A-Level Biology - OCR A

What is a light microscope used for?

Observing living and dead specimins

What are the pros and cons of a light microscope?

Pros: Cheap, portable, easy to use, can study living specimens. Cons: Limited magnification, poor resolution.

What is a laser scanning confocal microscope used for?

Creating a high resolution, high contrast image, at different depths of the specimen.

What is a transmission electron microscope be used for?

Observing the internal ultrastructure of cells under high magnification and resolution

What is a scanning electron microscope used for?

Viewing the surface of objets under high magnification and resolution

What are the pros and cons of an electron microscope?

Pros: Very high magnification and excellent resolution. Cons: specimen has to be dead, very expensive, very large, needs great skill and training to use.

What is the difference between a transmission and an scanning electron microscope?

TEM sends a beam of electrons through the specimen, the SEM bounces electrons off the surface.

What is the difference between light and electron microscopes?

Light uses lenses to focus a beam of light. Electron uses a beam of electrons, focused by magnets.

What is an eye piece graticule?

A small ruler fitted to a light microscope's eyepiece. It must be calibrated using a stage micrometer before being used to measure specimens.

What is a stage micrometer?

A millimeter long ruler etched onto a slide. it has 100 divisions, each of 0.01mm or 10 micrometers. It is used to calibrate the eyepiece graticule

Why do we stain specimens?

To provide more contrast, and make it easier to distinguish certain parts.

What is differential staining?

Using a stain to distinguish between either 2 different orgaisms, or between organelles of a specimin due to preferential absorbtion of stain.

What is the formula to calculate magnification?

Magnification \= Image size / Actual size

What is the formula to calculate actual object size?

Actual size \= Image size / Magnification

How do we work out image size?

Use a ruler and measure the image.

What is magnification?

A measure of how much larger the image of a specimen looks under the microscope

What is resolution?

The ability to distinguish between to adjacent individual points as separate.

What are the maximum resolutions of the different microscopes?

Light: 200nm; SEM: 10nm; TEM: 0.2nm.

What is the maximum magnification of the different microscopes?

Light: 1,500X; SEM: 100,000X; TEM: 500,000X.

What are the main structures of all eukaryotic cells?

Nucleus; nucleolus; cytoplasm; cytoskeleton; plasma membrane;mitochondria; Golgi apparatus; smooth endoplasmic reticulum; rough endoplasmic reticulum; ribosomes.

What is the structure and function of the nucleus?

Surrounded by a double membrane (the nuclear envelope). Contains chromatin (DNA wound around histones). Stores the human genome, controls the cell by providing instructions for protein synthesis.

What is the structure and function of the nucleolus?

Made of RNA, produces ribosomes.

What is the structure and function of the nuclear envelope?

A double membrane embeded with channel proteins forming pores. Separates the nucleus from the rest of the cell. Pores allow ribosomes and mRNA to leave the nucleus.

What is the structure and function of the rough endoplasmic reticulum (RER)?

A system of fluid filled membranes studded with ribosomes. Continuous with the nuclear membrane. Large surface area formed by folding, enables lots of protein synthesis. Proteins pinched off in vesicles transported to the Golgi apparatus.

What is the structure and function of the smooth endoplasmic reticulum (SER)?

A system of fluid filled membranes. No ribosomes. Contain enzymes for cholesterol, lipid and phospholipid synthesis.

What is the structure and function of the Golgi apparatus?

A stack of flattened membrane bound sacs. Vesicles from the RER join at the cis face. Here they are modified, by adding sugar to make glycoproteins, adding lipids to form glycolipids. Folding proteins into their 3D shape. Modified proteins are pinched off from the trans face into transport vesicles.

What is the structure and function of the ribosomes?

2 subunits, large and small. Made of rRNA . Proteins synthesised here.

What is the structure and function of the mitochondria?

Double membrane bound. Inner membrane folded into cristae within a fluid filled matrix. Contain own DNA and 70s ribosomes. Site of aerobic respiration.

What is the structure and function of the lysosomes?

Membrane bound sacs containing hydrolytic (digestive) enzymes. break down old organelles and foreign matter for reuse.

