TOPIC 4. MOVEMENT IN AND OUT OF CELLS

CEL MEMBRANE STRUCTURE

3.4.1 Identify the structure of a cell (or organelle) membrane The cell membrane is a phospholipid bilayer (double membrane),

which has embedded proteins and cholesterol It is described as a “fluid mosaic model” because it is always

moving and has a mosaic pattern to the way it looks due to the proteins that poke out through it.

The phospholipids look like balloons with two strings attached. The balloon part is called the “head” and likes water (hydrophilic),

the strings are called the “tails” and don’t like water (hydrophobic). Since the cytoplasm inside cells is mostly water, then the tails are

gathered together on the inside of the membrane away from the water and the heads face outward towards the water environment inside and outside the cell.

The proteins are periodically spread throughout the membrane and help with transport of specific molecules or ions across the membrane.

There are different proteins – some passively or actively transport things across the membrane, some are receptors.

Pumps actively transport across the membrane using cell energy (in the form of ATP)

Channels passively transport across the membrane using no cell energy.

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3.4.2 Explain the importance of the membrane proteins to transport across the membrane The presence of protein channels and pumps, as well as the

hydrophobic centre region of the membrane, allows cells to regulate (control) what can enter and leave the cell.

This protects a cell from pathogens This allows a cell to take in nutrients (endocytosis or diffusion or

active transport) This allows the cell to get rid of waste products (exoocytosis or

diffusion or active transport) It also allows one cell to synthesise a protein product and transport

it to another cell via exocytosis.

3.5.1 DIFFUSION

Define diffusion as: the net movement of particles 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.Diffusion requires a CONCENTRATION GRADIENT.

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3.5.2 Describe the importance of diffusion of gases, solutes, and water specifically as a solvent• Diffusion is important to organisms because it is the process by

which many useful molecules enter the body cells and waste products are removed.

• For example, in mammals, digested food molecules (amino acids, glucose) move down a concentration gradient from the intestine to the blood. Waste products such as carbon dioxide or urea travel by diffusion from body cells into the bloodstream.

• In mammals, oxygen diffuses from high concentration (in the alveoli in the lungs)) to a lower concentration (in the blood entering the lungs from the body) [it also diffuses from the red blood cells into individual cells which are low on oxygen]. Carbon dioxide diffuses from high concentration (in the blood) to a lower concentration (in the air sac).

• In plants, oxygen diffuses from plant cells into the air, while carbon dioxide diffuses from the air into the plant cells so that photosynthesis can take place.

3.5.3 State that substances move into and out of cells by diffusion through the cell membrane; this is a dynamic process (always happening)

• Cell membranes are semi-permeable.

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• The proteins and phospholipids in the membrane regulate what can move through depending on the size and charge of the substance.

Supplement3.5.4 State that the energy for diffusion comes from the kinetic energy of random movement of molecules and ions

Particle theory states that all matter consists of many, very small particles, which are constantly moving, or in a continual state of motion. The degree to which the particles move is determined by how much energy they have and their relationship to other particles

3.5.5 EXPLAIN the factors that influence the rate of diffusion, limited to surface area, temperature, concentration gradients and distance

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The factors that influence the rate of diffusion apply to BOTH simple AND facilitated diffusion

CONCENTRATION GRADIENT: the greater the concentration gradient (i.e. difference in concentration between one side of the membrane and the other), the greater the rate of diffusion

SURFACE AREA: the greater the surface area, the greater the rate of diffusion

TEMPERATURE: as temperature increases, the rate of diffusion increases since the particles move with greater kinetic energy

CONCENTRATION GRADIENT: the greater the concentration gradient (i.e. difference in concentration between one side of the membrane and the other), the greater the rate of diffusion

DISTANCE FOR DIFFUSION: The greater the distance that a particle must diffuse. The longer time taken for diffusion

FACILITATED DIFFUSION

FACILITATED DIFFUSION uses protein channels to allow SPECIFIC molecules to enter/leave the cell through diffusion, moving from a region of high concentration to a region of low concentration. This is also a passive mechanism (the cell does not expend energy). The same ‘rules’ affecting the rate of simple diffusion also apply to facilitated diffusion – rate will be affected by temperature, number of protein channels available, concentration gradient ad distance for diffusion.

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3.5 OSMOSIS and 3.7 CELL RESPONSES Core3.5.6 State that water diffuses through partially permeable membranes by osmosis. Osmosis is a special case of diffusion.

