WJEC AS Biology Unit 1.3 - Cell membranes and cell transport

Principal components of the plasma membrane

• Phospholipid bilayer

• Extrinsic and intrinsic proteins

• Cholesterol

• Glycoproteins and glycolipids

Plasma membrane structure (diagram)

Plasma membrane structure (description)

• Phospholipids form a phospholipid bilayer;

  • hydrophobic fatty acid tails point towards each other and meet in the middle

  • hydrophilic heads are on the outside, interacting with water inside or outside the cell

• Cholesterol is found between the fatty acid tails and helps maintain the stability of the membrane

• Intrinsic proteins span both layers of the phospholipid bilayer (carrier proteins/channel proteins)

• Extrinsic proteins are associated with one of the layers of the phospholipid bilayer

• The proteins may be attached to carbohydrates —> a glycoprotein. These form the glycolax and play an important role in cell recognition and signalling

Fluid mosaic model

• Model of the structure of biological membranes, in which proteins are studded through a phospholipid bilayer, as in a mosaic. The movement of molecules within a layer of the bilayer is its fluidity.

• Mosaic; embedded proteins. These proteins produce a pattern

• Fluid; the individual phospholipids can move within each phospholipid layer relative to each other

Phospholipids in the cell membrane

• important components of cell-surface membranes and form the basis of membrane structure because;

  • can form bilayer, with one sheet of pl molecules opposite another

  • inner layer of pls has its hydrophilic heads pointing in, towards the cell, and interacts with the water in the cytoplasm

  • outer layer of pls has its hydrophilic hydrophilic heads pointing outwards, interacting with the water surrounding the cell

  • hydrophobic tails of the two pl layers point towards each other, to the centre of the membrane

  • the pl component of a membrane allows lipid-soluble molecules across, but not water-soluble molecules

Proteins in the cell membrane

• scattered through the phospholipid bilayer of the membrane

• Two ways in which they’re embedded; extrinsic proteins or intrinsic proteins

• Move freely within the pl bilayer. The ease with which they move is dependent on the number of pls with unsaturated fatty acids in the pls

• Types;

  • enzyme or signalling protein

  • carbohydrate chain + glycoprotein —> glycolax; for cell recognition so cells group together to form tissues

  • Extrinsic protein

  • Intrinsic protein; carrier protein/pump or channel protein/hydrophilic channel

  • Receptor; for recognition by hormones

Extrinsic proteins

• on either surface of the bilayer

• Provide structural support

• Form recognition sites, by identifying cells

• Form receptor sites for hormone attachment

Intrinsic proteins

• extend across both layers of pl bilayer

• Include transport proteins, which use active or passive transport to move molecules and ions across the cell membrane

Glycolipids

• lipids with carbohydrate chains attached

• Occur in both plant and animal membranes

Glycoproteins

• Function as the receptors for chemical signalling

• + carbohydrate —> glycolax

• Exist in both plant and animal cell membranes

Channel proteins

• mechanism of facilitated diffusion

• Intrinsic proteins

Carrier protein

• mechanism of facilitated diffusion

• Intrinsic proteins

Cholesterol in the membrane

• exists only in animal cells

• Occurs between the fatty acid tails of the pls

• Make the membranes more stable at high temps and more fluid at low temps

Glycolax

• carbohydrate + glycoprotein —> glycolax

• Play an important role in cell recognition and signalling

• Some have roles as hormone receptors, in cell-to-cell recognition and in cell-to-cell adhesion

Hydrophobic areas of the membrane

• fatty acid tails

• Face inwards

Hydrophilic areas of the membrane

• phosphate heads

• Face outwards into aqueous solution

Properties of cell membranes

Functions of cell membranes

How structure and properties allow functions of cell membranes

Permeability of the cell membrane

Factors affecting permeability of the cell membrane

• temperature

• Organic solvents

• In active transport;

