Unit 1, Module 1, OCR Biology

Magnification: The degree to which the size of an image is larger than the object itself. Magnification = Image/Actual Size

Resolution: The degree to which it is possible to distinguish between 2 objects that are very close together. Higher resolution = More detail

Light Microscopes

• Objectivity lenses that can rotate round to view specimens at different magnifications.
• Magnification = Up to x1500
• Resolution = 200nm (this means that if 2 objects are seen closer together then 200nm, they will be seen as 1 object)
• Specimens can include small living organisms, thin sections of larger plants and animals and smear preparations of blood or cheek cells.

Preperation of specimens

• Staining- Coloured stains allows the specimen to be seen. 
• Sectioning- Specimens are embedding in wax then thin sections are cut without distorting the structure of the specimen.

Electron Microscopes have a better resolution then light microscopes, so we can look at objects at a higher magnification.

• Resolution = 0.1nm
• Image is projected onto a screen/photographic paper to make a black-and-white image. However, colours can be added afterwards using specialised computer software.
• 2 types, TEM and SEM.
• TEM- Electron beam passes through a thin sample, electrons pass through the denser parts of the sample less easily, so giving some contrast, final image produced is 2D and the magnification possible is about x500,000
• SEM- The electron beam is directed onto a sample (the electrons don't pass through the specimen, they "bounce off" the sample"), the final image produced is a 3D view of the surface of the sample, the magnification possible is about x100,000

Advantages - Achieve a better magnification due to better resolution, this means that more detailed images can be produced. The 3D images a SEM create can reveal detail of contours and cellular or tissue arrangments.                                                                               Disadvantages - Samples are placed in a vacuum so no live specimens can be viewed, they're extremely expensive and preparing samples requires a lot of skill and training.

Phospholipid: The basic structural component of plasma membranes (cell surface membranes) Consists of 2 layers of phospholipid molecules, proteins are embedded in this layer.

The Roles of Membranes

• Seperating cell contents from the outside environment.
• Seperating 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.

Nature of Phospholipids

The head is hydrophillic (water loving) and the tails are hydrophobic (water hating). This is due to the charges in the molecule, it causes the heads to face outwards, and the tails inwards when in water, making the structure of the plasma membrane stable without the phospholipids bot being bonded together. 

All membranes are permeable to water molecules because water can diffuse through the lipid bilayer. Always describe membranes as partially permeable, NEVER SEMI-PERMEABLE!!!

Fluid Mosaic: Refers to the model of cell membrane structure. The lipid molecules give fluidity and proteins in the membrane give it a mosaic (patchwork) appearance. The molecules can move about.

There is also cholesteral, glycoprotiens, glycolipids, channel proteins, carrier proteins and receptor sites (see "The Fluid Mosaic Model Continued").

Glycolipids: Small carbohydrate molecules attatched to phospholipids. Glycoprotiens are small carbohydrate molecules attatched to protiens. Both are also involved in cell signalling.

Cholesterol: Provides the membrane with stability as water molecules can't pass through it.

Channel Proteins: Allows molecules (such as glucose that are too large and too hydrophillic to pass through the phospholipid layer) to pass through these protein channels.

Carrier Proteins: Actively moves some substances across the membrane.

Receptor sites: Allows hormones to bind with the cell so that the cell "response" can be carried out (cell signalling). Important in allowing drugs to bind and affect cell metabolism.

Increasing temperature gives the molecules more kinetic energy, so they move faster. This increased movement gives gaps so contents can much easier pass through the wall. When the temperature is too high the bonds between the phospholipids and other components break and they allow a sudden increase of material in and out of the cell.

Cell signalling: Cells communicate with one another by signals. Some molecules signaling takes place within a cell, some signal from one cell to another. Receptors pick up these cell signals (receptors are often protein molecules or modified protein molecules).

Target cell: Any cell with a receptor for a hormone molecule (communication between cells is often mediated by hormones).

Medicinal drugs - Interfering with receptors

A number of medicinal drugs have been developed that can block receptors. E.g. Beta-blockers are used to prevent heart muscle from increasing the heart rate too high in people.

Hijacking receptors

Viruses enter cells by binding with receptors. HIV can infect humans because it can enter the cells of the immune system as it fits into the receptors on the cells surface of important immune cells. It will reproduce inside the cell, therefore destroying it.

