Cell Membrane – Definition, Function/Structure, Animal/Plant Cell

What is the difference between a plant cell and an animal cell? Which one is bigger? How does each type of cell differ from the other?

Cell Membrane - Definition, Function/Structure, Animal/Plant Cell

If you are looking to discover the answers to these questions, as well as learn all you need to know about the function and structure of each type of cell, then you are in the right place – read on for all you need to know!

Plant And Animal Cells: An Overview

Plant cells and animal cells are two types of cells found in living organisms. They both contain DNA and organelles, but they look very different.

Animal cells have a tough outer layer called the plasma membrane, whereas plant cells don’t.

Plant cells are usually smaller than animal cells, and the size of a plant cell depends on its type (e.g., root or leaf).

In contrast, animal cells tend to be larger because they contain more organelles.

Animal cells are also known as eukaryotic cells, which means that their nucleus contains nuclei.

Eukaryotes are divided into three main groups based on how many nuclei they contain: unicellular, multicellular, and metazoa. 

Unicellular eukaryotes include bacteria, yeast, and algae. Multicellular eukaryotes include fungi, plants, and animals. Metazoa is made up of animals, including humans.

Animal cells can be further subdivided into two categories: prokaryotic cells and eukaryotic 

cells. Prokaryotic cells lack a nucleus and instead have a single large mass of chromosomes inside the cytoplasm.

Eukaryotic cells, such as those found in plants and animals, contain a nucleus surrounded by a double-membrane envelope called the nuclear membrane.

What Is A Cell Membrane?

A cell membrane is a thin layer of lipid bilayer that surrounds each individual cell. It provides structural support for the cell and helps regulate what enters and leaves the cell.

The cell membrane consists mainly of phospholipids. Phospholipids are molecules made up of a phosphate group bound to a fatty acid.

The phosphate group binds tightly to water molecules, so it doesn’t mix well with oil or fat. Fatty acids are long chains of carbon atoms joined together by double bonds.

The phospholipid molecule is shaped like a cone. One end points toward the outside of the cell, while the other end points inward towards the center of the cell.

This shape allows the phospholipid to stretch out as much as possible without breaking apart.

When two phospholipid molecules come close enough together, they will bond together to form a single phospholipid layer. This process is called “lipid packing.”

When this happens, the phospholipid molecules pack themselves together in layers. These layers stack one on top of another until they reach the cell’s exterior.

The cell membrane is very strong. If you were to cut through the cell membrane, you would find that it is extremely difficult to break the membrane apart.

The cell membrane prevents harmful things from entering the cell. It also keeps harmful things such as bacteria, viruses, and toxins from leaving the cell.

Cell Membrane And Cell Communication

The cell membrane has receptors on its surface. Receptors are proteins that bind to certain chemicals.

Once the receptor recognizes the chemical, the receptor sends signals to the rest of the cell.


Receptors are important because they allow cells to communicate with each other. For example, when a nerve cell receives a signal from an adjacent muscle cell, it releases acetylcholine.

Acetylcholine then travels along the axon to the next neuron, where it stimulates the release of more neurotransmitters.

Receptors are also used to send messages to the body. For instance, hormones travel through the bloodstream to target organs.

They attach to special receptors on the surfaces of these organs. Hormones stimulate the organ to perform specific tasks.

Ion Channels

The cell membrane also contains channels that transport ions across the membrane.

Ion channels are protein-based structures that open and close, allowing different substances to enter or leave the cell.

Channels are important because they control how much sodium, potassium, calcium, magnesium, chlorine, and bromine move into and out of the cell.

Channels are also important for maintaining the electrical potential difference across the membrane.

Electrical potential differences are caused by the movement of charged particles across the membrane. 

Different types of ion channels have different functions. Some channels only let sodium pass through them.

Other channels only let chloride go through them. Still, others only let calcium go through them.

Ion channels are also important for controlling cellular metabolism.

Metabolism involves the breakdown of nutrients (such as glucose) and the production of energy (ATP). Glucose enters the cell via glucose transporters. 