  • presence of a specific pump for molecule

  • availability of oxygen to make ATP by aerobic respiration

• In facilitated diffusion;

  • presence of specific carrier proteins (—> selectively permeable)

• In simple diffusion;

  • conc gradient (greater = higher r. Of diff)

  • Size of molecules

  • Large surface area

  • Temperature

  • Lipid solubility

  • Distance

  • Must be non-polar

Draw a fluid mosaic model

Diffusion

The passive movement of a molecule or ion down a conc gradient from a region of high concentration to a region of low concentration

Rate of diffusion

(surface area x concentration gradient)/diffusion distance or length of diffusion path

Factors affecting the rate of diffusion

Facilitated diffusion

• The passive movement of molecules or ions down a conc gradient, across a membrane, by protein carrier molecules in the membrane

Factors affecting rate of facilitated diffusion

• presence of specific carrier proteins (—> selectively permeable)

Types of facilitated diffusion

• Channel proteins (ions)

• Carrier proteins (larger polar molecules)

Facilitated diffusion with channel proteins

• Ions (charged and hydrophilic molecules)

• Move through channel proteins in the membrane

• Lined with hydrophilic groups/water

• Passive process

• Ions move from a higher concentration—>a lower concentration the other side of the membrane

• Channels of the protein may open under certain conditions (gated)

Facilitated diffusion with carrier proteins

• Larger polar molecules such as amino acids and sugars

• Carrier proteins interact with the molecule to be transported, causing the carriers to change shape

• Examples of carriers include glucose permease and amino acid transporters

• Carriers=specific to the molecule they transport

• Different transporters for different molecules therefore membranes=selective

• For it to be facilitated, the molecules must move from a higher conc—>a lower conc

• Passive process, no energy required

Active transport

• The movement of molecules or ions across a membrane against a concentration gradient, using energy from the hydrolysis of ATP made by the cell in respiration

• Ions and molecules are moved from a lower to a higher concentration against the concentration gradient

• Required energy from ATP

• Occurs through intrinsic carrier proteins spanning the membrane

• Rate is limited by the number and availability of carrier proteins

Factors affecting transport across cell membranes overall

• Conc gradient

• Size of molecule

• Large surface area

• Temperature

• Lipid solubility

• Distance

• Must be non-polar

Active transport and respiration

• At higher conc differences across a membranes rate of uptake increases and reaches a plateau, at which the carrier proteins are saturated

• The rate of uptake is reduced with the addition of a respiratory inhibitor

Active transport and cyanide

• cyanide = respiratory inhibitor

• Prevents aerobic respiration + the production of ATP in the mitochondria

• Without ATP, active transport cannot occur

• Cyanide reduces active transport

Co-transport

• A transport mechanism in which facilitated diffusion brings molecules and ions, such as glucose and sodium ions, across the cell membrane together into a cell

• Type of facilitated diffusion that brings molecules into cells together on the same transport protein molecule

• Sodium-glucose co-transport is significant in absorbing glucose and sodium ions across cell membranes and into the blood in the ileum and the kidney nephron

Process of co-transport

• A glucose molecule and two sodium ions outside the cell attach to a carrier proteins spanning in the cell membrane

• The carrier protein changes shape and deposits the glucose molecule and the sodium ions inside the cell

• The glucose molecule and sodium ions separately diffuse through the cell to the opposite membrane

• The glucose passes into the blood by facilitated diffusion and sodium ions are carried by active transport

Osmosis

The net passive movement of water molecules from a region of higher water potential to a region of lower water potential through a selectively permeable membrane

Water potential

• The tendency for water to move into a system

• Water moves from a solution with higher water potential (less negative) to one with a lower water potential (more negative)

• Decreased by the addition of a solute

• Pure water = 0

Solute potential

• A measure of the osmotic strength of a solution. Reduction in water potential due to presence of solute molecules