What is the structure and function of the chloroplasts?

Double membrane bound. Inner membrane forms flat discs filled with chlorophyll called thylakoids, in stacks called granum. These are surrounded by a fluid matrix called the stroma. Contain own DNA and 70s ribosomes. Site of photosynthesis.

What is the structure and function of the plasma membrane?

Phospholipid bilayer, cholesterol, proteins, glycoproteins, glycolipids. Separates cell contents from external environment. Controls what enters and leaves the cell. Identifies cell as self. Acts as a receptor for various chemicals. site of chemical reactions.

What is the structure and function of the centrioles?

Present in animals only. Two bundles of microtubules at right angles. Used as an anchor point to separate chromosomes during cell division. Form the basis of cilia.

What is the structure and function of the cell wall?

Present in plants and fungi only. Bundles of cellulose fibres in plants. Chitin in fungi. Provides support and strength, maintaining the cell's shape. prevents the cell from bursting when turgid. Permeable to allow solutions through.

What is the structure and function of the flagella?

9 pairs of microtubules surround 2 lone microtubules. A membrane covers the whole thing. Used for moving unicellular organisms around.

What is the structure and function of the cillia?

9 pairs of microtubules surround 2 lone microtubules. A membrane covers the whole thing. Present in great numbers. Move together in a wafting pattern to move single celled organisms. When present on tissues waft to move fluids such as mucus.

Which structures are involved in the production of proteins?

Nucleus; RER; transport vesicle; Golgi apparatus

What is the structure and function of the cytoskeleton?

Microfilaments made of actin - allow cell movement and cytokinesis. Microtubules made of tubulinsscaffold like structures to give cell shape. Intermediate fibres give mechanical strength, maintaining integrity.

What do prokaryotic and eukaryotic cells have in common?

Plasma membrane; cytoplasm; ribosomes; DNa and RNA

What is different between prokaryotic and eukaryotic cells?

Prokaryotes have no: Nucleus; centrioles; membrane bound organelles (mitochondria, chloroplast, RER, SER, Golgi apparatus). Prokaryotes also have: peptidoglycan cell wall; smaller ribosomes; naked loop of DNA; plasmids.

How do prokaryotes divide?

Binary fission

State the three main factors that affect the need for an exchange system

Size, SA:Vol ratio and level of activity

Explain why smaller organisms have a lower demand for oxygen than larger organisms

Smaller organisms have a larger SA:V ratio than larger organisms; simple diffusion can be sufficient for unicellular organisms but due to many-layered multicellular organisms, it would be too slow. Multicellular organisms therefore need transport systems

Describe and explain three features of a good exchange surface

Large SA (folded walls; provides more space for relevant molecules to pass through); thin, permeable barriers (reduces diffusion distance); good blood supply (keeps high concentration gradients for rapid diffusion)

Describe how human alveoli are adapted to reduce diffusion distances

Alveolus wall one cell thick; capillary wall one cell thick; walls of alveoli/capillaries contain squamous (flattened) cells; caoillaries are in close contact with alveoli walls; capillaries are narrow to restrict RBC movement

Describe the role of surfactant in alveoli

Coats the internal surface of the alveoli to reduce cohesive forces between water molecules, preventing collapse

Describe the mechanism of inspiration

Diaphragm contracts (moves down and flattens); external intercostal muscles contract to push ribcage up and out; volume of thorax increased and so pressure decreases below atmospheric pressure; air rushes into lungs down a pressure gradient

Why do alveolar walls contain elastic fibres?

To stretch during inspiration and recoil , pushing air out, during expiration

Which tissue type comprises alveolar walls?

Squamous epithelium

The trachea is lined with ciliated epithelial tissue and goblet cells. Describe the functions of these.

Goblet cells produce mucus onto the tracheal lining, trapping dust and microorganisms. The cilia then beat and move the mucus away from the lungs and towards the throat

The trachea and bronchi are lined with cartilage. Describe why.

