Biology movement of substances in and out of cells

What is diffusion?

The net movement of particles from a region of high concentration to a region of low concentration , they move down a concentration gradient

What does passive mean?

Does not require energy in the form of ATP from respiration

Name substances that cannot diffuse through a cell membrane and why

Large molecules e.g. proteins , starch - they are too large to pass through the membrane

Name substances that can diffuse through a cell membrane

small molecules e.g. oxygen , glucose , amino acids , minerals

What 3 factors affect the rate of diffusion into cells?

Temperature , concentration gradient , surface area

How does the temperature affect the rate of diffusion?

By increasing the temperature particles gain more kinetic energy and therefore move faster

Why can’t we increase the temperature to increase the rate of diffusion in our bodies?

This may cause enzymes to denature

How does the concentration gradient affect the rate of diffusion?

Increasing the concentration gradient increases the rate of diffusion , the steeper the concentration gradient (bigger the difference between the high and low concentration) the greater the net movement of particles

Give an example of where an increased concentration gradient is utilised in plants

Pant cells use carbon dioxide for photosynthesis , as plant cells use this the concentration inside the plant cells drop , this increases the diffusion rate of carbon dioxide into the plant cells

How does the surface area affect the rate of diffusion?

The greater the surface area the faster the rate of diffusion , a bigger surface area allows more space for diffusion so more particles can move in a period of time

Give an example of where a bigger surface area is utilised in humans

The small intestine wall is highly folded increasing the surface area that is in contact with the blood stream , this increases the rate of diffusion of molecules produced in digestion such as glucose and amino acids

What is osmosis?

Osmosis is the net of movement of water particles from a higher water potential to a lower water potential across a partially permeable membrane

What is water potential?

The concentration of free water molecules (ones that are not interacting with a solute so are free to move around)

What substance has the highest water potential?

Pure water (as it contains no solutes and therefore all the water molecules are free to move)

What happens to an animal cell when placed in a surrounding solution that is concentrated (has a lower water potential than inside the cell)?

The animal cell loses water , because water moves by osmosis from a higher water potential inside the cell to a lower water potential outside the cell through the partially permeable cell membrane , the animal cell becomes crenated (it shrivels)

What happens to an animal cell when placed in a surrounding solution that is dilute (has a higher water potential than inside the cell)?

The animal cell gains water , because water moves by osmosis from a higher water potential outside the cell to a lower water potential inside the cell through the partially permeable cell membrane , the animal cell swells and may burst (called lysis)

A plant cell full of water is said to be ….

Turgid

What happens to a plant cell when placed in a surrounding solution that is concentrated (it has a lower water potential)?

The plant cell loses water , because water moves by osmosis from a higher water potential inside the cell to a lower water potential outside of the cell through the partially cell permeable , the plant cell becomes flaccid (soft) (and plant may droop/wilt)

What happens to a plant cell when placed in a surrounding solution that is dilute (it has a higher water potential)?

The plant gains water , because water moves by osmosis from a higher water potential outside the cell to a lower water potential inside the cell through a partially permeable cell membrane , the plant becomes turgid (firm)

Why do plant cells not lyse (burst) but animal cells do?

the plant cells have a cellulose cell wall which can with stand the turgor pressure

What is turgor pressure?

The pressure exerted onto the cell wall of a plant when water moves in and out of the cell

What is active transport?

The movement of particles from a low concentration to a higher concentration against their concentration gradient , this requires ATP and a carrier protein

What is meant by ‘active’?

Requires energy in the form of ATP from respiration

What is a carrier protein?

A special protein that spans a cell membrane - it transports molecules into cells.

Give an example of active transport in plants

Mineral ions are taken from the soil into the roots by active transport , this is because there is usually a lower concentration of minerals in the soil compared to the roots

Give an example of active transport in humans

In the small intestine some glucose is actively transported from the small intestine into the blood through villi (this ensures all the glucose in food can be absorbed (to be used e.g. respiration) and it does not leave our body in faeces) , in the proximal convoluted tubule of kidney nephrons, glucose is actively transported from the filtrate into the blood (this ensures all glucose is reabsorbed and it does not leave our body in urine)