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Saving energy?. Sayid has decided to save energy by staying in bed all day. How much of his energy do you think this will save?. Surprisingly, the answer is only about 30%.

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Saving energy?

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saving energy
Saving energy?

Sayid has decided to save energy by staying in bed all day.

How much of his energy do you think this will save?

Surprisingly, the answer is only about 30%.

The other 70% keeps his body temperature at 37°C (98.6 °F), and the solutions around his cells at just the right concentration.

what is homeostasis
What is homeostasis?

The body uses so much energy, even during sleep, because it must maintain a constant internal environment.

This process of keeping things the same is called homeostasis.

A series of automatic control systems ensures that the body maintains a constant temperature and steady levels of water, ions and blood sugar.

Homeostasis allows the body’s cells to work at their optimum.

why is water important
Why is water important?

The human body is about 60-70% water.

Water molecules and ions constantly move in and out of cells, and are essential for all life processes.

Dehydration (loss of too much water from the body) damages cells.

How is water gained and lost?

  • Water is produced by the body during respiration, and absorbed from food and drink.
  • Water is lost from the body in exhaled air, sweat, urine and feces.
what is osmosis



(dialysis tubing)



What is osmosis?

Osmosis is the diffusion of water moleculesfrom a low concentration solution to high concentration solution, across a partially-permeable membrane.

A partially-permeable membrane has holes in it that permit water molecules through but are too small to allow larger molecules through. Osmosis can be demonstrated using dialysis tubing filled with a solution and placed in a beaker of pure water.

dilute vs concentrated

pure water



dilute solution

Dilute vs. concentrated

During osmosis, water molecules diffuse from pure water or dilute solution to more concentrated solutions.

  • Dilute solutions have a high concentration of water molecules.
  • Concentrated solutions have a low concentration of water molecules.
osmosis and cells



cell wall

Osmosis and cells

Plant and animal cells are surrounded by a partially-permeable plasma membrane. This allows water and other small molecules to diffuse across.

Plant cells additionally have a strong cell wall surrounding the membrane which offers support and protection.

red blood cell

plant cell

osmosis and animal cells
Osmosis and animal cells

Animal cells do not have a cell wall. This means they respond differently to plant cells to the gain and loss of water.

In dilute solutions, osmosis can cause animals cells, such as red blood cells, to swell up and burst. This is called lysis.

In concentrated solutions, water loss causes the cells to shrink. When this happens to red blood cells, it is called crenation.

factors that affect passive transport
Whether a molecule can move through the membrane depends on:

the size of the molecule

the type of molecule (polar or nonpolar, ions, etc.)

Molecules move by one of the following methods: diffusion, facilitated diffusion, or osmosis.

Factors that affect Passive Transport:
egg in hypertonic solution
Egg in hypertonic solution

Water moved out of the egg to try to dilute the solutes outside of the egg and make the external environment have a higher water concentration to match the inside.

Chicken egg in syrup

egg in hypotonic solution
Egg in hypotonic solution

Water moved into the egg to reduce the water concentration compared to solutes outside of the egg and make the external environment have a lower water concentration to match the inside.

Chicken egg in tap water (or vinegar)

osmosis and animal cells2


Osmosis and animal cells

In order to remain healthy, animal cells need to maintain an isotonic water balance. This means that the water concentration both inside and outside the cell are equal.

The concentration of water and salt in the blood are controlled by the kidneys.

The kidneys are controlled by the portion of the brain called the hypothalamus.

waste removal
Waste removal

Several organs are important in removing waste from the body.

The lungs remove carbon dioxide.

The liver converts excess protein into urea.

The skin provides a surface for small amounts of water and salt to move out of the body.

The kidneys remove unwanted substances such as urea, excess water and salt.

what is urea
What is urea?

Excess amino acids in the body are broken down by the liver, producing a waste substance called urea.

This process is important because it converts toxic ammonia to urea, which is done using carbon dioxide.

Once formed, urea is transported by the circulatory system to the kidneys.

The kidneys filter the blood, removing urea and excess water and salt, which forms urine. Urine is stored in the bladder before being excreted from the body.

analysing urine
Analysing urine

Changes in the colour, clarity, pH and the presence of certain substances in urine can help doctors diagnose medical conditions:

  • Protein or red blood cells in urine can indicate kidney damage or disease, as these substances would not normally filter through the glomerulus.
  • Glucose in urine is often an indication of diabetes. A person with diabetes will have a high level of glucose in the blood.
controlling blood glucose
Controlling blood glucose

Between meals, blood glucose levels are topped up from stored deposits in the liver and muscles.

After a meal, blood glucose rises but quickly returns to normal. Where does the excess go? Why not leave it in the blood?

Excess glucose makes the blood plasma and tissue fluid around cells too concentrated. This can severely damage cells, for example, causing crenation in red blood cells.

However, low blood sugar levels can be equally as dangerous, as it can make cells swell up and burst. This is called lysis.