The nucleus: This controls all the activities of the cell. It contains the genes on the chromosomes that carry instructions for making the proteins needed to build new cells or organisms.
The cytoplasm: This is a liquid gel in which most of the chemical reactions needed for life take place, for example the first stages of cellular respiration.
The cell membrane: This controls the passage of substances such as glucose and mineral ions into the cells. It also controls the movement of substances such as urea and hormones out of the cell.
The mitochondria: These are structures in the cytoplasm where oxygen is used and most of the energy is released during respiration.
The Ribosomes: This is where protein synthesis occurs, making all the proteins needed in the cell.
Plant cells have all the features of a typical animal cell, but they also contain features which are needed for their very different way of life. All plant and animal cells have:
- A cell wall made of cellulose which strengthens the cell and gives it support.
Many, but not all plant cells have:
- Chloroplasts which are found in all green parts of the plant. Chloropyll- the pigment which makes the plants green is vital as it absorbs light energy in order to make food by photosynthesis. Root cells do not have chloroplasts because they are underground and do not photosynthesize.
- A permanent vaccuole which is a space in the cytoplasm that is filled with cell sap. This is important for keeping the cells rigid to support the plant.
Bacteria are single celled organisms which are much smaller than animal and plant cells.
A bacterial cell is made up of a cytoplasm surrounded by a cell wall. Inside the cell is the genetic material of the cell. The genes however are not contained in a nucleus. The genetic material is contained in a long strand of DNA known as the bacterial chromosome. This is usually circular and found free in the cytoplasm.
Many bacterial cells also contain plasmids, which are small circular bits of DNA that carry additional genetic information. Plasmids are widely used by scientists in genetic engineering.
Bacteria may have a slime capsule around the outside of the cell wall. Some types of bacterium have at least one flagellum, a long protein strand that lashes out and is used by the bacteria to move.
Although some bacteria cause disease, many are harmless. Some bacteria are very useful, e.g. we use bacteria in yoghurt, cheese, sewage treatment and in medicines.They are vital as decomposers in food chains, and in natural cycles such as the nitrogen cycle and the carbon cycle. They are also essential for a healthy gut.
Another type of microorganism that is very useful to people is yeast. Yeast are single celled fungi. Each yeast cell has a nucleus containing the genetic material of the cell, a cytoplasm and a cell membrane surrounded by a cell wall.
The main way in which yeasts reproduce is by asexual budding. This involves a new yeast cell growing out from the orignal cell to form a separate yeast organism.
Yeast cells are specialised to be able to survive for a long time, even when there is little oxygen available. When yeast cells have plenty of oxygen, they use aerobic respiration, where they use oxygen to break down sugar to produce energy for the cell. During this process they produce CO2 and water as waste products. However when there is little oxygen present, anaerobic respiration occurs, where yeast cells break down sugar into ethanol and CO2. This anaerobic respiration is sometimes known as fermentation.
Humans have used yeast for making alcohol and bread for a while now, Yeast is also vital for the production of antibiotics, such as penicillin, and for the carbon cycle. However yeast can also cause problems as it breaks down foods which people are storing.
Specialised cells are cells which have certain features in order to carry out their function. When a cell becomes specialised, its structure is adapted to suit the particular job it does. As a result, specialised cells often look very different to the normal plant or animal cell.
Sometimes cells become so specialised, they only have one function in the body, e.g. sperm cells, red blood cells and nerve cells. Some specialised cells such as egg and sperm work individually. Others work together as part of a tissue, and organ or a whole organism.
Specialised Cells: Fat Cells
If you eat more food than you need, your body makes fat and stores it in your fat cells. The fat can be broken down and used for energy when it is needed. Fat cells help animals, includind humans to survive when there is a short supply of food. Fat cells together form adipose tissue.
- They have a small amount of cytoplasm and large amounts of fat
- They have few mitochondria as fat cells require little energy
- They can expand upto 1000 times their original size as they fill up with fat.
Specialised Cells: Cone Cells
There are cone cells in the retina of your eye which enable you to see in colour. Cone cells have 3 main adaptations:
- The outer segment contains a visual pigment that changes chemically in coloured light, It needs energy to change back to its original form. The visual pigments are based on the Vitamin A in your diet.
- The middle segment is full of mitochondria which release energy to help reform the visual pigment. This lets you see continually in colour.
- The final part of the cone cell is the specialized synapse that connects to the optic nerve. When coloured light makes the visual pigment change, and nerve impulse is triggered. This makes its way along the optic nerve up to the brain.
Specialised Cells: Root Hair Cells
You can find root hair cells close to the tips of growing roots. Plants need to take in lots of water and dissolves mineral ions. The root hair cells help them to take up water and mineral ions more effectively. Root hair cells are always close to the xylem tissue, which carries up water and mineral ions into the rest of the plant. Mineral ions are moved into the cell by active transport.
Root hair cells have 2 main adaptations:
- The root hairs increase the surface area for water to move into the cell.
- The root hai cells have a large permanent vacuole that speeds up the movement of water by osmosis from the soil across the root hair cell.
Specialised Cells: Sperm Cells
Sperm cells are usually released a long way from the egg they are going to fertilise. They contain the genetic information from the male parent. Depending on the type of animal, sperm cells need to move through water or the female reproductive system to reach an egg. Then they have to break into the egg. Sperm cells have several adaptations to make all this possible.
- A long tail whips from side to side and helps move the sperm towards the egg.
- The middle section is full of mitochondria which provide energy for the tail to work.
- The acrosome stores digestive enzymes to break down the outer layers of the egg.
- A large nucleus contains the genetic information which will be passed on.
Diffusion is the net movement of particles from an area where they are at a high concentration to an area where they are at a lower concentration , down a concentration gradient. Difussion occurs until an equillibrium of particles has been reached. The particles which diffuse are usually gases or any substance in a solute.
