Additional Science-Biology Chapter 1

All animal and plant cells have some structures in common; they have:

• A nucleus - to control the cell's activities
• Cytoplasm - a liquid gel in which many chemical reactions take place (e.g. respiration)
• A cell membrane - controls the passage of substances in and out of the cell
• Mitochondria - structures in the cytoplasm where oxygen is used and most of the energy is released during respiration
• Ribosomes - where protein synthesis takes place

Plant cells also have:

• A cell wall made of cellulose which strengthens the cell and gives it support
• Chloroplasts - (found in green parts of plants) contain chlorophyll for photosynthesis
• A permanant vacuole - contains cell sap to keep cells rigid

Each reaction is controlled by an enzyme. Each enzyme is a protein which controls the rate of a specific reaction.

Enzymes are found particularly in the mitochondria (and chloroplasts in plants)

Enzymes controlling the reaction of:

• Respiration are found in mitochondria
• Photosynthesis are found in chloroplasts
• Protein synthesis are found on the surface of ribosomes

Single cells are the smallest living organisms. As organisms develop, some of their cells become specialized to carry out particular jobs (differentiation). Differentiation happens much earlier in the development of animals than it does in plants.

When cells become specialized its structure is adapted to suit the particular job it does. As a result, specialized cells often look very different to plant or animal cells. Sometimes, they become so specialized that they only have one function within the body (e.g. sperm, eggs, red blood cells and nerve cells).

Specialized cells are often grouped together to form a tissue. Connective tissue joins bits of the body together. Nervous tissue carries information around the body and muscles move the body about.

Cells --> Tissues --> Organs --> Organ Systems --> Whole Body

Cone cells are in the light sensitive layer of the eye (the retina). They make it possible to see in color.

• The outer segment is filled with a special chemical called visual pigment. This changes chemically in colored light, then changes back to its original form which uses up energy.
• The middle segment is filled with mitochondria which produces lots of energy. This means the visual pigment can reform so the eye can see continually in color.
• The final part of the cell is a specialized nerve ending or synapse. This connects to the optic nerve which carries impulses to the brain. An impulse is triggered which crosses the synapse when colored light makes the visual pigment change.

Fat cells are storage cells. If you eat more than you need, your body makes fat and fills up the fat cells.

• They have very little normal cytoplasm - this leaves plenty of room for large amounts of fat
• They have very few mitochondria as they use very little energy
• They can expand - a fat cell can end up 1000 times its original size as it fills up with fat

Root hair cells are found close to the tips of growing roots. They enable plants to take in water which they need.

• They increase the surface area for water to move into the cell
• A large permanent vacuole, affects the movement of water from the soil across the root hair cell
• They are always positioned close to the xylem tissue that carries water up into the rest of the plant

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 to pass on to the offspring.

• They have long tails with muscle-like proteins so they can swim towards the egg
• The middle section is full of mitochondria, which provide the energy for the tail to work
• The acrosome, which stores digestive enzymes for breaking down the outer layers of the egg
• They have a large nucleus which contains the genetic information to be passed on

Dissolved substances move into and out of cells across the cell membrane by diffusion, osmosis and active transport.

Diffusion is the net movement of particles from an area of high concentration to an area of lower concentration. It takes place because of the random movement of the particles of a gas or of a substance in solution in water. If there is a big difference in concentration between two areas, diffusion will take place quickly. In general, the bigger the difference in concentration, the faster the rate of diffusion will be. The difference between two areas of concentration is called the concentration gradient.

The overall or net movement = particles moving in - particles moving out

Concentration isn't the only thing that affects the rate of diffusion. An increase in temperature means the particles in a gas or solution move more quickly.
The larger the difference in concentration, the faster the rate of diffusion. E.g.:

• Diffusion of oxygen into the cells of the body from the blood stream as the cells are respiring (and using up oxygen).
• Diffusion of carbon dioxide into actively photosynthesising plant cells.
• Diffusion of simple sugars into amino acids from the gut through cell membranes.

Osmosis is the diffusion of water across a partially permeable membrane from a dilute (less concentrated) solution to a more concentrated solution. No solute molecules can move across the membrane. The cell membrane is partially permeable.

Water is needed to support cells and because chemical reactions take place in solution.

A cell is basically some chemicals dissolved in water inside a partially permeable bag of cell membrane. Water will move from a high concentration of water particles (in a dilute solution) to a less concentrated solution of water particles (in a concentrated solution) across the membrane of the cell.

If a cell uses up water in its chemical reactions, the cytoplasm becomes more concentrated and more water will immediately move in by osmosis. Similarly if the cytoplasm becomes too dilute because water is produced during chemical reactions, water will leave the cell by osmosis, restoring the balance.

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.

On the other hand, if the solution outside the cell is more concentrated than the cell contents, then water will move out of the cell by osmosis, the cytoplasm will become too concentrated and the cell will shrivel up.

Plants rely on well-regulated osmosis to support their stems and leaves. Water moves into plant cells by osmosis, making the cytoplasm swell and press against the plant cell walls. The pressure builds up until no more water can physically enter the cell, this makes the cell hard and rigid. So for plants, it is important that the fluid surrounding the cells always has a higher concentration of water than the cytoplasm of the cells.

But sometimes, plants and animal cells need to move substances such as glucose against a concentration gradient. For this there is another method of transport known as active transport which uses energy from respiration.