• Tendency of water to leave due to conc of dissolved solutes in cytoplasm

• Negative value

Pressure potential in osmosis in plant cells

• the pressure exerted by the cell contents on the cell wall

• Generates a force, pushing water out of the cell

• Positive value

• Makes the cell turgid

The water potential equation

• Water potential = pressure potential + solute potential

• Pressure potential = water potential - solute potential

• Solute potential = water potential - pressure potential

Plasmolysis

The retraction of the cytoplasm and the cell membrane from the cell wall as a cell loses water by osmosis

Hypotonic solution

• The surrounding solution has a lower conc of dissolved solutes and therefore a higher water potential than the cell

Hypertonic solution

• The surrounding solution has a higher conc of dissolved solutes and therefore a lower water potential than the cell

Isotonic solution

• The surrounding solution has the same water potential as the cell

Incipient plasmolysis

Where the cell membrane and cytoplasm are partially detached from the cell wall due to insufficient water to make cell turgid

Turgor and plasmolysis in plant cell osmosis

• Turgid;

  • cytoplasm pushed against cell wall

  • Water potential = 0

  • Pressure potential =-solute potential

• Incipient plasmolysis;

  • cytoplasm beginning to pull away from cell wall

  • pressure potential = 0

  • water potential of cell = solute potential

• Plasmolysed;

  • cytoplasm completely pulled away from cell wall

  • Pressure potential = 0

  • Water potential of cell > solute potential of the external solution

Osmosis in animal cells

• No cell wall and so pressure potential does not have to be considered

• Water potential therefore = solute potential

• May cause osmotic lysis or haemolysis

• Haemolysis; without a cell wall, as water enters a red blood cell, the cell bursts

Bulk transport

• A cell can transport materials in bulk into and out of the cell

• Exocytosis and endocytosis provide a mechanism for bulk transport across a cell membrane and these processes change the surface area of cells as they occur

Endocytosis

• The active process of the cell membrane engulfing material, bringing it into the cell in a vesicle

• Occurs when material is engulfed by extensions of the cell membrane and cytoplasm, surrounding it, making a vesicle Occurs

• Two types;

  • phagocytosis

  • pinocytosis

Exocytosis

• The active process of a vesicle fusing with the cell membrane, releasing the molecules it contains

• Digestive enzymes often secreted in this way

Phagocytosis

• The active process of the cell membrane engulfing large particles, bringing them in the cell in a vesicle

• The products are absorbed into the cytoplasm

Pinocytosis

• The active process of the cell membrane engulfing droplets of fluid, bringing them into the cell in a vesicle

Why phospholipids form a bilayer

Hydrophilic and hydrophobic properties

What determines the position of a protein in a membrane

Polarity

Collective name for glycoproteins and glycolipids on the outer surface of the membrane

Glycolax

Feature of the phospholipid bilayer that prevents polar and charged particles from crossing the membrane

the bilayer has a hydrophobic core that prevents the passage of polar molecules while allowing the relatively free diffusion of non-polar molecules

Name two gases that can cross the membrane by simple diffusion

• Carbon dioxide

• Oxygen

Name two vitamins that can cross the membrane by simple diffusion

• Vitamin A

• Vitamin K

How lipid solubility affects the rate of diffusion

the molecules which are soluble in lipids can diffuse easily across the cell membrane. Some molecules are hydrophilic and not soluble in lipids. Therefore, they do not diffuse easily across the cell membrane and require the involvement of specialized proteins for their transport

How the size of the molecule affects the rate of diffusion

The rate of diffusion is inversely proportional to the size of the molecules. This means that the smaller is the size of the molecule, the higher is the rate of diffusion

How the surface area of the cell membrane affects the rate of diffusion

As the surface area of the membrane increases, the rate of diffusion also increases, as there is more space for molecules to diffuse across the membrane

Adaptation of the cell membrane that increases surface area

Microvilli

How diffusion path affects the rate of diffusion

The shorter the distance the substances have to move, the faster the rate of diffusion

