Cells, specialisation and membrane transport

Cells, specialisation and membrane transport

The cell is the basic structural and functional unit of living organisms. Some organisms consist of one cell that carries out all essential processes, while multicellular organisms contain many specialised cells. The cell theory provides a foundation for studying growth, reproduction and disease: living organisms are made of cells, and new cells arise from existing cells through cell division.

A typical animal cell has a cell membrane, cytoplasm and a nucleus. The membrane controls exchange with the surroundings, while the cytoplasm contains the structures and reactions needed for life. The nucleus holds most of the cell’s genetic material and helps regulate activities. Mitochondria are important sites of aerobic respiration, transferring energy from food molecules into forms the cell can use.

A typical plant cell also has a cellulose cell wall and a large permanent vacuole. Many plant cells, particularly those in photosynthetic tissues, contain chloroplasts with chlorophyll. Not every plant cell has chloroplasts: root cells growing underground generally do not photosynthesise. The cell wall gives support, while the membrane remains the selectively permeable boundary that regulates passage of many substances.

Diffusion is net movement of particles from higher concentration to lower concentration as a result of random motion. It does not require direct energy expenditure by the cell. Osmosis is net movement of water through a selectively permeable membrane from higher water potential to lower water potential. Describing only ‘high to low concentration’ can be ambiguous unless the substance whose concentration is meant is identified.

Active transport can move substances against their concentration gradient using energy and membrane transport proteins. Root uptake of some mineral ions is a useful example. Cells can also be specialised: a root-hair cell has a large surface area for absorption, while a red blood cell has adaptations for carrying oxygen. Relate the structural feature to the function rather than merely listing a cell’s shape.

Microscopes help reveal cellular structure, but images need scale. Magnification equals image size divided by actual size, using the same units. A drawing should have clear continuous lines, suitable proportions and labels that point accurately to the structures. Do not invent structures that are absent from the specimen, and distinguish what you observe from the biological explanation you infer from it.

From a cell to a functioning organism

A specialised cell has structures suited to a particular task. A root hair cell has an elongated projection that increases the area available for water and mineral uptake. A palisade mesophyll cell contains many chloroplasts for absorbing light. A mammalian red blood cell has a flexible, biconcave form and, when mature, no nucleus; this leaves more room for haemoglobin and helps it pass through narrow capillaries. A nerve cell has a long extension that carries electrical signals over considerable distances. In each example, identify the feature, the immediate advantage it provides and the biological task it supports. Saying a cell is “adapted” without explaining that chain does not establish how the adaptation works.

Cells with related roles form tissues. Several tissues work together as an organ, and organs cooperate in a system. The small intestine is an organ containing muscle, epithelial and other tissues. It belongs to the digestive system but also exchanges absorbed substances with the blood and lymph. These levels describe organisation, not size alone. A unicellular organism still performs nutrition, respiration, excretion and response even though it has no multicellular organs. Do not assume that a structure absent from one organism makes that organism non-living.

Distinguishing transport mechanisms

Diffusion is the net movement of particles down their concentration gradient because of random motion. Movement continues in both directions at equilibrium, but there is no overall transfer. Oxygen can diffuse from an air space into a respiring cell when its concentration is lower inside. A larger concentration difference, higher temperature, greater surface area and shorter distance can increase diffusion rate. These factors explain why a large organism cannot rely solely on diffusion across its outer surface to supply deep tissues.

Osmosis concerns water passing through a selectively permeable membrane from higher water potential to lower water potential. In a simple comparison, this often means movement from a more dilute solution to a more concentrated one. The membrane is essential to the definition. Water moving across open ground is not osmosis. An animal cell placed in a sufficiently dilute surrounding solution may swell and burst because it has no cell wall. A plant cell develops turgor as the contents press against its wall. A sufficiently concentrated surrounding solution causes water loss, loss of turgor and, in stronger cases, plasmolysis, where the membrane pulls away from the wall.

Active transport moves a substance against its concentration gradient using energy supplied by cellular metabolism and suitable membrane proteins. Roots may absorb particular mineral ions from a dilute soil solution by this process. Both diffusion and active transport can occur in a living cell at the same time. The relevant question is which mechanism explains the direction and energy requirement of the particular movement, not whether the cell is living. Selective permeability also means that different substances cross at different rates; it does not mean the membrane simply has holes of one fixed size.

Making sense of an osmosis investigation

Imagine equal potato cylinders placed in several labelled sugar solutions. Measure initial and final masses after the same immersion time, gently blotting surface liquid before weighing. Keep cylinder dimensions, potato source, temperature and immersion time comparable. Percentage mass change is (final mass − initial mass) ÷ initial mass × 100. A positive change suggests net water entry under the conditions used, and a negative change suggests net water loss. A concentration giving approximately zero mass change estimates the external concentration at which there was little overall water movement. It does not prove that individual water molecules stopped moving.

A cylinder that changes from 5.0 g to 5.5 g gains 0.5 g, so its percentage mass increase is 0.5 ÷ 5.0 × 100 = 10%. If a different cylinder falls from 5.0 g to 4.6 g, its change is −8%. Comparing percentages is more useful than comparing gains alone when starting masses differ. Repeats help identify natural specimen variation and measurement error. Do not interpret a tiny change beyond the balance's resolution as strong evidence. Real tissues also contain living cells and multiple dissolved substances, so a classroom result is an estimate rather than a perfect measurement of cell contents.

Self-check and common errors

Explain why a plant wilts when it loses water: cells lose turgor, so soft tissues have less internal support. Explain why a cell membrane and a cell wall cannot be exchanged in an answer: the membrane regulates transfer, while the wall provides structural resistance. Finally, compare a root absorbing water with a root taking up a scarce mineral ion. The water may enter by osmosis; the mineral may require active transport. Avoid saying that osmosis is the movement of “a solution” or that plants do not have mitochondria. Plant cells respire as well as photosynthesise.