C-shaped rings of cartilage line these tubes, preventing collapse during inspiration. The C-shape allows food to pass down the oesophagus behind the trachea

Describe how the nasal cavity is adapted for exchange

Large SA with a good blood supply, warming air to body temperature; lined with hair (which secretes mucus) to trap dust and MOs, protecting from infection; moist surfaces to increase the humidity of the incoming air, reducing evaporation from exchange surfaces

Describe the roles of smooth muscle and elastic tissue in the airways

Smooth muscle can contract to constrict airways (not under conscious control, i.e. involuntary); elastic fibres elongate smooth muscle again, recoiling the airway to its original shape and size (dilates airway)

Describe precautions that must be taken when using a spirometer

Subject should be free of asthma; there should be no air leaks in the apparatus; mouthpiece should be sterilised; soda lime should be fresh and functioning

Describe what is meant by the term vital capacity and state the factors that it depends upon

The maximum volume of air that can be moved by the lungs in one breath; measured by taking one deep brearh and expiring all the possible air from the lungs. Depends on: size (height) of person; age/gender; exercise levels.

State the usual range for vital capacity

2.5 - 5.0 dm3

Describe what is meant by residual volume and state the standard volume

The volume of air that remains in the lungs even after forced expiration, i.e. the air that remains in the airways and alveoli (usually 1.5 dm3)

Describe what is meant by tidal volume stating a typical figure

Volume of air moved in and out with each breath at rest. A normal value would be 0.5 dm3.

State the equation for ventilation rate

Ventilation rate \= tidal volume x breathing rate (breaths per minute)

The normal breathing rate of a healthy 50 year old woman is 18 breaths per minute and her tidal volume is 500 cm3. During strenuous exercise, her ventilation rate increases to 45 000 cm3min-1 and she is breathing 30 times a minute. Calculate her tidal volume during this exercise and state how much higher than normal this figure is.

TV \= 1500 cm3 ...1 dm3 higher than normal

Most bony fish have 5 pairs of gills which are covered by a bony flap, known as the ...? What is the function of this bony flap?

Operculum; protects the gills and ensures a constant flow of water

Describe the structure of gills in bony fish

Two rows of gill filaments; these are slender branches of tissue known as primary lamellae and are attached to a bony arch. Each gill filament is folded into secondary lamellae providing a very large surface area

Describe ventilation in bony fish

Buccal cavity (mouth) can change volume; floor of mouth moves downwards, drawing water into the buccal cavity; mouth closes and water is pushed through the gills. As water is pushed from the buccal cavity through the gills, the operculum moves outwards

How is air supplied to respiring tisses in an insect?

Tracheal system

How does air enter the tracheal system in an insect?

Spiracle

The ends of tracheoles in insects are filled with tracheal fluid. What is the function of this fluid?

Gaseous exchange occurs between air in tracheole and the tracheal fluid

When an insect is active, what changes occur in the insect to increase their oxygen supply?

Tracheal fluid can be withdrawn into the body fluid to increase the surface area of the tracheole wall exposed to air

What is the fluid mosaic model?

The theory of the cell membrane formed from a sea of phospholipids emmeded with proteins.

What is a glycolipid?

A lipid with a carbohydrate molecule attached.

What is a glycoprotein?

A protein with a carbohydrate molecule attached.

State four functions of membranes at the surface of cells

Compartmentalisation (separates cell's components from its external surroundings ensuring conditions inside cell remain unaffected); regulates transport of material into and out of cells (endocytosis and exocytosis); has surface antigens so the body's immune system recognises the cell as 'self'; contains receptors for chemical signals (hormones, drugs etc); may be the site of chemical reactions

State 3 functions of membranes within cells

Compartmentalisation (maintains specific conditions for metabolic reactions to take place, e.g. photosynthesis in chloroplasts, aerobic respiration in mitochondria); inner mitochondrial membrane is highly folded providing a larger surface area for electron carrier proteins; thylakoid membranes within chloroplasts house chlorophyll, the primary pigment required for light absorption in photosynthesis

The principal design of the plasma membrane consists of two layers; what name is given to these two layers?

Phospholipid bilayer

Explain the orientation of phospholipids within the bilayer

Hydrophilic phosphate heads are orientated outwards (towards water); hydrophobic fatty acid tails are orientated inwards (away from water)

Describe the function of glycoproteins and glycolipids in the phospholipid bilayer

As markers or antigens; cell signalling; cell communication; call adhesion

Give 3 examples of intrinsic proteins

Channel proteins, carrier proteins, glycoproteins

Channel proteins and carrier proteins have what main role within the membrane?

