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Biology

The mitochondrion (plural mitochondria) is the energy-production centre of the cell.

Cells need energy to break down molecules and build new ones. Mitochondria produce energy using aerobic respiration.

Aerobic respiration produces adenosine triphosphate (ATP) molecules, which are used as an energy source by other parts of the cell.

A transmission electron microscope image and a schematic diagram of a mitochondrion.
A transmission electron microscope image and a schematic diagram of a mitochondrion.

Mitochondria are unusual in two important aspects:

  • Mitochondria contain their own genetic information.
  • Mitochondria multiply independently from the cell. This process is similar to the multiplication of bacteria.

Mitochondria are essential for all eukaryotic cells. It is commonly thought that mitochondria evolved from free living prokaryotes.

Two organelles are very important for the packaging and delivery of proteins to different parts of the cell:

  • The endoplasmic reticulum (ER) is a large network of membrane compartments. It is the largest organelle in most animal and plant cells.

    Newly made proteins enter the ER. When inside the ER, proteins can be modified. They are then delivered to the Golgi body.

  • The Golgi apparatus (Golgi body) exports proteins to other parts of the cell.

    The Golgi apparatus is a group of stacked membranes. Proteins are shuttled between membranes in small organelles called vesicles.

    The Golgi apparatus can be thought of as the postal centre of the cell. The ER puts the stamps on the proteins.
    The Golgi apparatus can be thought of as the postal centre of the cell. The ER puts the stamps on the proteins.

Carbohydrates are organic molecules composed of carbon, oxygen and hydrogen.

There are three common types of carbohydrate:

  • Monosaccharides are the simplest carbohydrates. They are sugars.

    Monosaccharides are small, water-soluble molecules . They are the building blocks (monomers) for larger carbohydrates.

    Glucose, fructose and galactose are common monosaccharides.

  • Disaccharides are formed from two monosaccharides . They are also sugars.

    Sucrose, lactose and maltose are common disaccharides.

  • Polysaccharides are long chains of monosaccharides . They are not sugars.

    Polysaccharides are insoluble in water. The structure of polysaccharides can vary:

    Cellulose is a polysaccharide which forms long, straight chains. Starch is a polysaccharide that forms coiled structures. Both are polymers of glucose!

Rice plants store energy in the form of starch in their grains.
Rice plants store energy in the form of starch in their grains.

Benedict's test is a test for simple sugars (e.g. glucose).

  • Grind up your sample and mix in a test tube of distilled water.
  • Add 2 cm$$^3$$ of Benedict's solution ( light blue) to the same amount of your sample.
  • Boil the solution for 2-3 minutes.

If the solution turns a brick-red colour, your sample contains glucose!

Iodine test: tests for starch.

  • Grind up your sample and mix in a test tube of distilled water.
  • Add a few drops of iodine (in potassium iodide solution) into the test tube.

If the solution turns a blue-black colour, your sample contains starch!

The presence of starch in a leaf is a good indicator of photosynthesis.

Active transport is an energy-consuming process that transports molecules against their concentration gradient.

Cells often require molecules that are present inside the cell at higher concentrations than outside. These molecules cannot move into the cell by diffusion.

Active transport moves molecules from an area of low concentration to an area of high concentration.

This is the opposite direction to movement by diffusion!

Cells use active transport to absorb useful molecules:

  • Root hair cells actively transport ions from the soil. This is important for water uptake in plants.
  • Villi in the small intestine actively transport glucose from the gut. This maximises nutrient absorption from food.
Intestinal villi have carrier proteins in the membrane that actively transport glucose in from the gut.
Intestinal villi have carrier proteins in the membrane that actively transport glucose in from the gut.

Osmosis is the diffusion of water molecules across a semipermeable membrane.

Osmosis is a special kind of diffusion that applies only to water molecules.

Like diffusion, osmosis is a passive process:

Water diffuses from an area of low solute concentration (high water potential) to an area of high solute concentration (low water potential).

Low solute concentration = dilute solution = high water potential!

