Cell-surface membrane

The cell-surface membrane forms a boundary between a cell and its surroundings. It is selectively permeable: it allows some substances to cross more readily than others. In this way, it controls the movement of substances into and out of the cell.

Structure of the cell-surface membrane

The membrane consists mainly of two layers of phospholipids, called a phospholipid bilayer, with proteins embedded in it or associated with it. Each phospholipid has a phosphate head and fatty acid tails. The head is hydrophilic, meaning it is attracted to water, while the tails are hydrophobic, meaning they avoid water.

The heads face the watery environments on either side of the membrane: the fluid inside the cell and the fluid outside it. The tails face inwards, away from water and towards one another. This arrangement forms the membrane’s boundary and restricts the passage of some substances.

Membrane proteins

Some membrane proteins help substances cross the membrane, contributing to its control of transport. Others take part in cell signalling: they receive signals from the cell’s surroundings and help the cell respond.

Mitochondria

Mitochondria are organelles that carry out key reactions of aerobic respiration. This process releases energy from nutrients using oxygen and transfers some of that energy to ATP, a molecule that supplies energy for activities in the cell.

Mitochondrial membranes and matrix

Each mitochondrion is surrounded by two membranes. The inner membrane is folded into structures called cristae. These folds provide a large surface area for reactions involved in aerobic respiration and ATP production.

The inner membrane encloses a space called the matrix. The matrix contains enzymes involved in respiration, as well as mitochondrial DNA and ribosomes. The DNA carries instructions for making some mitochondrial proteins, and the ribosomes help make those proteins.

Mitochondria and energy demand

The number of mitochondria in a cell can reflect how much energy the cell needs. For example, a muscle cell with a high energy demand for contraction may contain many mitochondria.

Chloroplasts

Chloroplasts are structures inside plant and algal cells where photosynthesis occurs. During photosynthesis, light energy is used to make sugars.

Each chloroplast is surrounded by two membranes. Inside it is a system of flattened membrane sacs called thylakoids. Thylakoids are often arranged in stacks called grana; a single stack is a granum.

The thylakoid membranes contain chlorophyll, a pigment that absorbs light for photosynthesis. The fluid surrounding the thylakoids is called the stroma. It contains enzymes involved in photosynthesis, as well as the chloroplast’s own DNA and ribosomes.

Golgi apparatus

The Golgi apparatus receives substances made in the cell, changes them where needed, and sends them to their destinations. It consists of a stack of flattened, membrane-bound sacs.

Substances, including proteins, arrive from the endoplasmic reticulum in small membrane-bound sacs called vesicles. The Golgi apparatus modifies and sorts these substances, then packages them into Golgi vesicles.

The vesicles transport their contents to other parts of the cell or to the cell-surface membrane. At the cell-surface membrane, a vesicle can fuse with the membrane and release its contents outside the cell. This release is called secretion.

Lysosomes

A lysosome is a membrane-bound organelle that contains digestive enzymes. These enzymes break down material taken into the cell, as well as worn-out cell components.

The enzymes are called hydrolytic enzymes because they break molecules apart by hydrolysis: water is used to break chemical bonds. The lysosome’s surrounding membrane separates these enzymes from the cytoplasm.

Material to be digested can be enclosed in a compartment within the cell. A lysosome can fuse with that compartment, releasing its enzymes inside. The enzymes then break down the enclosed material.

Eukaryotic ribosomes

Ribosomes are structures where cells make proteins. They are not surrounded by a membrane. Each ribosome is made of ribosomal RNA and proteins arranged in two parts: a large subunit and a small subunit.

During protein synthesis, a ribosome uses the information carried by messenger RNA (mRNA) to join amino acids in a particular order. The resulting chain of amino acids is called a polypeptide.

In eukaryotic cells, cytoplasmic ribosomes may be free in the cytoplasm or attached to the rough endoplasmic reticulum.

Rough endoplasmic reticulum

The rough endoplasmic reticulum (rough ER) is a network of flattened, membrane-bound sacs in a cell. Ribosomes attached to the outer surface give it a rough appearance.

The ribosomes make proteins. Many of these proteins enter the interior of the rough ER, where they fold into their functional shapes and undergo initial processing.

Small membrane-bound sacs called transport vesicles carry these proteins from the rough ER towards the Golgi apparatus. There, the proteins undergo further processing and sorting.

Eukaryotic cell walls

A cell wall is a layer outside the cell-surface membrane. It provides mechanical strength, helping the cell maintain its shape.

When water enters a cell, the cell contents push outwards against the wall. The wall resists expansion, helping to prevent the cell from bursting.

The cell wall is distinct from the cell-surface membrane, which lies just inside it. The membrane is selectively permeable: it controls which substances enter and leave the cell. The wall provides structural support rather than serving as the selectively permeable barrier.

Cell walls in different eukaryotes

Plant cell walls contain cellulose, while fungal cell walls contain chitin. Algal cells also have cell walls, but their composition varies between groups of algae.

Plant cell vacuole

A plant cell vacuole is a compartment surrounded by a membrane. It contains cell sap, a solution of water and dissolved substances. The membrane surrounding the vacuole is called the tonoplast.

In many mature plant cells, a large vacuole occupies the centre of the cell. Water in the vacuole helps the cell contents press outwards against the cell wall. The cell wall resists this pressure, keeping the cell firm. This firmness is called turgor and helps support the cell and maintain its shape.

Specialisation of eukaryotic cells

In a complex multicellular organism, different cells perform different functions. A specialised cell has features that help it carry out a particular role. These features may include its shape, the abundance of a cell structure, or the way structures are arranged.

For example, a root hair cell absorbs water from the soil. Its long, thin extension gives it a large surface area in contact with the soil, allowing more water to enter the cell.

A sperm cell has a tail that propels it towards an egg. It also has many mitochondria, which release energy needed for the tail to move. Both features are suited to the sperm cell’s role in reaching the egg.

To explain a cell’s adaptation, name a specific feature and link it to the function it helps the cell perform. For a root hair cell, stating that it has a long extension identifies the feature; explaining that the extension increases the surface area for water absorption shows how the feature helps.

Levels of organisation in multicellular organisms

In a complex multicellular organism, cells have different jobs. A specialised cell has features that help it carry out a particular function. Cells do not work alone: they are organised into tissues, organs and organ systems.

A tissue is a group of cells working together to carry out a function. An organ is made of different tissues working together. An organ system is a group of organs that cooperate to perform a broader function for the organism.

For example, muscle cells form muscle tissue in the stomach. Muscle tissue works with other tissues in the stomach, including its lining, to help the stomach carry out its functions. The stomach is an organ; it works with other organs, such as the small intestine, in the digestive system.

In this way, the activities of individual cells contribute to the work of tissues, organs and organ systems, connecting them to the functioning of the whole organism.