3.2 cells - transport across membranes

What happens to the structure of membranes inside or around the cell

Remains the same, either having a single phospholipid bilayer or a double

What are the 3 main functions of the plasma memrbane? IMAGE

1. To control the transport of substances into and out of the cell or organelles, allowing certain molecules to pass through but not others, they are partially permeable

2. Act as a receptor site to recognise chemicals which need to enter the cell or organelle

3. Separate off the cell from the environment and the different reactions of the cell from each other by forming organelles (compartmentalisation). Different concentrations can be maintained on either side of the membrane

Even though the cell wall is many times thicker than the plasma membrane, it is still freely permeable. Why is that?

Contains large pores

What is the behaviour of a membrane? IMAGE

Partially permeable, they do not let polar substances pass through easily and aqueous contents are prevented from escaping, this is why cell-surface membrane acts as an effective barrier to separate the cell from its environment. This suggests that it contains lipids (phospholipids), as these don’t mix with water

Give the description of a plasma membrane structure? IMAGE

• Membrane is about 7nm wide and is a fluid structure, having protein molecules floating about in the bilayer, forming a fluid mosaic structure

• The proteins may be in one layer (extrinsic), or across the whole membrane (intrinsic)

• They may be fixed in one place or floating in the phospholipid bilayer

• The proteins which span the membrane often have hydrophilic channels in the middle to allow transport of some substances more easily.

What is the function of glycoproteins in a plasma membrane?

Short, branching carbohydrate chains are attached to proteins in the membrane, acting as receptors for hormones and neurotransmitters or antigens

What is the function of Glycolipids in a plasma membrane?

Made up of carbohydrate covalently bonded to a lipid. Acts as recognition sites, helping to maintain the stability of the membrane and help cells attach to one another, so forming tissues

What is the function of Cholesterol in a plasma membrane?

Restricts the movement of other molecules making up the membrane and increases strength and stability of membrane by making them less flexible. It helps to prevent the loss of water and dissolved ions from the cell.

This is the fluid-mosaic model of membrane structure. Give details about this. IMAGE

Fluid refers to the fact that all different molecules move around. The fluidity means that cells are able to change their shape (e.g. phagocytes)

Mosaic because there are different types of molecule - carbohydrates and proteins float around within the bilayer

The molecules in the membrane can:

• Move sideways about the membrane

• Leave the membrane to form vesicles in the cell’s cytoplasm

• Join the membrane from vesicles in the cytoplasm

Which organelles have double membrane (envelopes) surrounding them?

What are the 3 basic mechanisms by which exchange is achieved?

Diffusion, osmosis and active transport. There is also a process called endo/exocytosis

Diffusion and osmosis is a passive process and doesn’t need energy

Active transport is an active process, requiring the cell to provide ATP

What is diffusion?

Movement of substances from a region of their higher concentration to a region of their lower concentration, with molecules moving down a concentration gradient (diff in conc. between 2 areas). As a result of diffusion, substances tend to reach dynamic equilibrium, where they are evenly spread, although still moving

Explain several factors of which the rate of diffusion is affected by. IMAGE ← ignore this and finish the answer

• Surface area where diffusion occur - High S.A. of membrane where diffusion occurs, greater rate

• Thickness of exchange surface -

• Concentration gradient -

• Size of diffusing molecule -

• Temperature -

• Water solubility -

What is the rate of diffusion proportional to?

Rate of diffusion ∝ (Surface area x difference in concentration) / thickness of exchange surface

What do you need to do to increase diffusion rate?

• Maximise Surface Area

  Maximise Concentration Difference

• Minimise The Thickness Of Exchange Surface

What is simple diffusion?

Diffusion directly across phospholipid bilayer, non-polar molecules can diffuse rapidly through as they are lipid soluble. Smaller molecules can also diffuse through more easily and rapidly as they can fit between phospholipid molecules.

However, bilayer is barrier to polar molecules, so difficult to pass through, unless they are so small that they can diffuse rapidly through bilayer. They instead use the intrinsic proteins to go through by faciliated diffusion.

What is faciliated diffusion?

