Life and Cells
Most human cells, like most animal cells, have the following parts - 1. Nucleus - contains genetic material that controls the activities of the cell. 2. Cytoplasm - gel-like substance where most of the chemical reactions happen. It contains enzymes that control these chemical reactions. 3. Cell Membrane - holds the cell together and controls what goes in and out. 4. Mitochondria - these are where most of the reactions for respiration take place. Respiration releases energy that the cell needs to work. 5. Ribosomes - the are where the protiens are made in the cell.
Plant cells usually have all the bits that animall cells have, plus a few extra things that animal cells don't have 1. Rigid cell wall - made of cellulose. It supports the cell and strengthens it. 2. Permanent vacule - contains cell sap, a weak solution of sugr and salts. 3. Chloroplasts - these are where photosynthesis occurs, which makes the food for the plant. They contain a green substance called chlorophyll.
Cells have structures that are specialised so they can carry out their function. Similar cells are grouped together to make a tissue, and different tissues work together as an organ. Organs have a particular job to do in the body. Groups of organs working together make up an organ system, like the digestive system. And finally, groups of organs and systems working together make up a full organism.
Specialised cells (part 1)
Most cells are specialised for their specific function within a tissue or organ. Paliside leaf cells are adapted for photosynthesis 1) Packed with chloroplasts for photosynthesis. More of them are crammed at the top of the cell - so they're nearer the light. 2) Tall shape means alot of surface area exposed down the side for absorbing CO2 from the air in the leaf. 3) Thin shape means that you can pack loads of them in at the top of the leaf Paliside leaf cells are grouped together to give the paliside layer of a leaf. - this is the leaf tissue where most of the photosynthesis happens.
Guard cells are adapted to open and close pores 1) Special kidney shape which opens and closes the stomata (pores) in a leaf. 2) When the plant has lots of water the guard cells fill with it and go plump and turgid. This makes the stomata open so gases can be exchanged for photosynthesis. 3) When the plant is short of water, the guard cells loos water and become flacid, making the stomata close. This helps stop too much water vapour escaping. 4) Thin outer walls and thick inner walls make the opening and closing work. 5) They're also sensitive to light and close at night to save water without losing out on photosynthesis. Guard cells are therefore adapted to thier function of allowing gas exchange and controlling water loss within the leaf organ.
Specialised cells (part 2)
Red blood cells are adapted to carry oxygen 1) Concave shape gives a big surface area for absorbing oxygen. It also helps them pass smoothly through capillaries to reach body cells. 2) They're packed with haemoglobin - the pigment that absorbes the oxygen. 3) They have no nucleus, to leave even more room for haemoglobin. Red blood cells are an important part of the blood.
Sperm and egg cells are specialised for reproduction 1) The main finctions of an egg cell are to carry the female DNA and to nourish the developing embryo in the early stages. The egg cells contains huge food reserves to feed the emryo. 2) When a sperm fuses with the egg, the egg's membrane instantly changes its structure to stop any more sperm getting in. This makes sure the offspring end up with the right amount of DNA. 3) The function of a sperm is basically to get the male DNA to the female DNA. It has a long tail and a streamlined head to help it swim to the egg. There are alot of mitochondria in the cell to provide the energy needed. 4) Sperm also carry enzymes in their heads to digest through the egg cell membrane. Sperm and egg cells are very important cells in the reproductive system.
Diffusion is the passive movement of particles from an area of high concentration to an area of low concentration. Diffusion happens in both liquids and gases - that's because the particles in these aubstances are free to move about randomly. The simplest type is when different gases diffuse through each other. This is what's happening when the smell of perfume diffuses through a room. The bigger the difference in concentration, the faster the diffusion rate. Cell membranes are clever because they hold the cell together but they let stuff in and out also. Substances can move in and out of cells by diffusion and osmosis. Only very small molicules can diffuse through cells membranes though - like glucose, amino acids, water and oxygen. Big molicules like starch and proteins cant fit through the membrane.
1) Just like diffusion in the air, particles flow through the cell membrane from where there's a high concentration to where there's a low concentration. 2) They're only moving about randomly, so they go both ways - but if there are alot more particles on one side of the membrane, there's an overall movement from that side. 3) The rate of diffusion depends on three main things: a) Distance - substances diffuse more quickly when they haven't as far to move. b) Concentration difference (gradient) - substances diffuse faster if there's a big difference in concentration. If there are lots more particles on one side, there are more there to move across. c) Surface area - the more surface there is available to move across, the faster they can get from one side to the other.