Cell Transport Flashcards
Cell requirements
○ To enable normal function all cells in the human body need to be in a relatively stable environment
○ Brain and retina cells are susceptible to slight variations in glucose, oxygen, and temperature
○ Skin cells can cope with a much wider range of conditions (e.g. temperature)
○ One of the ‘roles’ of the body system:
- Cellular respiration
□ Supply materials needed e.g. glucose and oxygen
□ Removal waste e.g. carbon dioxide and urea
- Homeostasis
□ Body system work together to make sure that the cellular environment is kept constant
□ The systems operate in such a way that is a particular factor becomes too high or too low they will function to return the factor to its best or optimal level
Cell membrane
○ Allows selective transport of materials across and into and out of the cell
○ The fluid mosaic model
- Membranes consist of a flexible double layer of lipids, with a the ability to flow and change shape
- Specialised molecules are embedded into the lipids like mosaic art
- The combination of the flexible and fluid phospholipid bilayer with its mosaic of proteins, cholesterol and carbohydrate chains
□ Cholesterol
-> Gave rigidity/structure
-> The amount of cholesterol levels determines how the body maintains homeostasis
Cell membrane structure
- Composed mainly of lipids and proteins
- Arranged into two layers that make up the single membrane - lipid bilayer
- Substances and molecules move through the membrane in different ways
- How they are moved depends in their size and is they are organic or inorganic
- Very small molecules (e.g. O2 and CO2 can pass through small gaps between lipids)
- Fat soluble substances such as alcohol and steroid hormones can diffuse cross the membrane by ‘dissolving into the lipid (fat)
□ ‘Like dissolves like’
-> Soap (fatty acid) dissolves oil (fatty acid) - Proteins are embedded through the membrane and they carry out most of the functions of the membrane
- They can be divided into two types
□ Integral proteins - bridge the two sides of the membrane
□ Peripheral proteins - embedded into the outside of the membrane
Transport across the cell
- Cell membranes are described as being differentially permeable/semi-permeable/selectively permeable
- They allow some ions and molecules to pass through, but restrict the movement of others - this is usually based on their size
- Passive and active processes
□ Materials may pass through a cell membrane in a number of ways
Passive processes
- No energy required
- Living and dead cells
-> Types:
◊ Diffusion
◊ Osmosis
◊ Facilitated diffusion
Diffusion
□ Movement of molecules from an area of high concentration to an area of low concentration
□ Diffusion only occurs when there is a difference in in concentration between two areas (e.g. a concentration gradient)
□ Can occur in almost any circumstance where molecules can move - it is not confined to cells
□ What is it
– In cells, molecules will diffuse across the cell membrane if they are small enough to fit through the gaps between the lipids
– Once there us no concentration gradient, although diffusion continues to occur due to the kinetic energy of the molecules, the ‘end’ or net result is no change
– Oxygen, carbon dioxide, and alcohol pass through the lipid bilayer by diffusion
Osmosis
○ Special type of diffusion that involves the diffusion of water across a cell membrane
○ Water molecules are small and can pass through easily
○ Occurs when water moves from an area of high concentration of water to an area of low concentration of water
○ In a solution if there is a high concentration of water (solvent) then there must be a low concentration of solute
- E.g. a dilute solution
○ In a solution if there is a low concentration of water (solvent) then there must be a high concentration of solute
- E.g. a concentrated solution
○ Is a passive process
- It does not require energy
Active processes
- Requires cell’s energy
- Only in living cells
- Types:
◊ Active transport
◊ Cytosis
Concentration of solutions
○ Vocab tells us how water will move across a semi-permeable membrane
○ When using these words it has to be a comparison
- Side B is hypertonic compared to Side A
○ Vocab to describe relative concentration of two solutions:
- Hypertonic
□ A hypertonic solution has a greater concentration of solutes than another solution
□ There is a net movement of water into cells
-> Plant cells will swell but won’t burst sue to the cellulose cell wall
- Isotonic
□ An isotonic solution has the same concentration of solutes as another solution
□ No net movement of solute but particles are still moving
- Hypotonic
□ A hypotonic solution has a lower concentration of solutes than another solution
□ Net movement of water out of cells
-> In a plant cell, the cell membrane shrinks away from the cell
Active transport
○ The movement of molecules from an area of low concentration to an area of high concentration
- E.g. against a concentration gradient
○ Active transport is an active process and requires the use of ‘cellular energy’
- Only occurs in living cells
○ Can only be used for ‘small’ molecules as they have to pass through the cell membrane
○ ‘Cellular energy’ is used to provide ‘power’ to proteins in the cell membrane that ‘ferry’ the molecules across the cell membrane
Cytosis
○ Is an active process in which materials move into or out of the cell enclosed as vesicles
○ Vesicles are bubble-like structures consisting of either a solid particle or liquid droplet enfolded in a piece of the cell membrane
○ The material and its surrounding piece of cell membrane is called a vesicle
○ It is used for large particles that can’t ‘fit’ through the pores or protein channels in the cell membrane
○ Is an active process that requires cellular energy
- Therefore only occurs in living cell membranes
Exocytosis
○ When the material/particle in a vesicle is moved from the inside of the cytoplasm to the outside
○ Materials/particles passed out of a cell by exocytosis may be a waste product or may be a substance that the cell has made for use elsewhere e.g. saliva from salivary gland cells
Endocytosis
○ Endocytosis is when the material/particle is moved from ‘outside’ the cell into the cytoplasm
○ If the material/particle is ‘solid’ it is called phagocytosis
○ If the material/particle is a ’liquid’ it is called pinocytosis
○ All cells carry out pinocytosis but phagocytosis is performed only by specialised cells e.g. White blood cells
Tosis
- Cytosis
- Exocytosis
- Endocytosis