What is osmosis?

Net movement of water molecules from a region of higher water potential to a region of lower water potential, through a partially permeable membrane (down a water potential gradient). Partially permeable membrane allow some solutes (small molecules, O₂,H₂O,CO₂) to pass but prevents others (large molecules, sucrose, starch, protein)

What could have a lower water potential Ψ?

Solution more concentrated on solutes rather water

What could have a higher water potential Ψ?

Very dilute solution of solutes, or pure water

Describe this image. IMAGE ← alongside filling the gap

Solute molecules can’t cross membrane easily, so A and B can’t come into equilibrium by diffusion of both solutes and water, so only water can move. Water molecules show a net movement from ____ because there is a higher concentration of water molecules in A than B

What is water potential?

Tendency of water to move from one place to another as a result of osmosis, measured in kPa

Pick the image of the table of water potential of solutions.

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Why do animal cells change in volume and shape more than a plant cell when placed in solutions of different water potential?

Doesn’t have a cell wall

What happens to a red blood cell in a hypertonic solution (concentrated solution / low water potential)? IMAGE

Concentrated salt solution has _____ water potential than cell so water leaves by osmosis, causing cell to shrink (or crenate)

What happens to a red blood cell in a hypotonic solution (dilute solution / high water potential? IMAGE

Dilute solution or pure water has ____ water potential than cell so water enter cell by osmosis, causing cell to swell and eventually bursts. If the pressure inside cell becomes too great, the bursting of the cell is known as osmotic lysis.

What happens to a red blood cell in a isotonic solution (same concentration and water potential)? IMAGE

An isotonic solution has same Ψ as the cell, so no net osmotic movement of water (there is no Ψ gradient) and so shape and size of cell remains the same

Why are plant cells prevented from large changes in volume?

Cell walls prevents cell from taking in too much water and bursting by osmotic lysis, due to the rigidity of the cell wall

What happens to a plant cell in a hypertonic solution (concentrated solution / low water potential)? IMAGE

Concentrated salt solution has a lower water potential than cell so water leaves cell cytoplasm and vacuole by osmosis, causing cytoplasm to shrink away from cell wall, leaving gaps between cell membrane and cell wall (which are filled with concentrated solution of salt). This is called plasmolysis.

What happens to a plant cell in a hypotonic solution (dilute solution / high water potential? IMAGE

Dilute solution or pure water will have a higher water potential than cell and so water enters cell by osmosis. Cytoplasm and vacuole gains some water, but because of the high tensile strength of the cellulose cell wall, cell won’t burst. The cell is said to be turgid.

What happens to a plant cell in a isotonic solution (same concentration and water potential)? IMAGE

No net osmotic movement of water (there is no Ψ gradient) and so shape and size of cell remains the same.

What happens to the mass of a piece of potato if it is placed in the solutions for a period of time, pick the table.

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What is active transport? IMAGE

How does endocytosis and exocytosis work? IMAGE

Sometimes, cells need to transport large amounts of material across their plasma membranes. These amounts are too large to move by faciliated or simple diffusion and active transport. To do this, cells make containers from plasma membrane itself. These sac-like structures are called vesicles. Cells can use vesicles to transport solids or liquids across plasma membrane either into or out of the cell. Process of importing material in vesicles is called endocytosis and process of exporting material in vesicles is called exocytosis. Both processes require ATP.

Explain co-transport.

In order to absorb glucose from lumen of intestines into epithelial cells, there must be a higher concentration of glucose in the lumen compared to the epithelial cell (for faciliated diffusion). However, there is usually more glucose in epithelial cells and this is why active transport is needed.

Explain the process of co-transport of glucose and amino acids

1. Sodium ions are actively transported out of epithelial cell into blood in capillary

2. This reduces sodium ion concentration of the epithelial cell

3. Sodium ions can then diffuse from lumen down their concentration gradient into epithelial cell. The protein the sodium ions diffuse through is a co-transporter protein, so either glucose or amino acids also attach and are transported into epithelial cell against their concentration gradient

4. Glucose then moves by faciliated diffusion from epithelial cell to blood

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