Inside the cell, glucose is broken down into pyruvate and NADH. Pyruvate is converted to lactate and NADH is oxidized to NAD+. Both processes produce ATP.

Ion channels play a role in regulating the flow of ions into and out of the mitochondria. Mitochondria are the powerhouses of the cell.

They generate most of the energy needed by the cell. The mitochondrial membrane contains voltage-dependent anion channels. 

Voltage-dependent anion channels regulate the amount of protons moving into and out of the matrix.

This process generates enough energy to fuel all the metabolic reactions within the mitochondria.

What Is Cell Membrane Structure?

The cell membrane is the thin barrier that separates the interior of a cell from the outside environment. It’s composed of lipids and proteins.

Lipids are molecules made of carbon atoms with oxygen and hydrogen atoms attached. These molecules form long chains that make up the lipid bilayer.

The protein portion of the cell membrane is responsible for holding the lipid layers together. This structure is called the cytoskeleton.

What Are Some Functions Of Cell Membranes?

There are several functions of cell membranes. First, they act as barriers that separate one part of the body from another.

They prevent substances from entering or leaving certain parts of the body.

For example, blood vessels are lined with endothelial cells. The endothelium acts as a barrier between the bloodstream and surrounding tissues.

If a substance were to enter the bloodstream, the endothelial lining would stop it before it could reach any other tissue.

Second, cell membranes help maintain the shape of the cell. For example, if you cut open a potato, you’ll see that its cells remain intact because they’re held together by cell walls.

Without cell walls, the potato would fall apart.

Third, cell membranes provide support for the cell. In fact, the main function of the cytoplasm is to hold the nucleus, organelles, and all of the rest of the cell’s components together.

Fourth, cell membranes protect the cell against damage. When the cell membrane becomes damaged, it releases chemicals that attract white blood cells to attack foreign invaders.

Finally, cell membranes transport materials throughout the body. For example, nerve impulses travel down the axon of a neuron. At the end of the axon, there is a synapse.

This area contains tiny openings called synaptic vesicles. These vesicles contain neurotransmitters that are released at the synapses.

Once inside the synaptic vesicle, these molecules are transported across the cell membrane to the next neuron.

Differences Between Animal Cells And Plant Cells

Differences Between Animal Cells And Plant Cells

Animal cells and plant cells have different structures. The differences help explain why animal cells and plant cells behave differently.

Animal Cell Structure

An animal cell has three major layers. The outer layer contains the plasma membrane. The middle layer includes organelles. The innermost layer consists of cytoplasm.

Plant Cell Structure

A plant cell does not contain an outer plasma membrane. Instead, it has a thick cell wall. Organelles are found only in the cytoplasm. There are no mitochondria or chloroplasts.

How Do Animals And Plants Differ?

Animals and plants differ because they have different functions. An animal needs energy to survive. To get this energy, animals use oxygen to burn food.

This process produces heat and carbon dioxide. Heat is released into the environment. Carbon dioxide combines with water to create carbonic acid.

This chemical reaction helps neutralize acids in the blood.

Animals also produce hormones. Hormones control many processes inside the body. These include growth, reproduction, digestion, sleep cycles, and mood swings.

Plants do not need oxygen to live. They obtain their energy from sunlight. Sunlight provides them with carbohydrates.

Carbohydrates provide the raw materials needed to build cells.

What Are Mitochondria?

Mitochondria are small bodies located within the cell. They play an essential role in cellular respiration.

Cellular respiration is the process by which living organisms extract energy from nutrients.

Mitochondria perform two basic functions:

They convert glucose into pyruvate. Pyruvate then enters the citric acid cycle. This process creates ATP (adenosine triphosphate).

ATP is a molecule that stores energy. It is used as fuel for muscle contraction.

They break down fats and other lipids. Fatty acids enter the mitochondrion. Then, enzymes called carnitines transfer the fatty acids across the mitochondrial membrane.

Carnitines attach themselves to coenzyme A. Coenzyme A is a compound that carries electrons around the cell.