A semipermeable membrane only allows certain molecules to pass through. In the case of osmosis, water is able to move through, but large molecules like proteins are not.

Visking tubing is often used as a semi-permeable membrane in experiments.

Diffusion is the net (overall) movement of particles from an area of high concentration to an area of lower concentration.

When you put a drop of red dye into a glass of water, the dye diffuses until it is at equal concentration everywhere in solution.

A difference in concentration of a particle between two areas is called a concentration gradient. Diffusion occurs down a concentration gradient.

Diffusion is a passive process; it does not require energy.

Diffusion is fundamental to how organisms function:

  • Proteins diffuse within the cytoplasm of cells.
  • Carbon dioxide diffuses from the air into cells in the leaf for photosynthesis.
  • Oxygen diffuses from the blood into tissues for respiration.

Organisms can control concentration gradients to determine the direction of diffusion.

Molecules diffuse in random directions, but the net movement is from high to low concentration.
Molecules diffuse in random directions, but the net movement is from high to low concentration.

Mitosis can be divided into four different stages. They are recognisable by the position of the genetic material:

  1. Prophase:
  2. Metaphase:
  3. Anaphase:
  4. Telophase:

At the end of mitosis, both nuclei are diploid; they contain two versions of each chromosome.

Processing of food in the alimentary canal is divided into separate stages.

Process Definition
Ingestion Physical intake of food
Digestion Breakdown of large, insoluble matter into small soluble molecules
Absorption Movement of digested molecules from the gut into the blood stream
Assimilation Processing of absorbed nutrients for use by the body
Egestion Removal of undigested material and unabsorbed material from the gut
The alimentary canal forms a factory line for the different digestive processes.
The alimentary canal forms a factory line for the different digestive processes.

Digestion, the process of breaking down food into usable parts, has two separate types:

Physical digestion uses physical means to reduce the size of pieces of food. This can also be called mechanical digestion.

Chewing with the teeth is a form of physical digestion.

By breaking the food into smaller pieces, physical digestion increases the surface area that enzymes can attack.

During chemical digestion, enzymes break large molecules into small molecules that can be absorbed and used by the body.

Proteins are composed of hundreds of amino acids. Chemical digestion separates protein into its amino acids, which can then be used to make new proteins.

Chewing is an example of physical digestion
Chewing is an example of physical digestion

The alimentary canal is arranged so that organs specialised for a particular process are positioned where that process would be most effective.

The ileum (specialised for absorption) is located after the stomach and the duodenum, where the majority of food is digested.

Below is a table illustrating the differences in structure and function between arteries, veins and capillaries.

Features Artery Capillary Vein
Walls Thick, muscular and elastic One cell thick Thin layer of muscle
Lumen Relatively small Very small (blood cells travel in single-file) Relatively large
Valves No No Yes
Pressure High Medium Low
Blood content Oxygenated Both Deoxygenated
Direction Away from heart Within tissue Towards heart
Arteries branch into capillaries, which combine to form veins.
Arteries branch into capillaries, which combine to form veins.

The aorta splits into multiple arteries that supply blood to specific organs and limbs. Blood is then pooled into the vena cava to return to the heart.

Below is a diagram showing some of the important blood vessels that deliver blood to and from the organs.

The arteries and veins supplying blood to specific organs have different names.
The arteries and veins supplying blood to specific organs have different names.

Red blood cells are adapted to carry oxygen from the lungs to tissues around the body:

  • Biconcave disc shape: increases surface area for oxygen diffusion to and from red blood cells.

    This shape also allows red blood cells to bend, necessary for squeezing through capillary vessels.

  • Biconcave means that the cell bends inwards (is concave) on both sides!
    Biconcave means that the cell bends inwards (is concave) on both sides!
  • Have no nucleus: increases the space inside the cell to carry oxygen.
  • Contain haemoglobin: an iron-containing pigment that binds to oxygen.

Much of the iron we need in our diet goes into producing haemoglobin.

The colour of blood depends on whether the haemoglobin in red blood cells is bound to oxygen. Oxygen-bound haemoglobin is bright red, while unbound haemoglobin is darker.

