B1 Cell Structure and Transport

Light microscopes use light and lenses to form an image of a specimen and magnify it. They let us see individual cells and large subcellular structures. E.g. nuclei. They are relatively cheap. 

Electron microscopes uses elctrons to form an image. They allow us to see inside subcellular cells. They have much higher magnification and resolution than a light microscope. They are very large and expensive and have to be kept in special temperature, pressure, and humidity- controlled rooms.

• 1km = 1000m                                                Real size = image size / magnification 
• 1m = 100cm                                                  Magnification = image size / real size 
• 1cm = 10mm                                                 Image size = magnification x real size
• 1mm = 1000μm
• 1μm = 1000nm  

Animal cells

• Nucleus- controls all the activities of the cell. Contains genetic material.
• Cytoplasm- a liquid gel in which organelles are suspended and where most of the chemical reactions take place.
•  Cell membrane- Holds the cell together and controls what goes in and out. E.g. glucose and mineral ions into the cells.
• Mitochondria- structures in the cytoplasm, where most of aeorbic respiration take place which release energy for the cells to work.
• Ribosimes- where protein synthesis takes place

Plant cells

• Cell wall- made of cellulose. supports and strenghtens it.
• Permanent vacuole- contains cell sap. Important for keeping the cells rigid.
• Chloroplasts- where photosynthesis occurs. They contain a green substance called chloroplast which absorbs the light needed for photosynthesis.

Cells can either be eukaryotic or prokaryotic.

• Eukaryotic cells are complex and include all animal and plant cells. Eukaryotes are are organisms that are made up of eukaryotic cells
• Prokaryotic cells are smaller and simpler, e.g. bacteria. A prokaryote is a prokaryotic cell. (it's a single-celled organism).

Bacteria

• Bacteria have cytoplasm and a cell membrane surrounded by a cell wall, but the cell wall does not contain the cellulose you see in plant cells.
• Prokaryotes have no nucleus- instead the bacteria has a singluar circular strand of DNA that floats in the cytoplasm.
• They may also contain small rings of DNA called plasmids. 
• Some types of bacterium have at least one flagellum (plural: flagella), that is a long protein strand that lashes about and help the bacteria to move around.

Nerve cells- are specialised to carry electrical impulses around the body of an animal. They have several adaptations such as: 

• Lots of dentries to make connections to other nerve cells.
• An axon that carries the nerve impulse from one place to another. Can be very long.
• Synapses are adpated to pass the impulses to another cell or between a nerve cell and a muscle in the body using special transmitter chemicals.

Muscle cells are specialised cells that contract and relax e.g. striated (striped) muscle cells. They have three main adaptations, they:

• Contain special proteins that slide over each other making the fibres contract.
• Contain many mitochondria to transfer the energy needed for the chemical reactions that take place as the cells contract and relax.
• Can store glycogen, a chemical that can be broken down and used in cellular respiration by the mitochondria to transfer the energy needed for the fibres to contract.

Sperm cells need to get the genetic material from the male parent to to the egg. They have several adaptations to do this:

• Have a long tail to help swim to the egg.
• The middle section is full of mitochondria, which transfers energy needed for the tail to work.
• The acrosome stores digestive enzymes for breaking down the outer layers of the egg
• A large nucleus contains the genetic information to be passed on.

Root hair cells are cells on the surface of plant roots, which grow into long "hairs". They help take up water and mineral ions efficiently for the plant. They have three main adaptations:

• They greatly increase the surface area allowing more water and mineral ions to be absorbed.
• The have a large permanent vacuole that speeds up the movement of water by osmosis from the soil across the root hair cell.
• They have many mitochondria that transfer the energy needed for the active transport of mineral ions into the root hair cells.

Phtotsynthetic cells have a number of adaptations including:

• They contain chloroplast that contain chlorphyll that trap the light needed for photosynthesis
• They are usually positioned in continuous layers in the leaves and outer layers of the stem of a plant so they absorb as much light as possible .
• They have a large permanent vacuole that helps keep the cell rigid as a result of osmosis. They also keep the leaf spread out so it catches as much light as possible.

Xylem is the transport tissue in plants that carries water and mineral ions from the roots to the highest leaves and shoots.The xylem is also important in supporting the plant. They are adapted to their function in two ways:

• The xylem cells are alive when they are first formed but a chemical called lignin builds up in spirals in the cell walls. The cells die and form long hollow tubes that allow water and mineral ions to move easily through them, from one end of the plant to the other.
• The spirals and rings of lignin make the xylem cells very strong and help them withstand the pressure of water moving up the plant.

Phloem is the speacialised transport tiissue that carries the food made by photosynthesis around the plant. The adaptations include: 

• The cell walls between the cells break down to form special sieve plates. These alow water carrying disolved food to move freely up and down the tube to where it is needed.
• Phloem cells lose a lot of their internal structure but they are supported by companion cells taht help to keep them alive. The mitochondria of the companion cells transfer th energy needed to move dissolved food up and down the plant in phloem.

