The net movement of molecules from a region of their higher concentration to a region of their lower concentration, down a concentration gradient, as a result of their random movement.

Is diffusion an active or passive process, and where do the particles get their energy?

Passive — the cell does not supply energy. The particles move using their own kinetic energy (random movement).

Why can a partially permeable cell membrane restrict diffusion?

It lets some molecules cross easily but others with difficulty or not at all. The simplest selection is based on size — smaller molecules can diffuse across, larger ones cannot.

Give three things diffusion helps living organisms to do.

Obtain many of their requirements (e.g. nutrients, oxygen) Get rid of many of their waste products Carry out gas exchange for respiration

List the four factors that affect the rate of diffusion.

Surface area to volume ratio Diffusion distance Temperature Concentration gradient

How does surface area to volume ratio affect the rate of diffusion?

The bigger a cell or structure, the smaller its surface area to volume ratio, which slows the rate substances move across its surface. Cells adapted for diffusion increase their surface area (e.g. root hair cells, cells lining the ileum).

How does diffusion distance affect the rate of diffusion?

The smaller the distance molecules have to travel, the faster diffusion occurs. This is why capillaries and alveoli have walls only one cell thick.

How does temperature affect the rate of diffusion?

The higher the temperature, the more energy molecules have, so they move faster. This causes more collisions against the membrane and a faster rate of diffusion.

How does the concentration gradient affect the rate of diffusion?

The greater the difference in concentration on either side of the membrane, the faster diffusion occurs, because more random collisions against the membrane happen on the higher-concentration side.

The net movement of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution) through a partially permeable membrane.

Is osmosis active or passive, and what does it move?

It is passive transport (no energy required), and it only applies to the movement of water molecules.

A measure of the tendency of water molecules to move. A dilute solution has a high water potential; a concentrated solution has a low water potential.

What happens to an animal cell placed in distilled water, and why?

Water enters by osmosis because the distilled water has a higher water potential than the cell. With no cell wall, the cell swells until the membrane stretches too far and it bursts (lysis).

What happens to an animal cell placed in a strong sugar solution?

It loses water by osmosis (the solution has a lower water potential than the cell) and becomes crenated (shrivelled).

What happens to a plant cell placed in distilled water?

It gains water by osmosis. The vacuole enlarges and pushes the cell membrane against the cell wall, making the cell turgid (high turgor pressure).

What happens to a plant cell placed in a strong sugar solution?

It loses water by osmosis. The vacuole shrinks and the cell membrane shrivels away from the cell wall — the cell becomes flaccid or plasmolysed.

Why does a plant cell not burst when placed in distilled water?

The supporting cell wall is strong and prevents the cell from bursting. It creates turgor pressure that resists further water entry, protecting the cell from lysis.

In the potato osmosis core practical, what is measured and how is the effect worked out?

The mass of each potato cylinder is measured before and after being placed in the sucrose solutions. The percentage change in mass is then calculated for each concentration.

In the potato practical, why does the cylinder in distilled water gain mass?

The distilled water has a higher water potential than the potato cells, so water moves into the cells by osmosis. This makes the cells turgid and increases the mass of the cylinder.

In the potato practical, why does the cylinder in concentrated sucrose lose mass?

The sucrose solution has a lower water potential than the potato cells, so water moves out of the cells by osmosis. This makes the cells flaccid and decreases the mass of the cylinder.

What does it mean if a potato cylinder shows no change in mass?

There was no net movement of water. The solution had the same water potential (concentration) as the cytoplasm of the potato cells, so there was no concentration gradient.

The movement of particles through a cell membrane from a region of lower concentration to a region of higher concentration, using energy from respiration.

Why does active transport require energy?

Particles are moved against the concentration gradient — the opposite direction to which they would naturally move by diffusion — so energy from respiration is needed.

What in the cell membrane carries out active transport?

Protein carrier molecules embedded in the cell membrane.

Compare diffusion and active transport in terms of energy and direction of movement.

Diffusion: passive, no energy needed, moves particles down the concentration gradient. Active transport: requires energy from respiration, moves particles against the concentration gradient.

How is glucose absorbed from the small intestine when its concentration in the blood is already higher than in the gut?

By active transport, which moves glucose into the bloodstream against the concentration gradient. Diffusion alone would stop once the gradient ran out.

How does active transport prevent glucose being lost in the urine?

Epithelial cells in the kidney tubules (nephron) actively reabsorb glucose back into the blood, so none is lost in the urine.

How do root hair cells take up mineral ions from the soil?

By active transport — ions such as magnesium are moved from a lower concentration in the soil to a higher concentration inside the cell cytoplasm, using energy from respiration.

Why do plants need the magnesium ions and nitrate ions taken up by active transport?

Magnesium ions: needed to make chlorophyll. Nitrate ions: needed to make amino acids for protein synthesis (and so growth).