Differentiation means changing into something different. Differentiation is a process where cells change their shape and size. Cells differentiate during development.

The process begins when stem cells divide. Each daughter cell has half the number of chromosomes as the mother cell. 

As the cells divide, the number of chromosomes decreases until all the cells have the same number of chromosomes.

Stem cells usually contain 23 pairs of chromosomes. After differentiation, cells have only 22 pairs of chromosomes.

How Do Plant Cells Have Different Types Of Membranes?

The primary difference between a plant cell membrane and an animal cell membrane is that plant cell membranes do not possess a rigid wall like animal cell membranes.

Instead, plant cell membranes are flexible and allow water to pass through them easily.

In addition, plant cell membranes are much thinner than animal cell membranes.

A typical plant cell has a diameter of 10 micrometers, while an average animal cell measures 25 micrometers wide.

Another major difference between plant cell membranes and animal cell membranes is that plant cell membranes are highly permeable.

Plants use this characteristic to move nutrients around within their bodies. 

For example, when a plant takes in sunlight, it uses photosynthesis to convert light energy into chemical energy.

During this process, some of the electrons produced during the conversion process leak out of the chloroplast and travel through the thylakoid membrane to the stroma. 

Here, the electron travels along the thylakoid network until it reaches the endoplasmic reticulum, where it is used to produce ATP.

This process allows plants to store solar energy in the form of chemical bonds rather than heat.

Because plants don’t need to generate heat to survive, they can grow in areas that would otherwise be too hot for other organisms.

In contrast to animals, plant cells have two types of membranes: plasma and tonoplast. 

Plasma membranes surround the outside of the cell. Tonoplast membranes are found on the inside of the cell.

Plasma Membranes

Plasma membranes consist of phospholipid bilayers. Each layer has a hydrophilic head group and a hydrophobic tail.

The hydrophilic heads face outward toward the surrounding environment and the hydrophobic tails face inward toward the center of the cell.

When a lipid molecule undergoes phase separation, the lipids will spontaneously organize themselves into different phases.

If one side of the lipid molecule is more polar than the other, then the whole molecule will separate into two layers. 

One layer consists of the nonpolar part of the molecule, which remains in the middle of the solution.

The second layer consists of the polar parts of the molecule, which remain near the surface.

When a lipid molecule undergoes such a phase transition, the polar portion of the molecule becomes exposed on the outer surface.

In this way, the entire lipid molecule forms a barrier between the interior and exterior of the cell.

Because the polar portions of the lipid molecules are exposed on the outer surface, the plasma membrane acts as a barrier between the inside and outside of the cell.

It also prevents substances from entering or leaving the cell. This makes the plasma membrane very important for maintaining cellular homeostasis.

Tonoplast Membranes

The tonoplast is located at the center of the cell and surrounds the nucleus. It is made up of stacks of flattened sacs called vesicles.

These vesicles contain various enzymes that help break down food into smaller pieces so that the cell can absorb them. 

When the cell breaks down its food, it produces waste products. To remove these waste products, the cell needs to transport them away from the cell.

The tonoplast helps with this by creating channels that allow waste products to flow out of the cell.

The structure of the tonoplast is similar to that of the plasma membrane. However, there are differences.

First, the tonoplast contains fewer proteins than the plasma membrane does. Second, the tonoplast lacks the protein pumps that exist in the plasma membrane. 

Third, the tonoplast is not as rigid as the plasma membrane. Instead, the tonoplast has a flexible structure that allows it to change shape easily.

Types Of Cells In Plants

Plants contain many different kinds of cells. These include:

  • Epidermal cells form the outer layer of plant tissue. They cover the entire leaf. Epidermal cells are very thin. They do not have nuclei.
  • Lignified cells provide support and protection. Lignified cells are hard and strong. They give plants their shape and strength.
  • Parenchyma cells hold water and minerals. Parenchyma cells store food. They are usually round.
  • Saprophytic cells break down dead material. Saprophytic cells release enzymes into the soil that break down organic matter. This allows microorganisms to use it as food.