3.5.7 State that water moves in and out of cells by osmosis through the cell membrane

3.5.8 Investigate and describe the effects on plant tissues of immersing them in solutions of different concentrationsDefine the COMPARATIVE terms to describe the solute concentrations of two solutions being compared one to the other: hypertonic, isotonic, hypotonic

3.5.9 State that plants are supported by the pressure of water inside the cells pressing outwards on the cell wallSupplement3.5.10 Define osmosis as the net movement of water molecules from a region of higher water potential (dilute/ HYPOTONIC solution) to a region of lower water potential (concentrated/ HYPERTONIC solution), through a partially permeable membrane

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3.7.1 Explain the effects on plant tissues of immersing them in solutions of different concentrations by using the terms turgid, turgor pressure, plasmolysis and flaccid The outside and inside of cells can be COMPARED using the terms:

hypotonic, hypertonic, and isotonic. Hypotonic solutions have less solute (and more water) RELATIVE

to the environment on the other side of the cell membrane. In contrast, hypertonic solutions have more solute (and less water)

RELATIVE TO the environment on the other side of the cell membrane.

Isotonic solutions have equal quantities of water and solute particles on both sides of the cell membrane.

Water moves BY OSMOSIS from hypotonic to hypertonic solutions. If two solutions separated by a membrane are isotonic, then there is

equal movement of water back and forth through the cell membrane – a dynamic equilibrium.

All cells try to maintain isotonicity with the fluid in which they are immersed.

When water moves into a cell, this increases the pressure inside the cell and thus the pressure on the inside of the cell membrane.

In plant cells, the pressure on the inside of the cell membrane created by cytoplasm is called “turgor pressure”.

When water moves out of a cell from then turgor pressure goes down.

In a plant cell, if water moves into the cell it becomes “turgid/stiff” due to the increase of turgor pressure on the cell membrane. This maintains plant shape. Plant cells cannot burst since their rigid cell wall prevents them from exploding.

In a plant cell, if water moves out of the cell it becomes wilted (plasmolysis).

In an animal cell, if too much water enters the cell, the cell can burst (cytolysis), due to a quick increase in turgor pressure.

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In an animal cell, if water moves out of the cell it shrinks (becomes flaccid), which can cause the cell to die.

3.7.2 Explain the importance of osmosis in the uptake of water by plants3.7.3 Explain the importance of osmosis on animal cells and tissues In plants, opening of stomata (to allow exchange of oxygen and

carbon dioxide) depends on osmosis of water into and out of the guard cells.

In animals, the movement of water back into the blood when it is being cleaned in the kidney depends on osmosis.

In animals, the movement of food into the blood from the intestine and waste out of cells is controlled by osmosis.

3.7.4 Explain how plants are supported by TURGOR PRESSURE within cells, resulting from a build up of water (pressure) acting against an inelastic cell wall

When water moves by osmosis into the plant cells, it causes the central vacuole to expand and push against the cell wall. This is defined as ‘turgor pressure’.

This increase in turgor pressure on the inside of the plant cell, helps to maintain the structure of the plant cell.

This increased pressure is called turgidity. The cell wall, composed of cellulose and lignin, is rigid and keeps

the cells from exploding from increased turgor pressure.

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3.6 ACTIVE TRANSPORT Core3.6.1 Define active transport as the movement of particles through a cell membrane from a region of lower concentration to a region of higher concentration (in other words AGAINST a concentration gradient) using energy from respiration

3.6.3 Explain how protein molecules move particles across a membrane during active transport

Active transport uses ATP (chemical energy) to change the shape of protein pumps, which then do some kind of work in the cell – typically transporting particles (e.g. ions, molecules) AGAINST a concentration gradient.

Supplement

3.6.2 Discuss the importance of active transport as a process for movement across membranes: e.g. ion uptake by root hairs and uptake of glucose by epithelial cells of villi and kidney tubules

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In plant cells, the root hair cells are able to take up mineral ions, even though there might be less outside the cell compared to the cytoplasm inside the cell using active transport.

Cell membranes of animal intestinal cells have protein pumps that use ATP to change shape and pump nutrients (e.g.glucose) into or out of the intestinal cells.

Cell membranes of animal kidney cells have protein pumps that use ATP to change shape and pump essential nutrients back into the blood.

ACTIVE TRANSPORT II: ENDOCYTOSIS AND EXOCYTOSIS

Endocytosis is the transport of nutrients/ molecules into the cell using ENERGY and an infolding of the cell membrane. Protein pumps are not used.

Exocytosis is transport of particles OUT of the cell by using ENERGY and an infolding of the cell membrane. Protein pumps are not used.

These processes are typically used to import nutrients, export waste products OR export a useful product of the cell in order to transport it to another cell.

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