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B1: Cell structure and transport (1.10: Exchanging materials (Materials…
B1: Cell structure and transport
1.1: The world of the microscope
Light microscopes
Use a beam of light to form an image
Can magnify objects up to 2000 times
Can be used to view living objects
Relatively cheap
Relatively easy to carry around
Electron microscopes
Use a beam of electrons to form an image
Can magnify objects up to 2,000,000 times
Cannot be used for living objects
Very expensive
Need to be kept in special conditions
Resolving power
A measure of the ability to distinguish between 2 separate points that are very close together
Magnification = image size / size of actual object
1.2: Animal and plant cells
Animal cells
Range in size from about 10-30 micrometres
Made up of
Nucleus
Controls the cells activities
Genes on chromosomes within the nucleus that carry the instructions for making proteins
Cytoplasm
A liquid gel in which organelles are suspended, and where many chemical reactions take place
Cell membrane
Controls movement of substances in and out of the cell
Mitochondria
Where energy is transferred during aerobic respiration
Ribosomes
Where protein synthesis takes place
Plant cells
Range in size from 10-100 micrometres
Made up of
Rigid cell wall
Made of cellulose
For support
Chloroplasts
Contain chlorophyll for photosynthesis - chloroplasts absorb light to make food
Permanent vacuole
Contains cell sap which keeps the cell rigid and supports the plant
1.3: Eukaryotic and prokaryotic cells
Eukaryotic cells
All animals (including humans), plants, fungi, and protista are eukaryotes
Eukaryotic cells contain a cell membrane, cytoplasm, and a nucleus - the nucleus contains chromosomes, which are made up of the genetic material called DNA
Prokaryotic cells
Bacteria are single-celled living organisms - these are examples of prokaryotic cells
Bacteria are very small (0.2-2 micrometres) and can only be seen with a powerful microscope
Bacterial cells have a small membrane and a cell wall surrounding the cytoplasm - the cell wall is different from a plants cell wall because it isn't made of cellulose
Bacteria do not have a nucleus - the genetic material is found in the cytoplasm as a long circle of DNA - some prokaryotic cells also contain extra small circular rings of DNA called plasmids
Some bacteria have a protective slime capsule, others have a flagella for movement
Not all bacteria are harmful but some cause diseases in animals and plants - bacteria may cause stored food to decompose
When bacteria multiply they form a colony - bacterial colonies can be seen with the naked eye
1.4: Specialisation in animal cells
Large organisms are made of many cells - as the organism develops, cells differentiate (become specialised) to form different types of cells - these specialised cells may work individually, or together as part of a tissue, organ, or a whole organism
Specialised cells may contain large numbers of particular sub-cellular structures
Examples of specialised cells
Muscle cells
Special proteins that slide over each other
Many mitochondria to transfer the energy needed for chemical reactions
A store of glycogen that can be broken down and used in respiration to transfer energy
Sperm cells
A middle section full of mitochondria to transfer the energy needed by the tail to move
An acrosome to store digestive enzymes to break down the outer layers of the egg
A large nucleus to contain the genetic information
A long tail that whips from side to side to move the sperm
Nerve cells
Many dendrites to make connections to other nerve cells
An axon to carry impulses from one place to another
Nerve endings or synapses, which pass impulses to other cells by producing transmitter chemicals
Many mitochondria in the synapses to transfer the energy needed to make the transmitter chemicals
1.5: Specialisation in plant cells
Root hair cells
Greatly increase the surface area available for water to move into a cell
Have a large permanent vacuole to speed up the movement of water by osmosis from the soil across the root hair cell
Have many mitochondria to transfer the energy needed for the active transport of mineral ions into the root hair cells
Photosynthetic cells
Have chloroplasts containing chlorophyll to trap the light needed for photosynthesis
Are often found in continuous layers in the leaf and outer layers of stems
Have a large permanent vacuole that helps keep the cell rigid
Xylem cells
Xylem tissue has 2 main functions -to support the plant -to transport water and mineral ions from the roots to the stem and leaves
Living when they're first formed
Then a chemical called lignin builds up in spirals in the cell walls
The cells die leaving long hollow tubes - water and mineral ions can move up the tubes
The spirals and rings of lignin make the tubes of xylem very strong
Phloem
The tissue that transports food made by photosynthesis to the rest of the plant - phloem cells form tubes but do not become lignified like the xylem
The cell walls between the phloem cells break down to form sieve plates
Phloem cells lose a lot of their structures but are kept alive by companion cells
