When an egg is fertilised it begins to grow and develop. At first there is a growing ball of cells. Then as the organism gets bigger some of the cells change and become specialised.
There are many different specialised cells:
- Fat Cell
- Cone Cell (from human eye)
- Root Hair Cell
- Sperm Cell
When a cell becomes specialised its structure is adapted to suit the particular job it does. As a result, they may look different to normal cells. Sometimes cells become so specialised that they only have one function within the body. For example, the sperm cell. Specialised cells are often grouped together to form a tissue. In many bigger organisms there is another level of organisation. Several different tissues work together to do particular jobs and can form an organ such as a heart or liver.
How do Substances get in and out of Cells?
Molecules move randomly because of the energy they have. Diffusion is the random movement of molecules from an area of hig concentration to an area of low concentration. The larger the difference in concentration, the faster the rate of diffusion.
The overall/net movement= particles moving in - particles moving out
- The diffusion of oxygen into cells of teh body from the blood stream as the cells are respiring. (and useing up oxygen)
- The diffusion of carbon dioxide into actively photosynthesising plant cells
- The diffusion of simple sugars and amino acids from the gut through cell membranes.
Osmosis is the movement of water. The movement of molecules is random and requires no energy from the cell. It is the diffusion of water across a partially permeable membrane from a dilute solution to a more concentrated solution. No solute molecules can move across the membrane. The cell membrane is particuarly permeable. Water is needed to support cells and because chemical reactions take place in solution.
Osmosis in Plants: Cytoplasm too dry - water will immediatley move in by osmosis. Cytoplasm too dilute - water will leave the cell by osmosis. However, it can also cause serious problems - if the solution outside the cell is more dilute than the cell contents, then water will move into the cell by osmosis. The cell will swell and may burst. However, if the solution outside the cell is more concentrated then water will move out the cell by osmosis. The cytoplasm will become too concentrated and the cell will shrivel up.
Osmosis in Animals: Water moves into plant cells by osmosis, making the cytoplasm swell and press agaisnt the plant cell walls. The pressure builds up until no more water can physically enter the cell. This makes the cell hard and rigid. The swollen state keeps the leaves and stems of the plant rigid and firm. So for plants it is important that the fluid surrounding the cells always has a higher concentration of water than the cytoplasm cells. This keeps osmosis working in the right direction. But sometimes plant cells and animal cells need to move substances such as glucose agaisnt a concentration gradient. For this there is another method of transport known as active transport which uses energy from respiration.
Animal and Plant Cells
Animal and Plant cells have some structures in common:
- Nucleus - controls the cells activities
- Cytoplasm - chemical reactions take place
- Cell Membrane - controls the movement of materials
- Mitochondria - where energy is realeased during aerobic respirtation
- Ribosomes - where proteins are made
Plant Cells also have:
- Rigid Cell Wall - for support
- Chloroplasts - contains chlorophyll for photosynthesis
- Permanent Vacuole - containing cell sap