Animal Cell Membrane Structure

Animal cells differ from plant cells in several ways. For example, animal cells have nuclei while plant cells do not.

Also, animal cells have a cytoplasm that contains organelles. Plant cells lack an internal compartment like the cytoplasm.

Animal cells also have three main components: the plasma membrane, the cytoskeleton, 

and the nucleus. The plasma membrane is composed of a lipid bilayer. Like the plasma membrane, the inner leaflet of the plasma membrane faces the inside of the cell.

On the other hand, the outer leaflet of the plasma membranes faces the outside of the cell.

Another difference between the plasma membrane and the nuclear envelope is that the former is much thinner than the latter.

The thickness of the plasma membrane varies depending on the type of cell. For example, red blood cells have a very thin plasma membrane.

On the other hand, white blood cells have a thicker plasma membrane.

The plasma membrane is connected to the rest of the cell via the actin microfilaments.

Microfilaments are long bundles of protein filaments that run through the cytoplasm of the cell. They are attached to both sides of the plasma membrane.

In addition to being connected to the plasma membrane, the actin microfilament system plays another role in the cell.

It provides structural support for the cell. Without this support, the cell would collapse under its own weight.

The second component of animal cells is the cytoskeleton. Cytoskeletons provide stability to the cell. In most cases, they are found only in animal cells.

There are two types of cytoskeletons: microtubules and intermediate filaments. Both are made of tubulin and actin proteins.

Microtubules are hollow tubes that form part of the mitotic spindle. During mitosis, the spindle pulls apart the chromosomes during cell division.

This process separates the genetic material (DNA) into daughter cells.

Intermediate filaments are found in all eukaryotic organisms. They are made of keratin proteins. Keratins are large proteins that make up hair and nails.

Intermediate filaments are also found in epithelial cells. Epithelial cells line the surfaces of organs such as the lungs, kidneys, intestines, skin, etc.

Nuclear Envelope Structure

Nuclear Envelope Structure

The nuclear envelope consists of two layers: the inner and the outer nuclear membrane.

The inner nuclear membrane surrounds the nucleus. The outer nuclear membrane forms a ring around the inner nuclear membrane.

The inner nuclear membrane is studded with pores called nuclear pore complexes.

These complexes allow small molecules to enter or leave the nucleus. Molecules larger than about 40 kDa cannot pass through the nuclear pore complexes.

The inner nuclear membrane is connected to the endoplasmic reticulum by bridges called lamins. Lamins are proteins that provide strength to the nuclear envelope.

Lamin A is associated with the inner nuclear membrane, while lamin C is associated with the endoplasmic reticulum.

The outer nuclear membrane is connected to chromatin. Chromatin is DNA packaged into nucleosomes.

Nucleosomes are structures that contain 147 base pairs of DNA wrapped around eight histone proteins. Histones are basic proteins that help package DNA into nucleosomes.

There are four major classes of histones: H1, H2A, H2B, and H3. Each class has several subclasses.

H1 Histones

There are three main types of H1 histones: H1a, H1b, and H1c. All three are located at the periphery of the nucleus.

  • H1a Histones

H1a histones are expressed throughout the body. Their expression levels vary depending on where in the body they are produced.

For example, H1a histones from muscle tissue are more abundant than those from brain tissue.

H1a-containing nucleosomes are present in all tissues. However, their abundance varies among different tissues.

For example, H 1a-containing nucleosome levels are higher in skeletal muscle than in heart muscle.

  • H1b Histones

H1b histones are found in testis and ovaries. They may play an important role in spermatogenesis and oogenesis.

  • H1c Histones

H1c histones are found in erythrocytes. Erythrocytes have no nucleus. Instead, they have a cytoplasm filled with hemoglobin.

Hemoglobin contains iron atoms. Iron atoms are needed for oxygen transport in red blood cells.

In addition to these three main classes of H1 histones, there are also several other minor variants.

Some of these variants are found only in specific tissues. Others are found globally across many tissues.


