened, membrane-bound sacs called cisternae. They are continuous with the outer nuclear membrane. Rough ER is studded with ribosomes. Smooth ER does not have these.
Rough ER transports proteins made on the attached ribosomes. Smooth ER is involved in making lipids.
A series of flattened membrane-bound sacs.
It receives proteins from the ER and modifies them. It may add sugar molecules. It then packages the modified proteins into vesicles for transportation. Some may go to the cell surface for secretion.
May be spherical or sausage-shaped. They have two membranes separated by a fluid-filled space. The inner membrane is highly folded to form cristae. The central part is the matrix.
Mitochondria are the site where ATP is produced during respiration. Almost all activities that need energy in the cell are driven by the energy released from ATP.
Spherical sacs surrounded by a single membrane.
Contains powerful digestive enzymes to break down materials - ex. lysosomes in white blood cells help break down invading microorganisms.
Tiny organelles that are in the cytoplasm (free) or bound to the ER. Each ribosome consists of two subunits. They measure about 18nm in animal cells.
Ribosomes are the site of protein synthesis in the cell. They act as an assembly line where coded information (mRNA) is used to assemble proteins from amino acids.
Small tubes of protein fibres. A pair can be found next to the nucleus in animal cells and in the cells of some protoctists.
Centrioles take part in cell division. They form spindle fibres, which move chromosomes during nuclear division (anaphase).
1. Instructions to make a hormone are found in a gene. Genes are sections of DNA in the chromosomes in the nucleus.
2. The nucleus copies instructions in the DNA into a molecule called mRNA.
3. mRNA leaves the nucleus through a nuclear pore and attaches to a ribosome. This may be attached to the rough ER or free.
4. The ribosome reads the instructions and assembles the hormone.
5. The assembled protein is pinched off in a vesicle and transported to the Golgi apparatus.
6. The Golgi apparatus packages the protein and may also modify it. The vesicle fuses with the cell surface membrane.
7. The membrane opens to release the hormone outside.
Prokaryotes do not have a nucleus. They are bacteria and are much smaller than eukaryotic cells (20-40um). Other differences between the two:
- Prokaryotes only have one membrane and do not contain any membrane-bound organelles such as mitochondria.
- They are surrounded by a cell wall made of peptidoglycan (or murein).
- They contain much smaller ribosomes.
- They have a single circular group of DNA. Many also contain very small loops of DNA called plasmids.
- DNA is not surrounded by a membrane. The general area they are situated is called the nucleoid.
- ATP production takes place in infold regions of the cell surface membrane called mesosomes.
- Some have flagella, but these have a different internal structure. They are made up of a spiral of protein called flagellin.
The Role of Membranes
- Separating cell contents from the outside environment
- Separating cell components from cytoplasm
- Cell recognition and signalling
- Holding the components of some metabolic pathways in place
- Regulating the transport of materials into or out of cells
Plasma membranes are partially permeable - permeable to water and other solutes.
The Fluid Mosaic Model
Singer and Nicholson (1972) - Describes the molecular arrangements in cell membranes. Main features:
- A bilayer of phospholipid molecules, the basic structural component, arranged with hydrophilic phosphate heads outwards, and hydrophobic fatty acid tails inwards. This creates a barrier that separates the cell contents from the outside world as many metabolic reactions take place in a water-based environment.
- Various protein molecules floating in the bilayer - some extrinsicly (partially embedded), others intrinsic (completely spanning the layer). Some completely freely, others bound to other components or to structures within the cell.
- Cholesterol, a steroid molecule, fits between fatty acid tails, providing mechanical stability such that substances cannot pass easily.
- Glycoproteins and glycolipids serve as markers for cellular recognition by the immune system. Glycoproteins can also act as an antigen, a binding site/receptor for a hormone, cell adhesion, and attach to water molecules to stabilise the membrane.
The Fluid Mosaic Model
Communication and Cell Signalling
Cell signalling is the communication between cells to trigger a response. Signal molecules fit into receptors on cell surface membranes because their shapes are complementary to the target cell.
Ex - insulin is released from the pancreas in response to increased blood sugar levels. These attach to insulin receptors on the plasma membranes of many cells, triggering the increased presence of glucose channels in the plasma membrane to allow the cell to take up more glucose from the blood, reducing blood glucose level.
Medicinal drugs have been developed that are complementary to the type of receptor molecule. Such drugs are intended to block receptors. Viruses enter cells by binding with receptors on the membrane. Some poisons also bind with receptors.
