Biology

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CELL MEMBRANE

- A vital component of all living cells, which separates the cell's contents

from the outside environment.

- Semi-permeable phospholipid bilayer, allowing the entry and exit of

certain molecules while blocking other molecules; selectively permeable.

- Other functions of the cell membrane:

o Physical barrier

o Communication and signaling

o Nutrient uptake and waste removal

o Cell shape and stability

o Cell-cell interactions

ATENEO DE DAVAO UNIVERSITY

Km 7 Central Park Blvd, Talomo, 8016 Davao City, Philippines

Tel No. +63 (82) 221.2411 local 8608

E-Mail: [e-mail] * www.addu.edu.ph

In Consortium with Ateneo de Zamboanga University and Xavier University

SENIOR HIGH SCHOOL – BIOLOGY AND EARTH & LIFE SCIENCE CLUSTER

1. PASSIVE TRANSPORT

o A type of cellular transport that moves substances across the cell

membrane without the need for energy expenditure.

o Occurs along the concentration gradient, which is the difference in

concentration of a substance between two areas, particularly from

an area of high concentration to an area of low concentration.

o No net movement will occur after the solute molecules have

reached dynamic equilibrium where the molecules continue to

cross the membrane in both directions but at equal rates.

o There are three types:

i. Simple Diffusion - net movement of a substance from an

area of higher concentration to an area of lower

concentration

▪ Does not require the input of energy. The concentration

gradient alone is sufficient to drive the movement of

molecules

▪ Molecules move freely and randomly, colliding with each

other and spreading out to achieve a uniform distribution

▪ Involved in various biological processes, such as the

delivery of oxygen, water, and other small molecules to

cells in the body

▪ Factors that affect simple diffusion:

● Temperature – higher temperatures, faster diffusion

● Concentration gradient – greater difference in

gradient, faster diffusion

● Molecular size and weight – smaller molecules

diffuse faster

● Solubility – more soluble molecules in a solvent

diffuse quicker

● Membrane permeability – membranes that are more

permeable to a particular molecule will allow for faster

diffusion

ATENEO DE DAVAO UNIVERSITY

Km 7 Central Park Blvd, Talomo, 8016 Davao City, Philippines

Tel No. +63 (82) 221.2411 local 8608

E-Mail: [e-mail] * www.addu.edu.ph

In Consortium with Ateneo de Zamboanga University and Xavier University

SENIOR HIGH SCHOOL – BIOLOGY AND EARTH & LIFE SCIENCE CLUSTER

ii. Facilitated Diffusion - allows molecules to move across a

cell membrane with the help of specific transmembrane

integral proteins

▪ Does not require energy

▪ Transport proteins assist in the diffusion of molecules,

either by carrier proteins or channel proteins.

● Carrier proteins – bind to specific molecules and

undergo a conformational or shape change, which

allows the molecule to pass across the proteins. Ex.

Glucose transporter

● Channel proteins – form pores or channels which

allow specific molecules to pass across the

membrane. Ex. Aquaporins

▪ Factors that affect facilitated diffusion:

● Concentration gradient – greater difference in

gradient, faster rate of diffusion

● Temperature – higher temperatures, faster diffusion

rates

ATENEO DE DAVAO UNIVERSITY

Km 7 Central Park Blvd, Talomo, 8016 Davao City, Philippines

Tel No. +63 (82) 221.2411 local 8608

E-Mail: [e-mail] * www.addu.edu.ph

In Consortium with Ateneo de Zamboanga University and Xavier University

SENIOR HIGH SCHOOL – BIOLOGY AND EARTH & LIFE SCIENCE CLUSTER

● Saturation – once all transport proteins are in use,

maximum rate of diffusion is achieved and cannot

increase further

● Selectivity – each channel or carrier protein can only

transport specific molecules

iii. Osmosis - diffusion of solvent, usually water molecules,

across a selectively permeable membrane, driven by the

concentration gradient

▪ Osmotic pressure – pressure of a solution needed to keep

it in equilibrium with pure water

▪ Passive process; does not require energy

▪ Occurs across a selectively permeable membrane, which

allows the passage of water molecules but restricts the

passage of solutes

▪ Regulates water balance in cells and uptake of water in

plant roots

▪ Factors that affect osmosis:

● Concentration gradient – the greater the difference

of concentrations, the faster the rate of osmosis

● Temperature – an increase in temperature increases

the rate of osmosis