The leaf is made up of three layers of tissue:

The upper epidermis consists of a single layer of transparent cells. These cells are transparent because they lack chloroplasts.

Upper epidermal cells are coated by a waxy covering called the cuticle. This reduces evaporation and protects the leaf from damage and infections.

Plants in hot environments have thick cuticles to reduce water loss.

The mesophyll is where photosynthesis occurs and the food products are stored. It contains large air spaces that facilitate gas exchange.

The lower epidermis forms a protective layer at the bottom of the leaf. It is punctured with small gaps in its surface called stomata.

The upper and lower epidermis protect the mesophyll.
The upper and lower epidermis protect the mesophyll.

There are some important differences between the two transport vessels in the plant vascular system: xylem and phloem.

Xylem Phloem
Cells Dead cells Sieve tube elements

Companion cells

Substance carried Water and mineral salts Sugars, amino acids, hormones
Direction Unidirectional (upwards) Bidirectional
Mechanism of transport Passive process (capillary action, transpiration) Active process (uses energy)

DNA (deoxyribonucleic acid) is the genetic material of the cell.

DNA contains the instructions for a cell development and function. Everyone, except identical twins, has different DNA. However, the basic structure is the same for most living organisms.

DNA is a large polymer (repeating subunits) formed from long chains of nucleotides. The sequence of nucleotides in DNA forms the blueprint of the cell.

If the DNA in a single human cell was stretched out, it would be over 2 metres long!

The DNA molecule actually consists of two separate strands. These strands (shown in green) wrap around each other. This structure is called a double helix.

The structure of DNA was discovered by Francis Crick and James Watson in 1953. This discovery revolutionised biology!

Selective breeding (or artificial selection) is the process of humans breeding animals and plants for desirable traits.

Humans have known for a long time that offspring can inherit traits from their parents (heredity). This has been used to breed better offspring.

For example, to breed better crops:

  1. Parent plants with the largest corn grains used for breeding.
  2. The F1 offspring with the largest corn grains are used for breeding.
  3. The F2 offspring with the largest corn grains are used for breeding.

This process generates an extreme artificial selection pressure for large corn grains. Over many generations the grains will get progressively larger.

Selective breeding is a slow process, though it is much faster than evolution!

There are problems with selective breeding. Artificial selection reduces the genetic variation within a population. This increases the susceptibility to disease.

Dogs have been selectively breed for different traits. These dogs are now almost unrecognisable as the same species!
Dogs have been selectively breed for different traits. These dogs are now almost unrecognisable as the same species!

Photosynthesis is the process that converts light energy into chemical energy (in the form of glucose).

Light energy (usually from the sun) is used drive chemical reactions in photosynthesis. The overall chemical equation for photosynthesis is: $$$\ce{6CO2 + 6H2O ->[\text{light}] C6H12O6 + 6O2}$$$

$$$\text{Carbon dioxide + Water $\xrightarrow{\text{light}}$ Glucose + Oxygen}$$$

Photosynthesis occurs in specialised organelles called chloroplasts.

A palisade cell in the leaf can contain over 100 chloroplasts!

Sugars produced by photosynthesis can be stored in the leaf as starch. Alternatively, they can be transported to the rest of the plant through vascular bundles (veins).

The energy plants get from glucose can be used to build new cells or transport substances around the plant.

Leaves trap the light energy needed for photosynthesis.
Leaves trap the light energy needed for photosynthesis.

The eye is a sense organ that detects light from the surroundings.

Structure Description Function
Sclera White, outside layer Protects the eye
Cornea Transparent part of the sclera Focuses light onto the lens
Iris Coloured structure with muscles Controls pupil size
Pupil Hole in the iris Allows light into the eye
Lens Flexible transparent disc Focuses light onto the retina
Retina Lining of back of eye Contains light-sensitive receptors
Fovea Small part of retina with high receptor density Sharp, clear vision
Choroid Black-coloured layer Prevents reflection from back of the eye
Optic nerve Bundle of nerve fibres Delivers information to the brain