Transport across cell membranes Flashcards
What does the cell membrane consist of?
- mainly phospholipid bilayer + protein
What does fluid-mosaic model mean?
- phospholipid molecules constantly moving = fluid
- different protein molecules unevenly distributed = mosaic
How is the phospholipid bilayer related to the partial permeability of the cell membrane?
- hydrophilic heads orientated inwards towards cytoplasm or outwards towards watery extra-cellular fluid
- hydrophobic tails orientated towards each other
- enables lipid soluble molecules to pass quickly through but restricts passage of water soluble ions / polar molecules
How are small molecules passed through the membrane?
- e.g. carbon dioxide + oxygen diffuse through gaps between phospholipids
- water passes through water protein channels by osmosis
What are the functions of the bilayer?
- allows non-polar substances through
- forms barrier to water/soluble + charged substances/large molecules
- fluidity allows it to form vesicles or fuse with other membranes (phagocytosis)
- maintains different environments on each side
What do carrier / channel proteins do?
Aid passage of water soluble ions + polar molecules across membrane
What are channel proteins?
Acts as channels to allow specific ions/molecules through
What are carrier proteins?
Act as carriers (can mould / change shape) + transport larger substances in and out
What are intrinsic proteins?
Span entire membrane (can be channel or carrier proteins)
What are extrinsic proteins?
Span either one phospholipid layer or are on surface of membrane
What are glycoproteins?
Carbohydrate proteins
What is the role of enzymes in membranes?
In membranes of epithelial cells in small intestine
What is the role of channel proteins?
Have specific tertiary structure, which together with its charge + size determines which molecules can be transported by facilitated diffusion
What is the role of carrier proteins?
Possess specific tertiary structure complementary to polar molecules - aids transport by facilitated diffusion + active transport
What is the role of receptors for hormones in membranes?
Have specific tertiary structure allowing specific hormone with complementary shape to attach to its binding site
How do glycolipids have a role in cell membranes?
- act as receptors (for specific hormones)
- act as antigens (cell recognition)
How does cholesterol play a role in the cell membrane?
- type of lipid
- molecules bind to fatty acid tails causing them to pack more closely together
- restricts movement of phospholipids = membrane less fluid / more rigid
- helps maintain shape of animal cells
What is diffusion?
The net movement of molecules from a high conc to a low conc until equally distributed
Is diffusion an active or passive process?
- passive
- does not require energy from respiration
What is the rate of diffusion proportional to?
Surface area x conc gradient / diffusion distance
What is rate of diffusion increased by?
- higher conc gradient
- large surface area (e.g. microvilli)
- short diffusion distance (e.g. single cell layer)
What is facilitated diffusion?
- involves use of channel + carrier proteins
- allows transport of polar molecules (e.g. glucose + amino acids) across membranes down a conc gradient through a membrane
- gaseous exchange occurs via diffusion
Is facilitated diffusion a passive or active process?
- passive
- does not require energy from respiration
- not inhibited by respiratory inhibitors (e.g. cyanide)
What is active transport?
- Movement of molecules/ions through a partially permeable membrane
- uses carrier proteins
- against conc gradient
