Lecture 3 - Functions of Cell Membranes

Which types of cells have a plasma membrane?

both prokaryotic and eukaryotic

Which cells have membrane-bound organelles?

eukaryotes

What are some major functions of cell membranes?

enzyme activity, transport, recognition, communication, attachment

What is one enzymatic function of membrane proteins?

some act as enzymes located in cristae

What is the function of receptor membrane proteins?

detect signals outside of the cell

What is the role of membrane proteins in cell recognition?

help cells recognize and communicate with each other

How can membrane proteins help maintain cell shape?

attach plasma membrane to cytoskeleton and ecm

What is the function of cell junctions?

allow cells to interact and remain connected

What role do membrane proteins have in transport?

control movement across the cell membrane

How is the lipid bilayer arranged?

phospholipids with hydrophilic heads facing water and hydrophobic tails facing away from water

Why is the orientation of membrane proteins important?

determines which part of the protein interacts with the cytoplasm, membrane, or ecm

What are cell junctions?

sites where neighbouring cells have direct physical contact

What are tight junctions?

join neighboring animal cells tightly

Where are tight junctions commonly found?

epithelial tissues in skin, lining of gut

What is the main function of desmosomes?

strongly attach neighboring cells

What are gap junctions?

junctions that provide a small gap for communication and small molecules to pass through

What are gap junctions made of?

6 connexin proteins

Where are gap junctions found?

embyros, liver, heart muscle cells

What are plasmodesmata?

connect neighboring plant cells

What is the function of plasmodesmata?

allow communication and movement of substances between cells

What property of the cell membrane controls what can cross it?

selective permeability

What are the two major categories of membrane transport?

passive transport, active transport

What is passive transport?

movement of molecules from high to low concentration without using the cells energy

What direction does passive transport occur relative to the concentration gradient?

down the concentration gradient, high to low concentration

What are the main types of passive transport?

diffusion, facilitated diffusion

Which types of molecules can easily cross the membrane by diffusion?

small non polar molecules such as o2, co2 and small lipids

Why can small non-polar molecules cross the membrane easily?

interact with the hydrophobic region of the phospholipid bilayer

What is facilitated diffusion?

require specific transport proteins to diffuse through membrane

What are the two types of transport proteins involved in facilitated diffusion?

channel proteins, carrier proteins

How do channel proteins transport molecules?

form a channel through the membrane to allow substances through

What is an example of a channel protein?

aquaporins, allow water to move across membrane

How do carrier proteins transport molecules?

bind to a specific molecule and change shape, moving the molecule across membrane

What is osmosis?

diffusion of water across a membrane

What is active transport?

movement of molecules against their concentration gradient

Does active transport require energy?

yes in the form of atp

What type of membrane protein carries out active transport?

carrier proteins

What is an example of active transport?

na/k pump

What is an important advantage of membrane transport proteins?

provide selectivity

How can transport proteins affect the rate of transport?

can increase the rate of transport

What happens to membrane transport proteins after transporting molecules?

can be continuously reused

What limits the rate of transport through membrane transport proteins?

number of transport proteins available

How are large molecules transported across the membrane?

using vesicles

What are the two major forms of vesicle transport?

exocytosis and endocytosis

What does exocytosis do?

move materials out of cell

What does endocytosis do?

move materials into cell

What are the basic steps of exocytosis?

material packaged in vesicle, moves towards plasma membrane, vesicle and plasma membrane fuse, contents released out of cell

What are the three types of endocytosis?

phagocytosis, pinocytosis, receptor mediated endocytosis

What is phagocytosis?

cell eating, cell takes in large particles

What is pinocytosis?

cell drinking, cell takes in extracellular fluid

What is receptor-mediated endocytosis?

membrane forms a vesicle to bring selected substances into cell