GCSEScienceEdexcelCombined ScienceBiologyFlashcardsKey Concepts in BiologyMovement of Substances Into & Out of Cells

Movement of Substances Into & Out of Cells (Edexcel GCSE Combined Science: Biology): Flashcards

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Diffusion
The net movement of molecules from a region of their higher concentration to a region of their lower concentration, down a concentration gradient, as a result of their random movement.
Is diffusion an active or passive process, and where do the particles get their energy?
Passive — the cell does not supply energy. The particles move using their own kinetic energy (random movement).
Why can a partially permeable cell membrane restrict diffusion?
It lets some molecules cross easily but others with difficulty or not at all. The simplest selection is based on size — smaller molecules can diffuse across, larger ones cannot.
Give three things diffusion helps living organisms to do.
Obtain many of their requirements (e.g. nutrients, oxygen)
Get rid of many of their waste products
Carry out gas exchange for respiration
Diffusion is the movement of molecules from a region of concentration to a region of concentration.
higher / lower
List the four factors that affect the rate of diffusion.
Surface area to volume ratio
Diffusion distance
Temperature
Concentration gradient
How does surface area to volume ratio affect the rate of diffusion?
The bigger a cell or structure, the smaller its surface area to volume ratio, which slows the rate substances move across its surface. Cells adapted for diffusion increase their surface area (e.g. root hair cells, cells lining the ileum).
How does diffusion distance affect the rate of diffusion?
The smaller the distance molecules have to travel, the faster diffusion occurs. This is why capillaries and alveoli have walls only one cell thick.
How does temperature affect the rate of diffusion?
The higher the temperature, the more energy molecules have, so they move faster. This causes more collisions against the membrane and a faster rate of diffusion.
How does the concentration gradient affect the rate of diffusion?
The greater the difference in concentration on either side of the membrane, the faster diffusion occurs, because more random collisions against the membrane happen on the higher-concentration side.
A capillary wall is only cell thick, which reduces the diffusion and speeds up diffusion.
one / distance
Osmosis
The net movement of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution) through a partially permeable membrane.
Is osmosis active or passive, and what does it move?
It is passive transport (no energy required), and it only applies to the movement of water molecules.
Water potential
A measure of the tendency of water molecules to move. A dilute solution has a high water potential; a concentrated solution has a low water potential.
What happens to an animal cell placed in distilled water, and why?
Water enters by osmosis because the distilled water has a higher water potential than the cell. With no cell wall, the cell swells until the membrane stretches too far and it bursts (lysis).
What happens to an animal cell placed in a strong sugar solution?
It loses water by osmosis (the solution has a lower water potential than the cell) and becomes crenated (shrivelled).
What happens to a plant cell placed in distilled water?
It gains water by osmosis. The vacuole enlarges and pushes the cell membrane against the cell wall, making the cell turgid (high turgor pressure).
What happens to a plant cell placed in a strong sugar solution?
It loses water by osmosis. The vacuole shrinks and the cell membrane shrivels away from the cell wall — the cell becomes flaccid or plasmolysed.
Why does a plant cell not burst when placed in distilled water?
The supporting cell wall is strong and prevents the cell from bursting. It creates turgor pressure that resists further water entry, protecting the cell from lysis.
A concentrated solution has a water potential and a dilute solution has a water potential.
low / high
In the potato osmosis core practical, what is measured and how is the effect worked out?
The mass of each potato cylinder is measured before and after being placed in the sucrose solutions. The percentage change in mass is then calculated for each concentration.
In the potato practical, why does the cylinder in distilled water gain mass?
The distilled water has a higher water potential than the potato cells, so water moves into the cells by osmosis. This makes the cells turgid and increases the mass of the cylinder.
In the potato practical, why does the cylinder in concentrated sucrose lose mass?
The sucrose solution has a lower water potential than the potato cells, so water moves out of the cells by osmosis. This makes the cells flaccid and decreases the mass of the cylinder.
What does it mean if a potato cylinder shows no change in mass?
There was no net movement of water. The solution had the same water potential (concentration) as the cytoplasm of the potato cells, so there was no concentration gradient.
In the osmosis practical we calculate the change in mass of each potato cylinder.
percentage
Active transport
The movement of particles through a cell membrane from a region of lower concentration to a region of higher concentration, using energy from respiration.
Why does active transport require energy?
Particles are moved against the concentration gradient — the opposite direction to which they would naturally move by diffusion — so energy from respiration is needed.
What in the cell membrane carries out active transport?
Protein carrier molecules embedded in the cell membrane.
Compare diffusion and active transport in terms of energy and direction of movement.
Diffusion: passive, no energy needed, moves particles down the concentration gradient.
Active transport: requires energy from respiration, moves particles against the concentration gradient.
Active transport moves particles from a region of concentration to a region of concentration.
lower / higher
How is glucose absorbed from the small intestine when its concentration in the blood is already higher than in the gut?
By active transport, which moves glucose into the bloodstream against the concentration gradient. Diffusion alone would stop once the gradient ran out.
How does active transport prevent glucose being lost in the urine?
Epithelial cells in the kidney tubules (nephron) actively reabsorb glucose back into the blood, so none is lost in the urine.
How do root hair cells take up mineral ions from the soil?
By active transport — ions such as magnesium are moved from a lower concentration in the soil to a higher concentration inside the cell cytoplasm, using energy from respiration.
Why do plants need the magnesium ions and nitrate ions taken up by active transport?
Magnesium ions: needed to make chlorophyll.
Nitrate ions: needed to make amino acids for protein synthesis (and so growth).
Root hair cells absorb mineral ions by active transport using energy released from .
respiration

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