Transport

Describe the position and role of cholesterol in the membrane

Interspersed between phospholipid molecules; regulates fluidity of membrane by coming between phospholipids preventing crystallisation. Also bonds to phospholipids, preventing it becoming too fluid.

Describe the structure of a phospholipid

Glycerol backbone; one hydrophilic phosphate head; two hydrophobic fatty acid tails

Define the term diffusion.

The net movement of a substance from an area of high concentration to an area of low concentration. It is passive, does NOT require ATP.

Define the term facilitated diffusion

Movement of molecules from a high concentration to a low concentration, across a partially permeable membrane, via specific channel or carrier proteins. It is passive, does NOT require ATP

What is a channel protein?

A protein which creates a fluid filled pore in the cell membrane through which ions and small polar molecules can pass.

What is a carrier protein?

A protein which changes shape to allow larger molecules to pass through the membrane. In facilitated diffusion this requires no energy, in active transport it requires ATP.

What types of molecules can diffuse directly across the phospholipid bilayer?

Small molecules such as oxygen and carbon dioxide; lipid-soluble molecules such as steroid hormones and alcohol

Describe the movement of water across the phospholipid bilayer

Water is polar and insoluble in lipid, BUT because it is present in such high quantities, significant direct, simple diffusion does occur. In membranes (e.g. collecting duct) where a very high rate of water movement is required, special channel proteins known as aquaporins are inserted into the membrane.

Describe the effects of temperature on membrane structure

As temperature increases, phospholipids will have more KE, so move faster and more, making the bilayer more fluid and it begins to lose its structure. This loss of structure allows molecules to cross it more easily. Carrier and channel proteins may become denatured, affecting membrane transport.

Describe the effects of ethanol on membrane structure

Non-polar ethanol molecules can insert themselves into the bilayer disrupting the structure, meaning the bilayer becomes more fluid and permeable

State 5 factors that affect the rate of simple diffusion

Temperature, surface area, diffusion distance, size of molecule, concentration gradient

Define the term active transport

Movement of molecules, against their concentration gradient (using energy liberated from ATP hydrolysis) using specific protein channels or carriers

Describe how carrier proteins are used in active transport

Molecule binds to specific site in carrier protein; ATP binds to separate binding site; carrier protein changes shape (conformational change) and transports molecule across membrane

Define bulk transport and give two examples

The movement of large molecules that are too big to pass across the plasma membrane. Endocytosis (phagocytosis or pinocytosis) brings large molecules INTO the cell, enclosed in a vesicle. Exocytosis transports large molecules OUT of cells.

Define and describe phagocytosis

The intake of solid particles into the cell by engulfing. Pseudopodia surround the particles, the membrane fuses together, to form a vesicle.

Define and describe pinocytosis

The intake of liquids into the cell by engulfing. The plasma membrane invaginates, and the membrane fuses around the substance, forming a vesicle.

Define and describe exocytosis

The bulk transport of particles too large to pass through the membrane, out of the cell. It works like a reversal of pinocytosis. A vesicle containing the substance fuses with the plasma membrane. The fused site opens, releasing the contents of the secretory vesicle.

Describe the role of ATP in bulk transport

ATP is required to provide energy for the movement of vesicles along microtubules of cytoskeleton (via motor proteins); ATP is needed to fuse vesicle membrane and plasma membrane together (PM is changing shape)

What is the main difference between active transport and facilitated diffusion?

Active transport requires ATP; facillitated diffusion is passive

Define the term osmosis

The movement of water molecules from a region of higher water potential to a region of lower water potential, across a partially permeable membrane

What is water potential?

The tendancy of water molecules to move from one region to another.

What substance has the highest possible water potential of 0 kPa?

Pure water

State the equation that links water potential, solute potential and pressure potential

WP \= SP + PP

As more solute is added to a solution, what happens to the solute potential and hence the water potential?

Solute potential decreases, hence water potential decreases