Metabolism: Cells taking in nutrients they need to survive (e.g. oxygen for aerobic respiration) and then giving out waste products.

Diffusion: The movement of molecules from a place of high concentration to a place of low concentration. Each molecule diffuses down it's own concentation gradient. The rate of diffusion depends on temperature, concentration gradient, movement, surface area:volume ratio, distance and size.

Lipid based molecules: Fat soluble molecules can simply pass through the bilayer.

Very small molecules and ions: Oxygen and carbon dioxide can pass through the bilayer.

Larger/charged molecules: Go through "facilitated diffusion" which uses channel proteins and carrier proteins that are shaped to only allow certain molecules through. This can be controlled by the cell.

Sometimes molecules need to be moved against their concentration gradient. E.g. Magnesium ions are often in very short supplyin soil. So, active transport is used.

Active transport: The movement of molecules or ions across membranes against the concentration gradient. Uses ATP to drive protien "pumps" within the membrane. This includes different types of carrier protiens. They differ from the proteins used in facilitated diffusion:

• they carry specific molecules one way across the membrane
• in carrying molecules, they use ATP to fuel the cell.
• they can carry molecules against the concentration gradient
• they can carry molecules at a much faster rate than by diffusion
• molecules can be accumulated either inside cells or organelles, or outside cells
• the carrier proteins change shape to ensure one-way flow

In or out? Solid or Liquid?

Endo - Inwards     Exo - Outwards      Phago - Solid material      Pino - Liquid material

A substance that can dissolve is called a solute, the liquid it dissolves in is a solvent and the 2 together form a solution.

Water potential, kiloPascals (kPA): the measure of tendency of water molecules to diffuse from one place to another. As solutes are dissolved, water molecules cluster around them, formaing a solution, this lowers the water potential as water can't pass through as easily.

Osmosis: Special kind of diffusion that refers only to the movement of water molecules by diffusion and across a partially permeable membrane.

The water potential of cells is lower than that of pure water, because of the sugars, salts and other substances dissolved in the cytoplasm. In plant cells, the large vacuole also contains dissolved substances.

Water has the highest water potential of 0kPA. Dissolving solutes in water reduces the water potential, so water potential values go down from 0 to negative numbers. The larger the negative figure, the more solute dissolved and the lower the water potential.

The cell cycle: The events that take place as one parent cell divides to produce 2 new daughter cells which then grow to full size.

Daughter cells form from a parent cell dividing, they must be able to carry out the same functions as the parent cell and therefor must carry the same genetic information.

Chromosomes are in the nucleus of eukaryotic cells. Each chromosome contains 1 molecule of DNA, which includes specific lengths of DNA called genes (so chromosomes hold the instructions for making new cells). Humans have 46 chromosomes (or 23 pairs).

The molecules of DNA that make up each chromosome are wrapped around proteins called histones. The DNA and the histone proteins together are called chromatin. The DNA of each choromosome must be replicated so 2 replicas are produced. Each an exact copy of the original, held together by a centromere.

G phases indicate growth of organelle numbers and overall cell size. S phase indicates synthesis of new DNA (DNA replication). M phase is nuclear division and cytokinesis.

Mitosis: The process of nuclear division where 2 genetically identical nuclei are formed from 1 parent cell nucleus. Important for asexual reproduction, growth, repair and replacement.

Prophase - Replicated chromosomes supercoil (shorten and thicken).

The nuclear envelope breaks down and disappears. An organelle called a centriole divides into 2 and each daughter centriole moves to opposite ends (poles) of the cell to form the spindle (a structure made of protein threads).

Metaphase - Replicated chromosomes line up down the middle of the cell (equator).

Each chromosome becomes attatched to a spindle thread by it's centromere.

Anaphase - The replicas of each chromosome are pulled apart from each other towards oppisite poles of the cell.

Each of the 'sisters' effectively become individual chromosomes. They assume V-shapes.

Telophase - Two new nuclei are formed.

As the sister chromatids reach the poles of the cell, a new nuclear envelope forms around each set. The spindle breaks down and disappears. The chromosomes uncoil so you can no longer see them underneath a miscrope.