If there is a big difference in concentration between two areas, difussion will take place quickly.Many particles will move randomly towards the area of low concentration. Only a few will move randomly in the other direction. However, if there is only a small difference in concentration between two areas, the net movement by diffusion will be quite slow. The number of particles moving into the area of lower concentration by random movement will only be slightly more than the number of particles that are leaving the area.
Net movement = particles moving in- particles moving out.
In general, the greater the difference in concentration, the faster the rate of diffusion. The difference in concentration between the particles is known as the concentration.
Temperatue also affects the rate of diffusion. The hotter the temperature, the more energy the particles have, and therefore diffusion takes places more quickly.
Diffusion in Living Organisms
Dissolved substances move in and out of cells by diffusion across the cell membrane, These include simple sugars, such as glucose, gases such as oxygen and waste products such as urea from the break down of amino acids in the liver.
The oxygen you need for respiration passes from the air into your lungs. From the lungs it enters your red blood cells through the cell membranes by diffusion. The oxygen moves down a concentration gradient from a region of high to low oxygen concentration. Oxygen then also moves from the blood cells into the cells of the body where it is needed by diffusion down a concentration gradient.
Carbon dioxide moves out from the body cells into the red blood cells and then into the air in the lungs by diffusion down a concentration gradient in a similar way.
Individual cells may be adapted to make diffusion easier and more rapid. The most common adaptation is to increase the surface area of the cell membrane. Increasing the surface area means there is more room for diffusion to place. By folding up a membrane of a cell or tissue lining of an organ, the area over which diffusion can take place is greatly increased. Therefore the rate of diffusion is also greatly increases, so that much more of a substance moves in a given time.
Diffusion takes place when particles can spread freely from one place to another. However, the solutions inside cells are separated from those outside by the cell membrane. The membrane does not let all types of particles through. Membranes that only let some particles through are known as partially permeable membranes.
Osmosis is a special case of diffusion. It is the movement of water from a dilute to a more concentrated solution through a partially permeable membrane which allows water to pass through.
How osmosis differs from diffusion.
Partially permeable cell membranes let water move accross them. Remember:
- A dilute solution of sugar contains a high concentration of water and a low concentration of sugar (the solute)
- A concentrated sugar solutiion contains a relatively low concentration water and a high concentration of sugar.
The cytoplasm of a cell is made up of chemicals dissolved in water inside a partially pemeable bag of cell membrane. The cytoplasm contains a fairly concentrated solution of salt and sugars. Water moves from a dilute solution(with a high concentration of water molecules) to a concentrated solution (with fewer water molecules) accross the membrane of the cell.
This special type of diffusion, where only water moves accross a partially permeable membrane is known as osmosis.
The concentration inside your body cells needs to stay the same for them to work properly. However, the concentration of the solutions outside your cells may be very different to the concentration inside them. This concentration gradient can cause water to move into or out of the cells by osmosis
- If the concentration of solutes in the solution outside the cell is the same as the concentration inside the cell, the solution is isotonic to the cell.
- If the concentration of the solutes in the solution outside the cell is higher than the concentration inside the cell, the solution is hypertonic to the cell.
- If the concentration of solutes in the solution outside the cell is lower than the concentration inside the cell, the solution is hypotonic to the cell.
Osmosis in animals.
If a cell uses up water in its chemical reactions, the cytoplasm becomes more concentrated. The surrounding fluid becomes hypotonic and more water immediately moves in by osmosis.
If the cytoplasm becomes too dilute because more water in made in chemical reactions, the surrounding fluid becomes hypertonic and water leaves the cell by osmosis. So osmosis restores the balance in both cases.
However, osmosis can also cause big problems in animal cells. If the solution outside the cell becomes much more dilute than the cell contents(hypotonic), water will move into the cell by osmosis. The cell will swell and may burst.
If the solution outside the cell becomes more concentrated than the cell contents (hypertonic), water will move out of the cell by osmosis. The cytoplasm will become too concentrated and the cell will shrivel up. Then it can no longer survive.
Once you understand the effect osmosis can have on body cells, it becomes clear why maintaining constant internal conditions is so important
Osmosis in Plants
Plants rely on osmosis to support their stems and leaves. Water moves into plant cells by osmosis. This causes the vaccuole to swell and press the cytoplasm against the plant cell walls. The pressure builds up until no more water can physically enter the cell. This pressure is known as turgor. Turgor pressure makes the cells hard and rigid, which in turn keeps the leaves and stems of the plant rigid and firm.
Plants need the fluid surrounding the cells to always be hypotonic to the cytoplasm to prevent water leaving the cell by osmosis. If this happens, the cell will become flaccid and will wilt.
If more water is lost by osmosis, the vaccuole and cytoplasm shrink, and eventually the cell membrane pulls away from the cell wall. This is plasmolysis. Plasmolysis is usually only seen in lab experimets. Plasmolysed cells die quickly unless the osmotic balance is restored.
Active transport allows cells to move substances from an area of low concentration to an area of high concentration. This movement is against the concentration gradient. As a result, cells can absorb ions from very dilute solutions. It also enables them to move substances such as sugars and ions from one place to another through the cell membranes.
It takes energy for the active transport system to carry a molecule across the membrane and then return to its original position. The energy needed comes from respiration.
Active transport is widely used in cells. There are some situations where it is particularly important. For example, mineral ions in the soil, such as nitrate ions are usually found in very dilute solutions. These solutions are more dilute than the solution within the plant. By using active transport, plants can absorb these mineral ions, even though it is against a concentration gradient.
Sugar, such as glucose, is always actively transported out of your gut and kidney tubules into your blood. This is often done against a large concentration gradient.
Different methods of transportation
- active transport