AQA Biology Unit 2

Cells

Most human cells like most other animal cells have the following parts:

• a nucleus which controls the activities of the cel
• cytoplasm where the chemical reactions take place
• a cell membrane which controlls the exchanges of substances in and out
• mitochondria where the majority of energy is released
• ribsomes where protein synthesis occurs

Plant cells also have a cell wall which strengthens the cell. Plant cells also often have chloroplasts which absorb light energy to respire, a permanent vacuole filled with cell sap

The chemical reactions inside cells are controlled by enzymes

Cells may be specialised to carry out a particular function.

• Dissolved substances can move into and out of cells by diffusion and osmosis. 

•  Diffusion is the spreading of the particles of a gas, or of any substance in solution, resulting in a net movement from a region where they are of a higher concentration. The greater the difference in concentration, the faster the rate of diffusion.  

• Oxygen required for respiration passes through cell membranes by diffusion.  
• Water often moves across boundaries by osmosis. Osmosis is the diffusion of water from a dilute to a more concentrated solution through a partially permeable membrane that allows the passage of water molecules

Photosynthesis is summarised by the equation: carbon dioxide + water (+light energy)  → glucose + oxygen

During phtosynthesis: 

• light energy is absorbed by chlorophyll found in the chloroplasts
• this energy is used by converting water and carbon dioxide into sugar in the form of glucose
• oxygen is released as a by-product

The rate of photosynthesis may be limited by:

• low temperature
• shortage of carbon dioxide
• shortage of light

Light, temperature and the avaliability of carbon dioxide interact and they all could be a limiting factor of photosynthesis.The glucose produced in photosynthesis may also be converted and stored as insoluble starch. Plant cells use some of the glucose produced during photosynthesis for respiration.

Plants roots absorb mineral salts including nitrates needed for healthy growth. For healthy growth plants need mineral ions including: nitrates (they form amino acids then proteins), magnesium (this is needed for chlorophyll production.)

The symptons shown by plants growing in conditions where mineral ions are deficient include: stunted growth if nitrate ions are deficient and yellow leaves if magnesium ions are deficient

Radiation from the Sun is the source of energy for most communities of living organisms. Green plants capture a small part of the solar energy which reaches them. This energy is stored in the substances which make up the cells of the plants. 

The mass of living material (biomass) at each stage in a food chain is less than it was at the previous stage. The biomass at each stage can be drawn to scale and shown as a pyramid of biomass. 

 At each stage in a food chain, less material and less energy are contained in the biomass of the organisms. This means that the efficiency of food production can be improved by reducing the number of stages in food chains.  

The efficiency of food production can also be improved by restricting energy loss from food animals by limiting their movement and by controlling the temperature of their surroundings. 

The amounts of material and energy contained in the biomass of organisms is reduced at each successive stage in a food chain because: 

• some materials and energy are always lost in the organisms’ waste materials  
• respiration supplies all the energy needs for living  processes, including movement.  Much of this energy is eventually lost as heat to the surroundings. 
• these losses are especially large in mammals and birds whose bodies must be kept at a constant temperature which is usually higher than that of their surroundings.