B1 Cell Structure and Transport

Light Microscopes

• Low magnification
• Cheap
• Can be used anywhere
• Observe live specimens

Electron Microscopes

• Higher magnification
• Finer detail
• Large
• Expensive
• Need to be kept in special conditions

Magnification = size of image / size of object

Animal cells

• NUCLEUS - controls all activities of the cell. Contains chromosomes which carry genes that carry instructions for making new cells and organisms.
• CYTOPLASM - a liquid gel in which the organelles are suspended. Chemical reactions take place here.
• CELL MEMBRANE - controls passage of substances in and out of cell such as glucose, mineral ions, urea and hormones.
• MITOCHONDRIA - where aerobic respiration takes place, releasing energy for cell.
• RIBOSOMES - where protein synthesis takes place.

Plant cells only

• CHLOROPLASTS - contain green substance chlorophyll. Absorbs light so plant can make food by photosynthesis.
• VACUOLE - filled with cell sap, keeps cells rigid for plant support.
• CELL WALL - made of cellulose that strengthens and supports plant.

Eukaryotic cells

• Plant and animal cells are eukaryotic because they have:
  • Cell membrane
  • Cytoplasm
  • Gentic information enclosed in a nucleus

Prokaryotic cells

• Bacterial cells are prokaryotic because they have:
  • Cytoplasm and cell memebrane surrounded by cell wall (not made of cellulose)
  • Genetic material found in a single loop of DNA 
  • Rings called plasmids (code for antibiotic resistance)

Nerve cells

• Dendrites - make connections to other nerve cells
• Axon - carries nerve impulse from one place to another
• Synapses - pass impulses between nerve cell and muscle cell in the body using special transmitter chemicals

Muscle cells

• Special proteins - slide over each other making fibres contract
• Mitochondria - transfer energy for chemical reactions as cells contract and relax
• Glycogen storage - chemical broken down and used in cellular respiration by mitochondria to transfer energy needed for fibres to contract

Sperm cells

• Long tail - helps sperm move
• Acrosome - stores digestive enzymes for breaking down egg
• Nucleus - contains the genetic info to be passed on.

Root hair cells

• Help to take up water and mineral ions more efficiently 
• Always relatively close to the xylem tissue
• Mineral ions move into the root hair cell by active transport
• Greatly increase surface area available for water to move into cell
• Large permanent vacuole - speeds up osmosis from soil into cell
• Many mitochondria - transfer energy needed for active transport of mineral ions into cell. 

Photosynthetic cells

• Main concern = photosynthesis and getting as much light as possible
• Chloroplasts - contain chlorophyll that trap light needed for photosynthesis
• Continuous layers - max light absorption
• Large permanent vacuole - keeps cell rigid through osmosis and with photosynthetic tissue it keeps the stem supported
• Large permanent vacuole - keeps leaf spread out to absorb light.

Xylem cells

• Tissue responsible for bringing water and mineral ions from the roots up to the leaves and shoots. 
• Lignin builds up in cell walls and cells die. Long hollow tubes formed to allow water and mineral ions to move easily in plant.
• Spirals and rings of lignin make them strong and able to withstand pressure of the water moving up plant. 
• Supports plant. 

Phloem cells

• Tissue responsible for carrying food made by photosynthesis around body of plant.
• Phloem cells form tubes but they do not die. Dissolved food moves up and down the tubes.
• Cell walls between cells break to form sieve plates - water carrying dissolved food can move freely up and down tubes.
• Companion cells - Mitochondria in companion cells transfer energy to make dissolved food move up and down. 

Substances move in and out of the cell across the cell membrane via diffusion.

Diffusion is the spreading out of particles of a substance in a solution or gas from an area of high to low concentration.

Rates of diffusion are affected by

• Difference in concentrations (concentration gradient) - BIG difference means FASTER diffusion. Diffusion occurs DOWN a concentration gradient.
• temperature - INCREASE means particles move FASTER

In living organisms

• Oxygen passes from lungs to red blood cells by diffusion (high to low). Oxygen passes from RBC's to body cells. 

Increasing the surface area of cell membrane means rate of diffusion increases.

Osmosis is the diffusion of water particles from an area of high concentration to an area of low concentration through a partially permeable membrane.

Dilute solution - has a high concentration of water and a low concentration of solutes. Concentrated solution - has a low concentration of water and a high concentration of solutes.

Isotonic - the concentration of solutes in the solution outside the cell is the same as the internal concentration, the solution is isotonic to the cell.

Hypertonic - the concentration of solutes in the solution outside the cell is higher than the internal concentration, the solution is hypertonic to the cell.

Hypotonic - the concentration of solutes in the solution outside the cell is lower than the internal concentration, the solution is hypotonic to the cell.

If the solution outside the cell becomes too dilute, water moves into the cell and the cell may burst

If the solution outside the cell becomes too concentrated, water moves out of cell and so the cell may become shrivelled.

Water moves into plant cells by osmosis. Water creates turgor pressure that keeps the cells rigid and firm.

Plants need fluid surrounding the cell to be hypotonic to the cell, with lower concentration of solutes and a higher concentration of water than the plant cells themselves.

If the solution outside the cells becomes hypertonic (to the cells), water will leave the cell and the cell will become flaccid.

Plasmolysis may occur when more water is lost. 

Active transport moves substances from an area of low concentration to high concentration  (against a concentration gradient)

Active transport requires energy 

• produced in cell respiration in the mitochondria

Examples

• PLANTS: Nitrate ions in soil (very dilute solutions), more than plant's cells so it can be absorbed by active transport (against concentration gradient)
• HUMANS: Glucose (used for cell respiration) absorbed from lower concentration in the gut into higher glucose concentration in the blood. 

Small organisms

• Large SA:V ratio
• Means short diffusion distances (easier for exchange)

Large organisms

• Small SA:V ratio
• Means distances become larger (harder for exchange)

Larger organisms have special surfaces where exchange of materials take place to move gases, food molecules and metabolic waste.

Effectiveness of an exchange surface can be increased by

• having a large surface area
• having a thin membrane or being thin to provide a short diffusion path
• efficient blood supply maintains a steep concentration gradient
• being ventilated makes gas exchange more efficient