Passive transport depends on the kinetic energy in water or gas molecules. Diffusion is the movement of molecules from a region of high concentration to low concentration of that molecule, down a concentration gradient. Diffusion is affected by:
- Temperature - the higher, the more kinetic energy
- Concentration gradient - the steeper, the faster
- Stirring/moving - increases movement of molecules
- Surface area - the more, the faster
- Distance/thickness - the more, the slower
- Size of molecules - smaller molecules/ions diffuse more quickly
Lipid based (fat soluble) molecules such as steroid hormones can simply pass through the bilayer, as can very small molecules non-polar such as oxygen and carbon dioxide. Some water molecules can also pass. Larger molecules or charged molecules cannot. They must enter through facilitated diffusion through channel proteins (allow one type of ion or polar molecules through) or carrier proteins (shaped so a specific molecule can fit, change to allow them through to the other side).
Sometimes a cell may need more of a particular substance to function properly. Diffusion is not quick enough to meet these needs. Active transport is movement against a concentration gradient across membranes, using ATP.
Certain carrier proteins act as pumps to allow this to happen. They are shaped complementary to the molecule. They differ from proteins used in facilitated diffusion (channels) in the following ways:
- They carry specific molecules one way across the membrane
- They use metabolic energy in the form of ATP
- They carry molecules against the gradient
- They carry molecules at a much faster rate of diffusion
- Molecules can be accumulated either inside cells or organelles or outside cells
Endocytosis and Exocytosis
Some cells need to move quantities of material in or out. When materials move into the cell, the process is known as endocytosis. Bringing materials out is known as exocytosis. These processes require energy, which is used to form vesicles.
The movement of water molecules from high to low water potential across a partially permeable membrane. Water potential is a measure of the tendency of water molecules to diffuse from one place to another.
- If the water potential is high, an animal cell's membrane would burst. It is described as being haemolysed.
- If the water potential is low, the cell contents of an animal cell will shrink and the membrane will wrinkle up.
- If the water potential is high, a plant cell's cell wall will prevent bursting. The cell is described as being turgid.
- If the water potential is low, the plant cell's cytoplasm and vacuole will shrink, and the membrane will pull away from the cell wall. It is described as being plasmolysed.
The Cell Cycle
The events that take place as one parent cell divides to produce two new daughter cells which grow to full size. The daughter cells are genetically identical to each other and the parent cell. The cell cycle is divided into stages:
- Interphase - Split into the Growth 1 phase (growth and normal metabolic roles), the Synthesis phase (DNA replication), and the Growth 2 phase (growth and preparation for mitosis).
- Mitosis - The nucleus divides and chromatids separate. Mitosis occupies only a small proportion of the cell cycle.
- Cytokinesis - The cytoplasm divides or cleaves.
- Growth Phase - Each new cell grows to full size.
The process of nuclear division where two genetically identical nuclei are formed from one parent cell nucleus. Used for growth, repair and asexual reproduction. Consists of the following stages:
- Prophase - The nuclear envelope breaks down and disappears. Chromosomes shorten and thicken (supercoil). The centriole breaks into two and moves to opposite poles of the cell to form the spindle.
- Metaphase - Chromosomes move to the equator of the spindle and becomes attached to the spindle fibres by their centromeres.
- Anaphase - The replicated sister chromatids are separated from each other once the centromere splits. The spindle fibres shorten, pulling the sister chromatids further from each other towards the poles. Each becomes an individual chromosome, each genetically identical to the original chromosome in the parent cell.
- Telophase - A nuclear envelope forms around each set of chromosomes and two nuclei and nuclear membranes are formed. The spindle breaks down and disappears. The chromosomes uncoil.
Bacteria do not do mitosis, as they do not have linear chromosomes or centrioles. In plants, only meristem cells can; they do not have centrioles, but protein threads are made in cytoplasm.
Stem Cells and Differentiation
Mitosis produces clones. They create stem cells - undifferentiated genetically identical daughter cells, carrying a full set of genetic information, meaning they are capable of developing into any one of the different cell types in an organism. They can be described as omnipotent or totipotent.
Different in plant cells and animal cells. In plants it starts with the formation of a cell plate where the spindle equator was. The cell then lays down new membrane and cell wall material along this plate. In animals, cytokinesis starts from the outside, 'nipping in' the cell membrane and cytoplasm along the cleavage furrow.
In yeast cells, cell division occurs through a process called budding. It produces a small 'bud' in which DNA is replicated into. The nucleus migrates to the bud and a spindle forms. The nucleus divides and is followed by cytokinesis.
Meiosis produces gametes (sex cells) containing only set of chromosomes (23 - the haploid number). Fusion of the male and female gametes results in a zygote containing two sets of chromosomes (diploid).