Diffusion is the spreading out of particles of a gas, or of any substance in a solution resulting in a net movement of particles from an are of higher concentration to an are of lower concentration down a concentration gradient. 

• The greater the difference in concentration, the faster the rate of diffusion. This difference between two areas of concentartion is called the concentraion gradient. 
• Temperature also affects the rate of diffusion. The higher the temperature the faster the particles move around, so diffusion happens more rapidly .

Dissolved substances move in and out of your cells by diffusion across the cell membrane. E.g. simple sugars, such as glucose, gases, such as oxygen and carbon dioxide. Cell membrane is semi-permeable.

Individual cells may be adapted to make diffusion easier and more rapid. The most common adaptation is to increase the suraface area of the cell membrane by folding up the cell, or the tissue lining the organ. Therfore the area over which diffusion takes place is greatly increased. Folds in the cell membrane form microvilli which increse the surface area of the cell.

Osmosis is he movement of water molecules across a partially permeable membrane from a region of higher water concenteration to a region of lower water concentration.

The concenteration of solutes inside your body cells need to stay at the same level for them to work properly. However, the concentration of the solutions outside your cells may be very different to the concentration inside them. This concentration gradient can cause water to move into or out of cells by osmosis

• If the concentration of solutes in the solution outside the cell is the same as the internal concentration, the solution is isotonic to the cell.
• If the concentration of solutes in the solution outside the cell is higher than the internal concentration, the solution is hypertonic to the cell. 
• If the concentration of solutes in the solution outside the cell is lower than the internal concentartion, the solution is hypotonic.

Osmosis will try to even out the concentration if its unbalanced. In some cases osmosis can damage animal cells if the concentration outside the cell changes dramatically.

1) Cut up a potato into identical cylinders, and get some beakers with different sugar solutions in them. One should be pure water and another should be very concentrated sugar solution. You can also have a few with concentrations in between.

2)Measure the mass of the potato cylinders, then leave each one in the solutions for 24 hours or so. 

3) Then you take them out, dry them and measure their masses again.

4) If the cylinders have drawn in water by osmosis, they'll have increased in mass. If water has been drawn out, they'll have decreased in mass. 

5) Draw out your results in a table.

• The dependent variable is the chip mass.
• The independent variable is the concentration of the sugar solution.
• All other variables (volume of solution, temperature, time, etc) must be kept the same in each case or the experiment isn't a fair test. (Control variables)]

6) Repeat the experiment to make sure the results are accurate.

Plants rely on osmosis to support their stems and leaves. Water moves into plant cells by osmosis. 

Turgor is the pressure inside a plant cell exerted by the cell contents pressing on the cell wall. Turgor pressure makes the cells hard and rigid, which in turn keeps the leaves and stems of the plant rigid and firm.

Plants need fluid surrounding the cell to be hypoyonic to the cytoplasm. If the solution surrounding the plant cells is hypertonic to the cell contents, water will leave the cells by osmosis. The cells will no longer be firm and swollen - they become flaccid as there is no pressure on the walls. At this point the plant wilts.

If more water is lost by osmosis, the vacuole and cytoplasm shrink, and eventually the cell membrane pulls away from the cell wall. This is plasmolysis. Plasmolysed cells die quickly unless osmosis restores the balance.

Active transport is the movement of substances from a dilute solution to a more concentrated solution against a concentration gradient, requiring energy from respiration. As a result, cells cn absorb ions from very ilute solutions. It also enables cells to move substances, such as sugars and ions, from one place to another though the cell membrane.

Energy is needed for the active transport system to carry a molecule across the membrane and then return to its original position. This energy is produced during cell respiration.

Active transport is widleyt used in cells. There are some situations where it is particularly important. E.g. mineral ions in the soil, such as nitrate ions, are usually found in very dilute solutions. These solutions are more dilute than the solution within the plant root hair cells. By using active transport, plants can absorb these mineral ions, even though it is against a concentration gradient.

As living organisms get bigger and more complex, their surface area to volume ratio gets smaller. This makes it increasingly difficult to exchange materials quickly enough with the outside world:

• Gases and food molecules can no longer reach every cell inside the organism by simple diffusion.
• Metabolic waste cannot be removed fast enough to avoid poisoning the cells.

There are various adaptations to make the process of exchange more efficient. The effectiveness of an exchange surface can be increased by:

• Having a large surface area over which exchange surface can take place.
• Having a thin membrane to provide a short diffusion path
• In animals, having an efficient blood supply moves the diffusing substance away from the exchange surfaces and maintains a steep concentration (diffusion) gradient.
• In animals, being ventilated makes gas exchange more efficient by maintaining steep concentration gradients.