The companion cells contain mitochondria that transfer energy to aid the movement of dissolved food in the phloem
1.6: Diffusion
Molecules in liquids and gases move around randomly because of the energy they have
Diffusion is the spreading out of the particles of a gas or a substance in a solution
The net movement into or out of cells depends on the concentration of the particles on each side of the cell membrane
Because the particles move randomly, there will be a net (overall) movement from an area of high concentration to an area of lower concentration
The difference in concentration between 2 areas is called the concentration gradient
The larger the difference in concentration, the faster the rate of diffusion
An increase in temperature cause particles to move faster, which also increases the rate of diffusion
Examples of diffusion
The diffusion of oxygen and glucose into the cells of the body from the bloodstream for respiration
The diffusion of carbon dioxide into actively photosynthesising plant cells
The diffusion of oxygen and carbon dioxide in opposite directions in the lungs, known as gas exchange
The diffusion of simple sugars and amino acids from the gut through cell membranes
1.7: 0smosis
Osmosis is the diffusion of water across a partially permeable membrane - just like diffusion, the movement of water molecules is random and requires no energy from the cell
The water moves from an area of high water concentration to an area of low water concentration
The cell membrane is partially permeable
Terms to compare concentrations of 2 solutions
If the 2 solutions have the same concentrations they are isotonic
The solution that's more concentrated (more solute and less water) is hypertonic
The solution that's more dilute (more water and less solute) is hypotonic
Osmosis in animals
Animal cells that are surrounded by a hypotonic solution will swell and possibly burst because water moves into the cell my osmosis
If the solution around animal cells is hypertonic then water moves out of the cells and they shrink
Animals need complex mechanisms to control the concentration of the solutions around their cells to avoid bursting or shrinking
1.8: Osmosis in plants
In plants, osmosis is the key to their whole way of life
Turgor pressure occurs when no more water can enter a cell due to the pressure inside
water moves into plant cells by osmosis > the vacuole swells > the cytoplasm is pressed against the cell wall > the cell becomes rigid > the leaves and stem become rigid
As long as the outside solution is hypotonic water moves in and keeps the cells rigid, which supports the plant
Plant cells in a hypertonic solution lose water and become flaccid, so the plant wilts
When plant cells are placed in hypertonic solutions, a lot of water leaves the cell - the vacuole and cytoplasm shrink, then the membrane pulls away from the cell wall - this is referred to as plasmolysis
1.9: Active transport
Cells may need to absorb substances that are in short supply (against the concentration gradient)
Cells use active transport to absorb substances across partially permeable membranes against the concentration gradient
Active transport requires energy from respiration to move substances against a concentration gradient
Cells are able to absorb ions from dilute solutions
Glucose can be absorbed out of the gut and kidney tubules against are large concentration gradient by active transport
People with cystic fibrosis have thick, sticky mucus because the active transport system in their mucus cells isn't working properly
1.10: Exchanging materials
Materials such as oxygen and soluble food molecules need to reach all cells, and metabolic waste materials must be removed efficiently
Small organisms have a large surface area to volume ratio - single celled organisms are tiny and can gain enough of materials such as oxygen by diffusion through their surface
As organisms increase in size, their surface area to volume ratio decreases
Large, complex organisms have many cells that are not in contact with the environment, so they have special exchange surfaces to obtain all the food and oxygen they need
Efficient exchange surfaces have a large surface area, thin membranes or a short diffusion path, and an efficient transport system - the blood supply in animals
Gaseous exchange surfaces in animals must be well ventilated - oxygen is absorbed by the alveoli in the lungs when air is drawn in during breathing - the alveoli have a large surface area and a good blood supply to carry the oxygen away and maintain a concentration gradient
The villi of the small intestine have a large surface area, a short diffusion path, and a good blood supply to absorb soluble food molecules
Fish have gills, which are the gaseous exchange surface between the water and the blood - a flap, the operculum acts as a pump to maintain the flow of water over the gills - the blood carries the oxygen away to maintain a concentration gradient
Plants have long, thin roots to increase the surface area for water absorption - the root hair cells increase the surface area even more
Plant leaves are modified for efficient gaseous exchange - the leaves are flat and thin with internal air spaces and stomata to allow gases in and out of the leaves