Cytokinesis: The splitting in 2 of the new cells.

Animal Cells:

• Most cells are capable of mitosis and cytokinesis.
• Cytokinesis starts from outside the cell.

Plant Cells:

• Only special cells, called meristem cells, can divide by mitosis.
• Don't have centrioles, the tubulin protien threads are made in the cytoplasm.
• Cytokinesis starts with the formation of a cell plate were the spindle equator was.

Remember P-MAT for the different phases.

Clones: Genetically identical cells or organisms derived from one parent.

Bacteria are prokaryotes. They have a single strand of DNA that is in the cytoplasm, not in the nucleus. They may also have small plasmids (small circular pieces of DNA). They may have genes that resist antibiotics. Because bacteria can swap plasmids, they are used in genetic engineering. Bacteria divide by binary fission (all bacteria in a single colony have been produced by one cell dividing).

Stem Cells: Have the potential to become any sort of cell type. Bone marrow in humans contains stem cells that can divide to produce blood and bone cells. Scientists hope to one day control what type of cell a stem cell turns into so they can be used in medical treatments.

Meiosis - Special adult cells that contain half the adult number of chromosomes (called gametes) fuse together (one male, one female) to produce a zygote. This then grows and divides by mitosis. One set of chromosomes = haploid cells Two sets of chromosomes = diploid cells (I remember this by thinking the sex cells are "hap"loid, hap sounds like hump :P).

Differentiation: The changes occuring in cells of a  multicellular organism so that each different type of cell becomes specialised to perform a specific function.

With any eukaryotic cell there are different organelles, each performing a particular function to help with the survival of that cell. Single-celled organisms have a large surface area:volume ratio to do more effective diffusion, Mulit-cellular organisms don't and therefor need specialised cells (cells that may have different properties to others so it can perform it's function better).

Erythrocytes (red blood cells) - Cells that are destined to become erythrocytes lose their nucleus, mitochondria, golgi apparatus and rough ER. Theyre packed full of the protein haemoglobin and the shape changes to become biconcave disks. This is done so that it can perform its function of transporting oxygen as effectivley as possible.

• Tissue: A collection of cells that are similar to each other and perform common functions.
• Organs: A collection of tissues working together to perform a particular function.

• Vacuole containing enzymes: To digest the wall of the embryo.
• Nuclues containing chromosomes: Where the genetic information is kept.
• Tail: Move the sperm to the embryo.
• Mitochondria (not seen in diagram): Provide energy.

Xylem Tissue - It's adapted to transport water and minerals up the plant and support it.

• Elongated cells that form long continuous tubes with a wide lumen.
• Walls reinforced and waterproofed by lignin. forms long continuous tubes with a wide lumen.
• Contains little cell content.

Phloem Tissue - It's adapted transport sugars and minerals.

• Sieve tubes to keep a continuous flow.
• Companion cells to provide energy.
• Elongated cells that form long continuous tubes.

Squamous epithilial tissue: Made up of flattened cellsto form thin, smooth flat surfaces. Found in the walls of the alvioli in the lungs to allow good diffusion.

Ciliated epithilial tissue: Made up of column-shaped cells often found on the inner surface of tubes such as the tranchea and bronchi (airways in the lungs). The 'exposed' surface is covered in tiny projections called cilia to move mucus.

Harvesting light - Laeves are major organs of photosynthesis in plants. They have adapted to do their function to be as efficient as possible:

• A transparent upper surface layer (the upper epidermis) lets light through.
• A palisade layer underneath consists of long, thin, tightly packed cells contain lots of chloroplasts.
• A spongy mesophyll layer has many air spaces to allow circulation of gases.
• A lower epidermis layer has pores called stomata. These allow gases to be exchanged between the leaf and the outside air. The stomata each have 2 guard cells that can swell to open the pore. When the guard cells are not turgid, the stoma closes.
• A leaf vein system containing xylem and phloem tissues supports the leaf as well as transport material.

Stomata and Gaurd Cells - Appear in pairs in the lower epidermis. They contain chloroplasts and their cell walls contain spiral thickenings of cellulose. When the cells become turgid, only the outer walls stretch because of the spirals in the walls of the inner edges. The 2 gaurd cells bulge at both end so a pore opens between them (stoma).