How concentration gradient affects the rate of diffusion

The greater the concentration gradient, the quicker diffusion takes place

How temperature affects the rate of diffusion

Higher temperatures increase the energy and therefore the movement of the molecules, increasing the rate of diffusion. Lower temperatures decrease the energy of the molecules, thus decreasing the rate of diffusion

State the differences between simple and facilitated diffusion

• In simple diffusion, molecules move without the assistance of membrane proteins, whereas in facilitated diffusion, membrane proteins assist molecules in their movement downward

• SD = small, non polar molecules, FD = large or polar molecules

State the similarities between simple and facilitated diffusion

• Passive processes

• Transport molecules or ions

• Occurs down a conc gradient

Substances transported by simple diffusion

• small non-polar molecules, including oxygen and carbon dioxide

Substances transported by facilitated diffusion (carrier protein)

Larger polar molecules such as amino acids and sugars

Substances transported by osmosis

Water molecules

Substances transported by active transport

Examples of molecules which the cell needs high concentrations of include ions, glucose and amino acids

Substances transported by facilitated diffusion (channel protein)

Ions (charged and hydrophobic molecules)

Examples of co-transport

Sodium-glucose co-transport; significant in absorbing glucose and sodium ions across cell membranes and into the blood in the ileum and the kidney nephron

No net movement of water meaning

Solutions are isotonic to each other

What happens if a red blood cell is placed in an isotonic solution

• Isotonic solution has the same concentration of dissolved solutes as rbc

• It has the same water potential as RBC

• There is no net movement of water into or out of the cell

What happens if a red blood cell is placed in a hypotonic solution

• Hypotonic solution has a lower concentration of dissolved solutes than the RBC

• It has a higher water potential than inside the RBC

• Water mixes from the higher water potential of the surrounding solution into the cell by osmosis

• The cell bursts due to osmotic lysis

What happens if a red blood cell is placed in a hypertonic solution

• Hypertonic solution has a higher concentration of dissolved solutes than the RBC

• It has a lower water potential than the RBC

• Water moves from the higher water water potential inside the cell into the external solution by osmosis

• The cell loses volume/becomes cremated (shrink + shape changes, usually with a ruffled or scalloped border)

Osmotic lysis

Bursting/rupturing of the cell membrane due to osmotic movement of water into the cell when in a hypotonic environment

Why osmotic lysis doesn’t occur in plant cells

The presence of a cell wall

Equation for water potential in plants

Water potential = pressure potential + solute potential

Pressure potential

The hydrostatic pressure exerted by the cell contents on the cell wall. Equal and opposite to the pressure exerted by the cell wall on the cell contents

Solute potential

Tendency of water to leave due to concentration of dissolved solutes in cytoplasm

Plasmolysed cells

Meaning of active in membrane transport

Requires energy in the form of ATP

Features of active transport across the cell membrane

• active process

• Molecules move from a lower conc —> a higher conc (against conc gradient)

• Requires energy in the form of ATP

• Examples include Na+/K+ pumps (Needed for nerve transmission)

• Because these cells need to produce a lot of ATP, they need a lot of oxygen to produce ATP in aerobic respiration

• Examples of molecules which the cell needs high concentrations of include ions, glucose and amino acids

Process of active transport

Why the rate of active transport and facilitated diffusion are limited

• number of carrier and channel proteins becomes limited as they are all occupied

Term for release of substances from vesicles by the vesicle membrane fusing with the cell membrane

Exocytosis

Tern that describes a cell engulfing a particle to enclose it in a vesicle with the cell membrane

Endocytosis

Effect of exocytosis on the surface area of a cell membrane

• Increases the surface area of a cell membrane by fusing intracellular vesicles to the plasma membrane

Effect of endocytosis on the surface area of a cell membrane

• decreases the surface area of a cell membrane

Forming glycolax from the extracellular surfaces of the proteins

The extracellular surfaces of the proteins can be glycosylated to form a glycolax