The daughter cells are not genetically identical to each other or to the parent cell. This is because each pair of homologous chromosomes (pairs containing the same genes but not necessarily the same alleles) separate into haploid cells.
Changes occurring in cells of a multicellular organism so that each different type of cell becomes specialised to perform a specific function.
Erythrocytes (red blood cells) and neutrophils (a type of white blood cell) have different roles but were produced from undifferentiated stem cells in the bone marrow. Differentiation in plants can be referred to the xylem or phloem. Both come from dividing meristem cells such as cambium.
How Cells Are Specialised
- They lose their nucleus, mitochondria, Golgi apparatus and rough ER
- Packed full of the protein haemoglobin
- They become biconcave discs, increasing surface area for transporting oxygen from lungs to tissues
- Contain enormous numbers of lysosomes which produce enzymes to ingest invading microorganisms
- Packed with mitochondria providing energy for movement of the undulipodia
- Has specialised lysomes (acrosomes) that release enzymes for penetrating the egg
- Very small, long and thin to help movement
- A single long undulipodium to propel the cell
How Cells Are Specialised
Root Hair Cell
- Has a hair-like projection that penetrates the soil and increases the surface area to absorb water and minerals from the soil
- Have no or very few chloroplasts
- Cells are elongated, thin and tightly packed
- They contain lots of chloroplasts that contain chlorophyll for photosynthesis
- Controlled by two guard cells
- They contain chloroplasts
- Cell walls contain spiral thickenings of cellulose
- When turgid, the outer walls stretch and the two guard cells bulge at both ends, opening up the pore
How Cells Are Specialised
Squamous Epithelial Tissue
- Made up of flattened cells which form a thin, smooth, flat surface. This makes it ideal for lining the insides of tubes such as blood vessels, where fluids can pass easily over them
- They form thin walls such as alveoli walls to provide a short diffusion pathway for gaseous exchange
- Cells secrete a basement membrane, made of collagen and glycoproteins, which holds them in place. It attaches epithelial cells to connective tissue
Ciliated Epithelial Tissue
- Made up of column cells and found on the inner surface of tubes, such as the trachea, uterus and oviducts
- Some cells produce mucus which traps small particles and microorganisms
- Covered with cilia which moves the mucus to the back of the throat to be swallowed. Cilia in the oviduct moves the egg along from the ovaries
Tissues, Organs and Organ Systems
Tissues - A collection of cells that are similar to each other and perform a common function. Examples include xylem and phloem in plants, and epithelial and nervous tissues in animals.
Organs - A collection of tissues working together to perform a particular function. Examples include the leaves in plants, and the liver in animals.
Organ System - A number of organs working together to perform an overall life function.
The Need For Cooperation - Example
The leaf is an organ that contains many cells and tissues that are required to work together for photosynthesis. Photosynthesis requires light, carbon dioxide, water and chlorophyll. As the products of photosynthesis build up, they need to be removed to where they are needed. The waste product, oxygen, must also be excreted. The leaf is adapted in these ways:
- Upper Epidermis - is transparent and thin to allow light to reach the palisade cells
- Palisade Layer - made of elongated, thin, tightly packed cells containing hundreds of chloroplasts as it is the main site of photosynthesis
- Spongy Layer - main gas exchange surface of the leaf; air spaces allow movement of gases and increases surface area for gas exchange
- Lower Epidermis - many pores called stomata that let carbon dioxide, oxygen and water to diffuse in and out
- Veins - a leaf vein system containing xylem and phloem tissues supports the leaf, as well as transport water into the leaf, and products of the photosynthesis out of the leaf to other parts of the plant.
Organisation of Tissue
Animal tissues are grouped into four main categories:
- Epithelial tissue - layers and linings
- Connective tissue - hold structures together and provide support
- Muscle tissue - specialised to contract and move parts of the body
- Nervous tissue - converts stimuli to electrical impulses and conducts those impulses
Xylem and Phloem
Xylem tissue consists of xylem vessels with parenchyma cells and fibres. Meristem cells produce small cells that elongate. Their walls become reinforced and waterproofed by deposits of lignin. This kills the cell contents. The ends of the cells break down such that they become continous long tubes with a wide lumen. This makes it well suited for transporting water and minerals up the plant. It also helps to support the plant.
Phloem tubes consist of seive tubes and companion cells. The meristem tissue produces cells that elongate and line up end-to-end to form a long tube. Ends do not break down completely but form seive plates. These allow movement of materials up and down the tubes. Companion cells are located next to each seive. They are metabolically active and play an important role in moving the products of photosynthesis up and down the plant.