Parts of a Cell
The different parts of a cell all have different functions:
- The Nucleus controls what the cell does and contains the genetic information. It has a membrane extending from it onto which ribosomes are attached.
- Ribosomes are where protein synthesis occurs.
- The Cell Membrane controls movement into and out of the cell.
- Cytoplasm is where chemical reactions take place. The Cytoplasm may also contain mitochondria which release the most energy in respiration.
- Chloroplasts (only found in plant cells) contain chlorophyll which absorbs light to make energy
- The Permanent Vacuole (only found in plant cells) is filled with sap, it helps support the cell
- The Cell Wall (only found in plant cells) is made of cellulose, this strengthens the cells
- Root hair cells have a tiny, hair-like structure which increases the surface area of the cell. This enables it to absorb water and ions more efficiently.
- Palisade cells are column-shaped cells found on the upper surface of the leaf. They contain chloroplasts for photosynthesis.
- Xylem cells are long, thin, hollow cells that contain no cytoplasm. They transport water through the stem and root.
- Nerve cells (neurones) have long slender axons, which can carry nerve impulses over distances as long as a metre.
- The Ovum or Egg cell is much larger than other cells so that it can carry massive food reserves, for the developing embryo.
- The Sperm cell is the most mobile cell because of it's tail. It uses this tail to travel from the vagina to the ovum.
- Red Blood cells have no nucleus. This means that they have more room for haemoglobin, which enables them to be able to carry lots of oxygen.
- White Blood cells can change their shape in order to engulf and destroy microbes which have invaded the body.
Diffusion and Osmosis
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 a solution, which results in a net movement from a region where they are at a higher concentration to a region where they are at a lower concentration.
The greater the difference in concentration, the faster the rate of diffusion.
Osmosis is the movement of water from a dilute solution (with a high water to solute ratio) to a more concentrated solution (with a low water to solute ratio) through a semi-permeable membrane. The membrane allows the water molecules through but not the solute molecules because they are too large.
The effect of osmosis is to gradually dilute the solution. This is what happens in root hair cells, where water moves from the soil into the cell by osmosis.
Green plants make energy using sunlight, by a process called photosynthesis.
Four things are needed for photosynthesis to take place:
- Light from the Sun
- Carbon Dioxide diffused from the air
- Water from the soil
- Chlorophyll in the leaves.
Two things are produced during photosynthesis:
- Glucose - used for energy via respiration
- Oxygen - released into the atmosphere as a by-product.
The glucose which is not used to provide energy is converted into insoluble starch, which is stored in the stem, leaves or roots.
Carbon Dioxide + Water ---(light + chlorophyll)--> Glucose + Oxygen
Factors Affecting Photosynthesis
Temperature, carbon dioxide concentration and light intensity interact to limit the rate of photosynthesis. Any one of them, at a particular time, may be the limiting factor.
When these factors are increased, the rate of photosynthesis increases, given that there is no limiting factor. However, when the temperature approaches 45°C, the enzymes controlling photosynthesis start to be destroyed and so the rate drops to zero.
Photosynthesis, and therefore growth, can me controlled when these factors are controlled. For example, a greenhouse can be modified in order to increase the growth rate of a plant, and to increase it's size and strength by:
- Increasing temperature (up to about 40°C)
- Increasing carbon dioxide levels
- Increasing light intensity.
Food Chains and Biomass
A food chain always starts off with a plant. The energy stored in this plant is then passed on to organisms which eat the plant. This transfer of energy can be represented as a food chain, for example:
Grass > Rabbit > Stoat > Fox
The mass of living material is known as biomass. At each stage of the food chain, the biomass is less than it was at the previous stage. This is because materials and energy are lost in a number of ways, such as running, mating, excreting and generating heat. Warm-blooded animals must generate heat because their bodies must be kept at a constant temperature, however this results in a lot of energy being lost as heat energy.
We can improve the amount of energy and biomass we receive from our food (improving the efficiency of food production) either by reducing the number of stages in a food chain or by limiting an animal's movement, and controlling it's temperature (battery farms). This means less energy is lost through heat and movement.
Recycling the Materials of Life
Living things remove materials from the environment for growth and other processes, but when these organisms die or excrete waste, these materials are returned to the environment.
The key to this is the micro-organisms which break down the waste and the decaying bodies. This decay process releases substances used by plants for growth.
An example of recycling the materials of life:
- A deer eats plants, then waste is produced, returning some materials to the environment.
- Eventually, the deer dies, and the decaying body is broken down by micro-organisms.
- The materials from this are then absorbed by plants, ready for another animal to eat.
The Carbon Cycle
The Carbon Cycle:
CO2 is removed from the atmosphere by green plants to produce glucose by photosynthesis. Some is returned to the atmosphere by the plants during respiration.
The carbon obtained by photosynthesis is used to make carbohydrates, fats and proteins in plants. When the plants are eaten by animals, this carbon becomes carbohydrate, fat and protein in animals.
Animals release CO2 into the atmosphere during respiration.
When plants and animals die, other animals and micro-organisms feed on their bodies causing them to break down.
When micro-organisms and animals feed off of dead plants and animals, they respire, releasing CO2 into the atmosphere.
Enzymes are produced by living cells. They have many functions inside and outside cells.
Enzymes are biological catalysts - they increase the rate of chemical reactions in an organism.
Enzymes are protein molecules, made up of long chains of amino acids. These are folded into a 3D shape, which lets other molecules fit into the enzyme.
Different enzymes work best at certain temperatures and pH levels.
High temperatures destroy most enzymes, which is why it is dangerous for a human's body temperature to go above 37°C.
Enzymes Inside Living Cells
Enzymes in living cells catalyse (speed up) processes - such as respiration, protein synthesis and photosynthesis.
The energy released during respiration is used to: (the four 'M's)
- build the larger molecules using smaller ones
- enable muscles to contract (animals)
- maintain a constant temperature in colder surroundings (animals and birds)
- make proteins in plants from amino acids (made from sugars and nitrates)