Lamins are intermediate filament proteins that form the nuclear lamina. There are three types of lamins: A, B, and C.

Lamins are attached to the inner surface of the nuclear membrane. Lamins also connect the inner nuclear membrane to the endoplasmic reticulum.

Lamins are divided into three groups based on their size and location within the cell. Large lamins (lamin A and C) are found between the inner and outer nuclear membranes.

Smaller lamins (lamin B1 and 2) are found inside the nucleus. Lamins can be found in both animal and plant cells.

Types Of Cells In Animals

There are a number of types of cells found within the body of animals:

  • Epithelial cells line the surface of organs and glands. They protect the organ from foreign objects.

Epithelium lines the walls of the stomach, bladder, and intestines. The lining of the lungs is made of epithelial cells.

  • Endothelial cells line the interior surfaces of blood vessels. They allow nutrients to enter and waste products to leave the bloodstream.

Muscle cells move throughout the body. They help control the heart rate, regulate blood pressure, and produce energy.

  • Muscles are responsible for moving your arms, legs, eyes, tongue, and lips.
  • Nerve cells transmit signals throughout the body. Nerves carry messages from one part of the body to another.

Nerve cells send signals to muscles or organs. They receive information from them.

  • Neurons are nerve cells. Neurons have long extensions called axons. Axons connect neurons to other neurons or to muscles.
  • White blood cells (WBC) fight infections. WBCs play a vital role in protecting the body from bacteria, viruses, fungi, parasites, and toxins.

Lymphocytes are white blood cells. Lymphocytes are involved in fighting infection. B-cells produce antibodies. T-cells destroy infected cells.

Toxins And Disease In The Cell Membrane

Many diseases result when toxins or pathogens damage the cell membrane. Toxins can be dangerous if they get inside the cell.

Pathogens can cause disease if they enter the cell without being destroyed.

For example, many bacterial infections begin when the bacteria penetrate the skin. Bacteria use toxins to kill host cells.

These toxins destroy the membranes of the host cells. This allows the bacteria to multiply rapidly.

Other examples of diseases and conditions that can be caused by damaged cell membranes include:

Acute Lymphoblastic Leukemia

Acute lymphoblastic leukemia occurs when white blood cells called leukocytes do not mature properly.

Leukocytes are part of the immune system. If they do not mature properly, they cannot fight infection.

Alzheimer’s Disease

Alzheimer’s disease results when proteins clump together forming plaques and tangles. Plaques form outside the cell and tangle inside the cell. Tangles form inside the cell.


Amyloidosis occurs when amyloid proteins clump together and build up in tissues. Amyloid proteins are normally produced by the liver.

When amyloid proteins accumulate in the liver, it causes cirrhosis.


Arthritis occurs when cartilage breaks down causing pain and swelling. Cartilage is a tough material between joints. It protects bones from rubbing against each other.


Asthma is a chronic lung condition that makes breathing difficult. Airway inflammation is one of the main symptoms of asthma.

Inflammation is caused by substances such as histamine and bradykinin. Histamine stimulates airways to contract.

Bradykinin causes smooth muscle around the bronchioles to constrict.

Role In Research

The cell membrane plays an important role in research. Scientists study how cells work using microscopes.

They look at what happens to molecules when they pass through the cell membrane. They also observe changes in the cell membrane.

Scientists can learn about the structure and function of the cell membrane by studying its components.

For example, scientists can identify which proteins make up the cell membrane. They can also determine whether there are any holes in the cell membrane.

They may even find out why some parts of the cell membrane are more permeable than others.

Scientists can also study the effects of drugs on the cell membrane. Some drugs act directly on the cell membrane. Others affect the body’s response to the drug.

Researchers must understand how these drugs interact with the cell membrane before they can develop new treatments for diseases like cancer.

Final Thoughts

Understanding cell membranes is an important element of mastering microbiology and biological processes, and this introduction should offer a clear insight into the main elements and concepts that you need to understand.

Jennifer Dawkins